14:45 〜 15:00
[SMP25-17] Petrological characteristics of the slab-mantle interface during incipient subduction: insights from Dalrympole Amphibolite, Palawan, Philippines
キーワード：Slab-mantle interface, melange, Dalrympole amphibolite, Thermobarometry
The slab-mantle interface in the earliest stages of subduction are preserved in some ophiolites as thin slivers of amphibolite to granulite-facies rocks collectively referred to as metamorphic soles. Observation of most metamorphic soles reveal that they are composed of relatively coherent slices which preserve high geothermal gradients (~25 °C/ km) (e.g. Agard et al. 2018). The boundary between the subducting slab and the overlying mantle in a mature subduction zone on the other hand, is typically composed of eclogite-blueschist blocks surrounded by serpentine and/or chlorite- rich matrix (e.g. Bebout and Penniston-Dorland, 2016). The cooler gradients (4-10 °C/ km; Guillot et al., 2009) and fluid-rich nature of this setting, allows the extensive mixing of crustal and mantle components resulting in a hybridized matrix composition (e.g. Bebout and Penniston-Dorland, 2016).
In this study, we present the petrological characteristics and P-T history of the Dalrympole Amphibolite which correspond to the slab-mantle interface of an Eocene subduction zone in its infancy (Encarnacion et al., 1995). This unit underlies the residual mantle peridotites of the Palawan Ophiolite composed of harzburgite-dunite interlayers. Similar to other metamorphic soles (e.g. Oman), the Dalrympole amphibolite is predominantly composed of high grade metamafic blocks (i.e. amphibolites ± Grt, Ep) with only minor metasedimentary units. Unlike most soles which are preserved as coherent sheets however, the Dalrympole Amphibolites is a melange complex with 1-2 m blocks surrounded by a highly sheared matrix. The matrix apparently represents a hybridized composition (Ky-Hbl-Bt-Ilm±Grt) resulting from the mixing of sedimentary and mafic components derived from the downgoing slab.
Grt porphyroblasts in the garnet amphibolite block 214-TK9, preserves prograde zonation marked by a Mn-rich, Mg-poor core (= Alm46-51Prp10-19Grs15-20Sps15-25) and a Mn-poor, Mg-rich rim (= Alm38-41Prp30-35Grs13-15Sps13-17). Applying Zr-in-Rt thermometry (Tomkins et al., 2007) and Qz-in-Grt barometry (Angel et al., 2017) to Rt and Qz inclusions respectively, in the core and rim of the garnet porphyroblasts consistently reflect prograde metamorphism from the core (~625°C, 11.5 kbar) towards the rim (~700°C, 13 kbar). The same thermobarometric techniques were applied to the matrix sample 214-TK2 in order to constrain the P-T conditions leading to the formation of the matrix. Rutile inclusions in Ky, and Qz hosted in Grt which are in turn included in Ky were used. The Grt (=Alm35-36Prp28-32Grs16-19Sps16-19) in this sample are fine grained and do not preserve clear zonation. P-T conditions preserved in the matrix (~700°C, 13 kbar) are comparable to the rim stage of the metamafic blocks.
The similar peak metamorphic conditions constrained in the matrix and the metamafic blocks in Dalrympole Amphibolite suggests that the matrix-forming deformation possibly started en route towards depths of around ~45 km (Jagoutz and Behn, 2013). Furthermore, the paleogeothermal gradient preserved in the Dalrympole Amphibolite (~16 °C/km) and its block-and-matrix structure are atypical of metamorphic soles but are also warmer than commonly observed for a developed subduction channel in a mature arc. These characteristics are more comparable to early high T melanges (e.g. Agard et al. 2018) suggesting that the Dalrympole Amphibolite may correspond to the slab-mantle interface of a young subduction complex already transitioning from the much warmer conditions when subduction was first initiated.
In this study, we present the petrological characteristics and P-T history of the Dalrympole Amphibolite which correspond to the slab-mantle interface of an Eocene subduction zone in its infancy (Encarnacion et al., 1995). This unit underlies the residual mantle peridotites of the Palawan Ophiolite composed of harzburgite-dunite interlayers. Similar to other metamorphic soles (e.g. Oman), the Dalrympole amphibolite is predominantly composed of high grade metamafic blocks (i.e. amphibolites ± Grt, Ep) with only minor metasedimentary units. Unlike most soles which are preserved as coherent sheets however, the Dalrympole Amphibolites is a melange complex with 1-2 m blocks surrounded by a highly sheared matrix. The matrix apparently represents a hybridized composition (Ky-Hbl-Bt-Ilm±Grt) resulting from the mixing of sedimentary and mafic components derived from the downgoing slab.
Grt porphyroblasts in the garnet amphibolite block 214-TK9, preserves prograde zonation marked by a Mn-rich, Mg-poor core (= Alm46-51Prp10-19Grs15-20Sps15-25) and a Mn-poor, Mg-rich rim (= Alm38-41Prp30-35Grs13-15Sps13-17). Applying Zr-in-Rt thermometry (Tomkins et al., 2007) and Qz-in-Grt barometry (Angel et al., 2017) to Rt and Qz inclusions respectively, in the core and rim of the garnet porphyroblasts consistently reflect prograde metamorphism from the core (~625°C, 11.5 kbar) towards the rim (~700°C, 13 kbar). The same thermobarometric techniques were applied to the matrix sample 214-TK2 in order to constrain the P-T conditions leading to the formation of the matrix. Rutile inclusions in Ky, and Qz hosted in Grt which are in turn included in Ky were used. The Grt (=Alm35-36Prp28-32Grs16-19Sps16-19) in this sample are fine grained and do not preserve clear zonation. P-T conditions preserved in the matrix (~700°C, 13 kbar) are comparable to the rim stage of the metamafic blocks.
The similar peak metamorphic conditions constrained in the matrix and the metamafic blocks in Dalrympole Amphibolite suggests that the matrix-forming deformation possibly started en route towards depths of around ~45 km (Jagoutz and Behn, 2013). Furthermore, the paleogeothermal gradient preserved in the Dalrympole Amphibolite (~16 °C/km) and its block-and-matrix structure are atypical of metamorphic soles but are also warmer than commonly observed for a developed subduction channel in a mature arc. These characteristics are more comparable to early high T melanges (e.g. Agard et al. 2018) suggesting that the Dalrympole Amphibolite may correspond to the slab-mantle interface of a young subduction complex already transitioning from the much warmer conditions when subduction was first initiated.