Michel Ballèvre

RbSr and 40Ar39Ar laser probe dating of high-pressure phengites from the Sesia zone (Western Alps): underscoring of excess argon and new age constraints on the high-pressure metamorphism

Abstract The combined use of the RbSr and 40Ar39Ar laser probe methods allows confirmation of the existence of excess argon in phengites from eclogites from the Sesia zone (Western Alps) despite systematic 40Ar39Ar plateau ages. 40Ar39Ar phengite ages from two different areas show integrated age spreads between 65.4 ± 0.3 Ma and 109.5 ± 0.4 Ma. The youngest 40Ar39Ar plateau age (65.9 ± 0.4 Ma) is concordant with two RbSr phengite-whole-rock isochron ages from the same outcrop which display a mean age at 64.2 ± 2.5 Ma and older than a Rb-Sr biotite-whole-rock isochron age at 53.0 ± 1.0 Ma. This age concordance, being incompatible with the concept of isotopic closure temperature, is probably fortuitous and probably results from an excess argon contamination. However, the concordance of the RbSr phengite isotopic closure temperature with the temperature reached during the eclogite-facies event suggests that the age at 64.2 ± 2.5 Ma could be a crystallization age probably contemporaneous with the eclogite-facies event in the Sesia zone. When compared to recent age data from the Piemont zone and the Dora Maira Massif, the entire data set (which needs to be confirmed by extra measurements) support diachronous high-pressure (HP) events in the Western Alps.

Diapirism, bulk homogeneous shortening and transcurrent shearing in the Archaean Dharwar craton: the Holenarsipur area, southern India

Abstract Qualitative strain data are spatially integrated in order to define Archaean foliation trajectories, and variations in strain intensity and ellipsoid type, in the Holenarsipur area, Dharwar craton, southern India. The study provides constraints on the bulk geometry, the progressive deformation and the kinematic history of this region of Archaean continental crust. Two major structural events have shaped the study area: (i) a regional diapiric event contemporaneous with a magmatic event, and (ii) a later sinistral transcurrent ductile shear event which has locally reoriented both foliations and stretching lineations. The entire Holenarsipur supracrustal belt underwent a common tectono-metamorphic history that included two metamorphic events. Each event corresponds temporally to a distinct strain field. The first, a regional amphibolite facies event, was contemporaneous with the main tectonic event and probably with the intrusion of Halekote trondhjemites. The second occurred under greenschist-facies conditions and was restricted to a late transcurrent shear zone at the eastern boundary of the greenstone belt. The strain pattern prior to transcurrent shearing cannot be explained by a thrusting event unless all trace of this event had been erased by the diapirism. We interpret all structures older than the transcurrent shearing to be the direct result of vertical movements linked to body forces, augmented by minor horizontal crustal shortening.

Correlation of the nappe stack in the Ibero-Armorican arc across the Bay of Biscay: a joint French–Spanish project

Abstract A correlation between allochthonous units exposed in the NW Iberian Massif and the southern Armorican Massif is carried out based on lithological associations, structural position, age and geochemistry of protoliths and tectonometamorphic evolution. The units on both sides of the Bay of Biscay are grouped into Upper, Middle and Lower allochthons, whereas an underlying allochthonous thrust sheet identified in both massifs is referred to as the Parautochthon. The Lower Allochthon represents a fragment of the outermost edge of Gondwana that underwent continental subduction shortly after the closure of a Palaeozoic ocean which, in turn, is represented by the Middle Allochthon. The latter consists of supra-subduction ophiolites and metasedimentary sequences alternating with basic, mid-ocean ridge basalt (MORB)-type volcanics, with inheritances suggesting the proximity of a continental domain. Seafloor spreading began at the Cambro-Ordovician boundary and oceanic crust was still formed during the Late Devonian, covering the lifetime of the Rheic Ocean, which is possibly represented by the Middle Allochthon. The opening of the oceanic domain was related to pulling apart the peri-Gondwanan continental magmatic arc, which is represented by the Upper Allochthon.

Synconvergence spreading of the higher Himalaya crystalline in Ladakh

In Ladakh (Suru valley, NW Himalaya), regional fabrics of the Higher Himalaya Crystalline result from a major deformation event that (1) is restricted to the Indian crust above the Main Central Thrust (MCT), (2) postdates piling up of units above the MCT, (3) ended while thrusting was still continuing on the MCT, (4) developed under high-temperature conditions following thickening, (5) resulted in an asymmetric gneiss dome, and (6) is associated with ENE-WSW to EW regional extension. We suggest that extension was accommodated by spreading of the hot basement accompanied and followed by its uplift within the overlying cover along a major detachment zone. The onset of the process appears closely related to strong thermal softening of the Higher Himalaya Crystalline and is broadly synchronous with the formation of the Himalayan leucogranites, around 25 Ma. The regional pattern suggests that the MCT zone could have acted as a decoupling zone between a thickening lower part and a thinning upper part of the Indian crust during Miocene times. The extension direction is consistent with bulk Himalayan kinematics involving NW-SE dextral wrenching along the western syntax, and eastward extrusion of Tibet. We suggest that exhumation of gneissic ridges which characterize Indian units above the MCT all along the Himalayan belt could reflect large-scale crustal instability developed from thermally anomalous layers whose spreading ability can favor synconvergence extension.

Geothermobarometry in albite-garnet orthogneisses: A case study from the Gran Paradiso nappe (Western Alps)

Abstract The aim of this paper is to estimate syntectonic P - T conditions within albite- and garnet-bearing orthogneisses. These rocks are generally characterized by the assemblage quartz + albite + biotite + phengite + CaFe-garnet + epidote + titanite. Garnet contains up to 55 mole per cent of grossular. K-feldspar is a relict magmatic phase. P - T conditions are estimated using several independent methods. First, it is shown that exchange reactions based on the FeMg partitioning between garnet and biotite or garnet and phengite cannot be used to estimate temperatures in these rocks, due to the high grossular content of garnet. Second, maximum and minimum pressures are constrained, respectively, by the occurrence of albite instead of jadeite + quartz and by the assemblage phengite + biotite + quartz. Third, phase equilibria in albite- and garnet-bearing metagranites are modelled in the system K 2 OCaOFeOAl 2 O 3 SiO 2 H 2 O. Equilibrium curves are calculated for the observed phase compositions. Uncertainties in P - T estimates mainly result from the choice of appropriate non-ideal solution models for the garnet. An application is developed for granites from the Gran Paradiso nappe (Western Alps). These granites show an heterogeneous deformation of Alpine age expressed by mylonitic shear zones cutting across weakly deformed domains. Estimated P - T conditions for the synkinematic assemblages are 10–16 kbar at 550±50°C.

The tectonometamorphic evolution of the Sesia–Dent Blanche nappes (internal Western Alps): review and synthesis

AbstractThis study reviews and synthesizes the present knowledge on the Sesia–Dent Blanche nappes, the highest tectonic elements in the Western Alps (Switzerland and Italy), which comprise pieces of pre-Alpine basement and Mesozoic cover. All of the available data are integrated in a crustal-scale kinematic model with the aim to reconstruct the Alpine tectono-metamorphic evolution of the Sesia–Dent Blanche nappes. Although major uncertainties remain in the pre-Alpine geometry, the basement and cover sequences of the Sesia–Dent Blanche nappes are seen as part of a thinned continental crust derived from the Adriatic margin. The earliest stages of the Alpine evolution are interpreted as recording late Cretaceous subduction of the Adria-derived Sesia–Dent Blanche nappes below the South-Alpine domain. During this subduction, several sheets of crustal material were stacked and separated by shear zones that rework remnants of their Mesozoic cover. The recently described Roisan-Cignana Shear Zone of the Dent Blanche Tectonic System represents such a shear zone, indicating that the Sesia–Dent Blanche nappes represent a stack of several individual nappes. During the subsequent subduction of the Piemonte–Liguria Ocean large-scale folding of the nappe stack (including the Roisan-Cignana Shear Zone) took place under greenschist facies conditions, which indicates partial exhumation of the Dent Blanche Tectonic System. The entrance of the Briançonnais micro-continent within the subduction zone led to a drastic change in the deformation pattern of the Alpine belt, with rapid exhumation of the eclogite-facies ophiolite-bearing units and thrust propagation towards the foreland. Slab breakoff probably was responsible for allowing partial melting in the mantle and Oligocene intrusions into the most internal parts of the Sesia–Dent Blanche nappes. Finally, indentation of the Adriatic plate into the orogenic wedge resulted in the formation of the Vanzone back-fold, which marks the end of the pervasive ductile deformation within the Sesia–Dent Blanche nappes during the earliest Miocene.

Garnet-chloritoid-kyanite metapelites from the Raspas Complex (SW Ecuador): a key eclogite-facies assemblage

The Raspas Complex (Ecuador) contains one of the few eclogitic bodies in the northern Andes. It consists of meta- peridotites, eclogites, and metapelites. The latter display three assemblages: (i) garnet + chloritoid + kyanite, (ii) garnet + chlori- toid and (iii) garnet + chlorite, in all cases with quartz and muscovite in addition. The growth of these assemblages was coeval with the main ductile deformation, and was followed by minor reequilibration (chlorite growth in garnet + chloritoid samples and chloritoid + quartz aggregates replacing garnet and kyanite in garnet + chloritoid + kyanite samples). Detailed microprobe anal- yses show increasing magnesian compositions for garnet (from core to rim) and chloritoid (inclusions within garnet compared to matrix grains) in kyanite-bearing samples. The above data are interpreted in the framework of the KFMASH system. Reaction progress along the divariant reaction Cld = Grt + Ky explains the change in chemistry of coexisting phases. The divariant Grt-Cld- Ky assemblage has a narrow stability field, and the P-T conditions are estimated at about 20 kbar, 550-600°C. Decompression, recorded by chloritoid-quartz pseudomorphs of garnet, probably occurred as temperature decreased.

Orthopyroxene-andalusite symplectites replacing cordierite in granulites from the Strangways Range (Arunta block, central Australia): A new twist to the pressure-temperature history

Andalusite has been identified as the aluminosilicate polymorph intergrown with orthopyroxene in fine-grained symplectites replacing cordierite in granulites from the Strangways Range, Arunta block, central Australia. The reaction cordierite = orthopyroxene + andalusite + quartz, which has never been described before, is possible under extremely low water activity (aH2O < 0.1–0.2) conditions at ∼500 °C and ∼4 kbar. Incursion of low a(H2O) fluids during exhumation of the granulites during the Alice Springs (c. 350 Ma) orogeny resulted in formation of orthopyroxene in the andalusite stability field. This interpretation is radically different from those of earlier workers, who suggested that the orthopyroxene-bearing symplectites formed at high temperature, in response to isobaric cooling, or even during a second high-grade event. The granulites, preserved in low strain domains between retrograde amphibolite facies shear zones, have previously been considered essentially unaffected by the lower-grade metamorphic conditions because they remained anhydrous. Our data show that localized low a(H2O) fluid access during exhumation produced the anomalous orthopyroxene-andalusite assemblage.

Detrital zircon geochronology in blueschist-facies meta-conglomerates from the Western Alps: implications for the late Carboniferous to early Permian palaeogeography

In the Western Alps, the Money Complex of the Gran Paradiso Massif, metamorphosed under blueschist facies during the Alpine cycle, is considered to be Permo-Carboniferous in age, but no palaeontological or radiometric data constrain this interpretation. A revision of the lithostratigraphy of the Money Complex allows recognizing a polygenic (graphite-rich) and a monogenic (graphite-poor) meta-sedimentary formation. Detrital zircon U–Pb geochronology in both meta-sedimentary formations shows that (i) the main population is Cambrian and Ordovician in age, (ii) the youngest grains are Silurian and Lower Devonian, and (iii) Carboniferous zircon grains are lacking. A careful study of the age distributions in the Alps suggests that potential source for the detrital material in the Money Complex is the Briançonnais basement. Late Carboniferous magmatism is widespread in the Helvetic Zone of the Alps. Permian magmatism is dominant in the Briançonnais, the Austroalpine and the Southalpine basements. The lack of Carboniferous zircons in the Money Complex suggests that the detritus was not shed from the Helvetic zone, which was separated from the Money basin by the Zone Houillère basin, where the main drainage pattern was developed from south to north and where the depocenters migrated northwards from the Upper Missisippian to Upper Pennsylvanian. We suggest that the Money Complex may had been located to the east of the main river drainage inside the Zone Houillère basin or alternatively may represent a small basin, located on the east of the Zone Houillère.

Geometry and kinematics of the Roisan-Cignana Shear Zone, and the orogenic evolution of the Dent Blanche Tectonic System (Western Alps)

The Dent Blanche Tectonic System (DBTS) is a composite thrust sheet derived from the previously thinned passive Adriatic continental margin. A kilometric high-strain zone, the Roisan-Cignana Shear Zone (RCSZ) defines the major tectonic boundary within the DBTS and separates it into two subunits, the Dent Blanche s.s. nappe to the northwest and the Mont Mary nappe to the southeast. Within this shear zone, tectonic slices of Mesozoic and pre-Alpine meta-sediments became amalgamated with continental basement rocks of the Adriatic margin. The occurrence of high pressure assemblages along the contact between these tectonic slices indicates that the amalgamation occurred prior to or during the subduction process, at an early stage of the Alpine orogenic cycle. Detailed mapping, petrographic and structural analysis show that the Roisan-Cignana Shear Zone results from several superimposed Alpine structural and metamorphic stages. Subduction of the continental fragments is recorded by blueschist-facies deformation, whereas the Alpine collision is reflected by a greenschist facies overprint associated with the development of large-scale open folds. The post-nappe evolution comprises the development of low-angle brittle faults, followed by large-scale folding (Vanzone phase) and finally brittle extensional faults. The RCSZ shows that fragments of continental crust had been torn off the passive continental margin prior to continental collision, thus recording the entire history of the orogenic cycle. The role of preceding Permo-Triassic lithospheric thinning, Jurassic rifting, and ablative subduction processes in controlling the removal of crustal fragments from the reactivated passive continental margin is discussed. Results of this study constrain the temporal sequence of the tectono-metamorphic processes involved in the assembly of the DBTS, but they also show limits on the interpretation. In particular it remains difficult to judge to what extent pre-collisional rifting at the Adriatic continental margin preconditioned the efficiency of convergent processes, i.e. accretion, subduction, and orogenic exhumation.