赣南碛肚山岩体年代学、地球化学特征及地质意义

Chronology,Geochemical Characteristics and Geological Significance of the Qidushan Pluton,Southern Jiangxi Province

赣南碛肚山岩体作为石头坪重稀土矿床的成矿母岩,目前对其侵位时代、岩石成因、岩石类型、构造环境的认识不足。本文对碛肚山岩体黑云母正长花岗岩开展锆石、独居石U-Pb定年,元素地球化学及Sr-Nd同位素研究。结果表明:碛肚山岩体黑云母正长花岗岩锆石、独居石U-Pb年龄分别为145.0±2.3 Ma、147±1.7 Ma,属早白垩世岩浆活动产物;岩石具有高SiO2(72.78%~77.67%),富钾(K2O/Na2O=1.07~1.38),低Al_(2)O_(3)(12.03%~13.18%),过铝质-强过铝质(A/CNK=1.08~1.25),REE配分模式为海鸥型,富集Rb、U、Th等元素,亏损Sr、Ba、Eu等元素,略高的锆石饱和温度(761~834℃),显示过铝质A型花岗岩特点。同时岩石具有高Rb含量(277.6×10^(-6)~901.3×10^(-6)),Rb/Sr比值(8.07~195.93),显示高分异花岗岩特点。综上认为碛肚山岩体为高分异过铝质A型花岗岩。εNd(t)变化范围为-4.1~-3.0(>-7.0),符合地幔物质参与成岩过程,但在成岩过程中主要提供热量,物质贡献较少。根据区域地质和构造图解,认为碛肚山岩体侵入与古太平洋板块俯冲后撤致使岩石圈伸展有关。

The Qidushan pluton in southern Jiangxi Province is the parent rock of heavy rare earth mineralization in the Shitouping deposit,but the understanding of its emplacement age,petrogenesis,rock type and tectonic environment is insufficient.In this paper,zircon and monazite U-Pb dating,rock geochemistry and Sr-Nd isotope of biotite syenite in the Qidushan pluton were studied.The results show that U-Pb ages of zircons and monazite are 145.0±2.3 Ma and 147±1.7 Ma,respectively,which belong to the Early Cretaceous magmatic activity.The rocks have high SiO2(72.78%~77.67%),rich potassium(K2O/Na2O=1.07~1.38),low Al_(2)O_(3)(12.03%~13.18%),peraluminous-strong peraluminous(A/CNK=1.08~1.25),and the REE classification pattern is seagull type.The enrichment of Rb,Th,U and other elements,the loss of Ba,Sr,Eu and other elements,and the slightly higher saturation temperature of zircons(761~834℃)are indicative of the characteristics of peraluminous A-type granite.In addition,the rocks show high content of Rb(277.6×10^(-6)~901.3×10^(-6)),Rb/Sr ratio(8.07~195.93),displaying high differentiation characteristics of granite.In summary,the Qidushan pluton is a highly differentiated peraluminous A-type granite.The variation ofεNd(t)ranges from-4.1 to-3.0(>-7.0),which is consistent with the involvement of mantle material in the diagenesis process.The mantle material mainly provides much heat but few material contributions in the diagenesis.According to the regional geology and tectonic illustrations,it is suggested that the intrusion of pluton is related to the lithospheric extension caused by the subduction and retraction of the ancient Pacific Plate.