Detrital Zircon LA-ICP-MS Ages of Tertiary Sandstones from Western Qaidam Basin:Constraints on the Uplift of Altun Mountains
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The Altun mountains is a collisional orogenic belt strongly modified by strike-slip-faults,constituting the current northern tectonic boundary of Qinghai-Tibet Plateau.Its uplift is remarkable part of the Cenozoic uplift of Qinghai-Tibet Plateau.This contribution presents LA-ICP-MS U-Pb dating results of detrital zircons,together with heavy mineral assemblages from sandstones of the Xiaganchaigou Formation(E13) and lower segment of the Xiayoushashan Formation(N12) from northwest Qaidam Basin.Our results show that there are significant Indosinian and Caledonian age populations in all samples.However,significant differences in age spectrums exist between the samples from Xiaganchaigou Formation ages and those from Xiayoushashan Formation.Indosinian peaks in the age spectrums of Xiaganchaigou samples are stronger than those of Xiaganchaigou,whereas the Caledonian peak gradually become stronger in Xiayoushashan samples.The stability coefficients of heavy mineral assemblages decrease rapidly from the Xiaganchaigou Formation to the Xiayoushashan Formation,indicating that their debris changed from a high maturity into a low maturity level and that provenance area for the Xiaganchaigou Formation is much farther than that of the Xiayoushashan Formation.All these results imply that the Altun had experienced significant uplift from early Oligocene to early Pliocene.Although the subduction-collision orogenes is of Altun orogenic belt was completed in the early Paleozoic,the currently exposed Altun mountains was primarily formed by much late tectonic movement,i.e.uplifting associated with strike-slip faulting in Cenozoic.This work also confirms the U-Pb age spectrum of detrital zircon,associated with heavy mineral assemblage analysis,is an effective tool in research of stratum provenance and basin-mountain coupling.Keywords:
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The Early-Middle Jurassic Yan'an Formation widely distributed in the northwestern margin of Ordos Basin,and the outcrops scattered limitedly with similar lithological features.Many disputes still exist about the source areas of Yan'an Formation due to the tremendous geological reformation since the Late Cretaceous.The zircon Laser-Ablation Inductively Coupled Plasma-Mass Spectrometry(LA-ICP-MS)U-Pb dating was used to determine the ages of the source areas of the Yan'an Formation in the region.There are three samples selected from the sandstone of Yan'an Formation in two areas,altogether 210 grains were dated,and 193 valid zircon ages were obtained.Most zircons from the samples of Helan Mountain showed circular belt images under the cathodoluminescence microscope and displayed the left-oblique distribution pattern of rare earth elements.Most of those zircons have the Th/U larger than 0.4indicating the magmatic origin,and a few zircons were from the metamorphic rock in the area.Most zircon U-Pb dating ages are mainly of the Paleozoic with the major peak in the Permian.The secondary ages are of the Proterozoic.The zircons from Zhuozi Mountain area are with the similar characteristics both in the images under the cathodoluminescence microscope and in the rare earth element distribution pattern.There exists much difference in the zircon ages between the two areas.The old ages in Zhuozi Mountain possess great proportions;66.7% are of the Late ArcheanProterozoic(mainly the Proterozoic);only 22.7% of the total ages belong to the Permian,and 10.6% are of the Early-Middle Triassic.Compared with the ages of surrounding ancient rocks,it is concluded that the main sources of the Early-Middle Jurassic in the northwest margin of Ordos Basin were the Proterozoic magmatic and metamorphic rocks in the Alashan Paleo-Block;for the north of Ordos Basin,the main source was the Hercynian granite in the Alashan Paleo-Block.The main provenance in Helan Mountain was the Hercynian magmatic rocks in the Alashan Paleo-Block,and the secondary source was the Proterozoic magmatic and metamorphic rocks(Helan Mountain complex)for its northwest.While the principal source area in the Zhuozirocks and Early-Middle Triassic magmatic rocks acted as the minor source.There is difference in provenance ages among the two studied areas and the Ciyaobao area.More researches from the sedimentary and petrology need to be done to further describe the provenance attribute.
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Abstract The Selin Co basin in the northern Lhasa terrane includes more than 3000 m of upward coarsening Lower Cretaceous strata, and the sedimentary sequence from the flysch to the molasse indicates the evolution of a foreland basin. Petrographic analysis shows that sandstones are rich in volcanic and sedimentary lithics and most of them fall into recycled orogen and magmatic arc. Uranium–lead (U–Pb) ages were determined for 435 detrital zircons from the Lower Cretaceous strata in the Selin Co basin. Relative probability of detrital zircon ages shows the Eshaerbu Formation was rich in zircon grains with the age of 125–140 and 160–180 Ma, and the Duoni Formation was dominated by one main age cluster of 125–150 Ma. Analysis of the potential provenances suggests the Early Cretaceous zircon grains were primarily derived from the Gangdese magmatic arc to the south. The youngest zircon ages in the lowermost exposure of the Eshaerbru Formation are c . 130 Ma, providing a maximum depositional age of sediments in the Selin Co basin. Collectively, our studies, together with previously documented Cretaceous thrusting in the Lhasa terrane, suggest the Lower Cretaceous Selin Co basin was deposited in a retroarc foreland basin. From 145–90 Ma, a retroarc foreland basin was presumed to develop in the Lhasa terrane, migrating from the south to the north. Crustal thickening, likely associated with the evolution of the retroarc foreland basin, was speculated to start in the Early Cretaceous in the Lhasa terrane.
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Detailed observations of CL images and LA-ICP-MS detrital zircon U-Pb ages of the metasandstone samples from Liangdang area of Western Qinling provide information about the provenance of Taiyangsi Formation. The ages of detrital zircons show that the sedimentary ages of the Taiyangsi Group range from 426 to 420 Ma, suggesting Ludlow to Pridoli period. The U-Pb detrital zircon ages can be assigned to four age groups: 500~420Ma, 955~550Ma, 1866~1227Ma and 3039~2132Ma. Early Paleozoic age group shows the most strong peak age of 438 Ma, implying that the material source was mainly from the north margin of Western Qinling tectonic belt. The detrital zircons with Neoproterozoic age were related to the magma of the north margin of Western Qinling tectonic belt and Northern Qilian orogenic belt in the Neoproterozoic. The detrital zircons with Mesoproterozoic age and Paleoproterozoic-Neoarchaean age were mainly from the basement of Northern Qilian orogenic belt and the north margin of Western Qinling tectonic belt. A comprehensive analysis reveals that the material source of Taiyangsi Formation was mainly from the north margin of Western Qinling tectonic belt.
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Dahalajunshan Formation volcanic- sedimentary rocks, widespread in Yili area, western Tianshan, are the most direct carrier in the study of Early Carboniferous palaeogeography of western Tianshan and Paleozoic orogenic evolution of the Tianshan.The age distribution of detrital zircons of sandstone in Dahalajunshan Formation was studied by using LA-ICP-MS U-Pb isotopic ages of zircons. The results show that the detrital zircons have a wide range of zircon ages from(321±2) Ma to(435±2) Ma, whichcan be divided into 2 groups: 321-372 Ma and 395-435 Ma, according to the age and frequency distribution characteristics. Based on these dating data, combined with the mineralogical characteristics and CL images of zircons as well as the regional geological data of the Wusun Mountain, the authors have reached the following conclusions:(1) The age of the deposition of Dahalajunshan Formation is not later than the later Early Carboniferous;(2) the detrital materials of the studied sandstone were mainly derived from volcanic and magmatic rocks related to the Wusun Mountain and Nalati Mountain;(3) during middle- late Devonian, western Tianshan orogenic belt experienced significant ocean- land transition events and entered into the post- collision rift extensional stage. Therefore, the formation of volcanic- sedimentary rocks of Dahalajunshan Formation was in a post- collision extensional setting.
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The Visean to Bashkirian profile is well exposed in the Wushi area,the northwestern margin of the Tarim Basin,Northwest China. Numerous sandstone samples were chosen to carry out U-Pb and Lu-Hf isotope test by LA-ICP-MS method. Based on detailed analysis of geochronology,heavy minerals,sandstone skeleton component,this research further explores sedimentary evolution and its geodynamic mechanism about the Tarim Basin and the periphery orogen in Carboniferous. Sandstone skeleton component analysis shows that sandstone grains are dominated by quartz,and volcanic-metavolcanic is lithic fragments. Its main provenance is the recycled orogen,and has the trend to migrate to the inner craton from the bottom to top. The main proportions of heavy mineral assemblages are stable heavy minerals,such as zircon,tourmaline,TiO2 minerals. Heavy mineral analysis reflects the distant source and relatively stable tectonic setting are possible. The geochronological results show that the four samples have approximately similar UPb age patterns and Hf isotopic compositions,mainly reflecting multiphase tectono-thermal events with age groups of 392 ~ 496 Ma,708~ 868 Ma,893 ~ 1044,with eHf( t) ranged from- 15 to 13. In addition,there are also a small amount of 1713 ~ 1917 Ma,2376 ~2606Ma age. The geochronological information and provenance system are very similar to Silurian to Devonian strata underlying the unconformity. Comparing with the main potential source areas,the main provenance is the northern Tarim palaeohigh. The redeposition of pre-Carboniferous strata in Tarim Basin,associated with the erosion source in the Kunlun-Altun orogen,may also have some contribution,and became the mainly provenance of detrital zircon of 893 ~ 1044 Ma before Early Visean; while the two facts,detrital composition maturity increased and the abundance of 893 ~ 1044 Ma decreased rapidly,suggested that the material from the latter source diminished,and the mature material from the palaeohigh raised after Late Visean. In other words,although the radiolarian information shows that the South Tianshan Ocean had closed after Early Visean,the provenance system didn't change greatly until Bashkirian,and the information about collision,orogenic events and their effects of the South Tianshan and the Tarim hadn't been recorded.
Geochronology
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