Silvana Martin

Variscan migmatites, eclogites and garnet-peridotites of the Ulten zone, Eastern Austroalpine system

In northwestern Trentino (Nonsberg-Ultental area, northeastern Italy) the strongly foliated high-grade gneisses, migmatites and the mafic and Spl/Grt-bearing peridotite lenses of the Ulten zone record metamorphic processes which could be related to the evolution history of the Central Europe Variscan belt. The high P-T conditions calculated for the gneisses (1.5 ± 0.5 GPa; 600 < T < 850°C, depending on the XH2O) are comparable to those calculated for the mafic rocks (1.4 GPa; 640 < T < 700°C) and for the Grt-peridotites (1.2 < P < 2.0 GPa; 630 < T < 814°C). These conditions should correspond to those under which the S1 deformation has developed the mylonitic foliation in the gneisses. The drop in P registered by the gneisses, mafic rocks and Grt-peridotites (down to P < 1.0 GPa; T < 900°C) and pointed out by a post-S1 migmatization, may be related to the relaxation and exhumation processes after thickening. The decompressional component of the exhumation history has induced a statical growth of Spl-pyroxene/Spl-Amph symplectites after Grt in the Grt-peridotites and of Cpx-Pl/Amph-Pl in the eclogites. Textures and T estimates obtained from primary pre-exsolution pyroxene porphyroclasts in Grt-peridotites suggest that these could come from a high-T mantle environment, possibly a mantle wedge overlying a subducting slab, and may have been incorporated as sliced fragments into the crust during the Variscan collision. Metasomatism in the Grt-peridotites seems to confirm this origin. This article demonstrates that the Ulten zone probably represents a deep fragment of the Variscan belt, slightly reworked during the Alpine orogeny. Hence, this poses the problem of the possible occurrence of Variscan remnants with a European affinity in the Austroalpine system (i.e., African plate).

Mantle–crust interactions during Variscan subduction in the Eastern Alps (Nonsberg–Ulten zone): geochronology and new petrological constraints

Abstract We have analyzed the Sm–Nd and Rb–Sr whole-rock and mineral isotope systematics of garnet peridotites and associated eclogites and migmatitic gneisses from the Nonsberg–Ulten zone of the Eastern Alps. The garnet peridotites include coarse-grained varieties, characterized by well-preserved to slightly modified mantle geochemical signatures, and finer-grained varieties enriched in amphibole and LILE. Hydration of some of the most strongly deformed, fine-grained peridotites by crustal fluids caused isotopic disequilibrium between the peridotite minerals, preventing accurate age determinations. The coarse-grained peridotites, the eclogites and the country migmatitic gneisses yield garnet–whole-rock and garnet–clinopyroxene Sm–Nd ages that indicate for all rock types an isotopic homogenization event at ca. 330 Ma. The similar ages suggest that all rock types shared a common history since the incorporation of the peridotites in the crust, and constrain the garnet-facies metamorphism of the peridotites, as well as partial melting of the crust, to an episode of crustal subduction at the end of the Variscan orogenic cycle.

Petrogenesis of kelyphites in garnet peridotites: a case study from the Ulten zone, Italian Alps

Abstract Spinel-bearing symplectites around garnet (kelyphite) record the transition from high-pressure garnet peridotite to low-pressure spinel peridotite. In the first part, we review the previous studies that have been devoted to kelyphite. Many authors have identified orthopyroxene, spinel, clinopyroxene and amphibole in kelyphite. They have emphasized the common existence of two concentric coronae: an outer spinel-poor corona on the olivine side and an inner spinel-rich corona on the garnet side. Chlorite and plagioclase have also been reported in low-grade kelyphites, whereas more complex kelyphite-forming associations, including phlogopite, ilmenite, calcite and perovskite, have been observed in kimberlite xenoliths and interpreted as resulting from metasomatic action of the kimberlite magma or deep fluids. Three different origins have been proposed for kelyphite: magmatic origin, exsolution from orthopyroxene and metamorphic reaction between garnet and olivine during retrogression. This last interpretation is by far the most common. Finally, we discuss the origins of other pyroxenes + spinel symplectites and clusters that occur in peridotite. In the second part, we study various kelyphites observed in the Ulten peridotites (Italian Alps). They are composed of two concentric coronae: (i) a narrow outer spinel-free corona, in contact with olivine, made of orthopyroxene ± amphibole ± clinopyroxene; (ii) a wide inner corona, in contact with garnet, which consists of vermicular spinel forming various symplectites with either orthopyroxene, amphibole or clinopyroxene. An attempt to balance the kelyphite-forming reaction is made through a material transfer study, in order to determine element mobilities and to evaluate the opening of the system during the reaction. Two different quantitative treatments are used: the first approach consists in estimating the stoichiometric coefficients by the least square method; the second is based on measurements of the stoichiometric coefficients of the kelyphitisation products by analysis of back-scattered electron and X-ray images. This study shows that the kelyphite-forming reactions range between two extremes: garnet + olivine → orthopyroxene + spinel + clinopyroxene and garnet + olivine + H2O → orthopyroxene + spinel + amphibole. This accounts for strong variations in the relative abundance of amphibole and clinopyroxene, which were correlated with local fluctuations of PH2O. It is demonstrated that the boundary between the two concentric coronae coincides with the former garnet–olivine interface. These coronae did not maintain the garnet and olivine compositions; they exchanged components during their growth. Moreover, the whole kelyphite system was slightly open, with gains in alkalis, water and Mg, and losses of Fe and Al. The behaviour of Mg and Fe is likely explained by their incipient redistribution between ferromagnesian phases during kelyphitisation, in relation with the temperature decrease. Change in garnet composition close to kelyphite strongly supports this hypothesis. Incipient Tschermakitic substitution in neighbouring minerals could also explain the slight opening to Si and Al. Kelyphite formation has many characteristics of diffusion metasomatism at microscale. μ–μ diagrams are used to illustrate some aspects of this metasomatism. Finally, pressure–temperature conditions of kelyphite formation in the evolution of peridotite during exhumation are discussed. Kelyphitisation is mainly related to a drop in pressure, as attested by a strong volume increase (ΔV/V=7.1).

Late Jurassic blueschist facies pebbles from the Western Carpathian orogenic wedge and paleostructural implications for Western Tethys evolution

In spite of the absence of ophiolitic slices at the surface, some traces of the lost Tethys ocean are recorded along the Pieniny Klippen Belt (PKB), a narrow decollement thrust system sutured at the transpressive boundary between the Outer and Inner Carpathians. The enigmatic precollisional evolution of Western Carpathians can be deciphered from some late Albian to Campanian flysch conglomerates which display chrome spinel grains, ophiolitic detritus and pebbles of blueschist facies tholeiitic metabasalts yielding a 40Ar/39Ar plateau age of 155.4±0.6 Ma. Other detrital components are represented by extrabasinal pebbles of limestones, arc volcanics, and igneous to metamorphic basement rocks from southern sources. Our results suggest a markedly northward extension of the sublongitudinal Triassic Vardar (Meliata) Ocean and its subduction since the late Middle Jurassic, supposedly balanced westward by coeval spreading in the Ligurian-Piedmont basin of the Apennine-Western Alpine Tethys. A lateral kinematic connection between these diachronous and roughly parallel Tethys branches was provided on the north by a left-lateral east-west trending shear zone running from the Swiss-Austrian Penninic domain to the Northern Carpathians. This reconstruction replaces the classic model of two paired North Penninic and South Penninic oceanic basins and eastern homologues with the Brianconnais-Hochstegen and Czorstin microcontinents in between. The Late Jurassic-Early Cretaceous evolution of the Carpathian active margin was characterized by subduction metamorphism and accretion of a wide orogenic wedge; in this time, the shallowing to deeply subsiding basins inferred from facies analyses on the sedimentary units of the PKB were likely floored by individual sections of the growing wedge. Later, some exhuming blueschist ophiolitic units of the wedge were uplifted to the surface and functioned in the Albian-Campanian as an “exotic ridge” supplying clasts to the forearc basin. Finally, the colliding wedge became a cryptic paleostructure when, since the latest Cretaceous, it disappeared beneath the Inner Carpathian orogenic lid and was incorporated within the eastward moving infrastructure of the Carpathian orocline.

Detrital Fission-Track Analysis and Sedimentary Petrofacies as Keys of Alpine Exhumation: The Example of the Venetian Foreland (European Southern Alps, Italy)

Sedimentary petrography and detrital apatite fission-track analysis of sediments from the Oligo-Miocene Venetian foreland succession (NE Italy) reveal a multi-stage evolution of the Alpine source areas. During the first stage of sedimentation (Chattian to Langhian), the foreland basin was filled by sediments derived from erosion of mainly crystalline Austroalpine units exhumed north of the Periadriatic Lineament. These sediments are characterized by litharenites with mean quartz content of about 61% and by a mean detrital apatite fission-track peak of about 30 Ma. The Permian-Paleogene sedimentary cover of the Southalpine crystalline basement (Dolomite region) was bypassed by the sediments and made only a minor input. From the Serravallian onwards, the foreland basin was incorporated into the Southalpine thrust belt. The compressional events of the Valsugana phase resulted in thrusting and uplift of the Southalpine domain, as documented by Tortonian fission-track ages in the hanging wall of the major regional thrust (Valsugana thrust fault). The Southalpine crystalline basement, unaffected by Alpine metamorphism, and its sedimentary cover became the main source of sediments, as revealed by the increase in extrabasinal carbonate grains and detrital apatite grains with Late Triassic-Jurassic fission-track ages. During this final stage of basin evolution, about 3000 m of sediments were deposited. These strata subsequently have been uplifted and eroded since the Pliocene, indicating a southward migration of the Alpine deformation front.

First results from GPS measurements on present day alpine kinematics

Abstract Present day displacements of a few mm/year result from a preliminary analysis of data from permanent GPS stations positioned along the flanks of the Alps. The largest rates occur across the Alps: the Zimmerwald — Torino line, in the Western sector, increases its length at a rate of 5.2 mm/year. The line is very nearly accommodated by the Grasse — Torino line, which decreases at a very similar rate. No statistically significant displacement is evident in the Eastern sector. Our data thus support the hypothesis that the present day motion of Adria against Europe favours the escape to the West and Southwest of the North-western sectors of Italy. As the analysis of GPS data progresses, it should be possible to constrain by GPS measurements the present location of the Adria rotation pole and better test the ‘rigid block’ assumption.

Tectono-magmatic evolution of sheeted plutonic bodies along the north Giudicarie line (northern Italy)

AbstractTectonized slices of foliated quartz-diorite/ quartz-gabbro rocks are exposed along the north Giudicarie line between Dimaro and Rumo (Western Trentino region, north-east Italy). They show geochemical and mineralogical similarities with the north-east corner of the Adamello batholith (Presanella pluton) and may be regarded as a northern apophysis lamella of Adamello. The intrusive bodies were emplaced within the Adria crust at a relatively shallow depth (approximately

Dissakisite-(La) from the Ulten zone peridotite (Italian Eastern Alps): A new end-member of the epidote group

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