P-Tconditions during emplacement, and D2 regional metamorphism, of the Ben Vuirich Granite, Perthshire, Scotland

Abstract

The 590+2 Ma Ben Vuirich Granite was intruded into late Proterozoic Dalradian rocks prior to the Grampian orogeny, during which it was affected by upper amphibolite facies regional metamorphism. Spotted cordierite and andalusite (chiastolite) hornfelses at the granite margin were altered to kyaniteand garnet-bearing assemblages during regional metamorphism. From the inferred mineralogy of the hornfels, together with the normative Qz Ab Or values of the granite and the application of a simple cooling model, we conclude that the country rocks immediately adjacent to the granite were hornfelsed at T =~6008C and P 4 2 kbar.The hornfelsed rocks were subsequently metamorphosed during the regional D2 event to form an equilibrium mineral assemblage of muscovite + biotite + garnet + plagioclase + quartz + kyanite (after chiastolite). Garnet which grew during this event shows unusual reversed chemical zoning, with Ca increasing systematically from core to rim as Fe, Mg and Mn decrease. A study of element partitioning between coexisting phases in equilibrium (including zoned garnets), and use of an internally consistent thermodynamic dataset, suggest that isothermal (T = 577+428C) compression (from P = 6.2+1.6 kbar to 9.0+1.9 kbar) occurred during crustal thickening. K-feldspar-plagioclase-quartz veinlets found in the hornfels close the granite contact are demonstrated to be of igneous origin.

Carbonate-orthopyroxenite lenses from the Neoproterozoic Gerf ophiolite, South Eastern Desert, Egypt: The first record in the Arabian Nubian Shield ophiolites

“…The Shinai tonalite composition, combined with experimental results of Tuttle and Bowen (1958), Luth et al (1964) and Ebadi and Johannes (1991), indicates its crystallization at H 2 O-vapour pressure 62 kbar (e.g. Winkler, 1979;Ahmed-Said and Tanner, 2000). Moreover, the absence of garnet and sillimanite, and the common presence of cordierite + andalusite ± anthophyllite-bearing assemblage in the Shinai hornfelses suggest pressure 61.9 kbar (e.g.…”

The Dalradian rocks of the north-east Grampian Highlands of Scotland

“…In addition, the chemical zonation patterns in D2 regional metamorphic garnets reported by Ahmed-Said and Tanner (2000) were interpreted by them to indicate that there was a marked increase in confining pressure as the garnets grew during D2, caused by the major crustal-thickening event that was taking place at the time.…”

“…A scanning electron-microscope and electronmicroprobe study of the small regional metamorphic D2 garnets has shown that they preserve an extremely unusual chemical zonation with, for example, Ca increasing from the core to the rim of the garnet (Ahmed-Said and Tanner, 2000).…”

Cummingtonite in the Dalradian of NE Scotland

“…Four phases of regional deformation are recognized within the Dalradian sequence by most workers (Harris et al, 1976;Harte et al, 1984;Mendum andFettes, 1985, Ahmed-Said andTanner, 2000). D1 and D2 resulted in the development and subsequent shearing of major recumbent folds, including the Tay Nappe, and produced the inverted Dalradian succession.…”

“…THE Dalradian Supergroup of Scotland and Ireland (Harris et al, 1994) is a thick sequence of Neoproterozoic marine sedimentary rocks and basic igneous rocks that was folded and metamorphosed during the Early Palaeozoic Caledonian Orogeny. Four phases of regional deformation are recognized within the Dalradian sequence by most workers (Harris et al, 1976;Harte et al, 1984;Fettes, 1985, Ahmed-Said andTanner, 2000). D1 and D2 resulted in the development and subsequent shearing of major recumbent folds, including the Tay Nappe, and produced the inverted Dalradian succession.…”