DBPapers
DOI: 10.5593/SGEM2015/B11/S1.001

DEFORMATION-BLASTESIS RELATIONSHIPS IN PARAUTOCHTHONOUS AND AUTOCHTHONOUS UNITS IN NW IBERIA (NORTH PORTUGAL): IMPLICATION FOR THE THERMAL EVOLUTION OF THE VARISCAN BELT

M.Ribeiro, R.Ramos, I.Bobos
Tuesday 4 August 2015 by Libadmin2015

References: 15th International Multidisciplinary Scientific GeoConference SGEM 2015, www.sgem.org, SGEM2015 Conference Proceedings, ISBN 978-619-7105-31-5 / ISSN 1314-2704, June 18-24, 2015, Book1 Vol. 1, 3-10 pp

ABSTRACT
The deformation in Central Iberian Zone (CIZ) and Galicia Trás-os-Montes Zone (GTMZ), the axial geotectonic zones of the Iberian Variscan Belt, is characterized by heterogeneity/partition, conditioned by previous anisotropies, either regional structures in the Precambrian basement or the stratification (S0) of the Lower Paleozoic sedimentary sequences involved in the Variscan Orogeny. In NW of Portugal, the overlapping of allochthonous and parautochthonous nappes (GMTZ) on the autochthonous sequence (CIZ) implies some differences in the tectono-metamorphic evolution, although ZGTM and CIZ metasedimentary units report globally a LP-HT metamorphic gradient. In GTMZ three deformation phases (D1, D2 and D3) are well marked, while in CIZ only the first and last of these phases (D1 and D3) are important.
Litological, structural and petrographic data from different nappes are presented. In the autochthonous and lower parautochthonous nappes, both essentially pelitic/semipelitic standing out some carbonaceous levels, the most pervasive folding phase is D1, with an axial cleavage, developed in low grade conditions. In general this cleavage is parallel to S0. In the upper parautochthonous nappe the stratification (S0) constraints the geometry of the variscan folds and associated foliation. The isoclinal character of D1 folds was accented by the tectonic transport of the nappes (D2). The pelitic and semipelitic rocks show a metamorphic differentiation with Q and M domains parallel either the S0 or to the main foliation (S1/S2). The last phase D3 develops a NW-SE heterogeneous vertical foliation in parautochthonous (ZGTM) and in autoctonous (CIZ).
The first metamorphic assemblage in LP-LT prograde conditions is syn- to late-kinematic relatively to D1/D2. A second metamorphic assemblage pre- to syn-D3 mark the metamorphic peak with muscovite + quartz + biotite I ± staurolite ± andalusite I ± cordierite I ± garnet ± sillimanite I. The isogrades from this thermal event are conditioned by the rise and emplacement of the synkinematic granites. Post-D3
porphyroblast of cordierite II, sillimanite II, andalusite II and biotite II are only related to thermal metamorphism close to the post-kinematic granites. A 4th retrograde and/or hydrothermal assemblage corresponds to muscovite II + quartz + chlorite II.

Keywords: Autochthon, Parautochthonous units, Metasedimentary rocks, Foliation, Blastesis.

PAPER 10.5593/SGEM2015/B11/S1.001 : DEFORMATION-BLASTESIS RELATIONSHIPS IN PARAUTOCHTHONOUS AND AUTOCHTHONOUS UNITS IN NW IBERIA (NORTH PORTUGAL): IMPLICATION FOR THE THERMAL EVOLUTION OF THE VARISCAN BELT

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