José Munhá

The Beja Layered Gabbroic Sequence (Ossa-Morena Zone, Southern Portugal): geochronology and geodynamic implications

The Beja Igneous Complex (BIC) is a major geological feature in SW Iberian Variscides, consisting of three main units developed during different stages of the oblique collision between the Ossa Morena Zone (OMZ) upper plate and South Portuguese Zone (SPZ) lower plate, namely: (1) ca. 355 Ma to ca. 345 Ma Layered Gabbroic Sequence formed in early stages of collision magmatism; (2) the ca. 335-330 Ma to ca. 320 Ma Cuba-Alvito (gabbro-diorite) Complex formed throughout the late-collision magmatic event; and (3) the Baleizão Porphyry Complex corresponding to the period of post-collision magmatism, ca. 300 Ma. The new SHRIMP U-Pb age of 342±9 Ma reported here for amphibole-bearing pegmatite dykes cutting the layered gabbros is interpreted as dating the development of late fluid-rich melts in the Layered Gabbroic Sequence, synchronous with Fe-Cu-Co sulphide deposition. The close agreement between this data and available amphibole 40Ar/39Ar ages of BIC, Beja-Acebuches Ophiolite and other geological units of the OMZ southern border, may be taken as evidence for a moderate to rapid regional crustal uplift episode at ca. 340±5 Ma; this data, coupled with structural constraints, also allow to estimate the age for the transition between the D2a – D2b deformation phases of Variscan continental collision. A complex wedge system within the SW Iberian Variscides developed during this collision, involving the OMZ upper plate to the north and the SPZ passive margin in the lower plate. The Évora-Beja-Aracena Domain, located in the upper plate above the N-dipping subduction zone, is re-interpreted as a retro-wedge domain that was kinematically coupled to the SPZ pro-wedge and subduction system. Retro-wedge growth is linked to upper plate uplift (early collision) and a late-orogenic wedge thickening. The early stages of magmatism in the retro-wedge are related to asthenospheric mantle upwelling induced by the slab break-off. Regional LP-HT metamorphism and subsequent magmatic events in the retro-wedge domain were caused by long term high heat flow sustained by (1) mafic magma underplating, (2) stacking of high-heat producing upper-crustal lithologies, and reinforced (3) by (moderate to) rapid crustal uplifting. Mass advection and orogenic architecture were strongly affected by asymmetric removal towards the lower-part foreland basin and by transient mechanical properties of the wedge system associated with the anomalous thermal regime.

The Moura Phyllonitic Complex: An Accretionary Complex related with obduction in the Southern Iberia Variscan Suture

The structure of the southernmost domain of the Ossa Morena Zone in Portugal (south sector of the Iberian Autochthonous Terrane) is strongly controlled by earlier deformation events. The first two deformation events correspond to tangential strain regimes, marked by subhorizontal milonitic foliations. These events seem to be directly related with the obduction/subduction process during the Variscan ocean closure and the emplacement of the Beja-Acebuches Oceanic Terrane. In this domain (Évora-Beja Domain), the upper tectono-stratigraphic unit (Moura Phyllonitic Complex) is mainly represented by phyllites and corresponds to a strongly imbricated complex, involving several layers of autochthonous sequence (mainly rocks of a volcano-sedimentary complex), but it also includes dismembered and scattered slices of ophiolites. The widespread greenschists facies overprint an earlier high-pressure metamorphic event (blueschists in the central sector of Évora-Beja Domain and eclogites in the western sector). With regard to its geochemical signature, the Moura Phyllonitic Complex includes amphibolites ranging from N-MORB to T/P-MORB (ophiolitic slices) and mafic alkaline and peralkaline metavolcanics (autochthonous slices). At macroscopic scale, the autochthonous sequence of the Évora-Beja Domain is almost complete in the eastern region, with a stratigraphic sequence ranging from Precambrian to Silurian/Lower Devonian. Towards WSW, the Moura Phyllonitic Complex progressively become tectonically discordant on the sequence below, just near the suture, where it superposes Precambrian levels. The overall evidences (tectonic, metamorphic and geochemical) allow the conclusion that the Moura Phyllonitic Complex is an accretionary complex related with the obduction process during earlier times of the variscan ocean closure.

Geodynamic evolution of the Continental Allochthonous Terrane (CAT) of the Bragança Nappe Complex, NE Portugal

The Braganca Massif, in particular its Continental Allochthonous Terrane (CAT), is a keystone in the comprehension of the pre-Variscan and Variscan geodynamic evolution of the Eurasian plate, in particular of the high-grade metamorphic evolution. The Braganca CAT comprises three types of granulites (recording quite distinct metamorphic histories) and paragneisses with enclosed eclogite boudins. It has undergone five characterizable, main deformation phases during two orogenic cycles, separated by the extensive intrusion of mafic/ultramafic layered bodies (types 2 and 3 granulites) into a pervasively deformed granulitic lower continental crust (type 1 granulites). Geochronological data obtained on high-grade metamorphic rocks from Braganca and other similar rocks in the Variscan fold belt have been typically interpreted in two different ways: polycyclical evolution, Precambrian and Paleozoic (Variscan), or monocyclical evolution (Variscan). This second hypothesis is in clear contradiction with field evidence, with tectonometamorphic data, and even with some geochronological data, as shown in this article. Thus we suggest that the tectonometamorphic evolution of the Braganca CAT, and, by comparison, of other similar CAT in the Variscan fold belt, took place during two orogenic cycles, one Precambrian (Grenvillian? Pan-African/Cadomian?) and another one Paleozoic (Variscan).

The Late Triassic‐Early Jurassic Volcanism of Morocco and Portugal in the Framework of the Central Atlantic Magmatic Province: An Overview

An overview on the Late Triassic-Early Jurassic Magmatic Province of Morocco and Portugal (TJMPMP) is presented. It comprises extrusive basalts, interbedded with clastic rocks sequences preserved in elongated rift basins, and their feeder dikes and sills. Paleontologic ages range from Upper Ladinian-Lower Carnian to the Sinemurian for the sediments, while available 40 Ar/ 39 Ar analysis yield a mean age of 200±1,6 Ma for the volcanics. The volcanologic characteristics of the TJMPMP are those of continental basaltic successions. It comprises subaerial lava flows and pyroclastic deposits, sometimes deposited in lacustrine environments, and feeder dikes, constituting an interesting volcanic sub-province of the Central Atlantic Magmatic Province (CAMP) as most preserved outcrops are extrusive volcanics. These rocks correspond to Low-Ti (TiO 2 11 suggesting that their source may be within the continental lithospheric mantle. The nucleation of the rifting process may have started at two different triple junctions, an RRR junction near Florida and a RRT between Africa-Iberia-America.

Morphology, internal architecture and emplacement mechanisms of lava flows from the Central Atlantic Magmatic Province (CAMP) of Argana Basin (Morocco)

Abstract The morphology, internal architecture and emplacement mechanisms of the Central Atlantic Magmatic Province (CAMP) lava flows of Argana Basin in Morocco are presented. The volcanic pile was produced by two volcanic pulses. The first, represented by the Tasguint Formation, corresponds to a succession of 3–13 individual flows created by 1–8 eruptions; the second, Alemzi Formation, is composed of 2–7 individual flows formed by 1–4 eruptions. These formations, geochemically distinct, are separated by thin silty or sandy horizons or by palaeosols. They include ‘compound pahoehoe flows’ and ‘simple flows’. The first type is almost exclusive of the lower formation, while the second type dominates the upper formation. The lava flows show clear evidence of endogenous growth or ‘inflation’. The characteristics of the volcanic pile suggest slow emplacement during sustained eruptive episodes and are compatible with a continental basaltic succession facies model.

The sedimentary record of recent (last 500 years) environmental changes in the Seixal Bay marsh, Tagus estuary, Portugal

The inner Tagus estuary is essentially a sedimentation basin that receives cohesive sediment from terrestrial, marine, biological and anthropogenic sources. Three short cores from one site in a marsh area of this estuary (Seixal Bay) were analysed for sedimentary, geochemical and micropalaeontological contents (benthic foraminifera and nannoplankton). The length of the cores represents about half a millennium of sedimentation. Textural analysis suggests a highly uniform mud sedimentation for most of the cores but geochemical, mineralogical and micropaleontological results indicate climatic and environmental changes and anthropogenic disturbance. Three Foraminifera zones were identified. The lower part of the lower zone indicates sedimentation in an open channel or a lower domain of an exposed high-energy sandflat. Sediments of the upper part of the lower zone and of the middle zone were deposited in a lower-energy environment, probably associated with a sheltered, vertically aggrading mudflat located within the Seixal Bay. Biological and mineralogical indicators suggest that periods of total or partial closure of this bay occurred. Clay minerals indicate that drier and colder conditions prevailed in the lower half of this zone evolving gradually to a wetter and warmer environment towards the top. The upper zone indicates persistence of low-energy sedimentation and evolution towards the present salt-marsh conditions. Anthropogenic pollution is clear in geochemical proxies at the top of the sedimentary column and was used for dating purposes.

Mantle eduction: tectonic fluidisation at depth

Abstract Recent direct observations in trench environments demonstrate that massive volumes of wet sediments are subducted. The gas phases in and near trench zones also show the evidence for subduction of organics and ocean fluids. Serpentine sea mounts also show that large quantities of serpentine are subducted. Thus in the upper layers of the subducting slab metamorphic processes must generate fluidized beds which may be extruded along the subduction faults or intruded by hydraulic fracture mechanisms into the overlying cover rocks. This process which we term eduction, may account for some of the exotic assemblages containing high-pressure rocks found in subduction assemblages, the equivalent of deep mud dykes and volcanoes. The same process may be associated with seismic events.

Gold mineralization in recent and ancient volcanic-hosted massive sulfides: The PACMANUS field and the Neves Corvo deposit

Gold mineralization in the Neves Corvo copper stockworks occurs in two geochemical associations: (1) Au+Co+Bi(±Te) and (2) Au+Cu+Ag(±Hg). Type 1 gold occurs in the deeper parts of the feeder zones. Its associated mineralogical assemblage is consistent with low sulfur activity of the fluid and high temperature (360°–400°C) conditions. The second type of gold geochemical association reflects pH increase and/or temperature decrease as the fluid moves upwards in the stockwork and reaches the base of the massive sulfide Neves Corvo is accompanied by high sulfidation parageneses. These might have resulted either from the long-lived maturation of the ore-forming system, which would have led to extreme zone refining effects, and/or from the late input of an external fluid component (possibly magmatic?) in the Neves Corvo hydrothermal system. In the PACMANUS hydrothermal field, native gold also occurs in both high and low sulfidation mineral associations. Gold occurs at the seafloor level either associated with: (1) chalcopyrite+sphalerite +tennatite, or (2) chalcopyrite+bornite ± covellite. At depth, instead, gold occurs in sphalerite as small native gold inclusions (silver-poor). Following the process of copper enrichment by zone-refining, gold frequently concentrates in the borders of the sphalerite grains.

Petrology of ultramafic xenoliths from Madeira Island

Ultramafic xenoliths from Madeira island are divided into dunite/websterite/wehrlite/clinopyroxenite (DWWC) and harzburgite/lherzolite suites; the harzburgite/lherzolite xenoliths show abundant deformational features and are more refractory (Fo = 90–91) than the DWWC suite (Fo = 77–88). DWWC xenoliths are spinel-bearing olivine ± orthophyroxene cumulates with intercumulus clinopyroxene and rare plagioclase, amphibole and phlogopite. Mineral chemistry and geothermobarometric data indicate that DWWC xenoliths crystallized at 1150–1300 °C from Madeiran alkalic basalts and accumulated in magma reservoir(s) located 36–45 km beneath the island. The harzburgite/lherzolite xenoliths are composed of olivine + orthopyroxene + spinel ± clinopyroxene ± (rare) phlogopite and display alkali feldspar or clinopyroxenite veins and crystal aggregates. The complex thermal evolution recorded by these xenoliths and the close similarity of clinopyroxene REE contents and calculated f O 2 values in both harzburgites and DWWC cumulates are attributed to recent infiltration of the harzburgites by melts trapped or crystallized within the mantle; these features, and the refractory bulk chemistry of the harzburgite/lherzolite suite, support the interpretation that these xenoliths represent depleted oceanic lithosphere variously modified by magmatism associated with the genesis of Madeira island. The association of these upper mantle xenoliths with cumulates crystallized from Madeiran magmas (DWWC) suggests that the harzburgite/lherzolite suite originated in the uppermost mantle above magma storage zone(s), probably near the boundary between the mantle and the overlying oceanic crust.

Osmium isotope systematics in the Iberian Pyrite Belt

New Re and Os analyses of pyrites from Aljustrel, Lagoa Salgada and Neves-Corvo deposits yield concentrations that are similar to those previously reported for the Rio Tinto and Tharsis deposits, all at the Iberian Pyrite Belt. Osmium contents in IPB pyrites increase from the deeper stockwork zones towards the upper massive sulphides. Pyrite samples from IPB sulphide orebodies plot on a major 187Re/188Os-187Os/188O linear array, yielding an age of 359±26 Ma and an initial 187Os/188O ratio of ca. 0.57. Five samples from Neves-Corvo sulphide-rich stockwork ores yield an age of 358 ±29 Ma and an initial 187Os/188O ratio of 0.49 ±0.07, in accord with palynological and isotopic age constraints for ore formation at the IPB. Pyrites from Neves-Corvo copper-tin ores have Re/Os ratios and osmium concentrations that are similar to those in pyrites from sulphide-rich stockwork ores; however, their highly radiogenic initial 187Os/188O ratios (4.89–7.85, if preserving original isotope system-atics) preclude derivation from the same sources as the remaining IPB sulphides. The overall Re-Os geochemical data conform to previous Sr, Nd and Pb isotopic studies supporting multiple sources for IPB metallogenesis.