Petrogenesis of the Poyi mafic-ultramafic layered intrusion,NE Tarim Plate

The Pobei rock body is located in northeastern Tarim Plate.As one of many small intrusions of Pobei body,the Poyi intrusion formed during the third stage of the magmatism and dated at 278±2Ma by the SHRIMP zircon U-Pb method.The Poyi layered intrusion,which is mainly composed of ultramafic rocks,develops cumulate texture and rhythmic layering.The magma differentiated thoroughly and formed many types of rocks,which range from dunite to quartz diorite.In the ultramafic rocks,all olivine and most orthopyroxene occur as cumulate phase,while small part of orthopyroxene,most clinopyroxene,brown hornblende and biotite are intercumulate.In the mafic rocks,all olivine and orthopyroxene are cumulate phase,while all brown hornblende,biotite and quartz are intercumulate.But the clinopyroxene and plagioclase may be cumulate or intercumulate.The ultramafic rocks are tholeiitic series and the mafic ones are calc-alkalic.Xenoliths occurred in the intrusion,geochemistry of trace elements and EMⅡ evolutionary trend of Nd-Sr isotopic composition,fully demonstrate that the contamination increased as magma evolving.At the same time,contamination promoted magmatic differentiation and brought about the transformation of geochemical series.Geochemistry of platinum group and thiophile elements,as well as isotopic composition of sulfur,show that sulfur mainly came from mantle-derived magma and sulfide formed during magmatic stage.The sulfide segregation began in the late stage of the crystallization of peridotites and went on as the magma evolving.The Fo value of olivine and FeO content of whole-rock show that the primary magma was picritic and formed by partial melting of axis plume.The main rocks of magmatic source should be garnet-pyroxenolite.The other Permian magmatite in northeastern Tarim Plate,which derived from asthenospheric or depleted continental lithospheric mantle,also should be part of the Tarim large igneous province(LIP).