3 years ago

The volcanic succession of Baoligaomiao, central Inner Mongolia: Evidence for Carboniferous continental arc in the central Asian orogenic belt

The volcanic succession of Baoligaomiao, central Inner Mongolia: Evidence for Carboniferous continental arc in the central Asian orogenic belt
The Baoligaomiao Formation, within the Hegenshan ophiolite-arc-accretion complex is an important segment to understand the tectonic evolution of the Central Asian Orogenic Belt (CAOB), world's largest Phanerozoic orogenic belt. In this study, we present an integrated study of zircon U-Pb isotopic ages, whole rock major-trace elements, and Sr-Nd-Pb isotopic data from the volcanic succession in the Baoligaomiao Formation. The volcanic succession can be divided into the lower sequence with zircon U-Pb ages in the range of 326.3Ma–307.4Ma and the upper sequence of 305.3Ma. The succession belongs to two suites: calc-alkaline volcanics and high-Si rhyolites. The calc-alkaline volcanic rocks include basaltic andesite through andesite and dacite to rhyolite and their pyroclastic equivalents. These rocks exhibit a well-defined compositional trend from basaltic to rhyolitic magma, reflecting continuous fractional crystallization. These rocks show obvious enrichment in LILEs and LREEs and relative depletion of HFSEs, typical of subduction-related magma. The calc-alkaline rocks have low initial 87Sr/86Sr (0.7023–0.7052), positive ɛNd(t) values (2.75–4.80), and their initial Pb isotopic compositions are 17.875–18.485 of 206Pb/204Pb, 15.481–15.520 of 207Pb/204Pb and 37.467–37.764 of 208Pb/204Pb, respectively. Geochemical and isotopic results suggest that the volcanic succession represents Carboniferous subduction-related, mature, continental arc volcanism. The outcrops of high-Si rhyolites are restricted to the northern edge of the continental arc, marking a transition zone between volcanic arc and back-arc basin, where they are interbedded with the calc-alkaline rocks in the lower sequence, and the upper sequence is composed only of high-Si rhyolites. The high-Si rhyolites have high SiO2 (71.12–81.76wt%) and varied total alkali contents (K2O+Na2O=5.46–10.58wt%), low TiO2 (0.06–0.27wt%), MgO (0.09–0.89wt%) and CaO (0.08–0.72wt%). Based on the presence of mafic alkali phenocrysts, such as arfvedsonite and siderophyllite, high Zr/Nb ratios (>10) and peralkalinity index (PI) near unity, the high-Si rhyolites can be classified as peralkaline comendites. The high-Si rhyolites are characterized by unusually low Sr and Ba, and high abundance of Zr, Th, Nb, HREEs and Y. They show geochemical characteristics similar to those of typical A2-type granites including their high K2O+Na2O, Nb, Zr and Y, and high ratios of FeOT/MgO, Ga/Al and Y/Nb. Our study suggests that the high-Si rhyolites were derived from discrete trachytic parent magma with fractional crystallization within shallow magma reservoirs. Their Nd-Pb isotopic characteristics are similar to those of the calc-alkaline arc rocks and are compatible with partial melting of pre-existing juvenile continental arc crust. We observe that the widespread eruptions of A2-rhyolitic magmas (305.3Ma–303.4Ma) following a short period of magmatic quiescence was temporally and spatially associated with voluminous intrusion of the bimodal magmas (304.3Ma–299.3Ma) in the pre-existing arc volcanic-plutonic belt (329Ma–307Ma). We envisage northward subduction and slab breakoff process resulting in an obvious change of the regional stress field to extensional setting within the Carboniferous continental arc running E-W for thousands of kilometers. Therefore, we propose the existence of an east-west-trending Carboniferous continental arc in the Hegenshan ophiolite-arc-accretion complex, with the slab breakoff event suggesting that the age of the upper sequence (305.3±5.5Ma) likely indicates the maximum age for the cessation of the northward subduction of the Hegenshan oceanic lithosphere.

Publisher URL: www.sciencedirect.com/science

DOI: S1342937X1630510X

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