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Continental Rifting |
Department of Earth Sciences, Parks Road, Oxford OX1 3PR, UK
1 Department of Geology, Royal Holloway and Bedford New College, Egham Hill, Egham, Surrey, TW20 0EX, UK
The alkali basalts from Shuqra, Yemen, contain a megacryst suite consisting of olivine, aluminous diopside, kaersutite, pleonaste, plagioclase, and apatite. Deformational kink bands in the olivines, and the existence of multi-phase inclusions (e.g. olivine + pleonaste, aluminous diopside + pleonaste, kaersutite + pleonaste, aluminous diposide + plagioclase) suggest that the megacrysts are derived from disaggregated plutonic rocks. Whole-rock compositional variation cannot have been generated by the fractionation of the phenocrysts present (olivine, augite, labradorite), but is readily explicable as a mixing line between alkali basalt melt and restite, represented by the megacryst suite. It is suggested that the source rock was a deep-crustal, or uppermost mantle, hydrous, and very alkalic, picritic, gabbroic, or syenogabbroic intrusion, which was remelted, perhaps by the injection of further picritic magma. Although the Shuqra rocks themselves do not appear to contain mantle xenoliths, the proposed mechanism of magma generation, if operating on an interleaved crust/mantle transition zone, could provide a general explanation for the occurrence of occasional alkali basalt flows which are extremely rich in spinel lherzolite inclusions.
