Jianghai Yang

A sedimentary archive of tectonic switching from Emeishan Plume to Indosinian orogenic sources in SW China

Upper Permian to Middle Triassic sedimentary rocks in the Youjiang Basin, SW China, record a change from a within-plate mafic-dominated source to the NW, to a mixed source involving magmatic arc and recycled orogenic detritus that lay to the west and east. Upper Permian and some Lower Triassic sedimentary rocks are characterized by relatively high contents of TiO2 and Nb, and low ratios of Al2O3/TiO2 and Th/Nb. Detrital zircons yield U–Pb ages of c. 260 Ma and have geochemical affinities to those crystallized from the within-plate type magmas. These features, combined with extensive basaltic lithic fragments and plagioclase grains in the sandstones, imply a sedimentary source dominated by the Emeishan Large Igneous Province exposed to the NW of the basin. The Early and Middle Triassic marked a significant change in sediment provenance, with rocks displaying geochemical affinity to upper continental crust. U–Pb ages and trace elements of detrital zircons from this material show input from multiple sedimentary sources including the subduction–collision rocks related to the Indosinian Orogeny to the west of the basin and the recycled Precambrian–early Palaeozoic sedimentary and granitic rocks in the South China hinterland to the east of the basin. The Early Triassic change in the nature of the source supplying detritus to the Youjiang Basin, together with regional sedimentary and tectonothermal events, suggests a tectonic transition from the Emeishan Plume to the Indosinian Orogeny in SW China. Regional stratigraphic correlation and provenance data suggest that the Emeishan basalts provided a large sediment influx for the riverine–littoral–marine systems in the Late Permian. This basaltic particulate and dissolved input may have contributed significant unradiogenic Sr to the Late Permian seawater. Supplementary material: Details of sample preparation, analytical procedure and results, and additional figures are available at www.geolsoc.org.uk/SUP18698.

From Subduction to Collision in the Northern Tibetan Plateau: Evidence from the Early Silurian Clastic Rocks, Northwestern China

The Qilian Orogen records early Paleozoic collisional suturing of the Qaidam Block and the Central Qilian Block to the North China Craton. The composition and U-Pb age of detrital zircons and the composition of Cr-spinels from the Early Silurian Lujiaogou and Angzanggou formations in the northern part of orogen indicate derivation from evolving oceanic and continental source terranes. Heavy-mineral chemistry indicates the incorporation of suprasubduction zone–type ophiolitic detritus in addition to continent-derived material. Integrating these chemical and age data with regional data on the duration of subduction-related magmatic activity, syn- and postcollisional granitic rocks, and high-pressure metamorphic rocks constrains the transformation from oceanic subduction to continental collision to 450–440 Ma. The collision resulted in a flood of detritus into the northern part of the orogen from the Central Qilian Block, which masked input from the intervening magmatic arc, implying rapid exposure of the block.

Depositional chemistry of chert during late Paleozoic from western Guangxi and its implication for the tectonic evolution of the Youjiang Basin

Continual deep-water sediments from the late Early Devonian to the Late Permian extended in wide areas of western Guangxi. We analyzed the major, trace, and rare earth elements of the Upper Paleozoic cherts in Badu, western Guangxi. High non-terrigenous SiO2 contents (Sinon-ter/Sibulk(%)> 80%) and pure chert components (> 70%) indicate a large extent of silicification in the Upper Paleozoic cherts, except for the Upper Devonian-Lower Carboniferous Luzhai Formation cherts, which have lower non-terrigenous SiO2 contents (avg. 71.8%) and pure chert components (40%–70%). The Al/(Al+Fe+Mn) ratios and Feter/Febulk(%) values of samples from the lowest horizon of the Pingen Formation are 0.05–0.26, 13.1%–14.5%, respectively, indicating hydrothermal origins. All other samples show high Al/(Al+Fe+Mn) ratios (0.39–0.81) and high Feter/Febulk(%) values (23.1%–186.8%), indicating non-hydrothermal origins. The Pingen Formation and Liujiang Formation cherts show slightly-moderately negative Ce anomalies (0.71±0.07, 0.81±0.08, respectively) and higher Y/Ho ratios (33.49±1.27, 36.10±2.05, respectively) than PAAS. This suggests that these cherts were deposited in the open marine basin, rather than in the intracontinental rift basin as previously assumed. The Luzhai Formation cherts may be deposited near the seamount or seafloor plateaus with no negative Ce anomalies (1.09±0.07) and no significant Y-Ho fractionation (Y/Ho=28.60±1.25). The Nandan Formation and Sidazhai Formation cherts were deposited in the open-ocean basin with moderately negative Ce anomalies (0.67±0.08, 0.73±0.11, respectively) and high Y/Ho ratios (36.01±1.00, 32.00±2.25, respectively). On the basis of our studies about cherts, we conclude that the Youjiang Basin originated as part of the Paleo-Tethys that controlled the depositional environments of cherts during late Paleozoic. The rift of the Youjiang Basin had occurred at least since the Early-Middle Devonian. The basin had a trend of evolving into an open-ocean basin during the Early-Middle Permian.

Reconstructing Early Permian tropical climates from chemical weathering indices

This study was financially supported by the National Natural Science Foundation of China (grants 41302083, 41572078), the National Basic Research Program of China (grant 2011CB808800), and the Fundamental Research Funds for the Central Universities (grant CUGL140402), China University of Geosciences (Wuhan). P.A. Cawood acknowledges support from NERC grant NE/J021822/1.

Geochemical significance of the Paleogene soda-deposits bearing strata in Biyang Depression, Henan Province

The Biyang Depression, lying in the eastern Nanxiang Basin, has a dustpan-shape with faults developed to the southwest and southeast. The Paleogene Hetaoyuan Formation in this depression hosts one of the oldest soda-deposits, the Anpeng deposit. These sodium carbonate-bearing strata consist of lutites, muddy dolomites, and bedded soda-deposits, which are dominated by nahcolite with little trona, mirabilite, and halite. In the top Unit 3 and lower Unit 2 of Hetaoyuan Formation where the sodium carbonates are concentrated, dolomitic shale, muddy carbonate, and sodium carbonate can be separated based on their distinct Al2O3, TiO2, Na2O, MgO+CaO, and LOI (loss on ignition) contents. By using Na2O/(MgO+CaO) ratio as an indicator for the relative concentration of Na carbonates to Mg-Ca carbonates, the muddy dolomites can be classified into two groups. One group with Na2O/(MgO+CaO)<0.6 exhibits a positive correlation between this ratio and Al2O3+TiO2 content, an proxy for terrigenous clastic input and the other group with Na2O/(MgO+CaO)>0.6 presents a negative correlation. As the clastic proportion decreases, the former indicates that it tends to form pure Mg-Ca carbonates reflecting evaporation less than or equal to replenishment for the lake water in a relative humid climate, and the latter reveals a trend to form sodium carbonates with lake water’s evaporation more than replenishment in a relatively dry climate. From muddy dolomite to sodium carbonate samples, with Na2O/(MgO+CaO) ratio increasing, the Al normalized Zr, Ti, La, K, Rb, and Ba tend to be enriched and the post-Archean Average Shale (PAAS) normalized positive Eu anomaly becomes more obvious. Integrated with the high B contents and the report of searlesite in the bedded sodium carbonates and the regional extensional tectonic regime, the above geochemical characteristics were interpreted to signify hydrothermal contribution to forming these soda-deposits through the deep fault systems of basin margins.