Precambrian detrital zircons in the Early Paleozoic Chinese Altai: Their provenance and implications for the crustal growth of central Asia

Abstract

The basement characteristic of the Chinese Altai is not firmly constrained, which has strictly hindered our understanding of the early crustal growth in the region. The presence of Precambrian detrital zircons in metasedimentary rocks and xenocrystic zircons in granitoids may be regarded as evidence for the existence of unexposed old basement rocks, but can also be explained as detritus originally derived from a nearby cratonic fragment. This paper reports detrital zircon U-Pb and Hf isotopic data for both low- and high-grade metasedimentary rocks from the Chinese Altai. Zircons from these samples have similar age patterns and Hf isotopic compositions. They are characterized by an overwhelming dominance of Neoproterozoic-Early Paleozoic ages, with prominent peaks around 465-542Ma and e{open} Hf(t) values varying from -25 to +15. Besides, a few zircons cluster at 1.8-2.0Ga and sparse ones yield discordant ages around 2.3-2.6Ga, and both give negative e{open} Hf(t) values between 0 and -28. Combined with previous results in the region, our data suggest that low- and high-grade metasedimentary rocks most likely derived from similar provenances. These data, plus some xenocrystic zircon ages from the granitoid rocks, are quite comparable with the age patterns of the micro-continents and arc terranes in western Mongolia. The zircon population of 440-580Ma is dominant for gneissic granitoids within the Neoproterozoic-Early Paleozoic terranes in western Mongolian and the Chinese Altai. The Precambrian zircons (>540Ma) resemble those from old rocks preserved in the Tuva-Mongolian (TM) block and its adjacent arc terranes. Therefore, we suggest that old zircons in the Chinese Altai more likely represent the detritus recycled from western Mongolia. Moreover, information from the TM-derived Precambrian zircons, combined with previous palaeomagnetic constraints, favors northern India as the potential origin for the TM block, from which a small micro-continent drifted northwards and incorporated into the CAOB in a time interval from the Neoproterozoic to Early Paleozoic. Accordingly, the crustal growth of the CAOB in western Mongolia and the Chinese Altai could be outlined by a single accretionary system, and secular amalgamation of magmatic arcs around a Precambrian micro-continent can account for the accretionary history of the region in the Early Paleozoic time. © 2011 Elsevier B.V.