Detrital-zircon geochronology of the northeastern Tibetan plateau

Abstract

U-Pb geochronologic analyses have been conducted on 413 detrital-zircon grains collected from 16 samples in the Altun Shan, Nan Shan, and Qilian Shan. The samples come primarily from quartz arenites and metaturbidites of Middle to Late Proterozoic age and from feldspathic and volcanic clast-rich sandstones of early Paleozoic age. Zircon grains in Proterozoic strata resting on Tarim basement yielded mainly 2.0–1.9 Ga ages, whereas Proterozoic strata of the Qaidam and Qilian terranes yielded mainly ca. 930–820 Ma and ca. 1.9–1.1 Ga ages. The younger grains were apparently shed from local igneous rocks, whereas the grains older than 1.1 Ga were shed from an undetermined continental source. Grains in the lower Paleozoic strata are mainly ca. 500–430 Ma and were shed from nearby plutonic and possibly volcanic rocks that formed in a magmatic arc setting. Our detrital-zircon ages are consistent with a model (first proposed by E.R. Sobel and N. Arnaud) in which early Paleozoic magmatism occurred within a single northeast-facing magmatic arc that was constructed across an assemblage of Middle to Late Proterozoic accretionary complexes, remnants of magmatic arcs, and shallow-marine strata. This arc system was accreted to the Tarim and Sino-Korean cratons during Silurian–Devonian time. The resulting suture has been reactivated as Tertiary thrust faults that currently define the structural and topographic margin of the Tibetan plateau. Our data also provide two new estimates for the offset along the eastern Altyn Tagh fault. A belt of Middle Proterozoic shallow-marine strata is offset by ∼400 km, whereas a belt of 490–480 Ma magmatic arc rocks is offset by ∼370 km. These values are generally similar to the 350–400 km offset reported in most previous studies.