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1Monazite recorded Paleoproterozoic granulite-facies metamorphism and Triassic fluid modification of the Weihai pelitic granulite,Sulu orogen显示文摘Monazite is an important accessory mineral in the pelitic granulites(Weihai area,Sulu orogen),and is also a powerful monitor for understanding the metamorphic evolution of the granulites.The pelitic granulites incongruously occur as lenses in granitic gneisses.The lithologies of the lenses gradually change from core to margin:The undeformed coarse-grained granulite,the foliated fine-grained granulite,the garnet-biotite-gneiss,and the migmatized granulite.The lens cores mostly preserve a peak granulite-facies metamorphic mineral assemblage of garnet+plagioclase(antiperthite)+quartz+sillimanite+biotite with accessory minerals of rutile,zircon,and monazite.The lens margins display a fluid-induced retrogression.In order to decipher the metamorphic processes of the pelitic granulites,a combined study of BSE imaging,U-Pb dating,and trace element composition for the monazites from the metapelitic lens were conducted.Monazites from the undeformed coarse-grained granulite only record a Paleoproterozoic age(1832±7 Ma,n=40).Monazites from the other lithologies yield the inherited Paleoproterozoic age and Triassic overgrowth age.For example,monazites from the migmatite yield intercept ages of 1818±10 and 211±22 Ma(n=56)with Triassic concordant age of 223.8±2.9 Ma.The Paleoproterozoic monazites are characterized by remarkable depletion in HREE and Y with obviously negative Eu anomalies,indicating their formation equilibrated with garnet and feldspar under granulite-facies conditions.During Triassic fluid modification,the monazite bright rims assimilated Th and Si but released U,HREE,Y,and P.This process resulted in that the Triassic overgrowth monazites have higher HREE and Y contents,and lower Th and U contents with relatively low Th/U ratios.Thus,the monazites in the pelitic granulites recorded a Paleoproterozoic metamorphic event and Triassic fluid modification.The Weihai pelitic granulites might have a tectonic affinity with the North China Craton.Therefore,the Paleoproterozoic pelitic granulites were mechanically drawn into the orogen during the Triassic continental collision,and subsequently were remoulded by the fluids during its exhumation.Zhiwu XIONG Haijin XU Pan WANG Junfeng ZHANG 2021Science China Earth Sciences2021,64,6:0
2苏鲁造山带威海泥质麻粒岩古元古代麻粒岩相变质作用和三叠纪流体改造的独居石记录显示文摘独居石是苏鲁造山带威海地区出露的泥质麻粒岩中非常重要的一种副矿物,也是用来探究麻粒岩变质演化的有力工具.威海地区的泥质麻粒岩呈透镜体状不协调地产出于花岗质片麻岩中.透镜体从核部到边部,岩性逐渐变化,依次为:未变形的粗粒泥质麻粒岩、面理化的细粒泥质麻粒岩、石榴石黑云母片麻岩和混合岩化麻粒岩.透镜体的核部保存有麻粒岩相峰期变质矿物组合:石榴石+斜长石(反条纹长石)+石英+矽线石+黑云母,以及副矿物金红石、锆石和独居石.而透镜体的边部则出现受流体改造形成的退变质组合.为了揭示该泥质麻粒岩透镜体的变质演化过程,文章对其中的独居石矿物进行了BSE成图、U-Pb定年和微量元素成分的综合研究.未变形的粗粒泥质麻粒岩中独居石只记录了古元古代的年龄(1832±7)Ma (n=40),而其他三种岩石中的独居石则记录有古元古代继承年龄和三叠纪新生年龄.例如,混合岩中的独居石U-Pb定年结果给出(1818±10)Ma的上交点年龄、(211±22)Ma (n=56)的下交点年龄以及(223.8±2.9)Ma的三叠纪谐和年龄.在微量元素特征上,古元古代独居石具有强烈HREE和Y的亏损以及明显的Eu负异常,表明这些独居石是和石榴石及长石在麻粒岩相条件下同期生长的.而在三叠纪流体改造过程中,独居石边部获得Th和Si元素而释放出U、HREE、Y和P,该过程使得三叠纪增生独居石具有更高的HREE和Y含量、低的Th和U的含量及相对低的Th/U比值.因此,泥质麻粒岩中的独居石记录了古元古代的麻粒岩相的变质作用和三叠纪的流体改造过程.威海地区泥质麻粒岩在构造属性上可能属于华北克拉通,在三叠纪的大陆碰撞过程中被卷入苏鲁造山带,并在之后折返过程中受到了流体的改造.熊志武 续海金 王攀 章军锋 2021中国科学:地球科学2021,51,6:0
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