新疆北山地区坡一岩体橄榄石成分特征及其对地幔源区的制约

Compositions of the olivine from the Poyi ultramafic intrusion in the Beishan area,Xinjiang:Constraints on the nature of its mantle source

坡一超镁铁质岩体位于塔里木板块东北缘与中亚造山带南缘拼贴处的北山地区,主要由纯橄岩、异剥橄榄岩、方辉橄榄岩、橄榄辉石岩和少量橄长岩组成,其中方辉橄榄岩和橄榄辉石岩赋含铜镍硫化物矿化。不同类型岩石中橄榄石的Fo值介于84%~89%之间,其中纯橄岩中橄榄石的Fo值最高可达89%,与幔源原生岩浆结晶橄榄石的Fo值(88%~91%)相当。岩体中异剥橄榄岩和纯橄岩中Fo大于86%的橄榄石结晶于硫化物熔体熔离之前,相比与饱满地幔熔体平衡的橄榄石,其具有更高的Ni和更低的Mn含量,指示坡一岩体母岩浆可能不是饱满地幔部分熔融形成的。利用坡一岩体纯橄岩全岩成分和其中Fo值最高的橄榄石成分估算出的母岩浆成分与夏威夷玄武岩的母岩浆成分类似,均表现为富Si和富Ni,显示其地幔源区可能存在辉石岩组分。利用橄榄石的Ni/(Mg/Fe)和Mn/Fe比值估算出地幔源区部分熔融时辉石岩组分贡献的熔体比例约为49%。

The Poyi ultramafic intrusion is located in the Beishan area,a junction between the northeastern margin of the Tarim Craton and the southern margin of the Central Asian Orogenic Belt.The intrusion is mainly composed of dunite,wehrlite,harzburgite and olivine pyroxenite,with minor amounts of troctolite.The Ni-Cu sulfide mineralization is mainly hosted in harzburgite and olivine pyroxenite.The olivine grains of the rocks have Fo contents ranging from 84%to 89%,and the highest Fo content of the olivine is similar to that of olivine crystallized from a mantle-derived,primary mafic magma(88%~91%).The olivine grains with Fo>86%likely crystallized from the melt prior to the onset of sulfide segregation,and they have higher Ni and lower Mn contents than those formed from the melts in equilibrium with the fertile mantle.Therefore,the parental magma of the Poyi intrusion was unlikely derived from the fertile mantle.Based on the composition of the most primitive peridotite sample and the olivine in the sample,the composition of the parental magma of the Poyi intrusion is estimated to have similarly high Si and Ni contents as the parental magma of the Hawaii basalts,the parental magma of the Poyi intrusion is thus assumed to have been produced by partial melting of the mantle with a pyroxenite component.The fraction of the pyroxenite component in the partial melts is estimated to be^49%,based on the Ni/(Mg/Fe)and Mn/Fe of the olivine from the most primitive peridotite sample of the Poyi intrusion.