Stratigraphic Position and Textural Analysis of Volcanic Rocks of Abbas Abad Volcano-Sedimentary Belt, NE Shahrood

  Abbasabad Volcano-Sedimentary Belt in the northeast edge of Central Iran structural zone extends from Davarzan in west of Sabzevar to Mayamay in east of Shahrood and then changed its trend to south, joined to Biarjomand – Toroud - Moalleman magmatic belt in south and southeast of Shahrood. Local stratigraphic sequence of this belt from buttom to top in Davarzan- Abbas Abad district are composed of Paleocene- Eocene terrigenous sedimentary rocks (conglomerate, sandstone, siltstone, shale), alternation of basic – intermediate lavas and related pyroclastics in association with interbedded sedimentary layers (tuffaceous shale, tuffaceous sandstone, shale, siltstone, sandstone, conglomerate and Nummulitic limestones) belong to middle-upper Eocene and Oligocene terrigenous sedimentary rocks (conglomerate, sandstone, siltstone, shale). Field and laboratory studies indicate the extrusion of basaltic-trachyandesitic lavas in an unstable basin, from sub-aerial to very shallow marine (lagoon and open sea) environment in middle-upper Eocene. The presence of imperforate foraminifera indicates a low-energy, upper photic, shallow shelf lagoon depositional environment. The co-occurrence of perforate foraminifera biota such as small Nummulites with imperforate foraminifera such as miliolids, indicates that sedimentation took place in an open shelf lagoon. In some layers, the presence of large flat Nummulitids indicate that the sedimentary environment was formed in an open marine environment under normal marine salinity conditions with open water circulation and low hydrodynamic energy. The studied volcanic rocks with a compotitional range from olivine basalt to trachybasalt, basaltic trachyandesite and trachyandesite show the liquid line of descent (melt differentiation) in a fractional series from olivine basalt to trachyandesite. Textural variety and continuous changes including appearance and disappearance of some minerals and disequilibrium textures indicate a complicate path of magma generation, take place of magma chamber processes and rising of mantlic magma in a continental margin subduction zone.