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



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Fernández Suárez, Javier and Corfu, Fernando and Arenas Martín, Ricardo and Marcos Vallaure, Alberto and Martínez Catalán, José Ramón and Díaz García, Florentino and Abati Gómez, Jacobo and Fernández Rodríguez, Francisco José (2002) U–Pb evidence for a polyorogenic evolution of the HP–HT units of the NW Iberian Massif. Contributions to mineralogy and petrology, 143 . pp. 236-253. ISSN 0010-7999

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Official URL: http://link.springer.com/journal/410


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) overlyingthe oceanic suture in the Allochthonous Complexes of the NW Iberian Variscan Belt. The rocks investigated are seven granulite- to eclogitefacies paragneisses and one leucosome within mafic highpressure granulites in the Ordenes and Cabo Ortegal Complexes of NW Spain. U–Pb datingof zircon, monazite, titanite and rutile reveal the presence of a pervasive Early Ordovician metamorphic event at ca. 500–480Ma 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. 500–485Ma. Subsequently, the rock ensemble underwent exhumation accompanied by partial meltingan d zircon growth at ca. 485–470Ma. Meltingof mafic 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. 390–385Ma). This Early Devonian event, at variance with the previous one, was not pervasive, but, rather, was localised in areas of intense Variscan tectonothermal 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). Accordingto previous structural studies and Ar–Ar datingof 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.

Item Type:Article
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(The original publication is available at www.springerlink.com)

Uncontrolled Keywords:U–Pb geochronological study; NW Iberian; Variscan Belt
Subjects:Sciences > Geology > Petrology
Sciences > Geology > Geodynamics
ID Code:17429
Deposited On:13 Dec 2012 10:28
Last Modified:05 Jul 2017 08:50

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