Geochronology and Genesis of Meta-felsic Volcanic Rocks from the Kangbutiebao Formation in Chonghuer Basin on Southern Margin of Altay, Xinjiang
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Abstract:
The Chonghuer basin is one of the fault-bounded volcano-sedimentary basins on the southern margin of the Altay orogenic belt, where the Kangbutiebao Formation (Fm.) stratigraphic unithosted many economic deposits outcrops. LA-ICP-MS U-Pb zircon analyses from two meta-rhyolites and one meta-tuff of the Kangbutiebao Fm. in the Chonghuer Basin yield weighted mean 206Pb/238U ages of 385.3±1.2Ma,398.1±1.8Ma and 405.6±2.2Ma, respectively, which can be interpreted as the eruption age of the Kangbutiebao silicic volcanic rocks in the Chonghuer Basin. The meta-rhyolites have similar geochemical features to meta-rhyolites in Kelang Basin and Maizi Basin, which are low-Ti rhyolite with obvious negative anomaly of Ti, P, Sr, Ba, Nb, Ta and Eu (δEu=0.25~0.54), obvious enrichment of Th, U, Pb, Zr, Hf and LREE. These features, together with the regional geological characteristics, indicate that these felsic volcanic rocks were generated by partial melting of lower crust on an active continental margin. These volcano-sedimentary basins are down-faulted Basin formed in compression tectonic environment of Early—Middle Devonian period.Keywords:
Felsic
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Andesites
Dacite
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In order to limit accurate formation time of Hongtubu basic volcanic rocks in the east segment of North Qilian orogenic belt,LA-ICPMS U-Pb isotope dating technique,combined with cathodoluminescence image(CL) study, was separately used to determine the zircons from pillow basalt and diabase dyke.Two main populations of zircons from Hongtubu pillow basalt were obtained,giving an average 206Pb/238U ages of(443.4±1.7)Ma and an upper-intercept 206Pb/238U-207Pb/235U age of(1 782±34)Ma.Meanwhile,LA-ICP-MS U-Pb analyses for zircon grains from the diabase dyke within Hongtubu basalts yielded a mean age of(385.7±7.9)Ma,and a group of apparent 207Pb /206Pb ages from(1 697±6)Ma to(2 692±4)Ma.With the relevant data synthesized,Hongtubu basic volcanic rocks were considered to form in the Late Ordovician,corresponding to the age of Chenjiahe medium-acid volcanic rocks which locate in the north of Hongtubu basic volcanic rocks.It was deduced that primitive magma of the basic volcanic rocks was contaminated in different degree by Longshan rock group.These new data have important significance for further research on the tectonic formwork in the east segment of the north Qilian orogenic belt,its tectonic evolution,and the connected relation of North Qilian and North Qinling orogenic belt.
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Geochronology
Greenstone belt
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Geochronology
Volcanic plateau
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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.
Petrogenesis
Andesites
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Dacite
Lithophile
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Felsic
Dacite
Adakite
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Continental arc
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The two-Mica syenogranite in the Keyinblak Cu-Zn deposit area is located in Chonghuer sedimentary volcanic basin near the north of Erqis tectonic belt in the southern margin of Altay.The postmagmatic hydrothermalism are closely related to the deposit.The age of(278.6±3.5) Ma(MSDW=2.8)for the pluton by LA-MC-ICP-MS attributes to late Hercynian.Adding to the Ar-Ar age of 245Ma for biotite in quartz vein in the deposit area and the regional deformation and metamorphism in the southern margin of Altay from late Permian to Trias,the deposit is definite to formed in the early-middle Permian.Synthesizing the former study,we suggest that The two-Mica granite has formed in post-collisional processes and may be related to the local large scale shear event,probably as a result of part melting of metasedimentary rocks in Kangbutiebao formation due to the underplating of magma from mantle source during the late period of crustal extension.
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Geochronology
Devonian
Volcanic arc
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