Detrital-Zircon Evidence for the Origin of the Late Quaternary Loess in Qingzhou,Shandong Province and Its Implications for the Evolution of the Yellow River

The loess depositions in Shandong Province are important parts of loess records in eastern China,but their origin and genetic linkage between the Yellow River and the Chinese Loess Plateau are still unclear.This paper presents the detrital-zircon evidence for the origin and provenance evolution of the Qingzhou loess in Shandong Province.The results show that:(1) the Qingzhou loess was sourced from the sandy lands within the North China Plain that were fed by the silty materials from the Chinese Loess Plateau,the mountainous region in central Shandong Province,and the Yanshan and Taihang Mountains;(2) the Qingzhou loess was mainly sourced from the loess materials transported by the Yellow River,and the basal age of the Qingzhou loess indicates the timing of intensified aridification of the North China Plain and integration of the Yellow River was not later than 0.5 Ma;and(3) the provenance and grain-size of the Qingzhou loess underwent a significant transition at ~0.22 Ma,which was related to the expansion of the sandy land within the North China Plain.Our results indicate that the origin of the loess on the North China Plain are obviously different from the loess on the Chinese Loess Plateau,and the increase of grain-size and accumulation rate in the loess records on the North China Plain at ~0.22 Ma was due to the environmental deterioration of the eastern China.

The Langjiexue Group is an in situ sedimentary sequence rather than an exotic block: Constraints from coeval Upper Triassic strata of the Tethys Himalaya(Qulonggongba Formation)

The Upper Triassic Langjiexue Group in southeastern Tibet has long been an enigmatic geological unit. It belongs tectonically to the northern Tethys Himalayan zone, but provenance signatures of the detritus it contains are significantly different from those of typical Tethys Himalayan sandstones. Because the Langjiexue Group is everywhere in fault contact with Tethys Himalayan strata, its original paleogeographic position has remained controversial for a long time. According to some researchers, the Langjiexue Group was deposited onto the northern edge of the Indian passive continental margin, whereas others interpreted it as an independent block accreted to the northern Indian margin only during final India-Asia convergence and collision in the Paleocene. This study compares the Langjiexue Group and coeval Upper Triassic strata of the southern Tethys Himalayan zone(Qulonggongba Formation). Our new provenance data indicate that Qulonggongba Formation sandstones contain common felsic volcanic rock fragments, minor plagioclase, and euhedral to subhedral zircon grains yielding Late Paleozoic to Triassic ages. These provenance features compare well with those of the Langjiexue Group. Because the Qulonggongba Formation certainly belongs to the Tethys Himalayan zone, the provenance similarity with the Langjiexue Group indicates that the latter is also an in situ Tethys Himalayan sedimentary sequence rather than part of an exotic block. Volcanic detritus including Late Paleozoic to Triassic zircon grains in both Langjiexue Group and Qulonggongba Formation are interpreted to have been derived from the distant Gondwanide orogen generated by Pan-Pacific subduction beneath the southeastern margin of Gondwana. The Qulonggongba Formation, deposited above marlstones of the lower Upper Triassic Tulong Group, is overlain by India-derived coastal quartzose sandstones of the uppermost Triassic Derirong Formation. Deposition of both the Qulonggongba Formation and the Langjiexue Group were most likely controlled by regional tectonism, possibly a rifting event along the northern margin of Gondwana.