Garnet Amphibolite and Trondhjemite From the Wakasa Area, SW Japan: Evidence for Partial Melting and Fluid–Rock Interaction in a Paleozoic Subduction Complex

ABSTRACT

Partial melting of basaltic oceanic crust in subduction zones is a key process in continental crust evolution, yet natural records of its incipient stage remain scarce. We report a garnet amphibolite–trondhjemite association from the Paleozoic subduction complex of the Wakasa area, SW Japan, where ultramafic rocks of the Oeyama Ophiolite are exposed. Garnet amphibolite and trondhjemite occur as boudinaged layers within pelitic schist associated with serpentinite. Mylonitized trondhjemite is locally overprinted by pegmatitic veins. Quartz‐in‐garnet Raman barometry and Zr‐in‐rutile thermometry indicate metamorphic conditions of ~550°C–710°C and ~0.8–1.5 GPa for the garnet amphibolite, corresponding to high‐pressure amphibolite‐facies metamorphism. Whole‐rock geochemistry suggests a MORB‐like basaltic protolith for the garnet amphibolite, whereas trondhjemite lacking pegmatitic veins exhibits trace element characteristics consistent with an origin involving partial melting of the garnet amphibolite under conditions close to the metamorphic peak. The estimated peak temperature overlaps the H ₂ O‐saturated solidus but is lower than that required for dehydration melting of basaltic crust, suggesting that H ₂ O‐fluxed melting is a plausible mechanism. Zircon U–Pb ages of 340 ± 2 Ma (Th/U < 0.01–0.05) from trondhjemite associated with pegmatitic veins probably record a later fluid‐related overprinting event. These observations are consistent with partial melting of subducted basaltic rocks followed by subsequent fluid activity in a Paleozoic subduction setting.