The influence of sediment recycling and basement composition on evolution of mudrock chemistry in the southwestern United States

This paper reports systematic changes in mudrock composition through time on a single continental crustal block. The changes reflect both sediment recycling processes and changes through time in the composition of crystalline material being added to the sedimentary system and are related to tectonic evolution as the block matures from a series of accreted arc terranes to a stable craton. The major and trace element distributions reflect different aspects of the provenance of the mudrocks in this study. Major elements record sediment recycling processes as well as changing proportions of sedimentary and first-cycle source rocks. With the exceptions of K2O (which tends to increase), and SiO2 and Al2O3 (which show no trend), most major oxides tend to decline in relative abundance in younger mudrocks. Patterns shown by the Index of Compositional Variability ([Fe2O3 + K2O + Na2O + CaO + MgO + MnO + TiO2]/Al2O3) and by K2O/Al2O3 indicate that the major oxide trends are due to decreasing proportions of nonclay silicate minerals and a concomitant increase in the proportion of clay minerals, probably due to decreasing input of first cycle detritus coupled with recycling of sedimentary material. Excursions from progressive trends, marked by increases in MgO, K2O, and CaO, reflect episodes of large-scale input of nonclay first-cycle minerals from crystalline source rocks due to large-scale basement uplift. The chemistry of low-solubility trace elements, in contrast, is not sensitive to recycling effects and reflects the composition of first-cycle input. Incompatible elements are progressively enriched relative to compatible elements in younger mudrocks, and values for chondrite normalised rare earths elements also increase. In addition, the Eu anomaly becomes systematically more negative in younger samples. These trends cannot be explained by diagenetic or weathering processes, and, therefore, indicate that the proportion of fractionated granitic first-cycle detritus being added to the sedimentary system becomes greater with time. These results confirm the importance of tectonic setting in controlling mudrock chemistry, and also demonstrate that there is a dynamic relationship between the tectonic evolution of a continental block and the composition of its sedimentary mantle. © 1995.