Generation of Continental Alkalic Mafic Melts by Tholeiitic Melt–Mush Reactions: a New Perspective from Contrasting Mafic Cumulates and Dikes in Central Tibet

Abstract

Abstract Alkalic basalts from continental regions have been widely used to fingerprint chemically and lithologically distinct components in the mantle. However, superimposed on mantle source effects are the effects of intra-crustal magma differentiation, which may be underestimated when exclusively considering fractional crystallization (FC) in melt-rich magma chambers. Here we present extensive mineralogical and geochemical data for mafic cumulates and dolerite dikes from central Tibet to demonstrate that tholeiitic magma differentiation occurred in crystal-rich mushes and melt-filled fractures. The mafic cumulates provide a snapshot of the percolation of water-poor (<0.9 wt% H2O), oxide-undersaturated, tholeiitic melts through slightly hotter mushes (50–60% crystals) with a framework of primitive olivine ± plagioclase crystals. The highly corroded olivines indicate that thermo-chemical disequilibrium between the later percolating melts and pre-existing crystal framework triggered melt–mush reactions in an open system. High-temperature (>1060°C) reactions were accompanied by a decreasing melt mass, causing the initial tholeiitic melts to evolve into alkalic mafic melts during slow cooling. This process involved the dissolution and reprecipitation of olivine, plagioclase, and clinopyroxene as well as the late crystallization of high-TiO2 (up to 3.4 wt%) clinopyroxene and kaersutite. In contrast, the same tholeiitic magmas (i.e. dolerite dikes) that intruded the cold country rocks along fractures produced silica-rich residual melts saturated in Fe–Ti oxides and sodic plagioclase by FC during rapid cooling. The residual melt was always in equilibrium with the instantaneous crystal during closed-system FC, so there was no reaction between the interstitial melt and adjacent crystal even after a high degree of crystallization. Alkalic mafic melts that can be formed by melt–mush reactions exhibit strong enrichment of incompatible element contents (e.g. Ti and alkali and rare earth elements) and ratios (e.g. Ce/Y), but little Si enrichment and Mg depletion, compared with the parental tholeiitic magma, which cannot be reproduced by simple FC. We argue that tholeiitic melt–mush reactions at mid- to lower-crustal pressures (≥0.5 GPa) are a new mechanism for the generation of interstitial alkalic mafic melts in continental mush reservoirs. Continental alkalic basalts could be produced by this mechanism if such interstitial melts were concentrated into melt lenses by compaction and then transported rapidly upward to the surface by focused flow.