Ricardo Arenas

Diachronous Variscan tectonothermal activity in the NW Iberian Massif: Evidence from 40Ar/39Ar dating of regional fabrics

Abstract Multigrain concentrates of hornblende and muscovite together with whole-rock slate/phyllite samples have been dated (27 analyses) using 40Ar/39Ar incremental-release methods along a systematic traverse across the various lithotectonic structural elements which comprise northwestern sectors of the Variscan Iberian Massif. Hornblende concentrates from amphibolites in the allochthonous Ordenes Complex yield plateau isotope-correlation ages of 425 Ma and 377 Ma. Muscovite concentrates and whole-rock slate/phyllite from this and the Cabo Ortegal Complex yield plateau ages which range from 367 Ma to 295 Ma. Analyses of similar material from the relative autochthon yield plateau ages between 359 Ma and 275 Ma. Muscovite concentrates from three late- to post-kinematic granitic stocks yield plateau ages between 309 Ma and 274 Ma. At least seven of the 40Ar/39Ar analyses from metamorphic rocks record variable thermal rejuvenation of intracrystalline argon systems associated with emplacement of proximal granitic stocks. The remaining analyses may be used to constrain the local age of various Variscan tectonothermal events. The oldest fabric ages are recorded in allochthonous units whereas the youngest fabric ages occur along the boundary between internal and external zones. Middle Devonian ages are recorded in the allochthon and suggest a chronological continuity with deformational events in the relative autochthon, where Variscan deformation initiated in the Upper Devonian and diachronously prograded eastward. The first deformational events recorded in the limit within the internal and external zones occured ca. 20–25 Ma later (lower Namurian). Variscan deformation systematically prograded diachronously eastward across the orogen as new crustal material was added along the front of the developing orogenic wedge. However, the entire orogen remained tectonically active with different structural features forming at different times and at different places. An average propagation rate of ca. 5 km/m.y. is suggested by consideration of a 20–25 Ma difference in correlative fabric ages and present separations.

Variscan accretionary complex of northwest Iberia:Terrane correlation and succession of tectonothermal events

The allochthonous terranes of northwest Iberia can be correlated with specific paleogeographic realms of the continental masses and intervening oceans involved in the Variscan collision. Assuming that the existing ophiolites represent the suture formed by the closure of the Rheic ocean, the units in the footwall to the suture correspond to the outer edge of the Gondwana continental margin, and the units in the hanging wall are interpreted as fragments of the conjugate margin, represented by the Meguma terrane. This correlation establishes a precise link between circum-Atlantic terranes, and makes it possible to draw a relatively simple scenario of the successive tectonothermal events recorded. Following the amalgamation of Avalon to Laurentia, the remaining outboard terranes underwent a progressive accretion to this continent that ended with the collision between Laurentia and Gondwana.

Variscan exhumation of a subducted Paleozoic continental margin: The basal units of the Ordenes Complex, Galicia, NW Spain

A structural and metamorphic study was carried out in the basal units of the Ordenes Complex in Spain, thought to represent a subducted part of the Paleozoic margin of Gondwana. According to their metamorphic evolution, this part of the margin was subducted at the onset of the Variscan Orogeny, becoming part of an accretionary complex developed below a colliding element built previously. Variations in the PT conditions of the first high-pressure metamorphic event along the units indicate a polarity of the subduction to the west. Subsequent underthrusting of more continental material blocked the subduction and triggered the ascent and exhumation of the basal units, whereas the convergence continued. Recumbent folds and thrusts developed along with successive normal detachments. Compressional and extensional structures were synchronous or alternated in time and together induced the thinning and tapering of the orogenic wedge and its lateral spreading. The unroofing took place locally under an inverted temperature gradient caused by a detachment which carried a part of the hot mantle wedge above the subduction zone over the subducted units.

The Vila de Cruces Ophiolite: A Remnant of the Early Rheic Ocean in the Variscan Suture of Galicia (Northwest Iberian Massif)

The Vila de Cruces ophiolite is one of the ophiolitic units involved in the Variscan suture of the northwest Iberian Massif. This ophiolite consists of a tectonically repeated succession of greenschist facies volcanic rocks, common alternations of metasediments of pelitic or siliceous character, and scarce orthogneisses, metagabbros, and serpentinites. The protolith age of a granitic orthogneiss that intruded the mafic rocks is dated at \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape

Tectonic Evolution of the Careón Ophiolite (Northwest Spain): A Remnant of Oceanic Lithosphere in the Variscan Belt

497\pm 4

Careón ophiolite, NW Spain: Suprasubduction zone setting for the youngest Rheic Ocean floor

\end{document} Ma (U‐Pb in zircons). This age can be considered a reference for the generation of the ophiolite. According to their contents of some of the most immobile trace elements, the greenschist and the metagabbros are derived from basaltic magmas with compositions similar to those of island‐arc tholeiites. The influence of a subduction zone in the generation of the original basaltic magmas can be deduced from the marked negative Nb anomaly observed in all the metabasic rocks of this ophiolite. The granitic orthogneisses can also be genetically related to the basic rocks because they are similar to granitic rocks generated in volcanic arcs. The Vila de Cruces ophiolite is interpreted as a suprasubduction zone ophiolite generated in Late Cambrian times, during the early stages of the opening of the Rheic Ocean. The ophiolite was probably generated in a back‐arc basin developed during the first stages of the pulling apart and later drift of one or more peri‐Gondwanan terranes, one of them represented by the upper allochthon of the northwest Iberian Massif.

Magmatism and early-Variscan continental subduction in the northern Gondwana margin recorded in zircons from the basal units of Galicia, NW Spain

New U–Pb data (zircon, monazite and rutile) obtained from rocks of the uppermost allochthon in the Variscan belt of NW Spain indicate that the hangingwall to the suture includes an allochthonous unit with a pre-Variscan tectonothermal evolution. This evolution is characterised by an Early Ordovician (498–500 Ma) bimodal magmatism followed almost immediately (493–498 Ma) by a Barrovian style metamorphism up to the granulite facies. The metamorphism subsequent to the igneous intrusions requires convergence and crustal thickening in order to generate the Barrovian facies pattern shown by the lithologies of the uppermost allochthon. The almost coeval magmatism and metamorphism, and the chemistry of the metabasites suggest an accretionary complex, probably related to a volcanic arc, as the most probable setting for the origin of the uppermost unit. The implications of the existence of Early Ordovician convergent plate boundaries are discussed in the context of Gondwana–Laurentia–Avalon interactions.

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

Analysis of the Careón Unit in the Ordenes Complex (northwest Iberian Massif) has supplied relevant data concerning the existence of a Paleozoic oceanic lithosphere, probably related to the Rheic realm, and the early subduction‐related events that were obscured along much of the Variscan belt by subsequent collision tectonics. The ophiolite consists of serpentinized harzburgite and dunite in the lower section and a crustal section made up of coarse‐grained and pegmatitic gabbros. An Early Devonian zircon age ( \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape