粤西新兴岩体的形成时代与成因研究:对古特提斯洋东支关闭时间的约束

Formation age and petrogenesis of the Xinxing pluton in western Guangdong:Constraint on the closure of the East Paleo-Tethys Ocean

华南板块南缘二叠纪与三叠纪之交的构造属性仍存在较大争议。对新兴岩体详细的野外调查和研究发现,新兴岩体的侵位时间为晚三叠世(240~224 Ma)而非侏罗纪,其主要岩性为细中粒—细粒斑状黑云母二长花岗岩。地球化学分析显示,新兴花岗岩具有高钾钙碱性、过铝质—强过铝质花岗岩特征,具轻稀土元素富集、重稀土元素相对亏损的右倾稀土配分模式,富集大离子亲石元素(Rb、U)而亏损高场强元素(Nb、Ta、Zr、Hf、Ti)。Sr−Nd、Lu−Hf同位素分析显示,新兴花岗岩εNd(t)值介于-11.5~-10.5,εHf(t)值介于-2.9~-10.3,具有壳源源区特征。本次研究表明,粤西地区印支期构造-岩浆活动可能开始于~250 Ma,华南板块南缘海西—印支期岩浆作用自晚二叠世(大容山岩体)一直延续到晚三叠世(新兴岩体),且晚三叠世仍存在强烈的岩浆活动。新的证据支持古特提斯洋东段分支的关闭时间在~250 Ma,而印支板块和华南板块的陆陆碰撞拼贴一直延续到240~224 Ma。

The tectonic framework on the southern margin of South China Block(SCB)between Permian and Triassic Periods has long been a controversial subject.The regional geological survey and research show that the Xinxing pluton in western Guangdong was emplaced in Late Triassic(240-224 Ma)rather than Jurassic,whose main lithology is medium-fine and fine-grained porphyritic monzonitic granite.The Xinxing granite is characterized by high potassium calc-alkaline and peraluminous-strongly peraluminous series,which is enriched in LREE,Rb,U,and depleted in HREE,Nb,Ta,Zr,Hf,Ti.It exhibits relatively lowεNd(t)(-11.5--10.5)andεHf(t)(-2.9--10.3)values,showing the crustal source affinity.Therefore it is suggested that the start of the Indosinian tectonic-magmatic event in Western Gongdong probably took place in~250 Ma.These new geochemical and geochronological evidence have furthermore restricted the timing of tectonic activities on the southern margin of SCB between later Permian(Darongshan pluton)and later Triassic(Xinxing pluton).Hence,the closure time of East Paleo-Tethys Ocean would be about 250 Ma,and the continental-continental collision between Indo-China Block and SCB continued till 240-224 Ma.