The geochemistry of shales, siltstones and sandstones of Pennsylvanian-Permian age, Colorado, USA: implications for provenance and metamorphic studies

A series of shales and sandstones found near the source of the Sangre de Cr isto, Belden, and Maroon Formations from central Colorado were examined pet rographically and were analyzed for major and selected trace elements, incl uding the REE. The sandstones from the Belden Formation have higher quartz/ feldspar ratios than do those from the Maroon and Sangre de Cristo Formatio ns. Also, the alkali feldspar (i.e., orthoclase, microcline, perthite)/plag ioclase ratio decreases in the order Sangre de Cristo Formation > Maroon Fo rmation > Belden Formation, but the CIW' (chemical index of weathering = mo lecular [Al2O3/(Al2O3 + Na2O)]*100) decreases in the order Belden Formation > Sangre de Cristo Formation > Maroon Formation. This suggests that the Be lden Formation had a more plagioclase-rich granitoid source and more intens e weathering of the source than did the Maroon and Sangre de Cristo Formati ons. Also, the variation in the elemental composition within the terrigenou s sediment may be explained in terms of the variation in the observed miner als. Elemental ratios critical of provenance are statistically the same bet ween the finer sediment of the Maroon and Sangre de Cristo Formations and f all within the range of a granitoid provenance, suggesting a similar granit oid source composition for the two formations. The fine sediment from the B elden Formation, however, has significantly more negative Eu anomalies and lower La/Sc and Th/Cr ratios than those of the Maroon and Sangre de Cristo Formations, suggesting a more differentiated granitoid source for the Belde n than for the Maroon and Sangre de Cristo Formations. Most elemental conce ntrations or ratios vary by a factor of 0.12 to 60 between adjacent fine an d coarse sediment (< 1-m distances). Thus, it is not recommended that metas edimentary sequences similar in composition to this study be examined to de termine element mobility during metamorphism as the variation due to sedime ntary processes is so large. (C) 2000 Elsevier Science B.V. All rights rese rved.