Diagenetic albitization in the Tera Group, Cameros Basin (NE Spain) recorded by trace elements and spectral cathodoluminescence

This paper deals with the diagenetic albitization of both plagioclases and Kfeldspars in the Tithonian fluvial sandstones of a rift basin (Cameros Basin). The sandstones in the lower part of the rift record have not suffered this albitization process.A clear relationship is observed between sodium contents, as the main element of some feldspars and their cathodoluminescence (CL) colour (the higher the sodium content, the lower is their CL intensity). In conclusion, albitization processes are detectable by decreased CL intensities and changes in the CL spectra. In addition, very different trace element compositions are obtained by laserablation when comparing trace elements of non-albitized feldspars in sandstones of the lower part of the rift record with those of albitized feldspars in sandstones of the infill top. Non-albitized K-feldspars show Rb, Sr, Ba, Pb contents of up to 1000 ppm. In contrast, very flat profiles of trace element contents (<250 ppm) are recorded in albitized feldspars (both K-feldspars and plagioclases). Thus, albitization implies feldspars impoverished in trace elements, including REE, which suggests that albitization is a dissolution and reprecipitation process. Further, albitized plagioclases show higher REE contents than albitized K-feldspars. We report here that REE patterns partly depend on the initial composition of the feldspar (K-feldspar or plagioclase) as a useful geochemical criterion for distinguishing albitized detrital plagioclases from albitized detrital K-feldspars.CL spectra from non albitized and albitized K-feldspars and plagioclases revealed marked differences. Non-albitized K feldspars present blue (main emission band at 460 nm) and brownish CL colours (590 nm), sometimes in the same grain. Brownish colours are related to weathering processes. The primary blue emission is related to Al-O--Al centres, enhanced probably by Al incorporation due to the coupledsubstitution of Ba2++Al3+¡êM++Si4+. Weathered K-feldspars present 4.8 times lower Ba content than fresh blue luminescent ones. The brownish colours are related to the external border or fractured grain zones, altered by weathering processes. Therefore, the observed 590 nm emission is assumed to be caused by structural defects resulting fromweathering and alteration Albitized K-feldspars are usually weak luminescent with a typical CL emission band at 620 nm. Sometimes, relicts of the original blue luminescence (460 nm band) are still present. The leaching of probably both Al and Ba can be responsible for the decrease in the blue band. The characteristic 620 nm band is also dominant in albitizedweak luminescent plagioclases. Two additional emission bands at 440 nm (Al-O--Al centre) and 565 nm (Mn2+) occur, when albitized plagioclases preserved their original CL characteristics (green CL colour). Another spectral peak at ca. 720 nm can be explained by Fe3+ activation due to Fe3+ - Al3+ substitution.The spectral CL measurements indicate that changes in luminescence due to albitization (620 nm emission) seem to be more related to structural defects than to trace element activation or quenching.