Abstract

Miocene postcollisional porphyry Cu deposits in southern Tibet are genetically associated with dacitic-rhyolitic intrusions with unusually high Sr/Y ratios (>40), which have been attributed to dehydration melting of garnet amphibolite in a thickened lower crust. To test this hypothesis and examine the hydration state of copper ore–forming high Sr/Y magmas, we utilize a geohygrometer for granitoid rocks, entailing zircon-saturation thermometry and H2O-dependent phase equilibria. The results show that these Tibetan high Sr/Y magmas had dissolved H2O contents >10 wt%, which considerably exceeds the water supply by dehydration melting of basaltic amphibolites (maximum of 6.7 ± 1.4 wt%). Our results indicate that high Sr/Y dacitic-rhyolitic magmas cannot be produced by dehydration melting of basaltic amphibolites. While H2O-added melting of basaltic amphibolites can produce high Sr/Y dacitic-rhyolitic melts, it does not yield high enough Mg# (>50) to match the Tibetan ore-forming porphyries. We propose an alternative model for the genesis of copper ore–forming high Sr/Y magmas in Tibet, and suggest that the high Sr/Y dacitic-rhyolitic porphyries in southern Tibet are residually H2O-enriched, high-pressure differentiation products of hydrous mafic partial melts of Tibetan mantle. This hypothesis is based on the previous investigation of Miocene mafic microgranular enclaves (mantle-derived melts), which define a fractionation trend with, and have Sr-Nd-Hf isotopic compositions similar to, the host Tibetan ore-forming porphyries.

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