期刊
LITHOS
卷 362, 期 -, 页码 -出版社
ELSEVIER
DOI: 10.1016/j.lithos.2020.105480
关键词
Central Asian Orogenic Belt; Late Paleozoic; Zircon U-Pb geochronology; Geochemistry; Tectonic setting
资金
- National Natural Science Foundation of China [41772233, 41772114]
- China geological survey projects [DD20189613, DD20190005]
- Basic Outlay of Scientific Research Work from the Ministry of Science and Technology of China [J1708, JYYWF20180901]
- Austrian Science Fund (FWF) [J1708] Funding Source: Austrian Science Fund (FWF)
Here we report new petrography, geochemistry, U-Pb zircon geochronology, and Hf isotope results for volcanic rocks of the Dashizhai Formation in the Dashizhai area, Keerqin Right Wing Front Banner, Inner Mongolia. The volcanic rocks have mafic to felsic bulk-rock compositions and include basalt, rhyolite, basaltic andesite, andesite, dacite, and rhyolitic tuff. Two dacite and rhyolitic tuff samples yield laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) Pb-206/U-238 zircon ages of 332 +/- 1Ma and 300 +/- 1Ma, respectively, suggesting that the timing of felsic volcanism ranged from the early to late Carboniferous. Mafic volcanic rock samples are enriched in large-ion lithophile elements (LILE) and light rare earth elements (LREE; La/Nb = 1.58-4.84), and are depleted in high-field-strength elements (HFSE; Th/Yb ratios <2.2) and heavy rare earth elements [HREE; (Tb/Yb)(N) ratios 1.27-1.91]. Based on these trace element characteristics, we propose that the mafic volcanic rocks represent fractionated, high-degree partial melts that were sourced from the subduction-modified and melt-depleted mantle wedge. The felsic volcanic rocks yield LILE- and LREE-rich compositions and arc-like HFSE- and HREE-poor signatures. Magmatic zircons recovered from the dated rhyolitic tuff and dacite samples yield positive epsilon Hf(t) values (+1.21 to +14.85) and two-stage Hf model ages of 1236 and 391 Ma, suggesting that arc-like felsic volcanism was sourced from, and/or interacted with, the older juvenile lower-crust underlying the Dashizhai Formation. Therefore, the mafic to felsic volcanic rocks from the Dashizhai area likely formed in a Carboniferous magmatic arc at a continental margin. Based on the isotopic and geochemical signatures of the Carboniferous-Permian volcanic rocks in the Dashizhai area, we preliminarily divide central and eastern Inner Mongolia into three zones: (i) North Zone, which formed at the magmatically active continentalmargin; (ii) Middle Zone, which formed in a magmatic arc peripheral to the continental marginal; and (iii) South Zone, which formed in an oceanic island arc. The age and geochemical signature of the Dashizhai Formation points to active, northward subduction during the early Carboniferous to the early Permian and suggests that the Paleo-Asian Ocean closed after the early Permian. (C) 2020 Elsevier B.V. All rights reserved.
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