湖南大义山晚侏罗世富硼型成锡矿A型花岗岩成因及地质意义

位于南岭成矿带和钦-杭成矿带(简称钦-杭带)交汇部位的湖南大义山锡矿是南岭地区典型的富硼型锡多金属矿床。为厘清大义山锡矿成矿动力学背景，深化南岭地区钨锡矿成矿机制，本文以大义山成锡矿黑云母二长花岗岩为研究对象，开展了系统的岩石学和地球化学研究。研究发现，大义山锡矿具有富硼的特征，成矿黑云母二长花岗岩发育电气石"囊包"，电气石在蚀变矿物中广泛发育，并与锡矿化紧密共生，这些特征表明成矿花岗岩具有富硼的特征，并发生了岩浆热液流体出溶。地球化学分析显示，黑云母二长花岗岩具有较高含量的Al2O3(13.67%~14.00%)、Na2O (3.65%~3.90%)、K2O (3.49%~4.24%)以及较高的FeOT/(FeOT+MgO)比值(0.95~0.97)、FeOT/MgO比值(18.75~32.69)、A/CNK比值(1.15~1.26)和10000×Ga/Al比值(3.48~4.08)，较低的CaO (0.54%~0.59%)、P2O5(0.04%~0.06%)含量和锆饱和温度(716~725℃)。球粒陨石标准化稀土元素配分具有明显的四分组效应，强烈的Eu负异常，微量元素富集Rb、Th、U、Ta和Nd等元素，亏损Ba、Nb、Sr、Eu等元素。表明成矿花岗岩具有高分异的特征，与A2型花岗岩特征相近。Nd-Hf同位素分析显示，黑云母二长花岗岩与南岭地区成锡矿花岗岩及钦-杭带A型花岗岩同位素特征基本一致，表明岩浆主要来源于中元古代壳源物质熔融，并有幔源物质的加入。综合本文及前人研究成果，推测南岭地区燕山期伸展作用与Izanagi俯冲板块开天窗或撕裂有关，在此背景下，软流圈物质上涌，引发下地壳物质熔融，形成了富硼的壳幔源混合型花岗质岩浆。岩浆中较高的硼含量促使岩浆发生强烈的结晶分异，并有利于晚期锡矿的形成。

Genesis and geological significance of Late Jurassic high-B ore-bearing A-type granite in the Dayishan tin deposit, Hunan Province

The Dayishan tin deposit is located at the intersection of Nanling metallogenic belt and Qin-Hang metallogenic belt. Within Dayishan area, there are more than seven medium to large scale Sn deposits distributed along the Dayishan intrusion. In order to clarify the genetic and geodynamic background of Dayishan ore-forming granite, systematically petrological and geochemical studies are carried out on the metallogenic biotite monzogranite. It was found that the Dayishan tin deposit is characterized by the existence of tourmaline-bearing nodules and extensively distributed tourmaline in the hydrothermal stage, indicating that the ore-forming granite was rich in B, and underwent magmatic hydrothermal fluid dissolution at the late stage. Geochemical analysis results show that the biotite monzogranite has high contents of SiO2 (73.58%~74.76%), Al2O3 (13.67%~14.00%), Na2O (3.65%~3.90%), K2O (3.49%~4.24%) with high ratios of FeOT/(FeOT+MgO) (0.95~0.97), FeOT/MgO (18.75~32.69), A/CNK (1.15~1.26) and 10000×Ga/Al (3.48~4.08). It is also characterized by low contents of CaO (0.54%~0.59%), P2O5 (0.04%~0.06%), and low zirconium saturation temperatures (716~725℃). The chondrite-normalized distribution of rare earth elements has obvious tetrad effect and strong negative Eu anomaly. It is enriched in Rb, Th, U, Ta and Nd, and depleted in Ba, Nb, Sr and Eu. These elemental signals indicate that the ore-forming granite has the characteristics of highly differentiated granite, which is similar to the characteristics of A2 type granite. Nd and Hf isotopic results show that the biotite monzogranite has negative εNd(t) values (-5.1~-2.7) and εHf(t) values (-7.03~-4.93), and the tDM2(Nd) and tDM2(Hf) values are 1.2~1.4Ga and 1.5~1.6Ga, respectively. These isotope values indicate that the magma source of the granite is mainly derived from the Mesoproterozoic crust with addition of mantle-derived materials. Comprehensively analyzing on this work and previous results, this paper speculated that the Yanshanian lithospheric extension in the Nanling district was mostly attributed to tear-off of the subducting slab of the Izanagi Plate. Upwelling of the asthenospheric magma, consequently, induced the melting of the lower crust and forming crust-mantle mixed B-rich granitic magma. The high contents of boron in the magma could promote the differentiation of the granitic magma and enhance the possibility of the formation of tin deposit at the hydrothermal stage.