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صفحه نخست /Petrogenesis of the Darvazeh ...
عنوان
Petrogenesis of the Darvazeh mafic-intermediate intrusive bodies, Qorveh, Sanandaj-Sirjanzone, Iran
نوع پژوهش مقاله چاپ‌شده
کلیدواژه‌ها
Qorveh .Sanandaj-Sirjanzone .Mafic-intermediaterocks .Crystal fractionation .Activecontinentalmargin . Intra-arc
چکیده
The Darvazeh intrusion in the northern Sanandaj-Sirjan zone in the Alpine-Himalayan orogenic belt is composed of olivine gabbro, gabbroic diorite and granitoid units that intruded Jurassic metamorphic rocks. Field relations, textures, and whole rock geochemistry indicate that most of the mafic to intermediate composition rocks are not cumulates and instead represent liquid compositions. The tholeiitic (or ferroan) gabbros and magnesian gabbroic diorites evolved along liquid lines of descent and have different parental magmas. This conclusion is based on separate trends, often with overlapping MgO contents, for Si, Fe, Ti, Ni, Cr, Zn, Cu and Sr. The mineralogy (late versus early magnetite and contrasting proportions of magmatic amphibole) of the two suites are also slightly different as is the trace element patterns (for example Th, U, Ti and Sr). The Mg# of mafic-intermediate rocks ranges from 46 to 73, implying that the ultimate source for the most mafic rocks was in the mantle, not the crust. Flat HREE patterns, Sm/Yb ratios (<2) show that melting occurred in the shallow mantle, above the stability field of garnet. Small negative Nb-Ta-Ti anomalies and enrichments of Cs, Rb, Pb, Pb, and Sr on mantle-normalized trace element diagrams are consistent with a subduction zone source for both the tholeiitic gabbros and the magnesian gabbroic diorites. Hence, we conclude that the parental mafic magmas were generated by partial melting of spinel peridotite in the mantle wedge above a subducting slab of oceanic lithosphere. However, the composition of plagioclase-olivine pairs suggest the gabbroic magmas were not water-rich, and their high Fe/Mg and Ti/V ratios show they were relatively reduced compared to typical arc magmas. For the gabbros, melting may have been initiated by decompression in an intra-arc extensional environment caused by slab roll back or oblique subduction that partially separated the Sanandaj-Sirjan zone from the Central Iran block. Components from the
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