Geochemistry of granitic melts produced during the incongruent melting of muscovite: Implications for the extraction of Himalayan leucogranite magmas

Abstract

Granites formed from the incongruent melting of muscovite preserve evidence of chemical disequilibrium, both between restite and melt, and within individual phases during crystallization. Miocene leucogranites from the Himalayas exemplify melt formation from a pelitic source through muscovite breakdown. Trace elements Rb, Sr, and Ba reside almost entirely in micas and feldspars in the protolith and therefore are appropriate for quantitative modeling. Equilibrium between melt and restite was established for Sr under fluid phase‐absent conditions, but some disequilibrium is suggested by Rb abundances, consistent with known diffusion rates for these elements. Abundances of Zr and Ce in the granites, which provide essential structural constituents of the accessory phases zircon and monazite, indicate that the Himalayan granite melts were saturated in these elements if formed at temperatures of 670–700°C but slightly undersaturated if formed at somewhat higher temperatures. Ion microprobe traverses of Y and heavy rare earth element abundances across single garnets from Himalayan leucogranites provide evidence for disequilibrium between melt and phenocryst during garnet growth, thus constraining the evolution of magmatic compositions during melt formation and crystallization. In some cases, progressive melting of the protolith and crystallization of garnet from the melt appear to have been concomitant, indicating that minimum solidus temperatures were maintained throughout the melting event. This suggests that thermal buffering rather than melt extraction provided the rate‐determining step in magma evolution.