Geochronology,Geochemistry And Petrogenesis Of The Late Neoarchean Granitic Rocks In Western And Southern Liaoning

This thesis presents systematic research on the Diaoyutai monzonite granite in the Xingcheng area of western Liaoning,the Chengzitan gneissic quartz diorite and the Ambo granitic gneiss in the Dalian area of southern Liaoning.Based on the field investigations,the mineralogy and petrology,zircon LA-ICP-MS U-Pb geochronology,whole-rock geochemistry,zircon in-situ Hf isotope and apatite Nd isotope analysis are carried out.We finally determine the formation age of the Late Neoarchean granitic rocks in the western and southern Liaoning region,clarify the nature of their source area and petrogenesis,discuss the tectonic background of the Late Neoarchean granitic rock formation in the study area,and reveal the tectonic setting of the Precambrian crustal growth and the Late Neoarchean magmatism in the northeastern part of the North China Craton(NCC).1.Formation age of the Late Neoarchean granitic rocks in western and southern Liaoning Zircons in the Diaoyutai monzonite granite in western Liaoning,the Chengzitan gneissic quartz diorite and the Ambo granitic gneiss in southern Liaoning all developed oscillatory zone and show relatively high Th/U ratios(0.24 ? 1.75),which indicate their magmatic origin.The LA-ICP-MS U-Pb dating results yield 207Pb/206 Pb ages of 2560 Ma ? 2495 Ma?2556 Ma ? 2476 Ma and 2523 Ma ? 2512 Ma,respectively.Their weighted mean 207Pb/206 Pb ages are 2519±9 Ma?2505±10 Ma and 2519±11 Ma,respectively,representing their formation in the Late Neoarchean.2.Geochemisty of the Late Neoarchean granitic rocks in western and southern Liaoning Based on the petrographical and geochemical data,the Diaoyutai monzonite granite in western Liaoning are characterized by high SiO2(73.05% ? 73.38%),low MgO(0.36% ? 0.37%)and high Na2O(3.48% ? 3.62%)contents,being typical of weak peraluminum and high potassium calc-alkaline granites.They are enriched in light rare earth elements(LREEs)and large-ion lithophile elements(LILEs)and depleted in heavy rare earth elements(HREEs)and high field strength elements(HFSEs),exhibiting ?LREE/?HREE = 19.9 ? 20.5,(La/Yb)N = 27.9 ? 28.9),and weakly negative Sr,P,Ti and Eu anomalies(Eu/Eu* = 0.45 ? 0.46).Compared with the Diaoyutai monzonite granite in western Liaoning,the Chengzitan gneissic quartz diorite and Ambo granitic gneiss in southern Liaoning have relatively low SiO2(61.85% ? 63.85%),and relatively high MgO(1.08% ? 2.83%)and FeOT contents(3.42% ? 5.49%),being typical of high potassium calc-alkaline series of metaluminum rocks.They are enriched in LREE,depleted in HREE(?LREE/?HREE = 10.7 ? 20.6,(La/Yb)N = 12.88 ? 28.88),and show weak Eu negative anomaly(Eu/Eu* = 0.80 ? 1.04).Moreover,the Late Neoarchean granitic rocks in the western and southern Liaoning both show obvious Sr,P,Ti depletion and Pb positive anomalies.3.Hf-Nd isotopic compositions of the Late Neoarchean granitic rocks in western and southern Liaoning Magmatic zircons in the Diaoyutai monzonite granite in western Liaoning all show positive ?Hf(t)values ranging from 1.0 to 4.5,and their t DM1 ages vary from 2782 to 2654 Ma;Magmatic zircons in Chengzitan gneissic quartz diorite also have positive ?Hf(t)values ranging from 0.4 to 3.1,and their t DM1 ages vary from 2798 to 2741 Ma;Magmatic zircons in Ambo granitic gneiss in southern Liaoning show relatively high ?Hf(t)values ranging from 4.5 ? 5.9 with the t DM1 ages vary from 2656 to 2595 Ma.The 147Sm/144 Nd ratios of apatite in Anbo granitic gneiss are between 0.0930 and 0.1516,with homogeneous 143Nd/144 Nd ratios of 0.5112 ? 0.5115 and initial ?Nd(t)values between-10.02 and 8.05.4.Petrogenesis and tectonic setting of the Late Neoarchean granitic rocks in western and southern Liaoning The Neoarchean granitic rocks in western and southern Liaoning are characterized by high SiO₂,low MgO and high Na₂O+K₂O contents,typical of metaluminum to weak peraluminum and high potassium calc-alkaline granites.They were characterized by enrichment in LREEs and LILEs and depletion in HREEs and HFSEs and weakly negative Sr,P,Ti and Eu anomalies.Magmatic zircons all have positive ?Hf(t)values ranging from 0.4 to 5.9,and their t DM1 ages vary from 2798 to 2595 Ma with a peak age of 2740 Ma,which is consistent to the most important Neoarchean crustal accretion event in the NCC.Combined with the regional tectonic evolution,we suggest that the Neoarchean granitic rocks in the western and southern Liaoning region were derived from partial melting of the juvenile lower crustal materials and formed in the arc environment related to the plate subduction.