花岗质岩石中黑云母成分区域性变化对深部物质示踪及成矿的约束:以秦岭地区为例

Regional variation of biotite composition of granitoid and its significance to deep crust component and mineralization:A case study on biotite from Early Mesozoic granitoid in the Qinling orogen

秦岭造山带早中生代花岗质岩浆及成矿作用非常发育,是探讨地壳深部物质组成及成矿关系的重要地区之一,学者已从岩石学、矿床学和地球化学等角度开展了众多研究,但对成矿作用与花岗岩和地壳深部物质组成的认识仍不太清楚。黑云母是中酸性岩中常见的暗色矿物,也是指示成矿可能性的重要矿物之一。因此,本文对秦岭该期花岗质岩石中黑云母成分开展研究,为解决该问题探索新途径。对秦岭早中生代花岗质岩石中黑云母成分的研究结果显示,北秦岭和南秦岭两个构造单元中黑云母成分和形成物理条件存在系统差异,揭示其岩浆物源有别,进而制约了其成矿种类特征。北秦岭早中生代花岗质岩石中黑云母的成分和形成条件变化范围小,以MgO(8%~13%)、TiO_(2)(3%~5%)和Cl(0.02%~0.6%)含量较高,F(0.2%~0.4%)含量较低,以及较高的氧逸度(log f O_(2)值为-16.96~-14.62)和温度(682~771℃)为特征;而南秦岭的成分和形成条件总体变化范围较大,具有MgO(3%~15%)、TiO_(2)(2%~4.5%)、Cl(0.01%~0.18%)含量较低,F(0.1%~1.6%)含量较高,以及较低的氧逸度(log f O_(2)值为-20.88~-15.08)和温度(536~754℃)。此外,研究还显示,黑云母的形成压力与岩浆演化程度和矿物组合相关,当岩石中出现黑云母+白云母±石榴子石组合时,压力较高。秦岭两个构造单元中黑云母成分和形成条件的差异,特别是氧逸度和Cl含量的明显不同,揭示各自的成矿种类和成矿潜力不同,如北秦岭较高的氧逸度和Cl含量,形成铜矿的潜力比南秦岭大。同时,两个构造单元中黑云母的成分差异,还揭示了岩浆物源及深部物质组成的不同,即北秦岭比南秦岭具有更为年轻的地壳,这与区域同位素填图示踪的深部物质组成差异基本一致。由此可见,对区域上同时代花岗质岩石中黑云母成分的研究,不仅可以揭示岩浆演化、岩浆结晶过程物理化学条件等,还可示踪深部物质组成的空间变化与差异及成矿种类和潜力,有望成为探测深部物质组成的新方法和了解区域成矿背景和潜力的新途径。

The Qinling orogen,which is well development of granitic magmas and mineralization in the Early Mesozoic,is one of the important areas to study the relationship of deep crust composition and mineralization.Many studies have been carried out from the petrology,ore deposit and geochemistry,but the understanding of mineralization and composition of granite and deep crust component is still unclear.Biotite is a common mineral in medium-acid igneous rocks and one of the important minerals indicating the possibility of mineralization.Therefore,biotite composition in granitic rocks of this period in the Qinling orogen is studied in this paper to explore a new way to solve this problem.The study of biotite composition in the Early Mesozoic granitic rocks of the Qinling orogen shows that there are systematic differences in biotite composition and formation conditions in the North Qinling and South Qinling,indicating that their magmatic sources are different,thus restricting the characteristics of ore-forming types.The composition and formation condition variation of biotite in the Early Mesozoic granitic rocks in the North Qinling are small,characterized by the high contents of MgO(8%~13%),TiO_(2)(3%~5%)and Cl(0.02%~0.6%)and low content of F(0.2%~0.4%).And high oxygen fugaticity(log f O_(2)value of-16.96~-14.62)and temperature(682~771℃).While these of the South Qinling vary widely,with low contents of MgO(3%~15%),TiO_(2)(2%~4.5%)and Cl(0.01%~0.18%)and high content of F(0.1%~1.6%).And low oxygen fugacity(log f O_(2)value of-20.88~-15.08)and temperature(536~754℃).In addition,the study also shows that the pressure of biotite is related to the degree of magma evolution and mineral association,and the pressure is higher when the biotite+muscovite±garnet association occurs in the rock.The difference of biotite composition and formation conditions in the two tectonic units of the Qinling orogen,especially the obvious difference of oxygen fugacity and Cl content,reveals the different mineralization types and ore-forming potential of the two units.For example,the higher oxygen fugacity and Cl content in the North Qinling,the potential of copper ore formation is greater than that in the South Qinling.At the same time,the difference of biotite composition in the two tectonic units also reveals the difference of magmatic source and deep composition,that is,the North Qinling has a younger crust than the South Qinling,which is basically consistent with the difference of deep component in this region by isotopic mapping.Therefore,the study of biotite composition in contemporaneous granitic rocks in a region can not only reveal the magmatic evolution and physical and chemical conditions of magmatic crystallization process,but can also trace the spatial changes and differences of deep composition as well as the types and potential of mineralization.It is expected to become a new method for deep component exploration,and a new way to obtain the component background and potential of regional ore-forming.