L. Barbero

Crustal origin of Hercynian peraluminous granitic batholiths of Central Spain: petrological, geochemical and isotopic (Sr, Nd) constraints

Abstract In Central Spain, it is possible to distinguish two main types of peraluminous late-Hercynian granites: the PI types and the PS types. The distinction between both types is made on the basis of differences in peraluminosity (PS types are more peraluminous than PI types) and also on the appearance of some characteristic mafic minerals; PS types have biotite, cordierite and monazite as the most typical accessory phase, whereas PI types have biotite, amphibole (in the less evolved facies) and allanite as the accessory mineral. Both granite types have similar trace element ratios and initial Sr, Nd and Pb isotopic signatures. Initial 87Sr/86Sr ratios of both types exhibit a large range from 0.7073 to 0.7193, whereas initial eNd varies in a restricted range from −5.4 to −6.6. The scarce associated basic rocks do not play a significant role in the chemical variability of these peraluminous granites which follow low pressure crystal fractionation trends from granodiorite/monzogranite parental magmas. Mixing and AFC modelling of Sr and Nd isotopic data reveal an unrealistically high mantle contribution. Based on major and trace element and isotopic data, an orthogneissic protolith for both granitic series is proposed. Nevertheless, none of the metamorphic country rocks of Central Spain has the appropriate Sr isotopic composition to satisfy the origin of these granitic rocks, and so, it is argued that progressive isotopic re-equilibration of crustal material during the granulization of the lower Hercynian crust, together with the possibility of isotopic disequilibrium during melting (as has been demonstrated in migmatitic terranes in nearby areas) may explain the isotopic differences between the granites and the high level metamorphic country rocks.

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

In the Hercynian Anatectic Complex of Toledo (ACT) the anatectic granitoids include leucogranitic leucosomes, leucogranitic massifs, and restite-rich granites. They show a broad range of initial Sr and Nd isotope ratios from 0.711 to 0.720 and 0.51164 to 0.51203, respectively, which clearly indicate the absence of isotopic homogenization in the melts. Broadly, the ranges reflect the isotopic variation of the metapelitic protoliths. If crustal melting occurs under water-undersaturated conditions, as is the case of the ACT, the generated melts do not isotopically and chemically equilibrate with the granulitic residuum. The preservation of heterogeneities could arise through a number of processes. (1) duration of the process:in which the presence of melts with disequilibrium features, and the high solid content of several of the granites in the ACT point to a very short-lived magmatic system, (2) limited diffusion in which poorly segregated and volatile undersaturated granitoids have a very restricted opportunity for isotopic and chemical homogenization, and (3) magma dynamics in which the low melting rates, the high restite and low water contents, and the silica-rich composition of these granites, together with the small volume and high viscosity values of the melts, seriously restrict the physical processes leading to homogenization.

A re-examination of the typology of peraluminous granite types in intracontinental orogenic belts

Conventional rock classification diagrams do not distinguish the variety of peraluminous rock series. Moreover, peraluminous granite types have not been clearly discriminated in recent revisions. The study of several peraluminous series in different intracontinental orogenic belts reveals that four distinct groups can be defined. Using an A-B diagram, these four groups are: (1) highly peraluminous granitoids ( hP ) characterised by high A values and typified by an increase in peraluminosity toward the most mafic varieties; (2) moderately peraluminous granitoids ( mP ) which occupy the intermediate field and generally show increasing peraluminosity towards the most felsic varieties; (3) low peraluminous granitoids ( IP ) which plot in the lowest part of the peraluminous field defining negative slope trends; (4) highly felsic peraluminous granites ( fP ) with poorly defined variation trends. In intracontinental orogenic belts, the genesis of peraluminous granitic series is favoured by the abundance of fertile crustal protoliths, mainly metapelites, metaigneous rocks and metagreywackes. The difficulty of attaining temperatures in excess of 950°C at lower crustal levels during the tectonothermal evolution of thickened crust, inhibits the partial melting of more basic sources. Although the physical parameters of the melting process influence their chemical and mineralogical characteristics, source rock composition ultimately determines the degree of peraluminosity of the granitic series.

Residence and redistribution of REE, Y, Zr, Th and U during granulite-facies metamorphism: behaviour of accessory and major phases in peraluminous granulites of central Spain

Accessory minerals are thought to play a key role in controlling the behaviour of certain trace elements such as REE, Y, Zr, Th and U during crustal melting processes under high-grade metamorphic conditions. Although this is probably the case at middle crustal levels, when a comparison is made with granulite-facies lower crustal levels, differences are seen in trace element behaviour between accessory minerals and some major phases. Such a comparison can be made in Central Spain where two granulite-facies terranes have equilibrated under slightly different metamorphic conditions and where lower crustal xenoliths are also found. Differences in texture and chemical composition between accessory phases found in leucosomes and leucogranites and those of melanosomes and protholiths indicate that most of the accessory minerals in melt-rich migmatites are newly crystallized. This implies that an important redistribution of trace elements occurs during the early stages of granulite-facies metamorphism. In addition, the textural position of the accessory minerals with respect to the major phases is crucial in the redistribution of trace elements when melting proceeds via biotite dehydration melting reactions. In granulitic xenoliths from lower crustal levels, the situation seems to be different, as major minerals show high concentration of certain trace elements, the distribution of which is thus controlled by reactions involving final consumption of Al-Ti-phlogopite. A marked redistribution of HREE–Y–Zr between garnet and xenotime (where present) and zircon, but also of LREE between feldspars (K-feldspar and plagioclase) and monazite, is suggested.

Granulite-facies metamorphism in the Anatectic Complex of Toledo, Spain: late Hercynian tectonic evolution by crustal extension

The Anatectic Complex of Toledo consists of a heterogeneous suite of high-grade metamorphic rocks, mainly granulitic migmatites of pelitic composition and orthogneisses, and different types of syn-orogenic granitoids with minor associated basic rocks. None of the metamorphic rock types records evidence of the early prograde history, and only the peak and retrograde P–T conditions can be estimated. The intrusion of the syn-orogenic granitoids is either slightly before or synchronous with the metamorphic climax. These rock types thus can be used to make P–T estimates of the peak and retrograde conditions. Microtextural analysis of reaction textures in conjunction with a petrogenetic grid, has enabled construction of an incomplete ‘clockwise’ P–T path that is characterized by an isothermal decompression after or during the metamorphic peak temperatures. The peak estimates from different geo-thermobarometers are 800° ± 50 °C and 4–6 Kbar, the highest temperatures recorded in this part of the Iberian Hercynian Belt. It is proposed that this high-T/low-P metamorphism is at least partially related to an extensional process, possibly due to gravitational collapse of previously thickened crust.

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

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 interlayered 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, conglomerates and calc-silicate rocks

Eclogite facies relics in metabasites from the Sierra de Guadarrama (Spanish Central System):P-T estimations and implications for the Hercynian evolution

Abstract Relics of HP-MT eclogitic assemblages related to the first metamorphic stage of the Hercynian orogeny in the Sierra de Guadarrama (Spanish Central System, SCS) are preserved as boudins of pre-Ordovician metabasites enclosed by felsic gneisses. Textures indicate a multi-stage metamorphic history starting in the MT eclogite facies (as deduced from the presence of omphacite and rutile included in garnet) and continuing through medium to low pressure granulite and retrograde amphibolite-greenschist facies. Thermobarometric calculations in the eclogitic paragenesis yield pressures of ~14 kbar for temperatures in the range 725-775°C. Thermobarometry for the subsequent granulitic stage indicates a significant drop in pressure (P < 10 kbar) for similar temperatures of ~750°C. Metabasites vary from gabbro to leucotonalites showing the typical Fe enrichment of the tholeiitic series. Chemical characteristics indicate a derivation from low-pressure crystallization of tholeiitic melts more enriched than typical MORB compositions. Their original location far from continental margins as evidenced by the absence of ophiolitic material in the area and their association with platform sediments suggests that eclogitization was related to intracontinental crustal subduction and thickening. The P-T conditions estimated in the metabasites for the first metamorphic stage are similar to ones deduced for the surrounding metasediments and suggest that the Hercynian crust could have reached a thickness of ~70–80 km, which is more than the double the present thickness.

Exhumation of the Sierra de Cameros (Iberian Range, Spain): constraints from low-temperature thermochronology

Abstract We present new fission-track and (U–Th)/He data from apatite and zircon in order to reconstruct the exhumation of the Sierra de Cameros, in the northwestern part of Iberian Range, Spain. Zircon fission-track ages from samples from the depocentre of the basin were reset during the metamorphic peak at approximately 100 Ma. Detrital apatites from the uppermost sediments retain fission-track age information that is older than the sediment deposition age, indicating that these rocks have not exceeded 110 °C. Apatites from deeper in the stratigraphic sequence of the central part of the basin have fission-track ages of around 40 Ma, significantly younger than the stratigraphic age, recording the time of cooling after peak metamorphic conditions. Apatite (U–Th)/He ages in samples from these sediments are 31–40 Ma and record the last period of cooling during Alpine compression. The modelled thermal history derived from the uppermost sediments indicates that the thermal pulse associated with peak metamorphism was rapid, and that the region has cooled continuously to the present. The estimated palaeogeothermal gradient is around 86 °C km−1 and supports a tectonic model with a thick sedimentary fill (c. 8 km) and explains the origin of the low-grade metamorphism observed in the oldest sediments.

Alpha-recoil tracks in natural dark mica: Dating geological samples by optical and scanning force microscopy

Alpha-recoil tracks (ART) are lattice defects caused by the a-decay of 238 U, 235 U, 232 Th, and daughter products. Visualization of etched ARTs in dark mica by phase-contrast microscopy allows dating of Quaternary geological as well as archaeological materials. Visualization of etched ARTs by Nomarski-differential-interference-contrast microscopy (NDICM) and scanning force microscopy (SFM) enables the access to areal densities (qa) of ART etch pits beyond 10 4 mm � 2 and thus the extension of the new ART-dating technique to an age range >1 Ma. The successful application of SFM as a new tool in geochronology could open the way to a field to be characterized as nanogeochronology. In order to visualize ARTs by NDICM and SFM, dark mica was etched with 4% HF at 21 C for 5–107 min. A linear relationship between qa and etching time (te) was observed for phlogopites from the Kerguelen Islands (French territory, Indian Ocean), and the Kovdor magmatic complex (Russia). The volume density (qv) of ART is a function of etching speed (veff ) and slope of the q a -growth curve. The ART-age equation allows the calculation of an individual q v -growth curve for the phlogopite analysed by us using the uranium and thorium content. The ART-ages were determined by combining the experimentally obtained volume density with the individual qv-growth curve.

Radiological risk assessment of naturally occurring radioactive materials in marine sediments and its application in industrialized coastal areas: Bay of Algeciras, Spain

Radioactivity levels in aquatic environments can be assessed through the study of superficial sediments. Anthropogenic activities may alter radioactivity levels leading to the anomalous accumulation of natural radionuclides in coastal areas. In this work, marine sediments from a significant area subjected to severe industrial development were collected in order to measure activity concentrations of 226Ra, 232Th and 40K by gamma spectrometry. Radium equivalent index (Raeq), absorbed gamma dose rate in air (D) and annual effective dose equivalent (H) were also calculated and used as risk assessment tools. Results showed low levels of radioactivity in marine sediments from the Bay of Algeciras, discarding any significant radiological risks. Furthermore, the obtained data set could be used as background levels for future research applications and development of environmental regulatory frameworks.