Pedro Castiñeiras

Correlation of the nappe stack in the Ibero-Armorican arc across the Bay of Biscay: a joint French–Spanish project

Abstract A correlation between allochthonous units exposed in the NW Iberian Massif and the southern Armorican Massif is carried out based on lithological associations, structural position, age and geochemistry of protoliths and tectonometamorphic evolution. The units on both sides of the Bay of Biscay are grouped into Upper, Middle and Lower allochthons, whereas an underlying allochthonous thrust sheet identified in both massifs is referred to as the Parautochthon. The Lower Allochthon represents a fragment of the outermost edge of Gondwana that underwent continental subduction shortly after the closure of a Palaeozoic ocean which, in turn, is represented by the Middle Allochthon. The latter consists of supra-subduction ophiolites and metasedimentary sequences alternating with basic, mid-ocean ridge basalt (MORB)-type volcanics, with inheritances suggesting the proximity of a continental domain. Seafloor spreading began at the Cambro-Ordovician boundary and oceanic crust was still formed during the Late Devonian, covering the lifetime of the Rheic Ocean, which is possibly represented by the Middle Allochthon. The opening of the oceanic domain was related to pulling apart the peri-Gondwanan continental magmatic arc, which is represented by the Upper Allochthon.

Thrust and detachment systems in the Ordenes Complex (northwestern Spain): Implications for the Variscan-Appalachian geodynamics

The allochthonous complexes of northwestern Iberia consist of a pile of units of Gondwanan and peri-Gondwanan provenance, and include oceanic lithosphere. The units are classiµed into upper, intermediate (ophiolitic), and basal. We present a detailed geological map and sections across the Ordenes Complex, together with a brief description of its units and a discussion of its structures. In the upper units, two complete cycles of burial and exhumation have been identiµed. The µrst cycle, of Early Ordovician age, records a convergent plate margin, possibly in a peri-Gondwanan domain. The second is Variscan, and the structural evolution of the three groups of allochthonous units re×ects progressive accretion to an active orogenic wedge. Continuous understacking of continental and oceanic fragments toward the west began with the upper units and ended with the basal units. The latter represent the outermost margin of Gondwana, and their subduction marked the closure of the intervening ocean, and the change from subduction to a collisional regime. Terrane accretion took place in the Devonian and, during the Late Devonian and the Carboniferous, the deformation progressed inboard of the Gondwana margin. Variscan emplacement of the allochthonous units occurred in two successive thrusting episodes. The µrst placed the basal units over the sedimentary cover of the Gondwana margin in what seems to follow a normal sequence of thrusting. The second carried the upper and ophiolitic units on top of the previous nappe pile and has an out-of-sequence character. A possible correlation of the Early Ordovician convergence, early Variscan accretion, subsequent oceanic closure, continent-continent collision, and renewed thrust activity during the late Carboniferous in northwestern Iberia is established with the Taconian, Acadian, and Alleghanian orogenies in the Appalachians.

40Ar/39Ar laserprobe dating of mylonitic fabrics in a polyorogenic terrane of NW Iberia

The tectonothermal evolution of a polyorogenic terrane in the Variscan belt of NW Spain has been constrained by 40Ar/39Ar laserprobe incremental heating experiments on mylonitic fabrics developed in major structures. Transitional levels between HP–HT and IP upper units in the Órdenes Complex where metamorphic and structural records demonstrate two cycles of burial and exhumation were selected for dating. Two groups of ages have been defined: (1) Silurian–Early Devonian, obtained from mylonites of the Fornás extensional detachment, here considered as the minimum age for the start of tectonic exhumation of the HP–HT units and an upper age-limit for the HP–HT event itself; (2) Early to Mid-Devonian, from structures related to the Variscan convergence in the area, which include top-to-the-east thrusts and extensional detachments. A single, younger Carboniferous age obtained from the uppermost allochthonous sequences possibly reflects the final stages of emplacement of the allochthonous complexes. Our data indicate a polyorogenic character for a part of the Iberian allochthonous complexes, including Variscan (sensu stricto) and Early Variscan convergence, as well as an older, Early Palaeozoic cycle.

The basal tectonic mélange of the Cabo Ortegal Complex (NW Iberian Massif): a key unit in the suture of Pangea

Recent field work and mapping in the lower units of the Cabo Ortegal Complex provided new data about the tectonic melange that appears in the lowest structural position: the Somozas Melange. This melange unit with average thickness of 1800 m is restricted to the eastern part of the complex, and is located at the advancing front of the allochthonous complexes of NW Iberia. Three rock units are involved in the melange: 1) an ophiolitic melange consisting of igneous rocks mixed with serpentinites; 2) a metasedimentary unit with phyllites and phyllonites, with scarce conglomerates, marbles and quartzites; 3) high-T metamorphic rocks with varied types of amphibolites and orthogneisses. Two granitic rocks within the ophiolitic melange were dated using U-Pb zircon geochronology at 527 ± 2 Ma and 499 ± 1 Ma. Two different series of igneous rocks can be distinguished in this melange. The first series consists of gabbros, diorites, granitoids and basalts-basaltic andesites with calc-alkaline affinities. The second series contains common basaltic rocks, diabasic dikes and gabbros with chemical compositions typical of island-arc tholeiites. Both igneous series shared a common geographic setting, but the island-arc tholeiites are younger than the calc-alkaline igneous rocks. The two igneous series were probably generated in a mature volcanic arc located along the periphery of Gondwana. In the metasedimentary unit, a conglomerate from a large tectonic block included in serpentinites yielded age populations of detrital zircons suggesting that the sediments were deposited along the periphery of the West-African Craton. This conglomerate contains a large number of zircons (n = 24) with ages ranging 630-464 Ma, probably representing the chronology of the Pan-African event, including the magmatic activity in the volcanic arc where the igneous lithologies involved in the melange were generated. The maximum age of sedimentation for this conglomerate is estimated as latest Cambrian – earliest Ordovician, and constraints the end of the magmatic activity in the volcanic-arc. Within the unit of high-T rocks, an orthogneiss yields a U-Pb protolith age of 485 ± 6 Ma, which is similar to other ages of igneous rocks in the basal allochthonous terrane in NW Iberia. The three rock assemblages forming part of the Somozas Melange may be linked to the evolution of a mature peri-Gondwanan volcanic arc. This volcanic arc was affected by pronounced extension which caused the opening of intra-arc basins, culminating the rifting and subsequent drift of the external parts of the arc during the opening of the Rheic Ocean. This opening started during a time interval constrained by the peak activity in a mature volcanic arc (c. 527-499 Ma) and the generation of intraarc basins around the Cambrian-Ordovician boundary. Tectonic melanges including high-P rocks have been classically related to subduction zone environments. Regional relationships in NW Iberia and the nature of the rock units involved in the Somozas Melange, suggest that two different subduction zones generated during oblique convergence and collision between Gondwana and Laurussia were active during the final stages of the assembly of Pangea. The first related to the underthrusting beneath Laurussia of the most external Gondwana margin (c. 370 Ma). The second subduction zone was a new one which accreted later remnants of a peri-Gondwanan arc and sediments of the continental margin below a layer of exhumed high pressure rocks. This oblique collision finished the closure of the Rheic Ocean and contributes to define the new oceanic domain located to the East of Pangea, the Palaeotethys.

Isotope geochemistry and revised geochronology of the Purrido Ophiolite (Cabo Ortegal Complex, NW Iberian Massif): Devonian magmatism with mixed sources and involved Mesoproterozoic basement

Abstract: In the Purrido Ophiolite (Cabo Ortegal Complex), new U–Pb zircon dating of the amphibolite G03-8 (by laser ablation inductively coupled plasma mass spectrometry) confirms the existence of a dominant Mesoproterozoic zircon population with a refined age of 1155 ± 14 Ma. However, the U–Pb zircon dating of two more amphibolite samples (by sensitive high-resolution ion microprobe) has provided new ages of 395 ± 3 Ma and 395 ± 2 Ma, respectively, interpreted as the crystallization age. Hf isotope data for zircon show that most of the Devonian zircons crystallized from a juvenile depleted mantle source. The Mesoproterozoic zircons have relatively juvenile Hf isotopic composition reflecting some influence of an older component. A few Devonian zircon crystals show evidence of mixing with an older component represented by the Mesoproterozoic zircons. The whole-rock Sm–Nd isotope data indicate an important heterogeneity in the composition of the Purrido amphibolites, only compatible with the generation of their protoliths from two sources. We interpret these puzzling data as resulting from the mixing of a Devonian mantle-derived magma with a Mesoproterozoic basement. These new data provide new perspectives in the interpretation of the most common ophiolites across the Variscan suture in Europe. Supplementary material: Analytical methods, zircon U–Th–Pb SHRIMP analytical data, zircon U–Th–Pb LA-ICP-MS analytical data, zircon Lu–Hf LA-MC-ICPMS analytical data and whole-rock Sm–Nd isotope data are available at http://www.geolsoc.org.uk/SUP18449.

Late Variscan metamorphic and magmatic evolution in the eastern Pyrenees revealed by U–Pb age zircon dating

Variscan migmatites cropping out in the eastern Pyrenees were dated together with Late Variscan plutonic rocks. Upper Proterozoic–Lower Cambrian series were migmatized during a thermal episode that occurred in the interval 320–315 Ma coeval with the main Variscan deformation event (D1). The calc-alkaline Sant Llorenç–La Jonquera pluton and the gabbro–diorite Ceret stock were emplaced during a later thermal episode synchronous with the D2 deformation event. A tonalite located at the base of La Jonquera suite intruded into the upper crustal levels between 314 and 311 Ma. The gabbro–diorite stock was emplaced in the middle levels of the series in two magmatic pulses at 312 and 307 Ma. The thermal evolution recorded in the eastern Pyrenees can be correlated with that of neighbouring areas of NE Iberia (Pyrenees–Catalan Coastal Ranges) and SE France (Montagne Noire). The correlation suggests a NW–SE-trending zonation where the northeasternmost areas (Montagne Noire and eastern Pyrenees) would occupy relatively more internal zones of the orogen than the southwesternmost ones. Supplementary material: Methods and U–Th–Pb isotopic and REE geochemical data for zircon are available at www.geolsoc.org.uk/SUP18703.

Petrogenesis of Ordovician Magmatism in the Pyrenees (Albera and Canigó Massifs) Determined on the Basis of Zircon Minor and Trace Element Composition

Zircon composition (U, Th, rare earth elements, and Hf) was tested as a tracer of petrogenetic processes in a set of metaigneous rocks from two pre-Ordovician massifs in the Pyrenees, Canigó and Albera. Two groups were differentiated after analyzing a number of elements in zircon: (1) Casemí gneiss and Marialles amphibolite and (2) subvolcanic metaporphyries and Cadí and Sureda orthogneisses. Casemí gneiss and Marialles amphibolite from the Canigó massif have high Th, Th/U, and Ce/Sm and low Yb/Gd and U/Ce that define linear trends in most of the plots used. The anomalous trend of the data in the Th/U-versus-Hf plot suggests mantle involvement in the origin of these rocks and the participation of fractional crystallization during their evolution. Zircon of the metaporphyries and the Cadí and Sureda orthogneisses exhibit similar characteristics despite a difference in age. Zircon has low Th, Th/U, Ce/Sm, and and high Yb/Gd and U/Ce, suggesting that this mineral grew in a melt formed by anatexis of a continental crust, with stable plagioclase. These petrogenetic data are consistent with the previously studied whole-rock geochemistry and Sr-Nd isotopes and confirm the use of zircon as a marker of petrogenetic processes in preference to a lithological tracer.

El Chichón Volcano (Chiapas Volcanic Belt, Mexico)Transitional Calc-Alkaline to Adakitic-Like Magmatism:Petrologic and Tectonic Implications

The rocks of the 1982 eruption of El Chichón volcano (Chiapas, Mexico) display a series of geochemical and mineralogical features that make them a special case within the NW-trending Chiapas volcanic belt. The rocks are transitional between normal arc and adakitic-like trends. They are anhydrite-rich, and were derived from a water-rich, highly oxidized sulfur-rich magma, thus very much resembling adakitic magmas (e.g., the 1991 Pinatubo eruption). We propose that these rocks were generated within a complex plate tectonic scenario involving a torn Cocos plate (Tehuantepec fracture zone) and the ascent of hot asthenospheric mantle. The latter is supported by an outstanding negative S-wave anomaly widely extending beneath the zone, from 70 to 200 km in depth. The adakitic-like trend would be derived from the direct melting of subducting Cocos plate, whereas the transitional rocks would have resulted from the mixing of two poles, one reflecting a mantle source, and the other, the already mentioned adakitic melts. The basaltic source would also account for the high sulfur content and δ34S values of the El Chichón system (about +5.8), as result of a contribution of SO2 in fluids released from an underlying mafic magma.

Local isobaric heating above an extensional detachment in the middle crust of a Variscan allochthonous terrane (Órdenes complex, NW Spain)

In one of the most studied Variscan exposures, the Ordenes allochthonous complex (NW Spain), the transition between medium-pressure (MP) and high-pressure (HP) units in the SW of the complex has been identified as an extensional shear zone: the Fornas detachment. Migmatitic paragneisses crop out discontinuously along that boundary, at the base of the MP ensemble (O Pino unit). The metamorphic reaction sequence, mass balance calculations, and phase diagram modeling investigated in these paragneisses are interpreted in terms of an approximately isobaric heating path (8 ± 0.8 kbar), from ∼650 °C to 740 °C, crossing into the melt- and K-feldspar–bearing stability fields. These anatectic conditions are evidenced by the presence of leucosomes through progressive muscovite and biotite melting reactions. Our results indicate that the heating path evidenced by the migmatitic paragneisses is directly related to the subtractive nature of the Fornas detachment, with heat transferred from the footwall to the hanging-wall unit.