N. Amel; M. Moayyed; A. Ameri; M. Vosoghi Abedini; M.H. Emami; M. Moazzen
Abstract
The Plio-Quaternary calc-alkaline magmatism in Azerbaijan, northwest Iran, occurs as stratovolcanoes, lava flows and domes consisting of andesitic basalts, andesites, dacites, rhyodacites, and pyroclastic materials. The volcanic rocks unconformably cover the Miocene sedimentary ...
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The Plio-Quaternary calc-alkaline magmatism in Azerbaijan, northwest Iran, occurs as stratovolcanoes, lava flows and domes consisting of andesitic basalts, andesites, dacites, rhyodacites, and pyroclastic materials. The volcanic rocks unconformably cover the Miocene sedimentary formations. The chemistry of the felsic-intermediate volcanic rocks indicates that the parent magmas are medium-K calc-alkaline and metaluminous in nature, and are distinguished by Na2O/K2O> 1. The volcanic rocks are enriched in LILE and LREE, and depleted in Y, Nb, and HREE, and display highly fractionated REE patterns, with no Eu anomaly. They display post-collision and continental margin arcs affinities. The post collision convergence between Arabia and Eurasia continental plates, starting in Miocene, resulted in significant shortening, thickening, and uplift of the crust in northwestern Iran and eastern Turkey, and shaped the present Iran-Turkey Plateau.
The thermal perturbation processes in the underlying asthenospheric mantle led to partial melting at a low degree, producing alkali- basalt magmas, with garnet remaining stable in the source region. The ascent of the basaltic magma and its emplacement in the lower crust resulted in the partial melting of the crustal materials and development of acidic magma. These processes led to the ascent and eruption of the felsic magmas prior to the mafic magmas. Mixing of mantle-derived mafic magmas with felsic magmas of crustal origin, produced hybrid magmas of intermediate composition. The occurrence of inverted volcanic sequences, where olivine-alkaline basalts are underlain by calc-alkaline felsic-intermediate rocks, are typical features of bimodal volcanic activities in the Plio-Quaternary times in Azerbaijan.
There are mineralogical as well as geochemical evidences that AFC-type processes were involved in the evolution of the Plio-Quaternary volcanic rocks in Azerbaijan. A comparison of geological and geochemical characteristics of the Plio-Quaternary volcanic rocks from northwest Iran with those from eastern Turkey indicates that the two areas share similar evolution paths in the petrogenesis of magmatic rocks.
A. Ameri; N. Ashrafi; H. Karimi Gharebaba
Abstract
The study area is located about 20 km to the east of the Heris town in northwest Iran, and lies in the Alborz-Azarbaijan geological zone. The occurrence of lava domes associated with thick lava flows, volcanic breccias, and ignimbrite layers indicate an important eruptive center in the study area. The ...
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The study area is located about 20 km to the east of the Heris town in northwest Iran, and lies in the Alborz-Azarbaijan geological zone. The occurrence of lava domes associated with thick lava flows, volcanic breccias, and ignimbrite layers indicate an important eruptive center in the study area. The investigated volcanic rocks are of lower-middle Eocene time and associated with upper Eocene I-type granitoid intrusions. Hydrothermal fluids derived from the granitoid rocks, considered to be cogenetic with the volcanic rocks, led to local alteration and mineralization in the volcanic rocks. According to chemical classification, the volcanic rocks are andesite, andesitic-basalt and trachy-andesite. The textures vary from porphyry to microlitic porphyry and hyalomicrolitic and the phenocrysts are mostly plagioclase, alkali-feldspar, augite, hornblende and biotite. Sieve texture and selective alteration in some plagioclases, occurrence of biotite and hornblende with opacitic rims, associated with other disequilibrium textures are characteristic features of the rocks. Plots of the samples on the SiO2 vs. Na2O + K2O, AFM, and SiO2 vs. K2O diagrams show that, similar to the associated granitoids, the volcanic rocks are high- to medium- potassium calc-alkaline in nature. Chemical characteristics of the rocks suggest enrichments in a subduction zone or crustal contamination, rather than a within-plate enrichment trend. Plots of Rb/Ba vs. Ti/Y, and Nb/Zr vs. Nb show that the chemical composition of these rocks were controlled typically by fractional crystallization and the role of magma mixing was not significant in the evolution of the rocks. Plots of the samples on tectonic setting discrimination diagrams of basalts and andesites, based on Th, Ti, Zr, Nb, and Sr contents, indicate that the studied rocks belong to calc-alkaline volcanic arcs. Compared to andesitic rocks from typical tectonic environments, the compatible and incompatible elements patterns for the andesitic rocks from the study area are more consistent with a post-collision volcanic arc setting.
