Geochronology and geochemistry of the ‘green-bean rock’ (GBR, a potassium-rich felsic tuff) in the western margin of the Yangtze platform, SW China: Significance for the Olenekian-Anisian boundary and the Paleo-Tethys tectonics

Abstract

The ‘green-bean rock’ (GBR) is a greenish or yellowish green Triassic potassium-rich felsic tuff containing prominent bean-like quartz grains, which is widespread in the Yangtze platform and Nanpanjiang basin, South China. This paper studies the age, origin and tectonic setting of the GBR from the western margin of the Yangtze platform through whole-rock geochemistry and zircon U--Pb geochronology and Hf isotopes. In contrast to the Nanpanjiang basin, where multiple GBR layers are developed and encompass the Olenekian-Anisian boundary (OAB), which is constrained to be at 247.1–247.3 Ma, the age of the single GBR layer in the western margin of the Yangtze platform and its relationship with the OAB have not been determined. U--Pb dating on zircons from the GBR in this study yielded an age of 247.49 ± 0.68 Ma, which is considered to be comparable to that of the OAB, and thus the GBR may be used as a marker for regional stratigraphic correlation. The volcanic ashes of the GBR have been previously suggested to be derived from the Siberian Traps Large Igneous Province or from the Shiwandashan-Qingfang suture zone to the south of the Nanpanjiang basin. However, the whole-rock geochemistry of the GBR, especially enrichment in LREE, Th and U and depletion in Nb, Ta, Sr, Ba, K, Rb and Ti, and strong to moderate negative Eu anomalies (δEu = 0.16–0.20), suggest that the volcanic ashes are more likely derived from the adjacent Sanjiang Orogen to the southwest. The zircon geochemistry, especially the negative εHf(t) values (−16.4 to −5.1) and old two-stage Hf model (TDM2) ages (1.60–2.30 Ga), are also similar to those from the Early-Middle Triassic felsic volcanic rocks in the Sanjiang Orogen. The subduction of the Paleo-Tethys Ocean was previously inferred to have started between Early Permian and Middle Triassic. The data obtained in this study, in combination with those compiled for igneous rocks and volcanic ashes from Middle Permian to Late Triassic from the region, suggest that the subduction initiation of the Paleo-Tethys Ocean began no later than ~254 Ma, in middle Late Permian.