The slab‐mantle wedge interface of an incipient subduction zone: Insights from the P‐T‐D evolution and petrological characteristics of the Dalrymple Amphibolite, Palawan Ophiolite, Philippines

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.