西准噶尔巴尔雷克蛇绿混杂岩带中玄武岩地球化学特征及大地构造意义

Geochemistry of Basalt from the Barleik Ophiolitic Mélange in West Junggar and its Tectonic Implications

西准噶尔地区巴尔雷克蛇绿混杂岩中的玄武岩与蛇纹岩、放射虫硅质岩和晚泥盆世铁列克提组的泥质粉砂岩与沉凝灰岩形成混杂堆积。对玄武岩进行详细的岩石地球化学研究表明,SiO2含量为42.15%～44.71%,高TiO2(3.17%～3.77%)、Na2O(1.73%～2.28%),低Al2O3(13.54%～14.31%)、K2O(1%～1.82%),MgO含量相对稳定(6.75%～8.14%),Mg#为43～46,属于碱性玄武岩系列。稀土总量∑REE=186×10-6～219.06×10-6,轻、重稀土分馏较为明显((La/Yb)N=11.37～12.62),无明显Eu异常(Eu/Eu*=0.96～1),稀土配分模式类似于OIB。相对富集LILE(如Rb、Ba、Th),亏损HFSE(如Zr、Hf),没有明显的Nb和Ta异常,具有高的Ti/Yb(7395～8724)和Zr/Yb(120～136)比值,为典型的OIB地球化学特征。综合研究认为玄武岩形成于弧后盆地的海山环境,其岩浆源区可能为EMI型富集地幔,即软流圈的上涌导致尖晶石相二辉橄榄岩地幔源区大比例部分熔融形成的玄武岩。在区域上,蛇绿混杂岩中的玄武岩所代表的泥盆纪古洋盆是西准噶尔古洋盆向北收缩的残余洋盆。

Basalts from the Barleik ophiolitic mflange in West Junggar occur with serpentinite, radiolarian chert, and muddy siltstone and tuff from the Late Devonian Tielieketi Formation. Detailed geochemical analyses show that the basalts have typical geochemical characteristics to those of OIB, with low SiO2 （42.15% -44.71%）, high TiO2 （3.17%-3.77%）, Na20 （1.73%-2.28%）, low Al2O3 （13.54%-14.31%）, K20 （1%-1.82%） and moderate MgO （6.75%-8.14%, Mg =43-46）. The REE distribution patterns are similar to those of OIB, with ∑REE= 186 × 10-6 -219.06 ）〈 10.6, LREE enrichment （（La/Yb）N ---- 11.37 ~12.62）, weak Eu negative anomaly （Eu/Eu =0.96-1）, high Ti/Yb and Zr/Yb ratios （7395-8724, 120- 136, respectively）. Comprehensive study also suggests that the basalts formed in an ocean island or a seamount setting, with the magma originating from EMI--type enriched mantle, i. e. upwelling of asthenosphere resulted in partial melting of a large amount of spinel lherzolite mantle and thus forming the sasalt. Regionally, the Devonian oceanic basin which is represented by the basalts of the Barleik ophiolitic n61ange occurs as a remnant oceanic basin formed by northward shrinking of the West Junggar Paleoceanic 3asin.