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

摘要

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 geochem-ical 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 ~(206)Pb/~(238)U ages of 474 ± 7 Ma, 453 ± 7 Ma and 436 ± 9 Ma 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 ± 11 Ma for a sillimanite gneiss and 437 ± 5 Ma 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 ± 2 Ma. A diorite from a weakly deformed diorite-granodiorite pluton in the low-P/T complex has a zircon age of 419 ± 10 Ma that correlates in time with regional collisional magmatism. An undeformed pegmatite dike cutting the low-P/Tmetamorphic complex has an age of 411 ± 8 Ma, 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.