Anatexis of former arc magmatic rocks during oceanic subduction: A case study from the North Wulan gneiss complex

Abstract

Migmatites are widespread in the North Wulan gneiss complex from the South Qilian–North Qaidam orogenic belt, but their petrogenesis and ages are poorly constrained. Here, an integrated study of petrography, whole-rock geochemistry, geochronology, zircon trace element and Hf isotope analysis deciphers the nature and timing of partial melting in migmatitic amphibole-biotite gneiss. Zircon U–Pb geochronology reveals that the protoliths crystallized at 506–494 Ma followed by metamorphism and anatexis at ca. 465 to 450 Ma. Hafnium isotope compositions of inherited cores and anatectic rims are very similar, suggesting that partial melting occurred in a relatively closed isotopic system and new zircon rims grew via dissolution–reprecipitation of pre-existing zircon cores. Anatexis occurred by water-fluxed melting of mafic-intermediate rocks through the breakdown of biotite and growth of peritectic amphibole. The protolith of the migmatites records Cambrian arc magmatism in an active continental margin, which was induced by northward subduction of the South Qilian ocean slab. Contemporary arc-like magmatism and high-temperature/low-pressure metamorphism in the region suggest that anatexis in the North Wulan gneiss complex likely took place in a continental arc setting, which reflects the reworking of former arc magmatic rocks in a late stage of oceanic subduction.