西昆仑奥依塔格石炭-二叠纪岩浆岩:弧后盆地的产物?

目前对西昆仑石炭-二叠纪火山岩分带性、形成环境、深部地幔源区特征还缺乏较好的约束。在区域地质填图、综合研究的基础上,将西昆仑石炭-二叠纪岩浆岩空间上分为南带、北带。本文展示了北带岩浆岩集中出露的奥依塔格地区玄武岩、辉绿岩、辉长岩的地球化学和Sr、Nd、Pb同位素以及与辉长岩共生的斜长花岗岩的锆石LA-ICP-MS测年数据,以约束该区基性岩形成的时代、构造环境和地幔源区特征,同时与库地玄武岩、阿羌基性火山岩进行了比较。目前的数据表明:(1)斜长花岗岩单颗粒锆石LA-ICP-MS测年得到313.6±1.6Ma、291.6±1.7Ma两组年龄,后者代表斜长花岗岩和辉长岩的侵位时代,前者可能代表玄武岩的年龄。(2)球粒陨石标准化稀土元素配分模式图中,玄武岩显示轻稀土略富集的向右缓倾模式,辉绿岩、辉长岩均为轻稀土略亏损的近平坦型;原始地幔标准化微量元素值均表现为大离子亲石元素相对富集,Nb、Ta谷明显,高场强元素中后半部分呈平坦型模式。(3)地球化学指标显示奥依塔格一带基性岩未受到或很少受到地壳物质混染,样品的Nd、Pb组成可以用来代表地幔源区的成分特点,Nd-Pb、Pb-Pb图解显示其代表的地幔源区具有"Dupal"异常,并于金沙江蛇绿岩中玄武岩代表的地幔源区有较高的一致性。(4)综合岩石地球化学、沉积组合认为奥依塔格基性岩形成于弧后盆地构造环境,区域对比,指出它与库地一些克沟组玄武岩、于田县阿羌组火山岩同为康西瓦-麻扎混杂岩带代表的洋盆向北俯冲,引发弧后盆地扩展的结果。

The origin of Carboniferous-Permian magmatic rocks in Oytag area, West Kunlun: Back-arc basin?

Zonation, tectonic setting and nature of mantle source region for the volcanic rock of Carboniferous-Permian in West Kunlun is poorly understood at present. Based on the field geological mapping and composited researches, the West Kunlun Carboniferous-Permian magmatic rock could be divided into two parts (north and south) in space. We present the LA-ICP-MS zircon dating of plagiogranite that concomitant with gabbro, geochemical and Sr-Nd-Pb isotopic results of basalt, diabase and gabbro from Oytag in the north belt where the magmatic rocks are centralized appeared in order to constrain their age, tectonic setting and nature of mantle source region. Simultaneously, we compare them with the basalt from Kuda and the basic volcanic rock from Aqiang. Present data show that the emplacement of plagiogranite and gabbro from Oytag occurred in Early Permian (291.6±1.7Ma) and the formation of basalt may occur in Late Carboniferous (313.6±1.6Ma). The REE geochemical characteristics indicate that the basalts are in the right dipping pattern, slightly enriched in LREE. Dolerites and gabbros are in nearly flat-type and have a little loss in LREE. Trace elements in primitive mantle-normalized values are expressed as large-ion lithophile elements relative enrichment, Nb and Ta elements are obvious trough and the HFSE in the latter half are flat-mode. Geochemical signatures suggest that crust contamination have played insignificant roles on the basic volcanic rock from Oytag, the Nd, Pb isotopic compositions of these samples can be used to present the characteristics of mantle source region. The scheme of Nd-Pb, Pb-Pb displays that the mantle source region what it represent for has Dupal anomaly and is highly in accordance with what the basalt of ophiolite in the Jinsha River. A body of evidence including geochemistry, sedimentary assemblage indicate that the basic volcanic rock of Oytag form in the back-arc basin tectonic setting. With consideration of regional comparison, we propose that the basic volcanic rock from Oytag, the basalt from Yixiekegou Formation in Kuda and volcanic rock from Aqiang Formation in Yutian County, result in the back-arc spreading originated in the Kangxiwar-Mazar ocean basin northward subduction.