中酸性岩Re-Os同位素方法研究及其在西藏驱龙斑岩矿床的应用

Re-Os isotpic system with unique geochemical character has the great potential to be a critical approach for disclosing the petrogenesis of the intermediate-acid rock. However, due to the low concentration of Re and Os in the intermediate-acid rock,it is difficult to measure Re-Os system in them accurately and experiment method has become the barrier for the application. According to the characteristic of intermediate-acid rock, this research will improve the digestion conditions, the procedures and TIMS measurement method according to build a specific Re-Os analysis method for the intermediate-acid rock. Qulong porphyric deposit occurs in continental collision enviroment, which is different from the classic porphyric deposit. The petrogenesis of this deposit is both the controversy and the hot pot. This research will apply Re-Os isotopic system in Qulong deposit, seek what mineral enrich Re and Os in intermediate-acid magmatic system, together with other isotopic system constrain the source of the magma, fine depict the magmatic evolution, and disclose the relation between petrogenesis and ore-forming process.

Re-Os同位素体系具有独特的地球化学性质理应成为中酸性岩岩石成因研究的重要手段，但由于中酸性岩的Re、Os含量极低，技术方法难以满足分析要求，限制了Re-Os体系在中酸性岩中的应用。本研究针对中酸性岩的特点进行溶样条件，实验流程和TIMS测量方法的改进，建立能够满足此类样品Re-Os分析测试的方法。西藏驱龙矿床是产出于大陆碰撞环境下的斑岩矿床，其岩浆成因存在争议，是研究的热点。本研究以驱龙矿床为应用对象开展Re-Os同位素研究，寻找中酸性岩中Re、Os的富集矿物，与其他同位素手段联合约束含矿岩浆起源，精细刻画岩浆演化过程，揭示成岩成矿关系。

Re-Os同位素体系具有独特的地球化学性质，能够成为中酸性岩浆岩成因研究的重要手段。西藏驱龙斑岩铜矿床产出于大陆碰撞环境，其岩浆成因存在争议。本项目针对中酸性岩的特点开展了溶样条件和实验流程优化，对中酸性岩中Re和Os的富集矿物开展了初步研究，对冈底斯带上各时期岩浆岩及地质端元的Re-Os同位素特征开展了初步对比研究，对驱龙斑岩矿床内几类与成矿相关的岩浆岩开展了研究。通过研究发现：（1）磁铁矿是中酸性岩浆岩中的一种富集矿物；（2）初步厘定了西藏冈底斯岩浆带主要地质端元的Re-Os同位素特征，代软流圈组成的达孜基性次火山岩(187Os/188Os)i为0.3244~0.4220，代表基性下地壳的也拉香波角闪岩包体187Os/188Os为0.1520~0.2236，代表上地壳的安多片麻岩187Os/188Os为2.3813~14.0935，曲水岩基代表新生铁镁质下地壳或楔形地幔被交代后部分熔融形成的玄武质岩浆，(187Os/188Os)i为1.2098~2.2938；（3）驱龙斑岩铜矿发育两类闪长岩包体，其中富铜闪长岩包体的Ba/Ta比值、Cu含量、Sr-Nd-Os同位素组成与指征地幔交代作用强弱的(Sr/P)N呈明显相关性，结合放射性成因明显的Os同位素组成（0.6462~2.2103）指示其楔形地幔经历俯冲板片物质交代部分熔融的产物，而富集Os同位素组成与相对亏损Sr-Nd-Pb同位素组成表明交代地幔的板片物质是洋壳熔体/流体。普通闪长岩包体的Os同位素组成相对亏损（0.1730~0.7308），且具有较高的Mg#和Cr、Ni含量以及较低的稀土总量，与达孜基性次火山岩类似，表明软流圈地幔是普通闪长岩包体的源区。模拟计算显示，下地壳熔体分别与两类包体岩浆混合形成了花岗闪长岩。此项研究拓展了Re-Os同位素的应用领域，为中酸岩浆岩的起源和演化研究提供了一种有力的同位素手段。