U-Pb evidence for a polyorogenic evolution of the HP-HT units of the NW Iberian Massif

Abstract

A isotope dilution thermal ionisation mass spectrometry U Pb geochronological study was carried out on the high-pressure and high-temperature units (HP HT units) overlying the oceanic suture in the Allo chthonous Complexes of the NW Iberian Variscan Belt. The rocks investigated are seven granulite-to eclogite facies paragneisses and one leucosome within mafi c high pressure granulites in the Ordenes and Cabo Ortegal Complexes of NW Spain. U Pb dating of zircon, monazite, titanite and rutile reveal the presence of a pervasive Early Ordovician metamorphic event at ca. 500 480 Ma and a later Early Devonian event at ca. 400 380 Ma. The U Pb ages, in conjunction with petrological and structural data, indicate that the high-pressure event recorded by these rocks is Early Ordovician in age. Monazite ages in the paragneisses suggest that peak metamorphic conditions were reached at ca. 500 485 Ma. Subsequently, the rock ensemble underwent exhumation accompanied by partial melting and zircon growth at ca. 485 470 Ma. Melting of mafi c granulites was coeval with this latter episode as indicated by zircon crystallisation age in the leucosomes dated at ca. 486 Ma. Based on these data and on the general features of magmatism and metamorphic evolution, it is proposed that this process took place at a convergent plate boundary within a peri Gondwanan oceanic domain. Monazite, titanite and rutile data in some of the samples studied show evidence of a second metamorphic episode that took place between ca. 400 and 380 Ma (with a peak at ca. 390 385 Ma). This Early Devonian event, at variance with the previous one, was not pervasive, but, rather, was localised in areas of intense Variscan tectonothennal reworking. It is claimed that this later metamorphic event was recorded by the U Pb system in areas where monazite and titanite growth was enhanced by fluid circulation in highly strained rocks (Variscan shear zones). According to previous structural studies and Ar Ar dating of fabrics, this Early Devonian episode took place as the HP HT units were deformed and thrusted upon the ophiolitic units in the early stages of the Variscan collision.

A rootless suture and the loss of the roots of a mountain chain: The Variscan belt of NW Iberia

“…When considered together with the chemistry of the Monte Castelo gabbro, these terrigenons sediments, rich in volcanic components, snggest a volcanic arc environment, althongh the absence of truly volcanic rocks points to a dissected magmatic arc [31], Detrital zircon ages in metagrey wackes yielded three age popnlations of 25-2,4 Ga, 2,1-1,9 Ga and 610-480 Ma [31], establishing an Early-Middle Ordovician maximnrn depositional age, 3,2, High-P and high-T upper units These nnits, in the complexes of Bragan"a, Cabo Ortegal and Ordenes, inclnde paragneisses and nltra basic metaigneons rocks, bnt the most characteristic are garnet-clinopyroxene grannlites and eclogites retro graded to the amphibolite facies, Gabbros occnr in several stages of transformation, from relatively little metamorphosed rocks to coronitic metagabbros, high-P grannlites and amphibolites [10], Snbophitic and diabase textnres indicate emplacement at relatively shallow levels, The basic rocks are tholeiitic metagab bros with a MORB geochemical signatnre [35], bnt geochemical stndies of the nltramafi c rocks are consistent with generation in an arc setting [71], U-Pb data in metabasic rocks have yielded 520-480 Ma, viewed as protolith ages [54], bnt conld also refl ect the imprint of nearly contemporaneons meta morphism [30,32,57], However, the high-P grannlite and eclogite facies metamorphism is dated, by U-Pb and 40 Ar/39 Ar methods, between 425 and 390 Ma [30,32,37,54,66,73], This event involved snbdnction [36], and was snccessively followed by decompression, partial melting, penetrative amphibolite-facies myloni tization, which was dated at 390-380 Ma [20,21,37], recnrnbent folding, and thrusting [35,42,52],…”

“…The ensialic arc later became involved in the Variscan collision, The Cambro-Ordovician arc-related metamorphism was preserved in the intermediate-P nnits, bnt was overprinted by a Silnrian-Early Devonian (425-390 Ma) tectonometamorphic event in the high-P and high-T nnits [30,32,37], This event involved snbdnction of part of the arc, and resnlted from accretion to Lanrnssia [37,38,45,48] while the Rheic Ocean was still open (Fig, 3c), …”

“…The Corredoiras orthogneiss is granodioritic to tonalitic [40], whereas a minor gabbroic Epizonal metasediments occnpy the nppermost structnral position and consist of metapelite and greywacke of tnrbiditic character, with alteruations of qnartzite and conglomerate. When considered together with the chemistry of the Monte Castelo gabbro, these terrigenons sediments, rich in volcanic components, snggest a volcanic arc environment, althongh the absence of truly volcanic rocks points to a dissected magmatic arc [31], Detrital zircon ages in metagrey wackes yielded three age popnlations of 25-2,4 Ga, 2,1-1,9 Ga and 610- The geochemistry of the plntonic and nltramafi c rocks, and the abnndance of volcanic components in the metasediments are featnres consistent with generation of the npper nnits in an arc setting, The metamorphic evolntion of the intermediate-P nnits points in the same direction, as the P-T paths inclnde isobaric heating related to the intrusion of large plntons, followed by isothermal bnrial implying crustal thickening [4,18,40], Pressnrization in the sillimanite fi eld and a snbseqnent anticlockwise evolntion are linked to active plate margms, Late Cambrian-Early Ordovician (500-460 Ma) magmatism is widespread in the npper and basal allochthonons nnits and the antochthon, whereas inherited zircon ages from orthogneisses of all of them are similar, and snggest derivation from the West African craton [57,72] (Fig, 3a) and may represent a peri-Gondwanan fragment drifted away to open the Rheic Ocean (Fig, 3b),, Detrital zircon ages, from low grade metasediments in the npper nnits, also record the major events in NW Africa [31]: this evidence points to a common basement and snggests that the island arc was ensialicThe ensialic arc later became involved in the Variscan collision, The Cambro-Ordovician arc-related metamorphism was preserved in the intermediate-P nnits, bnt was overprinted by a Silnrian-Early Devonian (425-390 Ma) tectonometamorphic event in the high-P and high-T nnits [30,32,37], This event involved snbdnction of part of the arc, and resnlted from accretion to Lanrnssia [37,38,45,48] (Fig, 3c), • the high rates of snbsidence dednced from the thick preorogemc succeSSIon;while the Rheic Ocean was still open• the volnrninons Early Ordovician magmatism,The dominant sediments are pelite, sandstone, qnartzite, and limestone in formations and gronps of large lateral extent Lithologic associations and facies indicate deposition in a passive continental margin, which sncceeded the Cadomian Andean-type activity [27], Differences in thickness in Middle Cambrian to Late Ordovician deposits indicate extensional activity in the margin (Fig, 3b) [48...…”

The eclogite facies gneisses of the Cabo Ortegal Complex (NW Iberian Massif): Tectonothermal evolution and exhumation model

“…leucosomes due to anatexis are not found), and the lepidoblastic minerals such as Bt are not highly concentrated in the foliation planes. The existence of previous Cambrian deformations in the upper units has been demonstrated by different authors using U-Pb geochronological data (Fernández Suárez et al, 2002;Abati et al, 2007;Díaz García et al, 2010). These deformations were developed during the generation of the protoliths in a Cambrian peri-Gondwanan magmatic arc.…”

“…This terrane is considered a remnant of a Cambrian peri-Gondwanan magmatic arc , rifted from the main continent (Gómez and affected by a polymetamorphic evolution developed during the magmatic arc stage and later during eo-Variscan times (Abati et al, 1999;Fernández Suárez et al, 2002). The ophiolitic units are generally subdivided in lower ophiolites and upper ophiolites, with Cambrian (ca.…”