C. Fernández-Nieto

Geochemistry of Precambrian and Paleozoic siliciclastic rocks from the Iberian Range (NE Spain): implications for source-area weathering, sorting, provenance, and tectonic setting.

Abstract The major and trace element chemical composition of Precambrian and Paleozoic shales and sandstones from the Iberian Range (Spain) has been investigated to determine the provenance and tectonic setting of these rocks, as well as to appraise the influence of the weathering, hydraulic sorting and recycling processes upon source rock signature. The samples studied belong to a prograde sequence from diagenesis to anchizonal grade, and are mainly composed of quartz, illite/mica phases and/or kaolinite, with chlorite, feldspars, and carbonates as minor components. The major element distribution (Chemical Index of Alteration (CIA) indices) reflects that recycling processes have been important in homogenizing the composition of shales and sandstones. The higher CIA indices observed in the Silurian and Devonian shales (80 and 85, respectively) compared with those of Precambrian to Ordovician and the Carboniferous age (about 70) indicate that their source area underwent more intense chemical weathering processes, possibly due to climatic and/or tectonic variations. Alternatively, the source area of the Silurian and Devonian shales may have been composed of recycled sedimentary materials. Mineral fractionation is mainly observed in coarser rocks through zircon, apatite and xenotime accumulation, although the shales are not free of these phases. The zircon content of the fine-grained rocks is not high enough to affect rare earth element (REE) contents, but phosphate minerals (apatite and xenotime) at least partially control the REE distribution. The influence of apatite and xenotime are slightly higher in the Precambrian shales as the phosphate concentration produces a decrease in the La/Sm ratios. Chondrite-normalized REE patterns and negative Eu anomaly size of the studied rocks are similar to that of Post-Archean Australian shales (PAAS) indicating that they originally come from a differentiated silicic source. The higher REE contents in the studied shales in relation to PAAS indicate that recycling processes in the Iberian Range sources were probably more intense than that of PAAS. The slight differences among the REE patterns of the different groups of shales probably do not reflect changes in source-area composition, but instead suggest variations in mineral sorting, chemical weathering and/or sediment recycling. The Th/Sc, Co/Th, Cr/Th, Cr/V, V/Ni ratios support a primitive silicic source for these rocks, with the higher Th/Sc and lower Co/Th in Cambrian and Ordovician shales indicating a higher proportion of felsic material in their primitive source area. The compositional maturity of analyzed sandstones is typical of cratonic environments and their La, Sc, Th and Zr contents reflect their passive continental margin setting.

Hydrothermal events in the Valle de Tena (Spanish Western Pyrenees) as evidenced by fluid inclusions and trace-element distribution from fluorite deposits

Abstract This paper provides further evidences from fluorite and calcite geochemistry and from fluid inclusion studies for the origin of the Valle de Tena fluorite deposits. The Valle de Tena, Spanish Western Pyrenees, contains two kinds of fluorite mineralizations: (1) vein-type deposits located in the vicinity of the Panticosa granite and enclosed by Devonian carbonate facies; and (2) carbonate-hosted mineralizations on highly silicified Lower Carboniferous limestones. Based on geological evidences the latter deposits can be referred to as Permo-Triassic. Microthermometrical measurements on two-phase, water-rich inclusions with high liquid to vapor ratios yield homogenization temperatures ranging from 100° to 200°C for the two deposit-types. Salinities range from 4.0 to 11.6 eq wt% NaCl for carbonate-hosted deposits and from 12.5 to 21.3 eq wt% NaCl for vein mineralizations. REE contents are variable and highest abundances are found in the fluorite from Lanuza and Tebarray (green flurite). These later fluorite mineralizations have primary REE patterns with positive Eu anomalies which possibly reflect the alteration of feldspar. The REE distribution patterns of fluorites and calcites from the carbonate-hosted mineralizations are different from those occurring as veins; the decrease of LREE in the former resulted in roof-shaped REE patterns. The negative Eu anomalies as well as the trace-element distribution allow us to deduce that the fluid from which these later Ca-minerals were formed was probably equilibrated with the sedimentary country rocks. The fluid inclusion, geochemical, geologic and preliminary stable isotope data support two temporally distinct hydrothermal systems accounting for the fluorite formation and suggest a sedimentary basin source for the ore fluids. High-salinity fluids (basement brines) are involved in the formation of the vein-type deposits, whereas the subsequent carbonate-hosted mineralizations were formed by mobilization of the fluorite veins by formation waters. The most likely fluorite deposition mechanism is cooling with minor contribution of fluid-rock interaction. In such context, magmatic activity is considered as heat source. The above mineralization events possibly span a time from Permian to Triassic which is in close agreement with the timing of hydrothermal processes documented throughout the Hercynian Europe.

TEM study of mineral transformations in fired carbonated clays: relevance to brick making

Abstract This study uses transmission electron microscopy (TEM) and analytical electron microscopy (AEM) supported by X-ray diffraction (XRD) and scanning electron microscopy (SEM) to investigate the mineralogical and textural changes produced in carbonated clays by firing. Sample bars were prepared using raw clays composed of quartz, illite and carbonates with minor amounts of smectite, chlorite, feldspars and Fe oxides. The raw samples were then fired at temperatures of between 800 and 1050°C. The XRD data show that increases in firing temperature result in dehydroxylation of clay minerals, carbonate decomposition and the formation of Ca-bearing silicates (e.g. gehlenite, wollastonite, pyroxenes and anorthite). The sizes of the Ca-silicate crystals make the use of the SEM inappropriate since they lie below the resolution threshold. However, TEM/AEM do provide the required textural and compositional characterization, revealing that there is a broad range of pyroxene compositions, some of which resemble fassaite, and that Ca/Mg ratios increase with temperature. The TEM also shows significant dehydroxylation and vitrification of the clay-rich matrix at T of ~800°C. Observed mineralogical and textural changes probably occurred in a system with a local disequilibrium much like small-scale, high-temperature metamorphic reactions (i.e. pyrometamorphism). The importance of these results is that they enable the selection of more appropriate raw clay composition and firing dynamics (temperature, firing duration and cooling rate) for both the brickmaking industry and brick conservation in the field of cultural heritage.

Assessment of technological properties of calcareous and non-calcareous clays used for the brick-making industry of Zaragoza (Spain)

The granulometric, mineralogical, and geochemical features of two different types of clayey raw materials (carbonated clays, CC; and non-carbonated clays, NCC) used for the brickmaking industry near Zaragoza (Spain) have been studied. Their particle-size distribution suggests that they may be classified as silty clays and are suitable for use as thin-walled hollow bricks. For technological testing, prismatic bars obtained by extrusion under vacuum conditions from raw materials were fired from 800 to 1050 °C. Technological determinations, such as linear shrinkage, bulk density, hydric tests, and an accelerated decay test, indicate that both CC and NCC are suitable to be used as bricks for building, although bricks made of NCC would be of better quality. In addition, pore-size distribution suggests that the durability of the highest temperature fired products will be higher because of an increase both in the dominant pore access radius and in the percentage of larger pore sizes.

Mineralogical and trace element composition of clay-sized fractions from Albian siliciclastic rocks (Oliete Basin, NE Spain)

Abstract Mineralogical and geochemical techniques have been used to determine the role of minerals in controlling the trace element composition of a set of clay-sized fractions from Albian siliciclastic rocks in NE Spain. These clay-sized fractions are composed of kaolinite and illite, minor quantities of quartz, and accessory heavy minerals. Kaolinite has a smaller crystal size than illite, accounting for its relative concentration in these fractions. The mm-sized heavy minerals are rutile, Fe-Ti- and Fe-oxides, zircon, pyrite, cassiterite, monazite and xenotime. Geochemical data indicate that most of the trace elements are relatively concentrated in clay-sized fractions, except for Zr, Hf, Y and HREE. Statistical treatment shows three different associations in the clay-sized fractions: (1) Rb, Cs and Ba with clay phyllosilicates, especially illite; (2) REE with Th, Y and P phases; and (3) Sc, Cr and V with Ti- and Nb oxides. Therefore, these data do not support the dominant REE control by clay mineralogy that other authors have reported.

Mineralogy and geochemistry of Devonian detrital rocks from the Iberian Range (Spain)

Abstract Two profiles in Devonian marine deposits have been studied, consisting of pelites, subgreywackes, greywackes and quartzites. Quartz and clay minerals are major components and feldspar and calcite are minor ones. Phyllosilicates in the fine fractions are kaolinite and illite; kaolinite has a high degree of ordering; illite is predominantly of a 1Md polytype, with low Na content and poor crystallinity and has a phengitic composition in greywackes, whereas in pelites it is muscovitic in composition. Both phyllosilicates may be inherited from a source area with intensive weathering processes, although illite may also be a diagenetic phase. These mineral characteristics indicate that the Devonian rocks did not reach the anchizone boundary in their post-depositional evolution. The chemical composition of pelites and subgreywackes reveals a high degree of chemical maturity. Chondrite-normalized REE patterns indicate a higher degree of weathering of these Devonian sediments than of Post-Archaean Australian Shales (PAAS), possibly as a consequence of sedimentary recycling processes. The REE patterns of the Devonian rocks in addition to the high Th/Sc, La/Sc and Th/Co ratios suggest a felsic composition of the primitive source area, probably a K-rich granite.

Geochemical, and stable and radiogenic isotope records in Devonian and Early Carboniferous carbonates from Valle de Tena, central Pyrenees (Spain): evidence for their diagenetic environments

Mineralogical, textural and geochemical investigations were made to determine the post-depositional evolution of Devonian and Early Carboniferous carbonates from Valle de Tena. The carbonate association is made up of low-Mg calcite, which occurs as micrite, spar cements, neomorphic patches and spar filling veinlets. Non-stoichiometric dolomite and ankerite occur as cements (dolomite also as replacements) in the Middle Devonian, post-dating calcite types. All these phases pre-date tectonic stylolites, indicating compaction after stabilization of the carbonate minerals. Strontium concentrations indicate that Early Devonian and Early Carboniferous micrites initially precipitated as aragonite; Middle and Late Devonian micrites precipitated as high-Mg calcites. Both precursors were diagenetically stabilized to low-Mg calcites through interaction with meteoric waters in phreatic environments. Trace elements in dolomite and ankerite indicate precipitation from Sr-enriched meteoric water. All studied carbonates, except Middle Devonian limestones, precipitated in reducing environments, which favoured incorporation of Fe and Mn. Late calcite generations precipitated from more saline waters than micrites. Light δ18O values in micrites suggest alteration mainly in meteoric-phreatic environments. The dolomites and ankerites precipitated from more 18O-depleted fluids than the calcites, suggesting a greater contribution from meteoric waters. Variations in δ13C of micrites represent primary secular trends, according to published δ13C variations. The δ13C oscillations within each succession probably relate to sea-level oscillations. Strontium isotopes also point to a meteoric origin of diagenetic fluids. Model calculations suggest that O and Sr isotopes equilibrated between calcites and fluid at relatively low water/rock ratios, whereas C isotopic signatures are inherited from limestones.