A Study of Ophiolitic Extrusive Sequence and Supraophiolite Series in Gez-Afchang Area, Tectonomagmatic Constrains

The extrusive sequence exposed in the vicinity of Gez village located in the NW Sabzevar، comprises a diversity of rocks which could be divided in three main parts. The lower part contains abundance hyaloclastic breccia and tuff، and the middle part comprises vesicular pillow lava and the upper part has an alternation of sheet flow and volcanic-sedimentary rocks. The intercalations of pelagic limestone have Late Cretaceous microfouna. The abundant of hyaloclastic breccia-tuff، sheet flow and aphyric vesicular pillow lava versus phyric pillow lava indicate the formation of this sequence in the fast spreading rate. The supra-ophiolite volcanic-sedimentary rocks located in the Afchang area contain an alternation of turbidites with lava flow، phyric-aphyric lava، chert-radiolarite and pelagic limestone. The paleonthology studies of those revealed Late Cretaceous age، which suffer this idea that the supra-ophiolite serie formed in a trough juxtapose the Sabsevar oceanic crust at Late Cretaceous. Based an geochemical data، the sheet flow of lower part and the pillow lava of the middle part show OIB characters but the lava flows of the upper part of extrusive sequence and the phyric، aphyric and lava flow of supra-ophiolite serie are depleted in Nb and slightly depleted in Zr could be comparable with subduction volcanism. Study of the tectonomagmatic diagrams verifies the OIB and island arc tendency and seems that the generation of the magma of the lower and middle parts influenced by mantle plumes. The magmatic source of the upper part of extrusive sequence and supraophiolite lavas is depleted which have different enrichement from the subduction components (fluids-melt) released from subducted slab. The tectonomagmatic setting of extrusive sequence and supra-ophiolite series can be justified with the senario of general subduction of oceanic slab beneath the centeral Iran microcontinent during Upper Crtaceous، towards in a back-arc basin.