THE ALTAI MOUNTAINS IN CHINA, MONGOLIA AND RUSSIA: A CONTINUOUS SUBDUCTION SYSTEM IN THE EARLY PALEOZOIC

The Altai Mountains extend from Russia, though northwestern China into western Mongolia. Paragneisses of this orogenic belt occur in China and Mongolia and they were previously regarded as Precambrian basement. However, detrital zircon ages are dominantly 465-542 Ma old, with εHf(t) values varying from −25 to +15, indicating that these rocks were deposited in the Early Paleozoic. Old detrital zircons do exist, a few cluster at 1.8–2.0 Ga and sparse ones yield discordant ages around 2.3–2.6 Ga. The low-grade metasedimentary rocks of this orogenic belt occur as the dominant sedimentary sequences in China, Mongolia and Russia, and show zircon age spectra similar to the paragneisses. An systematic comparison of zircon U-Pb and Hf isotopic data with surrounding continental blocks indicate that the Precambrian zircons resemble those from old rocks preserved in the western Mongolia and its adjacent arc terranes, thus the old sedimentary detritus were possibly derived from the micro-continents and arc terranes in western Mongolia. Voluminous granitic intrusions in the region mostly give positive εHf(t) values, supporting that the Altai orogenic belt was on an Early Paleozoic active continental margin and precluding the existence of unexposed Precambrian basement in the region. The whole mountain belt possibly represents a continuous Paleozoic active continental margin rifted away from an old continent, like the NE Japan.

The research is supported by Hong Kong Research Grant Council (HKU705311P, HKU704712P and HKU704013P), National Science Foundation (41273048), and a HKU CRCG grant. The work is a contribution to IGCP 592 by the Joint Laboratory of Chemical Geodynamics between HKU and CAS (Guangzhou Institute of Geochemistry).