义敦型镁铁-超镁铁岩的主要特征及其与蛇绿岩的对比

THE CHARACTERISTICS OF YIDUN-TYPE MAFIC-ULTRAMAFIC ROCKS AND THEIR COMPARISON WITH OPHIOLITES

“义敦型”相当于文献中通称的阿尔卑斯型,但却不是蛇绿岩。蛇绿岩代表洋壳和地慢的岩石组合,而义敦型镁铁-超镁铁岩体则是由陆壳下的地幔及其部分熔融的产物组成的。因此,蛇绿岩与义敦型岩体的构造含意不同,后者的分布与古缝合带无关。在造山带中区分开蛇绿岩和非蛇绿岩是至关重要的。义敦型镁铁-超镁铁岩组合的认定在理论上和找矿实践上都是有意义的。

Yidun-type mafic-ultramafic rocks are a new type, which were raised by the authors at the time when we studied the mafic-ultramafic rocks in this region. It is similar to Alpine type but it is not ophiolite. The Yidun-type mafic-ultramafic body is mainly composed of metamorphic-peridotite (harzburgite and lherzolite are common, and dunite is limited), and is associated with a small amount of cumulative rocks(clinopyroxene peridotite, pyroxenite, amphibole-pyroxenite and etc.), gabbro and diabase. Its wall rock includes neritic limestone, sandy slate and rhyolite. The petrographic characteristics and chemical compositions of metamorphic peridotite are similar to that of the same kind of rock on the oceanic floor and indicate that both of them are members of upper mantle. Particularly, except LREE depleted ones(few samples), most REE distributions are the LREE-enriched. Its characteristics are not the same as that of the same kind of rocks in ophiolite, but it is similar to that of mantle inclusions in alkaline basalt. Gabbro is rich in Fe, Ti, P and large ion lithophile elements (LILE) as well as LREE, and poor in Mg and Al. It is different from the gabbro in ophiolite, but is similar to gabbros in Alpine area and in oceanic islands which suggests that the Yidun-type gabbro is a product of low-grade partial melting. The characteristics of Yidun-type body mentioned above are similar to Alpine type body, but tectonic environment of their emplacemcnt is different from those of ophiolite, therefore, it is very difficult to explain them by nappes. It may be believed that they are not ophiolite but are products derived from mantle and emplaced as diapir from below continental crust into shallow part of crust. The authors think that 'Yi-dun-type' is of significance not only in theory, but also in ore-prospecting. We know that ophiolite is a rock assemblage with a particular structural implication, and represents residuals of oceanic crust and upper mantle; however Yidun-type body has no relations with evolution of oceanic basins, it is a member of mantle under continental crust. Although the features of metamorphic peridotite in Yidun-type body are similar to those in ophiolite, the ore-bearing features are different as they occur in different tectonic environments. Taking chromite as an example, due to relatively weak depleted degrees of Yidun-type body, important industrial deposites are rare, although chrome mineralization is commonly seen in the body. It is however favoutable for gold, asbestos, magnesite and precious stone deposites and so on.