新疆西准噶尔库鲁木苏石英闪长岩成因及地质意义

Origin and geological implications of Kulumusu quartz diorite in West Junggar,Xinjiang

西准噶尔库鲁木苏石英闪长岩LA-ICP-MS锆石U-Pb年龄为(286±10)^(294±4)Ma,加权平均值为290.8±6.2 Ma,表明岩体形成于早二叠世早期.岩体具低Si(w(SiO_2)=58.08%~61.57%),高Al,Ca,TFe,富Na贫K,Mg~#为35~38,ASI(铝饱和指数)比值为1.11~1.20且w(Na+K)/w(Al)<1.0,属于过铝质钙碱性系列.微量元素具高Sr,Y,Yb,Ba,w(Rb)/w(Sr)(0.06~0.13),低Cr,Ni,亏损Nb,Ta,P,Ti,富集Ba,Rb,K,U,Zr,Hf以及相对富集LREE,HREE呈平坦型分布,轻微的负Eu异常的特征.结合区域地质背景,认为石英闪长岩在受到先前俯冲流体交代过的软流圈上涌底侵影响下,垫托在地壳底部的残留洋壳发生部分熔融,且发生了一定程度的混合作用,岩浆在上升过程中发生了单斜辉石等基性矿物的分离结晶作用,并受到上地壳长英质物质的混染作用.西准噶尔地区进入后碰撞构造演化阶段,软流圈地幔存在不均一性,这可能与本区先前存在多条俯冲带的构造过程紧密相关.

The LA-ICP-MS zircon U-Pb age of Kulumusu quartz diorite in West Junggar is（286±10）-（294±4） Ma,with a weighted average of 290.8±6.2 Ma,indicating that the intrusions formation was in the early stage of early Permian.The intrusions possess peraluminous calc-alkaline characteristics,i.e.low Si（w（SiO2）=58.08%-61.57%）,high Al,Ca,TFe,sodium-rich but potassium-poor.Mg^# is 35-38,ASI（aluminum saturation index） ratio is 1.11 to 1.20,and w（Na＋K）/w（Al）〈 1.0.As for trace elements,they are characterized by high Sr,Y,Yb,Ba,high w（Rb）/w（Sr）（0.06-0.13）,low Cr;Ni,Nb,Ta,P,Ti are depleted and Ba,Rb,K,U,Zr,Hf are enriched.LREEs are relatively enriched,HREE plot is in a flat distribution,and Eu is of a slightly negative anomaly.Combined with regional geological backgrounds,it could be deduced that the quarts diorite originated from partial melting of residual oceanic crust under the bottom of the crust with the influence of the under-plating of up-welling asthenosphere metasomatized by the early subduction fluids,during which process the magma underwent fractional crystallization of clinopyroxene mafic minerals and was also contaminated by felsic minerals.Petrogenesis showed that when the Western Junggar evolved to a post collisional stage,the asthenospheric mantle might have had a high heterogeneity closely related to the multiple subduction process.