First Discovery of Late Triassic Tuffs in the South Qilian Basin: Geochemical Characteristics, Zircon LA-ICP-MS U-Pb Ages and Potential Source Regions

This investigation reports the first discovery of more than 70 tuff intervals in the Upper Triassic, South Qilian Basin. Petrographic and geochemical analyses were carried out on ten tuff samples and zircon U-Pb dating were on three. Thin section and X-ray diffraction (XRD) results indicate that the tuffs were composed of crystal shards and altered glass shards; crystal shards include plagio-clase and quartz. Most of the tuffs had been transformed into illite/smectite mixed-layers (I/S). In addition, calcite, pyrite, dolomite and siderite were also identified in some of the tuff samples. Analysis of major elements suggests that the tuffs are peraluminous high-K calcalkaline series. Trace elements indicate that the tuffs are enriched in high field strength elements (HFSE), including Th, U, Ta, Zr and Hf. Geochemical characteristics suggest that the tuffs originated from comendite pantellerite and rhyolite from within plate setting. Zircon U-Pb dating (236.0 +/- 1.7, 231.4 +/- 1.6, and 223.1 +/- 3.9 Ma) indicate that the tuffs were erupted in the Late Triassic. Comparative chronology and geochemical analyses suggest that the West Qinling belt and the East Kunlun belt are the potential source regions of these tuffs, and they originated from the within plate magma during a post-collisional period.