Petrogenesis and tectonic implications of Palaeocene (ca. 54 Ma) rhyolites in the western Lhasa Terrane, south Tibet: Constraints from geochemistry and Sr–Nd–Hf isotope compositions

Abstract

The widespread Linzizong volcanic successions (LVS) in the Lhasa Terrane was a magmatic response to the tectonic transition from Neo‐Tethyan oceanic subduction to the India–Asia collision, which has been the subject of many studies; however, studies on the LVS in the western Lhasa Terrane are scarce compared to those in the east, limiting our understanding of the tectonic transition. Zircon U–Pb ages of two rhyolitic samples from the Shiquanhe region in the west of the Lhasa Terrane indicate that at least part of the Zenong Group, previously thought to have formed during the Early Cretaceous, was actually erupted during the Late Palaeocene (ca. 54 Ma) and belongs to the LVS. The rhyolites are characterized by high SiO2 and Al2O3, moderate K2O and Na2O, and low MgO, CaO, and FeO contents, enrichment in Rb and Th, depletion in Sr, Ba, Eu, Nb, and Ta, and have relatively uniform whole‐rock εNd(t) (−1.90 to −0.89) and zircon εHf(t) (+0.95 to +5.03) values. These geochemical characteristics indicate that they were formed by the partial melting of juvenile continental crust with subsequent plagioclase, biotite, amphibole, and Fe–Ti oxide fractionation. Combined with the results of previous studies, our data show that the western Lhasa Terrane underwent intensive Early Cenozoic crustal growth, similar to what occurred in the east, due to underplating of mantle‐derived magma resulting from the roll‐back and eventual break‐off of the Neo‐Tethys slab after the initial continental collision.

Origin of Authigenic Quartz in Organic-Rich Shales of the Dalong Formation (NW Middle Yangtze Basin), South China: Implications for Pore Network Development

“…For instance, quartz precipitated by low-temperature groundwater exhibits a maximum δ 30 Si value of 1.41‰, while modern seawater biogenic quartz has δ 30 Si values ranging from 0.52 to 3.23‰. Quartz sourced from hydrothermal vents displays δ 30 Si values ranging from −1.54 to 0.83‰, and secondary enlarged quartz produced during diagenesis has δ 30 Si values ranging from −0.22 to 0.35‰. − In the study area, the δ 30 Si values of shale from the Dalong Formation range from 0.06 to 1.35‰ (Figure ), with an average of 0.68‰. Combined with the characteristics of lithofacies, the primary source of silica in the Dalong Formation shale is of marine biogenic origin.…”