THE COMPOSITION OF SHALES FROM THE ORDOS BASIN, CHINA - EFFECTS OF SOURCE WEATHERING AND DIAGENESIS

The Ordos basin, situated in northwestern China, is the biggest Mesozo ic terrigenous sedimentary basin developed on the North China Platform . The geochemical data imply that shales from the Ordos basin were mai nly derived from a source area with an average Archaean granite compos ition. High Na2O contents, very limited variation of Ti/Zr, and ratios of (Gd/Yb)(N) higher than 2.0 indicate that Ordos shales are less mat ure than typical post-Archaean shales such as NASC and PAAS. Highly va riable CIA values (chemical index of weathering)indicate that most Ord os shales are less weathered than NASC and PAAS. Ratios of (La/Sm)(N) and total REE contents for the Ordos shales are positively correlated with CIA, whereas those of (Gd/Yb)(N) inversely vary with CLA. These s uggest that the weathering of source rocks is responsible for the REE fractionation in the shales of the Ordos basin. Depletion of Eu in cla y-sized fractions of the Ordos shales decreases with enhanced crystall inity of illite. Considering that ratios of Fe3+/Fe2+ in the shales de crease with the increase of crystallinity of chlorites, we propose tha t Eu anomalies in shales are directly related to the reduction of iron during burial diagenesis. (C) 1998 Elsevier Science B.V. All rights r eserved.