Geochronological and geochemical study of gneiss–schist complexes and associated granitoids, Beishan Orogen, southern Altaids

Abstract

The tectonic nature of metamorphic terranes and their role in orogenesis are problematic. Here we present new U–Pb ages and geochemical data for widespread metamorphic rocks and associated granitoids from Northwest China. Orthogneisses from the metamorphic complexes have crystallization ages of ∼457, ∼452, and ∼526 Ma. One paragneiss (schist) has a maximum depositional age of 312 ± 7 Ma. Three foliated granites were emplaced at ∼450, ∼349, and ∼410 Ma, and all lack inherited Precambrian ages. The metamorphic terranes may have undergone multiple petrotectonic events as revealed by the metamorphic ages. Both the orthogneisses and granitoids show enrichment in large ion lithophile elements (LILEs) and light rare Earth elements (LREEs), and depletion in high field strength elements (HFSEs), which indicate that they formed in a subduction-generated accretionary arc setting. Our study demonstrates that the metamorphic terranes in the Beishan area, originally considered as Precambrian basement with suspected Neoarchaean to Palaeoproterozoic ages, are actually parts of early Palaeozoic arcs. The protoliths were probably metamorphosed arc plutonic and sedimentary rocks. Combined with other studies, we speculate that the Beishan Orogen formed by progressive arc accretion during the latest Neoproterozoic to early Palaeozoic time. This new interpretation has implications for other high-grade metamorphic terranes within orogens that have been assumed to represent ancient or pre-existing micro-continental blocks. If so, the importance of collision as a mechanism of mountain building has been overestimated, and the accretionary process as a mechanism of continental growth has been underestimated.