Showing 5 results for Rhyolite
Dr Morteza Khalatbari Jafari, Mr Meysam Akbari, Dr Jalil Ghalamghash,
Volume 2, Issue 1 (7-2016)
Abstract
The study area is comprised of lavas and volcanoclastic rocks with basaltic andesite, andesite, trachy-andesite, trachyte-trachydacite, dacite and rhyolite-ignimbrite compositions which erupted in five episodes in an aqueous to sub-aerial environments. All the volcanic-sedimentary series intruded by younger acidic sub-volcanic massifs. The intermediate-acidic volcanic rocks recorded evidences of magma mixing and mingling. In discrimination magmatic diagrams, the lavas represent calc-alkaline, high-K calc-alkaline and shoshonitic characteristics. In spider diagrams, the basic-intermediate samples follow continental arc pattern. The spider diagrams of acidic samples follow the patterns of peraluminous rhyolite and upper crust. REE and spider patterns of the volcanic rocks, display enrichment of LREE, LILE relative to HREE, and clear depletion of HFSE (such as Nb, Ta and Ti), which signifies subduction zones. After collision of the Arabian and Iranian plates and shortening of the Alborz in Eocene, by subsequence lithospheric delamination, partial melting occurred in sub continental lithospheric mantle (SCLM). The resulted melt accumulated in the upper crust, and formed shallow magma chamber(s). Partial melting of the upper crust produced acidic magma. It seems that magma mixing and mingling had important role in generation of the basic-intermediate rocks
Ms Marzieh Veis;arami, Dr Mahmoud Sadeghian, Dr Habibollah Ghasemi, Ms Sakineh Shekari, Dr Mingguo Zhai,
Volume 4, Issue 2 (1-2019)
Abstract
Majerad igneous-metamorphic complex is located in the southeast of Shahrood and in the northern edge of the central structural zone of Iran. Metarhyolites are important compositional constituent of this complex and cropped out in the form of domes and/or intercalations with calcitic and dolomitic marbles, metapelites and metapsammites. Foliation and lineation were developed in metarhyolites due to metamorphism and mylonitisation. Based on the geochemical evidence, metarhyolites have medium to high-K calc-alkaline nature. These rocks are enriched in light rare earth elements (except Eu) with respect to heavy rare earth elements in primitive mantle normalized REEs diagram, and enriched in U, Th, Pb and K, and depleted in Zr Ti, P, Sr, and Nb in primitive mantle normalized spider diagram. Metarhyolites are dry or high temperature rhyolites and formed in intracontinental extensional setting. U-Pb dating of separated zircons from the metarhyolites, indicating 587.6 to 508 Ma (Late Neoproterozoic-Early Cambrian), but according to the age of granitoid which cutts this complex (553 ± 3.8 Ma), Late Neoprotrozoic (Ediacaran) is more reasonable age for the metarhyolites, which endorses their belonging to crystalline basement and Gondoanwan terrains of Iran.
Dr Morteza Khalatbari Jafari, Mrs Zinat Kilani Jafari Sani, Dr Jafar Omrani,
Volume 5, Issue 1 (4-2019)
Abstract
The Eocene volcanic rocks exposed in the NW Sechangi area of Lut block, include pyroclastic deposits and lavas. These volcanic rocks are basalt, basaltic andesite, andesite, andesite-trachyandesite, trachydacite, dacite and rhyolite-ignimbrite in composition. Based on our field observations, the volcanic rocks erupted in four stages in the aquous to subaerial environments. The volcanic rocks are exposed as alternation of acidic and basic-intermediate rocks resembling bimodal volcanism. According to our petrographical investigations the acidic rocks include pyroxenite microenclaves which probably originated from the remains of a sub-continental lithospheric mantle. In the discriminating magmatic diagrams the volcanic lavas show shoshonitic to high-K calc-alkaline trends. In the normalized REE patterns and spider diagrams they display enrichments of LREE and LILE relative to HFSE, comparable to subduction-related volcanism. In the tectonomagmatic diagrams they cluster within the continental margin field. Interpretation of the geochemical diagrams indicates that the basic-intermediate volcanic rocks were produced by partial melting of lithospheric mantle which were previously enriched by the subduction components (fluids-melt) effects of which may not be distinguished from the crustal effects. The acidic rocks record some crustal elements which indicate involving of the crust in the generation of these rocks. It appears that after the closure of the Neotethys branches in Late Cretaceous and subsequent lithospheric thickening in the Lut block, the Eocene volcanic rocks in the Sechangi area were formed in collision extensional environment.
F Faeze Ghodratishahmohamadi, M Mohsen Nasrabady, A Akbar Ahmadvand, K Kazem Gholizadeh, P Parviz Armani,
Volume 8, Issue 1 (9-2022)
Abstract
The Rizu Formation of the Zarand area consists of felsic (rhyolite-trachyte) and mafic (microgabbro-dolerite) magmatic suites, overlain by dolomites of the Desu Formation. Felsic magmatic units are investigated in this research. The rhyolite-trachytes of the Rizu Formation are composed of alkali feldspar microphenocrysts that have been enclosed by fine grained and micro granular groundmass consisting of quartz, alkali feldspar and plagioclase. Whole rock geochemical data indicate that the samples are metaluminous with high FeO/MgO ratio, Na2O+K2O and HFSE, and low CaO, Ba and Eu contents that are in line with geochemical characteristics of anorogenic granites. According to the tectonic discrimination diagrams, the felsic rocks of the Rizu Formation were formed in within plate to continental rift settings. They show the same geochemical signatures as those of anorogenic rhyolites of the East African rift and Basin and Range province of the west coast of North America. In addition, the rhyolite-trachytes of the Rizu Formation suggest an A1 subtype on the basis of anorogenic granite discrimination diagrams, derived from fractional crystallization of an OIB-type mantle melt in a within plate (failed rift) environment. Their association with microgabbros is also reminiscent of bimodal magmatism in a continental back arc rift formed by the subduction of Prototethys beneath the Iranian plate during the Late Neoproterozoic-Cambrian
Eng. Raziyeh Khalili, Dr. Ghodrat Torabi,
Volume 8, Issue 2 (12-2022)
Abstract
The Gooreh Mountain is located about 55 km northwest of Anarak (northeast of Isfahan) and in the western margin of the Central-East Iranian microcontinent (west of the Yazd block). The field studies show that the Eocene volcanic rocks in this area present two lithologies of gray-colored dacite and light-colored rhyolite. Dacite lavas cross-cut the rhyolite ones and are younger. Petrography indicates that dacites contain the major minerals of plagioclase (labradorite and bytownite), clinopyroxene (augite) and quartz, and rhyolites are mainly composed of plagioclase, sanidine and quartz. Chlorite, calcite, hematite and limonite are the secondary minerals in both rock units, formed by the alteration of clinopyroxene, plagioclase and magnetite. The main textures of these rocks are porphyritic, glomeroporphyritic, anti-rapakivi, sieved and poikilitic. The crystallization order of minerals in dacites is magnetite, clinopyroxene, plagioclase and quartz, and in rhyolites, is plagioclase, sanidine and quartz. Whole rock geochemical analyses show that the studied volcanic rocks of the Gooreh Mountain are sub-alkaline (calc-alkaline magma series) in nature. Moreover, the chondrite and primitive mantle-normalized diagrams are characterized by the enrichment of LREEs and LILEs and depletion of HFS elements (e.g., Ti, Nb and Ta). The negative TNT (Ti, Nb and Ta) anomalies probably indicate subduction-related volcanic arc magmatism. The dehydration of subducting slab and partial melting of the mantle wedge spinel peridotites produced a basic magma that, during ascent through the lower and middle continental crust, led to melting of amphibolites and formation of acidic magmas.
