Geology,geochemistry and petrogenesis of late Precambrian granitoids in the Central Hijaz Region of the Arabian Shield |
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| Authors: | N J Jackson J N Walsh E Pegram |
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| Institution: | (1) Saudi Arabian Deputy Ministry for Mineral Resources, c/o Watts Griffis and McOuat Ltd., P.O. Box 5219, Jeddah, Saudi Arabia;(2) Department of Geology, Kings College, Strand, London WC2, UK |
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| Abstract: | Late Precambrian granitoid rocks occurring within a 44,000 km2 area of the western Arabian Shield are subdivided on the basis of geology and petrology into older (820 to 715 Ma) and younger (686 to 517 Ma) assemblages. The older assemblage contains major complexes which can be assigned to either one of a granodioritic or trondhjemitic petrologic association. The earliest granitoid rocks are trondhjemitic tonalites (trondhjemite association), depleted in Ba, Ce, F, La, Li, Nb, Rb, Y and Zr compared to granitoids of the slightly younger granodiorite association, which are related to a calcic, calc-alkaline suite of rocks ranging in composition from gabbro through monzogranite. The plutonic rocks of the older assemblage were probably emplaced in the cores of contemporary island arcs.The younger plutonic assemblage is dominated by three, geochemically distinct, coeval granitic associations: the alkali granite, alkali-feldspar granite and monzogranite associations. The alkali granite association is composed of perthite granites (alkali granites and genetically related alkali-feldspar granites). Rocks of this association are marginally peralkaline or metaluminous and are characterized by low contents of Ba, Co, Li, Rb, Sc and Sr, and high contents of Be, Cu, F, REE, Nb, Sn, Y, Zn and Zr. The alkali-feldspar granite association is mainly composed of alkali-feldspar granites and syenogranites. Rocks of this association are marginally peraluminous or metaluminous and contain low Ba, Sr, and high F, Rb, Sn, Th and U. The monzogranite association consists mainly of monzogranites and granodiorites. Rocks of this association are peraluminous or marginally metaluminous and have the highest contents of Ba, Cu, Co, Li, Sc, Sr, Ta, and V, and the lowest contents of REE, Nb, Rb, Sn, Th, U, Y, Zn and Zr of the three granitic associations.These voluminous granitic magmas, together with the felsic component of a coeval sequence of bimodal volcanic rocks, are partial melts of the earlier island arc terrain produce during a prolonged fusion event. Subsolvus,  highca granites of the monzogranite association have I-type features and represent partial melts of previously unfused crust, while low-Ca perthite granites of the alkali granite and alkali-feldspar granite associations have A-type features and represent partial melts of previously fused crust.This type of petrogenetic model can account for much of the petrologic diversity of the Pan-African granitic terrain of the Arabian Shield. |
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