The Sediment And U-Pb Geochronology Features Of Detrital Zircon In Luochuan Sequence And Its Provenance Study

Luochua loess-paleosol sequence that lies on the Luochuan tableland, the main body of the loess plateau, is known as the direct product of the climate change in the Quaternary, the study of which on sedimentology, geochemical and paleomagnetic is the effectual approach to research the climate change of the quaternary. As common existing mineral in the eolian loess of quaternary, zircon's physical chemistry characteristics is stable, it has high closure temperature and strong ability of anti-jamming, it cannot be easily effected by airslake and pedogenesis once deposited, it is the most preferable mineral used to do the U-TH-Pb isotopic dating, and has been used into the REE tracer research. This article is trying to research the provenance evolution of Luochuan loess-paleosol and its climate change of the time, using the phenotype, Granularity, U-Pb ages integrated of detrital zircon in the red clay in the 11th and lower layers of Luochuanloess-paleosol with the exposed rock U-Pb zircon age combination in the Orogenic belt in northwest China.Understandings are obtained by following:a). Quaternary loess-paleosol and the Neogene red clay contains large amounts of detrital zircons, which has greater hardness.chemical stability, Its surface eolian transport character-istics and CL images shows that the zirons in Luochuan is formed by long-term eolian transpirt and mixture source.b).Luochuan detrital zircons has low percentage of<20μm fine particles,mainly distri-buted in the grain size between 20-60μm,there is larger size difference with the whole-rock, the reason of which may be due topedogenes is a greater impact on the whole rock particle size, rather than the size of zircon, In the detrital zircon of L24 loess, the particle content of coarse particles larger than 60μm, the median diameter, average particle size significantly increased comparing to other layers, which may be due to the phased uplift of the Tibetan Plateau; Neogene red clay increased in large size zircon content, it should be relevant to Exposed basement rocks in near source.c). Through the analysis on detrital zircon ages in 8 samples of loess-paleosol show that the peak age of zircons from Luochuan loess are in four time periods 250-450Ma, 750-900Ma,1800-1900Ma and 2500Ma. With the comparison analysis between Quaternary paleo wind direction and the zircon age in the orogenic belt in northwest China, we can find that provenance of loess deposition in a stable minor changes, its provenance area may be located in the Central Asian Orogenic Belt, Mountains in Inner Mongolia, eastern margin of the Qilian Mountains and the Qinghai-Tibet Plateau, and the red clay and loess provenance area has most of theoverlap, the provenanc earea may be locatedin theteritory of Inner Mongolia Mountains,Qilian Mountains and the eastern edge of Qinghai-Tibet Plateau, the lack of the Central Asian Orogenic Belt Source.