A detailed structural traverse across the basement rocks of the Eastern Desert of Egypt shows that they consist, apart from intrusions, of four broadly recumbent tectonic units. The lowest, of arkosic metasediments of continental shelf facies, is exposed in a dome. This unit is overlain by an allochthonous ophiolitic mélange containing complete and dismembered ophiolitic masses in a matrix of deep-oceanic graphitic pelites and turbidites. A near horizontal, schistose to mylonitic fabric, most intense near and below the base of the mélange, indicates extensive lateral tectonic transport of the ophiolitic mélange over the shelf metasediments, but the mélange originated as an olistostrome before being tectonically transported. The mélange extends over an area of at least 10 000 km 2 . It is locally overlain by calc-alkaline volcanics, in which deformation is less intense but increases, and schistosity flattens downwards, indicating some translation over the mélange. Unconformable on the mélange and calc-alkaline volcanics is a molasse-facies series, itself also locally strongly deformed. Late tectonic granites preceded, and locally post-date, the molasse-facies sediments. Still later, diapiric peralkaline riebeckite granites locally up-domed the recumbent structures. The tectonic evolution is related to a late Proterozoic subduction zone to the SE of the region.

The deformed, low-grade, metasedimentary-volcanic schist belts of NW Nigeria, and the voluminous granitoid plutons which invaded them, are expressions of late Proterozoic-early Phanerozoic activity in the terrain separating the W African and Congo cratons. Recent interpretations of Nigerian geotectonic evolution have invoked two generations of schist belt, one a product of Kibaran ( c . 1100 Ma) ensialic processes, the other due to Pan-African (700–450 Ma) marginal basin development. The detailed histories of the Anka Belt (Pan-African), Maru Belt (Kibaran) and Birnin Gwari Belt (unknown age), and the plutons emplaced in them, are documented here on the basis of new field, chemical and isotopic data.* Each belt represents a dominantly quiet-water sedimentary environment but volumetrically minor lithologies reveal important differences between the belts. The volcanic rocks and early minor intrusions have strong affinities with those of destructive plate margins. The schist belts were deformed congruently and simply before c . 750 Ma, following the development of flat-lying, possibly thrust-related structures in the Maru and Birnin Gwari Belts. Subsequent plutonism, beginning c . 750 Ma ago, has the calcalkaline, I-type characteristics of subduction zones but younger plutons have mildly peralkaline compositions due to thickening crust.

Weakly deformed, low grade, Lower Palaeozoic metasediments from central Wales contain abundant stack-like intergrowths of chlorite and white mica that closely resemble stacks described from the Devonian Hunsruckschiefer of West Germany; the Ordovician Martinsburg Slate, New Jersey, U.S.A.; and elsewhere. Several theories have been proposed to explain the origin of such stacks, including a detrital origin; strain-controlled growth of chlorite on a detrital mica nucleus; and strain-controlled intergrowth during metamorphism. None of these satisfactorily explains the central Wales stacks. A detrital origin is precluded by the presence of many stacks with shapes too delicate to have survived transportation, and a lack of hydrodynamic equivalence between the stacks and the clastic host grains. Features inconsistent with strain-controlled growth are constant alignment parallel to bedding but non-systematic orientation with respect to tectonic cleavage, their common occurrence in undeformed rocks, and petrographic evidence that they precede the tectonic cleavage. It is proposed that the stacks formed during diagenesis and low-grade metamorphism, and before the onset of deformation, through mimetic growth on a primary bedding fabric composed of clay minerals.

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Synopsis A previously unreported supracrustal suite, the “Cnoc an t-Sidhean Supracrustals”, together with at least three generations of acid and basic orthogneiss form the Lewisian complex at Stoer. Both supracrustals and orthogneisses underwent Badcallian granulite facies metamorphism and polyphase deformation, including the formation of major Badcallian recumbent folds.