Geochronology And Geochemistry Of The Mesozoic Qingshan Group Volcanic Rocks In Shandong Province

This PhD thesis deals with Early Cretaceous Qingshan Group volcanic rocks from Shandong Province including Luxi,Tan-Lu fault zone and Jiaodong areas by means of combined study of petrography,whole-rock major and trace element,Sr-Nd-Hf isotope,and zircon U-Pb geochronology.The results are used to constrain the timing of magmatism,magma genesis,and to explor the crust-mantle interaction of lithosphere evolution process beneath the southeastern North China Craton.Zircon U-Pb dating results show that Qingshan Group volcanic activity in Luxi and the Tan-Lu fault zone began contemporaneously at approximately 129 Ma,which is slightly earlier than Qingshan Group volcanism in Jiaodong,which began at approximately 126 Ma.Qingshan Group volcanism in the fault zone was constrained between 129 Ma and 97 Ma,lasted longer than in Luxi(129?115Ma)and Jiaodong(126?107Ma).Volcanic activity in the fault zone ended significantly later than the end time of volcanic activity on both side of the fault zone.And the duration of volcanic eruption in this weak zone was probably longer than in the relatively stable nearby area.These suggeust the Tan-Lu fault zone may play an important role in magma activity and lithospheric thinning.The Qingshan Group volcanic rocks from Linqu in Luxi area are mianly intermediate rocks,with SiO2 contents varied between 54.41%?67.81 wt.%,and constitute mainly andesite-trachyandesite-trachyte-trachydacite association.These volcanic rocks belong to calc-alkaline series.Geochemically,the rocks are characterized by enriched LREE and LILE,and depleted HFSE.These volcanic rocks have enriched Sr-Nd isotopic compositions((87Sr/86Sr)t = 0.704346-0.705831,?Nd(t)=-14.6?-9.5),and have low Hf isotopic composition(?Hf(t)=-29.3--21.6).The rocks have the similar Mg#values to the melts produced by metabasite,with large range of Cr(26.7×106?239×10-6)and Ni(10×10-6?69.7×10-6)content,and show significant mineral textural disequilibrium.Studies further indicate that these volcanic rocks from Qingshan Group in Linqu area may be the products of magma mixing between ancient North China Craton lower crust-derived melts and enriched lithospheric mantle-derived melts.The Qingshan Group volcanic rocks from Jiaodong area have wide SiO2 contents varied between 54.41%?67.81 wt.%,and comprise basaltic trachyandesites,trachyandesites,trachydacites,dacites,and rhyolites.The volcanic rocks are characterized by high K20(2.04-6.70 wt.%)and total alkalis(Na2o ?K20;5.29-10.88 wt.%)that plot within the field for shoshonitic rocks.All the rocks are enriched in LREE and LILE,and are depleted in HFSE.The rocks have high initial 87Sr/86Sr ratios(0.706760-0.708898),low?Nd(t)values(-19.5 to-12.5),and negative ?Hf(t)values(-31.3 to-19.7),similar to the Early Cretaceous Jiaodong mafic dikes and intermediate-mafic volcanic rocks.These geochemical and isotopic characteristics indicate that the volcanic rocks formed through simultaneous fractional crystallization and crustal assimilation of a basaltic magma derived from metasomatized mantle.The metasomatized mantle source might have been produced by the recycling of subducted Yangtze Craton crust into the ancient lithospheric mantle of the North China Craton.The Qingshan Group volcanic rocks along Tan-Lu fault zone have wide SiO2 contents varied between 50.51wt.%?75.63wt%,and are composed of basaltic trachyandesite,trachyandesite,trachydacite,andesite,dacite,and rhyolite.The volcanic rocks have high Na2O + K2O concentrations(5.94-11.33 wt.%),indicating that they are alkaline.All the rocks are characterized by enriched in LREE and LILE,but are depleted in HFSE.Anqiu and Gaoqiao volcanic rocks have wide initial 87Sr/86Sr ratios(0.707678-0.711259),variable negative ?Nd(t)values(-15.2 to-11)and ?Hf(t)values(-19.5--2.2).The varied isotopic compositions and the abundance inherited zircons,together with mineral disequilibrium features,suggest the Anqiu and Gaoqiao volcanic rocks were produced by the mixing of magmas from enriched lithospheric mantle and lower crust.The enriched lithospheric mantle originated from the ancient lithospheric mantle beneath the North China Craton(NCC),which was modified by the subduction of Yangtze crustal material.Juxian and Junan voalcanic rocks have initial 87Sr/86Sr ratios of 0.707502-0.709238,low ?Nd(t)values of-20.5 to-16.1 and negative zircon ?Hf(t)values of-24.6--16.4,similar to granitoids in Sulu orogenic belt,suggesting they derived from similar crustal sources.Combined the existence of the Neoproterozoic inherited zircons and the isotope characteristics,we propose that the Juxian and Junan volcanic rocks were dominantly derived from partial melting of the Subducted Yangtze crust but with contributions from the mantle.The results show that the more Yangtze crust material contributed to the voalcnic rocks near Sulu orogenic belt,and more North China crust material contributed to the voalcnic rocks far away from Sulu orogenic belt.The Qingshan Group volcanic rocks from Shandong Province record complex mult-stages of crust-mantle interactions during lithospheric evolution beneath the southeastern NCC.Firstly,the lithospheric mantle was metasomatized by the melts-fluids derived from the subducted YC crust,resulting in the formation of enriched lithospheric mantle.This stage of crust-mantle interaction significantly modified the composition of the lithospheric mantle beneath the southeastern NCC.Then,the enriched-mantle-derived magmas underplated into the lower crust,trigging the crust anatexis.The enriched-mantle-derived magmas,crust-deived magmas and their mix/contamination form the Qingshan Group volcanic rocks.