湘南中生代钨锡大规模成矿控制因素:锆石年龄和微量元素的启示

Control of the large-sized Mesozoic W-Sn mineralization in southern Hunan:Insights from zircon geochronology and trace element geochemistry

以湘南地区震旦系、泥盆系、石炭系和白垩系碎屑岩中碎屑锆石及印支期—燕山期花岗岩中岩浆锆石为对象,通过锆石U-Pb年龄和微量元素地球化学研究,探讨湘南地区中生代钨锡大规模成矿的控制因素。锆石年龄结果表明,碎屑锆石年龄缺乏2.0 Ga左右的谱峰,显示湘南地区震旦纪和晚古生代沉积盆地物源具有华夏陆块的亲缘性,不具扬子陆块亲缘性,可以代表研究区华夏地块岩浆活动的记录;碎屑锆石年龄峰值记录了华夏陆块2.5 Ga、1.3~1.1 Ga、950 Ma、800 Ma、650 Ma和430 Ma六期前中生代构造-岩浆事件,对应早前寒武纪陆壳形成、Grenville碰撞造山运动及Rodinia超大陆的聚合-裂解过程。锆石微量元素分析结果表明,同一构造-岩浆事件锆石的δEu、LREE/HREE、Y/Ho、Nb/Ta值变化幅度均较大,LREE/HREE和Nb/Ta值不随时代变新而变小(反而有变大的趋势),表明岩浆分异程度与时代无关。但是,锆石∑REE、Y、Th、U、Nb和Ta含量,以及LREE/HREE和Nb/Ta值随时代变新而增高,表明岩浆源区的成分成熟度逐渐增高。湘南地区钨锡矿化规模随着时代变新而增大,在中生代达到顶峰,这一趋势与地壳成熟度的不断增高密切相关。锆石微量元素特征可作为示踪钨锡成矿规模的重要指标。

Based on detrital zircons from Sinian,Devonian,Carboniferous and Cretaceous clastic rocks and magmatic zircons from Indosinian-Yanshanian granites in southern Hunan,the control of Mesozoic W-Sn large-sized mineralization in southern Hunan are discussed through zircon U-Pb geochronology and trace element geochemistry.Zircon geochronology shows that the detrital zircon age is lacking of the peak at 2.0 Ga,indicating that the provenance of the Sinian and Late Paleozoic sedimentary basins in southern Hunan is related to the Cathaysia block,but not to the Yangtze block,which can represent the record of the Cathaysia block magmatic activity in the study area.The age spectrum peaks of detrital zircons records six periods of pre-Mesozoic tectonic-magmatic events of the Cathaysia Block,including 2.5 Ga,1.3~1.1 Ga,950 Ma,800 Ma,650 Ma,and 430 Ma,respectively.These periods correspond to the early formation of Precambrian continental crust,Grenville collision orogeny event,and the amalgamation and splitting of the Rodinia supercontinent.The zirconδEu,LREE/HREE,Y/Ho,and Nb/Ta values of the same tectonic-magmatic event in South Hunan vary greatly.With the age getting younger,the LREE/HREE and Nb/Ta values tend to be larger,suggesting that the magma differentiation degree has nothing to do with age.The zircon∑REE,Y,Th,U,Nb and Ta contents as well as LREE/HREE and Nb/Ta values increase with ages becoming younger,suggesting that the compositional maturity of source area increases accordingly.The results indicate that the scale of tungsten-tin mineralization in southern Hunan increases with times and reaches its peak in Mesozoic,which is closely related to the increasing crustal maturity.The zircon trace elements can be used as an important index to trace the scale of W-Sn mineralization.