Diagenetic Albitization in the Tera Group, Cameros Basin (NE Spain) Recorded by Trace Elements and Spectral Cathodoluminescence
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Chemical Geology 312–313 (2012) 148–162 Contents lists available at SciVerse ScienceDirect Chemical Geology journal homepage: www.elsevier.com/locate/chemgeo Research paper Diagenetic albitization in the Tera Group, Cameros Basin (NE Spain) recorded by trace elements and spectral cathodoluminescence Laura González-Acebrón a,⁎, Jens Götze b, Donatella Barca c, José Arribas d, Ramón Mas e, Carlos Pérez-Garrido f a Dpto. Estratigrafía, Facultad de Ciencias Geológicas (UCM), C/ Jose Antonio Novais 2, 28040 Madrid, Spain b Institut für Mineralogie, Freiberg, Brennhausgasse 14, 09599, Germany c Università degli Studi della Calabria, Campus di Arcavacata, Vía P. Bucci, 87036, Arcavacata di Rende, Cosenza, Italy d Dpto. Petrología y Geoquímica, Facultad de Ciencias Geológicas (UCM) — Instituto de Geociencias (IGEO, CSIC-UCM), C/ Jose Antonio Novais 2, 28040 Madrid, Spain e Dpto. Estratigrafía, Facultad de Ciencias Geológicas (UCM) — Instituto de Geociencias (IGEO, CSIC-UCM), C/ Jose Antonio Novais 2, 28040 Madrid, Spain f Centro Nacional de Investigación sobre la Evolución Humana (CENIEH), Paseo de Atapuerca s/n, 09002 Burgos, Spain article info abstract Article history: This paper deals with the diagenetic albitization of both plagioclases and K-feldspars in the Tithonian fluvial sand- Received 26 September 2011 stones of a rift basin (Cameros Basin). The sandstones in the lower part of the rift record have not suffered this Received in revised form 13 April 2012 albitization process. A clear relationship is observed between sodium contents, as the main element of some feld- Accepted 16 April 2012 spars and their cathodoluminescence (CL) color (the higher the sodium content, the lower is their CL intensity). In Available online 26 April 2012 conclusion, albitization processes are detectable by decreased CL intensities and changes in the CL spectra. Editor: J.D. Blum In addition, very different trace element compositions are obtained by laser ablation when comparing trace ele- ments of non-albitized feldspars in sandstones of the lower part of the rift record with those of albitized feldspars Keywords: in sandstones of the infill top. Non-albitized K-feldspars show Rb, Sr, Ba and Pb contents of up to 1000 ppm. In Albitization contrast, very flat profiles of trace element contents (b250 ppm) are recorded in albitized feldspars (both K- Feldspar feldspars and plagioclases). Thus, albitization implies feldspars impoverished in trace elements, including REE, Plagioclase which suggests that albitization is a dissolution and reprecipitation process. Further, albitized plagioclases Cathodoluminescence show higher REE contents than albitized K-feldspars. We report here that REE patterns partly depend on the ini- Laser ablation ICP-MS tial composition of the feldspar (K-feldspar or plagioclase) as a useful geochemical criterion for distinguishing Trace elements 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 colors (590 nm), sometimes in the same grain. Brownish colors are related to weathering processes. The primary blue emission is related to Al–O−–Al centers, enhanced probably by Al incorporation due to the coupled substitution of Ba2+ + Al3+ ↔M++Si4+. Weathered K-feldspars present 4.8 times lower Ba content than fresh blue luminescent ones. The brownish colors 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 from weathering 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 albitized weak luminescent plagioclases. Two additional emission bands at 440 nm (Al–O−–Al center) and 565 nm (Mn2+) occur, when albitized plagioclases preserved their original CL characteristics (green CL color). Another spectral peak at ca. 720 nm can be explained by Fe3+ activation due to Fe3+–Al3+ substitution. The spectral CL measure- ments 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. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved. 1. Introduction ⁎ Corresponding author. Tel.: +34 913944785; fax: +34 91394485. Feldspars are particularly common minerals in siliciclastic sedi- E-mail addresses: [email protected] (L. González-Acebrón), ments. The albitization of detrital K-feldspar and plagioclase is one [email protected] (J. Götze), [email protected] (D. Barca), [email protected] (J. Arribas), [email protected] (R. Mas), of the most important diagenetic changes that occur in feldspathic [email protected] (C. Pérez-Garrido). sandstones (McBride, 1985; Milliken, 2005). The albite grains that 0009-2541/$ – see front matter. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2012.04.012 L. González-Acebrón et al. / Chemical Geology 312–313 (2012) 148–162 149 replace the original detrital K-feldspar or plagioclase are highly pseu- character of the feldspars, as well as the luminescence relicts, has be- domorphic and can hinder provenance interpretations. come an additional criterion to recognize albitization processes. In this The cathodoluminescence (CL) of feldspars is a useful tool for paper, we demonstrate that also weathering processes can produce a interpreting the genetic conditions of rock formation and alteration decrease on the luminescence of K-feldspars, and we have been able (Owen, 1991; Götze et al., 1999; Slaby et al., 2008). Feldspars formed to characterize both processes in terms of CL spectroscopy. under varying conditions can show different luminescence properties The blue emission of feldspars has been related to structural defects depending on the crystallization environment and trace element up- as Al–O−–Al centers (Marfunin and Bershov, 1970; Marfunin, 1979; take during growth or recrystallization (Götze et al., 1999; Kayama et Finch and Klein, 1999; Götze et al., 2000; Slaby et al., 2008)orTicenters al., 2010). Although feldspars exhibit substantial CL color variation (Mariano et al., 1973; Lee et al., 2007). In contrast, Mn2+ causes the yel- (Owen, 1991; Finch and Klein, 1999), CL in feldspars has yet to be low emissions of feldspars (Geake et al., 1971, 1973; Lee et al., 2007)and thoroughly exploited in sedimentary petrology. Fe3+ red or infrared emissions (Finch and Klein, 1999; Krbetschek et al., Plagioclase is often yellow-green but may show a variety of emission 2002). The luminescent color produced by Fe3+ activation is red alone colors. Variations in plagioclase colors as a function of plagioclase com- but in many cases other activators compete with Fe3+ and the feldspar position were described by Mora and Ramseyer (1992). Thus, calcic pla- may exhibit blue, green or yellow CL (Mariano, 1988). Other elements gioclase (An87–97) is distinguished by its yellow color, and intermediate that have been cited as potential activators of CL in feldspars are Tl, Pb, plagioclase (An39–65)appearsgreentoyellow.Microclineandorthoclase Cu, Cr and rare earth elements (REE) (Mariano et al., 1973; Marshall, exhibit usually blue CL (Owen, 1991). In sandstones, authigenic feldspar 1988; Götze et al., 1999). Among the REE, Eu has been reported to acti- is generally nonluminescent (Kastner, 1971; Kastner and Siever, 1979). vate CL in plagioclases (Mariano and Ring, 1975). According to Marshall In addition, authigenic feldspar, which is assumed to form at low tem- (1988), however, REE rarely occur in sufficient amounts in natural feld- perature, seems to have low concentrations of trace elements (Escobar spars to be CL activators and REE contents are usually higher in plagio- and Mariano, 1976). clases than in K-feldspars (Götze et al., 1999). Several petrographic studies have described albitized K-feldspars or In the present study conducted on feldspathic sandstones from the plagioclases as nonluminescent (Kastner, 1971; Saigal et al., 1988; Cameros Basin (NE Spain), we try to gain new insight into the possible Ramseyer et al., 1992). The grains become totally nonluminescent or trace elements or defects of importance in feldspar luminescence and present weak remains of their original luminescence color in case of their variation through feldspar alteration processes. For this, we com- partial albitization (Boles, 1982; Ramseyer et al., 1992; Ochoa, 2006; pared CL spectra of feldspars to trace element amounts in an effort to Caja et al., 2008). Nonluminescent areas are common in the fractured elucidate whether trace element activation is possible or structural de- grains and on cleavage traces, generating nonluminescent lines in fects can better explain the differences of luminescence among fresh, the feldspars (González-Acebrón et al., 2010a). The nonluminescent weathered and albitized feldspars. Fig. 1. Geological map of the Cameros Basin. The positions of the stratigraphic sections of the Tera Group are also marked. Sections in the western basin area have been studied by Arribas et al. (2003). Modified from Mas et al. (2002).