The Geochronology And Petrogenesis Of Adunchulu Granites In Xinbaerhu Right Banner,inner Mongolia

Granitoid is a significant characteristic of orogenic belt,implying a wealth of information on tectonic evolution and dynamics of orogenic belts.Erguna Massif is located at the east section of Xing'an Mongolian Orogenic Belt in north China,on which a large area of granites and volcanic rocks play an important role in the study of Mongolia-Okhotsk tectonic domain evolution.Combining the previous study achievements,and carrying out the systematic study of field geological investigation,isotope chronology,petrology,mineralogy,geochemistry of granites,this dissertation discusses the geochronology,petrogenesis,and geotectonic environment of Adunchulu granites in Xinbaerhu Right Banner,Inner Mongolia.The region of Adunchulu mainly develops granite porphyry and medium-grain syenogranite.The LA-ICP-MS isotope U-Pb dating results show that they were formed in the early Late Jurassic,with the emplacement and crystallization time of 162-158 Ma.The analysis of major elements shows that these granites belong to peraluminous rocks in high potassium calc-alkaline series,with characteristics of rich K₂O,Na₂O,Al₂O₃ and poor TFe O,Mg O,Ca O.The high differentiation index and low solidification index indicate that the magma experienced strong crystallization differentiation to the late stage of magmatic evolution.The analysis of trace elements shows that these granites have characteristics of LILE(Rb,Th,K,Pb,Nd)enrichment in different degrees and slight HFSE(Ta,Nb,Zr,Hf,Sm)depletion.The conspicuous depletion in P and Ti is related with the crystallization differentiation of accessory mineral(sphene and apatite).The strong negative anomaly of Nb and Ta suggests that the granitic magma might evolve from the partial melting of crustal rocks.The chondrite-normalized REE patterns have a medium-slope right-inclined“V”shape with enrichment in LREE and depletion in HREE.The conspicuous Eu negative anomaly indicates that there was plagioclase crystallization differentiation during the magmatic evolution or more plagioclases were left in the source of granitic magma.The comprehensive analyses indicate that the granite porphyry is I-type granite with high differentiation,while the medium-grain syenogranite is A-type granite.They make up Complex I-and A-type granites.The magma might be associated with partial melting of crustal rocks,and the crystallization differentiation made a difference in the process of magmatic evolution.Controlled by Mongolia-Okhotsk tectonic domain in the Late Jurassic,Adunchulu granites were induced by the lithospheric extensional after the closure of Mongolia-Okhotsk Ocean and orogenesis,suggesting that the Mongolia-Okhotsk tectonic domain evolution was coming to a close.In Quaternary,the internal and external forces have had effect on the granites exposed on the surface,finally forming the typical granite stone eggs.