湘东南印支期与燕山早期花岗岩成矿能力差异与岩石地球化学特征关系探讨

Relationship of petro-geochemical characteristics to metallogenic capacity differences between Indosinian and Early Yanshanian granites in southeastern Hunan

湘东南地处南岭中段北部,为华南重要中生代有色金属矿集区。区内主要矿床均与燕山早期花岗岩有关,印支期花岗岩极少形成规模矿床。已有研究表明区域构造体制不同可能是成矿差异的主要原因,即燕山早期花岗岩形成于后造山伸展构造体制,印支期花岗岩形成于后碰撞弱挤压构造体制。本文研究发现,相对印支期花岗岩而言,燕山早期花岗岩在岩石地球化学特征方面明显具有更好的W、Sn多金属成矿条件:成矿元素W、Sn,放射性生热元素U、Th,挥发分元素F以及稀有金属元素Be、B、Li、Rb等组分的含量及碱性程度更高,岩石氧化程度(Fe2O3/FeO比值)更低;(La/Yb)N值和δEu值更低,Rb/Sr比值更高,反映岩浆分异演化程度更高。因此,除构造体制差异外,花岗岩岩石地球化学特征差异也应是两时代花岗岩成矿差异的重要原因之一。两阶段花岗岩岩石地球化学特征差异的形成可能与构造-岩浆演化历史和构造环境差异有关。

Located in the northern part of the central segment of the Nanling Range, southeastern Hunan is a nonferrous mineralized concentration area in South China. All important deposits are related to Early Yanshanian granites, whereas a few Indosinian granites have formed large-size deposits. Studies show that the difference in tectonic settings is one of the principal factors responsible for the metallogenic differences, that is, the Early Yanshanian granites were formed in an extensional post-orogenic setting, while the Indosinian granites were formed in a weakly compressional post-collisional setting. It is also demonstrated that the Early Yanshanian granites have better W-Sn-polymetallic metalloginitic geochemical conditions than the Indosinian granites in that the former contain richer ore-forming elements W, Sn, radioactive calorific elements U, Th, and volatile elements F, Be, B, Li, Rb, are more alkaline, and have lower Fe2O3/FeO and （La/Yb）h ratios and δEu values and higher Rb/Sr ratios, which indicate that the magma has experienced a higher degree of crystallization differentiation. Therefore, the petro-geochemical differences between the two kinds of granites with different ages should constitute one of the chief causes for metallogenitic differences besides the tectonic regimes. The petro-geochemical differences are likely to be related to the differences in tectonic-magmatic evolution and tectonic setting.