Petrology,Geochronology And Geochemistry Of The Triassic Intrusive Rocks In The Helong Area Of Eastern Jilin Province

Helong area of eastern Jilin Province is located in the eastern intersection area of the northern margin of North China and Xing'an-Mogolia Orogenic Belt,which is also the intersection area of the Paleo^Asian Ocean and the circum^Pacific regimes.There is a controversy over when transition from the Paleo-Asian oceanic regime to the circum-Pacific tectonic regime took place.This thesis studies on the petrology,geochronology and geochemistry of the Triassic intrusive rocks in the Helong area of eastern Jilin Province.These data are used to elucidate their formation time,petrogenesis and tectonic setting,and reveal the transition time from the Paleo-Asian oceanic regime to the circum-Pacific tectonic regime.Zircon U-Pb dating results indicate that the Nantian medium^to fine^grained granodiorite in the Helong area of eastern Jilin Province was formed in 243.0±1.70Ma?the Middle Triassic?,the Yongxin medium-to fine-grained monzogranite was formed in236.1±0.89 Ma?the Middle Triassic?,and the Fudong porphyritic monzogranite was formed in 224.6±2.5Ma?the Late Triassic?.The Middle Triassic granodiorite and monzogranite have similar geochemical characteristics.They have SiO₂=66.68⁷3.71wt.,Al₃O₂=14.29¹6.41wt.,Na₂O=3.55⁴.90wt.,MgO=0.21¹.64wt.,TFe₂O₃=1.58⁴.44wt.,and Mg^#=0.19⁰.42.They belong to the medium-to high-K calc-lkaline series.Moreover,these Middle Triassic granitoids are enriched in LREE and LILE,and depleted in HREE and HFSE,with weakly negative Eu anomalies.Additionally,the Middle Triassic granitoids contain high Sr(436×10^-61161×10^-6)but have low Y(5.3×10^-613.1×10^-6)and Yb(0.63×10^-61.60×10^-6)concentrations,all of which are indicative of adakitic magma.Combined with the major elements,the Middle Triassic monzogranite and granodiorite belong to the comagmatic evolutionary series experienced the crystallization differentiation,whose primary magma was derived by partial melting of a thickened region of the lower continental crust.Therefore,they were formed in a compressional tectonic setting related to closure of the Paleo-Asian Ocean.The Late Triassic Fudong porphyritic monzogranite have Si O₂=61.55⁶6.46wt.,Al₃O₂=15.60¹6.52wt.,and Na₂O=4.08⁴.32wt..They belong to the high-K calc-alkaline series.They are enriched in LREE and LILE,and depleted in HREE and HFSE,with weakly negative Eu anomalies.Furthermore,the Late Triassic monzogranite also contains high Sr(542⁷97×10^-6)but has low Y(12.5²2.4×10^-6)and Yb(12.5²2.4×10^-6)concentrations,showing adakitic characteristics.However,the monzogranite has high MgO,TFe₂O₃ and Cr contents,and hosts mafic micro-granular enclaves,indicating that the primary magma was derived by the partial melting of a delaminated region of the lower continental crust.It is possible that the ascending melts interacted with mantle.We propose that the Late Triassic monzogranite was formed in a post^orogenic extensional environment after closure of the Paleo-Asian Ocean.In summary,the eastern Jilin Province has been influenced by the Paleo-Asian oceanic regime during the Middle-Late Triassic,instead of the circum-Pacific tectonic regime,implying that the Paleo-Pafic plate did not subduct beneath the eastern Jilin Province in the Middle-Late Triassic.