The Layos Granite, Hercynian Complex of Toledo (Spain): an example of parautochthonous restite-rich granite in a granulitic area

Abstract

ABSTRACf: The Layos Granite forms elongated massifs within the Toledo Complex of central Spain. It is late-tectonic with respect to the F2 regional phase and simultaneous with the metamorphic peak of the region, which reached a maximum temperature of 800-850°C and pressures of 400-600 MPa. Field studies indicate that this intrusion belongs to the "regional migmatite terrane granite" type. This granite is typically interJayered with sill-like veins and elongated bodies of cordierite/ garnet-bearing leucogranites. Enclaves are widespread and comprise restitic types (quartz lumps, biotite, cordierite and sillimanite-rich enclaves) and refractory metamorphic country-rocks including orthogneisses, amphibolites, quartzites, con glomerates and caJc-silicate rocks.These granites vary from quartz-rich tonalites to melamonzogranites and define a S-type trend on a QAP plot. Cordierite and biotite are the mafic phases of the rocks. The particularly high percentage of cordierite (10%-30%) varies inversely with the silica content. Sillimanite is a common accessory mineral, always included in cordierite, suggesting a restitic origin. The mineral chemistry of the Layos Granite is similar to that of the leucogranites and country-rock peraluminous granulites (kinzigites), indicating a close approach to equilibrium. The uniform composition of plagioclase (An2S)' the high albitic content of the K-feldspar, the continuous variation in the Fe/Mg ratios of the mafic minerals, and the high Ti content of the biotites (2·5-6·5%) suggest a genetic relationship.Geochemically, the Layos Granite is strongly peraluminous. Normative corundum lies between 4% and 10% and varies inversely with increase in Si02. The CaO content is typically low «1·25%) and shows little variation; similarJy the LILE show a limited range. On many variation diagrams, linear trends from peraluminous granulites to the Layos Granite and associated leucogranite can be observed. The chemical characteristics argue against an igneous fractionation or fusion mechanism for the diversification of the Layos Granite. A restite un mixing model between a granulitic pole (represented by the granulites of the Toledo Complex) and a minimum melt (Jeucogranites) could explain the main chemical variation of the Layos Granite. Melting of a peiitic protolith under anhydrous conditions (biotite dehydration melting) could lead to minimum-temperature melt compositions and a strongly peraluminous residuum.For the most mafic granites (61-63% Si02), it is estimated that the trapped restite component must have been around 65%. This high proportion of restite is close to the estimated rheological critical melt fraction, but field evidence suggests that this critical value has been exceeded. This high restite component implies high viscosity of the melt which, together with the anhydrous assemblage of the Layos Granite and the associated leucogranites, indicates H20-undersaturated melting conditions. Under such conditions, the high viscosity magma (crystal-liquid mush) had a restricted movement ca...

Geochemical and isotopic disequilibrium in crustal melting: An insight from the anatectic granitoids from Toledo, Spain

“…Nevertheless, in several places there are kilometer-sized outcrops of these granites that are considered to be superposed sheets of similar restite-rich granites. A petrographic, mineralogical, geochemical, and petrogenetic discussion of these granites is given by Barbero and Villaseca [ 1992)] They proposed a restiteunmixing model to explain the mineralogy and chemical variability, in which the restitic end-member was represented by the granulitic migmatites of the area, and the liquid end-member by the Cervatos leucogranites.…”

“…A detailed discussion of the petrography, mineral chemistry and petrogenesis of these granitoids is given by Barbero and Villaseca [1992] and Barbero [1992].…”

“…In contrast, the restite-rich Layos granite has actual concentrations of Zr and LREE t greater than the calculated saturation concentrations. This is a consequence of the large amount of entrapped restitic material present in these granites, whose geochemistry has been modeled in terms of mixtures of restite and leucogranitic melt [Barhero and Villaseca, 1992] lO equilibrium values, these cannot be interpreted in terms of equilibrium. This is because it is necessary to take into account the fact that feldspars can make an important contribution to the total LREE budget in granulite facies rocks [Reid, 1990].…”

Petrographic and geochemical study of the stone warrior stelae from central Iberia: Linking the geological record and archaeological heritage

“…At least one thin section was made for each stele, as well as of the sampled metasedimentary and granitic rocks cropping out in the surrounding area, for comparison. Previous geological studies carried out within the studied area were also used (Andonaegui & Barrera, 1984;IGME, 1985aIGME, , 1985bIGME, , 1985cIGME, , 1987IGME, , 1992IGME, , 2008aIGME, , 2008bIGME, , 2008cIGME, , 2009aIGME, , 2009bIGME, , 2009cITGE, 1989;Merino Martínez, 2014;Villaseca et al, 2014) These geochemical data can be found in Andonaegui and Barrera (1984), Andonaegui (1990), Barbero and Villaseca (1992), Villaseca and Barbero (1994), Barbero (1995), Andonaegui and Villaseca (1998) Figure 4a and Table 3), suggesting that these rocks present an excess of aluminum with respect to calc-alkali elements, which is responsible of the presence of aluminum-rich minerals (i.e., biotite, muscovite, cordierite, tourmaline, andalusite, sillimanite and/or garnet). The main petrographic and geochemical features of each stele, according to its lithological nature, are described below.…”

Insights into the complexity of crustal differentiation: K2O‐poor leucosomes within metasedimentary migmatites from the Southern Marginal Zone of the Limpopo Belt, South Africa

“…The Bandelierkop formation leucosomes exhibit both these characteristics strongly. The compilation of metasedimentary rock mafic residuum and leucosome compositions was sourced from: Bea and Montero (1999), Barbero and Villaseca (1992), Barbey et al (1990), Braun et al (1996, Carrington and Watt (1995), Chavagnac et al (2001), Del Moro et al (2000, Johannes et al (2003), Martignole and Martelat (2003), Mengel et al (2001), Sawyer et al (1999), , Villaseca et al (2001), White et al (2004), and Whitney and Irving (1994) et al (2014), the mechanisms that control the formation of high-temperature K 2 O-poor felsic leucosomes have not yet been tested by thermodynamically constrained modelling. One reason for this is the difficulty of designing phase equilibrium modelling experiments to predict disequilibrium behaviour.…”