加里东期白马山岩体锆石SHRIMPU-Pb年龄、地球化学特征及形成构造背景

Zircon SHRIMP U-Pb dating and Geochemistry of Caledonian Baimashan pluton and its tectonic significance

白马山岩体位于雪峰造山带的南东缘,其主体为加里东期花岗岩,部分印支期和早燕山期花岗岩。加里东期花岗岩主要位于白马山岩体中、东部,可划分为4个侵入次。对第二次侵入的细中粒斑状角闪石黑云母花岗闪长岩(γδSb)中的锆石进行了SHRIMP U-Pb年龄测试,得到的年龄为411±4.5Ma,此年龄反映出志留纪末—泥盆纪初的一期岩浆事件。其地球化学特征表明该期花岗岩为S型花岗岩,源岩主要为中地壳酸性岩石。第一次侵入的英云闪长岩(γδοSa)仅有一个小侵入体,以残留体形式存在于第三次(ηγSc)侵入体中,表明白马山岩体未经历更早的大规模岩浆活动,据此推断加里东期白马山岩体为志留纪后期广西运动的产物。区域上,城步-新化大断裂以西发育早志留世前陆盆地砂、泥质沉积,断裂以东至湘东南则缺失志留系,暗示广西运动前断裂以东已逆冲抬升并遭受剥蚀,断裂以西则形成前陆盆地。

Baimashan pluton in the southeastern of Xuefeng orogen is mainly composed of Caledonian granites with minor Indosinian and Yanshanian granites. The Caledonian granites are mainly located in the central and eastern of Baimashan pluton, which can be divided into four periods. U-Pb zircon dating of the second-intruded fine-grained to medium-grained porphyritic amphibole biotite granodiorite （γδSb） was carried out and yield weighted average age of 411±4.5 Ma, which indicates one magmatic event in late episode of Silurian to the early stage of Devonian. Geochemical characteristics indicate it belongs to S-type granitiod and suggest that it should come from acid rocks of middle crust. The first-intruded tonalite （γδoSa） is a little intrusion, which relicted in the third intrusion. It indicates that Baimashan granites hadn＇t experienced earlier large-scale magmatism, so we can inferred that Caledonian Baimashan granites was a product of the late Silurian deformation Guangxi Movement. The west of Chengbu-Xinhua Fault was a foreland basin regionally, which deposited sand and muddy sediments. The east part of this fault of the southeastern of Hunan Province was lack of Silurian sedimentary. It suggested the east part of Chengbu-Xinhua Fault had been thrust uplift and denudation, while the west part formed foreland basin.