Continental weathering in the Early Triassic in Himalayan Tethys, central Nepal: Implications for abrupt environmental change on the northern margin of Gondwanaland

The geochemistry of Triassic mudstones in the Himalayan Tethys sequence, central Nepal, was studied with respect to changes in sedimentary facies, grain size, and source rocks. The Triassic sedimentary facies of mudstone and carbonates show deposition in offshore to hemiplegic environments. The rare earth element (REE) pattern of the Permian and Triassic mudstones suggests uniformity correlatable to average shale. The major element geochemistry of the Early Triassic Griesbachian-early Smithian mudstones indicates a sediment supply from strongly weathered sources with the chemical index of alteration (CIA) values of 76–81. However, the mudstones in the late Smithian show weakly weathered sources with CIA values of 68–74. The lower part of the Middle Triassic Anisian mudstones return to Early Triassic paleoweathering levels. There are no significant relationships among lithofacies, the grain size of the sediments, and CIA values. Thus, the abrupt change of the degree of paleoweathering in the Early Triassic, late Smithian time, suggests a dramatic decrease in continental weathering, which is related to a predominantly arid climate in the northern marginal area of Gondwana.ArticleJOURNAL OF ASIAN EARTH SCIENCES.79, Part A:288-301(2014)journal articl

The Quaternary Kurobegawa Granite: an example of a deeply dissected resurgent pluton

H.I thank M. Yukawa for her help with sample preparation and LA-ICP-MS data collection, and Y. Hirata for sampling. U-Pb ages were calculated using an Excel spreadsheet provided by S. Sakata. English was improved by M. Coble. Comments by O. Bachmann, J. Wotzlaw and an anonymous reviewer were helpful to improve the manuscript. The Kansai Electric Power Co., Inc., and the Japan Ministry of the Environment gave us permission for the sampling. This work was partly supported by JSPS KAKENHI Grant Number JP16K05617. AC acknowledges the research grant "Beca Puente" and the financial support of a "Plan Propio" grant from the University of Granada Vicerrectorate of Research and Transfer. This is the IBERSIMS publication n. 88.The Quaternary Kurobegawa Granite, central Japan, is not only the youngest known granitic pluton exposed on the Earth’s surface, it is one of few localities where both Quaternary volcanics and related plutons are well exposed. Here, we present new zircon U–Pb ages together with whole rock and mineral geochemical data, revealing that the Kurobegawa Granite is a resurgent pluton that was emplaced following the caldera-forming eruption of the Jiigatake Volcanics at 1.55 ± 0.09 Ma. Following the eruption, the remnant magma chamber progressively cooled forming the voluminous Kurobegawa pluton in the upper crust (~ 6 km depth) until ~ 0.7 Ma when resurgence caused rapid uplift and erosion in the region. This is the first study to document the detailed spatiotemporal evolution of resurgent pluton for a Quaternary caldera system. Our new findings may contribute significantly to understanding the fate of active caldera systems that can produce supereruptions.Ministry of Education, Culture, Sports, Science and Technology, Japan (MEXT) Japan Society for the Promotion of Science Grants-in-Aid for Scientific Research (KAKENHI) JP16K05617University of Granada Vicerrectorate of Research and Transfe