Dominique Gasquet

Two-phase orogenic convergence in the external and internal SW Alps

The NW–SE-trending sector of the SW Alps includes the Dora Maira massif where Tertiary eclogites record ultrahigh pressures and rapid exhumation. Along a NE–SW crustal cross-section (Italy–France) compiled pressure–temperature–time data in internal zones are correlated with Tertiary stratigraphy in external zones to reconstruct orogen evolution, revealing a coherent two-phase convergence history. During the first, subduction–accretion phase (Eocene, 55–34 Ma) rapid north–south plate convergence caused the subduction and exhumation of high-pressure and ultrahigh-pressure (UHP) rocks in a steady-state subduction channel. This coincided with the north to NNW migration of an underfilled flexural basin across the European foreland. Nappe stacking within the subduction channel did not create significant relief, implying that primarily subduction forces generated this flexural basin. From 34 Ma onward, the second, collisional phase was characterized by slower NW–SE plate convergence. The internal units of the SW Alps underwent considerable anticlockwise rotation as they became involved in a NW–SE-oriented sinistral transpression zone between the European and Adriatic plates. To the north of the orogen the North Alpine Foreland Basin became overfilled as a result of high sediment supply from increasing orogen relief. In contrast, in SE France active flexure of the European plate appears to have ceased and sedimentation became limited to small thrust-sheet-top basins created by continuing gentle NE–SW shortening. Internal units were exhumed slowly from depths of c. 20 km, principally by erosion. In the SW Alps, the transition between these two phases was marked by the rapid subduction and exhumation of the Dora Maira UHP unit. Assuming lithostatic pressure, this unit would have been exhumed from 100 km depth, requiring a rate that exceeds that generated by plate convergence. Therefore, either exhumation was accelerated by additional stresses (locally generated by transpression, slab breakoff or high density contrasts) or, more controversially, the ultrahigh pressure occurred at a considerably shallower depth as a result of local overpressure.

Géochronologie U-Pb sur zircon de granitoïdes éburnéens et panafricains dans les boutonnières protérozoïques d'Igherm, du Kerdous et du Bas Drâa (Anti-Atlas occidental, Maroc)

Abstract U-Pb ages obtained from zircons of granitoids from the Proterozoic western Anti-Atlas confirm the existence of Palaeoproterozoic magmatism in the inliers of Igherm (Ait Makhlouf granite: 2 050±6 Ma) and Bas Drâa (Sidi Said granite: 1 987±20 Ma) and neoproterozoic magmatism in the inliers of Bas Drâa (Taourgha granite: 575±4 Ma) and Kerdous (Tarcouate granodiorite: 583±11 Ma, Tarcouate gabbro-diorite: 560±2 Ma). The emplacement of these granitoids is therefore polycyclic: Eburnian and Panafrican.

40AR/39AR DATING OF SHEAR ZONES IN THE VARISCAN BASEMENT OF GREATER KABYLIA (ALGERIA). EVIDENCE OF AN EO-ALPINE EVENT AT 128 MA (HAUTERIVIAN-BARREMIAN BOUNDARY) : GEODYNAMIC CONSEQUENCES

A ductile shearing event is recognized in gneisses and barite veins of the crystalline internal massifs of the Alpine Maghrebides in Algeria. Dating of shearing movement is carried out by 40Ar/39Ar laser-probe experiments performed on synchronous neoformed micas. The proposed age at 128 Ma is constrained by a two-component mixing model for the saddle-shaped age spectra on phengites and by a plateau age at 128.3±0.3 Ma on muscovite. This Hauterivian–Barremian boundary age records a strong reworking of the Alboran plate (input of detritals in the flysch basin) and surrounding areas. It can be correlated with the J magnetic anomaly in the Atlantic and with the distensive events in the Pyrenees and external Alps. Therefore, isotopic ages (80–130 Ma) obtained on the Alpine–Mediterranean basement could reflect interferences between this event at 128 Ma and late-Alpine stages.

Paleocene adakite Au-Fe bearing rocks, Mezcala, Mexico: evidence from geochemical characteristics

The Au–Fe mineralized granitoids at Mezcala district have a porphyry texture with a quartz+feldspar microcrystalline matrix and phenocrysts of plagioclase, quartz (with reaction rims), hornblende and biotite. The primary minerals are oligoclase–andesine, microcline and h-quartz. The accessory minerals are biotite, hornblende and, in minor amounts, apatite+zircon+sphene+titanomagnetite. Some intrusive rocks present abundant hornblende autoliths. Based on the petrography and bulk geochemistry of these granitoids, they are classified as monzonite, tonalite (the most abundant) and granodiorite with a strong calc-alkaline trend in potassium (K2O=3.8% average). The bulk and trace elements chemistry is SiO2=63.8%, Al2O3=15.83%, Fe2O3+MgO+MnO+TiO=6.52%; V=76.7 ppm, Cr=50.2 ppm, Ni=19.7 ppm, Sr=694 ppm. These granitoids show a strong depletion in heavy rare-earth elements (HREE), with average values of Yb=1 ppm and Y=13 ppm, this being the characteristic geochemical signature for adakite. The trace elements content suggests that the adakite granitoids from Mezcala were formed within a tectonic framework of volcanic arc related to the interaction between the Farallon and North America plates. This interaction occurred during the Paleocene after the Laramide Orogeny (post-collision zone) in a fast convergent thick continental crust (>50 km) subduction regime. The original magma is interpreted as being the product of partial melting of an amphibolite–eclogite transition zone source with little contribution of the mantle wedge. Along with the hydration processes, a metallic fertility also took place in the area. The geochemical signature of the adakites within the

Melting and undercooled crystallisation of felsic xenoliths from minor intrusions (Jebilet massif, Morocco)

Abstract Granitic and gneissic xenoliths within microdiorite dykes from the Palaeozoic Jebilet massif (Morocco) contain up to 40% of granophyric intergrowths that occur as films along quartz-feldspar contacts and form an interconnected grain boundary network. The composition of the feldspar megacrysts — i.e. large embayed crystals surrounded by the granophyric intergrowths — implies high-temperature crystallisation ( T >900 °C) and the sievetexture (or finger-print texture) indicates that these crystals underwent incipient melting. Moreover, the microstructural position and bulk composition of the granophyre are similar to that of glass in partly molten felsic xenoliths from lavas or pyroclastics. It is argued that the granophyre originated after partial melting of the xenoliths when incorporated into the dioritic magma. Crystallisation of that melt resulted in most cases in the formation of vermicular and/or cuneiform granophyre, which testifies to moderate degrees of undercooling. A few occurrences of plumose granophyre i.e. very thin intergrowths that are close to quartz-bearing spherulites, indicate that some xenoliths crystallised at higher degrees of undercooling. Together with granophyric xenoliths, a number of microgranular inclusions also occur. These are characterized by phenocrysts of quartz and feldspar in a microgranular groundmass. Finger-print texture in feldspar suggests that these inclusions, like the granophyric xenoliths, underwent ultra-metamorphism and melting. The microgranular groundmass is most probably indicative of crystallisation at lower degrees of undercooling, but the relationships between the two types of xenoliths still remain to be explored in detail.

Degassing as the Main Ore-forming process at the Giant Imiter Ag-Hg Vein Deposit in the Anti-Atlas Mountains, Morocco

The giant Imiter epithermal Ag–Hg vein deposit in the Anti-Atlas Mountains of southern Morocco formed during a major episode of mineralization linked with Ediacaran volcanism at ca. 550 Ma. Silver was deposited during two main epithermal mineralizing events referred to as epithermal-quartz (ESE-Qz) and epithermal-dolomite (ESE-Dol) stages under distinct stress fields (i.e., WNW-ESE and N-S shortening directions), and is confined to the late Neoproterozoic, N60-90° E-trending, transcrustal Imiter fault zone. Economic orebodies are aligned mainly along the interface between sedimentary and volcanic units of lower and upper Cryogenian age. The ore mineralogy consists principally of Ag–Hg amalgam, argentite, polybasite, pearceite, tetrahedrite-tennantite, proustite-pyrargyrite, imiterite, acanthite, arsenopyrite, pyrite, sphalerite, and galena. Gangue constituents are dominated by quartz (ESE-Qz stage) and dolomite (ESE-Dol stage). Wall-rock alteration is well developed and includes silicification and dolomitization, and minor propylitization and kaolinitization. Fluid inclusion data indicate that the mineralizing fluids evolved through time, from a mean temperature of ~180 °C and salinity of ~10 wt% NaCl during ESE-Qz stage I, to a mean temperature of ~165 °C and salinity of ~24 wt% NaCl equiv during ESE-Dol stage II. Calculated trapping pressures, in the range of 1.1–0.9 kbar, exclude fluid unmixing “effervescence” as a viable ore depositional mechanism. Conversely, halogen compositions suggest the involvement of magmatic brines and evolved seawater. Stable (C, O, S) and radiogenic (Pb, Re/Os) isotope data, together with noble gas isotope compositions, are consistent with various degrees of mixing between mantle and crustal sources along the fluid flow path. Collectively, these data suggest that degassing of CO2 and SO2 during epithermal mineralization and related fluid/rock interactions led to local redox-potential decreases and pH increases that resulted in preferential deposition of massive amounts of native Hg-rich silver instead of Ag and Hg sulphide minerals.

The Polymetallic (W–Au and Pb–Zn–Ag) Tighza District (Central Morocco): Ages of Magmatic and Hydrothermal Events

The W–Au, Pb–Zn–Ag, and Sb–Ba deposits of the polymetallic Tighza-Jbel Aouam district (central Meseta, Morocco), hosted in Paleozoic rocks surrounding late Variscan granite stocks, have been considered of magmatic-hydrothermal origin. The spatial distribution of the mineralization was attributed in early studies to zoning around a supposed hidden batholith. New geophysical data (El Dursi 2009) and U/Pb geochronology on zircon and monazite grains (this study) allow revision of this model, giving insights of a more complex setting and history for the Tighza-Jbel Aouam district. The W–Au mineralization formed at 295–280 Ma and is related to a magmatic event visible only in a large hydrothermal biotitic alteration halo, thus suggesting the presence of a hidden batholith. This mineralization cuts the granitic stocks that are dated at 320–300 Ma. From the occurrence of large veins, stockworks, sheeted veins, and disseminations in skarns, the W–Au deposit is considered similar to a porphyry-type deposit. The currently mined Pb–Zn–Ag deposit, which is spatially separated from the W–Au deposit, developed during an epithermal magmatic-hydrothermal episode dated at 254 ± 16 Ma. The polymetallic district of Tighza-Jbel Aouam thus appears to contain Cordilleran-style, porphyry-type mineralization (W–Au) followed by epithermal mineralization (Pb–Zn–Ag), both being related to pulses of calc-alkaline magmatism. Late Variscan and Permo-Triassic transpressive tectonics in the region localized magma emplacement and the generation of genetically associated hydrothermal fluids, with the magmas originating in the mantle and the continental crust.

Edough-Cap de Fer Polymetallic District, Northeast Algeria: II. Metallogenic Evolution of a Late Miocene Metamorphic Core Complex in the Alpine Maghrebide Belt

During the late Oligocene-early Miocene, three main hydrothermal events formed polymetallic deposits of the Edoug-Cap de Fer in the Edough massif of the Alpine Maghrebide belt. At ca. 17 Ma, the Karezas As (lollingite)-F (fluorite)-W (scheelite) deposit formed at a depth of ca. 2 km and temperatures of ca. 450–500 °C, from mixing between magmatic-hydrothermal hypersaline fluids issued from a concealed rare-metal granite and several metamorphic fluids derived from the metamorphic core complex. Slightly later, at ca. 16 Ma, the intrusion of microgranites produced high-enthalpy, liquid-dominated geothermal fields at the basement-Kabylian flysch boundary, with Numidian flysch acting as an impermeable lid and host for “mesothermal” polymetallic vein fields (Ain Barbar, Mellaha, Saf-Saf). Temperatures as high as ca. 350–375 °C were attained in the deep parts of the Ain Barbar field, at depths of ca. 1.3–1.5 km, accompanied by massive input of sodium that formed metasomatic plagioclase-rich hornfels (Chaiba domain); higher in the Cretaceous flysch aquifer, influx of hydrothermal fluids (300–270 °C) produced hydrothermal metamorphic assemblages of quartz-chlorite, calcite-chlorite, wairakite-chlorite, and epidote. The source of these hot fluids was a basement of the Edough type, in which heat advection was likely related to emplacement of a granite batholith at depth. Concomitant with the paleogeothermal circulations, fault activity created N170° E-trending fracture zones that progressively channeled fluid flow, with the development of propylitically altered linear zones and ore precipitation (Zn–Pb–Cu) at temperatures between 330 and 285 °C. At ca. 15 Ma, renewed magmatic activity (subvolcanic rhyolite dikes) was associated with the generation of new and shallow (ca. 800 m depth) geothermal fields, wherein convected surficial fluids (meteoric and possibly seawater) formed “epithermal” deposits including polymetallic quartz veins, quartz-stibnite metasomatic deposits in marble, and quartz-arsenopyrite-gold showings, at mostly lower temperatures of 300–250 °C. Excepting the Karezas skarn, for which a magmatic origin of the tungsten is likely, the metals deposited by the different hydrothermal systems were mainly sourced in rocks of the metamorphic core complex and its tectonically emplaced cover of Cretaceous flysch. Only a minor contribution of metals came from the magmatic rocks, as shown by lead isotope data for the Ain Barbar area. In particular, amphibolite of the Marble Complex in the Edough sequence may have been a major source of copper and the epithermal antimony (and gold?). The Edough-Cap de Fer district is directly linked to the evolution of the Edough metamorphic core complex. Although metallogenic activity began after the end of ductile deformation, metamorphic fluids derived from the core complex seem to have played a key role in the first stages of the hydrothermal circulation and related mineralization (Karezas W skarn, mesothermal polymetallic veins). However, the role of the late Miocene magmatism, induced by collisional processes through slab break-off and/or lithospheric delamination, was of equal importance in the genesis of the Edough-Cap de Fer metallic deposits, being the source of the heat advection responsible for hydrothermal convection during the meso- and epithermal mineralization. Finally, it appears that the transition from extension (related to opening of the Algerian-Provencal oceanic basin) to transpression (when the collision resumed), at the end of the Miocene, was the ultimate control on the mineralizing events in the Edough metamorphic core complex.

U/Pb Ages of Magmatism in the Zgounder Epithermal Ag–Hg Deposit, Sirwa Window, Anti-Atlas, Morocco

The Zgounder epithermal Ag–Hg deposit (Jbel Sirwa inlier, Anti-Atlas, Morocco) is hosted in Neoproterozoic volcanosedimentary and magmatic rocks in a Pan-African belt. Strongly altered U-rich zircons were analyzed for in situ SIMS U–Pb ages of multiple rhyolitic intrusions spatially associated with the Ag–Hg mineralization. Morphological features, internal microstructures, and chemical characteristics allow the discrimination of two types of zircon, magmatic and hydrothermal. U/Pb data reveal four main geologic events: (1) ca. 815 Ma corresponding to inherited cores of zircons that reflect the reworking of Neoproterozoic crust during the Pan-African (~885–555 Ma) orogenic cycle; (2) 610 ± 7 Ma; (3) 578 ± 4 Ma corresponding to the emplacement-crystallization of multiple rhyolitic intrusions and the large magmatic input within the upper crust of the Pan-African chain during the late Neoproterozoic; and (4) 564 ± 15 Ma, a less-precise age obtained on hydrothermally altered domains that may be related to a dissolution-reprecipitation mechanism in U-rich magmatic zircons. Presently, this last age represents the best estimate for the timing of hydrothermal albitization/mineralization in the Zgounder epithermal Ag–Hg deposit.

Edough-Cap de Fer Polymetallic District, Northeast Algeria: I. The Late Miocene Paleogeothermal System of Aïn Barbar and Its Cu–Zn–Pb Vein Mineralization

In northeast Algeria, the internal Edough massif of the Alpine Maghrebide belt, is an inlier of basement rocks under a cover of Cretaceous (Kabylian) and Cenozoic (Numidian) flysch nappes. During the late Oligocene-early Miocene, the Edough massif was an Oligo-Miocene metamorphic core complex involving the basement rocks (Pan-African gneiss, marble, amphibolite) and its Paleozoic cover. In a short time interval from latest Burdigalian to early Langhian (ca. 17–15 Ma), felsic intrusive rocks were emplaced in the basement and its tectonic cover under progressively shallower conditions (granite to rhyolite) that define the Edough-Cap de Fer magmatic district. At Ain Barbar, during intrusion of microgranites at ca. 16 Ma, a high-enthalpy, liquid-dominated geothermal system was active in the Cretaceous flysch reservoir, with Oligo-Miocene Numidian flysch serving as an impermeable cap. Temperatures as high as ca. 350–375 °C were attained in the deep parts of the Ain Barbar paleogeothermal field, at a depth of ca. 1.3–1.5 km. Input of massive amounts of sodium resulted in the formation of metasomatic plagioclase-rich hornfels (Chaiba domain), whereas higher in the Cretaceous flysch aquifer, invasion of hot fluids (300–270 °C) was associated with hydrothermal metamorphism (quartz-chlorite, calcite-chlorite, wairakite-chlorite, and epidote domains). The source of these hot fluids was a basement of the Edough type, in which advection of heat was likely related to emplacement of a granite batholith at depth. Concomitant with the paleogeothermal circulations, fault activity created N170° E fracture zones that progressively channelled fluid flow, with related development of linear propylitically altered zones and precipitation of Zn–Pb–Cu sulphides at temperatures between 330 and 285 °C. At ca. 15 Ma, renewed magmatic activity (subvolcanic rhyolite dikes) was associated with a new and shallower (ca. 800 m depth) geothermal system, involving the convective circulation of surficial fluids (meteoric and possibly seawater) at temperatures between 300 and 250 °C. Epithermal quartz and sulphides were deposited in the same vein systems as in the previous mineralization stage, but remained uneconomic. However, concomitant formation of massive adularia during alteration of the Chaiba rhyolite produced an economic K-feldspar body mined for ceramics.