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9. A hidden Oligocene pluton linked to the Periadriatic Fault System beneath the Permian Bressanone pluton, eastern Southern Alps.

Author

Visonà, Dario, Nimis, Paolo, Fioretti, Anna M., Massironi, Matteo, and Villa, Igor M.

Subjects
*IGNEOUS intrusions, *OLIGOCENE Epoch, *GRANODIORITE, *DIORITE, *ORTHOCLASE, *MAGMATISM, *METASOMATISM
Abstract

The Bressanone (Brixen) pluton, cropping out at the culmination of the Southalpine indenter between the North Giudicarie and the Pustertal-Gailtal faults, is mainly composed of Permian granodiorite to granite, with minor gabbros and diorites in its southern part. New U–Pb SHRIMP zircon ages reveal two distinct crystallization episodes at 289.7 ± 3.2 and 280 ± 2.2 Ma, respectively. The pluton is affected by a hydrothermal potassic to sodic + Cu metasomatic alteration, which has long been ascribed to a late phase of the Permian magmatism. In contrast with this hypothesis, we report new 39Ar–40Ar data for different generations of metasomatic K-feldspar, which indicate formation ages between 35.3 ± 0.3 and 27.8 ± 0.5 Ma. This interval overlaps with the ages of the widespread 'Periadriatic' calc-alkaline magmatism, which extends from the Western to the Eastern Alps straddling the Periadriatic Fault System. The observed hydrothermalism has geochemical characteristics compatible with those of the coeval calc-alkaline Periadriatic magmatism. These data altogether suggest the release of fluids from a hidden intrusion during the main stage of the Alpine orogenic magmatism. Our results provide the first evidence of Oligocene magmatic activity in the Southern Alps east of the Giudicarie Line, bridging the gap between the western-central and eastern Alpine magmatic plutons. A fairly continuous Oligocene magmatic belt straddling the Periadriatic Fault System is consistent with a triggering of the magmatism by slab steepening or slab break-off, either of which are considered to be an essential driver for the Miocene lithospheric rearrangement in the Eastern Alps. Our finding is particularly relevant given that the outcropping area of the Bressanone pluton is centred above the imaged subducting lithosphere gap that separates the Western-Central and the Eastern Alps, hence at a location where mantle upwelling should have been easier although no relevant magmatism was found to date. [ABSTRACT FROM AUTHOR]

Published
2022
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12. Occurrence and Origin of Andalusite in Peraluminous Felsic Igneous Rocks

Author

Clarke, D. Barrie, Dorais, Michael, Barbarin, Bernard, Barker, Dan, Cesare, Bernardo, Clarke, Geoffrey, El Baghdadi, Mohamed, Erdmann, Saskia, Förster, Hans-Jürgen, Gaeta, Mario, Gottesmann, Bärbel, Jamieson, Rebecca A., Kontak, Daniel J., Koller, Friedrich, Gomes, Carlos Leal, London, David, Morgan, George B., VI, Neves, Luis J. P. F., Pattison, David R. M., Pereira, Alcides J. S. C., Pichavant, Michel, Rapela, Carlos W., Renno, Axel D., Richards, Simon, Roberts, Malcolm, Rottura, Alessandro, Saavedra, Julio, Sial, Alcides Nobrega, Toselli, Alejandro J., Ugidos, Jose M., Uher, Pavel, Villaseca, Carlos, Visonà, Dario, Whitney, Donna L., Williamson, Ben, and Woodard, Henry H.

Published
2005

23. Structural evolution, metamorphism and melting in the Greater Himalayan Sequence in central-western Nepal

Author

Carosi, Rodolfo, Montomoli, Chiara, Iaccarino, Salvatore, and Visonà, Dario

Abstract

Joining geological mapping, structural analysis, petrology and geochronology allowed the internal architecture of the Greater Himalayan Sequence (GHS) to be unraveled. Several top-to-the-south/SW tectonic–metamorphic discontinuities developed at the regional scale, dividing it into three main units exhumed progressively from the upper to the lower one, starting from c.40 Ma and lasting for several million years. The activity of shear zones has been constrained and linked to the pressure–temperature–time–deformation (P–T–t–D) evolution of the deformed rocks by the use of petrochronology. Hanging wall and footwall rocks of the shear zones recorded maximum P–Tconditions at different times. Above the Main Central Thrust, a cryptic tectonometamorphic discontinuity (the High Himalayan Discontinuity (HHD)) has been recognized in Central-Eastern Himalaya.The older shear zone, that was active at c.41–28 Ma, triggered the earlier exhumation of the uppermost GHS and allowed the migration of melt, which was produced at peak metamorphic conditions and subsequently produced in abundance at the time of the activation of the HHD. Production of melt continued at low pressure, with nearly isobaric heating leading to the genesis and emplacement of andalusite- and cordierite-bearing granites.The timing of the activation of the shear zones from deeper to upper structural levels fits with an in-sequence shearing tectonic model for the exhumation of the GHS, further affected by out-of-sequence thrusts.

Published
2019
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25. Normal-senseZZ shear zones in the core of the Higher Himalayan Crystallines (Bhutan Himalaya): evidence for extrusion?

Author

Carosi, R, Montomoli, C, Rubatto, Daniela, Visonà, Dario, Carosi, R, Montomoli, C, Rubatto, Daniela, and Visonà, Dario

Abstract

Recent fieldwork in western Bhutan, dedicated to unravelling the tectonic structure of the mid-crustal rocks, indicates a complex deformation pattern in the Greater Himalayan Slab (GHS). A system of normal shear zones, striking NE-SW and steeply to moderately dipping to the SE, has been recognized within this extruding slab or wedge of crystalline rocks. The zones are characterized by well developed shear-sense indicators pointing to a top-down-to-SE sense of shear. The main Barrovian metamorphic minerals are bent and stretched by extensional shear bands and associated deformation mechanisms indicate a range of brittle-ductile deformation conditions. Normal shear zones are concentrated in the middle-upper part of the GHS and indicate a thrust-transport-parallel lengthening of the core itself. Vorticity analysis highlights a non-coaxial flow with pure and simple shear acting together during deformation (mean vorticity number bracketed between 0.63 and 0.76). These data, when compared to available data near the tectonic boundaries of the GHS, indicate an increasing component of pure shear towards the interior of the GHS. The ages of zircon overgrowths and monazites from a slightly deformed granite, 20.5 ± 0.5 Ma, and a mylonitic granite deformed into the shear zones, 17.0 ± 0.2 Ma, bracket the age of shear zone formation at close to 17 Ma. According to our data, the normal shear zones could well accommodate the pure shear component of deformation localized in the inner part of the extruding wedge/slab and is compatible with a channel flow model.

Published
2006

26. Surgeon island granite SHRIMP zircon ages: a clue for the Cambrian tectonic setting and evolution of the Palaeopacific margin of Gondwana (northern Victoria Land, Antarctica)

Author

Fioretti, A, Caponi, G., Black, Lance, Varne, R, Visonà, Dario, Fioretti, A, Caponi, G., Black, Lance, Varne, R, and Visonà, Dario

Abstract

In northern Victoria Land (NVL), Antarctica, the palaeopacific margin of Gondwana is made up of the inboard Wilson (WT) and the two outboard Bowers (BT) and Robertson Bay (RBT) terranes. The occurrence of a Cambro-Ordovician magmatic arc in the WT argues

Published
2005

27. Grenville-age magmatism at the South Tasman Rise (Australia): A new piercing point for the reconstruction of Rodinia

Author

Fioretti, Anna Maria, Black, Lance, Foden, J., Visonà, Dario, Fioretti, Anna Maria, Black, Lance, Foden, J., and Visonà, Dario

Abstract

A U-Pb zircon sensitive high-resolution ion microprobe age of 1119.4 ± 8.5 Ma, obained from a quartz syenite dredged from the South Tasman Rise (Australia), provides the first direct evidence of the presence of Grenville-age magmatic rocks along the cent

Published
2005

33. Occurrence and Origin of Andalusite in Peraluminous Felsic Igneous Rocks

Author

CLARKE, D. BARRIE, primary, DORAIS, MICHAEL, additional, BARBARIN, BERNARD, additional, BARKER, DAN, additional, CESARE, BERNARDO, additional, CLARKE, GEOFFREY, additional, EL BAGHDADI, MOHAMED, additional, ERDMANN, SASKIA, additional, FÖRSTER, HANS-JÜRGEN, additional, GAETA, MARIO, additional, GOTTESMANN, BÄRBEL, additional, JAMIESON, REBECCA A., additional, KONTAK, DANIEL J., additional, KOLLER, FRIEDRICH, additional, LEAL GOMES, CARLOS, additional, LONDON, DAVID, additional, MORGAN, GEORGE B., additional, NEVES, LUIS J. P. F., additional, PATTISON, DAVID R. M., additional, PEREIRA, ALCIDES J. S. C., additional, PICHAVANT, MICHEL, additional, RAPELA, CARLOS W., additional, RENNO, AXEL D., additional, RICHARDS, SIMON, additional, ROBERTS, MALCOLM, additional, ROTTURA, ALESSANDRO, additional, SAAVEDRA, JULIO, additional, SIAL, ALCIDES NOBREGA, additional, TOSELLI, ALEJANDRO J., additional, UGIDOS, JOSE M., additional, UHER, PAVEL, additional, VILLASECA, CARLOS, additional, VISONÀ, DARIO, additional, WHITNEY, DONNA L., additional, WILLIAMSON, BEN, additional, and WOODARD, HENRY H., additional

Published
2004
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34. Leucogranite intruding the South Tibetan Detachment in western Nepal: implications for exhumation models in the Himalayas.

Author

Carosi, Rodolfo, Montomoli, Chiara, Rubatto, Daniela, and Visonà, Dario

Subjects
ALASKITE, DETACHMENT reactions, SHEAR zones, URANIUM-lead dating
Abstract

The most popular models regarding the exhumation of the Greater Himalayan Sequence (GHS), such as extrusion, channel flow, critical taper and wedge extrusion, require prolonged activity of the two bounding shear zones and faults, the Main Central Thrust (MCT) and the South Tibetan Detachment (STD). We present the crystallization age of an undeformed leucogranite that intrudes both the GHS and the Tethyan Himalaya Sequence (THS). Zircon and monazite U-Pb ages in the leucogranite give ages between 23 and 25 Ma constraining, at that time, the end of shearing along the STD. Our results limit the contemporaneous activity of the MCT and STD to a short period of time (~1-2 Ma) and thus argue against exhumation models requiring prolonged contemporaneous activity of the MCT and STD. [ABSTRACT FROM AUTHOR]

Published
2013
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35. Eocene partial melting recorded in peritectic garnets from kyanite-gneiss, Greater Himalayan Sequence, central Nepal

Author

Carosi, Rodolfo, Montomoli, Chiara, Langone, Antonio, Turina, Alice, Cesare, Bernardo, Iaccarino, Salvatore, Fascioli, Luca, Visonà, Dario, Ronchi, Ausonio, and Rai, Santa Man

Abstract

Anatectic melt inclusions (nanogranites and nanotonalites) have been found in garnet of kyanite-gneiss at the bottom of the Greater Himalayan Sequence (GHS) along the Kali Gandaki valley, central Nepal, c.1 km structurally above the Main Central Thrust (MCT). In situU–Th–Pb dating of monazite included in garnets, in the same structural positions as melt inclusions, allowed us to constrain partial melting starting at c.41–36 Ma. Eocene partial melting occurred during prograde metamorphism in the kyanite stability field (Eo-Himalayan event). Sillimanite-bearing mylonitic foliation wraps around garnets showing a top-to-the-SW sense of shear linked to the MCT ductile activity and to the exhumation of the GHS. These findings highlight the occurrence of an older melting event in the GHS during prograde metamorphism in the kyanite stability field before the more diffuse Miocene melting event.The growth of prograde garnet and kyanite at 41–6 Ma in the MCT zone, affecting the bottom of the GHS, suggests that inverted metamorphism in the MCT zone and folded isograds in the GHS should be carefully proved with the aid of geochronology, because not all Barrovian minerals grew during the same time span and they grew in different tectonic settings.

Published
2015
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39. Tectonic discontinuities within the Greater Himalayan Sequence in Western Nepal: insights on exhumation mechanisms.

Author

Carosi, Rodolfo, Montomoli, Chiara, and Visonà, Dario

Subjects
KINEMATICS, SHEAR zones, EXHUMATION, INTERMENT
Abstract

The article explores the kinematics of several shear zones within the Greater Himalayan Sequence (GHS) in Western Nepal. These include a ductile top-to-the southeast (SE) shear zone north of Jumla village and the Toijem shear zone located in Dolpo village, said to be responsible for the exhumation of the hanging wall rocks before the period of exhumation by channel flows caused by the contemporaneous activity of the Main Central Thrust (MCT) and the South Tibetan Detachment System (STDS).

Published
2011

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