The high-grade Tseel Terrane in SW Mongolia: An Early Paleozoic arc system or a Precambrian sliver?

Abstract

The high-grade metamorphic Tseel Terrane in southwestern Mongolia is dominated by a strongly deformed sequence of migmatitic gneisses which has previously been interpreted as an early Precambrian continental sliver. However, the protolith age and timing of high-grade metamorphism remain poorly constrained. To resolve this problem, high-grade gneisses selected from the western Tseel Terrane were dated in the present study. Zircons with oscillatory zoning from three paragneisses predominantly yielded early Paleozoic ages between 450 and 550Ma, with prominent peaks at 471 and 506Ma. U–Pb dating of zircons from a granitic gneiss revealed that the protolith was emplaced at 420Ma. Zircons from a leucocratic granite vein gave a concordant age of 385±3Ma, close to ages of metamorphic zircons in the gneissic rocks, which is interpreted to record a high-temperature thermal event in the region. These results indicate that the Tseel Terrane principally consists of early Paleozoic rocks which underwent a high-grade metamorphic event in the Middle Devonian. In addition, SHRIMP dating of dominant magmatic zircons from an associated schist yielded a concordant U–Pb age of 495±5Ma, similar to those of the major zircon group in the high-grade paragneiss. A felsic tuffaceous sample yielded a U–Pb zircon age of 511±4Ma, documenting early Paleozoic volcanism which probably contributed to the deposition of the sediments in the region. Geochemically, high-grade paragneisses and low-grade schists show close similarities, and their common features suggest that their protoliths may represent significant erosion products of Cambrian arc magmatic rocks with minor old crustal components that were deposited in an active margin setting. The granitic gneiss with subduction-related chemical characteristics further favors an arc setting for the Tseel Terrane. Accordingly, our data combined with previous work suggest that the Tseel Terrane represents the constituent part of an early Paleozoic arc system situated on the southern margin of North Mongolian Domain. This arc system extended from western Mongolia to the Chinese Altai because various geological features of these two regions have close similarities.