Volumetric and compositional estimation of the Choiyoi Magmatic Province and its comparison with other Silicic Large Igneous Provinces
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Large igneous province
Silicic
Dacite
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Vergara and Munizaga (1974) reported an age of 760,000 years for a sample of rhyolite-dacite obsidian from the mainly late Oligocene to Miocene Central Valley volcanic belt at Ancud, Chiloe (Fig. 1). They interpreted this sample as representing “a last and isolated volcanic event in this series”.
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SHRIMP Zicon U-Pb Dating and Its Geological Implication for the Xiangshan Porphyric Dacite-Rhyolitic
The Xiangshan volcanic complexes is located at the intersection of two fault systems:the first one,with northeast strike,is named the Yongfeng-Fuzhou fault and the other,with nearly north-south strike,is called the Yihuang-Ningdu fault.The volcanic eruption can be divided into two events.The lower part of the first event was dominated by volcanic sediments,with a 70 m thick interlayer of tuff lava,while the major component of the upper part was porphyritic dacite-rhyolite.Distinct from the first event,the second one was produced a rhyolite-tuff with volcanic sedimentary laminas at bottom and porphyroclastic rhyolite lavas on top.Previous reports have provided many dates for these strata,such as 140.3 Ma for the porphyroclastic rhyolite lavas in the second event and the 136.0 Ma for the porphyritic dacite-rhyolite in the first event.However there is some conflict due to the low accuracy of the data,for the porphyroclastic rhyolite lavas should be younger than the porphyritic dacite-rhyolite.In this paper,the SHRIMP zircon U-Pb dating technique has been applied to the porphyritic dacite-rhyolite in the first event of Xiangshan volcanic complexes,and we obtain an 206Pb/238U age 136.6±2.7 Ma(MSWD=1.8).Then we can confirm that the eruption time of the porphyritic dacite-rhyolite should be about 136.6 Ma.Combined with the former reported ages,it seems more reasonable that the actual ages of the porphyroclastic rhyolite lavas should be a little younger than that formerly reported.
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The Hoshino gold area is located in the western part of the Hohi volcanic zone, northern Kyushu. Volcanic rocks in this area vary from andesitic rocks in the north to dacite and rhyolite in the south. The basement is constituted by metamorphic rocks of pre-Cretaceous age. The volcanic rocks of Pliocene age were subdivided into eight volcanic units. Seven fission track ages of zircons from five volcanic units have been determined, using the external detector method. The age data obtained, combined with some previously reported ages, show that two main volcanic activities have occurred in the area. The first volcanic activity took place around 4.3 Ma, and resulted into the deposition of the Hoshino Andesite and the Ikenoyama Conglomerate. The second main volcanism started around 3.5 Ma, and was characterized by the eruption of the Shakadake Andesite and the Reiganji Andesite at the early stage, then, by more acidic rocks of the Takeyama Andesite, the Hyugami Dacite and the Kuroki Rhyolite at the later stage. The main volcanism in the area ceased around 2.6 Ma.
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The Sandaowan rhyolite-dacite porphyry,located to the west of the Golmud River,about 55 km from south of Golmud City,is an intrusive vein of volcanic craters.It intruded into the clastic rocks,named Habalayigou group as one of the Nachitai unit in this region.A sample No.B727-1 was collected from the Sandaowan rhyolite-dacite porphyry rock.We used the LA-MC-ICPMS method of Zircon U-Pb Dating in this article.The 25 zircons from No.B727-1 yield a mean value of(425.9 ± 2.6) Ma of 206Pb/238U,which is interpreted as the age of the volcanism.The result shows that the volcanic crater was one of the eruptive center during Late collision of the Early Paleozoic in this region.Besides,it reveals a low time restrict to the Habalayigou group,implying that the Habalayigou strata formed before the Middle Silurian.These results conflict with previous 40Ar /39Ar whole rock ages of 147.96Ma for the rhyolite-dacite porphyry from the same rock.
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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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The chemical and isotopic compositions of sills at West Mageik Lake are contrasted with high‐silica rhyolite erupted in 1912 at the nearby Valley of Ten Thousand Smokes to gain insight into the petrogenesis of evolved magmas in the Aleutian arc. A high‐silica rhyolitic sill has an ε Nd value (+8.8) similar to an adjacent dacite porphyry stock, but the elevated Al, Sr, and 87 Sr/ 86 Sr of the rhyolite suggest that fractionation of a parental dacite magma was accompanied by significant contamination by Sr‐rich sediments, probably of Paleozoic age. The chemical and isotopic character of the West Mageik Lake rhyolite suggest that the system was of small volume, making it easily affected by crustal contamination. The fact that the 8 km³ of high‐silica rhyolite erupted at the Valley of Ten Thousand Smokes shows only a slight Sr isotopic shift from associated dacite and andesite suggests it was part of a larger system that could assimilate small masses of crust without having its composition significantly affected.
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