大别山宿松变质带花岗片麻岩的锆石U-Pb年龄和Hf同位素成分

宿松变质带位于大别碰撞造山带南部，是一个相对低级变质的构造岩石单位。该带岩石类型丰富，如云母石英片岩、花岗片麻岩、变玄武岩/斜长角闪岩、变质辉长岩、异剥钙榴岩、大理岩、石墨片岩和含磷岩系等。然而，它们的形成时代和岩石成因一直存在较大争议。为此，重点对大别山宿松变质带中花岗片麻岩进行了锆石SHRIMP U-Pb定年及Hf同位素分析。锆石定年结果表明，这些花岗片麻岩的原岩形成时代包括晚太古代(2.5~2.7 Ga)和新元古代(770~830 Ma)两大类，经历了晚三叠纪的绿帘角闪岩相变质作用。新元古代花岗片麻岩的原岩是由经历了约2.0 Ga变质作用的晚太古代岩石在新元古代大陆裂解过程中发生重熔作用形成的。鉴于花岗片麻岩的新元古代岩浆锆石年龄记录与大别造山带超高压正变质岩的原岩形成时代一致，结合变质作用特点和已有资料与区域地质背景，进一步证明宿松变质带花岗片麻岩等参与了扬子板块的三叠纪俯冲，并发生了绿帘角闪岩相变质作用，局部可能达到高压榴辉岩相变质作用，是扬子俯冲板块在较浅深度折返的岩片。由于大陆碰撞→造山后的逆冲、推覆以及构造堆叠与剥蚀作用，造成宿松变质带中有可能卷入较深俯冲的岩片。

Zircon U-Pb Ages and Hf-isotope Compositions of Granitic Gneisses from the Susong Metamorphic Zone in the Dabie Orogen,China

The Susong metamorphic zone is a relatively low grade metamorphic unit, located in the southern part of the Dabie collisional orogen, China. This zone is mainly composed of mica-quartz schist, granitic gneiss, meta-basalt/amphibolite, meta-gabbro, rodingite, marble, graphite schist and phosphate rock series, etc. However, the forming ages and petrogenesis of these rocks are still largely controversial. For this reason, the zircon SHRIMP U-Pb dating and Hf isotope analysis on granitic gneisses from the Susong metamorphic zone in the Dabie orogen were performed. The zircon dating results reveal that the precursor ages of these granitic gneisses can be divided into two types, i.e. Late Archean (2.5-2.7 Ga) and Neoproterozoic (770-830 Ma), and that they experience epidote amphibolite facies metamorphism in Late Triassic. The protoliths of Neoproterozoic granitic gneisses are derived from remelting of the Late Archean rocks with ~2.0 Ga metamorphic overprinting during Neoproterozoic continental breakup. In view of the agreement of Neoproterozoic igneous zircon age records of granitic gneisses with the protolith ages of Dabie ultrahigh pressure meta igneous rocks, combined with their metamorphic characteristics, published data and regional geological setting, the granitic gneisses from the Susong metamorphic zone are further documented to be involved in Triassic subduction of the Yangtze Block and underwent epidote amphibolite-facies with possibly local high pressure eclogite-facies metamorphism, and then detached and offscraped from the subducting crust of the Yangtgze Block at shallow depths. Because of thrusting up, nappe, tectonic stack and erosion after continental collision and orogenesis, the relatively deeply subducted rocks may have been mixed into the Susong metamorphic zone.