准噶尔洋洋内弧岩浆作用:西准噶尔百口泉早石炭世火山岩地球化学及Sr-Nd-Hf同位素研究

Junggar Intra-oceanic Arc Magmatism:Constrains from Geochemistry and Sr-Nd-Hf Isotope Geochemistry of the Early Carboniferous Baikouquan Volcanic Rocks in West Junggar,NW China

西准噶尔中部晚古生代的构造-岩浆作用被广泛认为与准噶尔洋的洋内俯冲有关。然而,对于重建洋内俯冲历史的弧相关岩浆活动,缺乏可靠的岩石学和地球化学证据。本文对西准噶尔百口泉包古图组火山岩进行了锆石U-Pb年代学和Lu-Hf同位素、全岩地球化学和Sr-Nd同位素组成的综合研究,旨在为西准噶尔中部早石炭世构造演化提供约束。通过对代表性玄武安山岩样品LA-ICP-MS锆石U-Pb年代学分析,获得年龄为342.2±1.0 Ma,表明其形成于早石炭世。岩石地球化学研究显示,百口泉火山岩具有钙碱性特征,SiO_(2)含量为48.38%~54.45%,具有高Al_(2)O_(3)含量(19.19%~19.78%),相对富集轻稀土元素,并显示富集LILE(如Rb、Ba、Sr),亏损HFSE(如Nb、Ta、Zr、Hf),与典型岛弧火山岩特征相似。根据Sm/Yb-Sm与Sm/Yb-La/Sm模拟计算模型,表明百口泉火山岩是由尖晶石+石榴石相二辉橄榄岩亏损地幔源发生部分熔融(熔融程度约为10%~20%),并由俯冲岩石圈释放的流体改造而成。同时样品具有低(^(87)Sr/^(86)Sr)i值(0.704593~0.704852),正ε_(Nd)(t)值(6.4~6.8)和ε_(Hf)(t)值(8.7~14.9),与西准噶尔中部早石炭世岛弧火山岩和达尔布特蛇绿岩类似,表明其源区为亏损的软流圈地幔。结合前人研究成果,认为百口泉火山岩形成与洋内俯冲的岩浆作用有关,是晚古生代西准噶尔洋盆西北向俯冲消减过程中的产物。

Intra-oceanic subduction is considered to be one of the most important contributors to the crustal growth of the immense Central Asian Orogenic Belt(CAOB),the study of which can deepen the understanding of plate tectonic processes and the history of continental growth.The Late Paleozoic tectono-magmatism in the central West Junggar has been widely considered to be related to the intra-oceanic subduction of the Junggar Ocean.However,robust petrologic and geochemical evidence for arc-related magmatism,which is critical to reconstruct the intra-oceanic subduction history,is still unavailable.A comprehensive study of geochronological,geochemical,and Sr-Nd-Hf isotopic compositions of the volcanic rocks from the Baogutu Formation in the Baikouquan region was carried out,aiming to provide constraints on the early Carboniferous tectonic evolution of the central West Junggar.LA-ICP-MS zircon U-Pb analysis of a representative basaltic andesite sample yielded an early Carboniferous age of 342 Ma.The volcanic rocks are calc-alkaline,with low SiO_(2)(48.38%to 54.45%)and high Al2O3(19.19%to 19.78%),and are relatively enriched in light rare earth elements and large ion lithophile elements(e.g.Rb、Ba、Sr),but depleted in high field strength elements(e.g.Nb,Ta,Zr,Hf),these characteristics are similar to those of typical island arc volcanic rocks.The Sm/Yb-Sm and Sm/Yb-La/Sm covariation diagrams indicate that the Baikouquan volcanic rocks were derived from 10%to 20%partial melting of a garnet and spinel lherzolitic depleted mantle source that modified by fluids released from the subducted oceanic lithosphere.They have high positive ε_(Nd)(t)values(+6.4 to+6.8),medium initial ^(87)Sr/^(86)Sr ratios with the range of 0.704593 to 0.704852 and variable ε_(Hf)(t)values(+8.7 to+14.9),which are similar to those of the early Carboniferous island arc volcanic rocks and the Dalabute mafic rocks in West Junggar,indicating they were derived from depleted asthenosphere mantle in a normal subduction environment.Combined with the palaeogeography and previous research results,we suggest that the formation and tectonic setting of these volcanic rocks are related to the intra-oceanic subduction magmatism,which are resulted from the southeastward subduction of the late Paleozoic ocean basin in Western Junggar.