Breccia of gabbros and dolerites - petrology - St. Kilda: an illustrated account of the geology
|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.].|
Chapter 6 Breccia of gabbros and dolerites: petrology
Keywords: textural range, annealing, hydrothermal minerals
The gabbros and dolerites of the Breccia display a wide range of textures and grain-size, from coarse to fine and occasionally glassy types, including fresh rocks and hydrothermally altered varieties. Coarse gabbros are rarely fresh, most showing evidence of some alteration with amphibole, chlorite and serpentine developed in place of olivine, veins of albite visible in the plagioclase crystals (Figure 10A), and significant amounts of epidote in a few places. Fresh olivine (ranging from Fo65 to Fo60) however remains in some gabbros as the cores of rounded grains with marginal alteration to fibrous chlorite (pycnochlorite) and amphibole (cummingtonite) with a scattering of magnetite grains. Feldspar compositions generally range from An72Ab28 to An40Ab60 and most grains are zoned over part of this range, but some show more extreme zoning to rims of An26Ab74 in areas of residual crystallisation. Late veins of albite (An5Ab95) occur throughout the breccia but alkali feldspar is found only in interstitial patches with quartz in a few gabbros, a scarcity reflected in the low content of K2O in the chemical analysis (column 3). Orthopyroxene (En67Wo2Fs31), like olivine, is uncommon and susceptible to alteration, but clinopyroxene is abundant, is relatively fresh and has a limited range of composition from En42Wo44Fs14 to En40Wo40Fs20 (Figure 5). Ilmenite and magnetite are common and show considerable variation in their contents of MnO, Cr2O3 and V2O3. Tiny amounts of iron and copper sulphides occur in the groundmass. Gabbros with this mineralogy make up perhaps one fifth of the Breccia and are of similar abundance to gabbros thought to be fragments of the Western Gabbro (EW). The latter in general are so recognised on the basis of mineralogy and structure but some blocks, notably on Mullach Mor, Mullach Geal and on Soay, have suffered more intense metamorphism than the rocks on the west coast. They consist of large crystals (relics of the original gabbro) which have been broken and partly recrystallised to form a matrix of tiny granules of olivine, pyroxene, feldspar and opaques (Figure 10B). The composition of the relict olivine and the newly-formed granules are very similar (Fo65 to Fo62), but there is considerable variation in the compositions of the other minerals. In some areas, granules of Ann lie adjacent to a large crystal of that composition whereas, in another part of the slide, granules of An65 exist next to zoned larger grains; the rocks are markedly inhomogeneous both texturally and compositionally. These petrographical features are consistent with highly localised brittle failure, granulation and varying degrees of recrystallisation in a hot, but essentially solid, gabbro.
The commonest basaltic dolerite on Glacan Mor is dark grey with a sharp, subconchoidal fracture. It consists of sparse plagioclase phenocrysts in a groundmass of granular to subhedral pyroxenes, small laths of plagioclase (An50Ab50), and opaque minerals which range in size from 2 to 50 µm (Figure 10C). Some patches of rock are richer in amphibole, chlorite and magnetite than others and may represent volatile-rich regions of the basalt. At different times the basalt has been fractured and intruded by a variety of different veins, some consisting of opaque minerals, some of amphibole, or chlorite, or epidote, or clay minerals and yet others of mixtures of these minerals. They indicate a considerable range and persistence of hydrothermal activity. Dolerites throughout the Breccia display a range of textures (Figure 10C), (Figure 11A) and (Figure 11B) which suggest that in different places the dolerites have undergone varying degrees of sub-solidus recrystallisation. In some the texture is recognisably igneous and represents a magma that has chilled against cool host rock's; in others the lath-like form of the feldspar is preserved but the shape of the interstitial pyroxenes and opaque minerals is distinctly granular and indicates that the basaltic magma was intruded into a substantially hotter environment (Figure 11A). In (Figure 11B) the texture is entirely granoblastic with no obvious trace of an igneous texture and it is likely that after initial solidification this particular metabasite remained at a high sub-solidus temperature long enough to undergo complete recrystallisation. Similar ranges in texture occur in the coarser rocks also and some gabbros on Boreray and Soay have ophitic textures modified by recrystallisation to smaller and more rounded grain sizes.
Hydrothermal alteration affects all rocks in the Breccia to greater or lesser degree, and some dolerites have been extensively altered to fine-grained chlorite-albite- epidote- sphene- carbonate assemblages. In addition to these minerals prehnite, pyrite and zeolites occur in amygdales, found in a few dolerites, and small amounts of pink epidote occur as a thin coating on some joints. The pink colouration is caused by up to 0.8% MnO. The breccias and microbreccias consist of rounded and irregular fragments of dolerite, gabbro, basaltic glass, feldspar and pyroxene crystals in a finely comminuted or glassy base that has undergone partial recrystallisation. Again local conditions have probably determined whether this is minimal (cool conditions), extensive (hot conditions with devitrification of glass and formation of subhedral crystals), or whether enough water and carbon dioxide were present to form hydrous minerals and carbonates. The rarity or absence of quartz and alkali feldspar, either in the original igneous textures or as veins, is further evidence that the Breccias predate the granitic rocks abundantly exposed nearby.
The chemical analyses of three common rock types in the Breccia show that the fine-grained flinty dolerite of Glacan Mor (column 1) is very similar (except in content of water and fluorine) to a dolerite from Mullach Geal (column 2) that has undergone partial recrystallisation (Figure 11A), and that both are similar to a gabbro from Boreray, despite evidence of hydrothermal alteration (Figure 10A). Both dolerite analyses indicate that the rocks are tholeiites with particularly low contents of potassium and titanium, suggesting oceanic affinities.
|Chemical analyses of dolerites and gabbro (EK)|
|Major Elements Oxide, wt%||1||2||3||Minor Elements (ppm)||1||2||3|
|* n.d. = not determined
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