Extensive "halo naevi" phenomenon and regression of melanin during nivolumab treatment in metastatic melanoma: A predictor of a better outcome?

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Pervasive Eclogitization Due to Brittle Deformation and Rehydration of Subducted Basement: Effects on Continental Recycling?

The buoyancy of continental crust opposes its subduction to mantle depths, except where mineral reactions substantially increase rock density. Sluggish kinetics limit such densification, especially in dry rocks, unless deformation and hydrous fluids intervene. Here we document how hydrous fluids in the subduction channel invaded lower crustal granulites at 50–60 km depth through a dense network of probably seismically induced fractures. We combine analyses of textures and mineral composition with thermodynamic modeling to reconstruct repeated stages of interaction, with pulses of high‐pressure (HP) fluid at 650–670°C, rehydrating the initially dry rocks to micaschists. SIMS oxygen isotopic data of quartz indicate fluids of crustal composition. HP growth rims in allanite and zircon show uniform U‐Th‐Pb ages of ∼65 Ma and indicate that hydration occurred during subduction, at eclogite facies conditions. Based on this case study in the Sesia Zone (Western Italian Alps), we conclude that continental crust, and in particular deep basement fragments, during subduction can behave as substantial fluid sinks, not sources. Density modeling indicates a bifurcation in continental recycling: Chiefly mafic crust, once it is eclogitized to >60%, are prone to end up in a subduction graveyard, such as is tomographically evident beneath the Alps at ∼550 km depth. By contrast, dominantly felsic HP fragments and mafic granulites remain positively buoyant and tend be incorporated into an orogen and be exhumed with it. Felsic and intermediate lithotypes remain positively buoyant even where deformation and fluid percolation allowed them to equilibrate at HP

Contrasting styles of (U)HP rock exhumation along the Cenozoic Adria-Europe plate boundary (Western Alps, Calabria, Corsica)

Since the first discovery of ultrahigh pressure (UHP) rocks 30 years ago in the Western Alps, the mechanisms for exhumation of (U)HP terranes worldwide are still debated. In the western Mediterranean, the presently accepted model of synconvergent exhumation (e.g., the channel-flow model) is in conflict with parts of the geologic record. We synthesize regional geologic data and present alternative exhumation mechanisms that consider the role of divergence within subduction zones. These mechanisms, i.e., (i) the motion of the upper plate away from the trench and (ii) the rollback of the lower plate, are discussed in detail with particular reference to the Cenozoic Adria-Europe plate boundary, and along three different transects (Western Alps, Calabria-Sardinia, and Corsica-Northern Apennines). In the Western Alps, (U)HP rocks were exhumed from the greatest depth at the rear of the accretionary wedge during motion of the upper plate away from the trench. Exhumation was extremely fast, and associated with very low geothermal gradients. In Calabria, HP rocks were exhumed from shallower depths and at lower rates during rollback of the Adriatic plate, with repeated exhumation pulses progressively younging toward the foreland. Both mechanisms were active to create boundary divergence along the Corsica-Northern Apennines transect, where European southeastward subduction was progressively replaced along strike by Adriatic northwestward subduction. The tectonic scenario depicted for the Western Alps trench during Eocene exhumation of (U)HP rocks correlates well with present-day eastern Papua New Guinea, which is presented as a modern analog of the Paleogene Adria-Europe plate boundary

U-Pb dating of magmatic zircon and metamorphic baddeleyite in the Ligurian eclogites (Voltri Massif, Western Alps)

U-Pb geochronology with ion microprobe (SHRIMP) analysis has been carried out on eclogite-facies rocks of the Beigua Unit, an ophiolitic slice of the Voltri Massif, Western Alps. The investigated samples are eclogites and high-pressure metasomatic rocks (metarodingites and centimetre-sized Ti-clinohumite-bearing dykes). Zircon contained in an eclogitic metagabbro and a metarodingite preserves magmatic zoning patterns and trace element compositions. The zircon ages of 160 ± 1 and 161 ± 3 Ma are interpreted to date the crystallization of the gabbroic protoliths. Ti-clinohumite dykes in the same unit contain baddeleyite crystals in textural equilibrium with Ti-clinohumite, diopside, chlorite and magnetite, which form the eclogite-facies assemblage in these rocks. Baddeleyite also contains inclusions of such minerals, indicating its formation at high pressure. The baddeleyite has cathodo-luminescence intensity and chaotic patterns similar to metamorphic zircon. It contains a significant amount of Hf (1.3-1.7 wt% , traces of Ti, Y, Nb, Ta, REE, U and Th. Its chondrite-normalised trace element pattern has strong enrichment in middle REE, positive Ce-anomaly and small negative Eu-anomaly. This represents the first report of baddeleyite formed during regional metamorphism, and suggests that this mineral could (re)crystallize easier than zircon under low-temperature, high-pressure conditions. The age of the baddeleyite is interpreted as likely dating the eclogite-facies metamorphism in the Beigua Unit at 33.6 ± 1.0 Ma. This age is very close to the Early Oligocene age of the overlying Tertiary continental breccias and conglomerates, which contains clasts of high-pressure rocks. This sedimentary record, which is unique for Alpine high-pressure units, is direct evidence of fast exhumation of the Beigua eclogites. The young age for the HP metamorphism of the Beigua ophiolite makes a revision of either the palaeogeography prior to collision, or of the subduction setting in the entire region, necessary

From passive margins to orogens: The link between ocean-continent transition zones and (ultra)high-pressure metamorphism

A lithostratigraphic association consisting of serpentinized mantle rocks, continent-derived allochthons, mid-oceanic ridge gabbros of Jurassic age and post-rift sediments, typical of an ocean-continent transition, is found in the eclogitic Piemonte units, in the Western Alps. In situ U-Pb geochronology was performed on zircons from an orthogneiss sampled at the bottom of a sliver of continental basement, in contact with serpentinites. Primary magmatic zircons of Permian age were overgrown by a second generation of zircon at ca. 166-150 Ma, likely related to melt infiltration associated with the intrusion of the underlying gabbroic body. This indicates that continental basement slices and oceanic basement rocks were already juxtaposed in the Jurassic and they were probably part of hyper-extended crust related to the opening of the Tethys. Therefore, the complex lithological association described here, which is also characteristic of several (ultra) high-pressure mélange zones worldwide, was acquired prior to the orogenic event, during which it was only partly reworked. Ocean-continent transitions are in positions favorable to reach (ultra)high-pressure conditions, following negatively buoyant oceanic lithosphere into subduction, and then being accreted to the orogen, in response to the arrival of more buoyant continental lithosphere, resisting subduction. The ocean-continent transition is now found in the immediate footwall of a 500-m-thick shear zone, which accommodated multiple episodes of deformation during Eocene-Oligocene time, suggesting an important link between Alpine deformation and rift-related structures

Was the Valaisan basin floored by oceanic crust? Evidence of permian magmatism in the Versoyen Unit (Valaisan Domain, NW Alps)

The Versoyen Unit (Western Alps) and its mafic rocks have been long considered the remnants of the oceanic crust that supposedly floored the Valaisan basin during the Cretaceous. Here we present U-Pb dating of zircons from a metaleucogabbro and a metagranite from the Versoyen Unit challenging this view. Magmatic zircon cores yield Permian ages of 267±1 and 272±2 Ma, respectively, which are interpreted as dating the crystallization of the magmas. Older inherited crystals and rare Cretaceous zircon rims (̃110-100 Ma) are also present. The young rims are characterized by very high U and REE contents. We speculate that the Cretaceous ages are related to a thermal/fluid event possibly induced by the opening of the Valaisan basin. The proposed Permian age for the Versoyen magmatism, together with the lack of geochronological evidence for a Cretaceous oceanic crust in the Valaisan domain sensu stricto, may force to reconsider the oceanic nature of the Valaisan Basin. We propose a model in which the Versoyen Unit is unrelated to and pre-dates the extensional tectonics that led to the formation of the Valaisan Basin and the Cretaceous deposition of sediments on this Permian basement. The Permian ages for the Versoyen intrusives correlate with extensive Permian intra-plate magmatism related to lithospheric stretching prior to the break-up of Pangea. The Versoyen Unit becomes the most external Alpine terrane that displays traces of this Permian basic magmatism. Traces of Cretaceous magmatism are preserved in the more internal Chiavenna and Balma units, located in the Central and Western Alps, respectively. However, several lines of evidence suggest that such units may have been unrelated to the Valaisan Basin. Therefore, we propose a new palaeogeographic scenario for the western Tethys, where two independent basins, the Valaisan Basin and the Chiavenna/Balma Ocean, were located between the Briançonnais micro-Plate and the European Plate sensu stricto

Constraints on the thermal evolution of the Adriatic margin during Jurassic continental break-up: U-Pb dating of rutile from the Ivrea-Verbano Zone, Italy

The Ivrea–Verbano Zone (IVZ), northern Italy, exposes an attenuated section through the Permian lower crust that records high-temperature metamorphism under lower crustal conditions and a protracted history of extension and exhumation associated partl

Constraints on the thermal evolution of the Adriatic margin during Jurassic continental break-up: U-Pb dating of rutile from the Ivrea-Verbano Zone, Italy

The Ivrea-Verbano Zone (IVZ), northern Italy, exposes an attenuated section through the Permian lower crust that records high-temperature metamorphism under lower crustal conditions and a protracted history of extension and exhumation associated partly with the Jurassic opening of the Alpine Tethys ocean. This study presents SHRIMP U-Pb geochronology of rutile from seven granulite facies metapelites from the base of the IVZ, collected from locations spanning ~35km along the strike of Paleozoic fabrics. Rutile crystallised during Permian high-temperature metamorphism and anatexis, yet all samples give Jurassic rutile U-Pb ages that record cooling through 650-550°C. Rutile age distributions are dominated by a peak at ~160Ma, with a subordinate peak at ~175Ma. Both ~160 and ~175Ma age populations show excellent agreement between samples, indicating that the two distinctive cooling stages they record were synchronous on a regional scale. The ~175Ma population is interpreted to record cooling in the footwall of rift-related faults and shear zones, for which widespread activity in the Lower Jurassic has been documented along the western margin of the Adriatic plate. The ~160Ma age population postdates the activity of all known rift-related structures within the Adriatic margin, but coincides with extensive gabbroic magmatism and exhumation of sub-continental mantle to the floor of the Alpine Tethys, west of the Ivrea Zone. We propose that this ~160Ma early post-rift age population records regional cooling following episodic heating of the distal Adriatic margin, likely related to extreme lithospheric thinning and associated advection of the asthenosphere to shallow levels. The partial preservation of the ~175Ma age cluster suggests that the post-rift (~160Ma) heating pulse was of short duration. The regional consistency of the data presented here, which is in contrast to many other thermochronometers in the IVZ, demonstrates the value of the rutile U-Pb technique for probing the thermal evolution of high-grade metamorphic terrains. In the IVZ, a significant decoupling between Zr-in-rutile temperatures and U-Pb ages of rutile is observed, with the two systems recording events ~120Ma apart