NEOPROTEROZOIC TO PALEOZOIC GEOLOGY OF THE ALTAI OROGEN, NW CHINA: NEW ZIRCON AGE DATA AND TECTONIC EVOLUTION

Abstract

We present a synthesis and a new account of the geological and tectonic history of the terranes of the Chinese Paleozoic Altai orogen together with new, single zircon ages for granitic and rhyodacitic rocks. A central terrane consists of Neoproterozoic to Silurian, amphibolite facies, metasedimentary rocks, and abundant Devonian-Carboniferous granites. The presence of Precambrian basement is indicated by Sinian fossils, our xenocryst ages, and published Nd mean crustal residence ages of granites. Felsic arc-type lavas on the southern margin of the terrane have a mean (207)Pb/(206)Pb zircon age of 505 Ma, reflecting the time of arc volcanism, and the presence of xenocysts with ages between 614 and 921 Ma suggests derivation by intracrustal melting. Accordingly, we suggest that a Cambro-Ordovician continental magmatic arc was built on the southern margin of the central terrane by northward subduction. A low-grade Ordovician Andean-type arc with a continental basement is situated above a normal fault on the northern side of the central terrane, and a low-grade Late Silurian to Early Devonian island arc on its southern side is succeeded southward by a terrane with Proterozoic basement overlain by Devonian to Carboniferous basins. During continent-arc collision high-grade gneisses of the central terrane were thrust southward over the Late Silurian to Early Devonian island arc with formation of inverted, Barrovian-type metamorphic isograds. The collisional processes caused exhumation of the high-grade central terrane and consequent emplacement of abundant granites derived by mixed arc-crust melting. This new model has major implications for the crustal and tectonic evolution of the Altaids.