جستجوی مقالات مرتبط با کلیدواژه « Eocene Volcanic rocks » در نشریات گروه « زمین شناسی »

The Kuh-e-Kalut-e-Ghandehari is located about 40 Km northwest of the Anarak (northeast of Isfahan). This area is a part of the Central Iran that is situated in west of the Yazd block; near to the Ashin ophiolite and drastic directional changes of the Great Kavir Fault. In the northwest of Anarak, the Ashin ophiolite is covered by the Cretaceous limestones, and crossed by the Eocene volcanic rocks. The Upper Eocene gabbro-diorite stock cross cuts the Eocene volcanic rocks. Andesites are the predominant unit of the Eocene volcanic rocks. They are grey to dark- grey in colour and have amphibolite xenolith. The main textures of these volcanic rocks are porphyritic, poikilitic, trachytic, sieved texture and glomeroporphyritic. The main phenocrystic minerals of andesites are plagioclase, amphibole and clinopyroxene situated in a matrix of the same minerals and sanidine, magnetite and apatite. Secondary minerals are formed as a result of the hydrothermal alteration and comprise zeolite, chlorite, epidot, sericite, kaolinite and calcite. Mineral chemistry indicates that plagioclases are andesine to labradorite (An= 30-52%), K-feldspars are sanidine (Or= 68-71%), amphiboles are calcic with pargasite composition (Mg#= 0.58-0.71), and clinopyroxenes are diopside (Mg#= 0.91-0.93) in composition. Whole rocks chemical analyses indicate that these andesite and trachy-andesite rocks have calc-alkaline property. The chondrite and primitive mantle-normalized REE and multi-element patterns show LREE and LILE enrichment and HFSE (Ti, Ta, Nb) depletion, which are characteristics of the subduction-related magmas. These data indicate that the primary magmas were genarated by medium degrees of partial melting of the lithospheric mantle spinel lherzolite. Subduction of the Central-East-Iranian Microcontinent (CEIM)-confining oceanic crusts (Naein and Ashin oceanic crusts) are possibly the reason of this volcanism. Geochemistry of these volcanic rocks are similar to the arc-related ones.

The Eocene volcanic rocks in Jirande area are dominated by basalt, andesite, trachyt and trachyandesite, which range from basic to intermediate rocks. Plagioclase as major mineral in igneous rocks makes up the largest percentage mineral in these rocks. Clinopyroxene and olivine in basaltic rocks and amphibole in acidic rocks are the prevalent minerals. In petrological studies, the existence of glomeroporphyry, sieve, corrosion gulf, skeletal, and also reaction rims confirm the role of crustal contamination and fractional crystallization in the evolution of the study rocks. Also, variation diagrams of trace elements, spider diagrams together with AFC modeling and ratio-ratio diagrams of trace elements, highlights the role of crustal contamination and magmatic fractionation in the genesis of the rocks studied. The parent magma of these rocks, have a composition like as the melts derived from enriched mantle, and plots in the field of 10-20 percent partial melting of a garnet-spinel lherzolitic source in the depth of 90 to 110 km.

Eocene volcanic rocks in SW of Choupanan are exposed along the Choupanan fault as lava with andesite, trachyandesite to dacite composition and pyroclastic rocks. This area is a part of Central Iran (Yazd block). Andesite is the most predominant rock unit of the area and is composed of feldspars (andesine, oligoclase, and sanidine), amphibole (magnesio hastingsite, magnesian hastingsite, magnesio hornblende, and ferropargasite), mica (biotite and phlogopite), quartz, clinopyroxene (diopside, augite), garnet, opaque minerals (ilmenite, magnetite, titanomagnetite, and hematite), zircon, sphene and apatite. Minerals which are produced by alteration are calcite, chlorite (diabantite), and prehnite. SiO2 content of these rocks ranges 55.1 to 61.1 wt% and LREEs exhibit an evident enrichment. These volcanic rocks present a transitional affinity (between calc-alkaline and shoshonitic) and geochemically are similar to the volcanic arc rocks. The field, petrography and geochemical studies reveal that the studied andesites are formed by same petrogenetic processes and affected by magma mixing and contamination during crystallization.

The Eocene volcanic rocks of the Posht-e-Badam block represent very good exposures in Saghand (Khoshoumi Mountain)، Alahabad and Chapedony areas (Yazd province). These rocks were developed during five volcanic phases: 1- Volcanic rocks of Alahabad area which are formed during three various phases: 1-1: The central andesitic phase; 1-2: The first andesitic fissure phase; and 1-3: The second andesibasaltic fissure phase. 2- The Chapedony andesite to andesibasaltics. 3- The Saghand (Khoshoumi Mountain) andesic rocks. On the base of field observations، the evidences of contamination are clearly noticed in the volcanic rocks of Saghand area which point to slower rate of magma ascending. The lowest volume of Eocene volcanic rocks exposed in Alahabad area. These rocks developed by two fissures and one central eruption. The presence of the highest volume of the Eocene volcanic rocks in Chapedony area displays active Chapedony fault during the Eocene time. The absence of contamination evidences in the rocks of Chapedony and Alahabad areas indicate the higher rate of magma ascending in compare to that of Saghand area. The occurrence of xenoliths and xenocrysts، the formation of fine grained clinopyroxenes around xenocrystic quartz، the sieved and anti-rapakivi textures، the gulf corrosion of quartz and the oscillatory zoning of plagioclases reveal magmatic contamination in Saghand area. The chemistry of clinopyroxenes and biotites display that these rocks، in nature، are similar to the calc-alkaline continental volcanic arc rocks.