Geochronology and geochemistry of late Paleozoic volcanic rocks from eastern Inner Mongolia, NE China: Implications for igneous petrogenesis, tectonic setting, and geodynamic evolution of the south-eastern Central Asian Orogenic Belt

Abstract

Here we report new petrography, geochemistry, UPb 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) 206Pb/238U zircon ages of 332 ± 1 Ma and 300 ± 1 Ma, 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 εHf(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 continental margin; (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.