Source-to-sink system of Late Permian post-collisional stream-dominated alluvial fans under humid paleoclimate along the Irtysh-Zaisan suture zone, Central Asian Orogenic Belt

摘要

Because of few reports on the Upper Permian in the NW Junggar terrane, Northwest China, the recently identified Upper Permian Zaya conglomerate in the Saur area can provide insights into tectonics and paleoclimate at that time. The succession consists of predominant composite cobble-pebble conglomerate (-80%) and subordinate (gravelly) sandstone (-20%), showing sedimentary structures of stream-dominated alluvial fans and the unique lithofacies in the Saur area. This conglomerate was originally classified as a part of Lower Carboniferous, but its youngest detrital zircon U-Pb age is 265.3 ? 1.7 Ma and the succession shows the deformational patterns different from those of the Triassic sequences in the West Junggar terrane. Thus the conglomerate was deposited during the Late Permian. The Zaya conglomerate was unconformably deposited upon the Lower Permian volcano-sedimentary strata over the Devonian?Carboniferous Saur Arc (SA) and Saur Forearc Basin (SFB) on the northern margin of the Kazakhstan terrane, which currently juxtaposes with the Devonian Buerjin Accretionary Complex (BAC) on the southern margin of the Altai terrane to north. The contributions of the SFB + BAC and SA to the Zaya conglomerate are 89.6?96.7% and 3.3?10.4%, respectively. The conglomerate clasts are derived from Permian?Carboniferous volcano-sedimentary rocks in the SA, but the sandstones contain not only volcanic materials from the SA and SFB but also metamorphic and ultramafic detritus from the BAC. This is in support of the prePermian amalgamation of the Altai-Kazakhstan terranes. Plant fossils and organization of conglomerate would suggest a relatively humid paleoclimate with abundant rainfall.