Provenance of the Lower Jurassic pyroclastic sediments in the Zigui Basin:Implication for crustal thickening in the eastern Qinling orogenic belt

The Late Triassic witnessed significant collisional orogenic events in the Qinling orogenic belt,accompanied by magma underplating and tectonic deformation.These processes potentially resulted in substantial crustal thickening and uplift of the Qinling orogen.However,due to the absence of igneous rock records from this period in the eastern section of the Qinling orogen,the changes in crustal thickness during this orogenic process have not been thoroughly investigated.A series of foreland basins emerged during the Early Mesozoic to the south of the East Qinling orogenic belt.These basins have preserved clastic sedimentary rocks derived from the uplift and erosion of the orogenic belt.These sedimentary records serve as crucial records to reconstruct the evolutionary history of the Qinling orogen.To further clarify the collisional orogenic process of the Qinling orogenic belt,this study conducted in situ volcanic lithic fragment geochemistry,detrital zircon U-Pb chronology and trace element composition analysis on the sandstones of the Lower Jurassic Tongzhuyuan Formation in the Zigui Basin.The results suggest that the sandstones,which exhibit a significant abundance of volcanic lithic fragments,has a characteristic detrital zircon age group of 250–200 Ma,indicating a major provenance from the Triassic volcanic rocks.Combined with regional correlation and paleocurrent analysis,the detrital zircon U-Pb age data show that the source area of volcanic rocks should be in the Qinling orogenic belt to the north of the basin.This interpretation is further supported by the Triassic granitic rocks exposed in the western part of the orogenic belt,representing the magmatism during the Triassic collisional orogenesis in the Qinling orogen.Based on the co-varying relationships between present-day crust thickness with the chemical compositions of granite rocks and zircons,the La/Yb ratio of volcanic lithic fragments in the Tongzhuyuan Formation and the Eu/Eu*ratio of detrital zircons with Triassic ages indicate that the Qinling orogen experienced crustal thickening during the Late Triassic,reaching its maximum thickness of 60–70 km at ca.220–210 Ma.This crustal thickening in the eastern Qinling orogen is temporally consistent with that in the western orogen as recorded by the Triassic granitic rocks and may be related to large-scale crustal shortening and magmatism during the collisional orogeny.

Devonian-Carboniferous Pillow OIB-Type Basalts in the Youjiang Basin,SW China:Implications for the Eastern Extension of the Paleo-Tethys Branch Ocean

Late Paleozoic pillow basalts are well preserved in the Youjiang Basin of SW China.In this study,we conducted geochemical and Sr-Nd isotopic study on the Late Devonian and Late Carboniferous basalts in Napo area,Youjiang Basin.The analyzed samples comprise moderate SiO2,TiO2,Mg#,and REE contents,with slightly positive or negative Eu anomalies,similar to the chemical nature of OIB(Oceanic island basalt).All basalts show low(^(143)Nd/^(144)Nd)iand(^(87)Sr/^(86)Sr)ivalues,close to EMI-type(I-type Enriched Mantle)magma source.Late Carboniferous basalts display higher(^(87)Sr/^(86)Sr)i,Nb/Th,and lowerε_(Nd)(t),Zr/Nb values than those of the Late Devonian basalts,implying progressively decreased enrichment components in the magma source processes.Such evolution trend reflects a continuous lithospheric thinning event,supporting a progressive rifting setting of the Youjiang Basin along the southwestern margin of the Yangtze Block in Late Paleozoic.Combined with regional geological background,the Youjiang Basin is viewed as a continental rift basin related to the opening of the Paleo-Tethys Ocean in Late Devonian and became a passive continental margin basin in Carboniferous.The Paleo-Tethys branch ocean probably extended further to the southern Youjiang Basin,likely corresponding to the Song Ma suture zone.

Climate paleogeography knowledge graph and deep time paleoclimate classifications

The climate paleogeography,especially the climate classifications,helps to interpret the global and regional climate changes and intuitively compare the climate conditions in different regions.However,the application of climate classification in deep time(i.e.,climate paleogeography)is prohibited due to the usually qualitatively constrained paleoclimate and the inconsistent descriptions and semantic heterogeneity of the climate types.In this study,a climate paleogeography knowledge graph is established under the framework of the Deep-Time Digital Earth program(DDE).The hierarchical knowledge graph consists of five paleoclimate classifications based on various strategies.The classifications are described and their strengths and weaknesses are fully evaluated in four aspects:“simplicity,applicability,quantifiability,and comparability”.We also reconstruct the global climate distributions in the Late Cretaceous according to these classifications.The results are compared and the relationships among these climate types in different classifications are evaluated.Our study unifies scientific concepts from different paleoclimate classifications,which provides an important theoretical basis for the application of paleoclimate classifications in deep time.

Radiolarian and Detrital Zircon in the Upper Carboniferous to Permian Bancheng Formation,Qinfang Basin,and the Geological Significance

The Bancheng Formation exposed along the Shift Reservoir nearby Bancheng Town, Qinfang Basin, southern Guangxi, is mainly composed of thin-bedded chert. The radiolarian assemblages in the studied section suggest it was a pelagic setting and the age of the Bancheng Formation is Late Carboniferous to Early Permian. The detrital zircon U-Pb ages from the section are characterized by a Permian peak at -282 Ma. Detrital zircon provenance analysis suggests that the Permian detrital zircons in the Bancheng Formation were likely from volcanic-magmatic arc rocks related to the subducfton of the Paleo-Tethyan Ocean. The long-lived deposition （from Upper Devonian to Middle Permian, about 125 Ma） of the radiolarian cherts in the Qinfang Basin was comparable with that deposited in Ailaoshan Ocean. The radiolarian assemblages in Qinfang Basin show a Tethyan affinity. Together with the Permian subduction-related arc volcanic rocks and the E-MORB type basalts to the northwest of the study area, our data support the existence of a Permian arc-related basin in the Qinfang area.

Conodont Biostratigraphic Constraint on the Lower Taiyuan Formation in Southern North China and Its Paleogeographic Implications

The Late Paleozoic Taiyuan Formation in North China is mainly composed of a mixed shallow-marine carbonate and terrigenous clastic deposits.Its basal limestones have been constrained in the late Pennsylvanian to Early Permian.To further constrain the age of the lowest Taiyuan Formation,we obtained two genera and 16 species of conodonts from the bottom limestones of the Taiyuan Formation in two sections in Henan Province,southern North China.The fauna includes Idiognathodus hebeiensis,Streptognathodus isolatus,S.elongatus,S.cf.longus,S.acuminatus,S.cf.recreatus,S.cf.cristellaris,S.bellus,S.invaginatus,S.wabaunsensis,S.glenisteri,S.conjunctus,S.binodosus,S.fuchengensis,S.nodulinearis,and S.sp.A genus of Streptognathodus dominated conodont assemblage with some Idiognathodus elements.It indicates a late Gzhelian(latest Pennsylvanian)age for the lowest Taiyuan Formation in southern North China.This is consistent with recently published high-precision zircon U-Pb ages from ash layers.Based on conodont biostratigraphy,the basal limestones of the Taiyuan Formation in Henan Province(southern North China)can be correlated with the upper part of Miaogou limestones of the Taiyuan Formation in Shanxi Province(northern North China).This correlation might reflect a significant sea-level rise in North China,possibly corresponding to a deglaciation event at the Permo-Carboniferous transition.