Geochemistry Of Mesozoic Magmatic Rock And Gold Mineralization Events During Rapid Lithospheric Thinning Beneath The Jiaodong Peninsula

Granite is an important part of the continental crust,and its origin is closely associated with the evolution of the continental crust.The Linglong granite is the most important Mesozoic pluton in the Shandong Peninsula,eastern China.However,its petrogenesis remains hotly controversial,particularly with regard to the nature of source region and geodynamic setting.Although some geochronological,geochemical and isotopic data were reported before to discuss the petrogenesis of the Linglong granite,the nature of source region and geodynamic setting are still controversial.On one hand,because the Linglong granite is located in southern margin of the North China Block and near to the North China Block(NCB)-South China Block(SCB)suture zone,its magmatic-tectonic affinity remains equivocal.Traditionally,geoscientists considered that the Linglong granite was derived from partial melting of the Neoarchean lower crust of the NCB.Recently,some investigators argued that the Linglong granite was derived mainly from partial melting of the subducted SCB on the basis of presence of Neoproterozoic inherited zircons which are absent in the NCB but a prominent feature in the SCB.On the other hand,the potential geodynamic mechanism for formation of the Linglong granite has been suggested to be controlled by the collision of the SCB and NCB in Triassic or to the subduction of the paleo-Pacific plate.Our new zircon U-Pb dating results reveal that the Linglong granite was emplaced in the Late Jurassic(157?160 Ma).In addition,abundant inherited zircons are identified in the granite with four age peaks at?208,?750,?1800 and?2450 Ma.The Linglong granite is weakly peraluminous I-type granite,and is characterized by high SiO2,Sr and La,but low MgO,Y and Yb contents,strongly fractionated REE pattern and high Sr/Y and La/Yb ratios.Rocks exhibit high initial 87Sr/86Sr ratios(0.7097 to 0.7125),low ?Nd(t)(-17.7 to-20.3)and variable zircon ?Hf(t)(-22.2 to-8.7)values.Zircon saturation temperatures reveal the magma temperatures are 760±20 ?,the lowest Tzr value of 740 ? may be close to initial magma temperature of these inheritance-rich rocks.Interpretation of the elemental and isotopic data suggests that the Linglong granite has some affinities with the adakite,and was most likely derived from partial melting of thickened lower crust without contribution of mantle components.The presence of a large number of inherited zircons and variable Sr-Nd-Hf isotopic compositions reveal that the Linglong granite probably has multiple sources consisting of the lower crust of both South China Block and North China Block,as well as the collision-related alkaline rocks and UHP metamorphic rocks.The continental arc-rifting related to the Izanagi plate subduction was the most likely geodynamic force for formation of the Jurassic Linglong adakatic granite in the Shandong Peninsula.Mafic dikes,controlled by regional lithosphere extension,play an important role in understanding the continental dynamics.Mesozoic lamprophyres are widely present in gold province in the Jiaodong Peninsula.In this study,we analyzed major and trace elements and Sr-Nd-Pb isotopic compositions of lamprophyres from the Linglong and Penglai Au-ore districts in the Jiaodong Peninsula,in an attempt to better understand Mesozoic lithospheric evolution beneath the eastern North China Craton.These lamprophyre dikes are calc-alkaline in nature,and characterized by low concentrations of SiO2,Ti02 and total Fe2O3,high concentrations of MgO,Mg#and compatible element,enriched in LREE and LILE but variably depleted in HFSE.They display initial 87Sr/86Sr ratios of 0.709134-0.710314,?Nd(t)values of-13.2 to-18.3,206Pb/204Pb of 17.364-17.645,207Pb/204Pb of 15.513-15.571 and 208Pb/204Pb of 37.995-38.374.Interpretation of elemental and isotopic data suggests that the Linglong and Penglai lamprophyres were derived from partial melting of a phlogopite-and/or amphibole-bearing lherzolite in the spinel-garnet transition zone.The parental magma might have experienced fractionation of olivine and clinopyroxene,and minor crustal materials were incorporated during ascent of these mafic magmas.Before?120 Ma of emplacement of these calc-alkaline lamprophyres,the ancient lithospheric mantle was variably metasomatized by hydrous fluids rather than melts from subducted/foundered continental crust.It is proposed that continuous modification by slab-derived hydrous fluids from the Paleo-Pacific plate converted the old cratonic lithospheric mantle to Mesozoic enriched lithospheric mantle.Geodynamic force for generation of these lamprophyres may be related to large scale lithospheric thinning coupled with upwelling of the asthenosphere beneath the North China Craton.Continental arc-rifting related to the Palaeo-Pacific plate subduction is favored as a geodynamic force for the cratonic lithosphere detachment.Lithospheric thinning and destruction of the North China Craton have been topics of active discussion throughout the last two decades,but the specific deep-level processes associated with the thinning of the lithosphere are still being actively debated.Two major hypotheses,thermo-mechanical-chemical erosion and rapid lithospheric delamination,have been suggested for the mode of lithospheric thinning.The former implies that the lowermost lithosphere is gradually eroded by upwelling asthenosphere,which represents a slow process lasting at least 100 Myr.The latter assumes that the thick lithosphere suddenly foundered into the underlying convecting mantle,and this marks a short event lasting about 10 Myr.In order to better understand the mechanism of lithospheric thinning beneath the NCC,this study addresses the following two key questions:(1)When did the magma source transfer from lithospheric to asthenospheric mantle sources;(2)Was this transition a relatively "slow",or a "rapid" process.Here we report co-occurrence of low-Ti(TiO2<1.1 wt%,Ti/Y<270)and high-Ti(TiO2>2 wt%,Ti/Y>370)types of lamprophyres in the Jiaodong Peninsula,eastern North China Craton in order to address this issue.Low-Ti lamprophyres are depleted in HFSE and enriched in Pb,both typical subduction signatures.We suggest they were derived from partial melting of an ancient and enriched lithospheric mantle,which was previously modified by slab-derived hydrous fluids.In contrast,the high-Ti lamprophyre has trace element patterns similar to many oceanic basalts with depletion of Pb but little or no HFSE depletion.We infer that they originated from partial melting of a convective asthenospheric mantle.Zircon U-Pb dating shows that both types of lamprophyres intruded the eastern NCC about 121 Ma ago.Their indistinguishable ages thus appear to record a rapid transition from lithospheric to asthenospheric mantle source,suggesting further that the lithosphere beneath the eastern North China Craton was removed,potentially delaminated ca.121 Ma ago beneath Jiaodong Peninsula.The detachment of cratonic lithosphere is likely related to continental arc-rifting which resulted from Palaeo-Pacific plate subduction in the Mesozoic.The Jiaodong Peninsula is the largest gold metallogenic region in China.On the basis of field observation,electron probe microanalysis,trace elements,platinum metals,halogen elements and S-Pb-He-Ar isotopic data of the Jiaojia gold deposit,we discuss the metallogeny and geodynamic setting of the gold mineralization.The Jiaojia gold deposit is mainly hosted in Mesozoic granitoids and controlled by NNE-trend or NE-trend fault zones.The ore-forming fluids had low salinity and high CO2 contents,and the He-Ar isotopic systematics in fluid inclusion show fingerprint of mantle-derived fluid input.The ore-forming materials were derived from multiple sources and show clear evidence for crust-mantle mixing.We argued that the Jiaojia gold deposits are orogenic gold deposits,formed in an accretionary orogeic setting.The Jiaojia gold deposit is likely formed in an active continental margin related to subduction of the Pacific plate in early Cretaceous.As the consequence of roll-back of the Pacific slab,the lithosphere was rapidly delaminated,which led to intensive crust-mantle interaction and extensive magmatism with formation of gold deposits in Jiaodong Peninsula.