MINERALOGY AND GEOCHEMISTRY OF MIOCENE DEPOSITS AT ALCUBIERRE-SIERRA,CENTRAL SECTOR OF THE EBRO BASIN, SPAIN

Two profiles in Miocene fluvio-lacustrine deposits consist of sandy, m arly, lutitic and carbonatic levels constituted by variable percentage s of quartz, calcite and clay minerals as major components, and feldsp ars, dolomite and occasionally gypsum and anhydrite as minor ones. The clay minerals are inherited and consist mostly of micas, with minor q uantities of chlorites, pyrophyllites and kaolinites. The crystalloche mical parameters of the micas indicate muscovitic compositions and the ir uniformity through both the different rocks and their silt and clay fractions suggest the same provenance source area, possibly located n orthward. Clay minerals concentrate preferentially Li, Sc, V, Cr, Co, Ni, Cu, Zn, Rb, Cs, Ba, Zr, Hf, Th, U and REE whereas the authigenic c arbonates concentrate Mn and Sr. The Sc, Cr, Th, Y, Zr and REE values in clay minerals indicate that the provenance source area of these dep osits was similar in composition to the average continental upper crus t, probably as a result of sedimentary recycling processes. Zeolitic l evels constituted by different proportions of analcime and smectite as major components outcrop at the top of the profiles. The analcimes sh ow anhedral to euhedral morphologies, with grain-size ranging between 1 and 20 mum, and Si/Al ratios ranging from 2.2 to 2.5. The smectites are dioctahedral and beidellitic in composition. The zeolitic levels p resent significant chemical differences relative to the other ones, su ch as higher overall REE contents, more pronounced negative Eu anomali es and higher (LaNb)n, Th/Sc and La/Sc ratios, suggesting a different provenance source area. Their chondrite-normalized REE patterns reflec t the possibility that the starting materials were pyroclastic eruptiv e rocks originating from intracrustal partial melting. The variable an alcime and smectite percentages are attributed to variations in H+/(Na + + K+) and K+/(Na + Ca2+ + Mg2+) activity ratios and silica and water activities in the pore-waters during diagenetic processes.