浙江桐庐自碎二长花岗斑岩的特征和成因

在中国东部的中生代断陷盆地中经常有大量次火山花岗斑岩类与陆相的中酸性火山岩共生,有时并与浅成侵入体共同构成火山侵入杂岩。其中有些次火山花岗斑岩的产状和结构比较特殊,对其成因有过长时间的激烈争论。确切地了解这些次火山花岗斑岩的特征,弄清它的形成机理是火山岩地区岩石学、基础地质及找矿工作中亟待解决的重要课题。浙江桐庐的自碎二长花岗斑岩就是这样一种次火山花岗斑岩。

The characteristics and the origin of the autoclastic monzonitic granite-porphyry, Tonglu, Zhe jiang

There is an autoclastic monzonitic granite-porphyry in the Tonglu volcanic basin of Zhejiang province. This rock has intruded into the boundary fault on the southern margin of the basin as an akmolith or stretches along the fault of central basin floor. There is a trasitional relation between it and the outside rhyodacitic rocks with same chemical composition. This rock exhibits the mor- tar texture. In its matrix, the characteristic texture of acidic hypabyssal rocks is seen to be common. As an outstanding feature, the rock without exception shows the peculiar "pearlitic edge" grown around the margin of crystalloclastic potash feldspars and it may be taken as a texture sign indicated that this rock has been crystallized under superepizonal environment. The structural state of these potash feldspars possesses the character of both volcanic and subvolcanic rocks. The chemical compositions of biobite and hornblende state clearly that the formation temperature of autoclastic monzonitic granite-por- phyry is lower than that of rhyodacitic volcanic rocks but formation pressure of the former is higher than the latter. Therefore it is a special variety of subvolcanic rock and was formed under the overlying strata of volcanic rocks or the extrusive phase and near the conduit of the magmas. This subvolcanic rock often occur in the composite caldere and in the volcanic dome inside the fault depression basin.