大兴安岭吉峰科马提岩地质地球化学特征

THE GEOLOGICAL AND GEOCHEMICAL CHARACTERISTICS OF THE JIFENG KOMATIITES IN THE DA HINGGAN LING MOUNTAINS

野外地质调查和室内岩石学研究表明 ,大兴安岭北段吉峰林场一带变质超基性岩为具有典型鬣刺结构的科马提岩。科马提岩系列由橄榄质科马提岩、玄武质科马提岩及拉斑玄武岩、辉长岩等岩石组成。科马提岩显示了从超镁铁质到镁铁质地球化学趋势 ,拉斑玄武岩具有从富镁到富铁的趋势 ,而上覆长英质火山岩则遵循钙 -碱趋势。科马提岩稀土配分型式为类似于南非超镁铁质科马提岩的平坦型或轻稀土略富集而重稀土平坦型。科马提岩系列 8件样品的Sm Nd同位素数据构成一条相关性较好的等时线 ,等时线年龄为 1 72 7Ma± 74 7Ma ,INd=0 5 1 0 72 5± 0 0 0 0 0 798,εNd (t) =6 94± 1 5 6 ,表明科马提岩形成于中元古代早期 ,其源区为亏损的软流圈地幔。这一地壳增生事件可能与松嫩地块从西伯利亚地台南缘裂解有关。

Field geological survey and geochemistry indicate that the metamorphic ultrabasic rock in Jifeng area of the north Da Hinggan Ling Mountains is komatiite with spinifex texture. The komatiite series consists of the ultramafic komatiite, basaltic komatiite, tholeiite and gabbro. The komatiite displays a geochemical trend from ultramafic to mafic,and the tholeiite shows a geochemical trend from rich Mg to rich Fe. The felsic igneous rock overlying the komatiite series displays a calc alkaline trend. The komatiite REE patterns show smooth or slightly rich in LREE, similar to that from South Africa. The eight samples composed of komatiite, tholeiite and gabbro yield a reasonable isochron age of 1727Ma±74 7Ma with I Nd =0 510725±0 0000798, ε Nd ( t) =6 94±1 56. It is suggested that the Jifeng komatiites were derived from the depleted rheosphere mantle during the early Mesoproterozoic. This crustal accretion event might be related to the splitting of the Songnen Massif from the southorn continental margin of the Siberia Massif.