火成碳酸岩成因的多元同位素制约－以湖北武当地块碳酸岩杂岩体为例

Carbonatites are magmatic rocks containing more than 50% modal carbonate minerals and less than 20 wt% SiO2 ultimately derived from the mantle. They have been found on all continents ranging in age from Archean to the present.Though the role of the mantle in carbonatites genesis is well established,There is still no consensus on the issues as to the exact mantle domain(lithospheric mantle or asthenosphere) from which the carbonatitic melts are generated; the relationship between the closely co-existed silicates and carbonatites and the evolutional paths of the carbonatitic magma. We'll carry out mineralogical, petrological and geochemical (including radiogenic isotopes, stable isotopes and noble gas isotopes)investigationson two suites of syenite-carbonatite complexes from Miaoya and Shaxiongdong, which intruded in the southern Qinling orogen.The exact intrusion ages of the syenites and carbonatites will be dated with aim to ensure whether they were formed simutaneously.On the basis of the acquired geochemical data,firm constraints will placed on the petrogenesis of the carbonatites and the associated syenites.The mantle domain(lithospheric mantle or asthenosphere) from which the carbonatitic melts originated will be located.A time scale constraints will also be put on the mantle source and the carbonatitic magma activities based on the Lu-Hf isotopic systematics. Finally,the geochemical dynamic settings and the relationship with the evolution of the Qinliang orogen will be discussed.

碳酸岩是一类富含碳酸盐矿物（体积＞50％），SiO2含量少于20％的岩浆岩，尽管碳酸岩起源于地幔的观点已经被普遍认同，但对其具体形成过程，如地幔源区物质组成、与空间上密切共生的硅酸岩间的关系、碳酸岩岩浆如何演化等问题仍存在广泛争论。项目选择侵入于南秦岭造山带武当地块中的庙垭、杀熊洞正长岩-碳酸岩杂岩体为研究对象，通过详细的矿物学、岩石学、地球化学、微区原位同位素年代学和多元同位素地球化学（包括放射成因同位素、稳定同位素和稀有气体同位素）示踪研究，确定该地区碳酸岩与硅酸岩的活动时代，判断碳酸岩与空间上密切共生的硅酸岩间是否具有成因联系，揭示碳酸岩的成因机制以及碳酸岩岩浆的演化规律，判断碳酸岩源区的地幔属性和碳酸岩岩浆的起源深度，在此基础上示踪大陆地幔源区的组成和特征，并对地幔中碳酸岩交代作用的时间尺度提供制约。揭示碳酸岩杂岩体形成过程与秦岭造山带构造演化间的关系。

庙垭、杀熊洞岩体是出露于南秦岭武当地块内的典型碳酸岩-碱性杂岩体，我们在野外地质调查基础上，对这两个杂岩体开展了系统的地质年代学，岩石地球化学以及同位素地球化学研究。庙垭岩体为正长岩和碳酸岩共生，杀熊洞岩体则为辉石岩、正长岩和碳酸岩共生。高精度的年代学结果表明庙垭正长岩、杀熊洞辉石岩和正长岩都形成于435 Ma，而庙垭碳酸岩形成于229Ma，杀熊洞碳酸岩都形成于226Ma，即两个杂岩体内的硅酸岩和碳酸岩都分别是同一期岩浆作用的产物。辉石岩和正长岩锆石Hf-O同位素以及碳酸岩全岩Sr、Nd同位素结果表明，正长岩和辉石岩源于一个亏损的地幔，且地幔源区受到俯冲的大洋板片下地壳来源熔体的交代作用，正长岩是辉石岩分离结晶作用的产物。碳酸岩也来源于一个相对亏损的地幔，其较低的Mg同位素组成表明俯冲的沉积碳酸盐物质对其地幔源区有重要贡献。本项研究首次证实了空间上共生的碳酸岩和碱性岩并不一定具有成因联系，而是不同时代、不同地球动力学背景背景下的产物。打破了人们通常认为的碱性岩-碳酸岩杂岩体中两者有成因联系的传统认识，对进一步认识碳酸岩的成因起到了重要推动作用。