Timing,Petrogenesis,and Setting of Paleozoic Synorogenic Intrusions from the Altai Mountains,Northwest China: Implications for the Tectonic Evolution of an Accretionary Orogen

摘要

The Altai Mountains are a key area for understanding the development of the Altai Tectonic Collage and accretionary orogen.However,the orogenic processes,particularly their early stage,have not been well understood.In this work,we undertake zircon U-Pb dating of six Paleozoic synorogenic plutons in order to better define the early magmatic and tectonic evolution of the Chinese Altai Mountains.The results revealed three Paleozoic granitic plutonic events at ca.460,408,and 375 Ma.These ages,along with the structural patterns of the plutons,suggest two periods of regional deformation,460-410 Ma and 410-370 Ma.The granitoids mainly follow the tholeiitic and calc-alkaline trends and are mostly I type.Sr-Nd isotopic analyses indicate that the sources of the granitoids contain both old continental and younger (juvenile) mantle-derived components.Chemical,isotopic,and structural features suggest that the plutons were formed mainly in continental arc settings and that the subduction and accretion processes began at ca.460 Ma and culminated at ca.408 Ma.Thus,the Altai orogen was mainly built up during early-middle Paleozoic time,rather than during late Paleozoic time.Furthermore,the southern Altai terrane comprises not only Silurian to Devonian island arcs but also old continental fragments.With these new constraints,we present a new model to account for the tectonic evolution of the Altai orogen.This model proposes that early-middle Paleozoic Altai orogenic processes could have experienced formation of an active continental margin,the splitting of this margin to form a back-arc oceanic basin,and the final closing of the back-arc basin.Consequently,the opening and closure of back-arc basins along active margins is probably a common process in the central Asian accretionary orogen.