湘东北地区石蛤蟆花岗岩体SHRIMP U-Pb年龄及地球化学特征

分布于湖南东北部的石蛤蟆岩体侵位于新元古代地层中。由微细粒斑状黑云母花岗闪长岩和细粒斑状黑云母二长花岗岩等两期侵入体组成。通过锆石SHRIMP U-Pb法测得岩体侵位年龄为157±2 Ma(2σ),MSWD=0.98,成岩时代为晚侏罗世。SiO2=68.26%～68.53%,K2O/Na2O=1.37～1.59,岩石属镁质、准铝质-微过铝质、高钾钙碱性-钾玄岩系列;岩石明显富集大离子亲石元素,亏损高场强元素,Rb/Sr较低(0.40～0.56);ΣREE较高(171.48～183.81),Eu为弱负异常(δEu=0.86～0.93),(La/Yb)N=27.11～45.87;具较高的εNd值(-5.11)和高T2DM(1.63 Ga)。综合研究表明,石蛤蟆花岗岩为混合源高钾钙碱性花岗岩类(KCG),其花岗岩浆有大量幔源物质加入。讨论认为岩体形成于构造体制转换下的地球动力学背景,是造山晚期张弛作用下的产物。

SHRIMP zircon U-Pb dating and geochemical characteristics of Shihama granite pluton in northeastern Hunan province

Distributed in northeastern Hunan province and intruded in Neoproterozoic strata, Shihama granite pluton is mainly composed of micro-fine-grained porphyritic biotite granodiorites of early stage and fine-grained porphyritic biotite monzonitic granites of late stage.SHRIMP zircon dating of the granites gives an age of 157±2 Ma which belongs to Late Jurassic.The granites belong to magnesian,metaluminous-weakly peraluminous,high-K calc-alkaline-shoshonite series,with SiO2=68.26% ～68.53% and K2O/Na2O=1.37～1.59.The rocks show enriched large-ion lithophile elements,depleted high field-strength elements,low values of Rb/Sr(0.40～0.56),high values of ΣREE(171.48～183.81),with δEu=0.86～0.93,(La/Yb)N=27.11～45.87,εNd=-5.11,T2DM=1.63 Ga. Studies indicate that the Shihama granites belong to high-K calc-alkaline granites(KCG) originated form the crust-mantled mixed source ivolving large amount of mantle source materials.It is believed that the granite pluton is the result of tectonic extension during the late period of orogenetic movement and is formed in the geodynamic background related to the transition of tectonic regime.