Disclosing the most intense magmatic flare-up in southern Tibet: Insights from study of the Pangduo volcanic-plutonic complex
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Quartz monzonite
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Silicic magma systems are of great scientific interest and societal importance owing to their role in the evolution of the crust and the hazards posed by volcanic eruptions. MELTS is a powerful and widely used tool to study the evolution of magmatic systems over a wide spectrum of compositions and conditions. However, the current calibration of MELTS fails to correctly predict the position of the quartz + feldspar saturation surface in temperature, pressure and composition space, making it unsuitable to study silicic systems. We create a modified calibration of MELTS optimized for silicic systems, dubbed rhyolite-MELTS, using early erupted Bishop pumice as a reference. Small adjustments to the calorimetrically determined enthalpy of formation of quartz and of the potassium end-member of alkali feldspar in the MELTS calibration lead to much improved predictions of the quartz + feldspar saturation surface as a function of pressure. Application of rhyolite-MELTS to the Highland Range Volcanic Sequence (Nevada), the Peach Spring Tuff (Arizona–Nevada–California), and the late-erupted Bishop Tuff (California), using compositions that vary from trachydacite to high-silica rhyolite, shows that the calibration is appropriate for a variety of fluid-bearing silicic systems. Some key observations include the following. (1) The simulated evolutionary paths are consistent with petrographic observations and glass compositions; further work is needed to compare predicted and observed mineral compositions. (2) The nearly invariant nature of silicic magmas is well captured by rhyolite-MELTS; unusual behavior is observed after extensive pseudo-invariant crystallization, suggesting that the new calibration works best for relatively small (i.e. <50 wt %) crystallization intervals, comparable with what is observed in volcanic rocks. (3) Our success with rhyolite-MELTS shows that water-bearing systems in which hydrous phases do not play a critical role can be appropriately handled; simulations are sensitive to initial water concentration, and although only a pure-H2O fluid is modeled, suitable amounts of water can be added or subtracted to mimic the effect of CO2 in fluid solubility. Our continuing work on natural systems shows that rhyolite-MELTS is very useful in constraining crystallization conditions, and is particularly well suited to explore the eruptive potential of silicic magmas. We show that constraints placed by rhyolite-MELTS simulations using late-erupted Bishop Tuff whole-rock and melt inclusion compositions are inconsistent with a vertically stratified magma body.
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The Yamakogawa Rhyolite, which erupted in the early Quaternary period in central Kyushu, Japan, comprises seven units, three contain of which spatter and stretched pumice. Our fieldwork shows that these are the deposits of strombolian fire-fountains and rheomorphic tuff. Such deposits derived from silicic magma have been previously described and still are controversial. Some of the reasons given for their formation were exclusively peralkaline composition and high-magmatic temperature. The chemical analyses of the Yamakogawa Rhyolite show nonperalkaline composition and low-magmatic temperature. Moreover, the mineral assemblage of the Yamakogawa Rhyolite suggests that its water content was indistinguishable from other rhyolitic deposits. This is the first report that demonstrates that eruption of silicic magma as fire-fountain and pyroclastic flow with rheomorphism is not, necessarily, restricted to peralkaline composition, high-magmatic temperature and low-water content rhyolite.
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Two mid-Miocene (16.5-15 Ma) rhyolite volcanic centers in eastern Oregon, the Buchanan rhyolite complex and Dooley Mountain rhyolite complex, were investigated to characterize eruptive units through field and laboratory analysis. Results of petrographic and geochemical analysis add to field observations to differentiate and discriminate the eruptive units. Additionally, new geochemical data are used to correlate stratigraphically younger and older basalt and ash-flow tuff units with regional eruptive units to constrain the eruptive periods with modern Ar-Ar age dates.
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<p>The Qaradagh batholith in northwest Iran mainly comprises granodioritic rocks, which makes more than 50% of the batholith. This lithology is the first intrusive pulse within this batholith and the oldest Tertiary magmatism in the region, though other younger pulses of granite, diorite, quartz-diorite, syenite, quartz-syenite, monzonite, quartz-monzonite, quartz monzodiorite, monzogranite and gabbro intruded the main body. These magmatic rocks have intruded the Upper Cretaceous and Paleogene sedimentary, volcano-sedimentary and igneous rocks.</p><p>The Qaradagh batholith hosts vein-type and some local stock-work type Cu&#8211;Au&#8211;Mo mineralization, especially in its central parts, while skarn-type deposits have been formed at its contacts with peripheral carbonate rocks. Its extension towards the north into the neighboring south Armenia (which is part of the South Armenian Block) is known as the Meghri&#8211;Ordubad pluton (MOP), which hosts several large porphyry Cu&#8211;Mo deposits and other precious and base metal mineralizations. U&#8211;Pb geochronology on the zircons separated from the granodioritic unit yielded a weighted <sup>206</sup>Pb/<sup>238</sup>U mean age of 43.81 &#177; 0.18 (MSWD=1.38) and a Pb*/U concordia age of 44.04 &#177; 1.00 Ma (MSWD= 24), which correspond to Middle Eocene.</p><p>Since the Qaradagh batholith and especially its earliest magmatic phase are considered as the oldest plutonic event of the Cenozoic age in northwest Iran, thus this investigation testifies to the fact that intrusive activities of Tertiary in this region has commenced in Middle Eocene, contrary to the opinion of the majority of authors who believe that plutonism in this region occurred during Oligocene.</p><p>However, this age is much older than the molybdenite Re&#8211;Os ages of quartz-sulfide veins hosted by granodioritic rocks (25.19 &#177; 0.19 to 31.22 &#177; 0.28 Ma), indicating that mineralization in this batholith is related to another much younger intrusive phase, and even to several phases, as the published ages of molybdenites from various veins and mineralized zones show a large interval. Comparing the obtained age with those from the MOP in southern Armenia indicate that southern part of the MOP is almost coeval with the emplacement of the granodioritic rocks in Qaradagh batholith.</p><p>The U and Th contents of the zircons range from 17.1 to 1534.0 and from 4.9 to 641.0 ppm, respectively, with Th/U ratios between 0.66 and 5.82 (mean of 1.26), indicating a magmatic source. Meanwhile, the &#949;Hf<sub>(t) </sub>values of the zircons range from 8.7 to 11.1 with the mean of 9.5, which are plotted between the CHUR and the Depleted Mantle evolution lines, indicating a juvenile and homogeneous magmatic source and the predominance of mantle-derived magmas with limited crustal assimilation.</p>
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Composite dolerite-rhyolite dikes traverse the Galway Granite batholith and its adjacent envelope. The dikes pertain to the Teach D6ite suite and were previously considered to be of Carboniferous age. New and extended examination of field relationships supports recent radiometric dating for an intrusive period that overlapped with the final consolidation of the Galway batholith. Regional crustal extension produced a complex pattern of fissuring, controlled by various preexisting structures, which permitted ascent of mantle-derived melts into and around the Galway batholith. Ponding of mafic magma at an intermediate level facilitated crustal partial melting and the generation of high-silica, high-alumina rhyolitic melts. The two contrasting magmas then rose into common or proximate dike fissures, rhyolitic injection immediately following that of dolerite. Magma storage in stratified chambers occasionally resulted in the development of a hybrid magma layer, but in all cases minor mingling and mixing beween dolerite and rhyolite magma continued up into the dikes. Rhyolite geochemistry precludes a genetic relationship with the Galway granitoids, despite a few instances where granitic material was entrained into rhyolitic magma. Introduction and setting The 400Ma Galway Granite batholith was emplaced into 470Ma island-arc orthogneisses in the Connemara sector of the Caledonides. This emplacement was followed by the intrusion of two hypabyssal suites: earlier microphyric ('porphyry') dacite dikes (Kinahan 1869; Mohr 2003) and a later complex nexus of dolerite dikes, the Teach D6ite (TD) suite (Mitchell and Mohr 1987; Fig. 1). The numerous and widespread dacite dikes have consistently been considered the youngest igneous rocks pertaining to the Galway batholith (Wager 1932; Wright 1961; Harvey 1967; Coats and Wilson 1971; Senior 1973; Leake 1974). However, new work summarised here suggests that the subsequent TD dikes were a final manifestation of the magmatic episode responsible for the batholith. The regional pattern of TD dikes comprises three major linear trends (Fig. 1). In the central part of the Galway batholith and its northern envelope, the NNE-trending Seanabhain system of dikes is intimately associated with the Shanawon Fault that separates the central and western blocks of the batholith (Feely and Madden 1988; Mohr 1993; Callaghan 1999). The na hUillinni dike system, 3.5km west of the Seanabhan system and parallel to it, projects much farther NNE into the orthogneiss envelope (Fig. 1). Secondly, a grid of ENE-trending dikes Irish Journal of Earth Sciences 22 (2004), 15-32. © Royal Irish Academy 15 This content downloaded from 207.46.13.174 on Sun, 10 Jul 2016 05:04:50 UTC All use subject to http://about.jstor.org/terms 16 Irish Journal of Earth Sciences (2004)
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In the Middle Jurassic Nelson Batholith, southern British Columbia, young 40 Ar/ 39 Ar ages (i.e., 50–60 Ma) and distorted isobaric surfaces in the batholith suggest the possibility of Paleocene granitic plutonism. We present the results of a study undertaken to evaluate this possibility. Geochemical criteria successfully distinguish a suite of granitoids within the Nelson Batholith that differ from Nelson granites of similar SiO 2 content. The granitoid suite is composed of 71.6–75.7 wt.% SiO 2 leucocratic biotite granite and quartz monzonite with strong enrichments in alkaline, alkaline earth, and rare earth elements. Nd and Pb isotopic compositions suggest that biotite granite and quartz monzonite are not related. Biotite granite yields a U–Pb age of 158.9 ± 0.6 Ma (concordant zircons). Quartz monzonite crystallized at 61 ± 1 Ma, based on interpretation of titanite and zircon analyses. Zircons from this sample lie along a line from 61 to 160 Ma and demonstrate the presence of Middle Jurassic inheritance. Based on its petrographic and isotopic similarity to other Middle Jurassic plutons in the Nelson Batholith – Valhalla Complex area, we include the 159 Ma biotite granite with the Jurassic plutonic suite. This result demonstrates that magmatism in southern British Columbia was active at least until the early Late Jurassic (Oxfordian). The Paleocene (61 Ma) quartz monzonite that intrudes the southern Nelson Batholith is the structurally highest occurrence of "Ladybird" granite yet documented in southern British Columbia. Comparison of new and published geochemical and isotopic data for Paleocene granitoids throughout the southern Omineca Belt, British Columbia, suggests that these granitoids were not derived from a single, old crustal source.
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