塔里木板块东北部坡一镁铁质-超镁铁质层状侵入体岩石成因

坡一侵入体位于塔里木板块东北部坡北岩体内,是该岩体第三阶段岩浆活动形成的十几个小侵入体中的一个,锆石U-Pb年龄为278±2Ma。该侵入体属于以超镁铁质岩石为主的层状岩系,堆晶结构与韵律性堆晶层理非常发育。岩浆分异充分,形成了从纯橄岩到石英闪长岩的多种岩石类型。在超镁铁质岩石中,所有的橄榄石和大部分斜方辉石是堆晶相,少量斜方辉石是填隙相,大部分单斜辉石、褐色普通角闪石和黑云母是填隙相。在镁铁质岩石中,橄榄石和斜方辉石全部是堆晶相,单斜辉石与斜长石既可以是堆晶相,也可以是填隙相;褐色普通角闪石、黑云母和石英均为填隙相。超镁铁质岩石属拉斑玄武岩系列,镁铁质岩石属钙碱性系列。侵入体中大量存在的捕掳体、微量元素地球化学、Nd-Sr同位素组成的EMⅡ型演化趋势,充分证明了同化混染作用伴随岩浆演化过程而逐渐增强,并不断促进了岩浆的分异,而且导致了岩石化学系列的转化。PGE和亲硫元素地球化学以及硫同位素组成证明,硫主要来自于岩浆,硫化物形成于岩浆阶段,岩浆未经历过早期硫化物熔离作用,硫化物熔离起始于橄榄岩相结晶的晚期阶段,并伴随着此后的岩浆演化过程而继续熔离。硫化物熔离是岩浆自身演化和同化混染共同作用的结果。橄榄石Fo分子含量和全岩FeO含量显示,原生岩浆是苦橄质岩浆;源区物质应该是石榴石辉石岩;岩浆生成于地幔柱轴部。在塔里木板块东北部还存在分别来自于软流圈和亏损型大陆岩石圈地幔的二叠纪岩浆岩,它们都应该是塔里木大火成岩省的组成部分。

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).