Phengite‐Derived Geochemical Signatures of Rear‐Arc Magmatism

ABSTRACT The term ‘rear‐arc’ designates volcanic sequences that are spatially distal to the volcanic arc axis within trench‐arc‐basin systems. Magmas of rear‐arc volcanoes form at depths over 100 km, considerably deeper than volcanic front magmas (~80 km). The geochemical fingerprint of the magma also differs in three ways: it is (1) enriched in TiO 2 and K 2 O; (2) enriched in large‐ion lithophile elements (LILEs: e.g., Ba, Rb, Sr) and LREEs; and (3) and has less depletion of Nb and Ta. An analysis of the pressure–temperature path of the subducted slab and associated phase transitions suggests that rear‐arc magmatism comprises a series of geochemical processes initiated when phengite forms K‐omphacite at pressures above 3 GPa. This would provide free water and additional potassium; a partial melt of phengite, clinopyroxene and coesite would produce a large amount of melt in water‐saturated conditions. The melt and fluid flowing into the mantle wedge would partially melt mantle rocks, producing a magma somewhat similar to volcanic front material but enriched in potassium and LREEs. In water‐saturated conditions, this fluid would decompose rutile (mainly TiO 2 ) and release Ti, Nb and Ta into the melt and transport LILEs into the magma.