Permo-Triassic Tectonic Setting Of The South China Block:Evidence From Contemporary Deposite Records

Permian to Triassic tectonothermal activity constitutes a key event in the assembly of Southeast Asia. This event is often termed the Hercynian-Indosinian orogeny in South China. Its effects include widespread igneous activity and metamorphism, and a regional unconformity among Upper Triassic and its underlayer, and an disconformity/uncormity among Upper Permian and its underlayer in the South China Block. In terms of paleogeographic reconstructions, this time period corresponds with the termination of subduction of the Paleo-Tethys beneath the southwestern margin of the South China Block and the beginning of subduction of the Paleo-Pacific along the southeastern and eastern margins. However, in detail the paleogeographic reconstructions are controversial.A number of studies have highlighted how sedimentology and detrital records of sedimentary basins can inform paleogeographic reconstructions by constraining the position, age, temporal and spatial trends, and tectonic setting of the sediment source. A series of Late Paleozoic-Early Mesozoic foreland basins is inferred to have developed along the southern margin of the South China Block in response to Permo-Triassic orogeny. These include the Shiwandashan (Qinfang Trough in Late Devonian to Middle Permian) and Yong’an basins. Petrologic data from Lower Devonian-Middle Permian chert rocks in the Qinfang Trough, detrital modal, whole rock geochemistry, and in situ detrital zircon geochemistry and U-Pb age data from Upper Permian to Lower Triassic clastic succession in the Shiwandashan and Yong’an basins, in combination with regional geological constraints, provide new insight into evolution and tectonic setting of the South China Block during Permo-Triassic orogeny. The main conclusions are summarized as follow:The Late Paleozoic siliceous rocks in the Qingfang Trough, southeastern Guangxi were well developed. Based on the analysis of major elements, trace elements and rare earth elements of Late Paleozoic siliceous rocks,, it was concluded that the Qinfang Trough was a continental margin basin in Late Palezoic. The Late Paleozoic siliceous rocks contain high content of SiO2 and high silicification degree; the Frasnian and Lower-Carboniferous siliceous rocks have lower Al/(Al+Fe+Mn) value (0.07-0.81) and higher U/Th value (0.16-10.1), whereas the Famennian and Lower-Middle Permian siliceous rocks have higher Al/(A1+Fe+Mn) (0.4-0.95) value and lower U/Th value (0.1-2.1), illustrating that the Frasnian and Lower-Carboniferous siliceous rocks have influenced by the hydrothermal process but the Famennian and Lower-Middle Permian siliceous rocks are biogenic origin; Ce/Ce*, LaN/YbN, and LaN/CeN values for Upper Devonian siliceous rocks are 0.78-1.08,0.4-1.52 and 0.88-1.35, whereas these values for Lower Carboniferous siliceous rocks are 0.55-0.91,0.12-1.8 and 1.12-1.79, and the Lower-Middle Permian siliceous rocks have Ce/Ce*, LaN/YbN, and LaN/CeN values of 1.01-1.62,0.72-2.71 and 0.62-1.9; Geochemical characteristics of the Late Paleozoic siliceous rocks in the Qinfang Trough indicate that the Qinfang Trough have expand as epicontinental sea during the Late Devonian with hydrothermal activity, then expanded again in the Early Carboniferous with siliceous rocks formed in deep sea basin environment, and in the Early Permian the trough shrinked gradually to a continental margin basin. In a conclusion, the Qinfang Trough is more like a sea basin and may be belong to the Aulacogen of the Paleo-Tethys Ocean in Late Paleozoic, and an ocean crust had never been formed.The Shiwandashan Basin, locates in the southwestern margin of the combination place of Yangtze Block and Cathyasian Block, at the transition zone between the Paleo-Tethys Ocean and Paleo-Pacific Ocean. Upper Permian to Triassic siliciclastic succession in the Shiwandashan Basin, South China, accumulated in response to a Permo-Triassic orogeny. Two unconformity bound were kept among Upper Permian succession with its underlay and Upper Triassic succession with its underlay, reflect tectonic processes associated with tectonothermal activity along the margins of the South China Block. The petrology, geochemistry and geochronology of this succession, along with north and northwest-directed paleocurrent indicators, reveal an evolving provenance related to erosion and reworking of Precambrian basement in the Yunkai Massif. The Upper Permian to Lower Triassic sandstones within the basin are quartz dominated, which along with their middle CIA values, Th/Sc and Zr/Sc ratios indicate a weak-middle multi-cycled source. The Upper Triassic sandstones contains higher quartz, CIA values, Th/Sc and Zr/Sc ratios than that of Upper Permian to Lower Triassic sandstones, indicate a strong multi-cycled source. Sandstone clasts in the Upper Permian conglomerates display age patterns similar to nearby Cambrian-Silurian strata. Other clast types (limestone, mudstone and cherts) are from the Early Paleozoic strata within or adjoining the basin. Detrital zircon age spectra of the strata display prominent age groups at 1200-800 Ma,650-500 Ma and 460-420 Ma, and are inferred to have been derived from basement units and Early Paleozoic rocks similar to those exposed in the Yunkai Massif to the south and southeast of the Shiwandashan Basin and/or from reworking of the Paleozoic units around the basin.Triassic sandstones show a similar detrital zircon age pattern as well as a 300-250 Ma age group whereas 650-500 Ma age group only contained in the Upper Triassic sandstone,300-250 Ma detrital zircons were likely derived from magmatic rocks located to the south of the basin whereas 650-500 Ma detrital zircons are probably derived from the recycled Early Paleozoic succession in the Yunkai Massif. The Late Permian marks a significant change in the paleogeography of the Shiwandashan Basin from an older deep marine chert succession to a terrestrial to shallow marine environment receiving an influx of clastic detritus related to uplift and erosion to the south of the basin. The Lower Triassic and Upper Triassic units within the basin record a further pulse of sediment influx including detritus derived from approximately syn-sedimentary magmatic activity and again material from Early Paleozoic recycled strata. Yunkai Massif, located in the southeast of the basin, underwent uplift in Late Permian and provided majority detritus for the basin in the Late Permian to Early Triassic. Integration of provenance data with regional geological information, magmatic and metamorphic records to the south of the Shiwandashan Basin suggests the basin was converted from a pre-Late Permian deep marine extensional basin to a Late Permian to Lower Triassic foreland basin and to Upper Triassic molasse basin(post-orogeny para basin) which corresponds with terrane accretion and the termination of Paleo-Tethys Ocean subduction along the southwest margin of the block.Northeast trending Yong’an Basin, located in southeast South China Block, preserves a Permian-Jurassic, marine to continental, siliciclastic-dominated, retro-arc foreland basin succession. Modal and detrital zircon data, along with published paleocurrent data, sedimentary facies, and euhedral to subhedral detrital zircon shapes, indicate derivation from multicomponent, nearby sources with input from both the interior of the block to the northwest and from an inferred arc accretionary complex to the southeast. The detrital zircon U-Pb age spectra range in age from Archean to early Mesozoic, with major age groups at 2000-1700 Ma,1200-900 Ma, 400-340 Ma,300-260 Ma and 250-210 Ma. In addition, Early Jurassic strata include zircon detritus with ages of 200-170 Ma. Regional geological relations suggest Precambrian and early Paleozoic detritus was derived from the inland Wuyi Mountain Region and Yunkai Massif of the South China Block. Sources for middle Paleozoic-early Mesozoic detrital zircons includes input from beyond the currently exposed China mainland. Paleogeographic reconstruction in East Asia suggest derivation from an active convergent plate margin along the southeastern rim of the block that incorporated part of Southwest Japan, and is related to the subduction of the Paleo-Pacific Ocean.Integration of our data with data from the contemporaneous Youjiang Basin suggests a record of continuous subduction of the Paleo-Pacific Ocean along southeast margin of the South China Block and termination of subduction of the Paleo-Tethys beneath its southwest margin in Permo-Triassic. The Yong’an Basin was a retro-arc foreland basin setting in Permian-Jurasic which related to the subduction of the Paleo-Pacific Ocean on southestern margin of the South China Block. The closure of Jinshajiang-Ailaoshan-Song Ma Ocean basin and subsequence collision of outboard blocks results in that Youjiang and Shiwandashan basins transformed as foreland basin in Late Permian and Middle Triassic, respectively, inferred a diachronous closure of the Paleo-Tethys Ocean and oblique collision of outboard blocks, decreasing from southeast to northwest. Subduction of the Paleo-Pacific along the southern margin of the South China Block in the Permian might be linked to the final subduction and diachronous closure of the Paleo-Tethys beneath the southwest margin of the block from southeast to northwest and the oblique collision of the outboard blocks in the south Asian.