Geochronology and geochemistry of volcanic rocks in the Arbasay Formation, Xinjiang Province (Northwest China): implications for the tectonic evolution of the North Tianshan
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Abstract:
The extensively exposed late Carboniferous volcanic and volcaniclastic successions along the northern margin of the North Tianshan are called the Arbasay Formation. We present field-based mapping, petrography, zircon cathodoluminescence (CL) images, and U–Pb dates, as well as whole-rock geochemical data for these rocks, in order to constrain their formation age and petrogenesis, and understand the geodynamic setting. Conspicuously, the Arbasay Formation shows typical basalt–andesite–dacite–rhyolite volcanic series, and is dominated by andesites with a small amount of basalts and rhyolites based on the geological profile. U–Pb isotopic dating using the LA-ICP-MS method on zircons reveals that the volcanic rocks in the Arbasay Formation formed at 308–305 Ma, that is, late Carboniferous, rather than early Permian as previously proposed. Geochemically, the volcanic rocks mainly belong to the calc-alkaline series and have arc-like geochemical compositions. They are enriched in LREEs ((La/Yb)N = 2.9–7.5) and LILEs (K, Rb, Ba) and depleted in HFSEs (Nb, Ta, Ti). In the tectonic discrimination diagrams, the basalts mainly fall into the area of continental arc. Given our U–Pb dating results, geochemical characteristics, and the regional geological framework, we propose that the late Carboniferous volcanic rocks originated from the arc-related setting, not the intracontinental rift-related setting. They are possibly the major constituents of a continental arc that is formed with the southward subduction of the North Tianshan Oceanic lithosphere.Keywords:
Petrogenesis
Andesites
Geochronology
Continental arc
Island arc
Dacite
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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Fractional crystallization (geology)
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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 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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Abstract Potassium-argon total rock, sanidine, and plagioclase ages are presented for 24 rhyolite, dacite, andesite, and basalt rocks of the Upper Cretaceous, Mt Somers Volcanics of mid Canterbury. Ages are grouped as follows: (1) total-rock ages of andesites and dacites: 45–94 Ma, but with a significant concentration at 82–91 Ma; (2) plagioclase ages of andesites and dacites: 87–93 Ma; (3) sanidine ages of pitchstone flows and rhyolitic intrusives: 88–99 Ma. The wide range of andesite total-rock ages, which do not correlate with co-existing plagioclase ages, indicate that substantial argon loss (up to 50%) may, in a few cases, have occurred from a second potassic phase, most probably volcanic glass. The majority of total-rock/plagioclase pairs show concordant ages, which in turn are similar to sanidine ages of closely associated rhyolitic rocks. Detailed stratigraphic evidence and potassium variations in the feldspars show that argon loss (about 5%) has occurred in most plagioclases (K < 0·8%) and some sanidines (K < 7%), causing younger ages than anticipated. In addition, despite favourable potassium concentrations (7% < K < 0·9%) of their feldspars, andesites known to predate rhyolites none the less yield younger ages. To some extent this pattern is explained by slight differences in the andesite-rhyolite sequences at the three main eruptive centres at Mt Somers, Malvern Hills, and Rangitata Gorge. Both andesite and rhyolite volcanism at Mt Somers (94–98 Ma) may predate rhyolites in the Malvern Hills area, 91–94 Ma. Mt Somers Volcanics extend from about 98 Ma (upper Albian) to at least 92 Ma (Turonian). Owing to the slight argon loss problem, the younger age limit for volcanism cannot be assessed. A basalt sample from Malvern Hills, dated as 37 Ma, is probably related to widespread tholeiitic Oligocene volcanism in North Canterbury, rather than to Mt Somers Volcanics. Mt Somers Volcanics show age similarities with more basic alkaline volcanics of inland Marlborough (and may form part of an Upper Cretaceous volcanic arc in the South Island). More distant age correlations can be made with rhyolites on Lord Howe Rise (Tasman Sea) and in central Marie Byrd Land, West Antarctica.
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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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