OR/15/026 Intrusive igneous rocks

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Gunn, A G, Mendum, J R and Thomas, C W. 2015. Geology of the Huntly and Turriff Districts. Sheet description for the 1:50 000 geological sheets 86W (Huntly) and 86E (Turriff) (Scotland). British Geological Survey Internal Report, OR/15/026.
Figure 4 Distribution of pre-tectonic and syntectonic mafic-ultramafic intrusive rocks in the Huntly district.

Intrusive igneous rocks are a major feature of the Huntly and Turriff districts (Figure 1). The Ordovician mafic and ultramafic igneous intrusions that constitute the North-east Grampian Basic Subsuite, a member of the Scottish Highlands Ordovician Suite, form a significant and distinctive element of the geology of this part of Scotland (Figure 4). The Huntly, Knock and Insch gabbro-peridotite plutons form the major intrusions but there are numerous smaller bodies and a wide development of contaminated mafic rocks and interleaved metasedimentary and mafic-ultramafic rocks. The syn- to late-tectonic plutons are also important as potential hosts of economic deposits of nickel, copper and platinum-group elements (PGE). Academic studies and commercial exploration programmes since the 1960s have complemented work by BGS and led to an improved understanding of the nature and origins of the mafic and ultramafic intrusions in the region (Gunn, 1997)[1].

Read (1919[2], 1923[3]) divided the mafic-ultramafic igneous rocks of the district into two main categories: an ‘Older Series’ that were intruded prior to regional deformation and metamorphism; and a ‘Younger Series’ that postdated these events (Figure 4). The ‘Older Series’ included ultramafic, mafic and felsic intrusions that were generally highly deformed and pervasively metamorphosed. They outcrop as discontinuous concordant sheets and pods in a belt that runs south-south-west from the Banffshire coast at Portsoy for over 30 km. They have been designated here as the Succoth–Brown Hill type intrusions. Read’s ‘Younger Series’ included a similar range of compositions, here dominated by the mafic and ultramafic rocks of the Huntly, Knock and Insch plutons.

Igneous rock nomenclature used in this report is based upon the latest BGS Rock Classification Scheme (Gillespie and Styles, 1999[4]; Gillespie et al., 2012[5]).

Pretectonic granites

Keith–Portsoy Granite

Several lenticular bodies of foliated augen granite ranging from a few metres to 350 m thick have been intruded into Appin and Argyll group metasedimentary rocks in the north-west part of the Huntly–Turriff district. They form part of a more extensive network of sheets and lenses of granite that extends from a kilometre west of Portsoy, south-westwards to beyond Keith [NJ 433 505], approximately coincident with the anastomosing south-east-dipping Keith Shear Zone. The main outcrops were previously known as the ‘Windyhills Granite’ (Read, 1923[3]).

The granite bodies are mostly poorly exposed, and typically form yellow-brown, cream and grey, weathered, friable, rounded masses with wide jointing. However, in parts they are represented by collections of subangular to rounded, relatively unweathered boulders. The best exposures are found in the Bowie Burn, around [NJ 4865 5633], in the Burn of Aultmore between [NJ 4602 5294] and [NJ 4596 5251], and around Over Windyhills [NJ 4923 5672] and Nethertown [NJ 4928 5725] farms, from where they extend up the western flank of Lurg Hill [NJ 507 575].

Where fresh and unfoliated, e.g. on parts of Lurg Hill, the rock is a grey and pink mottled, macroporphyritic, granite with minor muscovite. More commonly it is strongly foliated with a marked down-dip extension lineation, with conspicuous feldspar augen (after phenocrysts) and abundant muscovite. Pink potash feldspar megacrysts, up to 30 mm long, are ubiquitous and smaller subsidiary plagioclase megacrysts also occur in parts. The granite typically contains abundant small grey, fine-grained dioritic and metasedimentary xenoliths. The pervasive foliation is defined by aligned biotite, muscovite, recrystallised elongate to ribbon quartz, and by the preferred orientation of the feldspar megacrysts. It strikes north-east and dips generally south-east at 25° to 40°, subparallel to the margins of the granite sheets, and the bedding and regional cleavage. Where granite sheets intrude semipelites, their margins are commonly diffuse, and the semipelites share a similar fabric and extension lineation with the granite. In some areas pink feldspar porphyroblasts are locally developed in semipelitic rocks marginal to the granite, e.g. in the Burn of Aultmore at [NJ 4590 5334].

Eight representative samples were collected from fresh exposures the along its outcrop of the Keith–Portsoy Granite. The localities selected stretched from the Banffshire Coast south-west via Boggierow Quarries (Sheet 96W – Portsoy district) to Lurgbrae in the Huntly district and Muldearie, Keith, Glass and the Hill of Bellyhack in the Glenfiddich district (Sheet 85E). The samples were analysed at BGS in Keyworth using X-Ray Fluorescence Spectrometry on fused glass beads for the major elements, and pressed powder pellets for the trace elements. Full sample locations and analytical values are given in Appendix 1.The sample from the Banffshire coast section (GX 1717) was taken from a discordant lenticular granite dyke where it cuts an amphibolitic mafic sheet. Its geochemistry shows that the granite has assimilated a limited amount of mafic material in that it exhibits markedly low SiO2, K2O, P2O5 and Rb values and elevated TiO2, Al2O3, Fe2O3, MgO, Na2O, CaO, SO3, and Cu, Sr and Zr values. As such it is atypical for the granite and is excluded from the discussion below and from Table 3 that shows the mean composition of the Keith–Portsoy granite in comparison to that of Ben Vuirich and published mean values for the major typological classes of granitic rocks.

Table 3 Whole-rock geochemistry (mean values) of the Keith–Portsoy Granite, Ben Vuirich Granite, and the major typological classes of granite.
1: mean of 7 samples.
2: mean of 33 samples (from Tanner et al., 2006[6]).
3: mean of 578 S-type granites, 991 I-type granites, 421 felsic I-type granites and 148 A-type granites (Whalen et al., 1987[7]).
Keith– Portsoy1 Ben Vuirich2 S-type3 I-type3 Felsic I-type3 A-type3
SiO2 70.80 72.30 70.27 69.17 73.39 73.81
TiO2 0.59 0.60 0.48 0.43 0.26 0.26
Al2O3 13.92 12.96 14.10 14.33 13.43 12.40
FeO 3.51* 3.26 2.87 2.29 1.32 1.58
MnO 0.04* 0.06 0.06 0.07 0.05 0.06
MgO 1.08 0.74 1.42 1.42 0.55 0.20
CaO 1.21 1.51 2.03 3.20 1.71 0.75
Na2O 2.72 2.88 2.41 3.13 3.33 4.07
K2O 4.88 4.22 3.96 3.40 4.13 4.65
P2O5 0.16 0.16 0.15 0.11 0.07 0.04
Rb 185 152 217 151 194 169
Sr 165 162 120 247 13 48
Ba 533 720 468 538 510 352
Y 33 48 32 28 34 75
Zr 271 359 165 151 144 528
Nb 19 20 12 11 12 37
Ni 10 20 13 7 2 <1
Cr 43 26 nd nd nd nd
V 33 34 56 60 22 6

*Measured as Fe2O3 and Mn3O4

Figure 5 Niobium (Nb) v. Yttrium (Y) plot showing the Keith–Portsoy granite samples in relation to the fields of within-plate granites (WPG), volcanic arc and syn-collisional granites (VAG and syn COLG) and ocean-ridge granites (ORG).

The high SiO2 content (70.25 to 71.68 wt%; mean 70.80%) and restricted compositional range make it difficult to establish a clear lineage for the Keith–Portsoy Granite samples, and the absence of certain elements from the dataset, notably Ga, an important discriminator for A-type granitic rocks, precludes a full assessment of the typological affinity. However, the mean values for the Keith–Portsoy granite samples suggests it can be best described as exhibiting mild A-type characteristics, or characteristics transitional between A-type and felsic I/S-type granites. The granite has a high-K, peraluminous character, bordering on shoshonitic. On the Nb/Y diagram the analyses plot just in the within-plate field (Figure 5). As such, it displays notable compositional similarities with the Ben Vuirich Granite Pluton (Table 3), the ‘type’ intrusion of the approximately 600 Ma Vuirich Suite (Tanner et al., 2006[6]). Its similar composition and age (see below) suggest the Keith–Portsoy intrusion is part of the Vuirich Suite, a swarm of generally small, extension-related, broadly A-type granitoids that can be found stretching from the Appalachians to Scotland. Its sheet-like nature, strong foliation, and concentration in a roughly planar zone suggests that its emplacement, which occurred prior to the main regional deformation, was controlled by a pre-existing lineament. That zone was subsequently the focus for ductile shearing; granite samples from boreholes around Keith show distinctive asymmetrical augen ‘tails’ that imply that this later shearing was north-west directed (see Structure section). The related foliation and lineation correspond to the main secondary deformation phase in this area, here termed D2.

A concordant zircon U-Pb age of 599.9 ± 2.5 million years (Ma) was obtained from acicular euhedral zoned zircons separated from a foliated Portsoy Granite sample taken from Boggierow Quarries, about 1.6 km south-west of Portsoy. More internally complex, partly inherited zircons separated from a more strongly foliated granite sample from a borehole in the Keith area gave a discordant U-Pb age with a lower intercept at 600.8 ± 2.5 Ma and an upper intercept age at 1491 ± 25 Ma (Barriero, 1998[8]). The lower intercept age agrees with that obtained from the Boggierow sample, and confirms that the granite was intruded at about 600 Ma. The upper intercept suggests that the granite formed by partial melting of lower to middle continental crustal material previously subjected to a major tectonometamorphic event at around 1500 Ma.

The Neoproterozoic intrusion age for the Vuirich Suite granites corresponds to a period of rifting that marked a significant phase in the progressive thinning of the Rodinia supercontinent. The intrusion of the Vuirich Suite granites occurred coeval with voluminous localised basic magmatism, whose manifestation is well seen in the Tayvallich area of the south-west Highlands. These events took place some 30 to 50 Ma prior to rift-drift transition that signalled the formation of the Iapetus Ocean and formation of the separate continents of Laurentia and Gondwana (Dalziel, 1997[9]). In Scotland this last event marked the onset of dominantly turbiditic sedimentation, now manifest by Southern Highland Group metasedimentary rocks.

Pretectonic mafic–ultramafic rocks

In the Huntly district, lenses, pods and sheet-like bodies of mafic and ultramafic rocks crop out discontinuously within, or close to, the Portsoy Shear Zone between Brown Hill [NJ 440 367] and Drumnagorrach [NJ 522 525]. These rocks are considered to be pretectonic and are referred to here as the Succoth–Brown Hill (S-BH) type, after the large ultramafic intrusion of that name that lies some 4.5 km west-south-west of Huntly and extends south-westwards into the adjacent Glenfiddich district (Sheet 85E) (Styles, 1994[10], 1999[11]) (Figure 4).

Several small deformed and metamorphosed ultramafic bodies also occur mainly peripheral to the western part (Boganclogh sector) of the Insch Pluton. This concentration is termed the Boganclogh margin type, and is also interpreted as pretectonic (Styles, 1994[10], 1999[11]).

Succoth–Brown Hill type

The Succoth–Brown Hill mafic-ultramafic intrusion forms a poorly exposed elongate pod underlying an area of approximately 14 km2 (Gunn et al., 1990[12], 1996[13]). The boundaries of the intrusion are not exposed, but are interpreted as probably shear zones or faults. About 70 per cent of the complex is composed of metagabbroic rocks. The remainder consists of ultramafic rocks disposed in two belts along the northern and southern margins, the rocks along the latter being largely serpentinised.

Plate 1 Complex banding in metagabbroic rocks in the Succoth–Brown Hill pluton (P808816).

The metagabbroic rocks exhibit wide textural variation and include highly deformed, heterogeneous types, homogeneous, medium-grained variants with gabbroic texture and fine-grained, foliated mafic types (Plate 1 (P808816)). Penetrative foliations and mylonitic fabrics have mostly near vertical or steep east or south-east dipping orientations, probably reflecting the margins of the intrusion. Coarse-grained ultramafic intrusive rocks comprise variably altered and recrystallised clinopyroxene-rich and olivine-rich lithologies. Primary igneous features are poorly preserved as a result of later deformation, metamorphism and alteration. However, relatively undeformed modal layering that dips south-east at 30 to 40° is present in pyroxene-rich rocks in exposures near Red Burn [NJ 3448 8382].

It is not possible to deduce an original igneous ‘stratigraphy’ for the intrusion based on the distribution of rock types due to the deformation and the tectonic nature of the marginal zones and contacts. Magnetic and topographical data indicate that many of the units are discontinuous, their present extent being controlled by the occurrence of shear zones and later faulting. Elongate screens of quartzite have been mapped in the western sector of the intrusion. These may have been emplaced tectonically or may indicate that the igneous rocks were originally intruded as sheets.

Smaller bodies of S-BH type also occur at Whitehill [NJ 518 460], Brownhill [NJ 506 460] and about a kilometre north of Roadburn [NJ 513 453]. These intrusions lie within and adjacent to a major north-west-orientated faulted zone that effectively separates the Huntly and Knock plutons. Their external boundaries are inferred to be tectonic; north-east-trending shear zones occur parallel to the long dimension of the bodies and they are transected by the later north-west-orientated brittle faults. At Whitehill, heterogeneous, texturally diverse metagabbroic rocks are dominant, with subordinate discontinuous elongate pods and sheets of predominantly clinopyroxene-rich ultramafic rocks. Deformation fabrics are generally steeply dipping and lie subparallel to the mapped margins of the intrusions. Locally preserved primary igneous features indicate that these rocks were originally mafic and ultramafic cumulates. Relict igneous layering is preserved at Whitehill quarry [NJ 5186 4628] where thin, olivine-rich layers occur within clinopyroxenite.

In the Brownhill intrusion and its south-western extension towards Cairnie, the metagabbroic rocks are highly sheared within the PSZ, locally exhibiting a strong foliation. South of Cairnie, towards the River Deveron, metagabbroic rocks crop out in discontinuous sheets and lenses, one of which can be traced southwards into the Succoth–Brown Hill intrusion, These bodies lie close to the south-western corner of the Huntly pluton.

Serpentinised ultramafic rocks are found at several other localities within the PSZ. At the Hill of Milleath [NJ 470 425] sheared serpentinites, possibly metawehrlites, are poorly exposed on a prominent hill over an area of about 0.3 km2, forming an elongate pod about 1.2 km in length. A smaller pod of foliated serpentinite crops out adjacent to Ballochburn [NJ 489 479] at the western margin of the PSZ, close to the south-western corner of the Knock Pluton. The rock type here was originally rich in clinopyroxene, similar to the ultramafic rocks of the Succoth–Brown Hill intrusion. A further elongate body of ultramafic rocks up to 250 m wide, defined by ground magnetic survey data, extends from south of Drumnagorrach [NJ 5203 5251] in a south-westerly direction for about 1.8 km, again, along the western margin of the PSZ. Mineralogical studies indicate that these rocks were originally dunites and clinopyroxenites similar to those found in the Succoth–Brown Hill Intrusion. Many show evidence of an early stage mylonitisation under amphibolite-facies conditions.

Petrographical studies reveal certain common features within the S-BH type bodies:

  1. The ultramafic rocks are mainly clinopyroxene rich, originally either clinopyroxenites or olivine-clinopyroxenites. Subordinate wehrlites and dunites also occur.
  2. The order of primary crystallisation is either clinopyroxene-olivine-plagioclase or olivine-clinopyroxene-plagioclase.
  3. The mafic rocks were originally gabbros, but contain no evidence of original olivine or orthopyroxene.
  4. All the rocks exhibit evidence of multiple phases of recrystallisation and alteration. Primary magmatic textures are rarely preserved.

Microphotographs of typical S-BH type rocks are shown in Plate 3 (v, vi).

The pretectonic mafic and ultramafic rocks are also characterised by their distinctive mineral chemistry. In the ultramafic rocks of the Succoth–Brown Hill intrusion, clinopyroxenes, and the amphiboles which replace them, are rich in Ca and Mg, with Mg# around 90. Olivine compositions range from around Fo80 to Fo92. Although plagioclase ranges widely in composition (An90- An32), compositions are typically greater than An80 and, in some cases, even exceed An90. Styles (1994[10], 1999[11]) has shown that the ultramafic parts of the other S-BH type bodies in the region have comparable mineral chemistries.

Boganclogh margin-type ultramafic rocks

Small bodies of serpentinised and sheared dunite and harzburgite, which occur along the northern margins of the Insch Pluton, notably along the Boganclogh sector, are included in the Boganclogh margin type (Styles 1994[10], 1999[11]). In the Huntly district, elongate pods occur 1 km west of Old Merdrum [NJ 458 298], and at Leith Hall Home Farm [NJ 541 303 to 550 304]. A large body occurs on the Mount of Haddoch at the north-west corner of the Insch Pluton (Boganclogh Sector), but only just extends into the Huntly district at [NJ 434 297]. The rocks were originally harzburgites with subsidiary dunites, but most were completely serpentinised and recrystallised during shearing, destroying any original igneous fabrics.

The harzburgites contain orthopyroxene crystals up to 6 mm long, set in a groundmass of serpentinised olivine. These large orthopyroxenes have been deformed, resulting in bent crystals with some recrystallisation to finer-grained aggregates. The proportion of orthopyroxene varies from very low in the dunitic rocks up to about 20 per cent in the harzburgites. In some specimens, the olivine has almost completely altered to antigorite (serpentine), and the orthopyroxene to bastite (serpentine), or locally to amphibole. Where the serpentinites have been strongly deformed, the antigorite has recrystallised to define a strong planar fabric and secondary iron oxide has been streaked out, giving the rock a banded appearance. No mineral analyses have been obtained for these rocks in the district, but they are petrographically similar to the ultramafic rocks along the southern margin of the Boganclogh intrusion, which contain olivines of Fo92–89 and interstitial orthopyroxenes of En92.

Syntectonic mafic-ultramafic rocks

Intrusive rocks of the North-east Grampian Basic Subsuite form several major plutons, intrusion-swarms and sills in the East Grampian region. They are interpreted as having been intruded at or near the peak of metamorphism during the Early Ordovician and have been termed the ‘Younger Series’ (Read, 1923[3]), the ‘Younger Basic’ suite (Fettes and Munro, 1989[14]) and also the ‘Newer Gabbros'. The Huntly and Knock gabbro-peridotite plutons lie entirely within the Huntly district, and parts of the Insch Gabbro-peridotite Pluton also crop out along much of the southern margin of the Huntly and Turriff districts.

These intrusions are composed of a wide range of mafic and ultramafic lithologies of both cumulate and non-cumulate origin. Previous studies have focused mainly on the cumulate rocks, which consequently are better known than the non-cumulate rocks. Subsidiary intermediate and felsic lithologies are also present, notably in the insch Pluton. Clarke and Wadsworth (1970)[15] established a zonal classification for the cumulate rocks of the Insch Pluton based upon phase layering:

Upper Zone (UZ) - UZc: syenite
UZb: olivine monzonite, olivine monzodiorite
Uza: olivine ferrogabbro
Middle Zone (MZ) - norite, gabbro, quartz-biotite-norite
Lower Zone (LZ) - dunite, peridotite, troctolite, olivine-norite, olivine-gabbro

Associated cryptic variation in mineral chemistry indicates progressive fractionation of the magma upwards through the zonal sequence. This framework has been adopted by other workers as a basis for comparison between the intrusions in the region and for studying their petrogenesis.

Huntly Gabbro-peridotite Pluton

The Huntly Gabbro-peridotite Pluton underlies approximately 40 km2 around the town of Huntly. The external boundaries and internal structures of the intrusion are very poorly exposed. Major parts of the western margins of both the Huntly and Knock plutons are bounded by north- to north-east-trending shear zones that form part of the PSZ (Figure 4). The Central Huntly Shear Zone, that branches off the PSZ a few hundred metres south of Whitehill, transects the central part of the Huntly mass. Farther north the zone is coincident with the eastern flank of the Knock intrusion for at least 3 km. Evidence for shearing at the western boundary of the Huntly body comes from boreholes near Cairnford at [NJ 4861 4080] and at Drumdelgie [at around NJ 486 422] (Munro, 1984[16]). At Drumdelgie sheared and crushed cumulates occur close to metasedimentary rocks that lack evidence of hornfelsing, indicating tectonic juxtaposition. At the north-western boundary of the pluton mylonitic and foliated rocks with some brecciated zones make up a complex sheared margin and are exposed in sporadic exposure along the Burn of Cairnie to the south of Midtown [around NJ 492 444].

Ground magnetic surveys across the eastern and south-eastern margins of the intrusion suggest that magnetic gabbroic rocks, inferred to be olivine-bearing cumulates, are intercalated with a mixed nonmagnetic assemblage of altered and contaminated gabbroic and metasedimentary rocks. The northern boundary of the intrusion is inferred from magnetic and topographical discontinuities to be defined by faults trending north-west and east-north-east. Faults orientated approximately west-north-west and north-west also define much of the south-western margin of the intrusion. A gas pipeline trench extending for almost 7 km from the River Deveron at [NJ 485 405] towards the south-south-east to [NJ 545 379], about 650 m south of Cairn Hill, provided useful control on the position of the southern contact (Munro, 1984[16]; Munro and Gallagher, 1984[17] ). Along most of its length the trench exposed non-hornfelsed migmatitic metasedimentary rocks, locally mylonitic, but gabbroic rocks were recorded in a short section south of Craigwillie, around [NJ 51 39].

A wide variety of rock types have been identified within the Huntly intrusion reflecting primary magmatic variations, subsequent contamination, and later deformation and alteration. Four principal groups of rocks are recognised:

  1. cumulate rocks
  2. modified cumulate rocks
  3. contaminated igneous rocks
  4. pegmatitic rocks

Internal boundaries between these groups are not exposed, but their positions can be inferred locally from topographical features and magnetic data. The olivine cumulate rocks produce the highest magnetic field intensity and as a result their contacts can be defined relatively accurately. In contrast the other rock types (modified cumulates and contaminated/xenolithic lithologies) have generally low magnetic susceptibilities.

Cumulate rocks

Figure 6 Maps showing the distribution of the main component lithologies in the Huntly and Knock gabbro-peridotite plutons. a) Cumulate rocks ; b) Modified cumulate rocks; c) Contaminated igneous rocks; d) Pegmatitic and granitic rocks.

Mafic and ultramafic intrusive igneous rocks displaying cumulate textures are widely distributed in the Huntly pluton (Figure 6a). They may be recognised in hand specimen by their euhedral, commonly equigranular, medium-grained textures. The principal mafic rock type is olivine-gabbro, with minor troctolite, olivine-melagabbro and rare anorthosite. Ultramafic rock types are subordinate to those of mafic composition, but include plagioclase-bearing peridotite and melatroctolite with minor developments of dunite, peridotite and pyroxenite. The cumulate rocks locally display excellent primary igneous textures, principally layering and crystal lamination.

In the Huntly pluton cumulate rocks form several apparently discrete bodies. Some of these are defined on the basis of single outcrops or boreholes, locally supported by magnetic survey data. The largest outcrops are termed the West Huntly and East Huntly cumulate bodies, but smaller bodies also occur close to the eastern and southern margins of the pluton (Figure 6a).

West Huntly: the West Huntly body is the best known because of the relative abundance of natural exposures, and its outcrop in disused quarries, along forest tracks, and in boreholes drilled by BGS, the University of Aberdeen and by commercial exploration companies.

The cumulate rocks crop out over approximately 7 km2, mainly in Dunbennan Wood [NJ 495 415] and the Bin Forest [NJ 510 430], where they are cut by a number of faults. Boreholes through a narrow north–south band of ultramafic cumulate rocks close to the western margin of the body intersected a sheared contact, although local hornfelsing of the country rocks is evident. Olivine-gabbro is the dominant rock type of the West Huntly body with subordinate troctolite, melatroctolite, mela-olivine-gabbro and rare thin bands of anorthosite. Olivine-orthopyroxene-gabbro occurs in the eastern parts of its outcrop.

Plate 2 Magmatic layering in olivine-gabbro cumulates, Bin Quarry, Huntly Pluton (P008624).

Crystal lamination and igneous layering are common. The layering ranging from less than a centimetre up to several metres thick, and generally trends between north and north-north-east and dips steeply or near vertically (Plate 2). In the northern part of the body in the Bin Forest the layering dips to the west, whereas in its southern part around Dunbennan Hill, it dips to the east. Modal layering, reflecting the variations in the proportion of plagioclase to the mafic constituents, is the most widespread form, but grain size, phase and textural layering are also locally developed. Pseudosedimentary structures are locally preserved in the layered sequences. These include scour features, current bedding, and rip-up clasts. These features, together with the cryptic variation in mineral compositions and graded layering, all indicate a younging direction to the east. The northern section, in the Bin Forest, is thus interpreted as being overturned. Cyclic units, marked by repetitive phase layering with the onset of olivine crystallisation, are present on the metre scale, but larger scale cyclic units cannot be identified with confidence.

The primary magmatic features of the West Huntly cumulates are most clearly displayed in the disused Bin Quarry [NJ 498 431] and in Sinsharnie Quarry [NJ 490 440].

East Huntly: In the East Huntly body cumulate rocks underlie about 3 km2 straddling the River Deveron to the north-east of the town of Huntly (Figure 4). Coarse-grained olivine-gabbro and troctolite are the main rock types. Layering and other primary magmatic features are rarely observed: a small exposure on the north-west flank of Hill of Mungo [NJ 547 427] displays indistinct layering and crystal lamination with a north–south trend and near vertical attitude.

The western and southern margins of the body are marked by a sharp transition to a distinctive gabbro containing coarse ophitic clinopyroxene. To the north the cumulate rocks appear to become intercalated with increasing amounts of metasedimentary rocks. To the east they pass into granular gabbroic rocks comparable with those on the eastern flank of the West Huntly cumulate body.

Bridges: high amplitude ground and airborne magnetic anomalies delineate an elongate body of cumulate rocks about 800 m long and 100 m wide at Bridges [NJ 563 425], close to the eastern margin of the Huntly Intrusion. Two boreholes defined an eastern strip of ultramafic cumulates in contact with a western band of troctolites and gabbros. About 400 m to the south-east of Bridges at Costlyburn [NJ 564 416] another small body of gabbroic cumulate rocks is indicated by magnetic survey data and borehole evidence.

There is very little ground control on the distribution of mafic lithologies in the south-eastern part of the Huntly intrusion. Ground magnetic survey data indicate the presence of intercalated bodies of magnetic and nonmagnetic rocks in this sector. High amplitude magnetic anomalies are interpreted to be due to olivine cumulates, probably olivine-gabbro. Locally recrystallised and sheared noritic cumulates have been recovered in shallow boreholes at locations where the magnetic field intensity is low. It is suggested that this margin of the intrusion is characterised by intercalated gabbroic, noritic and metasedimentary rocks, in variable proportions, reflecting either the original intrusive form of the body or modification by shearing, or possibly a combination of both.

Craigwillie: A small isolated body of medium- to coarse-grained leucocratic gabbroic cumulates is exposed to the south-west of Craigwillie [NJ 511 396], close to the southern margin of the intrusion. Near vertical grain-size layering is well developed and trends nearly north–south. Exposure in a gas pipeline trench about 100 m to the south revealed foliated gabbroic rocks. These isolated exposures are interpreted as a small body of cumulate rocks bounded by shear-zones.

Mineralogy of cumulate rocks

The primary order of crystallisation is normally olivine-plagioclase-clinopyroxene-orthopyroxene; grain size ranges mainly from medium- to coarse-grained. Poikilitic textures, defined by pyroxene oikocrysts enclosing or partly enclosing plagioclase or olivine chadacrysts, are common. Ortho-cumulate (75–85% cumulate minerals, 25–15% groundmass) and mesocumulate (85–93% cumulate minerals) textures are widely observed, but adcumulates (93–100% cumulate minerals) are recorded only locally. Crystal lamination, defined by the alignment of primary crystals, is widely present especially in the western part of the West Huntly cumulate body.

Mineral composition data for cumulus olivine, plagioclase and clinopyroxene show a clear pattern of increasing fractionation in the West Huntly body from west to east. Olivine compositions range from Fo85 to Fo74, although local reversals in the general pattern do occur in association with some peridotitic units that show complex mixing textures. Cumulus plagioclase compositions range from An77 to An68 and mirror the changes in olivine compositions. Cumulus clinopy-roxenes are mainly augite with a similar trend of Fe enrichment from west to east in the West Huntly body.

Mineral compositions in the East Huntly body are generally more evolved than those in West Huntly, but a similar west to east fractionation trend can be tentatively identified. Mineral composition data from the other minor bodies of cumulate rocks in the Huntly intrusion are sparse but overlap with those from the East Huntly body.

Modified cumulate rocks

Extensive, but poorly exposed areas of the Huntly and Knock plutons appear not to be underlain by cumulate rocks, but such rocks have received little attention prior to the work of Fletcher (1989)[18]. Subsequently, field mapping, BGS Mineral Reconnaissance Programme surveys, and reevaluation of EVL borehole data, show that these ‘non-cumulate' rocks consist of a variety of rock types including modified cumulates and contaminated gabbroic rocks (Figures 6b, c). Each type commonly has textural affinities with adjacent units and irregular or gradational contacts.

West Huntly: the relationships between modified and unmodified cumulate lithologies are best seen in the West Huntly body. Cumulate rocks that display a range of igneous textures pass eastwards in the Bin Forest into rocks, firstly with an anhedral granular character and then through a range of intermediate types. This transition is best observed on the eastern flanks of the Bin between the Bin Quarry and the summit, but is also discernible in the eastern part of Dubenan Wood. Towards the east orthopyroxene appears as a cumulus phase, accompanied by an increase in olivine grain size. Aggregates of olivine crystals, up to 2 cm across, give rise to a glomeroporphyritic olivine-gabbro with a distinctive spotted appearance that can be mapped as a discontinuous band up to 50 m wide. Farther east these rocks become increasingly fine grained and develop an anhedral granular texture (Plate 3). Orthopyroxene becomes an increasingly important constituent and olivine gradually disappears. These rocks, termed granular gabbros, outcrop in a band about 1 km wide on the eastern flank of the Bin Forest. They comprise a heterogeneous group, composed mostly of fine-grained gabbroic and leucocratic noritic rocks. They are commonly partly or wholly recrystallised and show amphibolite-facies mineralogies. Although relict primary igneous textures are found locally, a foliation is widely developed and mylonitic fabrics are seen sporadically.

Plate 3 Photomicrographs of rocks from pre- and syntectonic mafic–ultramafic intrusions in the Huntly district.
1.Cumulate, West Huntly (P808811)
2.Glomeroporphyritic olivine-gabbro, west Huntly (P808813)
3.Granular/schistose gabbro, central Huntly (P808812)
4.Contaminated gabbro, central Huntly (P808814)
5.Clinopyroxenite/wehrlite, Succoth–Brown Hill (P808815)
6.Sheared metagabbro, Succoth–Brown Hill (P808810)

A large area underlain by granular gabbroic rocks extends from the east side of Dunbennan Hill eastwards under the town of Huntly and northwards from there in a band less than 1 km wide towards Haddoch [NJ 534 447]. In this area sporadic exposures of heterogeneous, fine- to medium-grained noritic rocks with minor clinopyroxene are found. Biotite is a common constituent, locally forming abundant coarse plates.

The best exposures of modified cumulates occur in two sections in the River Deveron north of Huntly, at Gibston — between [NJ 514 405] and [NJ 520 408], and near Huntly Castle — between [NJ 531 408] and [NJ 537 409]. Granular gabbroic rocks show discordant, possibly intrusive relationships to cumulate and ophitic gabbroic rocks in Battlehill Quarry [NJ 539 395] and in the Pirriesmill road cutting on the A96 Huntly bypass between [NJ 5315 3929] and [NJ 5330 3926]. Good exposures are also present to the east of the East Huntly cumulate body on and around the Hill of Mungo.

Linear topographical features and discontinuous north-trending aeromagnetic lineaments provide evidence for shear zones running through the central part of the Huntly intrusion. Field evidence for the shear zones is provided by sinuous mylonitic zones in fine-grained granular norites in a disused quarry south of Haddoch [NJ 5335 4440] and in the conspicuous north-trending foliation developed in recrystallised leucocratic gabbros near Robieston Croft [NJ 529 419].

East Huntly: in this area the transition westwards from cumulate rocks to granular gabbros is marked by the abrupt appearance of a distinctive ophitic variety of olivine-gabbro and by orthopyroxene-gabbro characterised by the presence of oikocrysts of clinopyroxene up to 3 cm across. These rocks have been termed ‘patchy cumulates’ by Fletcher (1989)[18]. Their texture varies from granular to cumulate over short distances and within individual exposures the two types may be interlayered. These transitional rocks occur in a narrow north-trending strip, 100 to 200 m wide, on the eastern side of the line of hills that include Backwood Hill [NJ 533 438], Crow Wood [NJ 535 425] and Deerpark Wood [NJ 536 420], all underlain by granular gabbroic and noritic rocks. To the south the ophitic rocks underlie a wider area and have been mapped in faulted contact with xenolithic rocks in the River Deveron at [NJ 5372 4088], close to its confluence with the River Bogie. They are widely observed in scattered exposures and as loose blocks in the southern part of Kinnoir Wood [NJ 540 410], on the Hill of Bruntstane [NJ 549 407] and the Hill of Greenfold [NJ 552 414], and in the intervening lower ground to the west.

Ophitic gabbros are also inferred to underlie a broader zone, 700 to 800 m wide, in the central part of the Huntly intrusion, although ground control in this area is poor. Gabbros with coarsely ophitic clinopyroxene, giving them a distinctive spotted appearance, crop out in the south at Gibston [NJ 516 411] and on the lower eastern slopes of Ordiquhill around [NJ 526 432]. Granular gabbros also outcrop sporadically within this zone. The ophitic gabbros marginal to the olivine-gabbro cumulates in East Huntly and elsewhere in the central part of the intrusion are mainly medium- to coarse-grained, although veins and irregular areas of pegmatitic gabbro are commonly developed. Variations in pyroxene content, crystal form and grain size locally produce weak layering. Both interlayered and gradational contacts with cumulate rocks and granular gabbros are observed locally.

Mineralogy of modified cumulate rocks

Cumulate and modified cumulate rocks in the Huntly intrusion belong to the Lower and Middle Zones of Clarke and Wadsworth (1970)[15].

In the West Huntly body the eastward transition from rocks with cumulus textures to anhedral granular varieties is associated with the appearance of orthopyroxene. The transition is accompanied first by the development of glomeroporphyritic olivine-gabbro, followed farther east by the disappearance of olivine and the passage into finer grained varieties of granular gabbro. The compositions of cumulus mineral phases in the modified cumulate rocks show a continuation of the eastward trend of increasing fractionation. Olivine compositions vary from Fo77 in the west to Fo50 in the granular gabbros in the east, with a corresponding change in plagioclase composition from An70 to An47. Iron enrichment trends in clinopyroxene and orthopyroxene are also present across the same area. The ophitic gabbros (patchy cumulates) in the eastern and central parts of the intrusion are characterised by relatively evolved mineral compositions with olivine in the range Fo72-Fo62 and plagioclase between An69 and An56.

Increasing physical alteration of the rocks is accompanied by mineralogical and lithological changes eastwards across the West Huntly body. There is an overall reduction in grain size associated with recrystallisation of original pyroxene and plagioclase producing rocks of granular character with mainly equant mineral grains. A secondary planar fabric or foliation is also commonly developed. The highest degrees of physical alteration and the greatest development of foliation in the intrusion occur adjacent to and within the Central Huntly Shear Zone, and on the eastern and western margins of the East Huntly cumulate rocks, away from the main cumulate bodies.

Contaminated and xenolithic igneous rocks

Contaminated rocks underlie a broad north–south tract, up to 1.5 km wide, in the central part of the Huntly Pluton (Figure 6c). They also form a series of bodies, commonly occupying high ground, within the Cowhythe Psammite Formation to the east of the mapped boundaries of the Huntly and Knock plutons. With the exception of a small area at Cuttle Hill [NJ 50 47] near the south-west corner of the Knock Pluton, contaminated igneous rocks are largely absent from the western side of the intrusions.

These rocks owe their origin to partial or complete assimilation of material of sedimentary origin and constitute a highly variable group. Several subgroupings may be distinguished locally, but variation is great and even on the scale of a single outcrop, several distinct lithologies may be identified. The rocks appear to have a close spatial association with granular recrystallised gabbroic rocks.

Contaminated gabbroic rocks are mainly medium-grained with considerable variation in texture and mineralogy (Plate 3). Orthopyroxene-bearing rocks predominate, the principal rock types being orthopyroxene-gabbro, clinopyroxene-norite and norite. They are commonly leucocratic containing more than 70 per cent plagioclase, with subordinate biotite, amphibole, garnet and Fe-Ti oxides. Minor quartz, cordierite, olivine, sillimanite, orthoclase, zircon, apatite, sphene, pyrite and pyrrhotite are present locally. They may have sharp or gradational contacts with xenolithic variants and recrystallised cumulates.

Xenolithic gabbros are highly contaminated heterogeneous rocks closely associated with other varieties of contaminated rocks and granular gabbros, forming either irregular patches or, locally, discordant dyke-like bodies. They are commonly medium to coarse grained and typically leucocratic. They contain locally abundant biotite and garnet, together with variable amounts of amphibole, orthopyroxene, cordierite, quartz, sillimanite, spinel, graphite, sulphides and apatite. The xenoliths are up to 2 m in size and may have sharp or diffuse boundaries. Internally they may be little disrupted or may be partly digested. Xenoliths are dominated by pelitic, semipelitic, quartzitic and calcsilicate lithologies, with subsidiary vein quartz, granite and graphitic pelite.

In the central Huntly zone contaminated rocks are well exposed at Boddum Hill [NJ 510 418], Ferny Knowe [NJ 510 423], Ordbrae [NJ 504 419] and Bin Moss [NJ 51 42]. Foliated variants occur at two localities within this zone. The first is on a knoll 300 m west of Gibston [NJ 513 411] where a strongly foliated and recrystallised leucogabbro containing quartz and aggregates of biotite and amphibole is cut by thin vertical mylonitic zones that trend approximately north–south and comprise fine-grained plagioclase, quartz and biotite. The second is a similarly foliated orthopyroxene-gabbro, spotted with biotite, which crops out about 2.5 km to the north, close to Roadburn, at [NJ 515 438]. Xenolithic variants are well exposed on elevated ground at Clean Hill [NJ 516 423], on the north-west flank of Ordiquhill [NJ 518 436], and at Thief’s Rock and Horse Rock [around NJ 522 440].

Xenolithic and contaminated gabbroic rocks are inferred to underlie the area around Cumrie [NJ 520 445] and Hill of Cormalet [NJ 525 451] at the northern end of the central Huntly zone. Exposure is rare in this sector but numerous, widespread large float blocks support the interpretation. The detailed mineralogy of these rocks has been documented by Droop et al. (2003)[19] and is described in the section on Metamorphism.

Good exposures of contaminated igneous rocks occur in the River Deveron to the north of Huntly where granular gabbros appear to be cut by xenolithic gabbroic rocks and locally gneissose contaminated rocks. An apparently intrusive relationship between xenolithic rocks and granular gabbros is also observed on the western flank of Ord Hill [NJ 503 424]. From this point a discordant body of xenolithic rocks, 70 to 80 m wide, can be traced discontinuously for at least 500 m in a west-south-westerly direction towards the River Deveron.

A highly varied assemblage of heterogeneous, locally veined, brecciated or foliated, hornfelsed metasedimentary rocks and contaminated xenolithic gabbroic rocks crop out in small bodies along the eastern flanks of the Huntly and Knock intrusions. They are exposed at Hill of Kinnoir [NJ 555 435], Elry Knowe [NJ 554 460], and between Barlatch Wood [NJ 554 471] and Milltown of Rothiemay [NJ 548 485]. Internal and contact relationships are not observed in outcrop.

In addition to the contaminated igneous rocks described above there are significant areas where there is limited evidence of interleaved or mixed gabbroic and metasedimentary rocks (Figure 6c). In the poorly exposed south-east part of the Huntly Pluton ground magnetic data and limited shallow drillhole intersections imply that the area is underlain by a mixture of gabbroic and noritic cumulate rocks (see above) with intervening areas of metasedimentary rocks. Similar lithologies are interpreted along parts of the north-eastern faulted margin of the Huntly Pluton, e.g. west of Haddoch [NJ 534 446], again in very poorly exposed ground. A further area of mixed gabbroic and metasedimentary rocks lies within the olivine-gabbro cumulates near the western margin of the Huntly Pluton.

Pegmatitic rocks

Pegmatitic dykes and sheets of gabbroic composition have been mapped at three localities in the eastern part of the Huntly intrusion: in a north-west-trending body cutting xenolithic rocks on the Hill of Kinnoir [NJ 555 436]; in a body of similar orientation cutting granular gabbros in Mungo Wood [NJ 551 430]; and in a body trending east-north-east in ophitic gabbros at the south end of Coniecleugh Forest [NJ 535 422] (Figure 6d). These bodies are up to 40 m wide and are composed of 1 to 10 cm crystals of plagioclase, pyroxene and ilmenite. Locally they contain xenoliths of granular gabbro. Elsewhere, they are cut by veinlets of granular gabbro or pyroxenite.

Irregular discordant bodies of graphite- and sulphide-bearing orthopyroxene-rich pegmatites occur in the West Huntly cumulate body. Two such bodies, up to 3.5 m wide, are exposed in the Bin Quarry [NJ 49 43]. They are heterogeneous in texture and composition. The principal silicate mineral is orthopyroxene, but some zones are highly feldspathic. The graphite and sulphide contents are also highly variable, each locally comprising up to 50 per cent of the rock. The wallrock adjacent to these bodies is altered over distances of up to 0.5 m with the development of biotite and chlorite. Rocks of this type have also been recorded from the River Deveron near Huntly Castle [NJ 538 410] and intersected in boreholes drilled in Dunbennan Wood and the Bin Forest.

Knock Gabbro-peridotite Pluton

The Knock Pluton underlies an area of approximately 25 km2 and lies to the north of the Huntly Pluton from which it is separated by a fault-bounded enclave up to 3 km wide (Figure 4). The enclave is poorly exposed and consists of low-lying ground underlain by Argyll Group metasedimentary rocks that have been intruded by several small mafic and ultramafic bodies. Natural exposure in the Knock Pluton is also sparse and information has been largely obtained from drilling and ground magnetic surveys conducted by Aberdeen University, EVL and BGS.

The Knock Pluton comprises a complex sequence of interlayered lenses and sheets of cumulates, modified cumulates, and contaminated and metasedimentary rocks. The sheets are discontinuous and steeply dipping. The lenticular form of some units is interpreted as typical and has been used as the basis for extrapolation of the varied lithologies over many parts of the pluton where other evidence is unavailable. The external boundaries of the intrusion are not exposed but the boreholes at Drumnagorrach, Claymires and in the south-eastern sector near Clashman Hillock [NJ 539 481] and Woodside indicate that the contacts are complex and commonly sheared. Elsewhere the positions of the external contacts are inferred mainly from ground magnetic survey data. Evidence from numerous boreholes indicates the presence of localised shear zones throughout the intrusion, with deformation apparently most intense in its south-eastern sector.

The complexity of the external contacts is demonstrated by exposures on Cuttle Hill [NJ 500 474] close to the south-west corner of the intrusion. The high ground here is underlain by granular, contaminated and locally xenolithic gabbroic rocks, locally strongly foliated. Two boreholes a few hundred metres to the west and north-west of Cuttle Hill intersected ultramafic, olivine-gabbro and gabbro cumulates up to 30 m thick within a banded sequence of metasedimentary rocks and contaminated and xenolithic gabbroic rocks. A strong foliation and shear zones are present in these sections, although the metasedimentary rocks are hornfelsed adjacent to their contact with the igneous rocks in one of the boreholes.

The nature and position of the eastern margin of the pluton is also difficult to delineate. Available evidence suggests that a single contact seems unlikely and that the proportion of metasedimentary material increases towards the east. Boreholes, sparse exposures and ground magnetic survey data provide some local information on the nature of this contact. A series of boreholes drilled near Glenburn Cottage [NJ 532 480] showed that cumulate lithologies passed eastward with a sharp transition into xenolithic gabbros and mica schists and gneisses. Approximately 1.4 km to the north near Claymires [NJ 533 494] boreholes intersected a sheared contact between clinopyroxene-norite and a sequence of biotite-gneiss and psammite to the east. Magnetic survey and borehole data also indicate that the northern boundary of the Knock Pluton is complex and has been a locus of deformation and shearing.

Cumulate rocks

Cumulate rocks in the Knock Pluton are dominated by troctolites and norites. The former occur mainly in the north-western section of the intrusion whereas the latter are more widespread in its central and southern parts. Boundaries of the cumulate bodies are defined chiefly on the basis of ground magnetic data, although borehole evidence indicates the common intercalation of different rock types. Olivine-bearing cumulates, troctolites, and subordinate olivine-gabbro, dunite and anorthosite underlie some 3 km2 in the western and north-western parts of the intrusion. Layering and crystal lamination are widely observed in float boulders but in-situ exposures are rare. Munro (1984)[16] reported laminated troctolites at Upper Fowlwood [NJ 5270 5270] with a vertical lamination striking north-east. Troctolite and olivine-gabbro cumulates were also reported from a borehole about 600 m farther to the south-east. The general north-east strike of magnetic features in this part of the intrusion may reflect primary magmatic structure or may reflect deformation features associated with the Portsoy Shear Zone.

The main tract of noritic and other orthopyroxene cumulates extends for more than 4 km in a north-easterly direction from near Ruthven [NJ 507 470] to near Parrock [NJ 530 510]. In the south-eastern part of the intrusion a narrower belt of orthopyroxene-bearing cumulates runs south-westwards from Cairnhill [NJ 532 487] for about 2 km. In this last body a sequence of at least 5 layered units, each 10 to 25 m in thickness, was reported by Fletcher (1989)[18] from a borehole near Glenburn Cottage, about 750 m west of Clashman Hillock. These units range from orthopyroxenite to norite and clinopyroxene-norite and exhibit clear cumulate textures.

Modified cumulate rocks

In contrast to the Huntly intrusion, modified and granular cumulates are not widely recognised in the Knock Pluton (Figure 6a,b). A poorly defined tract of granular gabbros and norites has been mapped close to the western margin over a distance of about 3.5 km, running south-westwards from Upper Fowlwood [NJ 527 530]. These mainly fine-grained rocks are partly or wholly recrystallised and amphibolitised and are heterogeneous, commonly foliated, and locally mylonitic. Remnants of primary igneous textures are sporadically observed. No contacts are exposed and relationships with adjacent rock types are unknown.

Compositions of cumulus minerals in the cumulate and modified cumulate rocks of the Knock Pluton are comparable with those from the East Huntly body. Olivine compositions lie in the range Fo80 to Fo61, plagioclase from An80 to An49, and orthopyroxene from En72 to En56. On this basis they may be assigned to the Lower and Middle Zones of Clarke and Wadsworth (1970)[15].

Contaminated igneous rocks

Extensive bodies of contaminated and xenolithic gabbroic rocks occur widely along the eastern and northern margins of the Knock mass (Figure 6c). Borehole evidence indicates complex interleaving of these rocks with cumulate lithologies and metasedimentary rocks, but it is rarely possible to delineate individual bodies with confidence. The contaminated igneous rocks have generally low magnetic susceptibilities, making them indistinguishable from the metasedimentary rocks where there is no exposure. In some marginal areas a heterogeneous assemblage of gabbroic and metasedimentary rocks is recorded. Such rocks appear to mark an irregular, at least partly deformed, transition between the intrusive igneous rocks and the surrounding metasedimentary rocks. A similar transition is inferred along the eastern margin of the Huntly intrusion on the basis of ground magnetic survey data and sparse borehole evidence.

Close to the delineated north-eastern margin of the Knock intrusion, contaminated gabbroic rocks are locally well exposed in discrete outlying bodies underlying the Bo Hill–Craigbourach area [NJ 560 515 to 566 528], by Ternemny [NJ 555 527], and around Barry Hill and Wether Hill [NJ 571 531 to 560 544]. These rocks comprise texturally and mineralogically varied, fine-grained biotite-bearing norites and orthopyroxene-gabbros which are locally xenolithic. Leucocratic, typically coarser grained varieties are widely observed to vein the other rock types, locally giving rise to apparent gneissose or migmatitic textures.

Contaminated, locally xenolithic gabbroic rocks crop out between Cuttle Hill [NJ 501 473] and Flooders [NJ 502 478] at the south-western corner of the Knock intrusion. They are commonly rich in biotite and locally strongly foliated. Deep drilling by EVL a few hundred metres to the west revealed a sheared and interbanded section through xenolithic gabbro, troctolite and metasedimentary rocks.

Enclaves of predominantly foliated, gneissose and commonly hornfelsed pelite and semipelite have been mapped within the main Knock intrusion in three areas, notably around Auchencrieve [NJ 523 480] where there is good borehole control. The enclaves are inferred to be narrow and lenticular, but in parts they are at least a kilometre in length. They are intercalated with variable, but subordinate, amounts of xenolithic, contaminated and cumulate gabbros. Locally, rafts of metasedimentary rock with maximum dimension exceeding 100 m have been identified from boreholes in the southern part of the Knock intrusion. However, in this area it is difficult to demonstrate an origin by assimilation of metasedimentary material rather than structural incorporation as a result of later shearing.

Rothiemay and Avochie intrusions

Ultramafic rocks underlie an area exceeding 1 km2 in the south-east corner of the Knock Pluton around the Bridge of Isla [NJ 530 467]. The best section is exposed in a railway cutting to the north-west of Rothiemay station [NJ 531 460]. Peridotite, melatroctolite and wehrlite are the dominant lithologies, with minor olivine-gabbro and troctolite. They commonly exhibit cumulate textures, layering and crystal lamination that trend approximately north-north-east and dip towards the west-north-west at about 50°.

In thin section olivines from the Rothiemay cutting have a distinctive rounded crystal shape. A poikilitic texture is locally well developed with large interstitial pyroxene plates. Small-scale modal variation gives rise to millimetre-thick layers of dunite, wehrlite and melatroctolite. The rocks show no significant evidence of deformation or metamorphism and are interpreted as local variants of Huntly-type cumulates. The mineral chemistry of the mafic and ultramafic rocks at Rothiemay is similar to that shown by the West Huntly cumulates but exhibits slightly more magnesian compositions. Most olivine compositions lie between Fo80-Fo85, and locally attain Fo90.

In the Avochie area, again at the south-east corner of the Knock Pluton, a sequence of mafic and ultramafic igneous rocks up to 400 m wide extends from just north-west of Dykehead [NJ 537 461] north for about 900 m to the River Deveron near Smithy Croft [NJ 535 470]. Gabbro, olivine-melagabbro, troctolite and melatroctolite, comparable with Huntly-type cumulate rocks, form in the eastern part of this sequence. Sparse borehole and surface observations indicate that the western and northern sections of this zone are underlain by foliated amphibolitic gabbros and xenolithic gabbros. A small body of dominantly ultramafic rocks occurs around Midplough Farm and the site of Avochie Castle [NJ 536 435], adjacent to the cumulate and contaminated gabbroic rocks. The ultramafic rocks range from poikilitic wehrlites, similar to those at Rothiemay, to clinopyroxenites with interstitial olivine, comparable to those from the Whitehill and Succoth–Brown Hill bodies. The exact relationships between the different rock units around Avochie are not known but the contact is interpreted as a ductile shear zone.

Insch Gabbro-peridotite intrusion

The Insch Pluton, which here includes a small part of its western Boganclogh sector, crops out over an area of about 80 km2 in the southern part of the Huntly and Turriff districts. The northern contacts of the pluton are tectonic, characterised generally by shear zones, but locally controlled by faults (Leslie, 1981[20], 1984[21]). However, a contact metamorphic aureole is preserved along the entire northern margin of the Insch Pluton; its width is variable, in part due to later east–west shearing. The aureole is some 300 m wide adjacent to the Boganclogh sector at the western margin of the district, but broadens rapidly to about 3 km at the western margin of the Bogie Devonian outlier.

The northern contact of the Insch intrusion trends generally east–west, except in the south-east part of the Turriff district where it has been is displaced by several significant faults (Leslie, 1984[21]). From Slack [NJ 579 311] to south of Largie [NJ 611 312] the contact is an eastward continuation of the northern margin of the Kennethmont Complex. It truncates the subzone boundaries within the Insch intrusion and also the north–south fault which runs from Brankston south towards the Alford and Inverurie districts (Gould, 1997[22]). In the vicinity of Largie [NJ 611 312 to 619 320], its north-east-trending margin may be an essentially intrusive contact.

The main displacements of the contact that occur south-east of Cross of Jackson [NJ 749 326] occur across north-east-trending faults, the largest of which displaces the pluton margin from near Lightnot [NJ 795 302] south-west to just south of Newton of Saphock [NJ 775 284]. The contact metamorphic aureole cannot be recognised in this sector, as a consequence of the higher grade of regional metamorphism. Contaminated and xenolithic mafic rocks also occur locally (e.g. near Newton of Saphock), indicating interaction between the country rock and the intrusion.

Cumulate rocks

Lower Zone: Lower Zone rocks are confined to small, isolated fault-bound outcrops of dunite and troctolite around Meldrum House [NJ 810 290] and Whigabuts [NJ 815 303] in the extreme south-east of the Turriff district.

The 1 km-wide belt of strongly foliated mafic rocks that separates the Lower and Middle Zone rocks in the Inverurie district crosses the south-east corner of Sheet 86E between [NJ 810 282] and [NJ 820 288]. As these rocks have been extensively recrystallised during amphibolite facies metamorphism, they cannot be readily assigned to the Lower or Middle Zones.

Ultramafic cumulates in the south-eastern part of the Turriff district are petrographically similar to Lower Zone cumulates in the main part of the Insch intrusion (Ashcroft and Munro, 1978[23]). They are dunites and troctolites with cumulus crystals up to 4 mm across. The composition of the cumulus olivine (Fo87-77) is significantly less magnesian than in the Boganclogh-margin ultramafic rocks, and plagioclase compositions range from An84 to An78.

In the Den of Wraes, north of Slack [NJ 576 317], a small elongate body of ultramafic rock occurs in Dalradian country rocks. The rock has been extensively amphibolitised, but originally was a poikilitic wehrlite or melanocratic olivine gabbro. Olivine compositions are Fo72-70, the actinolite has Mg# 81-86, and the kaersutite amphibole has Mg# of 76. The rock resembles the ultramafic cumulates from Rothiemay, at the south-east margin of the Knock Pluton.

Middle Zone: Middle Zone rocks underlie about 35 km2 in the southern and eastern parts of the Insch Pluton in Sheet 86 and consist of cumulate norites and granular gabbros, intermixed on scales of 1 to 100 m (Wadsworth, 1988[24]). The mapped trace of the Middle–Upper zone boundary indicates that the cumulate pile dips north at a low angle (probably <10°). Poorly developed modal layering dips at 10° to 50° to the west, north-west or north.

The rocks comprise three intimately associated varieties (Wadsworth, 1988)[24]:

  1. orthopyroxene-clinopyroxene-plagioclase-ilmenomagnetite cumulates,
  2. fine-grained granular gabbros
  3. porphyritic granular gabbros

Types a and b dominate the Middle Zone sequence. The cumulate rocks are relatively coarse-grained (2 to 4 mm) adcumulates. Cumulus pyroxenes show little or no zoning and plagioclase compositions are variable. There is little preferred orientation of cumulus grains or rhythmic variation in mineral proportions. Apatite is a cumulus phase in some norites.

The fine-grained and porphyritic granular gabbros differ only in that the latter contain abundant plagioclase phenocrysts. The granular gabbros are finer-grained (0.5 to 1 mm) than the cumulate rocks, and have textures varying from granoblastic to a granular mosaic of polygonal crystals. The transition from granular to cumulate gabbros may be sharp or can occur over some 5 to 10 m. Where visible contacts are sharp, there is no sign of one type chilled against the other. However, in general, contact relationships suggest that the granular gabbro intrudes cumulate norite. The quartz-biotite-norites that form the Middle Zone in the central part of the Insch Pluton crop out in the Huntly district over a very small area adjacent to the eastern contact of the Kennethmont Complex near Glanderston [NJ 589 289].

Although the mineral compositions are not particularly evolved, the Middle Zone rocks in the Turriff district are apparently at high ‘stratigraphical’ levels within the zone. However, Wadsworth (1988)[24] showed that cryptic variation within the Insch Middle Zone norites was only loosely related to position within the overall outcrop, probably due to the numerous faults and shear zones present.

Upper Zone: olivine ferrogabbros of UZa outcrop in the northern part of the Boganclogh sector of the Insch Pluton in the Huntly district. East of the Devonian Rhynie Outlier Upper Zone rocks of the Insch Pluton consists mainly of olivine ferrogabbros, with subordinate olivine monzonite and syenite, assigned to UZb and Uzc respectively. These higher subzones are confined to small areas around Coldhome [NJ 595 300] on Sheet 86W and on Fallow Hill [NJ 638 316] in Sheet 86E.

The apparently concordant boundary between the Middle and Upper Zones, marked by the incoming of cumulus iron-rich olivine, is the first marker horizon in the main part of the Insch intrusion. In the Turriff district it can be traced for over 11 km east-north-east from [NJ 641 287] to [NJ 752 318], where it is truncated by a north-north-west-trending fault. There are a few granular rocks in UZa, but they are not known from the UZb and UZc subzones. Rapid upward iron enrichment occurs in the olivines and pyroxenes of the Upper Zone, while the plagioclase, though more sodic than in the Middle Zone, continues to show a wide range of compositional zoning within individual crystals.

Where fresh, the olivine-ferrogabbro of UZa is a dark, blue-black, tough rock with only poor mineral layering. Their typical mineralogy is olivine + plagioclase ± clinopyroxene ± orthopyroxene + ilmenomagnetite + apatite, and the ferrogabbros are ortho- to mesocumulates. Pyroxene, especially orthopyroxene, is less abundant than in the Middle Zone rocks.

Olivine monzonite and subsidiary olivine monzodiorite (UZb) overlie the olivine-ferrogabbro on Fallow Hill [NJ 636 317] and north of the Shevock Burn near Foggieburn [NJ 587 297]. These rocks are very similar in appearance to the olivine-ferrogabbro, except for the euhedral crystals of orthoclase, scattered through the rock. This roughly coincides with the reduction in An content of the plagioclase to below 50 per cent and the appearance of zircon is a cumulus phase. Hence, the more plagioclase-rich UZb cumulates are olivine-monzodiorites rather than olivine-monzogabbros.

Syenite (UZc) crops out over an area of 3 km2 to the north of Wardhouse [NJ 593 290], and forms the summit of Fallow Hill [NJ 637 316]. The syenite is considerably more leucocratic than the UZb rocks. Orthoclase perthite is more abundant than plagioclase. Clinopyroxene is present in some specimens, but it is generally replaced and/or overgrown by amphibole, and biotite is always present. Apatite and zircon form small euhedral crystals.

The Upper Zone rocks of the Insch Pluton show a single trend of cryptic variation very similar to that shown by other highly differentiated tholeiitic layered intrusions. The most notable feature is the overlapping Mg/Fe ratios between the most evolved Middle Zone rocks and the least evolved Upper Zone rocks. Hay (2002)[25] used the major and trace element chemistry and cumulate mineralogy of the Insch Middle and Upper Zone rocks to show that there was straightforward upward differentiation to extreme compositions. No evidence of magma replenishment was found implying that the intrusion was closed during its final stages of crystallisation.

Contaminated and xenolithic rocks

These rocks are present where the original intrusive contacts of the Insch Pluton are preserved, and are also found around enclaves of metasedimentary rocks within the intrusion. The best developments are at Newtown of Saphock [NJ 776 283], Easter Saphock [NJ 775 283] and Mill of Boddam [NJ 624 303]. In the Middle Zone rocks of the eastern part of the Insch Pluton contamination is indicated by the occurrence of quartz-biotite-norites. These are typically associated with cordierite norites, which Gribble (1967[26], 1968[27]) has shown to be derived from an Al-rich dioritic partial melt produced by the assimilation of Dalradian pelitic metasedimentary rocks. Read (1966)[28] recorded black xenoliths of norite, together with pelitic and quartzose metasedimentary xenoliths, all within friable but fresh outcrops of olivine-ferrogabbro by the Mill of Boddam and in the neighbouring Kellock Burn. The norite consists of hyperthene, plagioclase feldspar and abundant iron oxide and locally shows relict mineralogical layering. It is generally coarse-grained but contains well-defined, medium- to fine-grained patches that vary in shape from irregular to elongate and even streaky. The dark grey to blue-black pelitic xenoliths range from 1 to 30 cm in length and are typically composed of cordierite, green spinel, plagioclase and biotite (± hypersthene). Cordierite-spinel hornfels, in places containing sillimanite is also present and shows alteration to diaspore. The siliceous xenoliths are grey and fine-grained and range up to 75 cm long. They consist of recrystallised quartz, with sparse skeletal hypersthene, iron oxide and feldspar.


Hay (2002)[25] carried out a detailed geochemical and isotopic investigation of the representative samples of the Insch (33 samples), Huntly (40 samples) and Belhelvie (38 samples) plutons. She also analysed 12 samples of Dalradian metasedimentary rocks. The resultant data was used to characterise the nature of the magmas and magmatic processes, the frequency of major influxes, level and type of magma contamination, and the origins of the mafic and ultramafic magmas. Her conclusions are summarised below.

Fractional crystallisation trends in differentiated igneous intrusions are normally illustrated by Harker diagrams showing the relative enrichment or depletion of major and trace elements concentrations against SiO2, or in mafic rocks, MgO. Such trends are difficult to assess in cumulate rocks as their major and trace element concentrations are controlled by the cumulus mineralogy and nature of the post-cumulus material. Indeed, Hay (2002)[25] showed that the major and trace element trends in individual plutons were readily explained by the nature of their cumulate mineralogy and post-cumulus material. All the plutons show increasing magmatic differentiation with ‘stratigraphical’ height and only the Huntly Pluton shows evidence of at least one new magma influx in its Lower Zone. The fractional crystallisation follows a tholeiitic trend and the lack of primary amphibole suggests that the magma was comparatively anhydrous.

The whole-rock geochemistry of the West Huntly granular gabbros is compatible with Middle Zone cumulates, implying little overall geochemical change during their textural modification. In contrast, the East Huntly granular gabbros and quartz-biotite-norites show more varied whole-rock geochemistries that reflect their marginal positions. The Insch Middle Zone norites show more evolved mineral compositions than the Huntly and Belhelvie Middle Zone rocks. They also have higher trace element contents, notable Ba, Zn and the LREEs, and higher 87Sr/86Sr ratios, all pointing to greater magma contamination in the Insch Pluton. The Insch Middle Zone trends continue into the Upper Zone sequence, which shows a straightforward upward differentiation to more extreme compositions implying that the intrusion was closed during the final stages of crystallisation.

The occurrence of orthopyroxene as a cumulus mineral prior to clinopyroxene suggests that crustal basement gneiss material was assimilated into the magma at an early stage, prior to emplacement. The Huntly and Insch fine-grained granular gabbros and gabbronorites require <5 per cent of contamination but the Insch porphyritic granular gabbronorites require between 5 and 20 per cent of crustal contamination. The Huntly Pluton shows evidence of contamination by the adjacent Dalradian Argyll Group metasedimentary rocks, particularly in its eastern upper part.

The most primitive Huntly and Belhelvie cumulates and the Insch non-cumulate rocks have Sr-Nd-O isotopic characteristics that imply their magmas were mantle derived, but their mineral compositions suggest that they were more evolved than primitive mantle magmas. Insch and Huntly samples plot within or close to the mantle array and the calculated parent magmas are slightly LREE-enriched. They are also enriched in LILE such as Ba and Rb and depleted in the HFSE, notably Nb and Zr. Together with their robust La/Yb ratios of >0.95, these features imply a subduction-related origin. The source magmas could have originated from the same depleted mantle source with some minor contamination by <1 per cent of subducted oceanic sediment.

Emplacement model

Cumulate rocks have been identified in several discrete bodies over a wide area in the Huntly Pluton. They comprise mainly olivine-gabbro with minor troctolite and peridotite. The order of primary crystallisation in these rocks is generally olivine-plagioclase-clinopyroxene-orthopyroxene. In the Knock Pluton, troctolite and norite cumulates are dominant, but the order of crystallisation is marked by the early appearance of orthopyroxene, before clinopyroxene. These rocks are inferred to have been produced by differentiation along the tholeiitic trend from a primary subalkaline basaltic magma. The assimilation of aluminous metasedimentary material may explain the onset of orthopyroxene crystallisation in West Huntly and its early appearance in the Knock Pluton.

Mineral chemistry data indicate that the majority of the cumulate rocks in the Huntly and Knock intrusions belong to the Lower and Middle Zones of Clarke and Wadsworth (1970)[15]. However, in contrast to the Insch Pluton, there is no overall coherent igneous ‘stratigraphy’ within the Huntly mass. In the West Huntly body both cryptic variation patterns and way-up indicators in the layered rocks suggest that the base of the intrusion now corresponds to the western margin with younging upwards towards the east. However at Bridges, similar rocks that also show relatively primitive compositions, young towards the west. Hay (2002)[25] has suggested that these layered cumulate rocks in the eastern part of the Huntly Pluton may represent roof cumulates. Alternatively they may be indicative of large scale folding within the intrusion (Fletcher, 1989)[18]. In the Knock Pluton microchemical data for plagioclase suggests that the base of the intrusion lies along the western contact, similar to Huntly. However borehole evidence from Drumnagorrach demonstrates the complex nature of the western margin of the Knock body with numerous mafic sheets and contaminated mafic rocks juxtaposed with highly foliated amphibolitised metabasic rocks, probably derived from the gabbros.

Major areas of the Huntly Pluton are underlain by cumulate rocks in which the primary igneous textures have been partially or wholly eliminated as a result of recrystallisation to finer grained granular lithologies. In some zones a pervasive planar foliation or cleavage has also been superimposed. These rocks are separated from the layered cumulate sequences by narrow belts of transitional rocks, mostly glomeroporphyritic olivine-gabbros or ophitic gabbros, which have been less strongly recrystallised. The origin of the granular gabbroic rocks in the ‘Younger Basic’ intrusions has been discussed by several workers (Read and Haq, 1965[29]; Clarke and Wadsworth, 1970[15]; Wadsworth, 1988[24]; Fletcher and Rice, 1989[30]; Gillespie et al., 1994[31]; Hay, 2002[25]). Various mechanisms for their formation have been proposed, but in the Huntly and Knock plutons we attribute the recrystallisation to shear-related deformation of the hot, ductile rock either immediately following or during the later stages of magma crystallisation.

Contaminated gabbroic rocks are widespread in both the Huntly and Knock plutons, with large areas of xenolithic rocks predominant towards their eastern margins. These rocks are derived from the assimilation of large quantities of metasedimentary rocks, preserved on a large scale either marking the roof zone of the intrusion or as a result of an emplacement mechanism that led to incorporation of a large amount of sedimentary material along this boundary. Elsewhere, for example in the central part of the Huntly Pluton, slow cooling of the magma allowed relatively complete digestion of the sedimentary xenoliths leading to the development of contaminated gabbros.

In summary, the distribution of lithologies in the Huntly and Knock plutons was controlled by a combination of magmatic processes involving magmatic fractionation and contemporaneous contamination associated with emplacement coeval with deformation and shearing. In the Huntly Pluton shearing was focused along north–south zones leading to widespread recrystallisation and associated grain size reduction, loss of primary igneous textures, and the development of a pervasive foliation in the most strongly deformed areas. Shearing and deformation were more widely distributed in the Knock Pluton resulting in the general obliteration of primary magmatic features and the lack of continuity of individual units along strike. Active shearing associated with the Portsoy Shear Zone at or close to the time of emplacement ( Stucture) would also explain several unusual features of the Huntly and Knock plutons including:

  1. the distribution and mineral chemistry of cumulate rocks
  2. the variable occurrence of granular gabbroic and noritic rocks
  3. the complex nature of the intrusion margins on both the western and eastern flanks
  4. the apparent shallow extent of the Huntly body implied by geophysical models.


Hay (2002)[25] obtained Sm-Nd whole-rock and mineral separate data that she used to define isochrons for samples from the Huntly and Insch plutons. The best data from the Insch Pluton, from a Middle Zone cumulate rock, gave a crystallisation age of 482 ± 9 Ma (MSWD = 0.26). Sm-Nd ages of 474 ± 12 Ma (MSWD = 2.4) and 467 ± 15 Ma (MSWD = 0.18) were obtained from the Huntly Pluton, interpreted to date crystallisation of a Middle Zone granular gabbro and a Lower Zone cumulate rock respectively. Dempster et al. (2002)[32] obtained a U-Pb TIMS zircon age of 470 ± 9 Ma from a syenogabbro from the Upper Zone of the Insch Pluton, again interpreted as a crystallisation age. They also presented a 207Pb/206Pb SHRIMP zircon age of 472 Ma from a clinopyroxene-bearing monzonite from the Morven–Cabrach Gabbroic Pluton. The low Pb and non-measurable U content and the presence of apatite and opaque oxide inclusions in the acicular zircons prevented more detailed analysis. These ages correlate well with U-Pb TIMS zircon ages of 474.3 ± 2.1 Ma and 471.5 ± 3 Ma from the Portsoy Gabbro-serpentinite Intrusion-swarm, obtained by Condon and Martin (cited in Oliver et al., 2008[33] as a personal communication) and Carty (2001)[34] respectively. Carty et al. (2012)[35] interpreted the age of the Portsoy gabbro emplacement as 471.3 ± 0.6 Ma, based on U-Pb TIMS analysis of three concordant fractions from magmatic zircon and also presented U-Pb titanite ages of 465.1 ± 0.9 Ma and 456 ± 4.5 Ma. When combined with data from the metamorphic porphyroblasts and fabrics, they proposed a model that D2 deformation and thrusting accompanied gabbro emplacement and was followed by rapid uplift. It seems clear that emplacement of these mafic and ultramafic intrusions of the North-east Grampian Basic Subsuite occurred during a short-lived episode of basic-ultrabasic magmatism during the Early Ordovician.

Comparison with pretectonic mafic-ultramafic intrusions

A comparison of lithological, mineralogical, deformational and geophysical features of the syntectonic mafic-ultramafic intrusions and the ‘Older’ pretectonic bodies, leads to the conclusion that they are not related (Table 4). Petrographical studies of rocks from the Succoth–Brown Hill type intrusions strongly imply that they have undergone a longer and more complex history than the Huntly and Knock plutons. This led Gunn et al. (1996)[13] to propose that the Succoth–Brown Hill type bodies were emplaced at an earlier time. The relative proportions and chemical compositions of mineral phases present and their crystallisation sequences also suggest that the intrusions were formed from magmas with different compositions. Furthermore, a marked structural discontinuity between the two types of intrusion is suggested by their contrasting geophysical features. Aeromagnetic and regional Bouguer gravity anomalies across the Portsoy Shear Zone confirm the presence of a major structural break along the south-west side of the Huntly intrusion. This area is also associated with a marked change in the regional strike of the Dalradian succession from north-east to north. In addition, the high-grade sillimanite-bearing schists and gneisses marginal to the Huntly and Knock plutons have not been reported from around the Succoth–Brown Hill intrusion (Beddoe-Stephens, 1990)[36]. This may be the result of movement along a late structure, or, if the S-BH Intrusion represents an older phase of intrusive activity, thermal contact effects may have been destroyed by the subsequent regional metamorphism.

Table 4 A comparison between the mafic-ultramafic rocks from Succoth–Brown Hill Intrusion, Boganclogh margin of the Insch Pluton, and Huntly and Knock plutons.
Succoth – Brown Hill Intrusion Boganclogh margin (Insch Pluton) Huntly and Knock plutons
Clinopyroxenite, wehrlite, dunite
Harzburgite, dunite, layered lherzolite
Dunite, peridotite, melatroctolite
Olivine-gabbro, troctolite
Internal contacts Tectonic Tectonic Magmatic, some tectonised
Magmatic features Rare primary magmatic features Rare primary magmatic features Magmatic features commonly preserved
Deformation and alteration Early high-temperature multi-phase recrystallisation and metamorphism; late alteration, commonly pervasive but little deformation. Early high-temperature multi-phase recrystallisation and metamorphism; late alteration, commonly pervasive but little deformation Simple deformation history. Deformation restricted to shear zones.
Mineral chemistry
Mafic minerals
Mg# 95-75, most ca. 90
In gabbro: >An80; ranges An90-32 and locally >An90
Mg# 95-90 Mg# 85-75
In gabbro: An75-55
Crystallisation sequence Clinopyroxene – olivine – plagioclase Olivine –orthopyroxene – clinopyroxene Olivine – plagioclase – clinopyroxene/orthopyroxene

It is not possible to quantify the age difference between the pre- and syntectonic intrusions on the basis of available evidence. As rocks within or adjacent to major shear zones have likely been affected by several deformation phases, possibly only separated by short periods of time, they commonly show complex deformational and metamorphic textures and mineralogies. These can be similar to those produced outwith the shear zone by several discrete regional metamorphic events. However, rocks adjacent to the Portsoy Shear Zone from the Knock area and from shear zones within the Huntly Pluton do not show complex textures, all the fabrics being essentially coplanar. The Succoth–Brown Hill type rocks may thus have been affected by an early regional metamorphic event prior to emplacement of the Huntly and Knock plutons, which show evidence of intrusion synchronous with or just following the peak of metamorphism.

The Boganclogh margin-type ultramafic rocks show evidence of a similar metamorphic and deformational history to the Succoth–Brown Hill type, and in the area to the north of Knock, in the Portsoy district (Sheet 96W), the two types occur close together. Styles (1999)[11] suggested that these two groups together may represent a dismembered arc-root complex and uppermost mantle from beneath an exhumed volcanic island arc. Their association with regional shear zones, such as those which border or transect the Insch Pluton and the Portsoy Shear Zone, suggests that these features may represent long-lived crustal-scale structures along which magma emplacement was focused. The Succoth–Brown Hill type rocks were probably derived from a calcalkaline magma with high initial contents of Ca and water leading to early crystallisation of clinopyroxene and formation of the characteristic clinopyroxenites (Styles, 1999)[11].

Other mafic and ultramafic rocks

Marnoch-Essendrum mafic and ultramafic intrusions

Read (1923)[3] showed a single outcrop of ‘Older’ gabbroic rocks extending from Pitfancy [NJ 602 435] northwards for about 8 km through Marnoch. However, the remapping and airborne magnetic survey data indicate a more complex situation. The principal exposures are in the River Deveron beneath, and to the west of, the Bridge of Marnoch [NJ 600 496 to 605 495]. Six sheets of coarse grained metagabbro, up to 100 m wide, have been delineated on the south bank of the river while only four are recognised on the north bank. This mismatch has been resolved by inferring an offset along an east–west fault along the river here.

Limited exposure and locally abundant boulders about 1 km to the west of Bridge of Marnoch indicate the presence of bodies of metagabbroic and subsidiary ultramafic rocks. The latter comprise amphibolitised clinopyroxene-olivine rocks. Both rock types show evidence of localised deformation, generally manifest as laterally discontinuous, small-scale shear zones.

To the south of Bridge of Marnoch there is sporadic evidence for the presence of similar rocks over a distance of about 9 km. Read (1923)[3] reported exposures near Hillbrae [NJ 601 474] and in a small quarry at Pitfancy [NJ 5904 4358]. Elsewhere the presence of mafic and ultramafic intrusive rocks has been inferred from float and wall boulders, with boundaries defined mainly by airborne magnetic data. Where ground control is absent, it is possible that the magnetic anomalies may relate to interbedded magnetite-rich metasedimentary rocks rather than igneous rocks.

On the basis of limited petrographical and mineralogical studies by Styles (1999)[11] these rocks cannot be grouped with either the pre- or syntectonic mafic-ultramafic intrusive rocks described above. They have been termed the Marnoch Mafic-ultramafic Intrusion-swarm and included within the North-east Grampian Basic Subsuite, but they are pervasively amphibolitised and appear to be unrelated to both the Insch, Huntly and Knock plutons and the Succoth–Brown Hill type intrusions. They also show no obvious relationship to the regional shear zones.

Metadolerite sheets west of the Porsoy Shear Zone

Generally concordant, sheet-like intrusions of metadolerite and metabasalt occur in a 1 to 2nbsp;km-wide zone, parallel to and north-west of the Portsoy Shear Zone. Mafic sheets are particularly common between Aswanley [NJ 437 395] and Coachford [NJ 465 455], but occur as far north-east as Netherton [NJ 497 489]. This major swarm lies adjacent to the Portsoy Shear Zone for some 56 km between Glen Fenzie, north Deeside, and the Huntly district.

The sheets are well exposed in sections along parts of the River Deveron and the Burn of Cairnie. Elsewhere, exposures are few, but where poorly exposed their presence is commonly indicated by float, particularly on the slopes of Both Hill [NJ 442 409] and Newton Hill [NJ 453 417]. Most sheets are several metres thick, but they vary from 1 m to over 100 m in width. The metadolerite and metabasalt are characteristically non-magnetic, precluding location of boundaries by magnetometer. The sheets represent crustal extension of over 20 per cent locally.

Where contacts with country rock are seen in this district, they are always concordant with bedding or regional foliation, although discordant contacts are recorded in upper Donside (Glenfiddich district – Sheet 75E). Contacts are usually sharp and some sheets have finer grained margins, but contact relationships are commonly obscured by shearing concentrated at the margins; contact metamorphic effects are rare.

Most of the sheets preserve igneous textures but some show a variably developed foliation, typically in discrete shear zones and at sheet margins. Many sheets are porphyritic, with plagioclase phenocrysts up to 5 mm wide comprising up to 30 per cent of the rock. Primary igneous phases have been replaced mimetically by an amphibolite-facies metamorphic assemblage consisting of amphibole (magnesio-hornblende), plagioclase (andesine-oligoclase with rims of albite), epidote, clinozoisite, titanite and ilmenite. Further recrystallisation and retrogression have resulted in widespread degradation of the igneous texture, with the development of plagioclase–quartz mosaics and an increase in epidote and clinozoisite. Whole-rock geochemical major and trace element analyses from the Donside mafic sheets show that they are subalkaline and relatively iron rich, with chemical compositions typical of continental tholeiites or within-plate basalts.

The age of the sheets is uncertain, but the available evidence favours emplacement after development of major folds, but prior to the peak of regional metamorphism. It seems most likely that they are part of the magmatic event that culminated in the emplacement of the major mafic plutons, occurred near the peak of metamorphism, and was linked to a major phase of contractional shearing across the Portsoy Shear Zone.

Syn- to late-tectonic granitic rocks

Avochie Granite

A small ovoid body, some 900 m long by 400 m wide, composed of white to grey, largely foliated, xenolithic, biotite granite, underlies the Hill of Avochie [NJ 541 466] and Brown Hill [NJ 543 470] on the east flank of the Deveron Valley and east of the Portsoy Shear Zone (Figure 6d). The steep, commonly subvertical foliation generally trends north to north-north-east but in the northern part of the body it trends north-west. In parts the granite displays a strong extension lineation, which in Avochie Quarry plunges 65° to the north-north-east. The granite is porphyritic (augened where foliated) in part and consists of turbid orthoclase, microcline, and subordinate oligoclase feldspar, biotite, quartz, and micropegmatite (Read, 1923)[3]. Feldspathic pegmatite veins and pods commonly cross-cut the granite.

Carvichen Granite

The Carvichen Granite forms a triangular shaped outcrop on the south-east edge of the Huntly Pluton (Figure 6d) encompassing Carvichen Cottages, [NJ 5408 3899], Cairnhill Farm [NJ 5445 3834] and Corsiestane Farm [NJ 5369 3864]. It consists of a white and grey to white and pink mottled, generally coarse-grained biotite granite with abundant metabasic and semipelite xenoliths. Like the Avochie Granite it is typically foliated and partly porphyritic, although potash feldspar megacrysts only reach about 1 cm in length. It is cut by coarse-grained, pink to white, pegmatitic leucogranite veins up to a metre thick, which are generally subhorizontal.

In thin section, potash feldspar megacrysts composed of turbid orthoclase and fresher microcline are commonly fringed by micropegmatite. The megacrysts lie in a matrix of quartz (with aligned inclusions), biotite, orthoclase and plagioclase feldspar (mostly oligoclase, some andesine). Primary muscovite is scarce and zircon, apatite and garnet are present as accessory phases. The quartz is recrystallised to fine-grained aggregates in parts.

Other granite intrusions

Small intrusions of foliated biotite-muscovite granite, lithologically similar to the Avochie and Carvichen bodies, occur as thin sheets or pods both within and adjacent to the Huntly and Knock Gabbro-peridotite plutons. Two small bodies of medium- to coarse-grained grey granite were formerly exposed in disused small quarries on the eastern bank of the River Bogie, immediately south of the Huntly Pluton at around [NJ 524 386], and some 460nbsp;m. South-south-east of Broadland Farm just west of the Huntly Pluton at around [NJ 483 410]. Read (1923)[3] records that the granite contained large pink garnets at this latter locality. A similar lenticular body occurs farther north on Meikle Brown Hill [NJ 5702 5192].

In the Knock Pluton borehole evidence shows that foliated granite sheets cross-cut contaminated/xenolithic gabbros, norites, and hornfelsed metasedimentary rocks. The thickest bodies occur north of Waulkmill at [NJ 5150 4844] and [NJ 5157 4860] (Munro, 1970)[37], and farther east near Auchincreive Farm [NJ 5223 4807] and by Cairnhill Farm at [NJ 5339 4858].

Foliated microgranite

A sheet of mylonitic feldspar-phyric microgranite forms a very distinctive marker over a distance of almost 8 km along the north-western margin of the PSZ. It outcrops between Backside [NJ 4109 3619] in the Glenfiddich district (Sheet 85E) and Terryhorn [NJ 468 402], close to the River Deveron in the Huntly district. Exposures occur in the Burn of Aswanley [NJ 448 391] and its outcrop is marked by the presence of abundant debris, including some large angular blocks, across Drumduan Moor [NJ 442 389] and on hillslopes to the north-east of Cairnagat [NJ 460 396]. The main sheet is up to 100 m thick and at least two thinner sheets are also present at Cairnagat.

This distinctive rock is pale pink and fine grained, with a very strong foliation although underformed rocks occur close to the northern margin in the Burn of Aswanley. Scattered phenocrysts of dark pink feldspar (both microcline and plagioclase), up to 5nbsp;mm, form augen in the most deformed examples and biotite is conspicuous within the foliation. In thin section a strong mylonitic fabric is evident, with well developed tails on the feldspar augen and the foliation defined by biotite, muscovite and ribbons of fine-grained granular quartz.

Aberchirder Granite

A pale grey biotite granite intrusion forms J-shaped body centred on Aberchirder. The granite underlies an area of about 4nbsp;km2 and is poorly exposed. It is cut by an east-north-east-trending fault that may also have partially controlled its emplacement. It is locally porphyritic and in parts shows some biotite alignment, possibly a combination of a weak igneous flow foliation and later minor tectonic effects. The granite intrudes greenschist-grade psammites and semipelites of the Macduff Slate Formation (Southern Highland Group) on its eastern side where a prominent hornfelsed zone containing cordierite and andalusite can be recognised. In a former roadside quarry at the south-east corner of Cleanhill Wood [NJ 6223 5173] a complex mixture of granite and hornfelsed pelite and gritty arenite of the Whitehills Grit Formation was documented by Read (1923)[3]. Cordierite and sillimanite were recorded.

Pankhurst (1974)[38] obtained a Rb-Sr whole-rock isochron from the Aberchirder Granite giving an age of 444 ± 9 million years (Ma), which he intepreted as dating its subsequent uplift and cooling. Oliver et al. (2008)[33] obtained a U-Pb zircon SHRIMP age of 450 ± 12 Ma that was interpreted as dating its intrusion. The Aberchirder Granite is similar in many respects to the Strichen, Longmanhill and Forest of Deer granites that give U-Pb zircon and monazite intrusion ages of about 475 to 465 Ma (Oliver et al. 2000)[33], so it is likely that the Aberchirder Granite was intruded at about 460 Ma. The initial 87Sr/86Sr ratio of 0.7157 ± 0.0008 shows that the granite was derived from a crustal melt or was contaminated strongly by crustal material.

Post-tectonic dioritic and granitic rocks

Kennethmont Quartz-diorite-granite Intrusion-Swarm

The Kennethmont Quartz-diorite-granite Intrusion-swarm is the most northerly member of the Alford Subsuite of the Scottish Highlands Ordovician Suite. It consists of a roughly triangular main quartz-diorite body with separate granite bodies at its north-east and south-west end. Its northern half is poorly exposed and underlies an area of about 7.5 km2 in the Huntly district, sandwiched between the eastern sector of the Insch Pluton and the Devonian Rhynie outlier. The intrusion has been described by Read (1923)[3], Sadashivaiah (1954)[39], Read and Haq (1965)[40], and Busrewil et al. (1975)[41].

The northern contact of the Kennethmont intrusion is the western continuation of the northern tectonic contact of the Insch Pluton. Narrow discontinuous pods of serpentinite of Boganclogh margin-type occur at the margin as well as up to 500 m to the north, implying that the contact is marked by a wide zone of shearing. The contact zone is exposed over about 5 km between the north of Leith Hall [NJ 532 303] and the Shevock Burn at Slack [NJ 581 310]. The rocks to the north of the contact are sillimanite-bearing hornfelses similar to those along the northern contact of the Boganclogh sector of the Insch Pluton.

The relationships with the Insch Pluton are unclear. The eastern contact is difficult to define due to poor exposure and the similarity in appearance of some dioritic rocks of the Kennethmont intrusion and the adjacent finer-grained quartz-biotite-norites of the Insch Pluton. The contact runs south from the Shevock Burn near Slack [NJ 580 310] to near Glanderston [NJ 582 290], where it displaced about 100 m eastwards by a fault. South of this fault diorites crop out in a small area on Hill of Glanderston [NJ 582 287].

The intrusion can be divided into three phases:

  1. grey, fine- to medium-grained tonalite containing abundant diorite xenoliths. The xenoliths include patches and later veins of coarser-grained grey granodiorite and granite,
  2. pink, coarse-grained quartz-syenite to syenogranite,
  3. pink, medium-grained granite.

The relative age of phases 1 and 2 is not known, but phase 3 cuts both phases 1 and 2. Phase 1 is considered to represent a diorite which has been intruded and hybridised by possibly cogenetic granitic magma (Busrewil et al., 1975)[41]. The diorite xenoliths are typically subrounded to blocky. The abundance of xenolithic material is variable; in places the tonalite is merely the host material in a vein complex enclosing the dominant dioritic blocks. Phase 2 is of uncertain origin. Although the more granitic elements are more quartz-feldspar rich than any proven Insch Upper Zone rocks, the more syenitic lithologies show some similarities to the UZc syenites in the Insch Pluton. Phase 3 may be part of the suite of granite and pegmatite sheets and veins which intrudes most of the syntectonic mafic intrusions of the North-east Grampian Highlands, including the Insch Pluton. The three phases are included together in the Kennethmont Complex partly for historical reasons, and partly because of the difficulty of delineating the outcrop areas of phases 2 and 3.

Pankhurst (1974)[38] obtained a Rb-Sr whole-rock isochron age of 453 ± 4 Ma, and an initial 87Sr/86Sr ratio of 0.7145 ± 0.0001 for the pink granite (Phase 3). He suggested that the parental Kennethmont diorite magma might be related genetically to differentiates of the Insch magma. The Rb/Sr age for granite in the Kennethmont intrusion has been corroborated by a zircon evaporation age of 457 ± 1 Ma from a sample collected just south-west of the summit of Barr Hill at [NJ 5765 3052] (Oliver, et al., 1998)[42]. However, Parry (2004)[43] has obtained U-Pb TIMS zircon and monazite ages of 472 ± 0.9 Ma and 471.5 ± 0.6 Ma respectively, from the Ord Hill Granite, a minor intrusion which cross-cuts the Boganclogh sector of the Insch Pluton west of Rhynie at [NJ 4876 2741]. This body and similar granitic minor intrusions appear to postdate the mafic and ultramafic units of the Insch Pluton and their hornfels.

Pegmatitic granites

Pegmatitic granites sheets and veins are present locally in the Huntly and Knock plutons (Figure 6d). They consist essentially of quartz, orthoclase (locally microcline), plagioclase (oligoclase-andesine), biotite and tourmaline. The pegmatitic bodies occur as low-angled sheets, lenticular pods and discordant steeply dipping dykes. A notably coarse grained example, 5 m thick and containing minor muscovite, is exposed near Ruthven at [NJ 5094 4707]. Pegamatites have been recorded in Battlehill Quarry [NJ 539 395], Haddoch Quarry [NJ 5336 4441] and also occur commonly in the foliated metabasic rocks and contaminated rocks of the Knock Pluton where they are up to a metre wide. Highly foliated examples were also intersected in a borehole at Drumnagorrach [NJ 5241 5242].

Granitic intrusions within the late-tectonic mafic-ultramafic intrusions

A distinctive cluster of pegmatitic granite and aplitic microgranite veins and sheets is restricted to the main mafic and ultramafic intrusions, particularly the Insch and Huntly plutons. Within the Turriff district, an aplitic microgranite body occurs near Wrangham [NJ 632 306], and a linear pegmatite body occurs near Loanend [NJ 720 290]. The thick quartz vein 400 m south of Jackstown [NJ 753 310] may also be related to this intrusive phase. These granitic rocks are possibly coeval with a granite pegmatite from Belhelvie (Aberdeen district) which has a Rb-Sr whole-rock isochron age of 467 ± 5 Ma (van Breemen and Boyd, 1972)[44].

Minor intrusions

A small number of quartz dolerite and tholeiite dykes assigned to the North Britain Late Carboniferous Tholeiitic Dyke Suite have been identified in the Huntly and Turriff districts. Exposures are rare, but the dolerite and tholeiite are characterised by very high magnetic susceptibilities, resulting in distinctive linear aeromagnetic anomalies (Gallagher, 1983[45]). All have a general west-south-west to east-north-east regional trend, although some local sectors are orientated west-north-west to east-south-east or east to west.

The most southerly dyke in the Huntly–Turriff district, formerly exposed at in a large quarry at Auchinbradie [NJ 621 300] near Upper Boddam, consists of a 13 m-wide body of quartz-dolerite with a distinct chilled margin against spheroidally weathered UZa olivine-gabbros of the Insch Pluton (Read, 1923)[3]. Ground magnetic surveys and rare exposures indicate that the same dyke, locally offset by faults, may extend to the eastern edge of the sheet near Backmoss [NJ 821 325]. Another major dyke with a general west-south-west to east-north-east trend occurs in the Rothienorman and Fyvie area where Read (1923)[3] reported four related exposures of dolerite; near Blackford House, 2 km west of Rothienorman at [NJ 701 356]; near Coshelly, 1 km south-east of Rothienorman at [NJ 730 352]; 900 m east-south-east of Hill of Petty Farm at [NJ 757 357]; and at Peth of Minnonie [NJ 792 363], 700 m west-south-west of St John’s Wells. The dolerites generally exhibit spheroidal weathering and Read (1923)[3] recorded chilled margins against the adjacent hornfelsed semipelite at the Peth of Minnonie Quarry.

A single lamprophyre dyke of probable Silurian or Devonian age was recorded by Fletcher (1989)[18] from the Huntly Pluton at the Bin Quarry [NJ 4978 4301].


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