Deformación y metamorfismo en la zona de Sanabria (provincias de Zamora, León y Orense, noroeste de España)

Abstract

The Sanabria Area belongs to the provinces of Zamora, León and Orense in the Northwest of Spain, and lies in the "Olio de sapo Anticline" near the Tertiary sedimentary cover of its south-eastern end. Materials represented in the area range from Precambrian and Lower Cambrian to Upper Silurian. The lowermost strata are grouped together into the "Serie de Porto", with quartzites, different varieties of schists (with sillimanite, staurolite, almandine, andalucite and chloritoid), and also with kyanite as a relict of a former high pressure metamorphism (pre-hercynian). There are also represented carbonate rocks (with diopside, olivine, epidote, tremolite, etc.), amphibole-gneises, piroxene-gneises, amphibolites (with hornblende, actinolite, cummingtonite, etc.). In the upper part of this series predominate the biotite or muscovite gneises with some quartzites and schists. These gneises have suffered a hercynian migmatisation and form the transition to the Olio de sapo formation. The total thickness of the Serie de Porto goes up to 1500 m. After a gradual transition zone concealed by migmatisation, comes the Olio de sapo formation, 1500-2000 m. thick and made up of silicic vulcanites with some detrital constituents. The big feldspar mega-crystals that can be seen in the deeper part of the "Anticline" were grown in solid state during the first metamorphic phase, since they are only seen in the more metamorphised (or with evidence of retrometamorphism) parts of the formation. On the other hand, these megacrystals are seen to include the primitive matrix and ashes together with remnants of former ordered micas. This formation is also affected by the regional migmatisation with formation of agmatites and nebulites in the Ribadelago area. Its transition to the overlying Lower Ordovician (Puebla Formation) is gradual and no evidence of disconformity or hyatus can be detected from the outcrops. The Puebla Formation is entirely made up of a monotonous schits series with some quartzite layers a few meters thick. In these quartzite layers can be found Cruzianas, Vexillum and Scolithus. After some 400 m. of these gray to violet or pale green schists, the armorican quartzite or Culebra Formation comes with a total thickness of less than 50 meters. On top of the Culebra Formation are pink schists and then deep blue schists where no determinable fossils have been found. In the upper part of this transitional series (San Pedro de las Herrerías Formation), there are more detrital components with a lighter color. This series is supposed to be of Middle to Upper Ordovician age and is unconformably overlained by a complex Series that has been dated as of Upper Wenlock age and named "Serie de San Vitero". It is made up of conglomerates, ampelites with graptolites, graywackes, crinoidal limestones, silicic vulcanites and lidites with graptolites. Both in the upper and lower conglomerates and in the graywackes are present metamorphic rock fragments that prove the existence of a prehercynian metamorphism. This series is interpreted as a molasse deposit of the first folding phase of silurian age. The presence of plant remains in the graywackes of the Serie de San Vitero and in comparable sequences of Galicia has led other authors to describe them as Carboniferous. After the microscopic study of the paleozoic material, the existence of two more metamorphic phases can be ascertained in this area. The second metamorphism was less intense than the first one and of intermediate pressure character. The third one was weak and affected a little part of the area, with cristallysation of biotite and chloritoid. After the second metamorphism the regional migmatisation took place. The second metamorphism and related migmatisation gave birth to most of the granitic rocks present in the area, among them, porphyritic granodiorites younger than the third phase and muscovite granodiorites younger than phase IV. There are also orthogneises interpreted as older that the phase I (Orthogneis de San Sebastián), and older than phase II (Gneis de Hermisende), and amphibole-quartzdiorites older than the migmatisation (Ribadelago Diorites). Late two-mica granodiorites and biotite granodiorites with megacrystals are also recorded. Some of them produced contact metamorphism. The tectonic history of the area resulted to be more complex than is generally believed in the current schemes for Galicia. Four important folding phases can be separated in time and space, all with different peculiarities. Phase I gives isoclinal Ε recumbent folds with NNE-SSW axes, with an associated metamorphism and probably important thrusting to the East. Phase II is also very important and gives also NE recumbent NW-SE trending folds with axial-plane schistosity. The associated intermediate pressure metamorphism almost obliterates the I phase cleavage and metamorphic minerals, being only possible to see the first one as "polygonal arcs" in the second axial-plane schistosity. The II phase structures lead to think of an important nappe or overthrust to the NE that moved the whole "Olio de sapo Anticline" and caused blastomilonitisation in the Serie de Porto. The overthrust would have evolved from a large recumbent fold similar to that of the Caurel Mountains (Lugo). Phase III is less intense and produced NW-SE trending folds with NE déversement and no associated cleavage. Phase IV produced penetrative strain-slip cleavage and a crenulatian apparent all over. It also gave birth to the megastructures seen presently. The strain-slip cleavage surfaces have a NE dip while the axial planes of the phase III are inclined to the SW. This IV phase carried an associated metamorphism and produced the orientation of muscovite granodiorites. There are also two later kinking-type phases. Two important fracture directions are also present: the NNE-SSW trending strike-slip fault of Calabor-Braganza and the E-W trending Padornelo-La Canda Fault of normal character. The second one is younger and made the first one to play again as a vertical fault essentially. The conclussions about the paleogeography of the paleozoic materials show the similarities existing between Lower Cambrian facies in the Nortwestern Iberian Peninsula and the differences arising in the Middle and Upper Cambrian that allow to divide them in two fundamental facies: volcanic facies predominant in the west, and sedimentary, predominant in the east, with occasional tectonic superpositions near the external zones. The transition between both facies seems nevertheless be more or less gradual and the thicker volcanic accumulation took place in the Olio de sapo area of deposit. In the Lower Ordovician, sedimentation comes again to be comparable in Northwestern Iberian Peninsula, and also in the Middle and Upper Ordovician and Lower Silurian when the first folding phase takes place. Thereafter, a molasse deposit is laid in both areas during the Upper Silurian, which is richer in iron in the external zones. Through comparison with the sedimentation in the external areas and with radiometric ages obtained in Galicia, it can be concluded that the aproxímate ages of the folding phases in Sanabria are: Silurian, Upper Devonian, Lower Westphalian and Upper Westphalian or Stephanian for the four principal ones. Fractures are tardihercinian. After the strong similarities existing between rocks in the Sanabria area and the Braganza and Moráis basic complexes, which are very close to the south, it is concluded that they are related as different parts of the same overthrust to the NE, which also moved the Olio de sapo· structure after the phase II and produced blastomylonitisation in both areas. The basic complexes, as well as the ones in Western Galicia, represent the ophiolites of the cambro-ordovician geosyncline that culminated with the silurian folding phase. Their roots can be placed in Western Galicia (blastomylonitic graben) or perhaps in an area located west of the present Atlantic Coast if we think of the two probable overthrusts suffered by these materials, the first one towards the East and the second one towards the Northeast. Its restriction to a defined geographic area (Complexes of Cabo Ortegal, Ordenes, Lalín, Braganza and Moráis) can be understood since the ophiolitic "suite" spreads out only in a definite extension of the internal zone and thins out and eventually disappears laterally. This lateral wedging is probably seen in the orthoamphibolites of the Serie de Porto in the Sanabria Area.