Detrital Records Of The Eastern Qinling Indosinian Orogenic Processes

The final collisional assemblage between North China and the South China blocks were finished during the Indosinian orogeny through the closure of the intervening Paleo-Tethyan branch ocean along the southern Qinling Orogen.After collisional orogeny,Qinling Orogen were propagated southward via large-scale thrusting toward the inland South China,forming the Dabashan thrusting-folding belt and converting the carbonate dominated marine deposition to terrestrial fluvial-deltaic deposition in the northern South China.While it has been widely accepted that this ocean-continental conversion and collisional orogeny occurred in the Triassic,debate remained regarding the exact timing and spatial processes of collision.Moreover,little was known about the Mesozoic exhumation and uplift of the Qinling Orogen.A series of foreland basins formed in the northern Yangtze margin along with the Qinling collisional orogeny.These foreland basins and depositions thereof provide important clues to track the mountain-basin interactions and orogenic processes in Qinling Orogen.The Zigui Basin constitutes an important part of the northern Yangtze foreland basin system and is situated to the south of Dabashan thrust-folding belt.In order to constrain the Indosinian collisional orogeny and mountain building in the Qinling Orogen,this study conducts an integrated sedimentologic,petrologic,zircon U-Pb geochronological,geochemical and Hf isotopic study on the Middle Triassic-Early Jurassic successions in Zigui Basin.In Zigui Basin there developed continuous Mesozoic successions related to the Qinling orogeny.The studied strata intervals are the Middle Triassic Badong Formation,Late Triassic Jiuligang Formation and Early Jurassic Tongzhuyuan Formation.Sedimentary and petrological data show that the Badong Formation was deposited in a coastal plain-tidal flat environment and sandy siltstones are rich in monocrystalline quartz grains with lots of sedimentary lithic fragments.The Jiuligang Formation was deposited in a delta environment,and sandstones are rich in monocrystalline quartz grains with few feldspar grains and lithic fragments.The conglomerates at the bottom of Tongzhuyuan Formation are alluvial fan-braided channel deposits and contain a large amount of siliceous gravels.There are a high content of polycrystalline quartz particles and a small amount of low-grade metamorphic and sedimentary lithic fragments in the conglomerate sandy matrix.Above the bottom conglomerates of Tongzhuyuan Formation,there develop riverine sandstones and mudstones.In addition to quartz and feldspar grains,sandstones of Tongzhuyuan Formation contain a large number of volcanic lithic fragments.From the Late Triassic to Early Jurassic sandstones,the content of polycrystalline quartz grains increased firstly and then decreased,reaching its maximum at the bottom of the Tongzhuyuan Formation.At the same time,the content of volcanic lithic fragments gradually increased and reaches the maximum in the sandstones of the middle Tongzhuyuan Formation.Detrital zircons of Badong Formation recorded three primary age groups of 950-650 Ma,500-350 Ma and 270-230 Ma,and two secondary age groups of 1950-1550 Ma and 2350-2150 Ma.Detrital zircons of Jiuligang Formation show three main age groups of 2100-1700 Ma,900-700 Ma,and 500-400 Ma,and two secondary age groups of 950-700 Ma and 250-200 Ma.The conglomerates at the bottom of Tongzhuyuan Formation have a U-Pb age distribution comparable to that for the sandstones of Jiuligang Formation.Detrital zircons in the lithic sandstones of Tongzhuyuan Formation only have a major age group of 250-200 Ma and a minor age group of 900-700 Ma.Combined with the southward paleocurrents measured in the Lower Jurassic of the basin and previously reported detrital zircon provenance data from the Triassic-Jurassic foreland basins on the northern margin of South China,it was interpreted that(1)the 2100-1700 Ma and 900-700 Ma detrital zircons would be originated from the South China block to the south,with the latter having another possible source in the Qinling orogenic belt to the north;(2)considering the high content of volcanic rock fragments in the corresponding sandstones,it is speculated that the 250-200 Ma zircon grains could be plausibly derived from the Qinling orogenic belt,denoting Triassic volcanic activity in this orogen.According to this comprehensive analysis,the sedimentary provenance of Zigui Basin may have changed from the South China block to the Qinling orogenic belt during the Middle Triassic-Early Jurassic.Detrital zircon grains with ages < 400 Ma show chemical affinities akin to those crystallized from arc/orogenic magmatism on the Th/U-Nb/Hf and Th/Nb-Hf/Hf diagrams.These grains have Th/Nb and U/Yb ratio and Hf isotopic compositions changing significantly during ?240-210 Ma and ?210-175 Ma periods.They display rapid increases in the ratios of Th/Nb and U/Yb and a significant decrease in their ?Hf(t)value from ?240 Ma to 210 Ma.From ?210 Ma to 175 Ma,zircon grains display rapid decreases in ratios of Th/Nb and U/Yb ratio and an increase in ?Hf(t)value.Combining geochronological,geochemical and isotopic data from the Mesozoic granitoid rocks in Qinling Orogen,the variations of zircon Th/Nb,U/Yb ratios and ?Hf(t)values indicate that the Qinling Indosinian collisional orogeny has initiated before ?240Ma,resulting in a significant thickening of the orogenic crust,and turned to post-collisional period at ?210 Ma with crustal thinning and orogenic exhumation.During the late Triassic-early Jurassic,there occurred large-scale southward thrusts and rapid orogenic uplift in the southern Qinling Orogen.The orogenic belt supplied a large number of syn-orogenic clasts with a volcanic origin to the basin.Based on these sedimentary records,this work provides time constraints for the start of collisional orogeny in the eastern Qinling Orogen at ?240 Ma and of large-scale southward thrusting and orogenic exhumation at ?210 Ma.This research not only gives insight into the sedimentary-tectonic evolution of the Qinling Orogen,but also provides new ideas and methods for basin-mountain coupling research.