primarily transverse input.
69 ] seamounts) would furnish a suitable sediment source On the
other hand, the detrital pyroxenes of the Falahill and Hazelbank Formations of the Gala area (present thesis), the Elvan Formation of the Abington area (Hepworth 1981) and the Red Island Formation of the Longford-Down Inlier (Sanders & Morris 1978) are non-alkaline, and thus more probably derived from a volcanic arc.
It is agreed that certain of the ferromagnesian minerals may have properties comparable to Bail Hill Volcanics, yet may have been derived from different sources. In support of this proposal, it is significant that clinopyroxenes of the Gala area do not show any evidence of sector-zoning as reported by McMurtry (1980) in the Bail Hill pyroxenes. Although the succession of the Southern Uplands and Longford-Down can not be strictly correlated with complete confidence, due to geographic separation and uncertainity over lateral equivalence of the fault blocks, the greywacke inputs, especially the 'basic-clast’ type, are in general petrographically comparable, and were thus derived from equivalent or identical source areas. This deduction implies that the source rocks themselves formed belts extending for over 300 kilometres parallel to the strike, A volcanic arc source is therefore supported for these non-alkaline detrital clinopyroxenes, contrary to McMurtry’s proposition; the Bail Hill
Û.
seamount was obviously able completly or exclusively to dominate the local sediment supply, but did not extend its influence laterally to any significant extent. The Marchburn Formation of West Nithsdale (Floyd 1975) and the Corsewall Formation of the
70
Rhlnns (Kelling 1962), lower in the succession than any unit proposed in the Gala area, also contain andesitic and clinopyroxene fragments; whereas these formations are either older or of comparable age (Llandeilo) to the Bail Hill volcanics, it seems unlikely that their detritus was derived from the Bail Hill area, especially since palaeocurrents from those areas are consistent in indicating a northwestward source.
It seems more plausible that a subduction complex, for instance the Ballantrae Igneous Complex, has contributed a significant proportion of the detritus of the ’basic-clast’ greywackes, which also include lavas of volcanic arc origin (Lewis & Bloxam 1977; Church & Gayer 1973; Wilkinson & Cann 1974; Bluck et al. 1980). It must, of course, be noted that the origin of the Ballantrae lavas is at present under debate; it has been argued that some sections of the lava in the Ballantrae complex also show oceanic affinities (Lewis & Bloxam 1977; Jones 1977) and may have been tectonically juxtaposed with volcanic arc types. It has also been suggested (Ryan et al. 1983) that the Deer Park Complex, West Ireland, represents a dismembered ophiolite, presumably a lateral correlative of the Ballantrae Ophiolites, and may have contributed as a source area for the
’basic~clast’ greywackes in the Irish sector (Sanders & Morris 1978).
The granitic fragments are mainly of two types; a) a foliated and sheared type which is likely to suggest either a Dalradian or a Grenvillian basement (Longman & Bluck 1979); and
71 -
b) unfoliated granites, some of which contain raicrocline and some micrographie and/or perthitic texture. It should be noted, however, that the reported Grenvillian age of the sheared granites is currently disputed; dating of first-cycle zircons have recently hinted at a ^180 Ma (Grampian) date (B. J. Bluck, pers. comm, to J. A. Weir). Two types of unfoliated granite fragments have been recognized, contrasting both in petrography and age (Bloxam 1968; Longman & Bluck 1979) firstly, a non-perthitic, subsolvus type (560 Ma) and secondly, a microperthitic, subsolvus type (470 Ma). These unfoliated types are considered to have been derived from a source area to the
SVI
margin of the Midland Valley. Some granites are free of K-feldspar and may represent the trondhjemitic granite of the Byne Hill, Girvan (Longman & Bluck 1979).The acidic volcanic fragments (rhyolite, quartz-porphyry,
"I
quartz-keratophyre and volcanic glass), like the crystalline basement itself, are not represented in surface exposure in the present day Midland Valley. It is considered that these rocks were eruptives related to either or both of the granitic events, that as such they overlay the basement complex, and that they were totally removed by erosion (Walton 1983).
The metamorphic fragments including quartzite, muscovite-schist and muscovite-biotite-schist are appropriate to a Dalradian source, an attribution which is also appropriate for at least the 480 Ma unsheared microperthitic granites.
72
Sedimentary assemblages are essentially intrabasinal and locally derived, though minor limestone and arkosic fragments also occur.
6.4 CONCLUSIONS
The main source area for the Falahill Formation was the
basic igneous rocks of a subduction complex and a volcanic arc,
1
j
with minor contributions from acid igneous rocks of the Midland I
J
Valley basement and possiblly metamorphic area of the Dalradian j basement. In the Heriot Formation the source area changed to a
dominantly. acid igneous and metamorphic terrain with subordination of the basic igneous source. This pattern was reversed within the Hazelbank Formation and re-established in the Fountainhall Formation, though with a lesser contribution from the metamorphic source. The metamorphic source in its turn assumed dominance in the Buckholm and Selkirk Formations, with progressive subordination of the basic contribution.
The main sources were a) a basic igneous source, partly subduction complex and partly volcanic arc; b) a metamorphic complex already unroofed to the level of the garnet isograd, inferentially the Dalradian basement; and c) an acidic igneous source, most probably granites of the Midland Valley basement of both 560 Ma and 470 Ma age (Longman & Bluck 1979). The possibility of the operation of a Grenvillian (1000 Ma) source awaits confirmation in the light of contrary, much younger indications, and must be regarded as not proven at present.
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Dominance of submarine fans derived from the respective sources controlled clast composition within the various groups. Thus the basic source prevailed within the Falahill Formation; the metamorphic/acid igneous terrain supervened in the Heriot Formation; the Hazelbank Formation reverted to the basic source, and the Fountainhall Formation to metamorphic/acidic source, which increased its dominance with the Buckholm Formation. The strongly silicic Selkirk Formation probably arose by recycling out of earlier formations by then incorporated into the inferred growing accretionary prism.
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7. STRUCTURE OF THE AREA 7.1 INTRODUCTION
The area displays marked contrasts in fold styles. To the NW, within the outcrops of the Falahill, Heriot, Hazelbank and Fountainhall Formations, the succession has been deformed into asymmetrical folds ranging from tight to open (Ramsay 1967) with modal trend of axial planes 030^-040° and dip 85-90°
(Fig. 7.2b). Some are overturned (Plate 7.2) mainly to the SE and a few (3 in 79) are NW-facing monoclinal folds. To the SE, within the outcrops of the Selkirk Formation and the contiguous part of the Buckholm Formation. The fold style ranges from close to open (Ramsay 1967), with one tight and one monoclinal fold. The majority (53%) of the observed folds plunge gently (mode 0-10°) NE, and a small number (23%) plunge SW. SW plunges appear to be uniformly distributed, rather than being concentrated within localized areas, A small proportion (40%) of the folds are steeply plunging (plunge: 030-65°); of these, 29% plunge NE and 11% SW. It should be noted that these plunges are not as steep as have been recorded elsewhere in the Southern Uplands (Rust 1965a; Weir
1968, 1979; Floyd 1975). The succession throughout the area youngs dominantly to the NW; the folds verge mostly to the SE
(Plate 7.1) and their long limbs young N1V; the monoclinal folds face NW. The ratio of the long limbs to short in asymmetrical folds ranges between 2:3 and 1:20, and the modal ratio is approximately 1:3.
iHERfOT FOUNTAINHALL VVALKERBUBN GALAÉHIELS SELKIRK Gala Area Km