Tab. 1+2: Representative electron-microprobe analyses of minerals from Du Toit Nunataks

An example of cordierite-bearing gneiss that is part of a high-grade gneiss-migmatite sequence is described from the Hatch Plain in the Read Mountains of the Shackleton Range, Antarctica, for the first time. The cordierite-bearing rocks constitute the more melanosomic portions of the metatectic and migmatitic rocks that are associated with relict granulite facies rocks such as enderbitic granulite and enderbitic garnet granulite. The predominant mineral assemblage in the cordierite-bearing rocks is chemically homogeneous cordierite (XMg 0.61) and biotite (XMg 0.47), strongly zoned garnet (XMg 0.18-0.11), sillimanite, K-feldspar (Or81-94Ab5-18An0.6), plagioclase (An28), and quartz. Inclusions of sillimanite and biotite relics in both garnet and cordierite indicate that garnet and cordierite were produced by the coupled, discontinuous reaction biotite + sillimanite + quartz = cordierite + garnet + K-feldspar + H2O. Various garnet-biotite and garnet-cordierite geothermometers and sillimanite-quartz-plagioclase-garnet-cordierite geobarometers yield a continuous clockwise path in the P-T diagram. The P-T conditions for equilibrium between garnet core and cordierite and between garnet core and biotite during peak metamorphism and migmatization were estimated to be 690 °C at 5-6 kb. This was followed by cooling and unloading with continuously changing conditions down to 515 °C at 2-3 kb. This low-pressure re-equilibration correlates with the pressure conditions evaluated by SCHULZE (1989) for the widespread granitic gneisses of the Read Group in the Shackleton Range. The associated relict enderbitic granulites representing low-pressure type granulite (8 kb; 790 °C) are comparable to similar low-pressure granulites from the East Antarctic craton. They were either formed by under-accretion processes after collision (WELLS 1979, p. 217) or they are a product of remetamorphism at P-T conditions intermediate between granulite and amphibolite facies. A model of a multiple imbrication zone with crustal thickening (CUTHBERT et al. 1983) is discussed for the formation of the relict granulites of the central and eastern Read Mountains which show higher pressure conditions (8-12 kb, SCHULZE & OLESCH 1990), indicating a Proterozoic crustal thickness of at least 40 km.

The metamorphic environments of the Strontian, Foyers and Dalbeattie intrusions, Scotland

Three metamorphic aureoles around intrusions of the Caledonian 'Newer Granite' suite are described. Each represents a different orogenic environment. The Strontian complex is intruded into sillimanite grade Moinian metasediments at the core of the orogen. The aureole comprises three zones; a transitional muscovite + sillimanite + K-feldspar zone, a sillimanite + K-feldspar zone and an inner cordierite + K-feldspar zone. Contact migmatization occurs in the inner part of the aureole. Zoning profiles from garnets in both regional and aureole assemblages show retrograde Mn-rich rims. Fe and Mg compositions are re-equilibrated to contact conditions. Apparent re-equilibration of Ca compositions results from increasingly ideal solid solution behaviour of Ca in plagioclase and garnet with increasing temperature. Temperatures of 690°C at 4.1 kbar (XH2O = 0.53) are estimated in the cordierite + K-feldspar zone, dropping to 630°C (XH2O = 0.69) at the sillimanite + K-feldspar isograd. The zones increase in width to the east, influenced by the regional thermal gradient at the time of intrusion. The timer-scale of the contact event, t2, relative to the regional, tl, - is estimated as t2/t1 = 101.1+ -0.7 and is consistent with Intrusion at an early stage of regional uplift and cooling. The Foyers complex intrudes Moinian rocks at a higher structural level. Regional assemblages range from garnet to sillimanite grade. Three contact zones are recognised; a sillimanite zone, a sillimanite + K-feldspar zone and an inner cordierite + K-feldspar zone. The limit of the aureole is marked by the breakdown of garnet which shows disequilibrium, both texturally, and in complex zoning profiles, within it. Temperatures of 660°C at 3.9 kbar (XH20 = 0.14) are estimated in the cordierite + K-feldspar zone? The Dalbeattie complex is at the margin of the orogen, intruded into low grade Silurian metasediments. Two zones are recognised; a biotite zone and an inner hornblende zone. Cordierite and diopside are present in the inner zone.