湘东丫江桥岩体时代与成因:来自LA-ICP-MS锆石U-Pb年代学、地球化学和Lu-Hf同位素制约

Geochronology and petrogenesis of the Yajiangqiao Granitic Pluton,eastern Hunan Province:constrains from LA-ICP-MS zircon U-Pb dating,geochemistry and Hf isotope

丫江桥岩体是湘东地区重要的成矿花岗岩,为精确厘定丫江桥岩体侵位时代、查明岩石成因,本文对丫江桥岩体中粗粒和中细粒黑云母二长花岗岩进行了LA-ICP-MS锆石U-Pb定年、全岩地球化学和原位锆石Lu-Hf同位素分析。获得LA-ICP-MS锆石U-Pb年龄为213.0±1.2Ma(n=12,MSWD=0.67)和212.1±1.2Ma(n=14,MSWD=0.09);岩石SiO_2含量为70.39%～72.88%和73.19%～74.85%;K_2O+Na_2O值为6.94%～7.95%和7.58%～7.96%;K_2O/Na_2O比值为1.25～1.70和1.45～1.58,A/CNK比值为1.05～1.07和1.07～1.09。稀土元素配分模式右倾,轻、重稀土分馏明显,(La/Yb)N值为13.76～24.52和7.12～11.01;负Eu异常,δEu值为0.34～0.61和0.26～0.33。富集Rb、K、Pb等大离子亲石元素,亏损Ba、Sr、Eu和高场强元素Nb、Ta、Ti等。锆石ε_(Hf)(t)值为-7.01～-0.90和-6.38～+6.85,二阶段Hf模式年龄(t_(DM2))为13.1～16.9Ga和12.7～16.6Ga。上述结果表明黑云母二长花岗岩为印支晚期岩浆活动产物,属弱过铝质花岗岩,岩浆主要来源于中元古界地壳物质重熔,可能有少量地幔物质参与。结合前人研究成果认为,黑云母二长花岗岩形成于后碰撞伸展的构造背景下,地壳减薄和压力降低以及地幔物质上涌的共同作用,导致地壳部分熔融形成花岗质岩浆,岩浆侵位冷却结晶,形成花岗岩体。

The Yajiangqiao granitic pluton is important metallogenic granite in eastern Hunan.In order to accurately determine the emplacement age and petrogenesis of the Yajiangqiao pluton,LA-ICP-MS zircon U-Pb dating,geochemistry,and Lu-Hf isotopic analysis were implemented for the medium-coarse grained biotite monzonitic granite and medium-fine grained biotite monzonitic granite of the Yajiangqiao pluton.LA-ICP-MS U-Pb zircon dating yielded two ages of 213.0±1.2Ma(=12,MSWD=0.67)and 212.1土1.2Ma(n=14,MSWD=0.09)for the Yajiangqiao biotite monzonitic granite.The two granites have SiO2 of 70.39%-72.88%and 73.19%?74.85%,and K2O+Na2O of 6.94%-7.95%and 7.58%-7.96%,with K2O/Na2O ratio of 1.25-1.70 and 1.45-1.58,and A/CNK ratio of 1.05-1.07 and 1.07-1.09,respectively.The chondrite-normalized REE patterns of the granites show are right oblique,distinct fractionation between LREE and HREE,with(La/Yb)N of 13.76-24.52 and 7.12-11.01 respectively,and moderate to obvious negative Eu anomaly,with theδEu value of 0.34-0.61 and 0.26-0.33.They are enriched in large ion lithophile elements(e.g.,Rb.K and Pb)and depleted in Ba,Sr,Eu and high field strength elements(e.g.,Nb,Ta and Ti).TheεHf(t)values range from-7.01-0.90 and 6.38-+6.85,and the two-stage Hf model ages(Zdm2)are 13.1-16.9Ga and 12.7-16.6Ga,respectively.The results above demonstrate that the biotite monzonitic granite was the product of late Indo-China magmatic activity,suggesting that they may belong to weak peraluminous granite.The magma derived mainly from the remelting of Proterozoic crust,likely involved with a minor amount of mantle material.Combined with the previous results that the biotite monzonitic granite formed in a post-collisional extension setting,this study concludes that the granitic pluton formed from the emplacement and crystallization of granitic magma due to crust thinning and fast pressure reducing,along with uprising of mantle material.