CAMBRIAN TO EARLY SILURIAN OPHIOLITE AND ACCRETIONARY PROCESSES IN THE BEISHAN COLLAGE

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.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 ²⁰⁶Pb/²³⁸U 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 Yueyashan-Xichangjing ophiolite. The Yueyashan-Xichangjing ophiolite was emplaced as a result of northward subduction of an oceanic plate beneath the Mazongshan island arc to the north in the late Ordovician to early Silurian. 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.