Increased Magmatic Water Content--The Key to Oligo-Miocene Porphyry Cu-Mo ± Au Formation in the Eastern Gangdese Belt, Tibet.

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. Oligocene and Miocene magmatic rocks crop out as small-volume intrusions and are associated with several large- to giant-sized porphyry Cu-Mo ± Au deposits, as well as numerous smaller porphyry and skarn deposits. In contrast, earlier, more voluminous Paleocene-Eocene magmatism is only known to be associated with three small porphyry deposits. Erosional loss of subvolcanic porphyry systems from the older Paleocene-Eocene sequence is not thought to be a controlling factor in this temporal difference of porphyry deposit distribution, because exhumation rates after 55 Ma were relatively low and coeval Linzizong volcanic rocks are extensively preserved in the Gangdese belt. Instead, we suggest that differences in magmatic history and petrogenesis led to this restricted temporal distribution of porphyry deposits. The Paleocene-Eocene magmas were generated during the onset of collision between India and Asia and were triggered by rollback of the Neo-Tethyan oceanic slab. This magmatism records the final stage of subduction of oceanic lithosphere beneath southern Tibet, and the relatively low water contents of these magmas may reflect final dehydration of the remnant Neo-Tethyan slab. After oceanic slab breakoff at ~40 to 38 Ma, the Greater India slab began to subduct beneath the Gangdese belt until hard collision with the Indian continent at ~35 Ma and breakoff of the Greater India slab at ~25 to 10 Ma. This last event is suggested to have caused asthenospheric mantle upwelling and partial melting of subduction-modified, hydrated Tibetan lithosphere. The resultant partial melts may have remobilized metals residual in the deep lithosphere from previous arc magmatism, giving rise to a suite of postsubduction porphyry-type ore deposits in the Oligo-Miocene. [ABSTRACT FROM AUTHOR]