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 Cristo, Belden, and Maroon Formations from central Colorado were examined petrographically and were analyzed for major and selected trace elements, including the REE. The sandstones from the Belden Formation have higher quartz/feldspar ratios than do those from the Maroon and Sangre de Cristo Formations. Also, the alkali feldspar (i.e., orthoclase, microcline, perthite)/plagioclase ratio decreases in the order Sangre de Cristo Formation > Maroon Formation > Belden Formation, but the CIW' (chemical index of weathering = molecular [Al2O3/(Al2O3 + Na2O)]*100) decreases in the order Belden Formation > Sangre de Cristo Formation > Maroon Formation. This suggests that the Belden Formation had a more plagioclase-rich granitoid source and more intense weathering of the source than did the Maroon and Sangre de Cristo Formations. Also, the variation in the elemental composition within the terrigenous sediment may be explained in terms of the variation in the observed minerals. Elemental ratios critical of provenance are statistically the same between the finer sediment of the Maroon and Sangre de Cristo Formations and fall within the range of a granitoid provenance, suggesting a similar granitoid source composition for the two formations. The fine sediment from the Belden 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 Belden than for the Maroon and Sangre de Cristo Formations. Most elemental concentrations or ratios vary by a factor of 0.12 to 60 between adjacent fine and coarse sediment (< 1-m distances). Thus, it is not recommended that metasedimentary sequences similar in composition to this study be examined to determine element mobility during metamorphism as the variation due to sedimentary processes is so large. (C) 2000 Elsevier Science B.V. All rights reserved.