| Late Paleozoic-Mesozoic sedimentary units within the basins of the south margin of the North China Craton?NCC?and the Qinling Orogen?QO?record the late Paleozoic to Mesozoic paleogeographic and tectonic evolution of the QO.In this study,new detrital zircon U-Pb ages and Hf-O isotopic data of the sedimentary rocks from the Benxi,Taishanmiao,Taizishan,Nanzhao and Mashiping formations is presented to constrain the deposition timing and sedimentary provenance.The above data,combined with regional research data,further reveals the palegeographic and tectonic evolution of the QO during the late Paleozoic to Mesozoic.1.Depositional ages of the late Paleozoic to Mesozoic sedimentary unitsDetrital zircons in quartz sandstone from the drillcore of the Benxi Formation are euhedral–subhedral,oscillatory zoned,and have high Th/U ratios?0.03–1.60,generally greater than 0.3?that are indicative of a magmatic origin,which is also supported by high contents of high rare earth elements?HREEs?.A total of 72 concordant analyses of 72zircons from 13H16A yielded 7 age populations at 2440,1788,1123,955,737,441,and344Ma.All 60 analyses of zircons from 13H16B yielded concordant ages between 3414and 335 Ma with 6 populations at 2440,1583,951,759,434,and 348 Ma.The two youngest single zircon ages determined during this study are 341±6 and 335±5 Ma.Geochronological data in this paper and previous regional geological data indicate that the Benxi Formation in the Sanmenxia area of the southern NCC was deposited during the late Carboniferous.Most zircons analyzed in this study from Mesozoic sedimentary rocks are euhedral–subhedral,and have oscillatory growth zoning or weak banding visible,indicating they have magmatic origins.This is consistent with their high Th/U ratios?0.04–3.56,generally>0.1?.The zircons from Taishanmiao?TSM?Formation and Taizishan?TZS?Formation have similar U-Pb age populations.The total population of TZS Formation shows five age populations at 1855,923,732,433,and 240 Ma.The youngest detrital zircon yielded a 206Pb/238U age of 234±3 Ma.The analysis of 70zircons of the TZS Formation yielded 65 concordant analyses and a range of ages between 2747 and 216 Ma.These ages form seven populations at 1938,1858,922,729,432,303,and 218 Ma,with the youngest zircon yielding a weighted-mean 206Pb/238U age of 216±3 Ma.Geochronological data suggest that the TSM and TZS Formations of the Nanzhao Basin in the NQO were both deposited during the Late Triassic.Two of the 70 analyses of zircons from Nanzhao?NZ?Formation were discordant,with the remaining concordant zircons yielding ages between 2392 and 110 Ma.The youngest detrital zircon has a 206Pb/238U age of 110±2 Ma.Thus,the NZ Formation was deposited during the Early Cretaceous.Three of the 70 analyses of zircons from Mashiping?MSP?Formation were discordant,with the remaining concordant analyses yielding ages of 2012–110 Ma that form eight age populations at 1850,1629,1418,1282,1110,836,440,and 122 Ma.The youngest zircon yielded a 206Pb/238U age of 110±2 Ma.Integrating all of data and previous researches indicate that the Maishiping Formation was deposited during the Early Cretaceous.2.Provenance and nature of the late Paleozoic to Mesozoic sedimentary source materialDetrital zircons from the Benxi Formation yield several main age populations:Archean-Mesoproterozoic,Neoproterozoic,Paleozoic.Archean-Mesoproterozoic zircons yield?Hf?t?values?–11.3 to+13.9?and?18O values?5.68‰–8.83‰?that are consistent with the Hf-O isotopic compositions of igneous and detrital zircons within NCC basement material.Neoproterozoic zircons have very variable?Hf?t?values?–19.1 to+12.5?and high?18O values?>6‰?,indicating that these Neoproterozoic detrital zircons were predominantly derived from the NQO.Paleozoic detrital zircons have variable?Hf?t?values?–19.9 to+15.3?,indicating derivation from the NQO.They also have?18O values between 5.23‰and 7.60‰that are in the range of compositions of mantle-derived zircons.The detrital material from the TSM Formation and TZS Formation showed similar age populations and were derived from Middle–Late Triassic,early Paleozoic,and Neoproterozoic sources,with minor amounts of material sourced from Paleoproterozoic to Mesoproterozoic terranes.Detrital zircons with concordant>1.5 Ga ages have?Hf?t?values?-6.2+3.0?,similar with the Hf isotopic compositions of igneous and detrital zircons within NCC basement material.The Neoproterozoic zircons analyzed in this study have variable?Hf?t?values?–20.2 to+1.5?that are close to igneous rocks Hf isotopic compositions within the QO.Paleozoic zircons yield?Hf?t?values from–15.0 to+7.6,combined with detrital zircons with ages of 470 and 440 Ma,suggesting they were derived from units within the NQO.However,the TZS Formation contains six late Paleozoic zircons(303–300 Ma;?Hf?t?from–15.0 to+6.8)and they are in round shapes,indicating they were mostly from the Xing-Meng Orogen Belt.All of the Late Triassic detrital zircons analyzed during this study have?Hf?t?values of–13.5 to–0.5,suggesting they were most likely from the SQO.The NZ Formation has a large detrital zircon age peak at 117 Ma and a weaker peak at 2270 Ma.Zircons with concordant>1.5 Ga ages have?Hf?t?values from–9.2 to+3.5,suggesting they were derived from NCC basement material.In comparison,the Early Cretaceous detrital zircons within this formation have negative?Hf?t?values from–17.1to–2.9,indicating that they were sourced from the QO.The MSP Formation detrital zircons yield a wide range with peak ages at 1850,1629,1418,1282,1110,836,440,and122 Ma.Detrital zircons with ages of 2012–1830 Ma have?Hf?t?values between–6.4 and+2.6.The Neoproterozoic and early Paleozoic zircons within this formation are most likely derived from the NQO and QO,whereas the Mesozoic zircons have?Hf?t?values from–24.6 to–11.8,identical to the Hf isotopic compositions of zircons from the QO.The MSP Formation also has similar age populations to the other sedimentary units outlined above,suggesting that they were derived from the recycling of previously existed sedimentary of the orogenic belt in this region.Zircon trace element compositions could provide new insights into parental melt types for different types of igneous rocks,crustal processes and settings.The detrital zircons in the study area were generally derived from intermediate–silicic igneous rocks,and the Triassic zircons formed in the arc-related/orogenic tectonic setting,whereas the Cretaceous zircons formed in both within-plate/anorogenic and arc-related/orogenic tectonic settings.3.late Paleozoic-Mesozoic paleogeographic and tectonic evolution of the Qinling OrogenThe presence of NQO-derived material within the Benxi Formation indicates that the NQO terrane was exposed prior to the late Carboniferous deposition of this formation.The northward driving force of the Mianlue Ocean caused the subduction of SQO beneath the NQO,resulting in the uplift of NQO,which provided sediments that were deposited in a series of basins to both the south and north of this area.The Late Triassic deposition of the TSM and TZS formations in the Nanzhao Basin involved material derived from the SQO,indicating that the QO terrane was undergoing significant uplift and denudation at this time.The northward-driving force of the YC meant that this subduction continued until the Late Triassic and the final closure of the Mianlue Ocean,which caused collision between the NCC and YC.Jurassic sediments are absent from the Nanzhao and Mashiping basins,which reflects the collision between the YC and NCC was contemporaneous with the significant uplift of the QO and southern NCC.Cretaceous sedimentary involved the deposition of poly-mict conglomerates of the Lower Cretaceous NZ and MSP formations,both of which sourced material from the NQO.The lithospheric root of this thickened crust was subsequently removed by delamination or some other form of destruction causing surface uplift.The coarse-grained nature of these foreland basin units were a response to the roll back and exhumation of the Paleo-Pacific Plate and collapse of the lithospheric root at this time. |
PDF Full Text Request