Magmatic and metamorphic development of an early to mid-Paleozoic continental margin arc in the southernmost Central Asian Orogenic Belt, Inner Mongolia, China

Abstract

The Bainaimiao arc in Inner Mongolia, China, comprises a weakly metamorphosed volcani-sedimentary sequence and a low-P/T metamorphic complex. In this study we present SHRIMP zircon ages and geochemical data to document the temporal and genetic relationships between these two tectonic units. Zircons from a rhyolite and two dacites (high-K calc-alkaline) of the volcani-sedimentary sequence yielded 206Pb/238U ages of 474±7Ma, 453±7Ma and 436±9Ma respectively, that we interpret as recording the timing of three volcanic arc episodes. Rocks from the low-P/T metamorphic complex yielded zircon ages of 462±11Ma for a sillimanite gneiss and 437±5Ma for a plagioclase–hornblende gneiss, reflecting two distinct anatectic events. Also from the low-P/T complex, the protolith age of a metadiorite is 438±2Ma. A diorite from a weakly deformed diorite–granodiorite pluton in the low-P/T complex has a zircon age of 419±10Ma that correlates in time with regional collisional magmatism. An undeformed pegmatite dike cutting the low-P/T metamorphic complex has an age of 411±8Ma, which postdates collision and provides an upper limit for the termination of orogeny. Accordingly, we conclude that the volcani-sedimentary sequence and the low-P/T metamorphic complex evolved coherently in the early to mid-Paleozoic and formed an integral continental margin arc in the southernmost Central Asian Orogenic Belt.