14:45 〜 15:00
[SMP27-11] Mixing, fluid Infiltration, Leaching and Deformation (MILD) processes on the slab-mantle wedge interface recorded in the Dalrymple Amphibolite
キーワード：Dalrymple Amphibolite, mechanical mixing, fluid infiltration, leaching, slab-mantle wedge interface
The boundary between the subducting slab and the overlying mantle wedge, i.e. slab-mantle wedge interface, is a dynamically evolving site of complex physical and fluid-related chemical processes which directly affect the chemistry of arc lavas. Localities preserving rocks that once represent the slab-mantle wedge interface thus provide unique insights to what comprises this boundary and the processes that occur on it. One such locality is the Dalrymple Amphibolite exposed at the base of the ultramafic section of the Palawan Ophiolite in Palawan Island, Philippines.
In this work, the whole rock composition of the blocks and the surrounding matrix of the Dalrymple Amphibolite is investigated to determine the protolith of the blocks and the effect of mechanical mixing and fluid infiltration in the matrix of this fossil slab-mantle wedge interface. The major and trace element contents of the metamafic blocks indicate their mid-oceanic ridge basalt origin similar to the mafic lavas of the crustal section of central Palawan Ophiolite.
The matrix surrounding the blocks exhibit highly variable mineral assemblage. In order to determine its petrogenesis, we identified groups of components/elements which behave similarly (Group 1 TiO2, Al2O3, Zr, Th and the light REE; Group 2 Cr, Ni and MgO) by calculating their correlation coefficients. These groups indicate mixing of metasedimentary (metased1) and metamafic (metamafic2A/EA) end-members to form the matrix. The mixing proportions of the end-members were estimated by employing regression analysis wherein the measured concentration of fluid immobile elements (Cr, Ni, Zr, TiO2 and Al2O3) in the matrix samples were fitted against a modelled concentration. The modelled composition of the matrix was determined by changing the end-member and their relative proportion until we obtained highest regression value (r2). The modelled and the measured matrix compositions were then used as the original (unmetasomatized) rock and the altered rock respectively in the isocon analysis, assuming that TiO2, Al2O3, Cr, Nd, Zr, and Hf are immobile.
This procedural workflow helped distinguish end-member components, estimate their mixing ratios, and determine the effects of infiltrating fluids. The metasediment / (metasediment + metamafic) ratios of the modelled matrix are high, generally above 50%. This indicates that the whole rock composition of the matrix was controlled by mixing of a subordinate amount of metamafic blocks in a metasedimentary-dominated shear zone. This is supported by the Cr-Nb content of rutile grains included in the matrix samples which indicate mixed metamafic and metapelitic signatures. The metamafic-metasedimentary dominated matrix in the Dalrymple Amphibolite contrasts with other high-pressure/temperature (P/T) type metamorphic terranes which are dominated by low-T minerals (serpentine, Mg-chlorite, and talc) derived from an ultramafic end-member, and could be reflective of conditions in warmer subduction zones.
Mass balance calculations indicate that the measured matrix compositions record losses in fluid mobile components (e.g. SiO2, CaO, P2O5). Losses in other elements/components (e.g. MnO, HREE) are more variable among matrix samples and are controlled by their mineral assemblage (e.g. presence or absence of garnet). This suggests a prograde fluid infiltration which likely occurred following the mixing process and preferentially leached out elements which are either fluid-mobile (e.g. SiO2, CaO, P2O5) or are not incorporated into the growing minerals in the matrix (e.g. HREE on garnet). Gains in K2O, Rb, and Ba in the matrix samples are meanwhile linked to the extent with which their minerals are replaced by secondary phases (e.g. kyanite replaced by muscovite). A later hydration event linked to retrograde metamorphic stage likely occurred and possibly masked the original loss of these fluid-mobile elements in the matrix samples during the earlier fluid-rock interaction.
In this work, the whole rock composition of the blocks and the surrounding matrix of the Dalrymple Amphibolite is investigated to determine the protolith of the blocks and the effect of mechanical mixing and fluid infiltration in the matrix of this fossil slab-mantle wedge interface. The major and trace element contents of the metamafic blocks indicate their mid-oceanic ridge basalt origin similar to the mafic lavas of the crustal section of central Palawan Ophiolite.
The matrix surrounding the blocks exhibit highly variable mineral assemblage. In order to determine its petrogenesis, we identified groups of components/elements which behave similarly (Group 1 TiO2, Al2O3, Zr, Th and the light REE; Group 2 Cr, Ni and MgO) by calculating their correlation coefficients. These groups indicate mixing of metasedimentary (metased1) and metamafic (metamafic2A/EA) end-members to form the matrix. The mixing proportions of the end-members were estimated by employing regression analysis wherein the measured concentration of fluid immobile elements (Cr, Ni, Zr, TiO2 and Al2O3) in the matrix samples were fitted against a modelled concentration. The modelled composition of the matrix was determined by changing the end-member and their relative proportion until we obtained highest regression value (r2). The modelled and the measured matrix compositions were then used as the original (unmetasomatized) rock and the altered rock respectively in the isocon analysis, assuming that TiO2, Al2O3, Cr, Nd, Zr, and Hf are immobile.
This procedural workflow helped distinguish end-member components, estimate their mixing ratios, and determine the effects of infiltrating fluids. The metasediment / (metasediment + metamafic) ratios of the modelled matrix are high, generally above 50%. This indicates that the whole rock composition of the matrix was controlled by mixing of a subordinate amount of metamafic blocks in a metasedimentary-dominated shear zone. This is supported by the Cr-Nb content of rutile grains included in the matrix samples which indicate mixed metamafic and metapelitic signatures. The metamafic-metasedimentary dominated matrix in the Dalrymple Amphibolite contrasts with other high-pressure/temperature (P/T) type metamorphic terranes which are dominated by low-T minerals (serpentine, Mg-chlorite, and talc) derived from an ultramafic end-member, and could be reflective of conditions in warmer subduction zones.
Mass balance calculations indicate that the measured matrix compositions record losses in fluid mobile components (e.g. SiO2, CaO, P2O5). Losses in other elements/components (e.g. MnO, HREE) are more variable among matrix samples and are controlled by their mineral assemblage (e.g. presence or absence of garnet). This suggests a prograde fluid infiltration which likely occurred following the mixing process and preferentially leached out elements which are either fluid-mobile (e.g. SiO2, CaO, P2O5) or are not incorporated into the growing minerals in the matrix (e.g. HREE on garnet). Gains in K2O, Rb, and Ba in the matrix samples are meanwhile linked to the extent with which their minerals are replaced by secondary phases (e.g. kyanite replaced by muscovite). A later hydration event linked to retrograde metamorphic stage likely occurred and possibly masked the original loss of these fluid-mobile elements in the matrix samples during the earlier fluid-rock interaction.