M Khalatbari Jafari; M Ghani
Abstract
In northwest Sabzevar the exposed extrusive sequence and supraophiolite series have interbedded pelagic limestone containing Late Cretaceous microfaunas. The extrusive sequence comprise of hyaloclastic breccia, tuff and sheet flow in lower part, aphyric and vesiculated pillow lava in middle part ...
Read More
In northwest Sabzevar the exposed extrusive sequence and supraophiolite series have interbedded pelagic limestone containing Late Cretaceous microfaunas. The extrusive sequence comprise of hyaloclastic breccia, tuff and sheet flow in lower part, aphyric and vesiculated pillow lava in middle part and volcanic sedimentary rocks upwards. The supraophiolite series contain alternations of volcanogenic sediments, turbidites and lavas. The study of chemical analyses of the lavas of the upper part of extrusive sequence and the lavas of supraophiolite series indicate similar geochemical affinity. These lavas have calc alkaline character and show clear depletions of Nb and definite depletions of Zr and Ti in the patterns of spider diagrams. In tectonomagmatic diagrams, they plot in the realm of subduction zone suggesting their formation above the subducted slab. Slight changes of HREE in the patterns of spider diagram can be attributed to partial melting of depleted mantle wedge which influenced by subduction components (fluids- melt) released from subducted slab causing enrichments of LREE. In comparison, the sheet flow and pillow lave of the lower and middle parts of extrusive sequence have OIB character without depletion of Nb, Zr and Ti. The patterns of their spider diagrams show enrichment of LREE indicating low partial melting of garnet-bearing enriched mantle. It seems a period of extension or arc rifting or slab roll-back have occurred before the arc is well developed. These process formed back-arc basin located close to arc. In this stage, arc magmatism stopped and caused the mantle plumes rising. It seems that the extrusive sequence exposed in northwest Sabzevar formed in transitional zone between arc to back-arc in Late Cretaceous.
M. Khalatbari Jafari; M. Ghani
Abstract
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 ...
Read More
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.
