Reactive Melt Flow in the Continental Arc Root: Insights for the Transition From Gabbronorite to Garnet Granulite in the Central Qilian Belt, NE Tibet

Abstract

Understanding reactive melt flow is crucial for advancing our knowledge of crustal differentiation; however, the mechanisms governing melt migration remain debated, particularly in deep magmatic arc environments. A composite sample from the Central Qilian continental arc, NE Tibet, preserves the transition from hornblende gabbronorite to garnet granulite, offering a rare opportunity to study reactive melt flow in the arc root. Thermodynamic modeling showed that the hornblende gabbronorite was metastable under lower-crustal conditions (6.2–8.2 kbar, 900–931°C). To equilibrate with the normal thermal regime of the middle to lower crust, it underwent near-isobaric cooling to 816 ± 16°C, whereas its transformation into garnet granulite occurred under higher pressure and temperature conditions (10.2–12.2 kbar, 833–865°C). The sample records melt-rock interactions during the transition from the magmatic stage to garnet granulite facies metamorphism. Reactive melts infiltrated grain boundaries, inducing mineral replacement via dissolution-precipitation and metasomatism. Enriched rare earth elements (REEs) in blue-green pargasite, reaction microstructures and hydrous products attest to melt-rock interactions involving Mg-Sr-REE-enriched silicate melts. Trace element mapping reveals a correlation between reaction microstructures and high-Sr plagioclase bands, highlighting grain boundary pathways for melt migration. Replacement microstructures illustrate permeable reactive melt flow pathways within the lower arc crust. Reactive melt flow enhanced chemical disequilibrium and mineralogical reorganization, driving textural maturation through coupled dissolution-reprecipitation. This pervasive melt-rock interaction mechanism likely governs both crustal differentiation and the development of high Sr arc magmatic signatures.