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MAGMATISM IN THE MESOZOIC EXTENDING OROGENIC PROCESS OF DA HINGGAN MTS

Shao Ji'an(Peking University, Beijing, 100871)Zhang Lüqiao(Inner Mongolia Institute of Geology, Huhhot, 010020)Mu Baolei(Peking University, Beijing, 100871)  
This paper explains briefly the Mesozoic extending orogenic process of Da Hinggan Mts , concentrating especially in petrologic and geochemical features of the late Mesozoic volcanic and plutonic rocks. There exists a suite of intraplate A type granites formed in an extensional environment. The volcanic rocks are potassic and trachytic. The granites and volcanic rocks have low N ( 87 Sr)/ N ( 86 Sr) values (0 704~0 708) and positive ε (Nd, t ) values (1~4). All of these are suggested as the results from the crust mantle miscible magma, which is created by underplating These rocks, analyzed jointly with the extending structures formed in the process of magmatic eruption and emplacement, can be considered as an important evidence of intracontinental extending orogeny Moreover, other evidences have been found by the authors in the area, including early Mesozoic mantle source mafic cumulate complex and intrusive rocks, Lower—Middle, Jurassic and Lower Cretaceous basic swarm dykes, and ultramafic hornblendite, limburgite, and alkaline olivine basalts of the Lower Cretaceous (130~120 Ma) Thus it is demonstrated that the late Mesozoic granite volcanic rock activity in the Da Hinggan Mts is only a stage of the whole magmatic evolution in an extensional settings
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