南岭大东山花岗岩的形成时代与成因--SHRIMP锆石U-Pb年龄、元素和Sr-Nd-Hf同位素地球化学

Age and Origin of the Dadongshan Granite from the Nanling Range: SHRIMP U-Pb Zircon Age, Geochemistry and Sr-Nd-Hf Isotopes

中国东南部南岭地区广泛出露以弱过铝质黑云母二长花岗岩和黑云母钾长花岗岩为主的燕山早期花岗质岩石,其成因有待进一步研究。大东山岩体岩性主要为黑云母二长花岗岩和黑云母钾长花岗岩,两个样品的SHRIMP锆石U-Pb年龄为165±2Ma和159±2Ma,与区域南岭系列的黑云母花岗岩的主要形成时代一致。花岗岩样品以高硅(SiO2>72%)、高钾(K2O/Na2O>1.6)、富碱(K2O+Na2O=7.36%~9.31%)和弱过铝质(集中于ASI=1.00~1.11)为特征。微量和稀土元素组成上,岩体富Rb,Th和LREE,贫Ba,Nb,Sr,P和Ti,Eu负异常显著(δEu=0.06~0.34)。多数样品的Zr,Ce,Nb和Y含量总和小于350×10-6,10000×Ga/Al值低于典型的A型花岗岩。同位素组成上,样品具有高Isr(0.7123~0.7193)和低εN(dt)(-9.3^-11.5)的特点,两阶段Nd模式年龄为1.70~1.89Ga;与全岩εNd(t)不同,岩浆锆石的εHf(t)具有较大的变化范围(-3.5^-11.8)。矿物学及地球化学结果表明大东山是一个高分异的I型花岗岩岩体。岩体岩浆很可能是由元古代火成岩石部分熔融形成,并伴随有少量年轻或新生幔源物质的加入,岩浆上升侵位的过程中发生混合、结晶分异作用。

Early Yanshanian （Jurassic） granites, which are dominated by weakly peraluminous biotite monzogranite and K-feldspar granite, are ubiquitous in Nanling Range in Southeast China. However, the petrogenesis of the granites remains controversial. The Dadongshan pluton consists of biotite monzogranite and biotite-bearing K-feldspar granite. SHRIMP U-Pb zircon dating of two samples yield ages of 165 ＋ 2 Ma and 159 ± 2 Ma, consistent with the major formation time of the ＂Nanling-series＂ biotite granites in the neighboring region. The granites are silica-rich （SiO2 〉 72%）, potassic （K2O/Na2O 〉 1.6）, weakly peraluminous （most have ASI=1.00 - 1.11） and enriched in alkalies （K2O＋Na2O = 7.4%～ 9.3%）. Samples are invariably enriched in Rb, Th and LREE, yet depleted in Ba, Nd, Sr, P and Ti, with distinct negative europium anomaly （ 8 Eu=0.06 ～ 0.34）. Low contents of Zr＋Ce＋Nb＋Y （〈 350 ppm） and low 10000 × Ga/A1 rule out their A-type affinity. Isotopically, samples show high and variable initial Sr ratios （0.7123 ～ 0.7193）, yet low and constant εNd（t） （-9.3 - -11.5）, corresponding to T20M age of 1.70 ～ 1.89 Ga. However, their zircon Hf isotopes show large variations, with εHf（t） values between -3.5 and -11.8. Mineralogical and geochemical features suggest that the Dadongshan granites are highly fractionated I-type granites. They were probably derived from partial melting of the Proterozoic igneous protoliths with minor addition of juvenile crust or newly mantle-derived magmas, accompanied by assimilation and fractional crystallization.