GEOCHEMISTRY OF TRIASSIC SANDSTONES (HIGH ATLAS OF MARRAKECH, MOROCCO): IMPLICATIONS FOR PROVENANCE, WEATHERING, AND TECTONIC SETTING.

The geochemical composition of fifteen fresh Triassic sandstone samples from two localities (Oukaimeden and Ourika Valley, Morocco) was carried out, in order to assess their provenance, weathering and tectonic setting. These sandstones are classified as litharenites and wackestones. The CIA (Chemical Index of Alteration; mean value GK= 57.6 and GI = 71.77) values and Al2O3- CaO +Na2O-K2O suggest that the source materials for these sandstones must have been subjected to low and moderate degree of weathering and reworking. Sandstones from studied areas had a high SiO2content in the range of 62-82% due to their mineralogical maturation. However, the negative correlation between SiO2content and the other major elements proposes another contribution of silica. AIT KEJOU sandstones show the presence of a small amount of clay minerals than IGHERMANE sandstones. The Upper Continental Crust (UCC) and North American Shales (NASC) normalization showed depletion in light rare earth elements (LREE) and enrichment in HREE. The negative europium (Eu/Eu*) anomaly support a felsic rocks provenance of the studied sandstones. The cerium (Ce/Ce*) anomaly indicated an oxidizing depositional environment. The depletion in LREE in relation to the HREE and low La/Lu ratio values suggest that the sandstones are as a result of fluvial/river channel system with a slight sea contribution. According to the provenance discrimination diagram, the sandstones were derived from the adjacent Cambrian basement units of Tifnout granitic massif, associated andesites and the granodiorites of the Precambrian basement units of Central High Atlas, located to the south of the studied areas. Discrimination diagrams suggested a passive margin tectonic setting for AIT KEJOU sandstones and an active continental margin tectonic setting for the IGHERMANE sandstones.