Coexisting wet and damp magma fractionation in the middle crust: An example from the Gangdese Batholith in southern Tibet

Abstract

Fractional crystallization of mantle-derived magmas is an effective mechanism for developing the chemical stratification of the continental crust at convergent plate margins. However, identifying such a mechanism is a challenge when the complementary cumulate rocks are missing. To address this issue, we present geochronological, mineral chemical, whole-rock geochemical, and zircon Hf isotopic data for the Kazi intrusive complex (hornblende gabbro and monzonite) in the Gangdese Batholith, southern Tibet. Zircon U–Pb dating shows that the Kazi complex formed at 94–90 Ma. The hornblende gabbro contains hornblende crystals with concentric zoning and the monzonite contains hornblende crystals with clinopyroxene inclusions. Mineral-based geothermobarometers indicate that the parental magmas of the Kazi complex first partially crystallized in the middle crust before being emplaced in the upper crust. Whole-rock major and trace element compositions, as well as those of calculated melts in equilibrium with early crystallized clinopyroxenes, reveal that the monzonites experienced clinopyroxene and plagioclase fractionation. The hornblende gabbros yield calculated melt major and trace element compositions in equilibrium with early crystallized hornblendes that are similar to those of adjacent adakitic granitic dikes, indicative of hornblende–melt separation in their generation. These features and the similar depleted zircon Hf isotopic compositions of the various lithologies show that formation of the Kazi complex was controlled primarily by coexisting wet and damp magma fractionation in the middle crust, and that the adakitic geochemical characteristics of some Late Cretaceous granitic dikes in the Gangdese Batholith could be due to hornblende-dominated fractionation of arc magmas.