Petrology of ultramafic and mafic rocks from the Lanzo peridotite body (Western Alps)

Pognante, Ugo; Rösli, Ursula; Toscani, Lorenzo

doi:10.1016/0024-4937(85)90025-8

Cited by 42 publications

(33 citation statements)

References 36 publications

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“…It was proposed that the northern body presents melt extraction lower than 6% and the lherzolite was equilibrated with T-MORB (Bodinier 1988). The central and southern domains present a variable percentage from 6% (central part) to 12% (south part) and have affinities with T to N-MORB (Pognante et al 1985;Bodinier 1988;Bodinier et al 1991). The southern body was interpreted as mantle asthenosphere, which rose from the garnet stability field as a high-temperature diapir emplaced at shallow levels, accompanied by a large degree of .…”

Section: Geological Settingmentioning

confidence: 99%

The variability of peridotite composition across a mantle shear zone (Lanzo massif, Italy): interplay of melt focusing and deformation

Kaczmarek

Müntener

2010

Contrib Mineral Petrol

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In this paper we present new data on the spatial variability of peridotite composition across a kilometerscale mantle shear zone within the Lanzo massif (Western Alps, Italy). The shear zone separates the central from the northern part of the massif. Plagioclase peridotite shows gradually increasing deformation towards the shear zone, from porphyroclastic to mylonitic textures in the central body, while the northern body is composed of porphyroclastic rocks. The peridotite displays a large range of compositions, from fertile peridotite to refractory harzburgite and dunite. Deformed peridotites (proto-mylonite and mylonites) tend to be compositionally more homogeneous and fertile than weakly deformed peridotites. The composition of most plagioclase peridotites show rather high and constant (Ce/Yb) N ratios, and Yb N that cannot be explained by any simple melting model. Instead, refertilization modeling, consisting of melt increments from spinel peridotite sources, particularly with E-MORB melt, reasonably reproduces the plagioclase peridotite whole rock composition. Combined with constraints from Ce-Nb and Ce-Th systematics, we speculate that peridotites such as those from Lanzo record pervasive refertilization processes in the thermal boundary layer. In this scenario, mantle shear zones might act as important areas of melt focusing in the upper mantle that separates the thermal boundary layer from the conductively cooled mantle.

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Section: Geological Settingmentioning

confidence: 99%

The variability of peridotite composition across a mantle shear zone (Lanzo massif, Italy): interplay of melt focusing and deformation

Kaczmarek

Müntener

2010

Contrib Mineral Petrol

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“…The preservation of primary textures also implies low-stress conditions within the subduction zone. Although HP shear zones have been described from many localities (Pognante et al, 1985;Austrheim, 1987;Boundy et al, 1992;Camacho et al, 1997), UHP shear zones are much rarer, although they have been reported from the Dabie Sulu region in China (Zhao et al, 2003). The lack of evidence for structures forming at UHP conditions may be due to metamorphic or deformational overprinting, continuous development and reactivation during exhumation, or an unspecific and/or unrecognizable microstructural record (Stöckhert, 2002;Zhao et al, 2003).…”

Section: Weakening and Detaching Subducted Crustmentioning

confidence: 99%

“…Eclogite-facies (HP) shear zones have been documented from a number of locations including the Western Gneiss Region and Bergen Arcs in Norway, and the Western Alps (Pognante et al, 1985;Austrheim, 1987;Boundy et al, 1992;Camacho et al, 1997). In the Bergen Arcs, mylonitic eclogitefacies shear zones transect coarser, apparently unaltered, granulite-facies rocks, suggesting that deformation, and arguably fluid, were necessary for triggering the eclogiteforming reactions (Austrheim and Griffin, 1985).…”

Section: Grain Size Reductionmentioning

confidence: 99%

Exhumation of (ultra-)high-pressure terranes: concepts and mechanisms

Warren

2013

Solid Earth

111

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Abstract. The formation and exhumation of high and ultrahigh-pressure, (U)HP, rocks of crustal origin appears to be ubiquitous during Phanerozoic plate subduction and continental collision events. Exhumation of (U)HP material has been shown in some orogens to have occurred only once, during a single short-lived event; in other cases exhumation appears to have occurred multiple discrete times or during a single, long-lived, protracted event. It is becoming increasingly clear that no single exhumation mechanism dominates in any particular tectonic environment, and the mechanism may change in time and space within the same subduction zone. Subduction zone style and internal force balance change in both time and space, responding to changes in width, steepness, composition of subducting material and velocity of subduction. In order for continental crust, which is relatively buoyant compared to the mantle even when metamorphosed to (U)HP assemblages, to be subducted to (U)HP conditions, it must remain attached to a stronger and denser substrate. Buoyancy and external tectonic forces drive exhumation, although the changing spatial and temporal dominance of different driving forces still remains unclear. Exhumation may involve whole-scale detachment of the terrane from the subducting slab followed by exhumation within a subduction channel (perhaps during continued subduction) or a reversal in motion of the entire plate (eduction) following the removal of a lower part of the subducting slab. Weakening mechanisms that may be responsible for the detachment of deeply subducted crust from its stronger, denser substrate include strain weakening, hydration, melting, grain size reduction and the development of foliation. These may act locally to form narrow high-strain shear zones separating stronger, less-strained crust or may act on the bulk of the subducted material, allowing whole-scale flow. Metamorphic reactions, metastability and the composition of the subducted crust all affect buoyancy and overall strength. Future research directions include identifying temporal and spatial changes in exhumation mechanisms within different tectonic environments, and determining the factors that influence those changes.

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“…Recent studies refined this model and proposed a conceptual model from porous to focused melt flow to explain the various igneous features in the Lanzo peridotite (Müntener and Piccardo 2003;Müntener et al 2005;Piccardo et al 2004): (a) a pervasive porous melt flow leading to the formation of impregnated peridotites; (b) focused porous flow where melt transport is mainly focused in replacive harzburgites and dunites; (c) initial crystallization and formation of megacrysts and small dikes corresponding to the ''indigenous'' group of Boudier and Nicolas (1972); ubiquitous formation of km-scale, discordant gabbroic and basaltic dikes. Previous studies (Bodinier 1988;Bodinier et al 1986;Pognante et al 1985) indicate that gabbro and basaltic dikes are derived from T-MORB to T-to N-MORB type magmas from the north to the south, respectively, and are compositionally similar to ophiolitic basalts of the Ligurian, Western and eastern central Alps (Beccaluva et al 1984;Bill et al 2000;Desmurs et al 2002).…”

Section: Geological Settingmentioning

confidence: 99%

“…The Lanzo massif is considered a part of the lithospheric mantle exhumed at the ocean floor during opening of the Jurassic-Piedmont Ligurian ocean (Lagabrielle et al 1989;Pognante et al 1985). Recent studies demonstrate that the presence of a high-temperature mantle shear zone between the north and the central part of the massif is related to rapid mantle exhumation.…”

Section: Geological Settingmentioning

confidence: 99%

Trace element chemistry and U–Pb dating of zircons from oceanic gabbros and their relationship with whole rock composition (Lanzo, Italian Alps)

Kaczmarek

Müntener

Rubatto

2007

Contrib Mineral Petrol

143

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The U-Pb ages and the trace element content of zircon U-Pb along with major and trace element whole rock data on gabbroic dikes from the Lanzo lherzolitic massif, N-Italy, have been determined to constrain crustal accretion in ocean-continent transition zones. Three Fe-Ti gabbros were dated from the central and the southern part of the massif providing middle Jurassic ages of 161 ± 2, 158 ± 2 and 163 ± 1 Ma, which argue for magmatic activity over few millions of years. Zircon crystals are characterized by high but variable Th/U ratios, rare earth element patterns enriched in heavy rare earths, pronounced positive Ce and negative Eu-anomalies consistent with crystallization after substantial plagioclase fractionation. The zircon trace element composition coupled with whole rock chemistry was used to reconstruct the crystallization history of the gabbros. A number of gabbros crystallized in situ, and zircon precipitated from trapped, intercumulus liquid, while other gabbros represent residual liquids that were extracted from a cumulus pile and crystallized along syn-magmatic shear zones. We propose a model in which the emplacement mechanism of gabbroic rocks in oceancontinent transition zones evolves from in situ crystallization to stratified crystallization with efficient extraction of residual liquid along syn-magmatic shear zones. Such an evolution of the crystallization history is probably related to the thermal evolution of the underlying mantle lithosphere.

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Petrology of ultramafic and mafic rocks from the Lanzo peridotite body (Western Alps)

Cited by 42 publications

References 36 publications

The variability of peridotite composition across a mantle shear zone (Lanzo massif, Italy): interplay of melt focusing and deformation

The variability of peridotite composition across a mantle shear zone (Lanzo massif, Italy): interplay of melt focusing and deformation

Exhumation of (ultra-)high-pressure terranes: concepts and mechanisms

Trace element chemistry and U–Pb dating of zircons from oceanic gabbros and their relationship with whole rock composition (Lanzo, Italian Alps)

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