甘肃龙首山岩带西井镁铁质岩体成因及其构造意义

Genesis of Xijing Intrusion from Longshoushan Terrane and the Tectonic Significance

西井岩体位于北祁连造山带以北,阿拉善地块西南缘的龙首山隆起带。以往的研究多以沿龙首山断裂分布的镁铁-超镁铁质岩带作为和金川岩体相关的岩浆事件进行,而本次选择西井镁铁质岩体进行了精确的地质年代学和地球化学研究,确定了西井岩体岩性主要为橄榄辉石岩和辉长岩,成岩时代为（421.0±9.0）Ma,可以和北祁连高压变质带榴辉岩年龄相对应;εNd（t）为4.06~5.52,（87 Sr/86 Sr）i为0.704 548~0.707 575,具有地幔岩石圈特征;微量元素及其同位素计算表明西井岩体经历了约10%的下地壳物质混染。据此得出西井岩体及其龙首山岩带早志留世镁铁质侵入岩体成因模式为：祁连洋壳连续俯冲过程中洋壳与陆壳分离,热的软流圈物质持续冲击地幔岩石圈的底部;由于热传导效应,大地热流沿着地幔岩石圈上升,使得80km深度的湿的橄榄岩层发生熔融,产生玄武质岩浆作用,玄武质岩浆上升过程中与下地壳物质发生约10%混染,形成西井岩体及其龙首山镁铁-超镁铁质岩带。

The Xijing mafic intrusion is located in Longshoushan terrane in the west of Alxa block and the north of Qilian orogenic belt.Previous interpretation for mafic-ultramafic intrusions along Longshoushan rift suggested that the genesis of these intrusions were related to the magmatism of Jinchuan intrusion.The major petrographic components of Xijing intrusion are olivine websterite and gabbro,and Xijing intrusion was formed in（421.0±9.0）Ma,which is consistent with the age of eclogites from North Qilian orogenic belt.The values ofεNd（t）and（87 Sr/86 Sr）iare 4.06-5.52and0.704 548-0.707 575 respectively,which lie in the field of lithospheric mantle.The calculation of isotope and trace elements indicates that Xijing intrusion was contaminated by the lower crustal materials in about 10%.At last,we interpret the evolution of Xijing intrusion and Longshoushan intrusion belt as follows：during Qilian continental subduction,the ocean lithosphere detached from the continental lithosphere,the hot asthenosphere impinged the base of the overriding mantle lithosphere near the breakoff point.Because of the conduction,the heat flow went up and caused the melting of lithosphere at solidus of a hydrated peridotite at a depth of 80 km,which produced basalt magmatism.During the ascent of magma through the mantle into the crust,the magma was contaminated with lower crust and formed the Xijing intrusion and Longshoushan mafic-ultramafic intrusion belt.