EARLY PALEOZOIC ISLAND ARC VOLCANISM OF THE CHINGIZ ARC (KAZAKHSTAN) AND SEDIMENTARY PROVENANCE: ABSENCE OF A CONTINENTAL SOURCE

Previous geological studies provide evidence suggesting that the early Paleozoic complexes represented within the Chingiz range of eastern Kazakhstan were formed in a volcanic arc environment. Although there are no regional exposures of complexes older than Cambrian, voluminous felsic volcanic complexes and intrusions of Cambrian age suggest Precambrian continental basement may have been a component of early Paleozoic arc activity. In order to discriminate between an active early Paleozoic intra-oceanic arc and a peri-continental arc environment we have undertaken a geochemical study targeting the voluminous andesitic volcanics and associated clastic rocks of Late Cambrian through Late Ordovician age within the Chingiz range. Isotope composition of Sr and Nd as well as concentration of rare-earth elements (REE’s) has been analyzed. In addition, we are attempting to correlate paleomagnetic findings (also presented at this meeting) with the constraints imposed by the geochemical data. Prelimanary results indicate that the majority of the volcanic suites are basaltic-andesite in composition and are predominately calc-alkaline in nature. Isotopic data and REE concentrations from volcanic lithologies suggest no contribution from continental crustal material. Geochemical analyses, of volcaniclastic sediments exhibit similar REE concentrations as the volcanic rocks related to the Chingiz volcanic arc, suggesting that the sedimentary units studied are derived primarily from intra-oceanic volcanics, with little to no continental contribution. While the geochemical data obtained suggest that there was no early Paleozoic continental source available to the basins sampled in this study, the idea of a continental mass proximal to the Chingiz volcanic arc cannot be excluded entirely. Our study was restricted to the center region of the Chingiz arc, therefore allowing for several scenarios where continental signatures may be represented within other, separate, arc-related sedimentary basins.