The Basin Translation From The Late Paleozoic To Middle Triassic Of The Youiiang Basin

The Youjiang Basin is located in the conjoining part of west Guangxi, south Guizhou and east Yunnan. Tectonically, it lies at the eastern end of the Tethyan tectonic domain at its juncture with the Paleo-Pacific tectonic domain. Various hypotheses for the tectonic nature of the Youjiang Basin have been proposed. During the Late Paleozoic-Middle Triassic, the Youjiang Basin was dominated by deep-water sediments including cherts, carbonates and clastic rocks associated with marine volcanic rocks. The petrogenesis and source of cherts, clastic rocks and volcanic rocks are very important to understand the tectonic evolution of the basin. Thus, we chose cherts, clastic rocks and volcanic rocks to conduct the study on systematic lithology (lithofacies), geochemistry and zircon U-Pb ages. The main objectives are to confirm their petrogenesis and tectonic settings relative to their generation and discuss the tectonic evolution of the Youjiang Basin during the Late Paleozoic-Middle Triassic. The main conclusions are summarized as follow:The Ce/Ce values of the cherts from the northern Youjiang Basin gradually decrease and their Y/Ho values gradually increase from the Devonian to the Early-Middle Permian, which the terrigenous materials input gradually decrease following the rifting of the basin. For example, the Ce/Ce and Y/Ho values of the Late Devonian cherts in the Ziyun, Puan, Luofu, Badu and Napo area are0.72±0.12and38.43±9.58, respectively; while, the Ce/Ce and Y/Ho values of the Early-Middle Permian cherts in the Ziyun and Jiuxu area are0.39±0.13and51.60±12.95, respectively. The Ce/Ce and Y/Ho values of the Early-Middle Permian cherts in the Hongyan area in the southern Youjiang Basin are0.68±0.19and41.83±13.27, respectively, being lower than the Ce/Ce values (1.03±0.15) and higher than the Y/Ho values (28.01±0.74) of those in the Xiniubao, which may reflect the latter cherts are influenced by higher amounts of terrigenous input from the Northern Vietnam block. The tectonic situation of the Youjiang Basin during the Early-Middle Permian was inherited during the early Late Permian and the Late Permian cherts in the Napo area have obviously negative Ce anomalies. The cherts and siliceous carbonates in the Ziyun and Tiane area have positive Ce anomalies, low Al2O3/TiO2values and high Eu/Eu values with mafic pyroclastic debris, associated with the intermittent terrigenous input from the Emeishan Large Igneous Province. The Devonian cherts were deposited in a rift basin, which were influenced by higher amounts of terrigenous input. The Ce/Ce values of the Early-Middle Permian cherts vary greatly from north to south of the Youjiang Basin, indicating that an open-ocean basin may be located between the Vietnam block and the Youjiang Basin during the Early-Middle Permian. Because the open-ocean basin stopped the terrigenous material from the Northern Vietnam block inputting into the north of the Youjiang Basin, and the Youjiang Basin was still coterminous with the open-ocean basin, the Early-Middle Permian cherts from the north of the Youjiang Basin retain the geochemical characteristics of the open-ocean basin cherts.The Upper Permian clastic rocks in the Ziyun and Tiane area on the northern margin of the Youjiang Basin have a geochemical affinity to Emeishan high-Ti basalt which show lower Al2O3/TiO2values (4.93～10.75), higher Eu/Eu values (0.57～1.10) and no negative anomalies of Nb, Ta and Ti. The source of the Upper Permian clastic rocks in the Napo area on the southern margin of the Youjiang Basin is related to the Emeishan large igneous province, based on the study of the dating and trace element compositions of detrital zircons and the bulk-rock geochemistry; while, the Al2O3/TiO2, Zr/Sc and Th/Sc values of the clastic rocks decrease from the bottom to the top of the section reflects the source change from felsic to mafic components of the Emeishan large igneous province. The provenance of the Late Permian clastic rocks indicates that the north of the Youjiang Basin was located in the passive continental margin.The U-Pb zircon age of a metabasite from the Babu ophiolite is358.9±5.5Ma, indicating that the Babu oceanic basin formed in the latest Devonian-earliest Carboniferous. The presence of abundant inherited zircons indicates that the initial oceanic basin was formed by the continental rifting. Both the Late Devonian and Late Carboniferous basalts in the Napo area show Ocean Island Basalt (OIB) affinity. Both the Nb/Th and Zr/Nb values of the basalts are closed to those of EMII, indicating that the magma resource was similar to EMII. The La/Yb and Sm/Yb values suggest that the Late Devonian and Late Carboniferous basalts were most likely derived from the partial melting of garnet peridotite. The Late Carboniferous basalts have higher Nb/Th values and lower Zr/Nb values relative to the Late Devonian basalts, indicating that the magma resource was changed from enriched lithospheric mantle to asthenosphere and were formed in the extension-thinning tectonic setting of the lithosphere. The early Late Permian basalts in the Napo area are geochemically classified into low-Ti (TiO2=0.70%～1.01%, Ti/Y=162～236) and high-Ti (TiO2=2.72%～3.59%, Ti/Y=391～683) types. The low-Ti basalts have A12O3, MgO, Mg#, Th/Nb, Th/La higher but TiO2, Fe2O3, P2O5,(La/Yb)N lower than those of the high-Ti basalts. The high-Ti basalts was formed from relatively primitive magmas generated by lower degrees of partial melting of garnet peridotite and have OIB geochemical characteristics, similar to the high-Ti basalts from Guangxi and high-Ti magic rocks in Funing, indicating the basalts were formed by a mantle plume. The low-Ti basalts in Napo, similar to the low-Ti magmatic rocks in Funing, have (La/Yb)N andεNd(t) lower, more flat chondrite-normalized REE patterns and more depletion in HFSEs than the high-Ti basalts, supporting an origin related to EMII-like source. The geochemical characteristics of the low-Ti and high-Ti basalts indicate that these basalts should be the products of the Emeishan large igneous province. The U-Pb zircon age of the Middle Triassic volcanics in the Napo area is241.2±1.9Ma. These volcanics characterized by relative enrichment in LREEs ((La/Yb)N=3.42～4.67) and LILEs (Rb, Ba, Th, U), pronounced negative Eu anomalies (Eu/Eu=0.60～0.74), pronounced depletion in HFSEs (e.g., Nb, Ta and Ti), relatively high (87Sr/86Sr)i values (0.710015～0.710069) and low εNd(t) values (-7.50～-7.26), were formed in the subduction-related arc setting. The geochemical characteristics of volcanics indicate that the N-MORB-type ophiolites representing the oceanic crust and OIB-type volcanics during the Late Paleozoic were formed by the continental rifting between the Vietnam block and the Youjiang Basin, and the island arc volcanics in the south of the Youjiang Basin during the Middle Triassic were formed by the oceanic crust subduction.On the basis of the previous region geological study, we conclude that there is a suture represented by the Babu ophiolite and island arc volcanics between the Youjiang Basin and the Northern Vietnam block. The Babu oceanic basin was the branch of the Paleo-Tethys which may be originally influenced by a mantle plume activity. The Late Paleozoic plume activity may result in the Northern Vietnam block drifting off the Yangtze block and the formation of the Babu Paleo-Tethys branch ocean. As following the lithosphere thinning resulting from the further activity of the plume during the Late Carboniferous, the typical OIB-type basalts which have more asthenosphere components were formed in the Napo area. The voluminous basalts caused by the plume activity were erupted along the southwestern margin of the Yangtze block and the low-Ti and high-Ti basalts were formed in the Napo area. The Youjiang basin was in extensional tectonic environments and the terrigenous materials input gradually decrease which result in the decreasing of the Ce/Ce values and increasing of the Y/Ho values of the cherts until the cherts have geochemical characteristics of the open-ocean basin cherts. The ocean basin began its southwest subduction beneath the Vietnam block during the Middle Permian and it turned into the compressional tectonic environments and the oceanic basin begun to close with the subduction of oceanic lithosphere and turn into the collision stage during the early Middle Triassic.The Youjiang Basin experienced rift basin (late Early Devonian to Late Devonian), passive continental margin (Early Carboniferous to Early Triassic) and foreland basin (Middle Triassic) tectonic stages.