MULTI-STAGE METASOMATISM OF MANTLE WEDGE PERIDOTITE: EXAMPLE FROM THE ALAG KHADNY ACCRETIONARY WEDGE, WESTERN MONGOLIA

Hydration and metasomatic reactions of mantle wedge are important to understand global material circulation and geological processes within the subduction zone. In this study, we present outstanding alteration textures of orthopyroxene in peridotites (associated with the eclogite body) from the Alag Khadny accretionary wedge, western Mongolia as an evidence for significant Ca metasomatism within the mantle wedge, and its relation to the hydration.

We perform petrological and geochemical analyses on seventeen peridotites collected from southern belt (mainly serpentinized peridotite with minor of limestones, and epidote-amphibolites) of the Alag Khadny accretionary wedge which is located in the Chandman area, western Mongolia, Central Asian Orogenic Belt. Northern belt of the accretionary wedge is composed largely of orthogneiss, and eclogite body with minor of micaschist, marble and small serpentinite bodies.

The peridotites contain primary olivine (Ol), orthopyroxene (Opx), clinopyroxene (Cpx), and spinel (Cr#=0.3-0.5). The peridotites were suffered by various extents of metasomatism and hydration. P-Ol was replaced by antigorite (Atg) with various extent, whereas primary Cpx was preserved without reactions. Notable feature is intense replacement of Opx (>60 vol.%). The replacement is divided into 3 types: secondary Cpx (S-Cpx) + tremolite (Tr); S-Cpx + Tr + S-olivine; and S-Cpx. Multi-trace element pattern shows enrichment of LILE, small Sr peak, and weak Zr depletion relative to REE, suggesting geochemical signatures of mantel-wedge peridotite.

Mass-balance calculation for the replacement shows the gains of Ca and water, and losses of silica, Mg and Fe. Due to the lack of Ca source in the primary minerals, it suggests infiltration of the external fluids causes significant amount of Ca metasomatism. The products (S-Cpx, Tr) of Ca metasomatism are commonly cut by Atg veins, indicating that main hydration occurred after Ca metasomatism. Atg after Ol suggests that hydration was occurred by Si-rich fluid. The stability field of mineral assemblage of the Opx replacement was consistent with the P-T conditions of eclogite bodies in the Alag Khadny accretionary (T= 590-630 °C and P=11-16 kbar), implying the hydration and metasomatism were caused by a supply of fluids derived from dehydration of the subducting slab.

Thus, mantle wedge peridotite in the Alag Khadny accretionary wedge records novel mass transport of Ca as well Si in subduction zone.