湘桂边界越城岭岩基北部印支期花岗岩锆石U-Pb年代学和地球化学特征

Zircon U-Pb Chronology,Geochemistry of the Indonesian Granitic Rocks from Northern Yuechengling Batholith in Guangxi-Hunan Junction

湘桂边界越城岭岩基北部印支期花岗岩时空分布和岩石成因目前还不明确.以该时期花岗岩为研究对象,进行了岩石学、锆石U-Pb年代学、地球化学和Nd-Hf同位素组成研究.研究结果显示,印支期花岗岩主要分布在湘桂边界以北区域,形成时代236~222Ma.岩性以含电气石的二长花岗岩为主,副矿物组合为锆石、(斜)黝帘石、磷灰石、石榴石,并具有高硅、富碱,贫钙、镁和磷,弱-强过铝质的地球化学特征.早阶段花岗岩源岩主要为变质杂砂岩,可能混入了少许新生地壳组分;晚阶段花岗岩源岩由不同比例的变质泥质岩石和变质杂砂岩组成.花岗岩具有负且稳定的锆石ε_(Nd)(t)值(-9.3^-10.6)和ε_(Hf)(t)值(-5.4^-11.9),反映源区平均地壳存留年龄为1.8Ga左右.花岗质岩浆的形成受控于华南印支期后造山加厚地壳背景下变沉积岩中云母类矿物的脱水熔融过程,其运移和就位与岩基旁侧深大断裂的松弛调整密切相关.

The temporal and spatial distribution of Indonesian granitoid in northern Yuechengling batholith is still uncertained.In this paper,a compilation of petrology,zircon U-Pb chronology,geochemistry and Nd-Hf isotopes was used on the granitoid to solve this uncertainty.The result shows Indonesian granitoid spreads in north area of the border with the age of 236-222 Ma,and mainly consists of medium to fine grained tourmaline-bearing monzogranites,with accessory minerals assemblage of zircon,zoisite（and/or clinozoisite）,apatite and garnet.The granitic samples have characteristics of high SiO2,alkaline,low CaO,MgO,P2O5 contents,and weak to strong peraluminous（A/CNK=1.07-1.17）,with enrichment of Rb,Th,U and strong depletion of Ba,Sr,Eu,Nb,Ti elements.Source of early stage of Indonesian granitic rocks mainly consisting of metagreywackes,with small amount of new crustal component.Those of late stage are inhomogeneous mixing of metagreywackes and metapelites.Granitic rocks of both stage have stable high negativeεNd（t）（-9.3--10.6）andεHf（t）（-5.4--11.9）ratios with source＇s average crustal residence age of ca 1.8 Ga.Integrated with Indonesian tectonic involvement of South China,formation of granitic magma is perhaps due to dehydration melting of mica-riched metasediments during post-orogenic thickenedcrust environment,migration and emplacement of the magma is controlled by relaxation of deep fault adjacent to the batholith.