Granitic magma heterogeneity induced by anatectic mechanism conversion: Evidence from cognate leucosomes, north Qaidam, NW China

Abstract The transition between fluid-present and fluid-absent melting mechanisms in granitic systems remains poorly constrained for intermediate scenarios involving limited free water, hindering understanding of the ways in which anatectic processes govern granite diversity. This study integrated petrological, geochemical, zircon U-Pb geochronological, and phase equilibrium modeling data from granitic gneisses in the northern Qaidam orogen to address this gap. Phase equilibria modeling revealed that the Xitieshan granitic gneisses experienced prolonged partial melting during exhumation, driven by coupled fluid-present and fluid-absent melting. Early exhumation triggered fluid-present melting via free water, generating Na-rich trondhjemitic melts through plagioclase consumption. Progressive melt accumulation then shifted melting to a fluid-absent regime dominated by muscovite breakdown, producing K-rich granitic melts. This mechanism transition during slab exhumation created two distinct synexhumation granites: K-rich mica granites and Na-rich trondhjemites. Zircon U-Pb ages (432 ± 6 Ma) and phase equilibria modeling confirmed that melt compositions evolved dynamically during decompression, with trondhjemitic melts dominating at higher pressures and granitic melts crystallizing at shallower depths. Sr-Nd isotopic data further revealed that both granite types originated from a single granitic gneiss protolith. Our results demonstrate that a single protolith can yield compositionally diverse granitic melts through sequential anatectic mechanisms under evolving fluid conditions. This process provides a unified model for coeval granite magmatism in exhumed continental slabs and highlights mechanism conversion as a key driver of granite diversity in collisional orogens.