An integrated field, geochemical and U-pb geochronological study of the southwest Hermitage Flexure (Newfoundland Appalachians, Canada) and the Sierra De Guadarrama (Iberian Massif, Central Spain) : a contribution to the understanding of the geological evolution of circum-Atlantic Peri-Gondwana

摘要

An integrated field and analytical study of three areas in Newfoundland and Spain was carried out to investigate the extent of the linkage between the geological evolution of the Peri-Gondwanan margin of the Iapetus Ocean, recorded in the southwest Hermitage Flexure of the Newfoundland Appalachians, and that of the Sierra de Guadarrama in the Central Iberian Zone of the Iberian Massif (European Variscan Belt). This study, while resolving the timing and character of the respective Appalachian and Variscan overprints, highlights the major importance of the Early Ordovician (ca. 480-470 Ma) magmatic events. -- The Cinq-Cerf gneiss is part of the westernmost extent of the Late Precambrian basement block of the Hermitage Flexure. New data from the Cinq-Cerf gneiss demonstrates that this is a composite unit formed by highly strained metasedimentary rocks and 675+12/-11 Ma granitic orthogneiss, locally intruded by weakly defomed 584+7/-6 Ma Hb1-bearing granodiorite and a transitional/volcanic arc tholeiitic 557+14/-5 Ma metagabbro. Although variably deformed during the intrusion of the synkinematic 431.5±1 Ma Western Head granite and the subquent, 420 Ma, greenschist facies overprint, this set of rocks preserve evidence of pre-Silurian deformation. This is consistent with a basement-cover relationship between the Cinq-Cerf-gneiss and the nearby low-grade 583-570 Ma volcanosedimentary rocks. The 675 Ma intrusive event provides a strong link with the Avalon Zone further demonstrating that Avalonian rocks (s.s.) were involved in the Early Paleozoic evolution of the eastern margin of the Iapetus Ocean. -- Further west the gneissic rocks of the Port-aux-Basques complex, generally adscribed to the Gander Zone, separate the Avalonian basement of the Hermitage Flexure from the suture with the peri-Laurentian margin. New data shows that the oldest set of intrusives, the Margaree orthogneiss, represent a 20 km long, 474-465 Ma mafic-felsic igneous complex overprinted by Silurian (417-410 Ma) upper amphibolite facies metamorphism and deformation. The geochemical signatures of the Margaree orthogneiss suggest that it formed in a transitional arc/back arc setting at the time of the major Late Arenig-Llanvirn back-arc rifting event in the peri-Gondwanan margin of the Northern Appalachians. -- In the Iberian Massif, the orthogneisses of the Sierra de Guadarrama form the southern portion of an enigmatic belt of pre-Variscan augen-gneisses that extends 600 km along the northern Central Iberian Zone (CIZ). Dating across the Berzosa-Riaza shear zone (BRSZ) demonstrates the presence of 468 Ma granites in the shear zone, 480±2 Ma volcanism in the low-medium grade hangingwall and coeval 488-477 Ma granitic batholiths in the high grade footwall. The 480 volcanism brackets the Sardic deformation in the CIZ between the Mid-Late Arenig. The BRSZ has an oblique extensional movement coincident with peak metamorphism (337-327 Ma), which developed during the Variscan D2 deformation (330-321 Ma), resulting in decompression and late-D2 growht of low P / high T assemblages (322 Ma) in the infrastructure. The shallow intrusion of the post-tectonic La Cabrera granite, dated at 292±2 Ma, seals the BRSZ and marks the end of the Variscan deformation. This new data demonstrates, for the first time, that the Early-Mid Carboniferous syn-collisional extension in the CIZ was coeval with extension along the entire hinterland of the Variscan belt. -- These Ordovician orthogneisses of the Sierra de Guadarrama are interpreted as relicts of a short-lived magmatic arc coeval with the subduction-related break-up of Avalonia from Gondwana. This new data illustrates the striking parallelism between the Early Paleozoic events in the Peri-Gondwanan side of the Northern Appalachians and the southern portion of the European Variscan belt.