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Department of Geology, University of Manchester, Oxford Road, Manchester M13 9PL, UK
Faculty of Earth Sciences, King Abdul Aziz University, P. O. Box 1744, Jeddah, Saudi Arabia
This paper examines the temporal and causal relationships between late Proterozoic deformation, plutonism and high-T, low-P regional metamorphism in a small area in the Nabitah mobile belt (Stoeser et al. 1984). The rocks have island-arc affinities and record the following sequence of events spanning c. 100–150 Ma: (i) calc-alkaline volcanism, (ii) calc-alkaline plutonism, (iii) intense ductile deformation, (iv) thermal peak of metamorphism and formation of mantled gneiss domes, (v) late-stage plutonism.
The P-T conditions at the peak of metamorphism are estimated from mineral assemblages in marbles and calc-silicate rocks to have been 620 ± 30°C, 3.2 ± 2 kbar, indicating a transient upper crustal mean thermal gradient of 54 + 64/-19°C km–1. No information is yet available on the P-T path, but a history of isothermal decompression is unlikely.
The cause of the metamorphism was probably the convective transfer of heat by the emplacement of high-level intrusions in an island-arc complex. The observation that the ductile deformation and the thermal maximum were almost coeval argues against a causal relationship, except in that whatever caused the deformation (an arc-continent collision?) may also have led to a reduction in magmatic activity and hence regional heat flow, thus terminating the metamorphism.
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  M. Brown Ridge-trench interactions and high-T-low-P metamorphism, with particular reference to the Cretaceous evolution of the Japanese Islands Geological Society, London, Special Publications, 1998; 138: 137 - 169. [Abstract] [PDF]
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