The Age And Compostion Of Basalts In Maar Lakes In Leizhou Peninsula,Southern China And Their Depositional Implication

Leizhou Peninsula in Southern China is a core region which is affected by the Asian monsoon circulation.The sedimentary sequences of three dry maar lakes in this region,namely Tianyang Lake,Qingtongyang Lake and Jiudouyang Lake can provide pivotal evidence for revealing the orbital scale evolution of Asian monsoon in low-latitude regions.In order to acquire the chronology control of sedimentary sequence and reveal paleoclimate implication of the sediment proxy indexes,it is fundamental to clarify the formation age of the Maar Lakes,and to understand the source of the sediments.In the core zone of the Asian monsoon,the abundant surface runoff and material transport under high temperature and humidity conditions may occur after the formation of the maar through the phreatomagmatic eruption.Thus,the formation age of the basalts at the bottom of the Maar Lakes can represent the the oldest age of the sediments.Furthermore,differences in compositions of the lake sediment can be used to reflect the occurrence of climatic cycles and tectonic events.Therefore,it is especially crucial to figure out the types and sources of the minerals during the process of analyzing sediment composition and interpreting the environmental proxies.On the one hand,the formation age of basalts at the bottom of the Maar lakes can be obtained by the K-Ar dating.At same time,it can be constrained by the weathering degree of the surface basalts which erupted at the same period as those at the bottom of the lakes.Comparing the elemental compositions between the surface weathering basalt profile and the sediments of Maar lakes,we can testify whether the sediments are mainly derived from weathering of the surface basalts.Through the clay mineral composition and element leaching status of surface weathering profile,we can qualitatively illuminate which materials can be provided to the lake by weathering and runoff supposing the surface basalt weathering is the main material source of lake sediments.In this study,we carried out K-Ar dating and geochemical analysis on these basalts,and also measured the clay minerals and major elements of the weathering soil profiles surrounding the Tianyang Maar lake.On the basis of our analyses,the following conclusions are drawn:K-Ar dating results show that the Tianyang maar lake was formed between 0.87 to.73 Ma.Meanwhile,we firstly report the formation age of Qingtongyang Lake and Jiudouyang Lake were formed at.82 a and 1.08 to 0.81 a,respectively.All of these Maar lakes are formed by the phreatomagmatic eruption at the Shimaoling period during the early Pleistocene to early middle Pleistocene in Southern China.The weathering degree of the typical weathering profile near the Tianyang Maar lake indicates that the age of the basalt parent rock is between 0.8 Ma and 1.33 Ma,which verifies the reliability of K-Ar age to some extent.Basalts from the bottom of the maar lakes belong to quartz tholeiites,which are also widely erupted around these three maar lakes in the same period.XRD analysis results of typical weathering profiles show that the weathered products of these basalts are mainly kaolinite,gibbsite and hematite.During periods of heavier rainfall,the physical and chemical weathering products could be transported into the lake basin along with the surface runoff and serve as the main sources of the sediments in these maar lakes.Comparing the Tianyang sediments and the basalts at the bottom of the lakes,it is found that they share the same Al2O3/TiO2ratios,which further indicates that the basalts around Maar lake can be used as the sources of the lake sediments.This result provides a fundamental understanding for the ongoing investigation of orbital monsoon precipitation variation in Pleistocene in Southern China.