Petrogenesis Of Mesozoic Granitoids And Crustal Evolution In Xingcheng Area, Western Liaoning Province

With the wide distribution of Mesozoic granitic magmatism, the Xingcheng area inwestern Liaoning Province, located in the eastern segment of northern margin of NorthChina Craton(NCC), is the ideal area to study the granitic petrogenesis, the nature ofmagma source and the tectonic attribute. In this paper, we do the systematic researchesabout the petrology, geochronology, geochemistry and Zircon Hf isotopic of granitic rocks.Based on the mentioned above, we have built the geochronological framework and havediscussed the petrogenesis, the nature of magma source and the tectonic setting of graniticrocks. And according to the information about the crust-mantle interaction recorded ingranitic rocks and the regional data, we also discuss the relationship between Mesozoicgranitic magma activities and lithosphere thinning, destruction of Craton mechanism inthe eastern segment of northern margin of NCC.The Mesozoic granitic magma activities in Xingcheng Area in western Liaoning aredivided into four stages: Late Triassic (230Ma~228Ma), early-middle Jurassic (194Ma~169Ma), Late Jurassic (161Ma~155Ma) and Early Cretaceous (139Ma~121Ma).The late Triassic granitic rocks are composed of adamellite, monzogranite and diorite.The samples have high Sr, Cr, Ni content and low Yb, Y content, and the sample have highMg#and high ratio of Sr/Y and La/Yb. The characteristics of inhomogenous negativevalue of εHf(t)(-6.40~+0.19)in magmatic zircons and ancient crustal TDM2values of 1.25Ga~1.67Ga, indicate that the Triassic granitic rocks were formed in a post-collisionalextensional tectonic setting after subduction of Paleo-Asian Ocean and the subduction ofYangtze Craton, and they are the product of partial melting of ancient lower crust underthe condition of the mantle-derived magma underplating, and geochemical features of thehigh Sr and low Y are inherited from the source of magma which is also enriched in Sr anddepleted in Y.The geochemical features and assemblages of rocks formed during Jurassic-earlyCretaceous are similar. The early-middle Jurassic granitic rocks are composed ofsyenogranite, monzogranite, quartz monzonite, granitic porphyry, granodiorite and quartzdiorite. The late Jurassic granitic rocks are composed of monzogranite, granitic porphyry,and quartz diorite. The early Cretaceous granitic rocks are composed of monzogranite,quartz syenite, granitic porphyry, and quartz monzodiorite. The samples are characterizedby the high SiO2content and total alkalis and are belonged to the high-K calc-alkalineseries. And the characteristics of quasi-aluminous to weakly peraluminous (A/CNK＜1.1,A/NK＞1.0) and the negative relationship between P2O5and SiO2are similar to thecharacteristics of I-type granite. The samples are enriched in LREE contents and K, Pband depleted in Nb, Ta, Ti and P. The characteristics of assemblage and geochemicalfeatures indicate the Jurassic-Cretaceous granitic rocks formed in the setting of activecontinental margin related to the subdction.The magmatic zircons of early-middle Jurassic granitic rocks have negative εHf(t)(-16.60~-4.15) and ancient crustal TDM2values of (1.49Ga~2.28Ga) which indicates theprimary magma is from the partial melting of ancient middle-lower crust. These rocksformed in the setting of active continental margin of subduction of Paleo-Pacific plate.The magma source of granitic rocks is from partial melting of the ancient lower-middlecurst caused by the underplating of mantle magma, which formed in the condition of fluidderived from the dehydration of subducted slab affected on the lithosphere mantle.The magmatic zircons of late Jurassic granitic rocks have negative εHf(t)(-26.24~-18.56) and ancient crustal TDM2values of (2.39Ga~2.87Ga) which indicates the primary magma is from the partial melting of ancient upper-middle crust. And these rocksmay be formed in the setting of active continental margin of intense subduction ofPaleo-Pacific plate. In the intense subduction, the mantle magma is formed in theinteraction of fluid derived from the dehydration of subducted slab with the lithospheremantle, then further underplated the ancient middle-upper curst and make the ancientmiddle-upper curst partial melting. In this setting, the late Jurassic rocks are distributedwidely in eastern North China.The samples in early Cretaceous have a wide range of values of Hf isotope:-24.92~-20.88(139Ma),-4.72~+6.22(130Ma),-17.30~-11.56(125Ma~121Ma), and thecharacteristics of Hf isotope indicate the source of early Cretaceous magma (middle-uppercrust, middle-lower crust or juvenile crust) is very complicated. The early Cretaceousgranitic rocks were formed in the lithosphere extensional environment when thePaleo-Pacific plate subducted/roll-backed quickly in the direction of NNW with highangle. The regional extension made the asthenosphere upwell and be unstable, and themagma of mantle was formed. Then wide mantle-crust interactions make the crust partialmelt. As a result, the source of early Cretaceous igeneous rocks which are distributedwidely in eastern North China Craton.The participation of depleted mantle components in the process of Late Triassic rockformation in western Liaoning area indicate the time of the lithosphere thinning in theeastern segment of northern margin of NCC begin from the late Triassic (~230Ma). Thedouble subduction of Paleo-Asian Ocean and Yangtze Craton in the early period is thereason of the lithosphere thinning. After the double subduction, the mantle-derived magmaupwell and underplate the lithosphere mantle and lower crust in a post-collisionalextensional tectonic setting. As a result, the lithosphere began to thin but the Cratondestruction was not obviously in this period. The participation of depleted mantlecomponents makes the continental crust slightly reforms and grows. The eastern segmentof northern margin of NCC was in the setting of active continental margin in Jurassic, andthe continuing dehydration and metasomatism of subduction slab changed the physical and chemical properties of the lithosphere mantle. The initial weak subduction inearly-middle Jurassic makes the lithosphere become thin but no obvious destruction ofCraton. And in this period, the continental crust reformed and grew slightly. As thesubduction became stronger during late Jurassic, the thickness of lithosphere in easternsegment of northern margin of NCC become thinner and the destruction of Cratonoccurred locally and the continental crust reformed and grew to some extent. In earlyCretaceous, the change of direction and angle of subduction of Paleo-Pacific led to theregional extension, which is the reason for massive mantle material upwelling and intensecrust-mantle interaction. And the lithosphere thinning and destruction of Craton reachedthe peak. As a result, the effects of reconstruction and hyperplasia of continental crust areobviously. In our opinions, the underplating mechanism of mantle-derived magmaoccurred mainly in the initial stage (early Mesozoic) of lithosphere thinning, and thedelamination of lithosphere mantle or lower crust coexisted while erosion action of mantlematerial occurred in the stage (late Mesozoic) of strong lithosphere thinning anddestruction of Craton.