Abstract Deciphering the formation and geodynamic evolution of high-pressure (HP) granulites in a collisional orogeny can provide crucial constraints on the geodynamic evolution of subduction-exhumation. To fully exploit the geodynamic potential of metamorphic rocks, it is necessary to constrain the metamorphic ages, although it is difficult to link zircon and monazite ages to metamorphic evolution. A good case study for understanding these geodynamic processes is felsic granulites in the Bashiwake area, South Altyn Tagh. Petrographic observations suggest that the studied felsic granulites have suffered multi-stage metamorphism, and the distinct metamorphic events were documented by compositional zoning and high Y + heavy rare earth element (HREE) concentrations in the large garnet porphyroblast. Zircon U-Pb dating yielded two major age clusters: one age cluster at ca. 900 Ma represents the age of the protolith for the felsic granulite, and another age cluster at ca. 500 Ma represents the post-UHT (ultrahigh temperature) stage based on the rare earth element distribution coefficients between zircon and garnet. Meanwhile, in situ monazites U-Pb dating yielded a weighted mean 206Pb/238U age of 482 ± 3.5 Ma, and the monazite U-Pb age was interpreted to be in agreement with the metamorphic zircon rims data, which together with zircon recorded the cooling time after the UHT stage. Whole-rock major and trace elements as well as Sr-Nd isotopes suggest that the protolith of the felsic granulite derived from partial melting of ancient crustal materials with the addition of mantle materials. Integrating these results along with previous studies, we propose that the felsic granulites metamorphosed from the Neoproterozoic granitic rocks, and the granitic rocks with associated mafic-ultramafic rocks suffered a common high-pressure–ultrahigh temperature (HP-UHT) metamorphism and subsequent granulite-facies metamorphism. A tentative model of subduction-relamination was proposed for the geodynamic evolution of the Bashiwake unit, South Altyn Tagh.
北祁连造山带是典型的早古生代增生型造山带,具有典型增生型造山带的沟-弧-盆体系。祁连地块位于北祁连造山带南部,由深变质的前寒武纪基底和新元古代到中生代的沉积盖层组成。在连祁地块北缘门源宝库河附近延伸数千米的变基性岩(主体为斜长角闪岩和角闪岩)中识别出了基性麻粒岩,峰期矿物组合为石榴子石+斜长石+钾长石+普通辉石+角闪石+钛铁矿+石英,相平衡模拟计算结果显示峰期温压条件为P=10.6kbar、T=800℃,峰期之后经历了近等温降压的P-T轨迹。锆石LA-ICP-MS U-Pb年代学结果显示变基性岩的原岩结晶年龄分别为1110±18Ma和1140±30Ma,变质年龄分别为478.2±3.9Ma和469±4.6Ma,结合近等温降压的P-T轨迹,469~478Ma的变质年龄应近似代表了麻粒岩相变质作用时代。结合区域资料,我们认为早古生代北祁连洋存在南北双向俯冲极性,祁连洋南向俯冲使得祁连地块前寒武纪基底在早古生代再活化。变基性岩的原岩年龄与祁连地块南部欧龙布鲁克地块(全吉地块)中发现的变基性岩和花岗质片麻岩原岩年龄及副片麻岩的变质年龄一致,均为~1100Ma,表明祁连地块与欧龙布鲁克地块一起卷入了格林威尔造山事件。;The Qilian orogenic belt is a typical Early Paleozoic accretionary orogenic belt with a typical trench-arc-basin system. The Qilian block, located in the south of the Qilian orogenic belt, is mainly composed of Precambrian basement overlain by Neoproterozoic to Mesozoic sedimentary sequences. The mafic granulites are identified from meta-mafic rocks (mainly plagioclase amphibolite and amphibolite) near the Baoku River on the northern margin of the Qilian block. The peak mineral assemblage is garnet+plagioclase+K-feldspar+augite+hornblende+ilmenite+quartz. The phase equilibria indicate that the peak P-T condition is ~10.6kbar and ~800℃ based on the compositions of garnet rim and its plagioclase inclusion. The petrographic observation, mineral chemistry and phase equilibria indicate that the mata-mafic rock has experienced a ITD P-T path after the peak stage. LA-ICP-MS U-Pb dating results of zircons show that the protolith crystallization ages of meta-mafic rocks are 1110±18Ma and 1140±30Ma, respectively, and the metamorphic ages are 478.2±3.9Ma and 469±4.6Ma, respectively. Combined with the P-T path, the metamorphic ages of 469~478Ma are considered to represent the age of granulite-facies metamorphism. Combined with the regional data, we believe that the North Qilian ocean had northward and southward subduction in the Early Paleozoic. The southward subduction of the Qilian ocean made the Precambrian basement of the Qilian block reactivated. The protolith ages of meta-mafic rocks are consistent with those of meta-mafic rocks and granitic gneiss, and the metamorphic age of paragneiss discovered in the Oulongbuluke micro-block, which indicates that Qilian block and Oulongbuluke micro-block were involved in the Greenville orogenic event.
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