THE CHARACTERISTICS OF MIOCENE FELDSPAR MINERALS AND THEIR TECTONIC AND PALEOCLIMATIC SIGNIFICANCES IN THE QAIDAM BASIN
Climatic change in the Miocene Epoch represents one of the special scenarios in the Cenozoic global cooling,of which the warming trend during Early to Middle Miocene had interrupted the natural rhythm on earth and climaxed in the Middle Miocene Climatic Optimum(MMCO).The Middle Miocene Climatic Transition(MMCT)following MMCO had turned the global climate into its previously normal cooling track.As the reversal event of the global cooling,even if MMCO could shed light on improving fidelity of climatic prediction under the global warming condition in the future,its impacts for regional environmental changes in the geological time scale remain unknown.This study focuses on mineral records to MMCO and MMCT based on 101 samples analysis to potassium feldspar and plagioclase concentrations at interval of about 0.1 Ma for the Miocene sediments from the Huatugou section(from 38° 22.0′ N,90° 52.9′E to 38°25.6′N,90° 53.8′ E)in the northwest Qaidam Basin,which was assigned to 21.3～11.3 Ma(thickness 3135 m)according to magnetostratigraphy and biostratigraphy previously.The Miocene is composed of the Xiayoushashan and Shangyoushashan formations.The former contains well sorted gray to red-brown silty clay,siltstone and fine-grained sandstone intercalated with sparse conglomerate layers,with numerous thin marl layers and abundant fossils including fish pieces and leaves in its lower parts.It could be interpreted to lacustrine sediments.It conformably overlies the Shangganchaigou Formation and lies conformably beneath the latter too.The latter is characterized by gray to brown,poorly sorted,clast-supported conglomerate intercalated with sparse sandstone and siltstone,which shows a general upward coarsening sequence.Large-scale planar cross-stratification and grain-sized suggest it was deposited in fluvial environment.Data results reveal that concentration of potassium feldspar and plagioclase are lower than their averages in the Earth surface.The feldspar/plagioclase values for silt particles in Xiayoushashan and Shangyoushashan formations in the Qaidam Basin spans 0.24～1.08,with average 0.75 which is higher clearly than that in the Earth surface(0.32).The climatic variation and oscillation recorded by potassium feldspar/plagioclase values in this study Basin match up to global deep-sea δ18O during early to Middle Miocene.Its climax indicates the MMCO which is supported by other observations in northern Tibetan Plateau,Tianshan,and Chinese Loess Plateau.Its variations in early to Middle Miocene suggest that the net moisture in soil had increased in the basin that period accompanying the global warming and reached to the climax during 17.4～15.0 Ma,even if evaporation was also increased along with"global warming".After that,net moisture decreased rapidly in pace with MMCT.And feldspar/plagioclase values in the Miocene also influenced by the provenance variation triggered by the tectonic movements in the northeastern Tibetan Plateau.Discordant shifts of potassium feldspar/plagioclase values to global deep-sea δ18O after 15.0 Ma imply that deformation of the surrounding mountains had driven new materials into the catchment which did not subject to long-time chemical weathering.We argue that feldspar minerals maintain higher concentration in the upper crust and parameter related them can document sensitively the chemical weathering controlled by the environmental moisture in arid central Asia,and then partly express regional moisture or precipitation.So,they offer a simple,convenient,and economical way to global comparison and data assimilation for advanced understanding to climatic changes.
Cenozoic global coolingpotassium feldsparplagioclasechemical weatheringMiddle Miocene Climate OptimumMiddle Miocene Climate Transitiontectonic movement
万方数据
维普
