Metasomatic enrichment of the sub-arc mantle: Evidence from the petrography and geochemistry of the xenolith-bearing andesites from Dinem Island, Batanes, Philippines

Abstract

The Babuyan Segment of the Luzon Arc in the Philippines is divided into an older west volcanic chain (WVC) and a younger east volcanic chain (EVC) based on the geochemical and geochronological characteristics of their volcanic rocks. Mantle and crustal xenoliths have been previously reported in the volcanic rocks of young Batan (EVC) and Sabtang (WVC). In this work, we present new petrographic and geochemical data on volcanic rocks from Dinem Island (EVC), which also host mantle and crustal xenoliths. These volcanic rock data are compared to the data of other volcanic centers in the region to better understand magmatic-metasomatic processes that occur beneath island arcs.The Dinem volcanic rocks are generally porphyritic andesites, with plagioclase and hornblende phenocrysts, minor clinopyroxene and Fe-Ti oxides. Correlation between major oxides (i.e., K2O, Na2O, TiO2, CaO and Al2O3) and trace elements (i.e., Ni and Cr) with FeO*/MgO suggests that the Dinem volcanic rocks are related by magmatic differentiation. These rocks also record enrichment in large ion lithophile elements (e.g. Rb, Ba, Th) and depletion in high field strength elements (e.g. Nb, Ta, Ti), which are typical of arc volcanic rocks. The volcanic rocks from Dinem also have elevated (La/Sm)N and Th/Yb and low Ba/Th values relative to WVC volcanic rocks. These trace element ratios and enriched LREE signature suggest that the Dinem and younger Batan volcanic rocks of the EVC were derived from a mantle wedge that is highly metasomatized by slab-derived fluids/melts relative to the source region of the WVC volcanic rocks.