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Mineralogy and Petrology - In the metamorphic Sanandaj-Sirjan Zone of western Iran, the “North Qorveh Volcanic Field” is constituted by Pleistocene scoria cones and associated deposits.... 相似文献
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A hypocrystalline silica-rich (63–67 wt.% SiO2, dacitic composition) lava flow (called G-lava) in the subaerial eruptive sequence of the Alborz Mountains (Razjerd district, Qazvin Province) of northern Iran, contains abundant (40–50 vol.%) 0.1- to 5.0-cm globules set in a matrix of rather similar composition and microtexture. Numerous globules have coalesced, showing triple-point junctions with 120° angles. Both phases in the G-lava (globules and matrix) contain similar microphenocrysts (plagioclase, ortho- and clinopyroxene and magnetite) in a trachytic groundmass. However, their mesostasis differ in colour, in composition, in the amount of glass and their amount of volatiles and silica: in the globules the mesostasis is darker and richer in SiO2 but is volatile poor. Other volcanic materials in the same unit are very similar in composition to the G-lava. The globular fabric was formed with two phases: one poor in volatiles (the globules), the other rich in volatiles (the matrix). The globules are slightly more silicic (66.9 against 64.6 wt.% SiO2), more potassic (3.7 againt 2.8 wt.% K2O) and more viscous (of the order of 103 to 104) than the matrix outside the globules. It seems that the two phases (globules and matrix) with different silica and volatiles contents and thus different vesicularities, viscosities and densities, were produced in the dacitic melt due to temperature and pressure drop and magmatic degassing in the volcanic conduit involved fluid-melt exsolution processes. Some of the volatile-rich melt was probably frothy during eruption, producing volcanic bombs and scoria. 相似文献
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The Zargoli granite, which extends in a northeast–southwest direction, intrudes into the Eocene–Oligocene regional metamorphic flysch‐type sediments in the northwest of Zahedan. This pluton, based on modal and geochemical classification, is composed of biotite granite and biotite granodiorite, was contaminated by country rocks during its emplacement, and is slightly changed to more aluminous. The SiO2 content of these rocks range from 62.4 to 66 wt% with an alumina saturation index of Shand [molar Al2O3/(CaO + Na2O + K2O)] ～ 1.1. Most of its chemical variations could be explained by fractionation or heterogeneous distribution of biotite. The features of the rocks resemble those which are typical to post‐collisional granitoids. Chondrite‐normalized rare‐earth element patterns of these rocks are fractionated at (La/Lu)N = 2.25–11.82 with a pronounced negative Eu anomaly (Eu/Eu* = 3.25–5.26). Zircon saturation thermometry provides a good estimation of magma temperatures (767.4–789.3°C) for zircon crystallization. These characteristics together with the moderate Mg# [100Mg/(Mg + Fe)] values (44–55), Fe + Mg + Ti (millications) = 130–175, and Al–(Na + K + 2Ca) (millications) = 5–50 may suggest that these rocks have been derived from the dehydration partial melting of quartz–feldspathic meta‐igneous lower crust. 相似文献
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