THE CONTROLS ON THE MAJOR AND TRACE-ELEMENT VARIATION OF SHALES, SILTSTONES, AND SANDSTONES OF PENNSYLVANIAN-PERMIAN AGE FROM UPLIFTED CONTINENTAL BLOCKS IN COLORADO TO PLATFORM SEDIMENT IN KANSAS, USA

Shales, siltstones, and sandstones of Pennsylvanian-Permian age from n ear the source in Colorado to those in the platform in eastern Colorad o and Kansas have been analyzed for major elements and a number of tra ce elements, including the REEs. The near-source sandstones are signif icantly more enriched (Student t-test at better than the 99% confidenc e level) in SiO2 and Na2O concentrations and more depleted in Al2O3, F e2O3 (total), TiO2, Th, Hf, Sc, Cr, Cs, REEs, Y, and Ni concentrations and La/Co and La/Ni ratios than the near-source shales and siltstones , most likely due to more plagioclase and quartz and less clay mineral s in the sandstones than in the shales and siltstones. There are no si gnificant differences in K2O and Sr concentrations and Eu/Eu, La/Lu, La/Sc, Th/Co, and Cr/Th ratios between the near-source sandstones and the near-source shales and siltstones. Samples of the Molas, Hermosa, and Cutler formations near the source that were formed in different en vironments in the same area contain no significant difference in Eu/Eu , La/Lu, La/Sc, Th/Sc, Th/Co, and Cr/Th ratios, so a generally silici c source and not the environment of deposition was most important in p roducing these elemental ratios. For example, Cr/Th ratios of near-sou rce shales, siltstones, and sandstones range from 2.5 to 17.5 and Eu/E u range from 0.48 to 0.78, which are in the range of sources of sedim ents derived from mainly silicic and not basic sources. Near-source sh ales and siltstones contain significantly higher (Student t-test) and more varied concentrations of most elements (Al2O3, Fe2O3, MnO, TiO2, Ba, Th, Hf, Ta, Co, Sc, REEs, Nb, Y) but significantly lower concentra tions of Na2O and Eu/Eu than platform shales and siltstones in Kansas (e.g., La = 65.7 +/- 40 and Eu/Eu = 0.55 +/- 0.07 in near-source sha les and siltstones and La = 23.7 +/- 8.7 and Eu/Eu = 0.64 +/- 0.08 in platform shales and siltstones). The SiO2 and CaO concentrations are not significantly different in platform shales and siltstones compared to the near-source shales and siltstones, so dilution of other minera ls by quartz and calcite is not the main reason for the lower concentr ation of most elements in the platform relative to the near-source sha les and siltstones. Rather the lesser concentrations of most elements in clay minerals of the platform shales and siltstones can account for the lower concentration of most elements compared to corresponding ne ar-source shales and siltstones. The lower concentrations of many elem ents in clay minerals in the platform shales and siltstones may be a r esult of having been derived from recycling of clay minerals from olde r rocks. The greater homogeneity of elemental concentrations of the pl atform shales and siltstones compared to those in the source is also c onsistent with homogeneous mixing of such recycled material. Also ther e is no significant difference in Th/Sc, La/Co, Th/Co, La/Ni, and Cr/T h ratios of the near-source sedimentary rocks in Colorado to the platf orm shales and siltstones in Kansas, and the latter are also consisten t with derivation from mostly silicic source rocks.