Late Paleozoic to Mesozoic Intrusions Distribution in the North Sanjiang Orogenic Belt,Southwest China:Evidence from Zircon U-Pb Dating and Geochemistry

A mosaic of terranes or blocks and associated Late Paleozoic to Mesozoic sutures are characteristics of the north Sanjiang orogenic belt(NSOB).A detailed field study and sampling across the three magmatic belts in north Sanjiang orogenic belt,which are the Jomda-Weixi magmatic belt,the Yidun magmatic belt and the Northeast Lhasa magmatic belt,yield abundant data that demonstrate multiphase magmatism took place during the late Paleozoic to early Mesozoic.9 new zircon LA-ICP-MS U-Pb ages and 160 published geochronological data have identified five continuous episodes of magma activities in the NSOB from the Late Paleozoic to Mesozoic:the Late Permian to Early Triassic(c.261-230 Ma);the Middle to Late Triassic(c.229-210 Ma);the Early to Middle Jurassic(c.206-165 Ma);the Early Cretaceous(c.138-110 Ma) and the Late Cretaceous(c.103-75 Ma).105 new and 830 published geochemical data reveal that the intrusive rocks in different episodes have distinct geochemical compositions.The Late Permian to Early Triassic intrusive rocks are all distributed in the Jomda-Weixi magmatic belt,showing arc-like characteristics;the Middle to Late Triassic intrusive rocks widely distributed in both Jomda-Weixi and Yidun magmatic belts,also demonstrating volcanic-arc granite features;the Early to Middle Jurassic intrusive rocks are mostly exposed in the easternmost Yidun magmatic belt and scattered in the westernmost Yangtza Block along the Garze-Litang suture,showing the properties of syn-collisional granite;nearly all the Early Cretaceous intrusive rocks distributed in the NE Lhasa magmatic belt along Bangong suture,exhibiting both arc-like and syn-collision-like characteristics;and the Late Cretaceous intrusive rocks mainly exposed in the westernmost Yidun magmatic belt,with A-type granite features.These suggest that the co-collision related magmatism in Indosinian period developed in the central and eastern parts of NSOB while the Yanshan period co-collision related magmatism mainly occurred in the west area.In detail,the earliest magmatism developed in late Permian to Triassic and formed the Jomda-Wei magmatic belt,then magmatic activity migrated eastwards and westwards,forming the Yidun magmatic bellt,the magmatism weakend at the end of late Triassic,until the explosure of the magmatic activity occurred in early Cretaceous in the west NSOB,forming the NE Lhasa magmatic belt.Then the magmatism migrated eastwards and made an impact on the within-plate magmatism in Yidun magmatic belt in late Cretaceous.