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A Petrogenetic Grid for Pelites in the Ballachulish and Other Scottish Thermal Aureoles

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A Petrogenetic Grid for Pelites in the Ballachulish and Other Scottish Thermal Aureoles J. geol. Soc. London, Vol. 142, 1985, pp. 7-28, 11 figs, 5 tables. Printed in Northern Ireland A petrogenetic grid for pelites in the Ballachulish and other Scottish thermal aureoles D. Pattison & B. Harte Grant Institute of Geology, University of Edinburgh, King’s Buildings, West Mains Road, Edinburgh EH9 3JW SUMMARY:The Ballachulish ‘Granite’ is acomposite Devonian intrusive complex sur- rounded by a distinctive thermal aureole developed in regionally deformed and metamorphosed Dalradian sediments on the W coast of Scotland. The most abundant rocks in the aureole are pelites, which show aprogression of assemblages from quartz-muscovite-chlorite upto a variety of high-grade assemblages involving combinations of cordierite, corundum, spinel and, rarely, hypersthene and garnet. Metamorphiczones have been mappedaround the granite, which are defined by the following reactions going upgrade: (1) Mu + Chl + Q = Cd + Bi + V Mu+Bi+Q=Cd+Kf+V (2) or { Mu+Cd=Q+Bi+As+V (3) Mu+Q=As+Kf+V Q+Bi+As=Cd+Kf+V (4) { or Mu+Cd=Bi+As+Kf+V (5) Mu=Cor+Kf+V (6) Bi+As=Cor+Kf+Cd+V The restricted occurrences of assemblages involving spinel, hypersthene and garnet do not allow higher grade zones to be mapped. Variations in the reaction sequence as a consequence of bulk compositionalfactors, in particular the development of quartz-bearing versus quartz-absent assemblages, are described. Details of the mineral assemblages from Ballachulish are combined with high-grade assemblage datafrom the Belhelvie,Lochnagar and Comrie aureoles to constructa comprehensiveschematic petrogenetic grid. The grid involves the minerals quartz, chlorite, muscovite, biotite, cordierite, alumino-silicate, K-feldspar, corundum, spinel, hypersthene and garnet, whose assemblage relationships are modelled in the system K,0-FeO-MgO-A1203- Si0,-H20 (KFMASH). This paper has two principal objectives: to summarize assemblages. For natural mineral assemblages, this is the sequence of pelitic assemblages developed in the accomplished by examining their variations in rocks of thermal aureole of the Ballachulish ‘Granite’ (Argyll- similar bulk composition at different grades of meta- shire),and to combine the assemblage information morphism. Consideration of the assemblages within a from Ballachulish with that from other Scottish modelframework of components and phases (con- thermal aureoles to develop a comprehensive schema- forming tothe Gibbsphase rule), followed by tic petrogenetic grid for thermal aureoles in general. systematization of the reactionboundaries between A great deal of work has gone into the construction different assemblages, using Schreinemakers’ analysis, of petrogenetic grids for regional metamorphic settings producesa schematic ‘grid’ or Schreinemakers’ net. atpressures above about 3 kbar (e.g. Albee 1965; Such a grid incorporates a great deal of relative P-T Harte & Hudson 1979; Carmichael,unpubl.). Much information. less precise is the knowledge of relative and absolute In the first part of the paper we present a detailed stabilities of commonmineral assemblages in low- analysis of the distribution of mineral assemblages pressure geological settings, such as thermal aureoles. found in thethermal aureole of the Ballachulish Another feature of existing petrogenetic grids is that ‘Granite’,Argyllshire, and then use these data quartz-absent assemblages have been largely neg- together with data from the aureoles of the Lochnagar lected, even though they are frequently developed in granite, Aberdeenshire, Belhelvie Gabbroic Complex, thermal aureoles. Aberdeenshire, and Carn Chois Diorite Complex In constructing such a grid, the most important step (Comrie Aureole), Perthshire, to construct a compre- is to establish the relative stabilities of mineral hensive schematic petrogenetic grid. The grid incorpo- Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/142/1/7/4888516/gsjgs.142.1.0007.pdf by guest on 30 September 2021 8 D. Pattison & B. Harte rates bothquartz-bearing and quartz-absentreaction the Lochaber and Ballachulish subgroups (see Table 1 sequences, both of which are well developed in the and Fig. 1). The pelites that enter the aureole range aureoles. from semi-pelites to black graphitic, sulphide-bearing The minerals involved includechlorite, quartz, slates. muscovite, biotite,cordierite, K-feldspar, alumino- Structurally, the Loch Leven area has undergone at silicate, corundum,spinel, hypersthene and garnet, least five deformational episodes during the Cambro- which may be modelledin the system K20-FeO- OrdovicianCaledonian Orogeny (Treagus 1974). In Mg0-A1203-Si02-H20 (KFMASH). the vicinity of the aureole, structures are largely of the Calibrating this schematicframework in P-T-X first deformation (D1), as manifested in themajor space involves both the direct experimental determina- NE-SW trending foldsand slides (see Fig. 1). tion of the P-T-X variations of specific reactions and Associated with the major D1 structures is a penetra- the use of chemical thermodynamics. In this paper we tive slaty cleavage developed in the phyllites, and a arenot concerned with the details of calibration, spaced cleavage in the dolomites and quartzites. Later focusing instead onthe construction of a schematic deformationepisodes are indicated by small-scale grid based on careful analysis of awide range of refolding and crenulation cleavages. naturalmineral assemblages which we hope will provide a solid framework for reliable calibration and Regional metamorphism other refinements. One major regional-metamorphic episode, approx- imately coeval with the D1-deformation, has affected Geological setting of the the rocks of the Ballachulish area. The grade increases Ballachulish area towards the SE, from a zone of chlorite-muscovite- quartz in the NW, into abroad zone where biotite Stratigraphy and structure appears sporadically, and finally into a garnet zone. The garnet-bearingzone is first developed in the The Ballachulish ‘Granite’, one of several composite pelites on the SE margin of the large Leven Schist belt calc-alkaline intrusions of Devonian age in the Scottish that runs through the centre of the map area (see Fig. highlands, intrudes lower Dalradian metasediments of l), and ceases abruptlyat the Ballachulish Slide. TABLE1: Stratigraphic units in the Ballachulish area Group Subgroup Formation Description Middle Dalradian BlairAtholl Cui1 slateBayDark greyslate Appinphyllite Grey phyllitewith flaggy interbeds ________________________________________------------------------------- Appinlimestone* Cream-coloured, banded limestone (Tiger rock) White dolomite Appin quartzite White, gritty, feldspathic, current-bedded Lower Dalradian Ballachulish quartzite (Appin Group) StripedFinelyinterbedded quartzite and black slate transition zone Ballachulish Black graphitic,sulphide bearing slate slate Ballachulish Dark pure limestone limestone Calcareous schist Light-coloured limestone Leven schist Grey phyllites and semipelites Lochaber Glen CoeGlen Fine-grained,well-bedded quartzite quartzite * This boundary is not very clear. Often the limestone units and the phyllite are interbedded, and the limestone is not always the first unit above the Appin quartzite. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/142/1/7/4888516/gsjgs.142.1.0007.pdf by guest on 30 September 2021 A petrogenetic grid foraureoles thermal 9 Garnet is absent in the Ballachulish Slate and Appin (sometimes with clinozoisite in the slightly more Phyllite to the SE of the slide, but reappears when the calcareous rocks); they are not significant components Leven Schist again outcrops. The localization of of the principal phases discussed below. This is also garnet-bearing assemblages to the Leven Schist sug- true of minor components such as FezO3, P205 and gests strong bulk compositional control. Ti02, which if present in sufficient quantity in the bulk composition give rise to additional phases (magnetite, The igneous complex apatite, rutile or ilmenite). The main exception is probably TiOz inbiotite, where Ti02 may reach In addition tothe main intrusive complex at 5 wt%. However, Ti02 is not an important constituent Ballachulish, there are numerous small mafic igneous of the other silicates and all rockscontain rutile or bodies which occur nearby.These include explosion ilmenite; thus, ignoring both TiOz and the oxide phase breccias and a seriesof coarsely crystalline pyroxene f does not affect the variance of individual assemblages. olivine * amphibole f biotite pipes (the Appinite In general, Thompson’s (1957) arguments for ignor- Suite, Bowes & Wright 1967). These bodies represent ing minorphases and components apply well, and the earliestphases of igneous activity in thearea, suggest that KFMASH is a reasonable model system. predating the main intrusive complex. Within this system may be represented all the principal The Ballachulish ‘Granite’ itselfis acomposite phases which are abundant in the rocks andlor show intrusive complex classified by Read (1961) as one of significant changes indistribution with grade. These the ‘Newer granites’ which occur throughout the principal phases include quartz, chlorite, muscovite, Scottish Caledonian Orogen. Essentially, the complex biotite, cordierite, K-feldspar, AlZSiO5 minerals, may be divided into an outer tonalitelquartzdiorite corundum, spinel,hypersthene and garnet.In addi- which is cross cut by acentral pink granitelquartz tion, at the time of metamorphism, it is assumed that monozonite (Streckeisen 1967). Both phases have no there
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