Basalto-plataformak: ezaugarriak, jatorria eta kontinenteen apurketa, eta suntsipen biologiko orokorrekin izan ditzaketen loturak

Basalto-plataformak egitura geologiko arras arruntak dira, basalto-kolada bakunen pilaketaz osatuta daude eta egitura zein konposizio zeharo homogeneoa erakusten dute. Agian, milioi bat kilometro kubikotik gorako bolumena izatea eta magmaren gehiengoa oso epe laburrean (< 1 Mu) azaleratzea dira basalto-plataformen ezaugarririk deigarrienak. Nahiz eta orain arte erupzio bolkaniko izugarri hauen zergatiak eta ondorioak garbi ez egon, gero eta ugariagoak dira plaka tektonikarekin eta Lur-mailako suntsipen biologiko orokorrekin nolabaiteko lotura erakusten dutela erakusten dituzten datuak. Lanaren hasieran, geologikoki bat-batekotzat har daitezkeen, erupzio hauen egitura eta ezaugarri petrologiko zein geokimikoak deskribatuko dira. Ondoren, basalto-plataformen sorrera prozesuen inguruan egun gailentzen ari diren bi ereduak deskribatuko dira, eta kontinenteen apurketa prozesuekin eta adin ezberdinetan izandako suntsipen biologiko orokorrrekin erlazionatzen dituzten datuak aipatuko dira

Tectonothermal analysis of high-temperature mylonitization in the

Mylonites in pelitic and quartzofeldsphatic gneisses from the Ouguela tectonic unit (Coimbra–Córdoba shear zone, SW Iberian Massif) have been studied as an example of high-temperature ductile deformation associated with transcurrent tectonics. Detailed microstructural and P–T analysis indicates that ductile deformation evolved from a metamorphic peak at approximately 650–750 °C and 7.5–9.5 kbar (quartzofeldsphatic gneisses) and 730–790 °C and 7.5–9.5 kbar (pelitic gneisses) to retrograde conditions at 500–575 °C and 4.5/5.5–6.5/7.5 kbar (quartzofeldsphatic gneisses) and 525–600 °C and 3.5/4.5–5.5/7.5 kbar (pelitic gneisses). Following the metamorphic peak, exhumation was very fast. The P–T trajectory, which does not reach the curve for granite melting, is distinct that of isothermal decompression. Instead, the progressive and contemporaneous decrease in pressure and temperature was a direct response to strong heat dissipation along the contacts between the ascending slice and the adjacent blocks. The horizontal component of exhumation path, calculated for middle and shallower crustal levels, sum to ca. 57 km to 94 km (for the pressure peak). Assuming this offset acted in the Viséan during a time interval of ca. 9 Ma, the estimated exhumation horizontal slip rate is in the order of 6.3 to 10.4 mm/yr, which corresponds to an exhumation oblique-slip exhumation rate of 6.6 to 10.7 mm/yr (for ductile deformation). These values indicate that the transcurrent tectonic displacements accommodated by these mylonitic are similar to those of modern intra-continental shear zones, such as the still active Karakoram Fault (8.3 mm/yr) in the Himalayas. The Coimbra–Córdoba shear zone is therefore a typical intra-continental transcurrent zone with ten-to-one hundred kilometre along-strike mass movement of material that aided the exhumation of deep crustal rocks. Study of this large-scale structure in the SW Iberian Massif is therefore central to models of orogenic deformation during the amalgamation of Pangea

Labak, intrusio sinbolkanikoak eta erlazionatutako metakin bolkanokastikoak

Magmen osaeraren eta tenperaturen arabera, magma mota bakoitzak prozesu ezberdinak jasango ditu eta ondorioz, magma hauen kanporaketan gertatzen diren prozesuak eta kanporatze moduak ere ez dira berdinak izango. Hori dela eta, laba mota bakoitzak ehundura eta litofazieen garapen ezberdina izango du. Hain zuzen ere, ezberdintasun hauek dira lan honetan aztertu nahi ditugunak. Honetarako, lehendabizi laba eta intrusio sinbolkanikoekin lotuta agertzen diren litofazie adierazgarri batzuk aztertuko ditugu eta ondoren osaera ezberdinetako labek ingurune ezberdinetan sorrarazten dituzten egiturak

Exhumation of high-pressure rocks in northern Gondwana during the Early

The Coimbra–Córdoba shear zone (CCSZ) represents a major intra-continental shear zone of the European Variscan orogen. The shear criteria found in metamorphic rocks of the CCSZ are consistent with sinistral transcurrent movements. Isoclinal and open folds with axes parallel to the stretching lineation are responsible for dip variations in the mylonitic foliation, but are related to the same kinematics. In selected outcrops of the Campo Maior unit (SW Iberian Massif, Portugal), boudins of high-pressure mafic granulites, high-grade amphibolites and felsic gneisses with long-axes parallel to the stretching lineation in the surrounded metamorphic rocks, were sampled together with the host migmatites for petrographic, geothermobarometric and U–Th–Pb SHRIMP in-situ zircon geochronology analysis. The results show that decompression associated with shearing and partial melting in the CCSZ began under granulite facies conditions during the Variscan orogeny (early Carboniferous: c. 340 Ma.). Peak metamorphic conditions in the mafic granulites (850–880 °C and 14.5–16.5 kbar), were followed by symplectitization at 725–750 °C and 12.5–14.5 kbar. Peak P–T conditions were 615–675 °C and 9.5–11.5 kbar in the high-grade amphibolites, 750–850 °C and 11.5–15.5 kbar in the weakly deformed gneisses, and 675–725 °C and 9–11.5 kbar in the sheared migmatites. Subsequently, temperatures and pressures decreased during amphibolite facies metamorphism coeval with mylonitization. Retrograde P–T conditions were 550–700 °C and 7–9 kbar in the high-grade amphibolite, 620–640 °C and 6–8 kbar in the gneisses, and 560–610 °C and 5–6.5 kbar in the migmatites. Zircon dating of the migmatites and gneisses indicate Ediacaran (c. 590 Ma) and Ordovician (c. 488–479 Ma) ages for the protoliths, and show that these rocks were part of the northern Gondwana margin with a West Africa Craton signature dominated by Paleoproterozoic (c. 2–1.8 Ga) and Neoproterozoic (c. 664–555 Ma) ages, and a characteristic lack of Mesoproterozoic (c. 0.9–1.7 Ga) ages. These rocks were probably subducted, and subsequently exhumed during the complex processes of Pangea formation. The high temperature–high pressure rocks of the Campo Maior unit were likely displaced by large-scale transcurrent movements within the CCSZ in the early Carboniferous. The CCSZ appears to represent a major shear zone in the SW Iberian Massif connected in some way to the Variscan suture zon

Relative timing of transcurrent displacements in northern Gondwana: U-Pb laser ablation ICP-MS zircon and monazite geochronology of gneisses and sheared granites from the western Iberia Massif (Portugal)

The Variscan belt of Western and Central Europe was formed by the oblique subduction of the Rheic Ocean and the collision of Laurussia with Gondwana during the Late Palaeozoic. We present field relationships and new U–Pb LA-ICP-MS zircon and monazite ages for Variscan gneisses and granites from a key section of the western Iberian Massif. The Martinchel section records the interplay of two kilometre-scale Variscan transcurrent shear zones active in the Gondwana basement of Pangaea: the Porto–Tomar fault zone (PTFZ) and the Coimbra–Córdoba shear zone (CCSZ). Different kinematic models have been invoked to explain the formation and evolution of these major Variscan structures mainly based on assumptions made in the absence of reliable radiometric ages. We show that: (1) ductile deformation and metamorphism were active in the CCSZ during the Visean–Serpukhovian (c.335–318 Ma) and created conditions for amphibolite facies metamorphism and coeval emplacement of granites; and (2) later ductile–brittle deformation related to dextral movements along the PTFZ overprinted the earlier foliation and folds derived from the CCSZ deformation, and deformed the previously intruded granites. U–Pb dating of zircon and monazites yield c.335 Ma ages for the ductile deformation developed under amphibolite facies metamorphic conditions in the Martinchel gneisses of the CCSZ. The gneisses were intruded by granites at c.335–318 Ma, and both were later deformed under ductile–brittle conditions by dextral motion on the PTFZ. The geometry of the Martinchel gneisses (typical of the CCSZ) changed from one of thrusting to one of normal faulting by refolding of the early foliation, stretching lineation and asymmetric structures related to the later PTFZ dextral shear episode. This pattern of interference is not fully considered in previous models and may lead to incorrect tectonic interpretations. According to our data and recently published ages, we suggest that the PTFZ was active after the Serpukhovian–Kasimovian since the c.318–308 Ma granites are deformed by north–south (170°) dextral shear planes. These data are critical to the interpretation of large-scale Carboniferous transcurrent displacements in northern Gondwana (Iberian Massif), and bear upon global models of crustal deformation that emphasize the importance of long-lived dextral movements during the collision between northern Gondwana and Laurussia following the closure of the Rheic Ocean

Crustal growth and deformational processes in the northern

The aim of this article is to present a compilation of available information on the Évora Massif based on structural mapping, whole-rock geochemistry, recognition of metamorphic mineral assemblages, and geothermobarometry. In our view, transcurrent movements responsible for strong orogen-parallel stretching were dominant and had a major role in the geodynamic evolution of this part of Ossa-Morena zone (southwest Iberian Massif). Cadomian and Variscan orogenic events separated by a period of intense rifting were the cause for the composite distribution of zones with contrasting metamorphic paths, the structural complexity, the variety of lithological associations, and the sequence of deformation events and magmatism. The proposed geodynamic reconstruction for this segment of the northern Gondwana continental margin includes three main stages in chronological order: (1) Neoproterozoic accretion and continental magmatic arc developing, dismantling, and reworking, followed by late-“orogenic” magmatism; (2) Lower Paleozoic crustal thinning, block tilting, and mantle upwelling, induced by generalized rifting, leading to the formation of marine basins with carbonate platform sediments and thick accumulations of volcaniclastic and terrigenous sediments, contemporaneous with normal and enriched mid-oceanic ridge basalt–type magmatism; and (3) Upper Paleozoic transpressional orogenesis resulting from obliquity of convergence and the geometry of the involved blocks. The third stage includes the tectonic inversion of Lower Paleozoic basins, crustal thickening, the exhumation of high- to medium-pressure rocks and partial exhumation of high-grade metamorphic lithologies (controlled by local transtension and major detachments), the formation of synorogenic basins fi lled with volcanicsedimentary sequences, and fi nally, the emplacement of late Variscan granodiorites and granites