Decoding Provenance and Tectonothermal Events by Detrital Zircon Fission-Track and U-Pb Double Dating: A Case of the Southern Ordos Basin

Multi-dating on the same detrital grains allows for determining multiple different geo-thermochronological ages simultaneously and thus could provide more details about regional tectonics.In this paper,we carried out detrital zircon fission-track and U-Pb double dating on the Permian-Middle Triassic sediments from the southern Ordos Basin to decipher the tectonic information archived in the sediments of intracratonic basins.The detrital zircon U-Pb ages and fission-track ages,together with lag time analyses,indicate that the Permian-Middle Triassic sediments in the southern Ordos Basin are characterized by multiple provenances.The crystalline basement of the North China Craton(NCC)and recycled materials from pre-Permian sediments that were ultimately sourced from the basement of the NCC are the primary provenance,while the Permian magmatites in the northern margin of NCC and Early Paleozoic crystalline rocks in Qinling Orogenic Collage act as minor provenance.In addition,the detrital zircon fission-track age peaks reveal four major tectonothermal events,including the Late Triassic-Early Jurassic post-depositional tectonothermal event and three other tectonothermal events associated with source terrains.The Late Triassic-Early Jurassic(225–179 Ma)tectonothermal event was closely related to the upwelling of deep material and energy beneath the southwestern Ordos Basin due to the coeval northward subduction of the Yangze Block and the following collision of the Yangze Block and the NCC.The Mid-Late Permian(275–263 Ma)tectonothermal event was associated with coeval denudation in the northern part of the NCC and North Qinling terrane,resulting from the subduction of the Paleo-Asian Ocean and Tethys Ocean toward the NCC.The Late Devonian-early Late Carboniferous(348±33 Ma)tectonothermal event corresponded the long-term denudation in the hinterland and periphery of the NCC because of the arc-continent collisions in the northern and southern margins of the NCC.The Late Neoproterozoic(813–565 Ma)tectonothermal event was associated with formation of the Great Unconformity within the NCC and may be causally related to the Rodinia supercontinent breakup driven by a large-scale mantle upwelling.

^(40)Ar/^(39)Ar Thermochronology Constraints on Jurassic Tectonothermal Event of Nyainrong Microcontinent

To reveal the Jurassic tectonothermal event occurring to the Nyainrong microcontinent which is gripped among the Bangong-Nujiang suture zone,^40Ar/^39Ar dating was carried out on the basement orthogneiss and Jurassic granitc gneiss in the microcontinent. In the heating stage, four sam- pies exhibited a flat plateau age, with the value Tp concentrated in the range of 166-176 Ma; isochron age Ti was concentrated in the range of 165-175 Ma, and their corresponding ages were the consistent within allowable range. The ages should be representative of the era of the final deformation of the Amdo gneiss and cooling emplacement of the magmatic rock in the Jurassic. The geochronological studies have shown that the final deformation of microcontinent crystalline basement and the cooling of the Mesozoic large-scale tectonothermal events occurred in late Middle Jurassic. In Middle Jurassic, Nyainrong microcontinent experienced strong tectonic movement. Combining with the geochronologi- cal with isotope geochemistry for the microcontinent, the cause of the tectonothermal event should be attributed to the collision between the Nyainrong microcontinent and South Qiangtang Block following the northward subduction of Bangong-Nujiang oceanic crust.

The ^(40)Ar-^(39)Ar ages of hornblendes in Grt-Pl-bearing amphibolite from the Larsemann Hils,East Antarctica and their geological implication

In this paper we reported the 40 Ar 39 Ar dating results of hornblendes in Grt Pl bearing amphibolite from the Larsemann Hills, East Antarctica. Their apparent ages respectively are 1586 Ma, 1011 1080 Ma, 761 Ma, 529 582 Ma. Their plateau ages of 1036 Ma and 554 Ma as well as an Ar Ar isochron age of 1010 Ma have also been obtained respectively. These isotopic dating results for the first time by the Ar Ar method for hornblendes completely record almost all the structural metamorphic thermal events that this region experienced, and provide an answer to the controversial question on the structural metamorphic thermal events of this region in recent several years, namely, which one is more important, the late Proterozoic 1000 Ma event (Grenvillian) or the early Palaeozoic 500 Ma event ( Pan African), as well as whether the former exists or not. The 40 Ar 39 Ar dating results of hornblendes show that the Larsemann Hills experienced a complicated poly metamorphic evolutionary history, and their protoliths were probably formed in early to mid Proterozoic. The late Proterozoic 1000 Ma event (Grenvillian) has been confirmed to be a predominant tectonothermal event whilst the early Palaeozoic 500 Ma event (Pan African) has been confirmed just to be the last strong tectonothermal event in this region.