Slab partial melts from the metasomatizing agent to adakite, Tafresh Eocene volcanic rocks, Iran

Geochemical and mineralogical characteristics of the Eocene volcanic succession in Tafresh area of the Urumieh-Dokhtar Magmatic Assemblage (UDMA) are unique in the 2000-km-length assemblage. Demonstrating rather steep rare earth element (REE) patterns and the widespread presence of amphibole (+biotite) phenocrysts are two distinct characters that dominate the Eocene volcanic succession of mainly andesitic composition. Coincidence of the geochemical and mineralogical characteristics of the whole volcanic succession with adakites, rather amphibole- (+biotite) rich dacitic (with 61-64 wt% SiO2) stocks and dykes, is considered as the key in unraveling the role of 'slab-derived melt contribution' in petrogenesis of the volcanic succession. Slab-derived melting has been an ongoing process that metasomatized some parts of the mantle wedge from which hybrid rocks (andesites) are derived. Basalts with distinct signatures of slab melt metasomatism are yet another support for the occurrence of slab melting. Interlayering of normal, island-arc-type calc-alkaline volcanic rocks with the slab-melt metasomatized basalts and hybrid andesites suggests that the slab melting has been motivated by the subduction. Formation of the Tafresh Caldera, the likely consequence of an explosive eruption, is compatible with the volatile-bearing nature of the adakitic volcanism in the study area. It is indicated by the ubiquitous presence of the hydrous minerals. Beneath the Tafresh area, in Eocene time, the subducting slab seems to have reached a critical high depth that is enough for the development of amphibolite-eclogite. The slab deformation, motivated by the geometry of subduction and/or the underlying mantle's steeper geotherms, is suggested to have resulted in the slab melting that helped develop a rock assemblage unique to the UDMA.