从花岗岩的Sm-Nd同位素探讨华南中下地壳的组成、性质和演化

AN EXPLORATION ON THE COMPOSITION, NATURE AND EVOLUTION OF MID-LOWER CRUST IN SOUTH CHINA BASED ON THE Sm-Nd ISOTOPIC DATA OF GRANITES.

现有 2 4 0个花岗岩体的Nd同位素资料表明 ,华南内陆花岗岩可能主要由地壳部分熔融而成。从壳源花岗岩提取源岩成分信息的方法是把这种花岗岩的同位素成分同出露的元古代弱变质地壳的相应资料作比较。野外关系说明 ,华南内陆花岗岩侵入毗邻的围岩 ,因此其形成深度应该比现在出露的深度大。因为这些花岗岩的定位深度一般在 5～ 15km之间 ,这些壳源花岗岩可能代表一种中地壳深度的熔融 ,它们的同位素成分应能提供在这一深度上地壳成分的信息。根据华南壳源花岗岩同出露的元古代弱变质地壳岩石的Sm -Nd同位素对比 ,华南内陆大部分地区在中地壳深度上存在中性至长英质成分的地壳 ,并且从早古生代到中生代不曾发生过明显变化。但是花岗岩浆的分异作用所导致的Sm、Nd分馏可能是亲石元素矿化的一个重要控制因素。

Fig.1 and Fig.2 show the distribution of the available Nd isotopic data of about 240 granite plutons in South China. The majority of depleted mantle model ages of granites range between 1.0-2.0 Ga, with an average of 1.5 Ga. The T DM ages of granites are consistent with the ages of the exposed Precambrian basement metamorphic rocks. The massive T DM values strongly indicate that the bulk of South China was formed during the Proterozoic. The majority of ε Nd (t)range between -5 and -10. The low ε Nd values require an ancient light rare earth element enriched source, as expected for rocks derived from Precambrian continental crust. Consequently, the source materials for granites in South China interior are suggested to be dominated by old and recycled crust. One approach towards extracting further information on the source composition from the granites derived from the crustal source is to compare their isotopic composition to the values for exposed, weakly metamorphosed, Proterozoic crust. Field relationships suggest that granites of South China interior are intrusives with respect to their immediate wall rocks and therefore must have been generated at greater paleo crustal depths than that represented by the current levels of exposure. Because these granites were generally emplaced at depths between 5 and 15 km, it is probable that the granites derived from the crustal source represent mid crustal melting and so their isotopic composition should provide information regarding the crustal compositions at this depth. The correspondence of Sm-Nd isotopic composition between the granites derived from the crustal source and the exposed weakly metamorphosed Proterozoic crust (Fig.3-7) reveals that there is an intermediate to felsic composition crust at mid crustal depth throughout South China interior, and the composition of mid crust has not significantly changed from the early Paleozoic to the Mesozoic. However, differentiation of the granitic magma could result in fractionation of Sm from Nd which is an important factor in controlling the production of lithophile mineralization.