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; P Konani
Abstract
Lava-dome shallow massive bodies located right around Nodeh Engelab, SE Jogatay in Khorasan Razavi province, cutting Eocene volcanic rocks in northern part of Sabzevar ophiolitic belt, are covered by Pliocene-Quaternary deposits. The margin of bodies comprise of lavas of basalt, phyric andesite basalt, ...
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Lava-dome shallow massive bodies located right around Nodeh Engelab, SE Jogatay in Khorasan Razavi province, cutting Eocene volcanic rocks in northern part of Sabzevar ophiolitic belt, are covered by Pliocene-Quaternary deposits. The margin of bodies comprise of lavas of basalt, phyric andesite basalt, fluidal andesitic compositions but the medial parts and roof of masses composed of phyric andesite-trachy andesite, phyric-aphyric dacites and quartz monzodiorite. No impacts of pyroclastic deposits have been observed around these masses. The sieve texture, oscillatory zoning in plagioclase crystals could be attributed to disequilibrium in magma chamber. The lava from margins of shallow massive bodies has calc alkaline and the rocks from middle parts of the bodies show calc alkaline to high-K calc alkaline affinities. The spider diagrams of margin lavas have substantial compliance with those middle parts of bodies and show enrichment in Rb, Ba, Th, La, Sr, and clear depletion in Nb and Ti. Despite some samples plot in adakitic domain, but they do not display chemical characteristic of index adakites. It seems that the parental magma of the studied shallow intrusions produced from partial melting of mantle wedge which have different enrichment and probably influenced by the components released from subducted slab. The other scenario could be attributed to the partial melting of metamorphosed mafic rocks at the top of subducted slab.
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.
M. E. Moslempour; M. Khalatbari-Jafari; T. Morishita; M. Ghaderi
Abstract
Mantle sequences exposed in west of Fannuj-Maskutan area comprise of lherzolite and porphyroclastic cpx-bearing harzburgite in the lower part and recrystallized fine-grained lherzolite with chromitite lenses in the upper parts of the sequence. Petrography studies and microprobe data show evidence ...
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Mantle sequences exposed in west of Fannuj-Maskutan area comprise of lherzolite and porphyroclastic cpx-bearing harzburgite in the lower part and recrystallized fine-grained lherzolite with chromitite lenses in the upper parts of the sequence. Petrography studies and microprobe data show evidence of melt/peridotite interactions, post-melting processes and subsolidus interactions associated with the appearance of two generations of deformed primary pyroxene-olivine and fine-grained pyroxene-olivine-amphibole neoblasts. Second generation of minerals formed as inclusion, interstitial and fine-grained. These two groups of minerals have different geochemical characteristics, i.e., the first group are comparable with abyssal peridotites and the second group are comparable to suprasubduction peridotites. Therefore, the chemical compositions of different generations of minerals show different petrogenesis for ultramafic rocks of the Fannuj-Maskutan ophiolitie complex. Interpretation of whole rock chemical data indicate that these rocks have a depleted MORB mantle source which underwent 5-15% partial melting. Rare earth element patterns normalized with chondrite standard values and compared with patterns of depleted MORB mantle (DMM), indicate enrichment in LREE/MREE ration and show U-shape patterns. Thus, the peridotites of the Fannuj-Maskutan ophiolitie have experienced multistage evolution and show characteristics of abyssal environment to suprasubduction zone. It might be said that transition from abyssal environment to suprasubduction has been affected by fluids derived from the subducted slab.
