Abstract

Petrographical and geochemical analysis of altered tuffaceous rocks of the Middle Triassic from the Iberian Range in Central Spain shows that pedogenic processes and subsequent diagenesis have strongly modified the original mineralogy and textures. These rocks are related to a magmatic episode that occurred at the Buntsandstein-Muschelkalk unconformity in this region, during the evolution of the Iberian Trough. The tuffaceous rocks accumulated in saline ponds in an open and terminal flood-plain setting. Their alteration is typified by neoformation of K-feldspar and illite. Precipitation of chert and carbonate cements and replacements were also important processes, resulting in intercalated chert beds and nodules. Much of the silica appears to be derived from extensive devitrification and weathering of glassy volcanic ash. X-ray diffraction analysis, microscopic observations, and EDX microanalysis reveal that the chert consists of various types of chalcedony, tridymite, micro and megaquartz, illite, K-feldspar, and small amounts of albite. In some of the nodules dolomite-ankerite is also common, enclosing anhydrite and barite relicts, which are partially silicified. Different levels of ammonium enrichment have been found in each petrofacies type: tuffs and tuffaceous rocks contain from 27 to 326 ppm (mean = 139) of NH 4 (super +) . Underlying arkosic sandstones with authigenic K-feldspar contain NH 4 (super +) ranging from 48 ppm to 191 ppm (mean = 124). The NH 4 (super +) was probably derived from decay of organic matter in the evaporitic environment of deposition and its incorporation into feldspar and illite as they formed from unstable volcanic glass, sanidine, plagioclase, and vitric volcanic fragments. This transformation could have occurred via the intermediate phases of smectite and zeolites. The tuffaceous rocks also have a high content of B, ranging from 61 ppm to 170 ppm (mean = 101). However, high levels of B are not positively correlated with high levels of NH 4 (super +) in the samples. Both B and NH 4 (super +) can be contained in either K-feldspar or illite, but it is not possible to know how much is present in each mineral without separating the minerals. Comparison of spidergrams of the tuffaceous rocks with those in the underlying arkosic sandstones indicates similar patterns with positive anomalies in La, Sm, and Y, and negative in Nb, P, Ti, and Yb, although the tuffaceous rocks are slightly richer in total REE. These data suggest that the distribution of REE must also be controlled by weathering and diagenetic reactions, and cannot be used for discrimination between volcanic and nonvolcanic sandstones.

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