Early Devonian back-arc extension in the eastern Central Asian Orogenic Belt: Evidence from a bimodal volcanic sequence from Xilinhot, central Inner Mongolia (North China)

The Early Devonian bimodal volcanic sequence is firstly recognized in the Xilinhot area, central Inner Mongolia (North China). Zircon U-Pb dating of,rhyolitic sample gives crystallization age of 407 +/- 2 Ma, which is interpreted as the extrusive age of this bimodal volcanic sequence. Basaltic samples belong to tholeiite series whereas rhyolitic samples are peraluminous. Basaltic rocks show typical N-MORB-like REE and trace elemental patterns, with depletion of LREEs and negligible anomalies of Eu (delta Eu= 0.83-1.00). They have initial Sr-87/Sr-86 ratios ranging from 0.7077 to 0.7086, and positive epsilon(Nd(t)) values from +7.5to +9.0. By contrast, rhyolitic rocks show enrichment in LREEs and LILEs but depletion in HFSEs, with negative Eu anomalies (delta Eu = 0.58-0.68). They have negative epsilon(Nd(t)) values from -6.7 to -7.7 and T-DM2 (Nd) values from 1695 to 1771 Ma. These elemental and isotopic data indicate that basaltic rocks were derived from a depleted mantle source with input of slab-derived fluids, whereas rhyolitic rocks might have been derived from remelting of Paleoproterozoic crustal materials. From our data and previous geological studies in this region, a back-arc setting was proposed for the Early Devonian bimodal volcanic rocks in the Xilinhot region. Subduction of the Paleo-Asian oceanic lithosphere caused opening of this back-arc basin and upwelling of mantle caused the formation of basalts and provided heat for remelting of crustal materials and formation of rhyolite.the (C) 2017 Elsevier Ltd. All rights reserved.