GEOCHEMISTRY OF CALC-ALKALINE VOLCANIC ROCKS FROM SOUTHEASTERN IRAN (KOUH-e-SHAHSAVARAN) (original) (raw)
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Geochemical constraints on the genesis of the volcanic rocks in the southeast of Isfahan area, Iran
Arabian Journal of Geosciences, 2009
Late Miocene-Pliocene to Quaternary calcalkaline lava flows and domes are exposed in southeast of Isfahan in the Urumieh Dokhtar magmatic belt in the Central Iran structural zone. These volcanic rocks have compositions ranging from basaltic andesites, andesites to dacites. Geochemical studies show these rocks are a medium to high K calc-alkaline suite and meta-aluminous. Major element variations are typical for calc-alkaline rocks. The volcanic rocks have SiO 2 contents ranging between 53.8% and 65.3%. Harker diagrams clearly show that the dacitic rocks did not form from the basaltic andesites by normal differentiation processes. They show large ion lithophile elements-and light rare earth elements (LREE)enriched normalized multielement patterns and negative Nb, Ti, Ta, and P. Condrite-normalized REE patterns display a steep decrease from LREE to light rare earth elements without any Eu anomaly. These characteristics are consistent with ratios obtained from subduction-related volcanic rocks and in collision setting. The melting of a heterogeneous source is possible mechanism for their magma genesis, which was enriched in incompatible elements situated at the upper continental lithospheric mantle or lower crust. The geochemical characteristics of these volcanic rocks suggested that these volcanic rocks evolved by contamination of a parental magma derived from metasomatized upper lithospheric mantle and crustal melts.
The Eocene volcanic rocks of northern Azerbaijan (N.W. Iran) are composed of four distinct associations: strongly undersaturated, alkaline, shoshonitic, and calc-alkaline. All four series axe closely associated in space and time. They show many features common with rocks of orogenic zones such as low TiO~, MgO, Nb and high K20 , Ba, Sr, and light REE abundances. The rocks within each series are predominantly related by fractional crystallization. The contents of major and trace elements in the rocks are consistent with their derivation from an enriched upper mantle by variable degrees of partial melting. It is suggested that this enrichment process is related to a subduction zone accompanying the closure of the South Tethyan Sea during the Cretaceous, while melting occurred along deep-seated faults reactivated during Early Tertiary times.
Contributions to Mineralogy and Petrology, 2020
A geochemical study was undertaken on basaltic lava flows intercalated with Oligocene to Miocene strata in the Sabzevar region, northern Iran, to examine their petrogenesis in a regional tectonic framework. The lavas are either aphyric or have phenocrysts and micro-phenocrysts of olivine and, to a lesser extent, clinopyroxene. Geochemically, the lavas are silicaundersaturated alkali basalts characterized by relatively high Mg# (~ 57-66) and Na 2 O/K 2 O (~ 2.0-6.7). They have distinctive trace element patterns characterized by strong rare-earth element fractionation, negative Nb-Ta and Zr-Hf anomalies and a positive Sr anomaly. Significant contamination by crustal materials either in the magma source or during ascent is ruled out on the basis of trace element compositions and Sr-Nd isotopic compositions (87 Sr/ 86 Sr = 0.7037-0.7048 and 143 Nd/ 144 Nd = 0.5128-0.5130), both of which differ markedly from continental crustal rocks. Phenocryst assemblage, analysis of multiple saturation points in lherzolite systems, and covariations of La/Yb with MgO of the studied lavas are generally consistent with an origin involving high-pressure fractionation of peridotite-derived melts. Primary magma compositions calculated by reversed fractionation of clinopyroxene and olivine for the relatively primitive samples (> 9 wt.% MgO), however, do not plot on the lherzolite multiple saturation points. Also, high-pressure fractionation predicts increasing trends of silica undersaturation and alkalinitiy with differentiation, and such trends are not indicated by the geochemical data. We suggest that the mixed trends shown by the data might be related to melt generation from both peridotite and silica-deficient pyroxenite sources, superimposed by variable degrees of high-pressure fractionation. The role of pyroxenite in magma genesis is indicated not only by the positive Sr anomaly shown by the trace element patterns, but also first-row transition element systematics of the studied lavas. The silica-deficient pyroxenites contributing to melt generation might have been transformed from mafic-ultramafic cumulates in subducted, lower oceanic crust, or might have formed in the lower crust or mantle lithosphere under continents during earlier magmatic episodes.
The style of volcanism of post-Eocene volcanism in the Alborz zone of northern Iran is different to that of Eocene volcanism (Karaj Formation). Indeed, the volcanic succession of the Abazar district, located in a narrow volcanic strip within the Alborz magmatic assemblage, is characterized by distinct mineralogical and chemical compositions linked to a complex magmatic evolution. The succession was produced by explosive eruptions followed by effusive eruptions. Two main volcanic events are recognized: (1) a thin rhyolitic ignimbritic sheet underlain by a thicker lithic breccia, and (2) lava flows including shoshonite, latite, and andesite that overlie the first event across a reddish soil horizon. Plagioclase in shoshonite (An48–92) shows normal zoning, whereas plagioclase in latite and andesite (An48–75) has a similar composition but shows reverse and oscillatory zoning. QUILF temperature calculations for shoshonites and andesites yield temperatures of 1035 C and 1029 C, respectively. The geothermometers proposed by Ridolfi et al. (2010) and Holland and Blundy (1994) yield temperatures of 960 C and 944 C for latitic lava, respectively. The samples of volcanic rock show a typical geochemical signature of the continental arc regime, but the andesites clearly differ from the shoshonites, the latites and the rhyolites. The mineralogical and chemical characteristics of these rocks are explained by the following petrogenesis: (1) intrusion of a hot, mantle-depth mafic (shoshonitic) magma, which differentiated in the magma chamber to produce a latitic and then a rhyolitic liquid; (2) rhyolitic ignimbritic eruptions from the top of the magma chamber, following by shoshonitic and then latitic extrusions; (3) magma mingling between the latitic and andesitic magmas, as indicated by the occurrence of andesite clasts within the latite; and (4) andesitic effusions. The youngest volcanic events in the Alborz zone show a close chemical relationship with continental arc rocks, indicating that they formed in a continental collision setting.
Plant Archives, 2021
The Ziaran volcanic Belt (ZVB), North of Iran contains a number of intra-continental alkaline volcanic range situated on South part of central Alborz Mountains, formed along the localized extensional basins developed in relation with the compressional regime of Eocene. The mid-upper Eocene volcanic suite comprises the extracted melt products of adiabatic decompression melting of the mantle that are represented by small volume intra-continental plate volcanic rocks of alkaline volcanism and their evaluated Rocks with compositions representative of mantle-derived, primary (or near-primary) melts. Trace element patterns with significant enrichment in LILE, HFSE and REEs, relative to Primitive Mantle. Chondrite-normalized of rare earth elements and enrichment in incompatible elements and their element ratios (e.g. LREE/HREE, MREE/HREE, LREE/MREE) shown these element modelling indicates that the magmas were generated by comparably variable degrees of partial melting of garnet lherzolite ...
Geochemistry of arc volcanic rocks of the Zagros Crush Zone, Neyriz, Iran
Journal of Asian Earth Sciences, 2001
The northeastern margin of the Tethyan Neyriz ophiolite complex in southwestern Iran is tectonically juxtaposed under cataclasticallydeformed island arc volcanic±volcaniclastic rocks. We document this arc component of the Zagros Crush Zone in the Neyriz area, and describe its petrographic and geochemical characteristics. The arc unit which we call the Hassanabad Unit, is tectonically intercalated with Cretaceous limestone in the cataclastic shear zone around the Hassanabad pass north of Neyriz.
2021
We report a petrographic and whole-rock geochemical characterization of the Cenozoic volcanic rocks cropping out in the Tafresh area of the central Urumieh-Dokhtar Magmatic Arc of Iran. The investigated rocks range mainly from basaltic andesite to dacite, and are considered to be genetically linked by (mostly) closed-system evolutionary processes involving fractionation of ferromagnesian minerals and plagioclase first, then of plagioclase and lesser amphibole (plus minor clinopyroxene) and finally of plagioclase with lesser alkali feldspar and minor amphibole. These represent a typical calcalkaline series emplaced in a subduction-related setting, producing the observed LILE-enriched and HFSE-depleted geochemical signature. The basaltic andesite compositions likely derived from an unsampled hydrous primitive melt equilibrated in a spinel-bearing metasomatized peridotite source, evolving at shallow to moderate crustal depths.
Current World Environment, 2015
Bala Zard volcanic rocks are erupted in the Lut Block in Eastern Iran and exposed about 270 Km north east of Kerman city. The volcanic rocks mainly consist of basalt, andesite, dacite, and rhyodacite. Petrographic studies show that several disequilibrium textures such as sieve texture and oscillatory zoning in plagioclase phenocrysts are present in some of the minerals of BalaZard volcanic rocks. Mineral chemistry studies revealed compositional changes along with the textural changes in minerals from the study area. The observed oscillatory zoning profile for the anorthite content (An) of plagioclase suggests repeated injections of mafic melts (replenishment) and mixing with preexiting magma and forming rocks with basaltic-andesite composition. The chemical composition of clinopyroxene phenocrysts exhibit calc-alkaline affinity and suggest that Bala Zard volcanic rocks formed in a subduction related volcanic arc.
Procedia Earth and Planetary Science, 2015
Tertiary basalts of NW Firoozeh in northeastern Iran are alkaline and evolve gradually towards the subalkaline andesites and dacites. Unvariant Sr isotopic ratios in the basalts, andesites and dacites indicate that the evolution of their parental melt towards progressively more differentiated melt occurred in the absence of crustal contamination. On the contrary, progressively higher Sr isotopic ratios in the alkaline basaltic trachyandesites to trachytes from NW Firoozeh suggest that crustal contamination played a significant role in the evolution of the alkaline rocks.
Chemical Geology, 2012
Miocene to Quaternary alkali basalts in the Lut-Sistan region, eastern Iran are spatially associated with two active, N-S-trending dextral strike-slip fault systems in the region, i.e., the Neh faults in the Sistan suture zone and the Nayband fault~200 km further to the west in the Lut block. Here, we present new 40 Ar/ 39 Ar ages, geochemical and Sr-Nd isotopic data for these rocks to decipher the petrogenetic processes responsible for their formation and regional tectonic implications. Our new ages indicate that the volcanism commenced at~14 Ma in the western Lut region and~11 Ma in the northern Sistan suture zone. The rocks are composed dominantly of hawaiites and mugearites with minor basanites and basaltic andesites. Petrographic observations and major and trace elemental variations suggest that the alkali basalts underwent variable fractionation of olivine, clinopyroxene and Fe-Ti oxides. Chondrite-normalized rare earth element and mantlenormalized trace element patterns of these rocks largely resemble those of ocean island basalts. High εNd (t) (+ 1.4 to + 3.6), low to moderate initial Sr isotopic ratios (0.7047-0.7073), and trace element ratios indicate that crustal contamination was insignificant in the petrogenesis. The rocks have neither geochemical features pointing to residual hornblende or phlogopite, nor arc-related signatures characteristic of the Iranian sub-continental lithospheric mantle. Thus, the alkali basalts most likely have asthenospheric origin. Modeling of REE suggests that they could have formed by low degrees of partial melting (~3-10%) of an enriched mantle source at garnet-stable depths. We propose that the east Iranian alkali basaltic volcanism was triggered by asthenospheric upwelling in an extensional setting, presumably caused by delamination of thickened lithospheric root following the Late Cretaceous collision between the Lut and Afghan continental blocks. Our results imply that two contrasting tectonic regimes coexist in Iran since the Middle Miocene, i.e., extensional in eastern and compressional in southwestern Iran.