In the Gangdese belt of southern Tibet, Paleocene-Eocene magmas record the final stage of Neo-Tethyan subduction and are associated with few economic porphyry deposits. In contrast, magmas formed during later stages of the India-Asia collision in the Oligo-Miocene are associated with several large porphyry Cu-Mo ± Au deposits, especially in the eastern part of the belt (~89°E–93°E). In a previous study, we showed that these Oligocene-Miocene magmas were more hydrous than the earlier Paleocene-Eocene magmas. In this study, we show that the later magmas were also more oxidized. Paleocene-Eocene rocks from the eastern Gangdese belt are characterized by low zircon Ce4+/Ce3+ ratios (mostly <50; 6.0–66.8; average = 25.7 ± 18.4, n = 26) and low to moderate ΔFMQ values (−1.2 to +0.8, average = −0.2 ± 0.8, n = 5; estimated from ilmenite-magnetite mineral pairs). In comparison, Miocene igneous rocks from the eastern Gangdese belt show higher zircon Ce4+/Ce3+ ratios (mostly >50; 32.3–141.9, average = 74.3 ± 30.1, n = 33) and higher fO2 values (ΔFMQ 0.8–2.9, average = 1.8 ± 0.8, n = 6; estimated from magnetite-ilmenite pairs).
Estimates of magmatic oxidation state from amphibole compositions also show an increase from ΔFMQ 1.2 to 2.1 (average = 1.6 ± 0.2, n = 40) in the Paleocene-Eocene to 2.0 to 2.8 (average = 2.5 ± 0.2, n = 58) in the Miocene. Sparse whole-rock Fe3+/Fe2+ ratios show the same general trend. The amphibole results are systematically shifted to higher ΔFMQ compared to data from magnetite-ilmenite pairs, but their trend is internally consistent.
The higher oxidation states (and water contents) of Miocene igneous rocks from the eastern Gangdese belt may explain their unique association with large porphyry deposits in Tibet, because it has been shown from other studies that the potential for hydrous calc-alkaline magmas to transport Au is maximized near ΔFMQ 1.0, and Cu at higher fO2 (ΔFMQ > 1.0). In comparison, the somewhat less oxidized and less hydrous Paleocene- Eocene magmas would have been less fertile for the formation of such deposits.
We suggest that these differences reflect the tectonomagmatic evolution of the Cenozoic Gangdese collisional orogen, from early collision-related magmatism in the Paleocene-Eocene to late collisional magmatism in the Miocene. Asthenospheric upwelling following slab breakoff in the Miocene caused interaction between mantle-derived magmas and previously subduction modified Tibetan lithosphere. The resultant evolved magmas had high oxidation states and water contents, which are favorable properties for the subsequent formation of magmatic-hydrothermal porphyry Cu-Mo ± Au deposits.