苏北牛山花岗片麻岩原岩成因类型、成岩时代及其地质意义

Genetic type and age of the protolith of the Niushan granite gneiss from northern jiangsu and its geologic significance

苏北牛山花岗片麻岩是变质变形的深成岩 ,它富含碱性长石 ,普遍含有碱性镁铁矿物 ,具有高硅、富碱、低铝、贫水 ,富含Nb、Zr、Y、Ga、REE及Eu明显亏损等特征 ,原岩成因类型属A型花岗岩。其Y /Nb、Yb /Ta、Ce /Nb等比值较高 ,10 0 0 0Ga /Al为 2 .4 6～ 3.4 5,碱度较低 ,原岩属A2 亚型花岗岩 ,是苏胶造山带碰撞造山后拉张环境下形成的碱性花岗岩。对其进行全岩Rb Sr等时线法测年 ,获得 797.7± 1.5Ma的地质年龄 ,揭示苏胶造山带于新元古代中期已开始向裂解转化。由此推断碰撞造山运动应发生于新元古代早期 ( 1.0～ 0 .9)Ga ,它与中国华北地块与扬子地块在晋宁期碰撞拼合相当 ,可与北美格林威尔造山运动对比 ,应是Ro

The Niushan granite gneiss is plutonic metamorphic rock. It is rich in alkali feldspar and commonly alkali mafic minerals, is characterized by high silicon, low aluminum, short of water and full of alkali, Nb, Zr, Y, Ga, REE as well as evident loss in Eu content. So, the granite gneiss belongs to an A type granite. It has higher retios of Y /Nb, Yb /Ta, Ce /Nb etc., and is lower in the retio of 10 000 Ga /Al and alkali, which indicates that the protolith belongs to A 2 subtype granite and is the alkali granite formed under the pullapart circumstance after the Su jiao orogenic belt colliding and raising up. The age of (797.7±1.5)Ma, which is based on the Rb Sr isochron method of whole rock, indicates that the Su jiao orogenic belt began to split from mid Neoproterozoic. Therefore, it can be concluded that the orogenic process should begin from early Neoproterozoic (1.0～0.9)Ga, which can be correlated with the colliding and splicing of North China block and Yangtze block, and can also be correlated with Grenville orogenic movement of North America. It should be part of the Rodinia supercontinent.