Study On The Source Of The Late Cenozoic Terrigenous Sediments In The Northern Chinese Tian Shan Foreland

Tectonic uplift history and its environmental effects of active orogenic belts under the background of extruded structures have always been a hotspot in geoscience research.The terrestrial sedimentary sequence in the foreland basin contains abundant information about tectonic uplift history and climate changes in the adjacent orogenic belt.In general,this set of sedimentary sequences in the foreland basin shows a tendency to thicken upward on the profile.Coarse-grained sediments generally appear in the upper part of the sedimentary sequence.The formation mechanism of this coarsening upward sediment exists debate on the structural cause theory or climate driving theory among relevant scholars,and there is no final conclusion.As an intercontinental extruded orogenic belt,the Tianshan Mountains have received Cenozoic deposits up to 5000 m in foreland,which has the characteristic of coarsening upward.The upper part of this profile is present as alluvial fan facies,composed of gray or black-gray grindstone,which is named as Xiyu conglomerate by Huang in the salt water ditch near the Kuqa River in the southern Chinese Tian Shan Mountains.The research on the Cenozoic deposits in northern Chinese Tian Shan foreland is focused on magnetostratigraphy study and sedimentology.On the Basis of previous studies,our work is focus on heavy mineral analysis and detrital zircon U-Pb chronology of the Cenozoic deposits in the Urumqi River section and Taxi River section,whose stratigraphic chronological framework was established by our project team before.Main achievements of this study are summarized as following:(1)The results of heavy minerals analysis show that the sedimentary components of Urumqi River section are mainly composed of mafic igneous rocks,supplemented by metamorphic rocks,and the depositional environment is oxidizing milieu.There are three types of heavy minerals combinations in the formation,Samples WZ1-WZ7(~5.8-4.2Ma): The dominant minerals are ilmenite-hematite-epidote-sashimi,and the characteristic minerals are barite;Samples WZ8-WZ10(~2.7-1.0Ma): the dominant minerals are ilmenite-hematite-chlorite-magnetite,and the characteristic mineral are hornblende;Sample WZ11-WZ12(~0.6-0.5Ma): the dominant minerals are hematiteilmenite-Epidote-magnetic hematite-white titanium stone,the characteristic minerals are hornblende-zircon-apatite-anatase.The heavy mineral characteristic index indicates that the granite or hornblende is rich in the source area.The sedimentary components in the Taxi River section are mainly composed of mafic volcanic rocks,supplemented by metamorphic rocks,with less intermediateacidic igneous rock(such as zircon,tourmaline,rutile,and apatite and so on),and the depositional environment is oxidized Environment,there are three types of heavy mineral combinations in the stratum,Sample TXZ0-TXZ1(~1.74-1.57Ma): The dominant minerals are hematite-epidote-magnetite-chrome-spinel,and the characteristic minerals are garnet-epidote-chromium spinel;Sample TXZ2-TXZ4(~3.37-2.34Ma): The dominant minerals are hematite-epidote-chromite-maghemite,and the characteristic minerals are garnet-white titanite-chromium spinel;Sample TXZ5-TXZ8(~6.64-3.83Ma): The dominant minerals are hematite-ilmenite-epidotemagnetite.The heavy mineral characteristic index shows that schist,hornblende or granulite is rich in the source area.(2)Based on the zircons U-Pb age spectrum and multi-dimensional scale graph,the zircon U-Pb isotopic ages range in the Urumqi River section are ~250-300 Ma,~300-340 Ma,~340-400 Ma and ~400-500 Ma.The main age range of sample WZ1 is dominated by ~250-300 Ma.The age distribution of samples WZ4,WZ8,WZ9 are similar,whose main zircon age range are ~250-300 Ma or ~300-340 Ma.Upward in profile,the proportion of ~250-300 Ma is increasing and the proportion of 300-340 Ma is decreasing.The age distribution of Sample WZ11 is Multimodal,including age range of ~400-500 Ma,~250-300 Ma,~300-340 Ma,~340-400 Ma,are almost equal.The distribution of WZ1 and WZ11 is also scattered in the multi-dimensional scale graph.Based on the zircon U-Pb age spectrum and multi-dimensional scale graph,the zircon U-Pb isotopic ages in the Taxi River section are mainly consist of ~170-200 Ma,~295-315 Ma,~315-335 Ma,~415-425 Ma and ~460Ma.The age spectrums are typical of double-peak,the main peak is ~295-315 Ma or ~315-335 Ma with little change,while the secondary peak changes significantly.The secondary peak of sample TXZ2,TXZ3,TXZ6,TXZ8 are ~415-425 Ma,while TXZ0's changes to ~460Ma.The multidimensional scale graph shows that TXZ0 is far away from others.Based on the Kolmogorov-Smirnov non-parametric test,this paper verifies the significance of differences between the late Cenozoic stratigraphic samples in the Urumqi River section and Taxi River section,According to the established magnetic stratigraphic chronological framework,the source of late Cenozoic sediments of Urumqi River section changes in ~5.8-4.6Ma and ~2.6Ma-0.5Ma,and the source of late Cenozoic sediments of Taxi River section changes in ~1.57-1.93 Ma.Both profiles have changes in sediment sources since ~2.0 Ma.(4)Based on the history of the quaternary glaciers in the Tianshan Mountains,this source adjustment may be caused by the strong erosion of the glaciers in the early Quaternary period of the Tianshan Mountains.Further analysis of five bedrock zircon samples according to DZMix and DZMds simulation in the Urumqi River Basin reveals that the sediment sources in Urumqi River section are mainly from the veins of the Tianshan Mountains during ~5.8 to ~2.6 Ma,after ~2.6 Ma,the sedimentary input of Houxia fault depression increased significantly.Combined with the natural geographical conditions and uplifting history of Urumqi River Basin,Houxia fault depression zone become an important source for piedmont deposit since the Quaternary period.