Metamorphism - St. Kilda
|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 9 Metamorphism
Keywords: granulite–zeolite facies range, mineralogy, textures
A prominent feature of the Western Gabbro is the network of veins that stand out on weathered surfaces (Figure 15A). These consist predominantly of amphibole and chlorite and range from widely-spaced thin veins, which appear to be joint infillings, to denser and more disoriented styles of veining, especially near the eastern margin of the Gabbro where igneous banding indicates that there has been relative rotation of discrete blocks. Veining in these highly sheared rocks may be accompanied by more pervasive zones of granulation, for example in the gabbro north of Mullach Bi and around Claigeann Mor. These penetrate all parts of the original, igneous texture and in places extremely tough rocks have resulted, composed of the same anhydrous gabbroic minerals but with a granulitic texture (Figure 10B). Highly localised shearing facilitated this high-temperature, anhydrous recrystallisation which appears to affect only the Western Gabbro. It may predate the metamorphism characterised by amphibole and chlorite veining, but possibly the two were generally synchronous, the veining representing recrystallisation along joints or fractures permitting ingress of water. Good examples of the latter are found on the slopes of the ridge north of Mullach Bi where some rocks have been sheared with development of hydrous minerals and others retain their igneous texture and show only minor alteration at vein margins (Figure 15B). The composition of the amphiboles in the veins ranges from actinolite to magnesiohornblende to edenite, and zoning of individual grains is common. In the sheared rocks the range extends to tremolite and tschermakite, both varieties commonly fringing or entirely replacing pyroxene and olivine grains. Local variation is marked and reflects the particular minerals and fluids in any one area which have contributed to a particular composition not only of amphibole but also of chlorite (diabantite, penninite and clinochlore are found in different areas), serpentine (magnesian and iron-rich varieties), biotite and spinel (with different Mg/Fe ratios) and talc. Green spinel (Mg0.6Fe0.4A12O4) is of sporadic occurrence but is associated with feldspathic rocks and generally surrounded by pale green amphibole (Figure 15C).
The high grade alteration described above is also present in large blocks of typical Western Gabbro, within the igneous breccia EK. However the most common secondary mineral assemblage encountered in the EK breccias of Soay, Boreray and Glacan Mor is amphibole-chlorite- epidote-albite, suggesting a low-grade, greenschist facies alteration involving hydration of mafic rocks. Petrographical evidence shows that this alteration postdates the granulite- and amphibolite-facies alteration of the gabbros, but since quartz and feldspar veining is conspicuously absent, it probably predates granitic magmatism in the St Kilda complex. The restricted nature of the alteration suggests it is due to hydrothermal activity, perhaps associated with the cessation of surface volcanism. Relatively high initial Sr87/Sr86 ratios in the Boreray gabbro (pp. 40–41), indicates that alteration was accompanied by minor enrichment in alkalis and perhaps other lithophile elements, but there is no evidence of widespread mineralisation in the breccias.
A distinctly different hydrothermal alteration affected the Mullach Sgar Complex and, to a limited extent, the Conachair Granite. Alteration is concentrated along mineralised fractures and faults (p. 37), and petrographically is less pervasive than that affecting the EK breccia. Typical mineral assemblages of apophyllite–chabazite–calcite–actinolite–epidote–prehnite–chlorite–quartz–albite occur in the MSC, while orthoclase–albite–quartz assemblages occur on fault planes cutting the Conachair Granite. Assemblages in the MSC indicate a very low-grade alteration, over a temperature range of 100°–260°C, and equivalent to the zeolite facies through to the prehnite–pumpellyite facies of regional metamorphism. These minerals occur in fractures in the MSC associated with NW–SE faults that cut both MSC and Conachair Granite. Thus alteration of the MSC must have occurred after consolidation of the Conachair Granite, and may therefore be the result of fluid release following uplift and early faulting of the Granite. Some zeolite-grade alteration may have accompanied the subsequent intrusion of late cone sheets and dykes.
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