Geochemistry and Nd isotopic composition of the Early Paleozoic flysch sequence in the Chinese Altai, Central Asia: evidence for a northward-derived mafic source and insight into Nd model ages in accretionary orogen

摘要

Although Nd isotopic composition of sedimentary rocks is considered as a useful tool to trace the provenance, their Nd model age (T DM) is usually misused to discuss the timing of crustal formation in accretionary orogen. In the Central Asian Orogenic Belt, an Early Paleozoic flysch sequence extensively occurs in the Chinese Altai and adjacent western Mongolian terranes. Sedimentary rocks in the northwestern Chinese Altai were collected from the flysch sequence for geochemical and Nd isotopic studies. Although these rocks show different geochemical characteristics (e.g., arenaceous and argillaceous), they all have relatively low CIA (Chemical Index of Alteration) values (49-75), indicating a relatively weak weathering source or the absence of compositionally mature alumina-rich minerals produced by chemical weathering processes. Their relatively high ICV (Index of Compositional Variability) values (0.95-1.35) reveal an immature source and indicate that these rocks were probably deposited in an active tectonic setting. The sedimentary rocks from the flysch sequence have high MgO, Sc, Co and low Y, Nb, which suggest significant addition of mafic rocks into the source. Modeling based on Nb-Cr and Zr-MgO contents displays nearly 20% to 50% addition of mafic materials. In combination with the age and distribution of mafic rocks in the Chinese Altai and adjacent terranes, a northward-derived mafic source is recommended for the sedimentary sequence. Although the sedimentary rocks have Nd model ages (T DM=1.44-1.57Ga) older than Devonian granites (T DM=1.6-1.1Ga), young detrital zircons from the sediments and inherited zircons from the granites (450-500Ma) suggest a mixed source consisting of both old crust components and juvenile or less evolved crustal components. This reveals that Nd model ages (T DM) of metasedimentary rocks and igneous rocks can hardly provide close constrains on the timing of crustal formation in accretionary orogen. With the progresses in adjacent western Mongolian terranes, a tectonic scenario dominated by subduction-accretionary process is suggested to interpret the formation of the Early Paleozoic flysch sequence in the Chinese Altai and adjacent terranes. © 2011 International Association for Gondwana Research.