Re-Os age for molybdenite from the Gangdese porphyry copper belt on Tibetan plateau: Implication for geody- namic setting and duration of the Cu mineralization

摘要

The Gangdese porphyry copper belt consists of one large and five middle-small deposits in addition to dozens of ore-bearing porphyry bodies. The belt trends 350 km long along the EW-striking Gangdese batholith, and locally occurs as a string of beads extending about 50 km within the SN-trending rifting zones (grabens) on the Tibetan plateau. Monzonitic granite-porphyry and quartz monzonitic porphyry, as dominant host rocks, are shoshonitic and potassic calc-alkaline. Available dating data indicate that the ages of the shallow-level emplacement for these porphyries vary from 10 Ma to 18 Ma, which are identical to that of associated potassic calc-alkaline volcanic rocks (10—15 Ma) and mafic dykes (13—18 Ma). The timing and duration of Cu mineralization events are constrained by Re-Os ages for molybdenites from three porphyry copper deposits in the Gangdese porphyry copper belt. Five molybdenite samples from the Nanmu deposit yielded an ~(187)Re-~(187)Os isochron with a highly precise age of (14.76±0.22) Ma; six molybdenites from the Chongjiang deposit gave an isochron age of (14.04±0.16) Ma. Re-Os model ages for two molybdenite samples from Lakang'e deposit vary from 13.5 Ma to 13.6 Ma, which are basically identical to isochron ages mentioned above. All the thirteen samples from these three deposits yielded a linear array in the isochron diagram with a higher correlation coefficient of 0.99719 and an isochron age of (14.39±0.22) Ma (1σ error), suggesting an coeval event of the Cu mineralization and a shared source of ore materials for the Gangdese porphyry copper belt. Compared with the longer-lived felsic magmatic-hydrothermal system, the Cu mineralization is a relatively short event with duration of about 0.5 Ma, and usually occurs in the later-stage of the complicate magmatic system. The emplacement age of the Gangdese porphyries indicates that they intruded after the rapid rising of the Gangdese Mountains at 21—23 Ma, and formed in a post-collision crustal extension environment. Constraint of the NS-trending rifting zones (grabens) on localization of these porphyry bodies implies that the minor east-west stretching dates back to approximately 18 Ma. Highly precise Re-Os age of the Cu mineralization event indicates that a significant east-west extension has commenced at about (14±0.5) Ma ago, which is identical to that of the normal faulting in central Tibet (13.5 Ma) and southern Tibet (14 Ma). The strong extension not only provided dilatant conduits for the migration and emplacement of the porphyry magmas pooling near the base of lithosphere, but also resulted in the rapid rising of a large volume of volatile, thus in turn constraining the temporal-spatial localization of the Gangdese porphyry Cu belt.