Provenance Analysis Of The Late Cretaceous Yunlong Formation In The Lanping Basin,Yunnan Province And Its Tectonic Implications

There are many intra-continental basins with evaporite series in SE Asia,and so many potash salt deposits are located in these basins,such as the Khorat Basin,the Vientiane Basin,the Muang Xai Basin,the Simao Basin and the Lanping Basin.Previous studies found there is a certain relationship of detritus among the Khorat Basin,the Muang Xai Basin and the Simao Basin.The Lanping Basin is located in the north of the Simao Basin.Previous studies have found that the Paleocene of the Lanping Basin is a red salt-bearing clastic rock series,which deposited with dry conditions in Mesozoic era and beared a set of gypsum-bearing and saltbearing strata called Yunlong Formation.In the southeast of the Lanping Basin,there is also a set of identical evaporating series strata called Mengyejing Formation in the Simao Basin.The two sets of strata can be compared from the lithology combination.Former scholar's research indicated that detrital material in Mengyejing Formation comes from different sources,however,clastic sediments of Yunlong Formation not been studied in detail and detrital zircon U-Pb geochronologic data has not been previously reported in the Lanping Basin.Therefore,the provenance analysis of the Lanping Basin can be a good complement to the source relationship between the basins,and can restore the ancient geographical environment and the general distribution of the ancient river system.In this paper,we report detrital zircon U-Pb geochronologic data of Yunlong Formation by LA-ICP-MS supplemented with sandstone petrography and geochemistry.Previous studies of whole-rock geochemistry have shown that detritus mainly comes from felsic source area,and are from a passive continental margin.The modal composition of the dated sandstone samples shows a “recycled orogen provencances” field in Qt-F-L and Qm-F-Lt diagram,indicating recycling of the detrital sediments during the mountain building process.Seven samples from different parts of the Lanping Basin were studied for U-Pb dating and zircon trace element.The zircon trace elements show that zircon is mainly derived from magmatic origin,and crystallized in an arc/orogeny environment.In addition,550 zircon ages were tested,and 502 effective single particle zircon ages were obtained.The results show that detrital zircon age of the oldest is 2733 Ma and the youngest is 105 Ma in the basin.Three major age populations were identified in the U-Pb age spectra: a Neoarchean to late Palaeoproterozoic group(composed of two distinct age populations at 2733-2384 Ma and 2057-1531 Ma),a Neoproterozoic group(1011-558 Ma),an Ordovician to Triassic group(also composed of two distinct age populations at 493-371 Ma and 340-203 Ma).Each of these age populations corresponds with one or several potential sources around the Lanping Basin.The Neoarchean to late Palaeoproterozoic zircons are primarily derived from the Qinling orogenic belt and the Yangtze plate.Recycling of sediments in the Yangtze plate may be the source of the Neoproterozoic zircon population.Sediments from the nearby orogenic belt most likely supplied the observed population of Ordovician to Triassic age zircons.This is consistent with the previous study of zircon Hf isotope.Through the comparative study of different regions of detrital zircons,the evidence from clastic provenance indicated there is a high degree of consistency between the Lanping Basin and the Simao Basin,and the two basins may begin link up in the late Cretaceous.