浙江龙游杂岩变质作用演化、锆石U-Pb定年及其构造指示意义

Metamorphic evolution,zircon U-Pb dating of the Longyou complex in Zhejiang Province and their tectonic implications

华夏地块加里东期构造热事件的属性问题一直存在较大争议,主要集中在是陆内造山作用还是陆间造山作用两种观点。龙游地区变质杂岩作为华夏地块东北部加里东期高级变质地体的重要组成部分,对其开展变质作用与锆石U-Pb年代学研究,可以为限定该期构造热事件的成因背景提供重要约束。在详细野外工作基础上,选择了5件代表性变质杂岩样品(2件长英质片麻岩样品和3件变基性岩样品)开展研究工作。通过岩相学与矿物化学,并结合定量相图计算,确定龙游地区高压基性麻粒岩经历了三个阶段的变质演化过程:(1)压力峰期阶段(M 1),矿物组合为Grt+Cpx+Pl+Hb+Qtz+Ilm,温压条件为855~865℃/1.04~1.15GPa;(2)温度峰期阶段(M 2),矿物组合为Grt+Cpx+Opx+Pl+Hb+Qtz+Ilm,温压条件为870~928℃/0.95~1.11GPa;(3)固相线降温、降压阶段(M 3),矿物组合为Cpx+Opx+Pl+Hb+Qtz+Ilm,温压条件为810~860℃/0.55~0.86GPa。总体构成顺时针样式的PT演化轨迹,反映其成因与造山作用有关。锆石U-Pb定年结果显示龙游地区长英质片麻岩碎屑锆石年龄集中于950~800Ma,高压基性麻粒岩碎屑锆石年龄集中于890~680Ma,显示物源主要来自于新元古代地体,并非古元古代变质基底。龙游地区变质杂岩主要记录了两个阶段的变质锆石U-Pb年龄,分别为460~440Ma和420~400Ma,前者可能代表了压力变质峰期阶段,后者则相当于温度变质峰期后的地体抬升冷却阶段年龄。综合各方面信息,可以认为华夏地块加里东期构造热事件很可能缘于增生后的扬子地块与某一未知陆块发生的陆-陆碰撞造山作用。

The nature of the Caledonian tectonic thermal event in the Cathaysia block remains controversial,with debates focusing on whether it was an intracontinental or intercontinental orogeny.To constrain the genetic background of this event,we investigated the Longyou metamorphic complex,an important component of the Caledonian metamorphic terrane in the northeastern Cathaysia block through metamorphic studies and zircon U-Pb geochronology.Based on detailed field geological investigations,five representative samples,including two felsic gneiss and three metamorphosed basic rock were selected.Petrographic observations,mineralogical analyses and phase equilibria modeling revealed three metamorphic stages in the high pressure basic granulite:(1)the peak pressure stage(M1),characterized by the mineral assemblages Grt+Cpx+Pl+Hb+Qtz+Ilm under P-T conditions of 855~865℃and 1.04~1.15GPa;(2)the peak temperature stage(M 2),characterized by Grt+Cpx+Opx+Pl+Hb+Qtz+Ilm under 870~928℃and 0.95~1.11GPa;(3)the solidus cooling and depression stage(M 3),characterized by Cpx+Opx+Pl+Hb+Qtz+Ilm under 810~860℃and 0.55~0.86GPa.Zircon U-Pb dating results show that detrital zircon ages in the felsic gneiss are concentrated between 950Ma and 800Ma,while those in the high-pressure basic granulite are concentrated between 890Ma and 680Ma,suggesting that the provenance is mainly from Neoproterozoic terranes rather than the Paleoproterozoic metamorphic basement.The Longyou metamorphic complex records two stages of metamorphic zircon U-Pb ages,ranging from 460Ma to 440Ma and from 420Ma to 400Ma,respectively;the former may represent the peak pressure stage,while the latter corresponds to the uplift and cooling stage.In summary,the Caledonian tectonic thermal event in the Cathaysia block is likely due to continental collision orogeny between the accreted Yangtze block and an unknown block.