内蒙古苏尼特左旗乌兰敖包基性-超基性岩的成因及其对含矿性的指示

Genesis of mafic-ultramafic rocks in Ulan Obo of Sonid Left Banner,Inner-Mongolia: Indication for metallogenic potentiality

乌兰敖包基性-超基性岩位于阿拉善北缘兴蒙造山带，由方辉橄榄岩、辉绿岩、辉长岩组成。地球化学分析表明：蛇纹石化橄榄岩为地幔橄榄岩，辉绿岩和辉长岩为拉斑玄武岩系列岩石，均具有与N-MORB相似的微量元素特征，但区别于N-MORB，3种岩石共同组成了蛇绿混杂岩。矿物学分析表明蛇纹石化橄榄岩中橄榄石为地幔橄榄岩中镁橄榄石（Fo=91.10~91.81），铬铁矿为铬尖晶石，具有高的Cr#和Mg#值分别为61.04~64.44、50.40~56.37）。单斜辉石出现在方辉橄榄岩、辉长岩及辉绿岩中，而斜方辉石只出现在方辉橄榄岩中，其中方辉橄榄岩中单斜辉石为顽透辉石，辉长岩中为普通辉石，而辉长岩中为次透辉石、贫钙普通辉石、普通辉石，斜方辉石均为斜顽辉石。辉长岩U-Pb年龄为344.5±1.5 Ma，表明该蛇绿混杂岩形成于早石炭世。地球化学及矿物学特征表明乌兰敖包蛇绿岩形成于俯冲早期的弧前环境中，属于SSZ型蛇绿岩。蛇绿混杂岩成因提供铬铁矿形成的有利条件，因此应围绕铬铁矿进行找矿工作。

Ulan Obo mafic-ultramafic rocks, located along the Inner Mongolia-Da Hinggan Mountains orogenic belt, consist of harzburgite, gabbros and diabase. Geochemical data indicate that serpentinized peridotites are mantle-derived peridotite. Gabbro and diabase are tholeiitic rocks and similar to N-MORB in LILE components. They together consist of ophiolitic complex. Mineralogical analyses reveal that the olivine in peridotite is forsterite（Fo=91.10~91.81）, and chromite has high Cr# and Mg#（61.04~64.44, 50.40~56.37） and belongs to pecotites. Clinopyroxene occurs in both mafic and ultramafic rocks, it is endiopside in peridotite and augite in diabase and is different in scale from sahlite to augite in gabbros. Orthopyroxene only occurs in harzburgite and belongs to cilinoenstatite. Zircon U-Pb weighted average age of gabbro is 344.5±1.5 Ma, suggesting that the ophiolite formed in early Carboniferous period. Geochemical and mineralogical researches suggest that Ulan Obo ophiolitic complex formed at the fore-arc setting of the Paleo-Asian Ocean in late Paleozoic period. Petrogenesis of Ulan Obo ophiolitic complex shows favorable conditions for chromite deposit formation,and therefore emphasis must be paid to the chromites in further exploration and prospecting.