赣南印支期柯树岭花岗岩体SHRIMP锆石U－Pb年龄、地球化学、锆石Hf同位素特征及成因探讨

SHRIMP zircon U-Pb dating,geochemistry and zircon Hf isotopic characteristics of granitoids in Keshuling granites,Jiangxi Province and their genetic analysis

利用SHRIMP锆石U-Pb定年方法测得赣南柯树岭-仙鹅塘云英岩-石英脉型钨锡多金属矿床柯树岭区段钻孔中的新鲜花岗岩的成岩年龄为251.5±6.6Ma和202±15Ma。柯树岭花岗岩属过铝质高钾钙碱性花岗岩;ΣREE较低(<80.16×10-6),与华南典型含铌钽矿化花岗岩ΣREE含量低的特点一致,是岩浆高度分异演化的产物;锆石εHf(t)值为-14.5～-1.3,Hf两阶段模式年龄TDM2为3.21～2.55Ga,表明该岩体起源于太古宙地壳物质的部分熔融;Al/(Mg+Fe)-Ca/(Mg+Fe)图解表明该岩体主要来源于变质杂砂岩和变质泥岩的部分熔融,但也混有一些变质玄武岩和变质英云闪长岩,两个正的εHf(t)值(+2.4和+3.0)显示岩浆在上升侵位过程中捕获了幔源残留锆石。包括柯树岭在内的华南印支期花岗岩是于后碰撞或碰撞晚期,在以被动侵位机制为主和挤压应力松弛的间隙环境中侵位的,为加厚地壳局部熔融的产物。

The Keshuling-Xian’etang greisen-quartz vein type tungsten-tin polymetallic deposit located in Chongyi County of Jiangxi Province is one of the medium-sized tungsten polymetallic deposits in South China.It is closely related to felsic Keshuling granite outcropped in South mining area.SHRIMP zircon U-Pb dating of a fresh granitic sample from the drilling core has yielded ages of 251.5±6.6 Ma and 202±15 Ma.The Keshuling granites fall into the peraluminous,high-K and calc-alkali category revealed by whole rock analysis.SiO2,total alkaline（Na2O＋K2O）,Fe2O3 and FeO contents of the Keshuling granitic intrusion are 73.51%74.23%,6.58%7.77%,0.01%0.07% and 0.49%0.95% respectively.K2O/Na2O and A/CNK ratios of the Keshuling granitic intrusion are 1.051.98 and 1.171.60（〉1.1）respectively.The low REE total content（ΣREE=43.98×10-680.16×10-6）demonstrates intense differentiation and crystallization at the petrogenic stage,which is the characteristics of typical Nb and Ta-bearing granites in South China.The（La/Yb）N and δEu of the Keshuling granites are 2.537.80 and 0.090.20 respectively.According to chondrite-normalized REE patterns,the REE of the Keshuling granitoids have ＂tetrad effect＂.Granites and pegmatites of ＂tetrad effect＂ are crystallization products of residual melting at the last stage of granitic magmatism.Intensive fractional crystallization can result in enrichment of volatile matters（H2O,F,Cl,B,P）,alkali metals（Na,K）and some ore-forming elements（REE,Nb,Ta,Li,Be）in residual melt.In the primitive mantle-normalized trace element spidergram of the Keshuling granites,defective values of Sr,Eu and Ti show that there existed crystallization and separation of feldspars and ilmenites.The 176Lu/177Hf and fLu/Hf ratios of zircons in the Keshuling granitic intrusion are 0.000 3730.003 818 and-0.99-0.88 respectively,which are lower than those of the upper crust.The 176Hf/177Hf ratios（0.281 8190.282 600）,εHf（t）values（between-14.5 and-1.3）and TDM2（3.212.55 Ga）are representatives of an Archean crustal reservoir.Ca/（Mg＋Fe）-Al/（Mg＋Fe）diagram indicates that Triassic granitoids in South China mainly originated from metagreywackes and pelites,with the involvement of a small amount of metamorphic basalts and tonalities.Two positive εHf（t）values（＋2.4 and ＋3.0）of zircons indicate that mantle sourced zircons were captured during the elevation and emplacement of felsic magma.Compared with previous achievements,it is suggested that the Triassic granitic rocks including the Keshuling granites in South China were formed in a post-collisional tectonic setting or at the late stage of syn-orogenic tectonic setting.During the relaxation after extruding process,the felsic magmas were fomed by partial melting of thickening crustal materials.