The mechanism of continental growth of the Altaids is currently under debate between models invoking continuous subduction-accretion or punctuated accretion by closure of multiple ocean basins. We use the Yueyashan-Xichangjing ophiolite belt of the Beishan Collage (southern Altaids) to constrain the earliest oceanic crust in the southern Paleoasian Ocean. Five lithotectonic units were identified from south to north: the Huaniushan block, a sedimentary passive margin, the structurally incoherent Yueyashan-Xichangjing ophiolite complex, a coherent sedimentary package, and the Mazongshan island arc with granitic rocks. We present a structural analysis of the accretionary complex, which is composed of the incoherent ophiolitic melange and coherent sedimentary rocks, to work out the tectonic polarity. A new weighted mean 206 Pb/ 238 U age of 534.4 ± 3.4 Ma from a plagiogranite in the Yueyashan-Xichangjing ophiolite indicates that the ocean floor formed in the early Cambrian. Furthermore, we present new geochemical data to constrain the tectonic setting of the YueyashanXichangjing ophiolite, and obtained seven U-Pb ages of Sandstone and granitoid and five Ar/Ar ages. The Yueyashan-Xichangjing ophiolite was emplaced as a result of northward subduction of an oceanic plate beneath the Mazongshan island arc to the north at about 420 Ma. Together with data from the literature, our work demonstrates that there were multiple overlapping periods of accretion existed in the Palaeozoic in the northern and southern Altaids. Therefore, a model of multiple accretion by closure of several ocean basins is most viable.
Key words:Beishan, Yueyashan-Xichangjing, ophiolite, Accretionary complex, Altaids. The Central Asian Orogenic Belt (CAOB) is characterized by amalgamation of various island arcs, ophiolites, accretionary prisms, oceanic plateaus and possibly some micro-continents, during the progressive subduction-accretion processes. Isotopic data suggest significant Phanerozoic crustal growth in the CAOB, whereas recycling of ancient crust is also addressed by some recent researchers. The Russian Altai is one of the key components of the CAOB, consisting of part of the Altai-Mongolian terrane (AM) in Russia, the Gorny Altai terrane (GA) and the Charysh-Terekta-Ulagan-Sayan suture (CTUSs) zone. Voluminous granitoids and their volcanic equivalents, as well as a series of Paleozoic sedimentary and meta-sedimentary rocks occur, but they have not been well investigated. Age and source nature of these rocks can provide critical constraints on the crustal growth, sedimentary basin evolution, and tectono-thermal events in Russian Altai area.In this study, two volcanic rocks and a biotite-gneiss along the CTUSs zone were collected for zircon U-Pb and Hf-isotope analysis. Zircons from the two volcanic rocks yield consistent crystallization ages of 420.9±1.4 Ma and 421.4±2.5 Ma, respectively, indicating a magmatic activity in the latest Silurian. These zircons are characterized by low initial 176 Hf/ 177 Hf isotopic ratios, with ε Hf (...