R Kohansal; M Ghorbani; S.M Pourmafi; M Khalatbari Jafari; J Omrani
Abstract
The Forumad ophiolite pillow lavas crop out at west of Sabzevar and at the periphery of north central Iran microcontinent. These lavas include Upper Cretaceous pelagic limestone with microfossils and are classified in two Late Turonian- Santonian and Coniacian- Late Campanian groups. These basaltic ...
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The Forumad ophiolite pillow lavas crop out at west of Sabzevar and at the periphery of north central Iran microcontinent. These lavas include Upper Cretaceous pelagic limestone with microfossils and are classified in two Late Turonian- Santonian and Coniacian- Late Campanian groups. These basaltic pillow lavas have tubular and flattened forms, with external (vitrified), medial (variolite) and internal (intergranuler) parts. Each part has its own texture with mineral composition of clinopyroxene ± plagioclase ± olivine. The medial part is characterized by formation of skeletal crystals of olivine and lesser extend of plagioclase, indicating high nucleation rate. The nature of Late Turonian- Santonian lavas is tholeiitic and Coniacian- Late Campanian lavas are tholeiitic and calc-alkaline. Despite of enrichment in LILE, relative depletion of HFSE, different enrichment in LREE and various linear trends of HREE which suggest the formation of pillow lavas in the arc environment and the heterogeneous mantle with varying degrees of partial melting, but low ratios of Th/ Yb, La/ Nb, Ce/ Nb and Ce/ Th indicate that they are more inclined to intermediate between IAB and MORB environments. Unlike Late Turonian – Santonian lavas, high ratios of La/Yb, Nb/ Yb in Late Coniacian – Late Campanian lavas represent mantle fertility and decrease of Nb/ Ta ratios as well as relative increase of Th exhibit the effect of partial melting of subducted slab in genesis of these lavas. The depth of partial melting was 40 to 70 km in the spinel lherzolite stability zone. It seems that the formation of Forumad ophiolitic pillow lavas is accrued in a marginal basin to a back arc basin of a supra-subduction zone.
R Kohansal; M Ghorbani; S.M Pourmoafi; M Khalatbari Jafari; J Omrani; S Zolfaghari; S Soleimani
Abstract
Abstract The Forumadophiolitic rocks as a part of Iran marginal microcontinentophiolites have been situated in the western part of Sabzevarophiolite. Field study shows extrusive sequence in this area with Late Cretaceous in age. This sequence has included basaltic pillow lavas with radiolarian chert ...
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Abstract The Forumadophiolitic rocks as a part of Iran marginal microcontinentophiolites have been situated in the western part of Sabzevarophiolite. Field study shows extrusive sequence in this area with Late Cretaceous in age. This sequence has included basaltic pillow lavas with radiolarian chert related to Late Turonian-Santonian, in contrast to Coniacian- Late Campanian age, when this sequence has composed of basaltic pillow lavas in lower - middle part and various tuffs in upper part. Sheet flows, hyaloclasticbreccias and pelagic limestone are intercalations between lavas and tuffs.Abundantly, pillow lavas with phyric texture are most predominant feature contrast with sheet flows. Whereas enrichment of LILE and depletion of some HFSE (Nb, Ta) exhibit subduction environment (island arcs) for pillow lavas of Forumad, and different enrichment in LREE to HREE displays unhomogenous mantle in the generation of them; the presence of two magmatic nature in lavas (tholeiitic and calc-alkaline), their positions in tectonomagmatic diagrams and clear enrichment of Th element in Late Turonian –Santonian lavas which are generally exhibition of ophiolitic extrusive sequence in Forumad area have been generated in a supra-subduction environment and back-arc basin.
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 ...
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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.
Y Vasigh; A Darvishzadeh; M Vosoughi Abedini; M.H Emami
Abstract
Heiran area is located in northwest of Ardabil and southwest of Caspian Sea. Field evidences indicate submarine volcanic activities in this area. The rocks in this area are of basaltic composition. The outcrops of pillow lavas, prisms, dykes and lava flows in different points are evidences showing the ...
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Heiran area is located in northwest of Ardabil and southwest of Caspian Sea. Field evidences indicate submarine volcanic activities in this area. The rocks in this area are of basaltic composition. The outcrops of pillow lavas, prisms, dykes and lava flows in different points are evidences showing the existence of oceanic crust in this area. Studies on other locations of southern margin of Caspian Sea as well as structural and petrological similarities between Heiran and these areas may confirm the fact that Heiran area is part of geo-suture of Caspian Sea southern margin. The submarine lavas in this area attributed to late Cretaceous-Eocene. With regard to petrographical characteristic, these rocks range from andesitic basalt to olivine basalt, and belong to alkaline series. The tectonomagmatic environment of these lavas is related to back arc basin. These magma originated from sub continental lithospheric mantle and formed in a supra subduction environment. During late Cretaceous-middle Paleogene, the closure of Sevan-Akera-Qaradagh led to the formation of marginal basin in the form of a back arc basin in the margin of Caspian Sea. The submarine lavas of Heiran likely originated from the volcanic activities in this marginal basin.
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 ...
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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.