新疆中天山南缘乌瓦门地区蛇绿岩中超镁铁岩的成因：来自岩石矿物学和地球化学的证据

摘要

本文对新疆中天山南缘乌瓦门蛇绿混杂岩中的超镁铁岩进行了岩石矿物学和地球化学研究，对其成因和形成环境进行限定。乌瓦门蛇绿混杂岩中的超镁铁岩为蛇纹石化二辉橄榄岩，由橄榄石（Fo=89.1~90.6）、斜方辉石（Wo0.4-2.4En87.2~90.7Fs8.9-10.9；Mg#=89.0~91.0）、单斜辉石（Wo49.1-51.3En16.0~48.4Fs0.9-4.3；Mg#=90.2~92.1）和尖晶石（Mg#=71.8~77.5；Cr#=9.3~13.4）组成。主量元素组成上，以低MgO（37.74%~41.34%）、高Al2O3（2.58%~3.39%）、高CaO（2.23%~3.68%）和高TiO2（0.05%~0.11%）为特征。微量元素上，亏损稀土元素（REE总量为1.73×10−6~4.63×10−6），亏损不相容元素（如，Rb=0.4×10−6~1.39×10−6，Zr=0.73×10−6~3.28×10−6，Hf=0.04×10−6~0.11×10−6），富集相容元素（如，Cr=2516×10−6~2793×10−6，Co=84.6×10−6~119×10−6，Ni=1641×10−6~2261×10−6）。矿物学和地球化学特征一致指示，乌瓦门蛇绿混杂岩中的超镁铁岩为经历过低程度（5%~10%）部分熔融作用的残余地幔橄榄岩，形成于洋中脊环境，是MOR型蛇绿岩中的地幔橄榄岩。%The Wuwamen ophiolite is located on the southern margin of Middle Tianshan Mountains, Xinjiang. In this paper, petrology, mineralogy and geochemistry of the ultramafic rocks from Wuwamen ophiolite were studied to constrain their origin and tectonic setting. The ultramafic rocks from Wuwamen ophiolite are serpentinized lherzolites composed of olivine (Fo=89.1-90.6), orthopyroxene (Wo0.4-2.4En87.2~90.7Fs8.9-10.9;Mg#=89.0-91.0), clinopyroxene (Wo49.1-51.3En16.0~48.4Fs0.9-4.3;Mg#=90.2-92.1) and spinel (Mg#=71.8-77.5; Cr#=9.3-13.4). They are characterized by relatively lower content of MgO (37.74%-41.34%), and higher content of Al2O3 (2.58%-3.39%), CaO (2.23%-3.68%) and TiO2 (0.05%-0.11%). The ultramafic rocks are also depleted in rare earth elements (REE;total REE=1.73×10−6-4.63×10−6) and incompatible elements (e.g., Rb=0.4×10−6-1.39×10−6, Zr=0.73×10−6-3.28×10−6, Hf=0.04×10−6-0.11×10−6), and enriched in compatible elements (e.g., Cr=2516×10−6-2793×10−6, Co=84.6×10−6-119×10−6, Ni=1641×10−6-2261×10−6). These data indicate that the ultramafic rocks from Wuwamen ophiolite are residues of mantle rocks after low degrees (ca. 5%-10%) of partial melting in a Mid-Ocean-Ridge (MOR) environment.