Breccia of gabbros and dolerites - 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 5 Breccia of gabbros and dolerites EK: Field description
Keywords: complex rock mixture, age relations, microbreccias, explosive vulcanicity
Viewed from Village Bay (Figure 8A), the basic rocks of Mullach Geal and Mullach Mor form sporadic outcrops and tors along the skyline above the quarry. They are the southermost part of an igneous Breccia (EK) that consists of basalt, dolerites and gabbros ranging in texture from fine-grained granular rocks to coarse and pegmatitic varieties, some with black pyroxenes up to 10 cm long which are prominent on weathered surfaces. Coarse and fine rocks are intimately mixed such that contacts can rarely be traced for more than a few metres. Gabbro-dolerite contacts may be sharp or transitional although both rock types show similar patterns of shearing and granulation. Finer-grained, flinty basalts cut both the gabbro and granular dolerite, often in tortuous fashion, and may be distinguished from the latter by their chilled contacts, abrupt grain-size variation and less intense shearing.
On Hirta the most extensive exposures of the Breccia are found on the cliffs of Glacan Mor and around Gob na h-Airde with similar rocks occurring low on the Cambir (Figure 8B). Boreray, Soay and the stacs also consist largely of igneous Breccia (Figure 9A). The dominant rock type in the Breccia is gabbro. Various kinds are present although much is texturally similar to the Western Gabbro (EW). These gabbros have been brecciated, veined and intruded by basaltic fluids which display a range of textures. Some have been chilled against cold gabbro blocks and show black glassy margins, others have come into contact with hot gabbros and have more crystalline edges, and yet others are fragmental rocks which consist of basalt and dolerite fragments of various sizes and compositions resting in a fine-grained matrix of the same material. In some rocks brecciation and veining by basalt has occurred repeatedly so that changes to wall-rock temperatures have resulted in different degrees of recrystallisation of the veins, and this has led to a variety of textures from doleritic to fine-grained granoblastic being developed over distances of a few metres. The granoblastic rocks are commonly hard and flinty, dark grey, black or dark green depending on the degree and nature of subsequent hydrothermal alteration. The most abundant basaltic rock on Glacan Mor is extremely fine-grained and splintery, almost black, and closely jointed. On some weathered surfaces the closely spaced jointing of the ,basalt veins contrasts with the less jointed gabbro blocks and reveals the inconstant and unpredictable nature of the contacts. The general fracture pattern suggests processes of formation which have involved the transport and mixing of large solid blocks of basic rock by mobile basaltic material. These structures are exposed in Glacan Mor and in places on the eastern rock shelves of Glen Bay, especially south of the tunnel where the straight, steeply dipping chilled margin of the Glen Bay Gabbro cuts across disoriented blocks in the Breccia. A type of gabbro common as blocks on Glacan Mor, Soay and Boreray is exposed for about 200 ft above the quarry in Abhainn Mhor. Here, it is an ophitic gabbro very variable in grain size and texture, largely consisting of feldspar and pyroxene in a coarse ophitic texture with minor but significant quantities of fine-grained and pegmatitic varieties. Amygdales, filled with green amphibole, chlorite and clay minerals are present in a few places and some form elongate groups parallel to mineral lamination displayed by the feldspar. The lamination and the zones of amygdales dip at about 20°N into Mullach Mor, roughly parallel to the inferred contact between the Breccia and the Mullach Sgar Complex.
On Boreray and Soay some large blocks of banded, layered and sheared gabbro within the breccia are indistinguishable from the Western Gabbro (EW) of Hirta and Dun. One block, texturally identical with the EW Type 2 gabbro on Ruaival, extends for about 100 m along the top of the main south-facing grass slope on Boreray. To the west and east of this block are dolerites and gabbros of varying shades of dark green and dark grey, generally fine-grained but with sporadic coarser patches where white feldspar and dark pyroxene or amphibole form conspicuous ophitic textures. In places along the ridge north of the summit of Boreray (Figure 9A), fragmental textures similar to those on Glacan Mor are visible on weathered surfaces and there is little doubt that the assemblages of rock types, their structures and the textures are the same as those on Hirta. Breccias on Soay show the same features, with dolerites of different kinds intrusive into a range of gabbros. A large raft of gabbro similar to the Type 1 variety of EW forms a prominent cliff at 700 ft facing the Cambir, and other blocks of similar type occur on the northern and western cliffs of Soay. Many blocks are sheared and show metamorphic features similar to the gabbros of Mullach Mor and Mullach Geal.
Evidence of the age of the Breccia relative to other phases of intrusive activity on St Kilda is indicated by field relationships on Hirta. On Mullach Geal dolerite and microgranite sheets cutting the Breccia are offshoots of the Mullach Sgar Complex (MSC) well exposed in the quarry beneath. The contact between the Breccia and MSC appears to dip gently north. On the cliffs east of Glacan Mor igneous breccias are cut by dolerites and granites of the MSC. In the overhanging cliffs above Na Cleitean both MSC and EK rocks are cut by veins of Conachair Granite (I). Thus the breccias appear to predate the Mullach Sgar Complex. Field evidence from the east side of Glen Bay suggests that breccias also predate the Glen Bay Gabbro (EG) which is chilled against it. However, within the Glen Bay Gabbro, a coarse texture with sub-vertical layering is developed at a distance of 100–120 m west of the contact, and the bulk of the Gabbro has clearly crystallised under plutonic conditions. It thus seems that chilling is more likely the result of collapse of a large block of cold country rock (EK gabbros and dolerites) into the magma rather than of upward intrusions into relatively cold country rocks in sub-plutonic conditions. The possible size of such a collapse block is difficult to estimate, but it seems to have been brecciated and veined by basalt and in places hydrothermally altered prior to making contact with the Glen Bay Gabbro. No doubt further brecciation and basalt veining (by EG) accompanied collapse. Thus the field evidence suggests that on Hirta some igneous brecciation was contemporaneous with the crystallisation of the Glen Bay Gabbro. The breccias have formed from layered gabbroic country (EW) rocks at fairly high sub-solidus temperatures which were close to or within magma conduits. Collapse following rapid evacuation of magma, perhaps accompanying surface eruption, is a likely feature of sub-volcanic magmatism. So also is the variety of dolerites and basalts intrusive in single or multiple veins, together with explosive microbreccias and hydrothermal alteration. These breccias, which are the most widespread rock type in the island group, are the most direct evidence of the explosive surface volcanism which was centred on St Kilda.
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