Petrology of lawsonite-, pumpellyite- and sodic amphibole-bearing metabasites from north-east Oman |
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| Authors: | ALEY EL-DIN K EL-SHAZLY |
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| Institution: | Department of Mineral Sciences, American Museum of Natural History, New York, NY 10024-5192. and Stanford University, Stanford, CA 94305, USA |
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| Abstract: | The assemblages chlorite-pumpellyite-lawsonite-albite-quartz, chlorite-lawsonite-quartz-epidote and chlorite-epidote-albite-quartz occur in metabasaltic blocks and veins in a metamorphosed tectonic mélange in the structurally highest unit of the autochthonous and parautochthonous section underlying the Semail ophiolite in Saih Hatat, north-east Oman. The pre-Permian basement of this section contains mafic units characterized by the assemblage crossite-epidote-chlorite-quartz-albite /el actinolite. These assemblages indicate a down-section increase in metamorphic grade from 'lawsonite-albite facies' conditions in the mélange to 'epidote-blueschist' facies conditions in the basement.
Application of empirically and experimentally based thermobarometers as well as petrogenetic grids calculated for a model basaltic system indicates that the P-T conditions of metamorphism ranged from 3 to 6 kbar and 250 to 300 C for the mélange and P > 6.8 kbar, T > 310 C for the basement units. Textural relations interpreted in the context of petrogenetic grids indicate that these units followed clockwise P-T paths of evolution. The estimated P-T conditions and down-section increase in metamorphic grade in central, western and northern Saih Hatat are consistent with the hypothesis relating metamorphism to the Late Cretaceous tectonic loading of the continental margin by an ophiolite slab < 18 km in thickness. These results contrast with field and petrological observations documented for blueschists and eclogites exposed along the eastern coast of Saih Hatat which may have formed at an earlier stage in response to an Early Cretaceous collisional event. |
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| Keywords: | high-P/T metamorphism lawsonite P-T evolution petrogenetic grids pumpellyite thermobarometry |
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