Early Paleozoic sedimentary record of the Chinese Altai: Implications for its tectonic evolution

Abstract

The Chinese Altai is a typical part of the Central Asian Orogenic Belt (CAOB), but its tectonic evolution is not well understood. Geochemical study of Early Paleozoic metasedimentary rocks was conducted to better constrain their provenance and tectonic environment. These rocks have relatively low CIA values (49 to 79), with ICV values (0.73-1.36) higher than that of post-Archean Australian average shale (PAAS), suggesting that they experienced relatively weak chemical weathering and were mainly derived from an immature source. Compared to PAAS and upper crust, the samples mostly have relatively lower high field strength elements (HFSE) (e.g. Zr, Hf, Nb, Ta) and large ion lithophile element (LILE) contents (e.g. Rb, Sr, Ba, Th, U, Pb). However, their relatively high Rb concentrations (> 40 ppm), low Rb/Sr (0.04-3.24) and Th/U (Th/U = 3-8) ratios are indicative of an acidic-intermediate igneous source that had undergone weak chemical weathering. All of the samples are moderately enriched in LREE (light REE) and show relatively flat Chondrite-normalized HREE (heavy REE) patterns (La N/Yb N = 3.2-12.6) with various negative Eu anomalies (Eu/Eu* = 0.4-0.9). Meta-arenaceous samples have moderate SiO 2 (average 72.9 wt.%), Al 2O 3 (average 11.5 wt.%), Fe 2O 3 T + MgO (average 7.16 wt.%) contents and moderate Al 2O 3/SiO 2 (average 0.16), K 2O/Na 2O (average 0.93) ratios, similar to those of graywackes from continental island arcs or active continental margins. However, meta-argillaceous samples have high SiO 2 (average 60.8 wt.%), Al 2O 3 (average 17.2 wt.%), Fe 2O 3 T + MgO (average 12.2 wt.%) contents and high K 2O/Na 2O (average 3.45), Al 2O 3/SiO 2 (average 0.28) ratios similar to mudstones from continental arc basins. This kind of tectonic environment is also manifested in tectonic discrimination diagrams including Th-Sc-Zr/10 and La-Th-Sc. These geochemical characteristics suggest that the metasedimentary rocks were deposited in an active continental margin or continental arc setting. This does not favor the previously proposed passive continental margin model. Therefore, this study supports an arc accretion model for the Chinese Altai. © 2008 Elsevier B.V. All rights reserved.