Difference between revisions of "Cambir Dolerite - St. Kilda: an illustrated account of the geology"

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[[File:P991499.jpg|thumbnail|Figure 7B Detailed view of poikilitic orthopyroxene and granoblastic feldspar and clinopyroxene. S 67640; field 1 mm wide, cross polarised light.]]
 
[[File:P991499.jpg|thumbnail|Figure 7B Detailed view of poikilitic orthopyroxene and granoblastic feldspar and clinopyroxene. S 67640; field 1 mm wide, cross polarised light.]]
  
== Chapter 3 Western Gabbro: petrology ==
+
== Chapter 4 Cambir Dolerite D ==
  
'''Keywords: mineralogy, texture, microprobe analyses'''
+
Keywords: mineral analyses, annealed textures, cooling history
  
Feldspar, clinopyroxene and olivine form a number of interlocking granular and poikilitic textures in the Western Gabbro and some are illustrated in the photomicrographs (30 ''µ''m thick sections of rock photographed through a polarising microscope). The optical properties and composition of many minerals may be determined from such sections, and textural details of their relationships within the rock give some idea of how the rock was formed.
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At the north west end of the Cambir the Western Gabbro is intruded by sheets and veins of the fine-grained Cambir Dolerite. It is exposed on the Cambir cliffs facing the island of Soay, and also near sea level below Mullach Bi where south-westerly-dipping sheets of similar rock cut the Western Gabbro. On weathered surfaces small variations in grain size give the Dolerite a streaky appearance and altered poikilitic olivine crystals form vague bands of elongate black blotches. Contacts with the Western Gabbro are sharp and transgressive to the banding and lamination but variable in attitude and orientation; overall the Dolerite probably dips steeply south-east. Outcrops of Dolerite near the grass-mantled gully below and west of the Cambir summit are variable in grain size and in places distinction from the Western Gabbro is difficult. Near the top of the gully at about 500 ft (160 m) there is an isolated intrusion of medium-grained gabbro similar to some on Soay. Like the Western Gabbro, the Cambir Dolerite is intruded by coarse gabbroic and pegmatitic veins and affected by the same kinds of faulting and shearing which gave rise to the network of veins.
  
Plagioclase, a mixture of anorthite (An, CaAl<sub>2</sub>Si<sub>2</sub>O<sub>8</sub>) and albite (Ab, NaA1Si<sub>3</sub>O<sub>8</sub>), is the most abundant mineral, generally forming 50–75% of the rock although in places it may be scarce ( < 30%) or, as in [[:File:P991489.jpg|(Figure 3B)]], concentrated in thin anorthosite bands (100%). In the Western Gabbro most grains have calcium-rich cores of composition between An<sub>88</sub>Ab<sub>12</sub> and An<sub>75</sub>Ab<sub>25</sub>, and one or more outer zones that vary between An<sub>80</sub>Ab<sub>20</sub> and An<sub>55</sub>Ab<sub>45</sub>. Sharply defined zone boundaries in some crystals indicate rapid changes in conditions of crystallisation and suggest that these grains have been moved into different parts of magma chamber, perhaps by flow in a silicate melt or crystal mush, whereas gradational zoning shown by a large proportion of the plagioclase indicates much slower changes in the chemical and physical environment during crystallisation. Rocks containing unzoned crystals are probably a result of static conditions where the process of ionic diffusion supplied material from the main body of magma to the growing crystals. Each plagioclase grain within the area of a thin section (2 cm<sup>2</sup>) does not necessarily show the same zonal or compositional pattern, some grains may be unzoned, some with 2 or 3 zones and a few with 5 or 6, and this suggests that these crystals have accumulated from different environments. In many gabbros the first-formed plagioclase crystals are calcic and, as the intrusion cools, more sodic compositions are added. This process may be recorded in the feldspars as a 'normal' zoning pattern where progressively more sodic zones crystallise towards the grain margin. Normal and reverse zoning patterns are common in the Western Gabbro feldspars but an unusual feature of many grains is that, regardless of their inner zonal variation, they have relatively calcic rims.
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More than 90% of the Dolerite is made up of plagioclase, augite and opaque minerals, and the rest consists of olivine, orthopyroxene and a small quantity of alteration products. Most plagioclase grains are small and unzoned, ranging in composition from An<sub>50</sub> to An<sub>68</sub> in different rocks; some larger crystals are more calcic with complex zoning patterns and may be xenocrysts derived from the Western Gabbro. Clinopyroxene grains, making up about 40% of the rock, have a relatively constant composition of En<sub>38</sub> Wo<sub>42</sub> Fs<sub>20</sub>, and are generally less magnesian than the augite(s) of the Western Gabbro. the opaque grains are mostly magnetite with variable contents of either ilmenite lamellae or ulvospinel (Fe<sub>2</sub>TiO<sub>4</sub>) and spinel (MgFeA1<sub>4</sub>O<sub>8</sub>) granules, and this is reflected in the TiO<sub>2</sub> content which in some grains reaches 25 % . Irregularly shaped ilmenite grains occur in association with magnetite but are not common. Orthopyroxene grains, of smaller size and shape to the granular clinopyroxenes, are found in a few rocks, but this mineral occurs more commonly as poikilitic platy crystals enclosing plagioclase. Olivine also occurs in this form but not generally in the same rock and this mutually exclusive development of olivine in some rocks and orthopyroxene in others may be related to local temperature and oxidation conditions. Commonly, magnetite mantles the poikilitic olivine grains and there is some alteration to chlorite in the interior; fresh grains however range in composition from Fo<sub>63</sub>Fa<sub>37</sub> to Fo<sub>53</sub>Fa<sub>47</sub> [[:File:P991495.jpg|(Figure 5)]]. The sheets of dolerite below Mullach Bi have a similar mineralogy but they are coarser, with granular olivines and an overall intergranular texture. Parts of the Dolerite on the Cambir also are intergranular but the main mass is granulitic in texture, resembling a metamorphic rather than an igneous rock. This texture is characteristic of basic rocks that have been heated and maintained at a high enough temperature for sub-solidus recrystallisation (annealing) of the minerals to have occurred. Thus the sharp contacts and the transgressive nature of the veins of Cambir Dolerite in the Western Gabbro indicate that it was a liquid or a crystal mush when intruded, while the textural evidence suggests that much of the Dolerite has suffered a complex cooling history.
  
In Type 2 gabbros (from Mullach Bi to Ruaival and Dun) olivine and plagioclase form a granular mosaic in which crystal shape has been determined by the mutual interference of grains during cooling and solidification of the rock. One can envisage a slowly cooling basic magma from which primary or cumulus crystals of olivine and plagioclase accumulated to the extent of about 60% of the mass. At this stage mechanical movement probably ceased and on further cooling of the liquid between the crystals (intercumulus liquid) more plagioclase and olivine formed on the primary grains, and large poikilitic clinopyroxenes grew into the remaining spaces. Nearly all the pyroxene in Type 2 gabbro was formed in this way, and its average size of 20 mm and its skeletal form is in marked contrast to the granular habit of pyroxene in some Type 3 gabbros [[:File:P991493.jpg|(Figure 4B)]], and the modified granular habit characteristic of Type 1 gabbro. In Type 3 gabbros near Mullach Bi and Claigeann Mor, a third kind of pyroxene is part cumulus and part intercumulus in origin [[:File:P991494.jpg|(Figure 4C)]], the cores having commenced crystallisation early and containing feldspars which are much smaller than those outside the pyroxene and the rims enclosing grains at a much later stage in their growth. These pyroxenes resist weathering to a greater extent than olivine and plagioclase and stand out on the gabbro surfaces creating a characteristic knobbly appearance [[:File:P991491.jpg|(Figure 3D)]].
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Although the chemical analyses of the dolerites on the Cambir (col. 1) and Mullach Bi (col. 2) are similar, the latter is slightly more magnesian and less silicic; there is some variation in the trace elements and it is possible that the example of dolerite from Mullach Bi is related more to the suite of gabbros and dolerites which form parts of Soay, Glacan Mor and Boreray. Certainly the range of sulphides found in this dolerite was greater than in the main Cambir dolerite and these include chalcopyrite, nickeliferous pyrite and a member of the linnaeite–violarite family (Ni<sub>1.8</sub>Co<sub>0.8</sub>Fe<sub>0</sub>.<sub>6</sub>)S<sub>4</sub>.
  
Despite these different habits, the compositional range of the clinopyroxene is small and, in terms of its three major components, the molecular percentages are: MgSiO<sub>3</sub>, 42–45; CaSiO<sub>3</sub>, 43–45; and FeSiO<sub>3</sub>, 11–15. These values were obtained by analysing grains in polished thin sections with an electron microprobe. On the pyroxene diagram [[:File:P991495.jpg|(Figure 5)]] they lie in the augite field, close to augites from gabbros in Glacan Mor, Boreray and Soay, but quite separate from clinopyroxenes in the Cambir Dolerite and the Glen Bay Gabbro. Orthopyroxene (hypersthene) and olivine compositions are shown on the same diagram and where these occur in the same slide as the augite this is indicated by a tie-line. Olivine composition in the Western Gabbro expressed as molecular proportions of forsterite (Mg<sub>2</sub>SiO<sub>4</sub>) and fayalite (Fe<sub>2</sub>SiO<sub>4</sub>) generally varies between Fo<sub>78</sub>Fa<sub>22</sub> and Fo<sub>70</sub>Fa<sub>30</sub> but individual crystals are unzoned. Orthopyroxene also shows a small range in composition and is typically found intergrown with opaque minerals [[:File:P991492.jpg|(Figure 4A)]].
 
  
Magnetite (Fe<sub>3</sub>O<sub>4</sub>) and ilmenite (FeTiO<sub>3</sub>) occur in small amounts as discrete irregular grains or intergrown with orthopyroxene [[:File:P991492.jpg|(Figure 4A)]]. In thin section they are opaque but examination in reflected light reveals a considerable amount of intergrowth. Microprobe analysis of the grains further indicates wide variation in magnetite composition with up to 25% TiO<sub>2</sub>, 19% Cr<sub>2</sub>O<sub>3</sub>, 1% NiO or 1% V<sub>2</sub>O<sub>3</sub> present in different grains, and up to 10% spinel (MgAl<sub>2</sub>O<sub>4</sub>) in the magnetite intergrown with orthopyroxene. Minor amounts of relatively pure magnetite occur as needles in clinopyroxene and as granules on the edges of altered olivine grains. Green spinel is a minor and sporadic constituent of Type 3 gabbros and occurs next to olivine or surrounded by amphibole and chlorite in zones of alteration. Its composition is (Mg<sub>0</sub>.<sub>6</sub>Fe<sub>0</sub>.<sub>4</sub>)Al<sub>2</sub>O<sub>4</sub> with possibly a trace of chromium. Minute quantities of sulphide minerals (mainly chalcopyrite with a little pyrite) occur either on the boundaries of larger grains or included in plagioclase.
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{| class="wikitable"
 
+
|-
The wide range in abundance of the minerals in different parts of the Western Gabbro is associated with a comparable range in whole-rock chemical composition, and it is difficult to determine the average composition of the gabbro. For this reason selected analyses which illustrate the range are given in the table and in [[:File:P991495.jpg|(Figure 5)]] the range of the olivine and pyroxene composition is set in the context of values obtained from analyses of these minerals in other St Kilda rocks.
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| colspan="6"  | '''Chemical analysis Western Gabbro'''
 +
|-
 +
|  | Major elements (Oxide, wt %)
 +
|  | 1
 +
|  | 2
 +
|  | Minor elements (ppm)
 +
|  | 1
 +
|  | 2
 +
|-
 +
|| SiO<sub>2</sub>
 +
|| 49.00
 +
|| 47.70
 +
|  | Li
 +
|  | 6
 +
|  | 2
 +
|-
 +
|| TiO<sub>2</sub>
 +
|| 1.30
 +
|| 1.00
 +
|  | V
 +
|  | 418
 +
|  | 314
 +
|-
 +
|| A1<sub>2</sub>O<sub>3</sub>
 +
|| 14.50
 +
|| 15.30
 +
|  | Cr
 +
|  | 240
 +
|  | 147
 +
|-
 +
|| Fe<sub>2</sub>O<sub>3</sub>
 +
|| 4.30
 +
|| 5.40
 +
|  | Co
 +
|  | 96
 +
|  | 57
 +
|-
 +
|  | FeO
 +
|| 8.20
 +
|| 8.60
 +
|  | Ni
 +
|  | 112
 +
|  | 108
 +
|-
 +
|  | MnO
 +
|| 0.20
 +
|| 0.20
 +
|  | Cu
 +
|  | 261
 +
|  | 169
 +
|-
 +
|  | MgO
 +
|| 6.50
 +
|| 8.10
 +
|  | Zn
 +
|  | <50
 +
|  | n.d.
 +
|-
 +
|  | CaO
 +
|| 12.20
 +
|| 12.30
 +
|  | Rb
 +
|| '''< '''10
 +
|  | < 10
 +
|-
 +
|| Na<sub>2</sub>O
 +
|| 2.50
 +
|| 2.30
 +
|  | Sr
 +
|  | 124
 +
|  | 117
 +
|-
 +
|| K<sub>2</sub>O
 +
|| 0.10
 +
|| 0.00
 +
|  | Y
 +
|  | 26
 +
|  | n.d.
 +
|-
 +
|| H<sub>2</sub>O<sup>+110</sup>
 +
|| 0.90
 +
|| 0
 +
|  | Zr
 +
|  | 36
 +
|  | 28
 +
|-
 +
|| H<sub>2</sub>O<sup>-110</sup>
 +
|| 0.10
 +
|| 0.10
 +
|  |
 +
|  |
 +
|  |
 +
|-
 +
|| P<sub>2</sub>O<sub>5</sub>
 +
|  | 0.04
 +
|  | 0.02
 +
|  |
 +
|  |
 +
|  |
 +
|-
 +
|  | ''Total''
 +
|  | 99.84
 +
|  | 101.82
 +
|  |
 +
|  |
 +
|  |
 +
|-  
 +
| colspan="6"  | Analyses: 1 by A. N. Morigi, A. E. Davis and K. A. Holmes, 2 by R. R. Harding
 +
|-
 +
|}
  
 
== [[St. Kilda: an illustrated account of the geology#References|References]] ==
 
== [[St. Kilda: an illustrated account of the geology#References|References]] ==
 
{{EGwalks}}
 
{{EGwalks}}
 
[[Category: 2. Northern Highlands]]
 
[[Category: 2. Northern Highlands]]

Revision as of 19:04, 5 November 2019

From: Harding, R.R. and Nancarrow, P.H.A. 1984. St. Kilda: an illustrated account of the geology. BGS Report Vol. 16, No. 7. Keyworth: British Geological Survey.].
Map 2 Cambir Dolerite
Figure 6A Sheet of fine-grained Cambir Dolerite dipping gently to the left cuts across Western Gabbro with white feldspar-rich bands dipping steeply to the right. Locality: Gob Chathaill at a height of 80 ft (25 m).
Figure 6B Western cliffs of the Cambir. Cambir Dolerite occurs beneath and south of the summit at heights of 300-500 ft (90-150 m).
Figure 7A Clinopyroxene (brown), plagioclase (grey) and magnetite (black) grains averaging 0.2 mm across form a granoblastic texture. Orthopyroxene with magnetite inclusions occurs near the centre of the picture. S 67640; western cliffs of Cambir; plane polarised light.
Figure 7B Detailed view of poikilitic orthopyroxene and granoblastic feldspar and clinopyroxene. S 67640; field 1 mm wide, cross polarised light.

Chapter 4 Cambir Dolerite D

Keywords: mineral analyses, annealed textures, cooling history

At the north west end of the Cambir the Western Gabbro is intruded by sheets and veins of the fine-grained Cambir Dolerite. It is exposed on the Cambir cliffs facing the island of Soay, and also near sea level below Mullach Bi where south-westerly-dipping sheets of similar rock cut the Western Gabbro. On weathered surfaces small variations in grain size give the Dolerite a streaky appearance and altered poikilitic olivine crystals form vague bands of elongate black blotches. Contacts with the Western Gabbro are sharp and transgressive to the banding and lamination but variable in attitude and orientation; overall the Dolerite probably dips steeply south-east. Outcrops of Dolerite near the grass-mantled gully below and west of the Cambir summit are variable in grain size and in places distinction from the Western Gabbro is difficult. Near the top of the gully at about 500 ft (160 m) there is an isolated intrusion of medium-grained gabbro similar to some on Soay. Like the Western Gabbro, the Cambir Dolerite is intruded by coarse gabbroic and pegmatitic veins and affected by the same kinds of faulting and shearing which gave rise to the network of veins.

More than 90% of the Dolerite is made up of plagioclase, augite and opaque minerals, and the rest consists of olivine, orthopyroxene and a small quantity of alteration products. Most plagioclase grains are small and unzoned, ranging in composition from An50 to An68 in different rocks; some larger crystals are more calcic with complex zoning patterns and may be xenocrysts derived from the Western Gabbro. Clinopyroxene grains, making up about 40% of the rock, have a relatively constant composition of En38 Wo42 Fs20, and are generally less magnesian than the augite(s) of the Western Gabbro. the opaque grains are mostly magnetite with variable contents of either ilmenite lamellae or ulvospinel (Fe2TiO4) and spinel (MgFeA14O8) granules, and this is reflected in the TiO2 content which in some grains reaches 25 % . Irregularly shaped ilmenite grains occur in association with magnetite but are not common. Orthopyroxene grains, of smaller size and shape to the granular clinopyroxenes, are found in a few rocks, but this mineral occurs more commonly as poikilitic platy crystals enclosing plagioclase. Olivine also occurs in this form but not generally in the same rock and this mutually exclusive development of olivine in some rocks and orthopyroxene in others may be related to local temperature and oxidation conditions. Commonly, magnetite mantles the poikilitic olivine grains and there is some alteration to chlorite in the interior; fresh grains however range in composition from Fo63Fa37 to Fo53Fa47 (Figure 5). The sheets of dolerite below Mullach Bi have a similar mineralogy but they are coarser, with granular olivines and an overall intergranular texture. Parts of the Dolerite on the Cambir also are intergranular but the main mass is granulitic in texture, resembling a metamorphic rather than an igneous rock. This texture is characteristic of basic rocks that have been heated and maintained at a high enough temperature for sub-solidus recrystallisation (annealing) of the minerals to have occurred. Thus the sharp contacts and the transgressive nature of the veins of Cambir Dolerite in the Western Gabbro indicate that it was a liquid or a crystal mush when intruded, while the textural evidence suggests that much of the Dolerite has suffered a complex cooling history.

Although the chemical analyses of the dolerites on the Cambir (col. 1) and Mullach Bi (col. 2) are similar, the latter is slightly more magnesian and less silicic; there is some variation in the trace elements and it is possible that the example of dolerite from Mullach Bi is related more to the suite of gabbros and dolerites which form parts of Soay, Glacan Mor and Boreray. Certainly the range of sulphides found in this dolerite was greater than in the main Cambir dolerite and these include chalcopyrite, nickeliferous pyrite and a member of the linnaeite–violarite family (Ni1.8Co0.8Fe0.6)S4.


Chemical analysis Western Gabbro
Major elements (Oxide, wt %) 1 2 Minor elements (ppm) 1 2
SiO2 49.00 47.70 Li 6 2
TiO2 1.30 1.00 V 418 314
A12O3 14.50 15.30 Cr 240 147
Fe2O3 4.30 5.40 Co 96 57
FeO 8.20 8.60 Ni 112 108
MnO 0.20 0.20 Cu 261 169
MgO 6.50 8.10 Zn <50 n.d.
CaO 12.20 12.30 Rb < 10 < 10
Na2O 2.50 2.30 Sr 124 117
K2O 0.10 0.00 Y 26 n.d.
H2O+110 0.90 0 Zr 36 28
H2O-110 0.10 0.10
P2O5 0.04 0.02
Total 99.84 101.82
Analyses: 1 by A. N. Morigi, A. E. Davis and K. A. Holmes, 2 by R. R. Harding

References

At all times follow: The Scottish Access Codeand Code of conduct for geological field work