Meso-Cenozoic Magmatism And Exhumation History Of The North China Craton

The North China Craton(NCC),the largest and oldest cratonic block in China,consist mainly of the Western Block(WB),Trans-North China Orogen(TNCO)and Eastern Block(EB).It preserves important records on cratonic evolution during the early Precambrian and subsequent multiple tectonic reactivation associated with lithospheric thinning and craton destruction during the Meso-Cenozoic,with concomitant eastward thinning of lithosphere from 200-250 km in the WB through 100-150 km in the TNCO to 60-100 km in the EB from west to east.Thus,the NCC offers a classic example for lithospheric thinning related to craton destruction through multiple tectonic events,with voluminous Late Mesozoic magmatism associated with craton destruction.This thesis focuses on Late Mesozoic magmatic rocks from Yishui,Juxian and Junan domains of the Shandong Peninsula in the eastern NCC,rhyolitic rocks from North Hebei Province(Chicheng basin)in the northern NCC,adakitic granitoids from North Taihang Mountain(Zhijiazhuang skarn iron deposit)in the central NCC,together with Paleoproterozoic and Late Mesozoic magmatic rocks from two flanks of the Tan-Lu Fault Zone(TLFZ)in the eastern NCC,with detailedly petrological,geochemical,zircon U-Pb geochronological and Lu-Hf isotopic,apatite U-Pb geochronological,and apatite low-temperature thermochronological studies to gain insights into the lithospheric thinning and craton destruction of the NCC.Zircon U-Pb data for magmatic rocks from Yishui,Juxian and Junan domains in the eastern NCC show ages in the range from 137 to 116 Ma with two peaks at 134 and 123 Ma.The Chicheng rhyolitic rocks from northern NCC show ages ranging of 144-114 Ma,with three peaks at 141,136 and 131 Ma.The Zhijiazhuang adakitic rocks from central NCC display ages of 138-112 Ma,characterized by multiple peaks at 134,129,125,121 and 118 Ma.These age data obtained in this thesis represent emplacement or eruption ages,and could be correlated with the tectonic events of westward PaleoPacific subduction during the Early Cretaceous.Furthermore,the age data from eastern NCC(Yishui,Juxian and Junan domains)also record numerous inherited zircon ages,with minor scattered Paleoproterozoic 207Pb/206 Pb spot ages of 2457,2350,2135,and 1939 Ma and the Neoproterozoic to Early Jurassic 206Pb/238 U age peaks at 762,532 and 179 Ma.The Paleoproterozoic inherited zircon grains are interpreted as xenocrysts captured from Precambrian crystallized basement of the NCC,the Neoproterozoic zircon grains inherited from reworked Yangtze Craton basement accreted to the margin of the NCC during the Triassic collision between NCC and Yangtze Craton.The Paleozoic zircons are also correlated with the source from Yangtze Craton and/or influenced by the Late Paleozoic southward Paleo-Asian Oceanic subduction.The Late Triassic to Jurassic zircons could be correlated with delamination of the thickened continental crust during deep subduction of the Yangtze continental crust and continentcontinent collision between NCC and Yangtze Craton.Zircon Lu-Hf isotopic data for studied magmatic rocks from eastern,northern and central NCC are characterized by negative ?Hf(t)values in the range from-35 to-5 and the Hf crustal model ages(TDMC)ranging of 3.7-1.5 Ga,with the ?Hf(t)values of-36.8 to-7.9 and TDMC values of 3776-1690 Ma for eastern NCC(Yishui,Juxian and Junan domains),-23.0 to-11.8 and 2650-1994 Ma for northern NCC(Chicheng basin),and-21.7 to-7.8 and 2560-1680 Ma for central NCC(Zhijiazhuang skarn iron deposit).The results suggest the involvement of heterogeneous sources.The major Early Cretaceous zircon population from northern and central NCC show very similar TDMC values between 2.5 and 2.0 Ga,indicating that the magma was mainly derived from reworked Paleoproterozoic crustal materials.The Early Cretaceous zircons from the eastern NCC display relatively wide TDMC values range from 3.0 to 2.0 Ga,implying the involvement of reworked Mesoarchean to Paleoproterozoic crustal materials in the magma source.These inherited Neoproterozoic to Early Paleozoic zircon grains from the eastern NCC exhibit TDMC values range of 3.0-2.0 Ga consistent with Early Cretaceous zircons,and thus further suggesting that reworked Mesoarchean to Paleoproterozoic crustal materials served as magma source.Geochemical studies on these igneous rocks carried out as part of this thesis also support partial melting of Mesoarchean to Paleoproterozoic crustal magma source,with likely minor input of lithospheric mantle components.Whole-rock geochemical data show that almost all the igneous rocks studied in this thesis belong to high-K calc-alkaline to shoshonite,peraluminous to metaluminous,and mixed I-and A-types granites for magmatic rocks from eastern NCC,A-type granites for rhyolitic rocks from northern NCC,and I-type granites for adakitic rocks from central NCC.These rocks show high concentration of light rare elements(LREEs)and relatively low contents of heavy rare earth elements(HREEs),and most of these rocks are characterized by negative Eu anomalies on their Chondrite normalized REE patterns,suggesting feldspar fractionation during the magma evolution.These rocks exhibit an overall enrichment in large ion lithophile elements(LILE;K,Rb,Sr,Ba,Pb)and relative depletion in the high field strength elements(HFSE;Nb,Y,Hf,Ta,Ti)with positive anomalies for K,La and Pb and negative anomalies at U,Ta,Nb,P and Ti on their primitive mantle normalized patterns,indicating subduction-related geochemical signature.The rocks from eastern and northern NCC show arc magmatic signature,and those from central NCC characterized by adakitic affinity.Almost all the Chicheng rhyolitic rocks from northern NCC show post-COLG geochemical signature and mixed signature of WPG and VAG.The Zhijiazhuang adakitic rocks from central NCC belong to a VAG without post-COLG tectonic setting.Most of magmatic rocks from eastern NCC show post-COLG and VAG tectonic setting,with a transition trend from VAG to WPG setting.All the rocks from northern and central NCC are also characterized by the subduction-related geochemical signature,whereas those from eastern NCC show transition trend from collision to subduction.Apatite U-Pb age data for eleven igneous rocks taken from two flanks of the TLFZ in the eastern NCC yield apatite U-Pb ages of Precambrian rocks with three group ages of ~2.3,~2.0,~1.8 Ga,indicating apatite recrystallization or thermal resetting in response to the Paleoproterozoic crustal growth events in the eastern NCC.The apatite U-Pb ages of Mesozoic rocks show age in the range of 162-112 Ma,likely recording shallow granitoids emplacement and associated rapid post magmatic cooling correspond to lithospheric thinning and craton destruction of the NCC.The apatite fission track(AFT)ages for the eleven igneous rocks yield two group cooling ages of Early Cretaceous(122-113 Ma)and Late Cretaceous to Early Paleogene(98-59 Ma),and associated thermal history models constructed in this thesis also reveal two rapid cooling stages of the Early Cretaceous(130-105 Ma)and Late Cretaceous to Paleogene(85-55 Ma).The compiled multiple thermochronological age data from previous studies in the Shandong Peninsula show Late Triassic to Neogene cooling ages,and response to multiple complex tectono-thermal history related to multiple tectonic events in the eastern NCC.The Early Cretaceous AFT ages and rapid cooling pulse response to the peak event of craton destruction.The Late Cretaceous AFT ages and rapid cooling pulse is interpreted as a response to continued lithosphere thinning,whereas the Paleogene likely represents the termination of craton destruction.Regional correlations show zircon U-Pb age data from this thesis are almost consistent with the compiled age data from Central Asian Orogenic Belt(CAOB),East China and North Korea,and NCC from previous studies.The ages correspond to multiple Paleozoic to Early Cretaceous tectonic events related to the Paleozoic PaleoAsian Oceanic Plate subduction,Permian collision between CAOB and NCC,Triassic collision between NCC and South China Craton(Yangtze Craton),and Jurassic to Early Cretaceous Paleo-Pacific Oceanic Plate subduction.By correlating the newly obtained geochronological,low-temperature thermochronological,isotopic,and geochemical data from this thesis with those from previous studies,the following inferences are made.1)Multiple tectonic events surrounding the NCC occurred during the Paleozoic to Mesozoic,including southward Paleo-Asian Oceanic subduction,northward subduction and collision with the South China Craton(Yangtze Craton),and westward subduction of the Paleo-Pacific plate,jointly lead to the craton destruction and lithospheric thinning of NCC but the Paleo-Pacific Plate subduction as the main driving force.2)The far-field effect related to the subduction of Paleo-Pacific Plate from the east resulting in craton destruction in the northern and central NCC as well as the peak event of lithospheric thinning and craton destruction of the NCC might have been the trigger for Early Cretaceous magmatism and mineralization of the Zhijiazhuang skarn iron deposit in the central NCC.3)The craton destruction processes might be correlated with lithospheric modification by multistage peridotite-melt interaction and lithospheric extension associated with Paleo-Pacific plate subduction.4)The westward Paleo-Pacific plate subduction with the slab rollback induced lithospheric extension and back-arc spreading,asthenospheric upwelling and interaction with the lithospheric materials,leading to the lithospheric thinning and partial delamination,replacement of colder lithospheric mantle,melting of the lithosphere mantle and modification of the mantle and its underplating beneath the lower to middle crust during the Early Cretaceous.5)The timing of cessation for craton destruction continued until the Early Cenozoic(Paleogene)and was not prior to Late Cretaceous.6)The eastern NCC underwent multiple thermo-tectonic reactivation during the Meso-Cenozoic,with the Early Cretaceous rapid uplift in response to peak event of craton destruction resulting from the Paleo-Pacific slab rollback in a back-arc extensional setting,the Late Cretaceous rapid uplift corresponding to continuing craton destruction triggered by the NNW-directed Pacific Plate subduction,and the Paleogene interval multiple rapid uplift indicating the termination of craton destruction correlated with dextral motion of the TLFZ induced by the Pacific Plate subduction when the subduction direction changed from NNW to WNW and/or due to a far-field effect of India-Eurasia collision.