Constraints on the early Mesozoic denudation of the Qinling orogen from Upper Triassic-Lower Jurassic successions in the Zigui Basin, central China

Abstract

The Qinling mountains are situated in central China and mark the collisional assembly between the North and South China blocks. To understand the early Mesozoic evolution of the Qinling orogenic belt, we present new paleocurrent data, sandstone petrology and detrital zircon U-Pb ages for Upper Triassic-Lower Jurassic sedimentary successions in the Zigui Basin in the northern margin of the South China block. The deltaic sandstones in the Upper Triassic Jiuligang Formation have high concentrations of quartz grains and the basal alluvial conglomerates of the Lower Jurassic Tongzhuyuan Formation contain abundant chert gravels with sandy matrixs rich in quartz grains. Both the Upper Triassic sandstones and Lower Jurassic conglomerate have two major detrital zircon U-Pb age groups of 2100–1700 Ma and 950–700 Ma with very few Phanerozoic ages. The overlying fluvial sandstones of the Tongzhuyuan Formation display relative enrichment in volcanic and metamorphic rock fragments. They are characterized by primary zircon U-Pb age groups of 250–200 Ma and 950–700 Ma. Consistent with the northwestward paleocurrents, zircon grains of 2100–1700 Ma and 900–700 Ma were likely derived directly or recycled from the Paleoproterozoic and Neoproterozoic basement of the South China Block, with the latter also possibly sourced from the Qinling orogen. The 250–200 Ma age group is understood to be associated with volcanic rock fragments and, considering the southward paleocurrents, indicates a volcanic source within the Qinling orogen. Along with the increased low-grade metamorphic rock fragments in sandstones, this significant provenance change in the Early Jurassic indicates the initial massive erosion and rapid southward propagation of the south Qinling orogen.