Initiation of Partial Melting in Nominally Anhydrous Eclogite‐Facies Garnet Pyroxenite During Exhumation

ABSTRACT Whether nominally anhydrous garnet pyroxenite can melt during exhumation from eclogite‐facies P–T conditions and, if so, what chemical composition such melts would have is largely unexplored. Here, we document petrographic, chemical, geochronological and thermodynamic modelling evidence for the initiation of in situ partial melting of nominally anhydrous eclogite‐facies garnet pyroxenite (garnet + omphacite/sodian augite + rutile) from the northern Sulu belt, China. The garnet pyroxenite forms the cores of amphibolitized boudins hosted in strongly deformed migmatitic felsic gneiss. The garnet pyroxenite is strongly foliated and comprises interlayered garnet‐rich and clinopyroxene‐rich layers. The latter contain abundant pockets (0.2–1 mm across) comprising volumetrically dominant plagioclase with euhedral grains of either clinopyroxene (Cpx + Pl leucosome) or amphibole (Amp + Pl leucosome), and locally grain‐boundary films of K‐feldspar (K‐bearing leucosome). These leucosome pockets are preferentially located at triple junctions between clinopyroxene grains and have microstructural features consistent with their crystallization from locally derived melt, including plagioclase seams extending along clinopyroxene grain boundaries in the garnet pyroxenite host. The calculated bulk composition of the leucosome pockets is consistent with melting through a process combining breakdown of clinopyroxene ± garnet, but requiring a contribution of mobile components (including H 2 O and K) from the host gneiss, yielding melts of gabbroic to monzonitic composition. U–Pb analysis of euhedral zircon cores yield Neoproterozoic ages of c. 740 Ma, whereas zircon rims yield Triassic metamorphic ages of 244–221 Ma, consistent with the late prograde to early retrograde evolution of the garnet pyroxenite during subduction of the Yangtze craton. Thermodynamic modelling shows that gradients in μ H 2 O and μ K 2 O (where μ is chemical potential) generated during decompression from eclogite‐facies P–T conditions drove diffusive transfer of material from the gneiss to the garnet pyroxenite. This interpretation is consistent with the elevated contents of large ion lithophile and light rare earth elements in most of the leucosome pockets relative to host clinopyroxene. Thirteen leucosomes are nepheline normative and five are hypersthene normative, suggesting that partial melting of nominally anhydrous garnet pyroxenite produces alkaline to subalkaline basaltic melts. Our findings indicate that clinopyroxene‐breakdown melting is an intrinsic consequence of suprasolidus decompression from eclogite‐facies P–T conditions. Although the volume of melt generated in the garnet pyroxenite was minor, our study may have wider implications for the genesis of alkalic OIB magmas.