西昆仑西北缘大洋斜长花岗岩带的岩石地球化学特征、成因及其构造环境

Geochemical characteristics,petrogenesis and tectonic setting of Oceanic plagiogranites belt in the northwestern margin of western Kunlun

大洋斜长花岗岩一般作为蛇绿岩的组成部分,发育规模往往较小,目前世界上不与蛇绿岩共生或有成因关系的斜长花岗岩实例较少,关于不发育蛇绿岩的大规模大洋斜长花岗岩的报道更是少见。本文通过岩石学、岩石地球化学、锆石UPb定年和Hf同位素研究,论证西昆仑造山带西北缘发育着大规模由英云闪长岩、石英闪长岩和奥长花岗岩组成的大洋斜长花岗岩带。以该斜长花岗带中奥依塔克岩体和萨罗依岩体为研究对象,测得其锆石LA-ICP-MS U-Pb年龄分别为322.8±2.2Ma、319.0±1.7Ma,锆石均具较高的正εHf（t）值（11.06~16.93）,模式年龄为320~540Ma（平均值为396.1Ma）,大于其形成年龄。岩石具低的Al2O3、高Ca O和Mg O含量,强烈的富钠贫钾,稀土元素总量较低,LREE稍亏损或平坦型配分型式,通过与世界典型大洋斜长花岗岩的对比,该斜长花岗岩带属于典型大洋斜长花岗岩,但相对具负Eu异常,富集K、Rb,亏损Nb、Ta元素。对该大规模斜长花岗岩的成因研究进行了探索,认为该斜长花岗岩带不是传统的玄武质岩浆结晶分异成因,而是在低压（〈0.1GPa）低温（737~781℃）条件下镁铁质洋壳发生部分熔融的产物,残留相为斜长石＋角闪石±斜方辉石±钛铁矿（无石榴石）。结合区域资料,西昆仑西北缘斜长花岗岩带是在晚石炭世弧后盆地的伸展裂解环境下以乌鲁阿特组（玄武岩夹沉积岩）为代表的早石炭世新生镁铁质洋壳部分熔融所形成的产物。

As a part of the ophiolite suite,Oceanic plagiogranite always crops outsporadically. At present,whether ophiolite＇s absence or no genetic relationship with ophiolite,oceanic plagiogranite is rarely reported,let alone making the large-scale ocean plagiogranite existing independently. Based on the petrology,petrogeochemistry,zircon U-Pb isotopic dating and Hf isotope,it is proved that the northwestern margin of western Kunlun exists large-scale ocean plagiogranite belt which composed of tonalite,quartz diorite and trondjemite. In this study,Oytag pluton and Saluoyi pluton are as the research subjects in the plagiogranite belt. As a result,all the U-Pb zircon LA-ICP-MS ages are 322. 8 ± 2. 2Ma and 319 ± 1. 7Ma,respectively. All the zircons have positive εHf（ t）values（ 11. 06 ~ 16. 93）,and the model ages（ 320 ~ 540 Ma,average = 396. 1Ma） are older than the formation age. Moreover,these plutons are characterized by low Al2O3,high Ca O and Mg O,with intensively rich in Na and poor in K. Furthermore,they also display low ∑REE concentration and little depleted-flat LREE pattern. According to the comparison with typical ocean plagiogranite around the world,these plagiogranites should belong to the typical ocean plagiogranite,yet implying depleted Eu anomaly,enriched in K and Rb,depleted in Nb and Ta. In conclusion,these plagiogranites should be derived by mafic oceanic crust＇s partial melting（ residual mineral phases is： Pl ＋ Hb ＋ Opx ＋ Ilm（ without Grt）） in the condition of low pressure（ 0. 1GPa） and low temperature（ 737 ~ 781℃）,other than simply the result of basaltic magma＇s crystallization. Besides,combining with regional geological data,these plagiogranites should be the products of partial melting of juvenile mafic oceanic crust which is represented by Wuluate Group（ mainly produced basalt with sedimentary rock） in Early Carboniferous, and subsequently appeared with Oytag back-arc basin rifting in Late Carboniferous.