U-Pb zircon geochronology from the basement of the Central Qilian Shan: Implications for tectonic evolution of northeastern Tibetan Plateau

Abstract:

Existing work on the basement evolution of the Qilian Shan-Nan Shan region of the northeastern Tibetan Plateau has severe limitations because of the lack of systematic geologic mapping. Geochronology studies must be put in the context of Cenozoic strain that has accumulated across this region. The aim of this study was to first establish the structural framework of a ~1500 km² region in the central Qilian-Nan Shan thrust belt through detailed geologic mapping and couple this mapping with a systematic U-Pb zircon geochronology. This study identifies two Cenozoic thrust systems in the region: (1) the Shule and (2) Tuo Lai thrust systems. We present LA-ICP MS U-Pb zircon ages of samples collected from these thrust systems that record Neoproterozoic to the Paleozoic magmatism. According to our U-Pb zircon dating results, the basement of the Qilian Shan has experienced at least three periods of intrusive activity since the Neoproterozoic: (1) 930±8.0 to 897±4.0 Ma, (2) 529±9.0 to 403±18 Ma, and (3) 383.0±6.0 Ma to 345.0±17 Ma, with the peak ages of 920 Ma, 475 Ma and 378 Ma, respectively. Zircons from high- and ultrahigh-pressure eclogites from the northern central Qilian have an igneous protolith with an age of ca. 916±22.8 Ma (2σ, MSWD=1.71). Most zircons display recrystallized rims, which suggests that eclogite-faces metamorphism occurred at ca. 485±22 Ma (2σ, MSWD=1.4). We integrate our U-Pb zircon ages with stratigraphic and paleontological data to discuss the origin and tectonic evolution of the Qilian Shan Precambrian basement in northwestern China. A Proterozoic continental quartzite sample yields detrital zircon age-clusters at ca. 1.3–1.2 Ga and ca. 2.0–1.7 Ga, and minor peaks scattered between ca. 2.7 and ca. 2.2 Ga. This age signature is very similar to our quartzite sample from the Baiyunerbo Group of the North China craton, in Inner Mongolia. This similarity provides further support for the idea that the Precambrian Qilian Shan basement was part of the North China craton.