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In this contribution, we present the petrological characteristics and pressure–temperature–deformation (P–T–D) history of the Dalrymple Amphibolite in Palawan Island, Philippines. This unit occurs below the residual mantle peridotites of the Palawan Ophiolite composed of interlayered dunite and harzburgite. The Dalrymple Amphibolite is predominantly composed of high-grade metamafic blocks with minor metasedimentary units surrounded by a highly sheared hybridized matrix (kyanite + Ca-amphibole + biotite + ilmenite ± garnet). Zr-in-rutile geothermometry and quartz-in-garnet geobarometry of the garnet amphibolite block reveal the P–T conditions of the prograde metamorphism from ~625°C, 11.5 kbar to ~700°C, 13 kbar. Zr-in-rutile geothermometer and the TZARS geobarometer (clinozoisite + rutile + quartz = anorthite + titanite + H2O) further indicate comparable peak metamorphic conditions for the amphibolite and epidote amphibolite (~605–710°C, 10–13 kbar) blocks. Geothermobarometry of the matrix surrounding the blocks reveals similar peak metamorphic conditions of ~700°C and 13 kbar to the garnet amphibolite blocks. The paleogeothermal gradient preserved in the Dalrymple Amphibolite (~16°C/km) and its block-in-matrix structure are atypical of metamorphic soles and are more akin to high-T mélange complexes interpreted to have formed during incipient subduction. These P–T estimates and its block-in-matrix structure suggest that the Dalrymple Amphibolite likely represents the slab–mantle wedge interface of a young arc complex already transitioning from the much warmer paleogeothermal gradients of the slab–mantle wedge interface when subduction was first initiated. The similar peak metamorphic conditions constrained in the matrix and the metamafic blocks in the Dalrymple Amphibolite indicate that the matrix-forming deformation started before the sequence reached peak metamorphic conditions at depths of around 45 km. Furthermore, the petrological and geochemical characteristics of this block-in-matrix sequence and the multiple generation of cross-cutting veins reveal the complex processes that occur in the slab–mantle wedge interface of an arc in its infancy. Our results highlight the prevalence of Ca-amphibole and kyanite over phyllosilicates (e.g., chlorite and serpentine) in the mélange matrix, mixing of crustal components along the slab–mantle wedge interface promoted by multiple deformation events and the ubiquity of fluids throughout its history. Subduction zones with similar geothermal gradients (e.g., warm subduction zones) possibly share these petrological characteristics.

We report contrasting pressure-temperature-time (P-T-t) paths of migmatites developed in the highest-grade metamorphic zone (Grt-Crd zone) and the contact metamorphic zone (Crd-Kfs zone) of the Mikawa area, Ryoke belt, SW Japan to discuss the complex P-T-D-t evolution of the middle crust that experienced pulsed granitoid intrusions. In the Grt-Crd zone, sillimanite-grade high-T metamorphic condition prevailed from ca. 97 Ma to 87 Ma, followed by cooling to ~500 oC, ~4 kbar. The intrusion of gneissose granitoids below the Grt-Crd zone isobarically reheated the Grt-Crd zone rocks again to the sillimanite-grade high-T condition at ca. 84 Ma. This was followed by ca. 71-70 Ma contact metamorphism. Ductile deformation that formed and folded the foliation of migmatites started before ca. 89 Ma and continued at least until ca. 84 Ma in the Grt-Crd zone. On the other hand, ca. 74 Ma age of the Crd-Kfs zone migmatite developed around the Inagawa Granodiorite in addition to ca. 70 Ma age of a syn-tectonic pegmatite vein revealed that the intrusion of “75-69 Ma granitoids” caused partial melting and locally triggered low-strain ductile deformation in their contact aureoles. Comparison with other areas of the Ryoke belt suggests that plutono-metamorphic evolution of the Mikawa and Aoyama areas are similar with each other in that ca. 80 Ma reheating events (i.c. contact metamorphism) are observed, while absence of separate reheating event postdating peak metamorphism in the Yanai area is a rather uncommon feature in the Ryoke belt.