塔里木东北缘坡十Ni矿化侵入体中橄榄石和铬尖晶石对成岩成矿及源区特征的约束

Olivine and Cr-spinel constraints on the petrogenesis, mineralization and source characteristics of nickel-mineralized Poshi intrusion, NE Tarim

坡十Ni矿化超镁铁侵入体的矿化岩相主要为第二侵入期次的（斜长）单辉橄榄岩、（斜长）二辉橄榄岩、 纯橄岩等岩相。坡十超镁铁岩的橄榄石成分变化范围较大， 橄榄石的Fo值在76.8～89.6之间， Ni含量为767×10^-6～4 580×10^-6。铬尖晶石的Mg^#值和Cr^#值变化范围分别为19.4～41.9和49.8～64.8， 原生铬尖晶石中Cr2O3和Al2O3表现为负相关， 蚀变改造的铬尖晶石则表现为正相关。橄榄石成分剖面显示坡十母岩浆处于一个动态的岩浆系统中， 成分稳定的新鲜岩浆的补给、 持续向上的动力及浅部橄榄石快速分离结晶，造成了不同深度橄榄石成分的不同变化。坡十侵入体母岩浆估算结果为MgO=14.49％， FeO=10.01％，模拟结果显示橄榄石中Ni含量的变化主要受橄榄石结晶分异和硫化物不混溶作用共同控制，其中橄榄石与硫化物熔体发生明显的Fe-Ni交换反应。坡十母岩浆中橄榄石分离结晶造成的硫饱和，是坡十硫化物熔离的重要因素。橄榄石高Fo值、母岩浆高MgO、超镁铁岩中斜长石发育、矿物高结晶温度和铬尖晶石成分的弧岩浆特征显示，塔里木东北缘坡十侵入体是俯冲交代的岩石圈地幔部分熔融形成的母岩浆的产物，表现出低压高温的演化特征，其中源区熔融机制可能与塔里木二叠纪地幔柱提供的热源或该区大规模拆沉作用造成的软流圈上涌有关。

The nickel-mineralized Poshi ultramafic intrusion is situated in the southern margin of Baidiwa fault. Sulfide mineralization mainly occurs in the（plagioclase-bearing）wehrlite, lherzolite and dunite. The olivine in Poshi ultramafic rocks show wide range of compositions, with Ni 767×10^-6～4 580×10^-6 and Fo 76.8～89.6. The Mg^# and Cr^# in Cr-spinel from Poshi intrusion vary from 13.2 to 43.2 and 48.8 to 64.8,respectively. The variations of Cr2O3 and Al2O3 contents show negative correlations in primitive Cr-spinel,while the relatively small variations of Cr2O3 and Al2O3 in altered Cr-spinel show positive correlations. The stratigraphic variations of olivine compositions indicate Poshi intrusion formed in a much dynamic environment. The replenishment of magma with stable composition, the continuous upward power and the fast crystallization of olivine in upper part,result in different variations of the olivine compositions in different depth. The MgO and FeO contents in estimated magma compositions for Poshi are 14.49% and 10.01%, respectively. Our modeling results show that the Ni contents in olivine are controlled by both olivine crystallization and sulfide segregation, and the olivine obviously react with sulfide by Fe-Ni exchange. The high Fo in olivine,high MgO in parental magma,mass of plagioclase occurring in ultramafic rocks,high mineral crystallization temperature and arc-like Cr-spinel compositions,indicate that the Poshi parental magma was derived from the previously-modified mantle,and the partial melting of the metasomatized lithospheric mantle is thought to have resulted from heating by mantle plume activity or by upwelling asthenosphere associated with lithosphere delamination.