Hercynian Basement Research Articles

3-D Seismic Improves Structural Mapping of a Gas Storage Reservoir (Paris Basin)

In the Paris basin, anticlinal structures with closure of no more than 80 m and surface area of a few km[sup 2] are used for underground gas storage. At Soings-en-Sologne, a three-dimensional (3-D) survey (13 km[sup 2]) was carried out over such a structure to establish its exact geometry and to detail its fault network. Various reflectors were picked automatically on the migrated data: the top of the Kimmeridgian, the top of the Bathoinian and the base of the Hettangian close to the top of the reservoir. The isochron maps were converted into depth using data from 12 wells. Horizon attributes (amplitude, dip, and azimuth) were used to reconstruct the fault's pattern with much greater accuracy than that supplied by interpretation from previous two-dimensional seismic. The Triassic and the Jurassic are affected by two systems of conjugate faults (N10-N110, inherited from the Hercynian basement and N30-N120). Alternating clay and limestone are the cause of numerous structural disharmonies, particularly on both sides of the Bathonian. Ridges associated with N30-N120 faults suggest compressive movements contemporaneous with the tertiary events. The northern structure in Soings-en-Sologne thus appear to be the result of polyphased tectonics. Its closure (25 m), which is associated either withmore » dips or faults, is described in detail by 3-D seismic, permitting more accurate forecast of the volume available for gas storage.« less

The genesis of the arsenopyrite gold veins from the Vall de Ribes District, eastern Pyrenees, Spain

In the district of Vall de Ribes, Cambro-Ordovician schists contain several disseminated gold-bearing arsenopyrite occurrences. Close to these occurrences quartz veins parallel to the Hercynian foliation also contain arsenopyrite gold mineralization. Most of the ore is, however, within brittle structures (fractures and thrusts) crosscutting the Hercynian foliation. They occur as veins enclosed in schists and dolomites, and as replacement bodies in dolomites, and are representative of a large number of gold-bearing sulfide deposits that are known to occur in the Hercynian basement of the Pyrenees.The postfoliation veins mainly consist of quartz, siderite, ankerite, arsenopyrite, pyrite, and several minor sulfides. Gold is found in close association with the sulfides. According to fluid inclusion and arsenopyrite geothermometry, the veins formed at a temperature around 350 degrees to 400 degrees C and 2 kbars pressure. Mineralizing fluids were CO 2 -free NaCl + CaCl 2 + (KCl) brines with salinities ranging from 10 to 26 wt percent NaCl equiv. The pH of the fluid was near neutral, the sulfur fugacity was constrained by the stability of pyrite and arsenopyrite, and the oxygen fugacity was close to or higher than pyrite-pyrrhotite-magnetite equilibrium. Under these conditions, and as suggested by available experimental data, gold was most likely transported as AuCl (super -) 2 . The high dependence of gold solubility on temperature suggests a cooling of the solution as the most efficient precipitating mechanism, which is in accordance with the close association of native gold to sulfides and quartz found in the veins.The enclosing and vein carbonates are strongly depleted in 13 C and 18 O (delta 13 C = -10ppm; delta 18 = + 13ppm in vein ankerite) with respect to the regional dolomite value (delta 13 C = -0.4ppm; delta 18 O = +24ppm) but show a positive correlation between delta 13 C and delta 18 O, interpreted as the result of an exchange process between regional dolomite and an external fluid. The delta 18 O values of quartz range from 9.8 in synfoliation quartz to 15.8 per mil in postfoliation vein quartz from the Tarragona mine. Calculated delta 18 O values of water in equilibrium with carbonates and quartz range from 6.7 to 10.8 per mil. The delta D values of chlorites associated with quartz vary between -45 to -61 per mil. The delta D values of these fluids gave values between -5 and -30 per mil. These delta 18 O and delta D data are compatible either with a fluid of metamorphic origin or seawater infiltrating through the metamorphic pile. The heavy sulfur isotope composition of disseminated arsenopyrite (delta 34 S = 11ppm) and vein arsenopyrite (delta 34 S from 13-17ppm) indicates a sulfur of crustal origin. The carbon isotope composition of vein carbonates (delta 13 C 2 O-CO 2 -NaCl fluid generated during metamorphism (at 500 degrees -550 degrees C and 5-6 kbars), contemporaneously to the main foliation, is favored. This fluid could contain metals in solution as evidenced by the deposition of native gold and sulfides in quartz veins parallel to the foliation. The subsequent rise of the thermal gradient from 25 degrees to 45 degrees -70 degrees C/km caused the thermal expansion of the fluids, their migration into faults and thrusts, and the evolution of their chemistry to a CO 2 -free brine by the unmixing of a CO 2 -rich phase. The transport of gold and other metals as chloride complexes was ensured throughout the process. Although a surficial fluid, i.e., seawater, infiltrating through the metasediments could acquire a similar isotopic composition, several geologic uncertainties make this hypothesis less reliable.

Seismic applications of a Paris Basin shallow geology database

The Paris Basin is a Cratonic basin characterized by relatively thin sedimentary series which are either slightly folded or unfolded. The area with the greatest subsidence is located east of Paris. In this central part of the basin, the stratigraphic series lying over the Hercynian basement vary from Permian to Tertiary ages (Fig. 1). The main source rocks are represented by the Liassic marls (Hettangian/ Toarcian). The reservoir rocks are usually located within the Trias, Middle Jurassic, Lower Cretaceous and even the Lias as has been revealed by a recent discovery in the Hettangian (WeIl of L'Orme 1990).

Origin of the Atrevida Vein (Catalonian coastal ranges, Spain); mineralogic, fluid inclusion, and stable isotope study

Atrevida is a Ba-F base metal vein located in the Catalonian coastal ranges of Spain about 100 km southwest of Barcelona. The vein cuts Paleozoic metasediments and granites of the Hercynian basement and Triassic red beds at the base of an overlying transgressive sequence. The Atrevida vein can be divided into three parts on the basis of mineralogy. The lower level contains sugarlike quartz + fluorite + minor barite. The middle level contains a breccia of banded barite + basement rocks cemented by a complex sulfide-arsenide assemblage with galena, Ni sulfide, Ag sulfosalt, and native silver veinlets. The upper level contains symmetrical bands of white barite. Late calcite and marcasite are found in geodes. The only alteration observed is quartz + sericite + chlorite after K feldspar and biotite where the vein cuts granite host rocks.Fluid inclusions in fluorite homogenize at 110 degrees C and have salinities of 21 wt percent NaCl equiv. Inclusions in sphalerite homogenize at 90 degrees C and have salinities of 22 wt percent NaCl equiv. Eutectic results in both minerals are as low as -59 degrees C, indicating that the trapped fluids are complex CaCl 2 -bearing polysaline brines. Fluid inclusions in late calcite homogenize at -0.3 degrees C, indicating that low-salinity waters invaded the vein system during calcite deposition.Sulfur and oxygen isotope analyses reveal that vein barites have delta 34 S values of 14 to 17.9 per mil (CDT) and delta 18 O values of 9.5 to 12.2 per mil (SMOW). Barite also occurs as cement in the Triassic red beds with delta 34 S = 15.3 per mil and delta 18 O = 10.6 per mil. Both the sulfur and the oxygen isotope compositions are within the ranges reported for Lower Triassic- to Lower Jurassic-age evaporites.Banded galena from the middle level of the vein has delta 34 S values between -6.3 and -8.9 per mil, and rimming anglesite has delta 34 S values of -21 to -30.9 per mil. Galena from the central breccia has delta 34 S values of -14.9 to -16.1 per mil and coexisting sphalerite is about -10 per mil. Quartz in the lower level of the vein has delta 18 O in the range 20.7 to 22.7 per mil.We infer that sulfate in the vein was derived directly from seawater or from a solution which dissolved evaporites as it descended through the Triassic section along the fault zone. Barite and sulfides were precipitated where the sulfate-bearing solution mixed with an ascending, Ba-rich brine. The reduced sulfur found in the vein may have been carried by the ascending brine (banded galena) and may also have been produced by bacterial reduction of sulfate from a surficial source (galena in breccia). Oxidizing, nonmarine waters were introduced into the hydrothermal system giving rise to anglesite rims on galena.Mineralization took place in the Late Triassic during an extensional tectonic regime characterized by a high-geothermal gradient. Atrevida, and similar barite veins found throughout the Catalonian coastal ranges, formed along basin margin faults near a paleosurface formed on Hercynian basement. On a broader scale, low-temperature veins are known to occur throughout western Europe and northern Africa. If Atrevida is representative of the group, then the formation of these veins may be linked to Mesozoic extensional tectonics.

Structure of the Variscan belt beneath the British and Armorican overstep sequences

Interpretation of gravity and magnetic data from southern England and of deep seismic data from the English Channel, Celtic Sea, and Paris basin provides a new picture of the buried Hercynian belt between France and England. The main structure corresponds to a mobile belt of crustal thickening that developed between the Midlands block, a northern block that had detached from the Armorican block prior to Hercynian time, and the Armorican block. The frontal collision between the two blocks recorded during the late Carboniferous was responsible for a pronounced narrowing of the belt between the Isle of Wight and Normandy. During collision, structural wedges were ejected in the channel and possibly also in the Hercynian basement beneath the Paris basin.

Geology and tectonics of the Betic Zone, SE Spain

The southern margin of the Iberian continent comprises a complex geological history, which can be traced back into the Proterozoic when a gneissic basement was formed. A cover of Nevado-Filabride rocks was deposited onto this Proterozoic basement during the Palaeozoic. The Permian intrusion of the Bédar granite suggests that the Palaeozoic sedimentary rocks now exposed in the Betic Cordilleras have been affected by the Hercynian orogeny, similar to that of Palaeozoic rocks in the adjoining Ossa Morena zone. The Hercynian basement was probably covered by Alpujarride carbonates in the Mesozoic, when rifting opened Tethys and intruded 140 Ma old basic dyke swarms. The Palaeozoic basement of the Betic Cordilleras comprises a suture of Late Cretaceous subduction (80–85 Ma B.P.). This can be inferred from (HP,LT)-metamorphic facies obducted in coeval thrust slices of Nevado-Filabride rocks. The Nevado-Filabride nappes include Mesozoic carbonates transformed into marbles (Macael). The suture was probably annnealed after the closure of Tethys. The Mesozoic boundary between Africa and Europe and the rifting in Tethys ceased during the Palaeogene when the Atlas Mountains were formed (36 Ma B.P.). Subsequently, relative movements between Africa and Europe in the Neogene were accommodated by strike-slip faults. These define the Iberian microplate moving eastward by Atlantic spreading and possibly subducting under Italy. Neogene deformation of Iberia is most intensive in southeastern Spain due to emplacement of a mantle diapir under the Alboran area. This occurred early in the Neogene (Aquitanian) and caused nappe spreading involving Alpujarride and Malaguide rocks. The subsequent Neogene history has now been studied in great detail and is reviewed here. It involves clear examples of calc-alkaline volcanism — most probably not related to subduction, seismic motion along strike-slip faults within the Iberian microplate, and syntectonic deposition in intermontane Neogene basins.

Comparative study of the Eastern Iberides (spain) and the Western Pyrenees (France) Mesozoic basins

Three main periods characterize the geodynamic evolution of the Mesozoic basins located in the Eastern Iberides and the Western Pyrenees: 1. (1) From Triassic to Albian times, the breakup of the Hercynian basement led to the general formation of different-sized half-grabens. The regional distensional stress which generated such basins was oriented northwest-southeast (Triassic and Jurassic), then northeast-southwest (Early Cretaceous), with a high activity in the Early Triassic, the Early Lias, the Kimmeridgian, the Barremian and the Late Aptian. During the Early Cretaceous period, extensional tectonics induced block tilting, as well as halokinetic movements of the Triassic evaporites. 2. (2) The Albian period put an end to this common evolution. The rifting movements lessened considerably in the Eastern Iberian Basin where continental to coastal sandstones were deposited, whereas they accelerated in the West Pyrenean Basin, leading to flysch accumulations, related to the openning of the Bay of Biscay. 3. (3) The Late Cretaceous was characterized by the same opposed evolution of a shallow carbonate platform in the Eastern Iberides and a deep, terrigenous flysch basin in the Western Pyrenees. During Mesozoic times, both on the platforms and in the basins, the depositional sequence organisation was closely linked to eustatic sealevel changes. Nevertheless, the local extensional process generated important modifications in thickness, particularly within the Lower Triassic, the Liassic, the Kimmerdigian and the Aptian series as well as within the Pyrenean Middle and Upper Cretaceous deposits. The structural framework of both Eastern Iberian and Western Pyrenean areas was induced by the deformation of the Iberian Plate, related to relative sinistral movements of Africa with respect to Europe. The later structural evolution (Tertiary tectogenesis) was strongly influenced by the inherited Mesozoic framework, both in the Eastern Iberides, where the extensional structures are preserved and in the Western Pyrenees, where the important compressional stress led to generalized reversal tectonics.

Study of paleoweathering on the Spanish hercynian basement Montes de Toledo (Central Spain)

Summary The Hercynian basement of the Iberian Peninsula consists of a series of late Hercynian blocks that were unleveled by the Alpine orogeny. The “Montes de Toledo” range is found in one of the elevated blocks of the central zone and shows ancient alterations affecting different rocks which are fossilized by sediments of different ages. Two of these weathering profiles, under the Plio-Pleistocene sediments of “Rana” formations are studied. One is developed on granodiorite with some intercalated metamorphic rocks and the other on a slaty series. In both profiles smectite is formed in a first weathering process. A second weathering process, yielding kaolinite and affecting “Rana” sediments is superimposed on older one. Both processes seem to be widely spread in the western part of the Iberian Peninsula.

Clays in paleoweathering processes: study of a typical weathering profile in the Hercynian basement in the Montes de Toledo (Spain)

AbstractA series of weathered Paleozoic slates was studied in an area southwest of the Montes de Toledo mountain range where the weathered slates are overlain by a raña formation and show three weathering levels. The fresh slate (at 30 m depth) is composed of ferrous chlorite, mica, quartz and feldspars. In the lowest part of the weathering profile the ferrous chlorite begins to evolve to smectite, and in the intermediate level, all the chlorite has been transformed into smectite. In the upper level (in contact with the raña formation), the smectites evolve to kaolinite, and the micas, which are unaffected at greater depths, begin to evolve to vermiculite, and there has been segregation of the free iron oxyhydroxides producing red, ochre and white zones. Two principal weathering processes, separated in time but superimposed, have been identified.

IL CARNICO DI LIERNA (COMO): STRATIGRAFIA E PALEOGEOGRAFIA

The Carnian sedimentary succession of the area, more than 4OO m thick and belonging to the Coltignone tectonic unit (Grigne Group), has considerable paleogeographic significance, for it documents the western termination of the deltalagoon-carbonate platform depositional system of the Lombard Prealps. The Carnian succession, directly overlying basinal (Perledo-Varenna Formation) to shallow-marine (Lierna Formation) carbonates of Ladinian age, is characterized by deltaic terrigenous deposits (Val Sabbia Sandstone), progressively replaced by lagoonal (Gorno Formation) to peritidal (Breno Formation) limestones. In the studied stratigraphic sections these three lithofacies alternate vertically at decametric scale, and are each characterized by different types of cyclothems at metric scale. High-frequency punctuated aggradational cycles (PAC) are best recognized at the top of the Val Sabbia clastic body, where the final transition to the Gorno lagoon is marked by arenite layers containing ferruginous ooids. Grains of this type have nor been previously reported from Southalpine arenites. The petrographic composition of the Val Sabbia Sandstone points to provenance from an active volcanic chain, but abundance of quartzose detritus and occurrence of perthitic K-feldspar and sedimentary rock fragments suggest extensive erosion of the Hercynian basement and Upper Carboniferous to Ladinian sedimentary cover. These peculiar characteristics confirm the existence of a small Lierna lobe separated from the deltaic systems of central Lombardy, and fed by a basement- high located southwest of Como lake.

The Cossato-Mergozzo-Brissago Line and the Pogallo Line (Southern Alps, Northern Italy) and their relationships with the late-Hercynian magmatic and metamorphic events

The Hercynian basement of the Southern Alps west of Lago di Lugano consists of two units: the Ivrea-Verbano, which is well known for its lower crust and upper mantle rocks and as the seat of a marked gravimetric positive anomaly, and the Serie dei Laghi, an intermediate to upper crustal metasedimentary and metagranitic complex. The contact between them is a subvertical tectonic discontinuity, the Cossato-Mergozzo-Brissago Line (CMB) and Pogallo Line, recently the subject of considerable interest as the possible analogue of a low-angle normal fault (later tilted to the present upright position), such as those increasingly revealed by the deep reflection seismics in thinned continental crust. Late Hercynian igneous activity mainly took place along this lineament. The contact is however very complex. The Cossato-lower Valsesia segment is poorly exposed and has been reactivated by a younger fault (Cremosina Line). In the Valsesia-Val d'Ossola (Toce River) segment the CMB Line is expressed as a strong structural and lithological contrast as well as by a narrow belt of mylonites; in the Val Cannobina-Lago Maggiore sector the mylonites are distributed over a considerable thickness mostly within the Serie dei Laghi. They are connected with large-scale ductile folding on nearly vertical axes (“Schlingenbau”). The CMB Line is dissected by several orthogonal and oblique subvertical faults (Grottaccio, Quarna, Val Lessa, Val Pogallo faults). The presence of appinitic calcalkaline dykes and stocks of about 285 Ma, intruded along particular segments of the CMB isolated by those faults, suggests that the attitude of the main tectonic boundary was upright at the time of intrusion. The intrusion apparently occurred in a transtensional environment and induced a temperature increase in the country rocks causing a low pressure metamorphic overprint in the Val Cannobina-Lago Maggiore segment and dehydration melting in a belt 2–3 km wide between Valsesia and Val d'Ossola. The Pogallo Line cuts across the CMB Line with a sinistral horizontal slip of about 11 km and is also characterized by amphibolite facies mylonites. The absence of mylonitic appinites and consistent geologic evidence point to formation of the fault before 285 Ma. A significant Mesozoic displacement along both faults is therefore excluded, even though post-Permian reactivations are demonstrated by the presence of Rb-Sr and K-Ar mica mixed ages along the CMB Line. A new evolutionary model can be proposed to account for most of the geological, petrological and geophysical data: Early Paleozoic continental accretion, subduction and magmatic arc followed by a more external, probably oblique, Hercynian reactivation inducing important transcurrent movements along CMB and Pogallo before the Early Permian magmatic phase.

Metallogenesis of the French part of the Variscan orogen. Part II: Time-space relationships between U, Au and SnW ore deposition and geodynamic events — mineralogical and UPb data

Mineralogical, geochemical, structural and isotopic (UPb) studies were carried out on representative uranium and gold deposits from the French part of the Variscan orogen. Results are compared with complementary data from the literature in order to propose a reconstruction of the metallogenesis in the Variscan terranes, with a particular emphasis on U deposits. The data show that the early metal concentrations or pre-enrichments in the terranes are affected by successive metallogenic stages which caused significant metal transport and deposition. Thus, the significant geodynamic events which affected the Variscan terranes were responsible for multistage metal deposition or alteration of the early concentrations and for the superimposed features of the metal deposits. UPb geochronological results complemented by a compilation of geochronological data on U and PbZnBaF deposits show that three major stages of U deposition took place in Western Europe in the periods 325–300 Ma, 290–260 Ma and 190–170 Ma. In addition, small uranium mineralization events have been dated at 425 Ma, 90–110 Ma and as Tertiary (about 40 Ma). The Permo-Stephanian stage is associated with late brittle deformation associated with major shear zone movements and constitutes the major stage of U concentration. The Jurassic phase has a prominent part in the hydrothermal remobilization of uranium and in FBa (PbZn) deposition throughout Western Europe, especially in the eastern part of the Massif Central. Other stages correspond to subordinate uranium migration within Variscan terranes during earlier or later geodynamic phases: the Silurian metamorphism in Vendée and the Mesozoic to Tertiary tectonic movements related to the Pyrenean or Alpine orogenies in the south eastern part of the Massif Central and in the Alps. Data available on Au and SnW (As, Sb) deposits have been considered for comparison with the U metallogenesis. Stages related to the deposition of such metals display significant similarities with those related to uranium deposition. They occurred mostly during Visean to Permian events related to the intrusion of the late Visean (SnW) and Late Carboniferous (SnW, LiF) granitoids and to the late brittle deformation stages which affected the Hercynian basement in the vicinity of major shear zones (Au, AsSb). However, the significant discrepancies between the physical-chemical conditions of solubilization, transport and deposition of the metal led to distinct deposit location independently of the metal source location within the terranes.

Late Hercynian shear zones in Sardinia

In the past few years two main late Hercynian shear zones have been identified in Sardinia: the Mount Grighini shear zone in the centre of the island and the Posada Valley shear zone in the north. Both are dextral wrench shear zones, affecting the Hercynian basement during the Late Carboniferous, and characterized by a simple shear deformation with an evolution from cataclastic to ultramylonitic rocks. The meso-and microstructural deformation features are discussed here. On the basis of these, two maps of the different deformative zones within the shear zones have been prepared and both displacements have been calculated. An attempt has also been made to correlate with other late Hercynian shear zones in continental Europe on the basis of common features and geological constraints.

Direct dating of Plio-Quaternary pumices by 40Ar/39Ar step-heating and single-grain laser fusion methods: the example of the Monts-Dore massif (Massif Central, France)

K-feldspars separated from Plio-Quaternary pumice flows of the Monts-Dore massif (Massif Central, France) give 40Ar/39Ar saddle-shaped age spectra. Laser-probe analysis of hand-picked single grains gives ages in agreement with those of overlying and underlying lava flows previously dated by the conventional K-Ar method. The laser-probe ages are lower than the minimum ages measured on the populations of grains by the step-heating method. As previously suggested by Lo Bello and co-workers for the pumice flow of Neschers belonging to the same volcanic massif, the saddle-shaped age spectra are ascribed to the different Ar-release patterns of two populations of K-feldspars: (1) young sanidine phenocrysts of the pumices; and (2) old K-feldspars plucked from the Hercynian basement during the explosive eruption. Measurements of both the granitic rocks from surrounding areas and the granitic xenoliths included in pumices give ages around 330 Ma and show that most of the xenocrysts included in the pumices did not lose significant amounts of argon during the eruption. With the 40Ar/39Ar step-heating method, we were able to detect contamination of Quaternary K-feldspars by Hercynian K-feldspars as low as 0.25ℵ. Because pumice flows of the Monts-Dore massif crop out over large areas, these new ages will be useful for establishing a precise stratigraphy of the European Plio-Quaternary.

Episodic discrete and distributed deformation: consequences and controls in a thrust culmination from the central Pyrenees

The Pic de Port Vieux culmination is a 0.5 km long thrust culmination in the footwall of the Gavarnie Thrust in the central Pyrenees. The culmination comprises a set of blind thrusts that imbricate a 50 m thick Mesozoic cover sequence, comprising Triassic red beds and Upper Cretaceous limestone, sandwiched between underlying Hercynian basement and the overlying Palaeozoic strata of the Gavarnie thrust sheet. An intense cleavage developed within the culmination associated with the oblique tip-lines of the thrusts as they climbed from the basement into the cover strata. Detailed mapping shows that the distributed strain associated with the fault tips locally re-activated earlier folded thrusts, creating upper detachments to the system, and produced a set of new overstepping isolated faults. The main detachment occurred along the roof of the culmination using the back-limbs of folds in the roof thrust. The movement on these detachments can be demonstrated to have been episodic, with periods of fault slip producing splays that were then folded during periods when distributed deformation was dominant. The main control on this episodic deformation is modelled as the result of feedback between increase in fluid pressure associated with inelastic porosity reduction during cleavage development, and fluid pressure bleed-off associated with fault zone dilatancy in the culmination.

Hydrothermal processes and ore-forming fluids in the Le Bourneix gold deposit, central France

The Le Bourneix gold deposit in the French Massif Central occurs within a shear zone in gneisses of the Limousin Hercynian basement. Gold-bearing quartz lenses formed during extension phases, after cataclasis of initially barren quartz postdating mylonites (Bouchot et al., 1989). The economic mineralization (arsenopyrite, gold, pyrite, galena, and Pb-sulfosalts), hosted by microsaccharoidal quartz, formed after crystallization of arsenopyrite and pyrite, synchronous with white milky quartz. A later paragenesis associated with hyaline quartz comprises Ag-rich gold, sphalerite, galena, tetrahedrite, and sulfosalts. Arsenopyrites have a range of compositions, with a mean As content equivalent to an equilibrium temperature with pyrite, of about 400 degrees + or - 30 degrees C. Hydrothermal alterations synchronous with ore deposition resulted in a phengite + chlorite association, and then a propylitic chlorite + carbonate assemblage.From fluid inclusion studies, it appears that the general evolution of the ore fluids is (1) circulation under high pressure (2 kbars ?) at 400 degrees + or - 50 degrees C of a low-salinity solution with minor volatile contents, (2) mixing of this solution with a CO 2 -rich vapor induced by a drop in pressure down to 0.6 kbars producing a heterogeneous fluid at 320 degrees C, and (3) circulation of volatile-free aqueous solutions at decreasing temperature (300 degrees - 120 degrees C) and salinity.Most of the economic ore probably formed at the transition from (1) to (2). Lead isotope data suggest a local source for metals, with mixing of poorly radiogenic lead from the basement orthogneiss with radiogenic lead from a supracrustal source.

Evolution of a Mid-Cretaceous carbonate platform, Gorbea (northern Spain)

Excellent exposure in the Gorbea Massif permits a stratigraphic cross-section to be made of the Aptian-Albian mixed terrigenous carbonate sequence in the Basque-Cantabrian region. Nine lithostratigraphic units have been differentiated and detailed sedimentological analysis enables the reconstruction of the paleogeographical evolution of this area through the time period in which rifting gave way to spreading in the Bay of Biscay. Special emphasis has been placed on the evolution of a major Albian carbonate buildup (Itxina), the evolution of which was accomplished through three main stages, which are best reflected on the western side of the buildup. First a ramp-type margin developed evolving into a platform-type and back again to a ramp-type carbonate bank margin. Carbonate platform initiation and demise were strongly influenced by siliciclastic input from southern deltaic systems whose efficiency was dependent on climatic factors and relative sea-level changes. The tectonic influence was crucial on the location of buildup growth. The structures that formed in the area can be assigned to the reactivation of Hercynian basement faults. This gave rise to tilted blocks whose tops where the site of growth for isolated carbonate platforms.

Geology of Porcupine Bank and Goban Spur, Northeastern Atlantic — Preliminary results of the Cyaporc submersible cruise

During the Cyaporc cruise, five submersible traverses and four dredge hauls were made up the steep western slopes of Porcupine Bank and Goban Spur on the continental margin west of Ireland. The major aims of the study were to examine the syn- and post-rift evolution of the margin during the Mesozoic and Cenozoic, and to compare the “basement” rocks of Porcupine Bank and Goban Spur with those of the Hercynian and Caledonian orogens of the western British Isles. On Porcupine Bank, sampling and observational data show a metamorphic basement overlain by a sequence of Upper Palaeozoic siltstones and sandstones. Mesozoic sediments are restricted to isolated patches of Lower Cretaceous shallow-marine deposits. On Goban Spur, the oldest rocks are weakly metamorphosed sandstones of Late Palaeozoic age (Hercynian basement). In contrast to Porcupine Bank, these are overlain by a thick sequence of shallow-water Barremian limestones deposited during the late Early Cretaceous rifting event. Upper Cretaceous and Tertiary sediments have only a patchy development in both areas. Comparison with onshore areas suggest that the Porcupine Bank Palaeozoic sequence is comparable with that of the Hercynian foreland seen in Ireland. The Palaeozoic sequence at Goban Spur, with granites intruded along its southern flank, is less easily compared with onshore sequences, but most closely resembles that seen in onshore southwest England. The development of a thick syn-rift sequence on Goban Spur resulted from faulting and subsidence related to crustal thinning during the Early Cretaceous rifting event. In contrast, seismic refraction results show little crustal thinning beneath Porcupine Bank, offering an explanation for the absence of Mesozoic subsidence and the lack of preserved Mesozoic sediments.

Diachronous metamorphic equilibria in the Hercynian basement of northern Sardinia, Italy

Diachronous metamorphic equilibria in the Hercynian basement of northern Sardinia, Italy

La structure de la marge occidentale de la Sardaigne et la fragmentation de l'ile au Cenozoique

Sardinia is commonly considered as part of a continental arc. During the Early Miocene (21-19 Ma) anticlockwise rotation of the island and oceanic drifting led to the opening of the Western Mediterranean Basin. Among the structures inherited from the rifting phase (Late Oligocene) the 30–40 km wide N-S Sardinian Graben is the salient tectonic feature of the island. It divides the Hercynian basement into two blocks: the main eastern massif, and on the western side the Nurra Massif and the Iglesiente-Sulcis. Further west, the continental margin extends 100 km towards the Western Mediterranean Basin. The western continental margin is divided by a large median NW-SE fracture zone into two areas: a northern part where the two sets of faults coexist, and a southern part where the NE-SW set progressively disappears towards the south. The southwestern margin of the island is therefore affected only by the remaining broadly NW-SE faults. The median fracture is a composite zone consisting of an array of normal faults locally featuring a narrow graben. It runs from the bottom of the slope (and presumably the ocean-continent boundary) to the southern end of a small NE-SW graben underlying the continental shelf of Oristano. At its northern end the Oristano shelf graben is bounded by another major NW-SE fracture, the south bounding fault of the Campidano Graben. This fault runs from the Gulf of Oristano to the Gulf of Cagliari, and separates the Iglesiente-Sulcis Block from the Eastern Sardinian Massif. The island is thus crossed from the bottom of the western slope to the Gulf of Cagliari by a continuous set of NW-SE enechelon faults. This en-echelon system is contemporaneous with the rifting phase in the Western Mediterranean Basin and delineates three crustal blocks through the Sardinian basement. Although this set of faults displays mainly extensional activity, apparent flower structures are present in some seismic records from the western margin, suggesting left-lateral strike-slip displacement. In Plio-Quaternary times, a new extensional event overprinted the earlier structures in the Campidano area. The fragmentation of the Sardinian basement is therefore still active. The displacement of blocks related to the opening of the NW-SE Plio-Quaternary Campidano Graben is indeed transformed at both ends by two strike-slip structures: along the Ichnusa Seamount at the southern end and, to the north, along a reactivated NE-SW Hercynian fault. The latter runs across the eastern massif (from the Gulf of Oristano to the Capo Comino to the east), and is accompanied by Plio-Quaternary basic volcanics. Hence, except for brief and low intensity Miocene compressive events, Sardinia appears to be mostly affected by extension and fragmentation from the end of the oceanic drifting to the Recent. Therefore, the Corso-Sardinian Block does not seem to transmit the compressional stress to Southern Europe due to the northward displacement of the African plate.