南秦岭东江口、柞水和梨园堂花岗岩类锆石LA-ICP-MSU-Pb年代学与锆石Lu-Hf同位素组成

东江口、柞水和梨园堂岩体位于商丹断裂南侧。锆石的LA-ICP-MS U-Pb年代学分析表明,东江口花岗闪长岩、柞水花岗岩、梨园堂石英二长岩和梨园堂花岗岩等4个样品的岩浆结晶年龄分别为246.8±2.5Ma(早三叠纪),233.6±1.3Ma(中三叠纪),956.1±4.5Ma(新元古代),203.6±2.2Ma(晚三叠纪)。锆石的Lu-Hf同位素原位分析结果表明,锆石的两阶段Hf模式年龄(tDM2)分别为1.4~1.6Ga、1.0~1.3Ga、1.0~1.3Ga和1.0~1.3Ga。勉略洋闭合(约250Ma)之后,扬子板块和华北板块发生碰撞,导致扬子陆块俯冲至南秦岭地块之下并发生小规模的部分熔融形成早-中三叠纪(246.8~233.6Ma)花岗岩类。碰撞结束(约220Ma)后,扬子陆块板片断离诱发软流圈物质上涌,同时俯冲的扬子陆壳开始折返,在地幔热和构造减压的条件下,俯冲陆壳及上覆岩石圈地幔发生广泛的部分熔融,形成不同程度具埃达克质地球化学特征的晚三叠纪(199.0~224.8Ma)花岗岩类及伴生的镁铁质包体。

Zircon LA-ICP-MS U-Pb dating and Lu-Hf isotopic system of Dongjiangkou, Zhashui, and Liyuantang granitoid intrusions, South Qinling belt, central China

The Dongjiangkou, Zhashui, and Liyuantang granitoid intrusions occur near the Shang-Dan fault. Zircon U-Pb dating yielded the following crystallization ages: 246.8±2.5Ma for the Dongjiangkou granodiorite, 233.6±1.3Ma for the Zhashui granite, 956.1±4.5Ma for the Liyuantang quartz monzonite, and 203.6±2.2Ma for the Liyuantang granite. The accompanied zircon in situ Lu-Hf isotopic analysis gave two-stage Hf model ages (t_DM2) of 1.4~1.6Ga, 1.0~1.3Ga, 1.0~1.3Ga, and 1.0~1.3Ga for these granitoids respectively. The closure of the Mian-Lue Ocean at about 250Ma was followed by the collision of the Yangtze Craton and North China Craton resulted in the formation of the Qinling orogenic belt, meanwhile, partial melting of crustal rocks took place during the subduction of the Yangtze plate and South Qinling massif and genrated the Early-Middle Triassic granitoids at 246.8~233.6Ma. Subsequently, at about 220Ma, the "Yangtze" slab was detached and induced upwelling of the asthenosphere. Widespread partial melting of the subducted continental crust and the upper lithosphere mantle produced adakitic and associated mafic inclusions in Late Triassic (199.0~224.8Ma).