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New Rb-Sr whole-rock data are presented which confirm a Grenvillian age for widespread, early, regional metamorphism of the Moine in the SW Highlands of Scotland. It further suggests that Caledonian metamorphic activity occurred between c.470 and 410 Ma ago. Previous isotopic analyses of mineral separates from schists, gneissose pelites and pegmatites are reviewed in the light of the new data, and the work of Purdy & Jaeger (1976). This suggests a cooling history, following the Caledonian metamorphism, which involved sequential uplift of crustal segments.

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Malay Basin, a northward-trending pull-apart extensional rift basin, formed during the late Eocene-early Oligocene and then underwent thermal subsidence and sedimentation during the early Miocene. Reorientation of regional stress fields during the mid-Miocene caused structural inversion resulting in east-west anticlines and half grabens.

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SUMMARY. Banded gneisses and migmatites in the Champira Dome contain assemblages of the biotitecordierite-almandine subfacies of the granulite facies. The banded gneisses represent a sequence of arkosic sediments metamorphosed to sillimanite-cordierite gneisses. The migmatites were derived from more argillaceous and potassic sediments, with lower oxidation ratios and lower Ba/Rb ratios. They consist of a quartzo-feldspathic leucosome, representing anatectic melt, with schlieren of the refractory minerals sillimanite, garnet, biotite, and oxide minerals. Cordierite developed from garnet and biotite, except in rocks of high FeO/(FeO + MgO) ratio. Both rock types contain assemblages of magnetite+ hercynite + corundum + ilmenite + hematite, formed by unmixing of high-temperature solid solutions. Rb-Sr studies of the banded gneisses gave an age of 2327 + 25 Ma, which is interpreted as the date of metamorphism, and an initial 878r/86Sr ratio of o.7o64 + 0.ooo3. The migmatite samples plot close to this 2327 Ma regression line, but the strontium isotopes were considerably disturbed, though not homogenized on the scale of sampling, 962 + 34 Ma ago. It is considered that the anatexis in the migmatites was contemporaneous with the metamorphism of the banded gneisses and that the 962 + 34 Ma event may be correlated with recrystallization of the migmatites accompanying the growth of cordierite. THE Champira Dome ( fig. IA) consists largely of banded sillimanite-cordierite garnet gneisses. It was emplaced as a tectonic wedge between ENEtrending boundary faults and appears to have a different structural and metamorphic history to that of the surrounding rocks (Peters,.i975). The purpose of this paper is to elucidate as far as possible the origins and metamorphic history of the rocks of the dome. Two suites of specimens were examined. Specimens MB 52 and 54-59 are banded gneisses. To the north-east of these ( fig. z B) is an area of migmatite . Both suites contain mineral assemblages characteristic of the