U-Pb geochronology and geochemistry of the Dashibao Basalts in the Songpan-Ganzi Terrane, SW China, with implications for the age of Emeishan volcanism

Abstract

Permian marine basalts (the Dashibao Formation) in the SongpanGanzi Terrane to the west of the Yangtze Block, SW China, yield a SHRIMP zircon U-Pb weighted mean age of 263 ؎ 2 Ma. The Dashibao basalts are characterized by high TiO 2 contents (1.73-4.65 wt. %) and Ti/Y ratios with a mean of 577, and OIB-like rare earth element (REE) and incompatible element patterns. Geochemical variation within the basalt succession allows division into two groups; Group 1 with an alkaline composition is distinguished by higher TiO 2 and P 2 O 5 contents, along with higher Ti/Y and Sm/Yb ratios than the underlying Group 2 that consists predominantly of tholeiitic lavas. Both groups possess weakly to moderately positive Nd (t) values (0.82 to 5.28), but the Group 2 tholeiitic basalts show relatively depleted signatures (most Nd (t) >2.5) when compared to their Group 1 counterparts ( Nd (t) <2.5). REE modeling is consistent with variable degrees of melting of primitive mantle within the garnet stability field, and reveals that the Group 1 alkaline basalts could have been generated by lower degrees of melting (5-11%) than the Group 2 tholeiites (up to 19%). The initial Nd isotope discrepancy is interpreted in terms of depleted asthenospheric involvement in the early stage Group 2 tholeiitic magma. Combined geochronology, petrography and geochemistry for the Dashibao Formation confirms that it was temporally and genetically associated with the Emeishan basalts, and is therefore an integral part of the Emeishan large igneous province. The new zircon U-Pb dating supports the view that the Emeishan volcanism could be a boundary event occurring at or around the Middle-Late Permian (the Guadalupian-Lopingian) transition, and thereby confirms the validity of a causal connection with the end-Guadalupian mass extinction.

Late Permian Bimodal Volcanic Rocks in the Northern Qiangtang Terrane, Central Tibet: Evidence for Interaction Between the Emeishan Plume and the Paleo‐Tethyan Subduction System

“…Schematic diagram illustrating the formation of the Zhouqiongmalu (ZQML) bimodal volcanic rocks during the Late Permian. Emeishan plume‐related magmatism occurred in the South China Block ([1], Emeishan large igneous province [ELIP]; e.g., Xiao et al, ; Xu et al, ; Z. C. Zhang et al, ), Songpan‐Garze Terrane ([2], Dashibao high‐Ti basalts; Li et al, ; Song et al, ; Zi et al, ), and the northeastern margin of the northern Qiangtang Terrane ([3], Yushu high‐Ti gabbros; e.g., Liu, Ma, Guo, Sun et al, ). Paleo‐Tethys subduction‐related magmatism occurred mainly in the central and southern parts of the northern Qiangtang Terrane ([4], back‐arc lavas, such as the ZQML bimodal volcanic rocks; [5], arc lavas; e.g., Yang et al, ) far from the South China Block.…”

“…Ages for the Late Triassic magmatic rocks are from Lu et al (), Wang et al (), Zhai, Jahn, Su et al (), and Wang et al () and references therein. Ages for the Permian magmatic rocks in eastern Tibet are from Li et al (), Liu, Ma, Guo, Sun et al (), Yang et al (), Zhang et al (), and Zi et al (). Ages for the Late Devonian to Early Carboniferous magmatic rocks are from Jiang et al () and Wang et al ().…”

Late Silurian to Late Triassic seamount/oceanic plateau series accretion in Jinshajiang subduction mélange, Central Tibet, SW China

“…For example, Silurian mafic blocks are near coeval to OIB blocks (426.8–450.7 Ma; Jian & Liu, ) or within‐plate amphibolite preserved as xenolith (403.7–439.3 Ma; Jian et al, ) in southern Jinshajiang Suture; Early Carboniferous OIB blocks (such as gabbros and basalts) were found in the Xijir Ulan region of western Jinshajiang Suture (Bian, Zheng, Li, & Sha, ), and coeval MORB‐EMORB ophiolites were also recognized in Ailaoshan Suture (320–346 Ma; Jian et al, , ; Lai, Meffre, Crawford, Zaw, Halpin, et al, ); Early Permian OIB rocks are coeval with the limestones in the study area (Figure a) that contain Early‐Middle Permian fossils such as Inozoans and Cydocyclius sp. (QIGS, ) and are slightly younger than Early Permian ophiolites in south segment of Jinshajiang Suture (294 Ma; X. F. Wang et al, ) or Ailaoshan Suture (295–298 Ma; G. Z. Li et al, ); finally, the Late Permian OIB rocks can well correspond to contemporaneous MORB in the Zhiduo mélange unit (252.5 Ma; Y. Liu et al, ), OIBs in the northernmost Jinshajiang Suture (258.4 Ma; Yong et al, ), and even in the Songpan–Ganzi complex (263 Ma; Zi et al, ).…”

“…Widespread Late Triassic granitic rocks are classified into I‐, S‐, and A‐types (Sigoyer et al, and references therein). The presence of Late Triassic andesites (Cai, Zhang, Xu, Shi, & Yuan, ; Xia et al, ), high‐Mg andesites (Q. Wang et al, ), and late Permian plume‐related oceanic island basalt (OIB; Zi et al, ) indicates that the Middle‐Upper Triassic marine gravity flow deposits were mainly underlain by ocean crust during deposition (Şengör & Hsü, ; Yin & Harrison, ; Zhou & Graham, ). The deformation age of Late Triassic turbidites is constrained by the intrusion of the Early Jurassic syn‐kinematic granite (Roger, Malavieille, Leloup, Calassou, & Xu, ) and prograde metamorphism that has monazite age peaks ranging from 191.5 ± 2.4 to 179.4 ± 1.6 Ma (Weller et al, ).…”

“…The presence of Late Triassic andesites (Cai, Zhang, Xu, Shi, & Yuan, 2010;Xia et al, 2017), high-Mg andesites (Q. Wang et al, 2011), and late Permian plume-related oceanic island basalt (OIB; Zi et al, 2010) indicates that the Middle-Upper Triassic marine gravity flow deposits were mainly underlain by ocean crust during deposition (Şengör & Hsü, 1984;Yin & Harrison, 2000;Zhou & Graham, 1996).…”