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《Acta Petrologica Sinica》 2007-05
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K-Ar and ~(40)Ar/~(39)Ar age dating of Nimu porphyry copper orefield in Central Gangdese:Constrains on magmatic-hydrothermal evolution and metallogenetic tectonic setting.

LI JinXiang QIN KeZhang LI GuangMing YANG LieKun Key Laboratory of Mineral Resources,Institute of Geology and Geophysics,Chinese Academy of Sciences,Beijing 100029,China  
Gangdese porphyry copper metallogenetic belt,distinct from classical porphyry copper deposits that were occurred in island arc and continent margin setting,were formed in post-collision setting and showed considerable prospective.This article systematically researches the porphyries of Bairong and Tinggong porphyry deposits from Nimu metallogenetic cluster,and confirms its evolving and intrusive sequence from monzogranite to ore-forming monzogranite porphyry to quartz diorite porphyrite to granodiorite porphyry.The K-Ar age for phenocrystic hornblende in porphyroid monzogranite from Bairong copper deposit is 16.9±2.4Ma.The K-At ages for phenocrystic biotites in quartz diorite porphyrite and granodiorite porphyry are 12.3±0.2Ma and 11.5±0.2Ma from Bairong porphyry deposit,13.8±0.2Ma and 13.5±0.3Ma from Tinggong porphyry deposit,correspondingly,its~(40)Ar/~(39)Ar plateau ages are 12.5±0.2Ma and 12.4±0.2Ma,14.9±0.2Ma and 14.2±0.2Ma respectively.These ages indicate quartz diorite porphyrite formed after porphyroid monzogranite,whereas before granodiorite porphyry. The ore-forming process is related to monzogranite porphyry,which the forming age is later than porphyroid monzogranite and slightly earlier than quartz diorite porphyrite and granodiorite porphyry.Sufficient differentiation and evolvement of the complexes contribute to the concentration of ore-forming fluid and gradually enrichment to ore in Nimu metallogenetic cluster.The K-Ar age and~(40)Ar/~(39)Ar plateau age for hydrothermal sericites from altered monzogranite porphyry respectively yield 11.8±0.2Ma and 12.0±0.1Ma in Bairong porphyry copper deposit,and represent the forming age of medium-low temperature alteration and late-mineralization,and its alteration age was consistent with the~(40)Ar/~(39)Ar plateau ages for phenocrystic biotites in quartz diorite porphyrite and granodiorite porphyry from Bairong porphyry copper deposits and published molybdenite Re-Os age was consistent with the~(40)Ar/~(39)Ar plateau ages for phenocrystic biotites in quartz diorite porphyrite and granodiorite porphyry from Tinggong porphyry copper deposit,which imply that the magma cooling and crystallization of quartz diorite porphyrite and granodiorite porphyry from Bairong and Tinggong porphyry copper deposits overlapped temporally with hydrothermal mineralization.Combined with the published ages,the duration limit of Bairong porphyry deposit approximately range from 0.5 to 5Ma,and≈4Ma for Tinggong porphyry deposit,≈5Ma for Chongjiang.The emplacement of porphyries sequence from Nimu metallogenetic cluster was later than post-collisional rapid uplift of Gangdese≈21Ma and≈15Ma the normal fault began to form in Central Gangdese,indicated ore-forming porphyries possibly emplaced the tectonic setting from thickened crust and the rapid uplift of Gangdese mountain to initial extension,namely,the porphyry copper deposits possibly occur in a transition tectonic setting between contractional deformation associated with Gangdese uplift and the initial onset of east-west striking extension.
【Fund】: 青藏973项目斑岩铜矿课题(2002CB412605);; 国家自然科学基金项目(40672068,40334044)资助成果
【CateGory Index】: P597;P618.41
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