Age Constraints On Thermochronometry Of Dongling-Lishu Fault Within Songliao Basin

This thesis takes Dongling-Lishu fault as study area,and the object of study is Huoshiling,Shahezi and Yingcheng formations as well as drillholes of Li 6 and other Wells at depth of 2000~3400 m from deep sedimentary strata.We mainly studies on detrital zircon fission track chronology,LA-ICPMAS zircon U-Pb geochronology and whole rock geochemistry,with aims of constrainting the formation age of Andesite within Huoshiling Formation and the deep sedimentary strata,respectively.By using LA-ICPMS zircon U-Pb geochronology,the dating result of andesites collected from Huoshiling Formation indicates that the upper limit of Huoshiling Formation elder than 136±5 Ma and the low limit younger than 136±5 Ma.In this study,the lithologic vary of the deep sedimentary strata from the drillholes are obvious.The strata can be divided into three rock sections based on lithology.The low section mainly consists of glutenites and conglomerates and dark mudstone rocks;middle section is a set of overflow-facies volcanic rocks which mainly contains tuffs,andesites and rhyolites,as well as a few of basalts;the upper section is a set of rock association consisted of conglomerates and mudstones.According to rock sequence and comparisions with main strata occupied in study area,we find that it is difficult to accurately constraint the formation age of the deep sedimentary strata.The detrital zircons fission track age dating of the sedimentary rock samples taken from the deep sedimentary strata yield 14 disaccorded mixed-distribution age groups.Firstly,we use gaussian fitting to decompose the mixed-distribution age groups into multiple separate age groups,and we successfully obtained 14 minimum pool age groups by using the P(F)test.Sample CL28A1:114.9 Ma;Sample CL28A3: 119.6 Ma;Sample CL28A4: 119.8 Ma;Sample CL28A5: 116.4 Ma;Sample CL28A7: 121.0 Ma;Sample CL28A9: 121.1 Ma;Sample CL28B2: 117.7 Ma;Sample CL28B3: 122.2 Ma;Sample CL28B4 : 122.6 Ma;sample CL28B6: 119.4 Ma;sample CL28B7: 114.9 Ma;sample CL28B8: 125.9 Ma;sample CL28B9: 123.7 Ma;sample CL28C1: 122.5 Ma.Among them,the smallest pool age group with age of 114.9 Ma represents the sedimentary low limit of the deep sedimentary strata.Additionally,6 sedimentary rock samples collected from Li 6 and other wells in deep sedimentary strata yield 6 detrital zircon U-Pb minimum age values/groups: 109±2 Ma(1 grain),122±3 Ma(2 grains),125±3 Ma(1 grain),141±3 Ma(1 grain),124±5 Ma(7 grains)and 115±8 Ma(13 grains).From the perspective of mathematical statistics,the 115±8 Ma detrital zircons age group is considered as the low limit of deep sedimentary strata.Combined with U-Pb geochronology and detrital zircons fission-track geochronology studies,further demonstrate that the deep sedimentary strata formed after ~115 Ma i.e.the late Aptian period of the Cretaceous.Therefore,we can conclude that there is Huoshiling formation occurred in southern of the Songliao basin,whereas the so-called Huoshiling Formation actually belongs to the top of the Shahezi Formation or the bottom of the Yingcheng Formation.Volcanic rocks from Yingcheng Formation within the southern margin of the Songliao Basin are mainly by I-and A-type rhyolite and andesite.Its rock association have the characteristics of a bimodal volcanic rocks,suggesting a stretching tectonic setting.Combined with the weighted mean ages range from 101.4 ± 1.9 Ma to 113.8 ± 1.5 Ma,indicates that these volcanic rocks are formed in an extensional tectonic setting during the Early Cretaceous.