Granitic gneisses and gneissic granites from the Central Terrane of the Chinese Altai Orogen: zircon ages and tectonic significance

Abstract

The granitic gneiss and gneissic granite are two major components of the Central Terrane of the Chinese Altai Orogen. The former is intruded by the latter and is considered to have Sinian or Ordovician ages. Samples of the granitic gneiss and gneissic granite were collected from the western part of the Central Terrane. Both of these rock types have granitic compositions with arc affinities, but can be chemically distinguished. For example, the granitic gneisses are characterized by high Mg, Ni, and Cr contents. Zircon U- Pb isotope analyses by LA-ICP-MS indicate that the granitic gneisses and gneissic granites were crystallized at 470-500 Ma and 380-420 Ma, respectively. Inherited zircons with Neoproterozoic ages are common and 2.5 Ga zircons also occur in the granitic gneisses, but only one gneissic granite sample contain some Neoproterozoic inheritance zircons. Amphibolites and greenschists also occur in the study area. These rocks are envisaged as metamorphic volcanic rocks and they have island arc geochemical features. Zircons separated from a amphibolite sample give 400 Ma U-Pb ages. Based on these results and geochemical data, we propose that an undefined Precambrian continental block with significant amount of Neoproterozoic rocks along its margin was intruded by the protolith of the granitic gneisses at 470-500 Ma in a continental magmatic arc environment. On the other hand, the gneissic granites were formed in an island arc environment about 100 Ma later, as evidenced by the absence of Neoprterozoic inheritance zircons and the contemporaneous island arc volcanism. This study is supported by Hong Kong RGC (HKU7040/04P).