Nature and Disposition of the Garnet-Biotite Boundary at Balquhidder, Perthshire, Scotland

Fawzy, Sherif M. A (1989) Nature and Disposition of the Garnet-Biotite Boundary at Balquhidder, Perthshire, Scotland. PhD thesis, University of Glasgow.

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The metamorphic rocks at Balquhidder are polyphase deformed and polymetamorphosed with the D1 - D4 sequential development of structures and the M1 - M4 development of minerals generally corresponding to that expressed in nearby areas. Garnet is found throughout the area and the Barrovian garnet-isograd must have been at least 2Km above the present topography. There is no garnet-biotite boundary present. The incoming of garnet was controlled by the bulk rock composition so that garnetiferous and non-garnetiferous schists can occur in the same, or adjacent outcrops. Textural studies have shown that two phases of garnet growth occurred. The more prominent of these was during M2 when three different types were formed (G2A, G2B, G2C), two only lithologies of the Pitlochry Schist (G2A, G2C) and the other (G2B) only in the Ben Lui Schist. Growth of garnet was related to particular mineral reactions, non-instantaneous nucleation and reaction partitioning. This has been substantiated by the determination, using the electron microprobe of chlorite, muscovite, garnet, biotite, albite and hornblende compositions. Garnet-biotite and garnet-hornblende pairs permit temperatures of M2 metamorphism to be determined: 530 C - 482 C for type G2A (almandine), 423 - 410 C for type G2B (almandine) and 370 C for type G2C (spessartine-rich). The 530 C estimate for G2A is considered to represent climactic conditions and, when considered with a pressure estimate of 7Kb based on the biotite-muscovite-chlorite-quartz geobarometer, indicates a typical Barrovian heat flow of c. 27 C/Km and mid-amphibolite-facies conditions. On the basis of regional correlation of structures, the M2 metamorphism (and D1 - D2) is related to the pre-590Ma Grampian Orogeny and was followed by elevation of temperature to its peak during crustal thickening during D1 (Tay Nappe formation). A second phase of garnet growth took place post-D2 - pre-D4 (possibly post-D2 - pre-D3). Temperature of development of G2-4 (?G2-3) was 377 C for a spessartine-rich type, i. e. in the green schist facies. The elevation of temperature is related to the c. 500Ma event demonstrated elsewhere in the Highlands on the basis of isotopic studies and during which crustal thickening is postulated. Previous interpretations of the presence of an inverted garnet isograd are not substantiated by this investigation. The structural development does not permit inversion of M2 or M2-4 geotherms by tectonic activity, while the distribution of garnet throughout the area does not permit the presence of any isograd to be demonstrated. Rather the whole area is within the garnet zone. Two of the previous interpretations, which suggest an inverted zonal distribution, recognized neither the composition - nucleation - reaction partitioning controls nor the polyphase development of garnet. One sets out a position for a "garnet isograd" that corresponds to the prominent development of G2A and G2C garnets in particular flat-lying units of the Pitlochry Schist. The other sets out a position for a "garnet isograd" that corresponds to the prominent development of G2B and G2-4 garnets in particular units of the Ben Lui Schist. These interpretations are rejected on the basis of the detailed evidence set out here. D4 and post-D4 mineralogical expressions only result in minor modifications to the earlier formed features. D4 is correlated with the regionally expressed c. 460Ma period of uplift and post-D4 mineral growth was both prograde and retrograde. The deformation and metamorphic history in the Balquhidder area generally corresponds to, and may be representative of, Caledonian activity in at least considerable parts of the SW Highlands.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Geology, Petrology
Date of Award: 1989
Depositing User: Enlighten Team
Unique ID: glathesis:1989-78011
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 30 Jan 2020 15:44
Last Modified: 30 Jan 2020 15:44

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