滇西腾冲大松坡锡矿区晚白垩世铝质A型花岗岩的发现及其地质意义

滇西大松坡锡矿区位于腾冲-梁河锡成矿带,该带分布着燕山早期、燕山晚期和喜马拉雅早期三个时代的花岗岩,其岩石地球化学特征及其与锡成矿作用的联系尚需进一步研究探明。本文选取该区古永岩体中的中细粒黑云母二长花岗岩和黑云母花岗岩中的钾长石进行了Ar-Ar定年;中细粒黑云母二长花岗岩和黑云母花岗岩的坪年龄分别为67.84±0.60Ma (2σ)和65.86±0.42Ma (2σ),显示前者略早于后者生成。对大松坡锡矿区古永岩体内花岗岩进行地球化学研究表明其属于高钾钙碱性系列和过铝质系列;轻重稀土分异较明显,无铈异常,负铕异常突出;Ba、Sr、P、Ti等元素亏损,U、Th、Ta、Rb、Pb等元素相对富集,10000Ga/Al范围为3.53～5.52;地球化学特征指示其为A型花岗岩,通过主、微量元素特征和构造环境判别图解进一步判别其为A2亚型(造山后)花岗岩,推断该花岗岩产于造山后的伸展构造环境。大松坡锡矿区云英岩和矿石的稀土及微量元素含量均低于该区古永岩体内花岗岩,但却有着相似的稀土配分模式和微量元素蛛网图分布型式,暗示它们有成因联系。大松坡云英岩型锡矿很可能是热液运移过程中萃取该区古永岩体内过铝质A型花岗岩中的成矿物质并沿张性构造裂隙充填而成。结合本次获得的Ar-Ar测年结果,推断大松坡云英岩型锡矿是该区古永岩体在燕山晚期-喜马拉雅早期岩浆活动的成矿响应,该区在68～65Ma至少存在一次与古永岩体内A型花岗岩关系密切的锡成矿作用。

Late Cretaceous aluminium A-type granites and its geological significance of Dasongpo Sn deposit, Tengchong, West Yunnan

Dasongpo tin deposit lies in the well-known West Yunnan Sn mineralization zone as well as in Tengchong-Lianghe Sn mineralization subzone.There are granites of three different ages including Early Yanshanian,Late Yanshanian and Early Himalayan.These granites have very close relationship with the metallogenesis of Sn-polymetallic deposits,though,the lithogeochemical characteristics and the connection between the intrusive and metallogenic need further research.In this paper,fine-grained biotite monzogranite and biotite granite of Guyong intrusive were selected in the area to make Ar-Ar age-dating.The K-feldspar of the finegrained biotite monzogranite gives a well-defined 40Ar/39Ar plateau age of 67.84 ± 0.60Ma(2σ) while the K-feldspar of biotite granite dating results give a well-defined 40Ar/39Ar plateau age of 65.86 ± 0.42Ma(2σ),showing that the fine-grained biotite monzogranite formed slightly earlier than the biotite granite.Dasongpo granites from Guyong intrusive belong to high-K calc-alkaline to shoshonite series and peraluminous series based on lithogeochemical characteristics,and which have significant differentiation between HREE and LREE,no Ce anomalies but very significant negative Eu anomalies.In addition,they are depleted in Ba,Sr,P,Ti and enriched in U,Th,Ta,Rb,Pb.The range of 10000Ga/Al is from 3.53 to 5.52.These geochemical characteristics all indicate that they are typical A-type granites.From major element,trace element geochemical characteristics,and discrimination diagrams of tectonic environments we can determine that they are A2-type(post-collisional) granites,inferring Dasongpo granites from Guyong intrusive produced in the collision extensional tectonic environment.Greisen veins and Sn ores of Dasongpo Sn deposit have less REE and trace element than Dasongpo granites from Guyong intrusive,but their chondrite normalized REE distribution patterns and primitive mantle normalized multi-element spider diagrams are very similar to those of the Dasongpo Guyong granites,indicating that there is genetic connection between them.The Dasongpo greisen-type Sn veins were possibly formed by extracting metallogenic material from Dasongpo Guyong A-type granites while the hydrothermal transporting and then filling the fracture to mineralize.We infer that,combined with the Ar-Ar dating,Dasongpo Sn deposit is the metallogenetic response to magmatism of Guyong intrusive in Late Yanshanian to Early Himalayan in this area,and there is at least one Sn mineralization which is closely associated with the A-type granites of Guyong intrusive during 68 ～ 65Ma.