Petrogenesis of calc-alkaline,shoshonitic and associated ultrapotassic Oligocene volcanic rocks from the Northwestern Alps,Italy

Along the Western Alps there is geological evidence of late-Alpine (Oligocene) magmatic activity which clearly postdates the Lepontine (Eocene-early Oligocene) metamorphism and related deformation of the Alpine nappe pile. This magmatic activity was notably delayed in relation to the most important convergent processes and may be related to buoyancy of lithosphere, tensional tectonics and thermal updoming subsequent to the collision between the Eurasian and African plates. The geochemical features of the rocks and the geophysical characteristics of the Alpine chain, suggest that: (a) shoshonitic and calcalkaline melts may have been generated by partial melting of metasomatized peridotitic material and subsequent fractional crystallization and crustal contamination; silicic andesites and latites, however, could have been also derived from metasomatized eclogite or deep continental crust material; (b) the ultrapotassic lamprophyres with high K, P, LREE, Th, Zr, U and high ⁸⁷Sr/⁸⁶Sr ratios were generated by partial melting of strongly metasomatized mantle; the varied Sr-isotopic ratios may partially also reflect additional radiogenic component from the continental crust following magma segregation from the source.     

The monzogabbroic intrusion in the island of Vulcano,Aeolian Archipelago,Italy

Two drill-holes were carried out during 1983–84 by the Joint Venture AGIP-EMS-ENEL on the island of Vulcano southwest of the Cratere della Fossa. After passing through pyroclastics and lavas of the young volcanic centres of Vulcano the drill-holes penetrated an intrusion of monzogabbro to leuco-monzogabbro composition. In one of the holes the top of the intrusion occurs at 1360 m and the intrusive rocks are found to the bottom of the well at 2050 m. At this depth the temperature exceeds 419 °C and the temperature gradients are sufficiently steep that magma could well be reached only a few hundred metres deeper. Lava of the South Vulcano centre is metamorphosed by the intrusion.A massive pyroclastic bed, underlying the welded scoriae deposits associated with collapse of the Caldera del Piano system, contains blocks of the intrusion. Radiometric data suggest an intrusion age of 30 000 years. Geophysical data indicate that the main intrusion is a shallow level and is located in the stretch of sea west of Mt. Lentìa.     

Coupling of oceanic and continental crust during Eocene eclogite-facies metamorphism: evidence from the Monte Rosa nappe,western Alps

High precision U–Pb geochronology of rutile from quartz–carbonate–white mica–rutile veins that are hosted within eclogite and schist of the Monte Rosa nappe, western Alps, Italy, indicate that the Monte Rosa nappe was at eclogite-facies metamorphic conditions at 42.6 ± 0.6 Ma. The sample area [Indren glacier, Furgg zone; Dal Piaz (2001) Geology of the Monte Rosa massif: historical review and personal comments. SMPM] consists of eclogite boudins that are exposed inside a south-plunging overturned synform within micaceous schist. Associated with the eclogite and schist are quartz–carbonate–white mica–rutile veins that formed in tension cracks in the eclogite and along the contact between eclogite and surrounding schist. Intrusion of the veins at about 42.6 Ma occurred at eclogite-facies metamorphic conditions (480–570°C, >1.3–1.4 GPa) based on textural relations, oxygen isotope thermometry, and geothermobarometry. The timing of eclogite-facies metamorphism in the Monte Rosa nappe determined in this study is identical to that of the Gran Paradiso nappe [Meffan-Main et al. (2004) J Metamorphic Geol 22:261–281], confirming that these two units have shared the same Alpine metamorphic history. Furthermore, the Gran Paradiso and Monte Rosa nappes underwent eclogite-facies metamorphism within the same time interval as the structurally overlying Zermatt-Saas ophiolite [∼50–40 Ma; e.g., Amato et al. (1999) Earth Planet Sci Lett 171:425–438; Mayer et al. (1999) Eur Union Geosci 10:809 (abstract); Lapen et al. (2003) Earth Planet Sci Lett 215:57–72]. The nearly identical P–T–t histories of the Gran Paradiso, Monte Rosa, and Zermatt-Saas units suggest that these units shared a common Alpine tectonic and metamorphic history. The close spatial and temporal associations between high pressure (HP) ophiolite and continental crust during Alpine orogeny indicates that the HP internal basement nappes in the western Alps may have played a key role in exhumation and preservation of the ophiolitic rocks through buoyancy-driven uplift. Coupling of oceanic and continental crust may therefore be critical in preventing permanent loss of oceanic crust to the mantle.     

Modelling of mantle postglacial relaxation in axisymmetric geometry with a composite rheology and a glacial load interpolated by adjusted spherical harmonics analysis

Although studies on glacial isostatic adjustment usually assume a purely linear rheology, we have previously shown that mantle relaxation after the melting of Laurentide ice sheet is better described by a composite rheology including a non-linear term. This modelling is, however, based on axially symmetric geometry and glacial forcing derived from ICE-3G and suffers from a certain amount of arbitrariness in the definition of the ice load. In this work we apply adjusted spherical harmonics analysis to interpolate the ice thicknesses of ICE-3G and ICE-1 glaciological models. This filters out the non-axisymmetric components of the ice load by considering only the zonal terms in the spherical harmonics expansion. The resulting load function is used in finite-element simulation of postglacial rebound to compare composite versus purely linear rheology. Our results confirm that composite rheology can explain relative sea level (RSL) data in North America significantly better than a purely linear rheology. The performance of composite rheology suggests that in future investigations, it may be better to use this more physically realistic creep law for modelling mantle deformation induced by glacial forcing.     

Tertiary age and paleostructural inferences of the eclogitic imprint in the Austroalpine outliers and Zermatt–Saas ophiolite, western Alps

The Austroalpine Sesia-Lanzo inlier and upper Austroalpine Dent Blanche, Mt. Mary and Pillonet outliers occur on top of the western-Alpine orogenic wedge and, as a whole, override the structurally composite ophiolitic Piemonte zone. Instead, the Mt. Emilius, Glacier-Rafray, Etirol-Levaz and other lower Austroalpine eclogitic outliers are inserted within the Piemonte zone, between its upper (Combin) and lower (Zermatt-Saas) tectonic elements, or within the latter. Rb-Sr dating on phengitic micas show that the eclogitic imprint in the lower Austroalpine outliers, conventionally regarded as Late Cretaceous by comparison with the Sesia-Lanzo inlier, is of Eocene age (49-40 Ma), like the underlying Zermatt-Saas ophiolite (45-42 Ma) between the Aosta valley and Gran Paradiso massif. ⁴⁰Ar-³⁹Ar plateau ages on the same mica concentrates of the ophiolitic Zermatt-Saas nappe (46-43 Ma) are consistent with Rb-Sr dating, whereas that on the Austroalpine Glacier-Rafray klippe (92 Ma) is influenced by argon excess. The lower Austroalpine outliers underwent the subduction metamorphism concurrently with the Zermatt-Saas nappe, 20-25 Ma later than the eclogitic Sesia-Lanzo inlier and blueschist Pillonet klippe. The temporal gap and present intra-ophiolitic position mean that the lower Austroalpine outliers were probably derived from an intraoceanic extensional allochthon (Mt. Emilius domain) stranded inside the Piemonte-Ligurian ocean far from the Dent Blanche-Sesia domain and Adriatic margin.     

Phase relations in the partial melting of the Baldissero spinel-lherzolite (Ivrea-Verbano zone,Western Alps,Italy)

The occurrence of various types of mobilizates in the Baldissero spinel lherzolite is due to partial melting of the same body. The study of the relationships between the peridotite and its mobilizates demonstrates that olivine did not take an active part in the fusion. Estimates of the degree of partial melting vary from 10% for the average composition, to 20% for the most depleted samples. These values refer to an initial pyrolitic composition, and thus are relative, as they can vary depending on the actual primary composition.The calculated composition of the liquid generated by partial melting is quite similar to that of a picritic basalt, and is practically the same irrespective of the 10% and 20% fusion. This fact provides strong evidence that melting took place at a unique invariant point of the natural system, producing a liquid with a remarkably constant composition.Projection of the liquid in the fo-an-di-si diagram is fairly well aligned with the modal compositions of the solid residua, but does not coincide with the minimum of the simplified system. The proposed solution is based on the enlargement of the spinel field (at constant pressure), due to the Cr content in this phase. Therefore, the position of the invariant minimum is not fixed, but rather controlled by the Cr content of the spinel. Is is suggested that, by an increase in the Cr content, spinel might at a given moment become refractory. Thus, saturated or over-saturated magmas are produced depending on the phase relations between olivine, orthopyroxene and clinopyroxene. This would happen in the case of very advanced fusions or in the case of fusion of already depleted peridotites.The relationships between mobilizates of different generations suggest a non adiabatic mantle upwelling.