Melt- versus fluid-induced metasomatism in spinel to garnet wedge peridotites (Ulten Zone, Eastern Italian Alps): clues from trace element and Li abundances

The peridotite bodies of the Ulten Zone (Upper Austroalpine, Italian Eastern Alps) are enclosed in Variscan migmatites and derive from a mantle wedge environment. They display the progressive transformation of porphyroclastic spinel peridotites (T=1,200°C; P=1.5 GPa) into fine-grained garnet–amphibole peridotites (T=850°C; P=3 GPa). Detailed bulk-rock and mineral trace element analyses of a sample suite documenting the entire metamorphic evolution of the peridotites revealed several stages of metasomatism. The spinel peridotites derive from a depleted mantle that became enriched in some large ion lithophile element (LILE) and light rare earth elements (LREE). The same signature pertains to clinopyroxene and orthopyroxene, indicating that this metasomatic signature was acquired at the recorded temperature of 1,200°C. Such a temperature is considerably above the wet peridotite solidus and hence the metasomatic agent must have been a hydrous melt. Moreover, the Li-enrichment of the spinel-facies pyroxenes (up to 24 ppm Li) reflects disequilibrium distribution after exchange with a presumably mafic melt. ^cpx/opx D _Li=3–7 and ^cpx/ol D _Li=2.7–8 indicate that the spinel-facies clinopyroxene hosts higher Li amounts than the coexisting minerals. LREE fractionation, variable LREE enrichment, LILE enrichment with respect to HFSE (average clinopyroxene Pb _N /Nb _N =16–90) in spinel lherzolites can be related to chromatographic effects of porous melt flow. The significant enrichment of pyroxenes from the spinel lherzolites in Pb, U and Li indicates that the metasomatic melt was subduction-related. All these features suggest that the spinel lherzolites formed a mantle wedge layer percolated by melts carrying recycled crustal components and rising from a deeper source of subduction magmas. The garnet + amphibole peridotites equilibrated at temperatures well below the wet solidus in the presence of an aqueous fluid. Bulk-rock trace element patterns display pronounced positive anomalies in Cs, Ba, Pb and U and moderate enrichment in Li, indicating addition of a crustal component to the mantle rocks. Amphibole hosts most of these trace elements. Clinopyroxene displays high LILE/HFSE (Pb _N /Nb _N =300–600), low Ce/Pb (1.4–2.7 in garnet-facies clinopyroxene compared with 2.6–24.5 in the spinel-facies one) and variable LILE and LREE enrichments. The coupled increase of modal amphibole, Sr and Pb, together with positive Pb–Sr and Pb–U correlations, further indicate that incompatible element influx in these samples was fluid-mediated. In the garnet-facies samples, amphibole and, interestingly, olivine have similarly high Li concentrations as clinopyroxene, leading to ^cpx/amph D _Li=0.7 and ^cpx/ol D _Li=0.7–0.8, the latter being up to ten times lower than in the spinel-facies rocks. Due to its high modal abundance, olivine is the main host of Li in the garnet–amphibole peridotites. The observed metasomatic features provide evidence for the infiltration of an aqueous fluid in the mantle wedge above a subducting slab. This fluid most likely derived from subducted crustal rocks that underwent partial melting. Successive retrograde re-equilibration during exhumation of the garnet peridotite is accompanied by garnet and clinopyroxene breakdown and amphibole formation. This process produced minor changes, such as an increase of HREE and Li in amphibole, and an increase of Li in olivine. The general trace element signature remains essentially unchanged during retrogression and further hydration, indicating that fluids with a similar composition to the one present at the garnet–amphibole peridotite formation, were responsible for increased amphibole formation. The combined evidence from the metamorphic and metasomatic evolution indicates that the peridotites experienced first corner flow in a mantle wedge, followed by subduction and finally entrapment and exhumation within a crustal slab. During their entire history the Ulten peridotites were percolated first by melts and then by aqueous fluids, which added recycled crustal components to the mantle wedge.     

Fluid-controlled crustal metasomatism within a high-pressure subducted mélange (Mt. Hochwart, Eastern Italian Alps)

The Nonsberg–Ultental Region of northern Italy contains a Palaeozoic mélange that was partially subducted during the Variscan orogeny. This mélange is constituted mainly by metapelites characterized by shale-type REE-patterns, displaying partial melting which began under high-pressure conditions. The resulting migmatites enclose minor slivers of mantle-wedge peridotites that have been incorporated into the mélange during subduction. Peridotites display important large ion lithophile elements (LILE) enrichment consequent to amphibole recrystallization contemporaneously with metapelite migmatization at P ≈ 2.7 GPa and T ≈ 850 °C in the garnet–peridotite field. Crustal and mantle (ultramafic) rocks of the mélange display the same Sm–Nd ages of about 330 ± 6 Ma, which dates both the metamorphic peak and the migmatization event. The zircon U–Pb age of the metasomatic amphibolitic contact between garnet peridotite and migmatite is identical (333.3 ± 2.4 Ma) within analytical errors. Therefore, metasomatism, migmatization and peak metamorphism are constrained to the same event. The presence of Cl-rich apatite and ferrokinoshitalite in the contact amphibolite, together with the trace-element patterns of peridotites, suggest that metasomatism was driven by Cl- and LILE-rich fluids derived from ocean water transported into the subduction zone by sediments and crustal rocks. These fluids interacted with the crust, prompting partial melting under water oversaturated conditions and partitioning LILE from the crust itself. Peridotites, which were well below their wet solidus temperature, could not melt but they recrystallized in the crustal mélange under garnet-facies conditions. Crustal fluids caused extensive hydration and LILE-enrichment in peridotites and severe Sm–Nd isotope disequilibrium between minerals, especially in the recrystallized peridotites. The proposed scenario suggests massive entrapment of crustal aqueous fluids at high-pressure conditions within subduction zones.     

Origin and emplacement of ultramafic–mafic intrusions in the Erro-Tobbio mantle peridotite (Ligurian Alps, Italy)

The Erro-Tobbio peridotites (Voltri Massif, Ligurian Alps) represent subcontinental lithospheric mantle tectonically exhumed during Permo–Mesozoic extension of the Europe–Adria lithosphere. Previous studies have shown that exhumation started during Permian times, and occurred along kilometer-scale lithospheric shear zones which enhanced progressive deformation and recrystallization from spinel- to plagioclase-facies conditions. Ongoing field and petrologic investigations have revealed that the peridotites experienced, during uplift, a composite history of diffuse melt migration and multiple episodes of ultramafic–mafic intrusions. In this paper we present the results of field, structural and petrologic–geochemical investigations into a sector of the Erro-Tobbio peridotite unit that preserves well this multiple intrusion history. Melt impregnation in the peridotites is evidenced by significant plagioclase enrichment and crystallization of unstrained orthopyroxene replacing kinked mantle olivine and clinopyroxene; impregnating melts were thus opx-saturated. Melt–rock interaction caused chemical changes in mantle minerals (e.g. Al decrease and REE increase in cpx; Ti and Cr# enrichment in spinel). Nevertheless, clinopyroxenes still exhibit LREE depletion (Ce_N/Sm_N = 0.006–0.011), indicating a depleted signature for the percolating melts. Melt impregnation was thus related to diffuse porous flow migration of depleted MORB-type melt fractions that modified their compositions towards opx saturation by mantle–melt interaction during ascent. The impregnated peridotites are intruded by a hectometer-scale stratified cumulate body, mostly consisting of troctolites and plagioclase wehrlites, showing gradational, interfingered contacts with the host mantle rocks. Subsequent intrusion events are revealed by the occurrence of olivine gabbros as decameter-wide lenses, variably thick (centimeter- to meter-scale) dykes and thin dykelets, which crosscut both the peridotite foliation and the magmatic layering in the cumulates. Overall, major and trace element compositions of minerals in the intrusives indicate that they represent variably differentiated cumulus products crystallized from rather primitive N-MORB-type aggregated melts. Slightly more evolved compositions are shown by olivine gabbros, relative to the troctolites and plagioclase wehrlites of the cumulate body. Mineral chemistry features (e.g. the Fo–An correlation and high Na, Ti, Mg# in cpx) indicate that the studied intrusive rocks crystallized at moderate pressure conditions (3–5 kbar, i.e. 9–15 km depth). Our study thus points to a progressive transition from porous flow melt migration to emplacement of magmas in fractures, presumably related to progressive change of lithospheric mantle rheology during extension-related uplift and cooling.     

The role of structural inheritance in continental break-up and exhumation of Alpine Tethyan mantle(Canavese Zone,Western Alps)

The Canavese Zone(CZ)in the Western Alps represents the remnant of the distal passive margin of the Adria microplate,which was stretched and thinned during the Jurassic opening of the Alpine Tethys.Through detailed geological mapping,stratigraphic and structural analyses,we document that the continental break-up of Pangea and tectonic dismemberment of the Adria distal margin,up to mantle rocks exhumation and oceanization,did not simply result from the syn-rift Jurassic extension but was strongly favored by older structu ral inheritances(the Proto-Canavese Shear Zone),which controlled earlier lithospheric weakness.Our findings allowed to redefine in detail(i)the tectono-stratigraphic setting of the Variscan metamorphic basement and the Late Carbonife rous to Early Cretaceous CZ succession,(ii)the role played by inherited Late Carboniferous to Early Triassic structures and(iii)the significance of the CZ in the geodynamic evolution of the Alpine Tethys.The large amount of extensional displacement and crustal thinning occurred during different pulses of Late Carbonife rous-Early Triassic strike-slip tectonics is wellconsistent with the role played by long-lived regional-scale wrench faults(e.g.,the East-Variscan Shear Zone),suggesting a re-discussion of models of mantle exhumation driven by low-angle detachment faults as unique efficient mechanism in stretching and thinning continental crust.     

Dating synmagmatic folds: a case study of Schlingen structures in the Strona-Ceneri Zone (Southern Alps, northern Italy)

The Strona-Ceneri Zone (Southern Alps) contains folds with moderately to steeply inclined axial planes and fold axes, and amplitudes of up to several kilometres (so-called 'Schlingen'). These amphibolite facies folds deform the main schistosity of Late Ordovician metagranitoids and are discordantly overlain by unmetamorphic Permian sedimentary rocks. Mutually cross-cutting relationships between these folds and garnet-bearing leucotonalitic dykes indicate that these dykes were emplaced during folding. Sm–Nd systematics and the strongly peraluminous composition of these dykes point to an anatectic origin. Pb step leaching of magmatic garnet from a leucotonalitic dyke yielded a 321.3±2.3  Ma intrusive age. Rb–Sr ages on muscovites from leucotonalitic dykes range from 307 to 298  Ma, interpreted as cooling ages during retrograde amphibolite facies metamorphism. Conventional U–Pb data of zircons from an older granodioritic dyke that pre-dates the Schlingen folds yielded discordant U–Pb ages ranging from 371 to 294  Ma. These ages reflect a more complicated multi-episodic growth history which is consistent with the observed polyphase structural overprint of this dyke. Schlingen folding was accompanied by prograde amphibolite facies metamorphism, during the thermal peak of which the leucotonalitic dyke material was generated by partial melting in a deeper source region from where these S-type magmas intruded the presently exposed level. Because partial melting may occur in a relatively late stage of a clockwise P–T–t path, or even during decompression on the retrograde path, we do not exclude the possibility that Schlingen folding had already started in Early Carboniferous time. Schlingen folds also occur in Penninic and Austroalpine basement units with a very similar pre-Alpine history, indicating that Variscan folding affected large segments of the future Alpine realm.     

P–T–t evolution of spinel–cordierite–garnet gneisses from the Sauwald Zone (Southern Bohemian Massif, Upper Austria): is there evidence for two independent late-Variscan low-P/high-T events in the Moldanubian Unit?

The Sauwald Zone, located at the southern rim of the Bohemian Massif in Upper Austria, belongs to the Moldanubian Unit. It exposes uniform biotite + plagioclase ± cordierite paragneisses that formed during the post-collisional high-T/low-P stage of the Variscan orogeny. Rare metapelitic inlayers contain the mineral assemblage garnet + cordierite + green spinel + sillimanite + K-feldspar + plagioclase + biotite + quartz. Mineral chemical and textural data indicate four stages of mineral growth: (1) peak assemblage as inclusions in garnet (stage 1): garnet core + cordierite + green spinel + sillimanite + plagioclase (An_35–65); (2) post-peak assemblages in the matrix (stages 2, 3): cordierite + spinel (brown-green and brown) ± sillimanite ± garnet rim + plagioclase (An_10–45); and (3) late-stage growth of fibrolite, muscovite and albite (An_0–15) during stage 4. Calculation of the P–T conditions of the peak assemblage (stage 1) yields 750–840°C, 0.29–0.53 GPa and for the stage 2 matrix assemblage garnet + cordierite + green spinel + sillimanite + plagioclase 620–730°C, 0.27–0.36 GPa. The observed phase relations indicate a clockwise P–T path, which terminates below 0.38 GPa. The P–T evolution of the Sauwald Zone and the Monotonous Unit are very similar, however, monazite ages of the former are younger (321 ± 9 Ma vs. 334 ± 1 Ma). This indicates that high-T/low-P metamorphism in the Sauwald Zone was either of longer duration or there were two independent phases of late-Variscan low-P/high-T metamorphism in the Moldanubian Unit.     

Calcareous nannoplankton, planktonic foraminiferal, and carbonate carbon isotope stratigraphy of the Cenomanian–Turonian boundary section in the Ultrahelvetic Zone (Eastern Alps, Upper Austria)

Ultrahelvetic units of the Eastern Alps were deposited on the distal European continental margin of the (Alpine) Tethys. The Rehkogelgraben section (“Buntmergelserie”, Ultrahelvetic unit, Upper Austria) comprises a 5 m thick succession of upper Cenomanian marl-limestone cycles overlain by a black shale interval composed of three black shale layers and carbonate-free claystones, followed by lower Turonian white to light grey marly limestones with thin marl layers. The main biostratigraphic events in the section are the last occurrence of Rotalipora and the first occurrences of Helvetoglobotruncana helvetica and Quadrum gartneri. The thickest black shale horizon has a TOC content of about 5%, with predominantly marine organic matter of kerogen type II. Vitrinite reflectance and Rock-Eval parameter T_max (<424 °C) indicate low maturity. HI values range from 261 to 362 mg HC/g TOC. δ¹³C values of bulk rock carbonates display the well documented positive shift around the black shale interval, allowing correlation of the Rehkogelgraben section with other sections such as the Global Boundary Stratotype Section and Point (GSSP) succession at Pueblo, USA, and reference sections at Eastbourne, UK, and Gubbio, Italy. Sediment accumulation rates at Rehkogelgraben (average 2.5 mm/ka) are significantly lower than those at Pueblo and Eastbourne.