OSL DATING OF VOLCANIC BAKED SEDIMENTS IN DATONG AREA, SHANXI PROVINCE OF CHINA

Abstract

Geochronology of Quaternary volcanic activities in Datong area,Shanxi Province,has long been studied in the past 40 ~50 years,but remains controversial.This paper will further explore this problem by using quartz OSL dating of the volcanic baked layer,which has been annealed to high temperature that resets the OSL clock back to zero and meets one of the most important precondition for OSL dating.Quartz grains from six samples were collected from the baked loess layer,with the thickness of 65cm on the overlying Quaternary volcanic basalt at Dongshuitong village(40°08'31.1N,113°46'37.1E; 1172m a.s.l.)in Yanggao County.Three lacustrine sediments that also have been baked were obtained from Yujiazhai village(39°58'26.5N,113°44'50.0E; 957m a.s.l.)in Datong County.Fine-grained quartz fractions were isolated from all samples by using fluorosilicic acid etching for 7 days.The OSL growth curve with regeneration doses were constructed by both SAR and SMAR protocol,and also dose recovery tests were carried out,which proved that SMAR is more suitable for fine-grained quartz OSL dating.Therefore all samples were dated by SMAR protocol and OSL ages were estimated.OSL ages of five samples from Dongshuitou section show that the OSL ages decreases gradually from the top to the bottom of the baked layer,probably indicating that the degree of baking was gradually reduced.Thus the age of the volcanic eruption is estimated to be 84.23.6ka that of the upmost OSL age,which has been baked mostly.The OSL ages,at about 170ka,of two samples from the top and the bottom of the baked layer near Yujiazhai village,should be regarded as the minimum apparent age,and of significant uncertainty,due to the saturation of OSL signal in both natural and regeneration dose.In a conclusion,the quartz OSL dating results of nine baked sediment samples show that there were at least two phases of volcanic eruption in Datong area during the Quaternary period.One of which took place in about 170ka or older,the other one in about 85ka.