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[en] Compositional zonation in ignimbrites is relatively common, and is often inferred to record gradual withdrawal by an eruption of a density-stratified magma chamber (with silicic magma towards the top and more dense, mafic magma at the bottom). We show that this model does not match observations at the ca. 0.1 Ma Zaragoza ignimbrite from Los Humeros caldera in central Mexico. Detailed petrologic studies reveal a more complex scenario: the ignimbrite exhibits a 'double' vertical zonation based on the compositions of pumice lapilli. We present evidence for mingling and limited mixing occurred during or immediately before the caldera-forming eruption. One possibility to explain the observations is that the ignimbrite eruption occurred in response to intrusion of a hybridized andesitic magma into a rhyodacitic magma chamber.
[en] Prospecting for hydrothermal type uranium deposits should be aimed at medium-to large-sized deposits, and be guided by mantle-sourced, superimposed, deep-sourced metallogenic theory and the establishment of a multifactor, composite, deep-sourced metallogenic model. The author suggests that hydrothermal uranium deposits may be classified into three genetic types, i.e. hydrothermal circulation concentration, postmagmatic hydrothermal and mantle fluid concentration. These types of uranium deposits are characterized by their own metallogenic features and are concentrated in the same mineralization-concentrated area forming a metallogenic series. Large-sized uranium ore fields and rich-large uranium deposits are usually closely associated with mantle-sourced metallogenesis and the formation of such uranium ore fields and deposits is characterized by specific and unique regional geologic environments. Recognition criteria of mantle-sourced metallogenesis are preliminarily proposed in the paper. It is pointed out that prospecting in the future should follow the metallogenic model proper for the specific genetic type, and the establishment of operable prospecting model to realize the model-guided prospecting. (authors)
[en] In the NW of the iberian Massif discordant late-Hercynian granitic plutons are well exposed, of which the Peneda-Geres massif is ne o the most representative. Similarly to other plutons known in the area, it exhibits a dominant sub-alkaline ferro-potassic affinity. Field, petrographic, mineralogical and geochemical data suggest that this massif which is composed of four main granitic units, three of which roughly concentric, results from the sub-synchronous emplacement of two distinct granitic magmas. The internal chemical-mineralogical evolutions observed are interpreted as primary trends in which the main petrogenetic process is fractional crystallization. (authors)
[en] Los Cuartos Granite crops out on the western flank of the Cumbres Calchaquies, east of Tafi del Valle, Tucuman Province, Argentina. It has a granitic to granodioritic composition, of two micas, being biotite dominant. The granite is medium grained and inequigranular, with rare K-feldspar megacrysts. It contains host-rock xenoliths with different degrees of assimilation. The granite intrusion produced contact metamorphism in the surrounding basement, with development of andalucite porphyroblasts and poikiloblastic biotite. The granite is calk-alkaline and peraluminous, and is K- and P-rich and Ca- and Napoor compared to other granitoids of the region. The pluton shows late-tectonic features with regard to the regional deformation. The granite possibly intruded during the Ordovician (Famatinian Cycle) and its emplacement was structurally controlled by the Tafi Megafracture. Los Cuartos Granite and other intrusive bodies of the region share similar petrographical, geochemical, structural and geochronological characteristics. (Author).
[en] Three main types of collapse calderas can be defined, 1) summit caldera: those formed at the top of large volcanoes, 2) classic caldera: semi-circular to irregular-shaped large structures, several km in diameter and related to relatively large-volume pyroclastic products, and 3) graben caldera: explosive volcano-tectonic collapse structures from which large-volume, ignimbrite-forming eruptions occurred through several fissural vents along the graben master faults and the intra-graben block faults. These in turn can collapse at least with three styles: 1) Piston: when the collapse occurs as a single crustal block; 2) Trap-door: when collapse occurs unevenly along one side while the opposite side remains with no collapse; 3) Piece-meal: when collapse occurs as broken pieces of the crust on top of the magma chamber.
[en] Accessory wolframite with an extraordinarily high rhenium content (2.93–3.52%) in ongonite from the Badzhal area in the Far East is described. The capacity of tungsten isomorphic substitution with rhenium in wolframite is studied. The appearance of rhenium-bearing wolframite is caused by specific conditions of the crystallization of rare-metal granite magmas.
[en] Metallogenic features of different types of uranium deposit in southern Jiangxi Province are summarized. It is considered that uranium metallogenesis in southern Jiangxi Province is characterized by multi-phases and multi-stages. It is suggested that controlling factors on regional uranium metallogenesis are mainly uranium-rich metamorphic basement, uranium-rich magma and faults. (authors)
[en] Mount Taftan is a double-peaked stratovolcano, located in southeast of Iran. This volcano constructed a number of calderas among which one of the most important is Anjerk. The magmas erupted from this multi-caldera complex range from andesi-basalt to dacite, but are dominated by andesite and dacite. Two terminal cones, Narkuh and Matherkuh, culminate at 4100 m and 3950 m, respectively. There are three evolutionary stages in the history of the volcanic complex (stage 1: Palaevolcanism, 6.95±0.72Ma, stage 2: Mesovolcanism, 6.01±0.15Ma and stage 3: Neo volcanism, 0.71± 0.03Ma). The eruptive products consist of lava flows, iguimbrites and pyroclastic rocks. The later include tuffs, nuees ardents, breccias and sometimes reworked as lahars. Textural and mineralogical data suggest that both magma mixing and fractional crystallization were involved in the generation of the andesites and dacites. The magmas erupted from this volcano show a calc-alkaline trend. The corresponding lavas are calk-alkaline with a potasic tendency.