~(40)Ar/~(39)Ar Ages of the Ore-bearing Porphyries of the Gangdese Porphyry Copper Belt and Their Geological Significances
QU Xiaoming, HOU Zengqian, LI ZhenqingInstitute of Mineral Resources, CAGS, China, Beijing 100037
The ore-bearing porphyries of the Gangdese porphyry copper belt in the southern Tibet plateau possess the essential characteristics of adakite and formed from partial melting of subducted Tethyan oceanic crusts under eclogite facies conditions. This paper utilizes 40Ar/39Ar step-heating technique to date the ages of biotite and plagioclase phenocrysts of the ore-bearing porphyries sampled from Chongjiang and Lakange deposits. The analysis results indicate that the two copper-Molybdenum deposits have highly consistent mineralization ages despite of their more than 100 km apart each other. The two biotite phenocrysts yield plateau ages of 13. 5 ± 1. 0 Ma and 13. 42 ± 0. 10 Ma, respectively. Meanwhile the two plagioclase phenocrysts yield their plateau ages of 12. 22 ± 0. 05 Ma and 12. 5 ± 1. 3 Ma, respectively. Within analysis error range these age values are consistent with the mineralization age (14 Ma±) of the porphyry copper deposits based on molybdenite Re-Os isochon ages of the corresponding deposits. From the emplacement timing and location of the ore-bearing porphyries it is infered that the Indus plate was subducted in steep angle beneath the Asia continent along the Yaluzangbo suture and enter mantle lithosphere. The Ew-and NWW-striking occurrences of both ore-bearing porphyries and copper mineralization zones suggest that the porphyry emplacement and copper mineralization occurred under a SN-striking extension regime and were synchronous with the uplifting process in which the plateau reached its highest elevation. Hereafter, the stress regime of the plateau changed into EW-striking extention and a series of SN-striking collapse structures formed under gravity effect.
【CateGory Index】： P597