新疆东天山黄山岩体岩石地球化学特征与岩石成因

LITHOGEOCHEMISTRY AND PETROGENESIS OF HUANGSHAN INTRUSION,EAST TIANSHAN,XINJIANG

黄山岩体位于东天山北部的土墩-黄山-图拉尔根镁铁-超镁铁质岩带中段,受康古尔塔格-黄山韧性剪切带控制,主要由橄榄岩、橄榄二辉岩、辉石岩、辉长苏长岩、辉长岩以及辉长闪长岩组成。岩石化学组成属拉斑玄武岩系列,普遍富集大离子亲石元素(Rb,Ba,Sr),亏损高场强元素(Nb,Ta,Ti)。岩体εNd(t)=+4.1^+9.2,除3件样品Sεr(t)为+2.2,+12.5和+15.4,大部分Sεr(t)=(-22.5^-4.5),Nd,Sr同位素组成基本属亏损型地幔特征;Pb同位素初始值(206Pb/204Pb)i=18.081~18.413,(207Pb/204Pb)i=15.441~15.513,(208Pb/204Pb)i=37.461 6~37.899,具有MORB亲和性。岩相学及岩石地球化学特征显示岩浆演化过程中主要发生了橄榄石、斜方辉石、单斜辉石和斜长石的分离结晶作用。岩浆演化晚期阶段发生了一定程度的同化混染作用,元素地球化学和Nd,Sr,Pb同位素体系证明,岩浆源区主体由软流圈地幔物质组成,同时也有一定量富集型岩石圈地幔组份加入。黄山岩体是岩石圈根部拆沉加热熔融和软流圈地幔上涌减压熔融的产物,这种地幔动力学机制应该对应于后碰撞伸展环境。

Controlled by the Kangurtag-Huangshan ductile sheer zone,the Huangshan intrusion is located in the central segment of Tudun-Huangshan-Tulaergen mafic-ultramafic rock belt,the northern section of the East Tianshan.The intrusion is characteristic of tholeiitic series and composed of peridotite,olivine pyroxenites,lherzolite,olivine gabbronorite,gabbros and gabbros-diorite.They are enriched in large ion lithophile elements（such as Rb,Ba,Sr） and depleted in high field strength elements（such as Nb,Ta,Ti）.The εNd（t） value of the rocks ranges from ＋4.1 to ＋9.2 and the εSr（t） value of most rocks from-22.5 to-4.5 except for three samples with the εSr（t） value of ＋2.2,＋12.5 and ＋15.4 respectively.Nd,Sr isotopic compositions indicate that the magma was derived from depleted mantle.The initial 206Pb/204Pb,207Pb/204Pb and 208Pb/204Pb is 18.081~18.413,15.441~15.513 and 37.461 6~37.899 respectively,indicating the MORB-affinity.Petrographic and lithogeochemical analysis suggests that the magma was experienced fractional crystallization of olivine,orthopyroxene,clinopyroxene and plagioclase and was subjected to crust assimilation contamination to a certain extent in the late stages of magmatic evolution.Characters of element geochemistry and Nb,Sr,Pb isotope shows that the magma source is composed of asthenosphere components combined with enriched lithospheric mantle.Huangshan intrusion is t resulted from the lithospheric delamination at the root and the upwelling of asthenosphere components,corresponding to post-collision extension setting.