Ordovician metasedimentary rocks are the oldest and most extensive sedimentary sequence in the Chinese Altai. They experienced two major episodes of deformation (D1 and D2) resulting in the formation of juxtaposed Barrovian-type and migmatite domains. D1 is characterized by a penetrative sub-horizontal fabric (S1), and D2 is marked by upright folds (F2) with NW−SE trending axial planes in shallow crustal levels and by sub-vertical transposition foliations (S2) in the high-grade cores of large scale F2 antiforms. In the Barrovian-type domain, successive growth of biotite, garnet and staurolite is observed in the S1 fabric. Kyanite included in garnet and plagioclase in the migmatite domain is interpreted to have formed also in S1. In the biotite and garnet zones, the spaced S2 cleavage is marked by biotite and muscovite, and in the staurolite and kyanite zones, the penetrative S2 fabric is characterized by sillimanite, locally with late cordierite. Phase equilibria modelling indicates that the S1 fabric was associated with an increase of pressure and temperature under Barrovian-type conditions in both domains. The S2 fabric was related to decompression, in which rocks in the biotite and garnet zones well preserve the peak assemblage and the higher grade rocks in the staurolite and kyanite zones reequilibrated to different degrees under high-temperature/low-pressure (HT/LP) conditions. The D1 metamorphic history is attributed to the progressive burial related to Early-Middle Palaeozoic crustal thickening and the Accepted ArticleThis article is protected by copyright. All rights reserved. metamorphism associated with D2 is interpreted to result from exhumation by vertical extrusion. The extrusion of hot rocks was contemporaneous with the formation of gneiss domes accompanied by the intrusion of juvenile magmas at middle crustal levels during the Middle Palaeozoic. Consequently, there is a genetic link between the Barrovian-type and migmatite domains related to continuous transition of the Barrovian-type fabric into the HT/LP one during development of domal structures in the southern Altai orogenic belt. This study has a broad impact on the understanding of the thermo-mechanical behaviour of accretionary orogenic systems worldwide. The lower crustal flow and doming of hot crust so far reported only in continental collisional settings seems to be also an integral mechanism responsible for both horizontal and vertical redistribution of accreted material prior to continental collision.

Zircons were separated from granitoids, gneisses, and sedimentary rocks of the Chinese Altai. Those with igneous characteristics yielded U-Pb ages of 280-2800 Ma, recording a long history of magmatic activity in the region. Zircon Hf isotopic compositions show an abrupt change at ~420 Ma, indicating that prior to that time the magmas came from both ancient and juvenile sources, whereas younger magmas were derived mainly from juvenile material. This may imply that the lithosphere was significantly modified in composition by a rapid addition of melt from the mantle. We suggest that this dramatic change was due to the onset of ridge subduction, which can account not only for the formation of voluminous granitoids, mafic rocks with complex compositions, and the association of adakite + high-Mg andesite + boninite + Nb-enriched basalt, but also for the coeval high-T, low-P metamorphism.Paleozoic, ridge subduction, Hf isotope, granite, Altai Zircon is ideal for Hf isotopic study, because the crystal chemistry of zircon allows entrain of large amounts of Hf, but not Lu, thus the radiogenic daughter of 176 Hf from the decay of 176 Lu is extremely low in zircon, and the 176 Hf/ 177 Hf ratios of the zircon reflect those of the precursor magma [1] . Significant advances have been made on zircon Hf isotopic systematics, for example the Laboratory of Lithospheric Evolution in the Institute of Geology and Geophysics, Chinese Academy of Sciences has contributed greatly to the technological aspects of this work.For this study, samples were collected from granitoids, gneisses, and sedimentary rocks of different age in the Chinese Altai and analyzed in the above-mentioned Laboratory of Lithospheric Evolution. Our results show an abrupt change in zircon Hf isotopic compositions at ~420 Ma, which may be result of an important regional geological event. Based on an evaluation of the regional geology, geochemical data, and metamorphic development, we suggest that the observed change may mark the onset of ridge subduction. We plan to test this model with further research.