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; 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).

The eastern Gangdese magmatic belt (between ~88°E and 94°E) in southern Tibet preserves a series of Cenozoic collision-related igneous rocks that have formed since the start of the India-Asia collision at ~55 to 50 Ma. The Paleocene-Eocene intrusive rocks have intermediate [La/Yb]_N ratios (average = 12.0 ± 8.6, n = 150, range = 1.15–64.0), intermediate to low Sr/Y ratios (mostly < 40), and negative Eu anomalies (average Eu_n/Eu* = 0.76 ± 0.19, n = 150, range = 0.23–1.33). They are mainly composed of equigranular pyroxene and plagioclase in mafic intrusions or porphyritic plagioclase and quartz in intermediate-felsic intrusions, with minor interstitial amphibole and biotite. In contrast, Oligo-Miocene intrusive igneous rocks have high [La/Yb]_N ratios (Oligocene: average [La/Yb]_N = 42.8 ± 17.0, n = 35, range = 23.2–93.2; Miocene: average [La/Yb]_N = 28.5 ± 8.8, n = 93, range = 6.7–49.1), high Sr/Y ratios (>40), and weak or absent Eu anomalies (Oligocene: average Eu_n/Eu* = 0.99 ± 0.27, n = 35, range = 0.67–1.58; Miocene: average Eu_n/Eu* = 0.95 ± 0.13, n = 93, range = 0.44–1.46), and their mineralogy consists mainly of plagioclase, quartz, and amphibole as phenocrysts. These geochemical and mineralogical characteristics suggest that the Paleocene-Eocene magmas were relatively dry and evolved primarily by fractionation of pyroxene and plagioclase, whereas the Oligo-Miocene magmas were more hydrous and fractionated significant amounts of hornblende and lesser plagioclase prior to upper crustal emplacement.