Mullach Sgar Complex - field description - 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 11 Mullach Sgar Complex. Field description gD, FM and PM
Keywords: basalt, granite, hybrid rocks, fragment shapes, textures, map
The Mullach Sgar Complex consists of fragmented intrusions of dolerite, microdiorite and granite, and of blocks of gabbro and dolerite derived from the Western Gabbro, the Breccia of Mullach Mor and the Glen Bay Gabbro. At its south-west margin, granites and dolerites are clearly intrusive into the Western Gabbro, the contact dipping 80°SW away from the centre of the Complex, but the northern margin of the complex is ill-defined in Gleann Mor and beneath Mullach Geal it probably has an irregular gentle northerly dip. Its eastern margin dips approximately 45° west and runs from Mullach Mor across to Glacan Chonachair where veins of the later intrusive Conachair Granite cut dolerites and granites of the Complex.
Excellent exposures on Na h-Eagan and in the Dun Passage have enabled four major phases of intrusion to be recognised (Figure 19). The first of these phases consists of grey-green dolerite, microdiorite and microgranite and occurs near sea level between Ruaival and Na h-Eagan. The component rocks of the first phase have suffered disruption by the subsequent phases, and brecciated and partially or completely hydridised fragments can be found throughout the Complex. The second phase is represented by dolerite, granodiorite and granite which is marginally coarser than the majority of rocks in the Complex and weathers more easily to a brown crumbly rock. It occurs on the rock shelves of the Dun Passage in elongate lenses trending south-west with a steep dip, and is also present in Abhainn a Ghlinne Mhoir. In the Dun Passage the Phase 2 rocks are intruded by fine-grained dark grey dolerites which characteristically form elongate lobate and generally parallel dyke-like masses, 10 cm to 10 m long in a granite matrix. These form Phase 3 and are approximately parallel to the margin of the Complex. Phase 3 dolerites are cut by Phase 4 microdiorite and granite which on Na h-Eagan forms a large composite dyke with chilled lobate margins dipping steeply south-west (Figure 18). Another composite dyke of microdiorite chilled against granite, with similar orientation, occurs on the rock shelves beneath the site of St Brianan's Church. Similar sheets of microgranite containing lobate inclusions of dolerite or microdiorite intrude the Western Gabbro 400 m north of Mullach Bi, on Claigeann Mor and in many places on Ruaival and Dun. Dykes consisting of a similar range of rock types with similar structures also intrude the granite and gabbro in Glen Bay. Xenoliths of gabbro are largely unaffected by the invading granites and dolerites, and at most appear to suffer only marginal alteration, where hydrous minerals have formed at the expense of olivine and pyroxene and some hybridisation by quartzofeldspathic material may be apparent. The major features of the Complex include the intimate association of mafic and felsic rocks (many in sheet structures), the large amount of shattering and net-veining and the rarity of shearing, the occurrence of mafic rocks chilled against felsic rocks, and the lack of felsic inclusions in the dolerites and microdiorites. The chilled margins of the lobate dolerites and the ubiquity of felsic rock between mafic fragments suggest that the sequence of magma intrusion was basaltic followed by granitic followed by basaltic and the complex may have developed in the following way. Firstly subsidence of the gabbro and dolerite country rocks (EW, EK and EG) took place and granitic magma welled up and occupied the fractures and space created by the subsidence. Then further subsidence and fracturing enabled alternate intrusion of granite and either basalt or microdiorite in a number of pulses or phases. Enough time must have elapsed for phase 2 rocks to solidify before phase 3 magmas were intruded, and perhaps movements of various competent blocks within the Complex contributed to the unusual lobate shape of many dolerite fragments.
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