La Provincia Magmatica Cenozoica Sudalpina, nota in letteratura come Provincia Vulcanica Veneta (VVP; NE Italia), è una delle aree magmatiche più vaste della placca Adria. Procedendo all’interno della VVP, da nord-ovest verso sud-est, si riconoscono cinque distretti magmatici: Val d’Adige, Monti Lessini, Marosticano, Colli Berici e Colli Euganei. Solamente in quest’ultimo distretto le rocce vulcaniche e subvulcaniche variano da subordinatamente basaltiche, a prevalentemente acide, soprattutto quarzo-trachiti e rioliti, mentre nei restanti distretti della VVP le vulcaniti affioranti sono esclusivamente lave indifferenziate, varianti in composizione da mela-nepheliniti a tholeiiti quarzo-normative. Solitamente i basalti alcalini ospitano peridotiti mantelliche. Nonostante gli approfonditi studi petrologici su questa provincia, i dati geocronologici delle attività magmatiche di ogni distretto sono scarsamente definiti o persino totalmente mancanti. La combinazione delle già note età biostratigrafiche con i nuovi dati radioisotopici 40Ar/39Ar, ottenuti dai prodotti magmatici della VVP, ha permesso la ricostruzione dell’evoluzione temporale dell’attività magmatica, la quale è stata discontinua durante un arco di tempo di circa 30 Ma (dal tardo Paleocene fino al primo Miocene). Inoltre, i) nuove tomografie isotropiche e anisotropiche della regione alpina e ii) recenti modelli numerici, rivelanti la rarità del magmatismo indotto dallo slab breakoff, hanno permesso la costruzione di un nuovo modello per il processo innescante il magmatismo della VVP. Quest’ultimo coinvolge la risalita di un flusso toroidale/poloidale indotto dal progressivo ritiro e verticalizzazione dello slab europeo subducente in seguito alla collisione continentale Adria-Europa. La migrazione sud-orientale del magmatismo potrebbe essere dovuta al movimento della placca Adria più rapido del ritiro dello slab europeo. La geochimica dei campioni mafici della VVP mostra una tipica firma OIB, oltre ad anomalie positive in Ba, Sr e P, probabilmente ereditate da una sorgente mantellica carbonatitica. Quest’ultima considerazione parrebbe confermata dalla caratteristiche geochimiche di una nuova suite di xenoliti mantellici provenienti dal distretto del Marosticano, che rivelano un’innaspettata origine da un mantello di tipo on-craton avente un’impronta da metasomatismo di fusi carbonatitici/silicatici ricchi in CO2. Secondo gli studi precedenti, gli xenoliti mantellici della Val d’Adige e dei Monti Lessini esibiscono caratteristiche di mantello litosferico off-craton variabilmente influenzato da metasomatismo silicatico Na-alcalino. Nonostante ciò, le porzioni di mantello on- e off-craton della VVP mostrano simili età Re/Os Archeane/Proterozoiche, suggerendo che il domino mantellico del Marosticano potrebbe essere interpretato come resti di una porzione di cratone Archeano/Proterozoico, la cui firma non è stata elisa dal metasomatismo carbonatitico/silicatico ricco in CO2, mentre gli xenoliti dei Monti Lessini e della Val d’Adige rappresenterebbero resti di domini circum-cratonici composizionalmente ringiovaniti dall’infiltrazione di fusi astenosferici, la cui risalita potrebbe essere stata indotta dal ritiro dello slab europeo dopo la collisione Adria/Europa.

The Cenozoic Southalpine Magmatic Province, known as Veneto Volcanic Province (VVP; NE Italy) is one of the widest magmatic areas within the Adria plate. It is formed by five main magmatic districts that are from north-west to south-east: Val d’Adige, Lessini Mts., Marosticano, Berici Hills, and Euganean Hills. Only in this last district volcanic and subvolcanic rocks range from subordinate basalts to prevalently acidic types, mostly quartz-trachytes and rhyolites, while for the rest of the VVP districts the outcropping volcanics are wholly undifferentiated lavas, ranging in composition from mela-nephelinites to quartz-normative tholeiites. Commonly alkaline basalts host mantle peridotites. Despite the extensive petrological studies on this province, the geochronological data of the related magmatic activities for each district are poorly defined or even totally missing. The combination of previous biostratigraphic data with new 40Ar/39Ar radioisotopic ages of VVP magmatic products allowed to reconstruct the temporal evolution of the magmatic activity, which was discontinuous and covers a time-span of about 30 Ma (from late Paleocene to early Miocene). In addition, i) new isotropic and anisotropic tomographies for the Alpine region and ii) recent numerical modelling revealing the rarity of the magmatism induced by slab breakoff allow to develop a new model for mechanism triggering the VVP magmatism. This involves the upwelling of toroidal/poloidal flow induced by the progressive retreat and steepness of the subducting European slab after the Adria-Europe continent collision. A likely explanation for the southeastward migration of the magmatism can be accounted for the overidding Adria plate moving faster than the European slab retreatment. The geochemistry of the VVP mafic samples shows a typical OIB signature, as well as positive Ba, Sr, and P anomalies, probably inherited by a carbonatite metasomatized mantle source. This last consideration seems to be confirmed by the geochemical features of the new occurrences of Marosticano mantle xenoliths that reveal an unexpected on-craton type mantle with fingerprint of carbonatitic/CO2-rich silicatic melt metasomatism. According to the previous studies Val d’Adige and Lessini Mts. mantle xenoliths exhibit characteristics of off-craton lithospheric mantle variably affected by Na-alkaline silicatic metasomatism. However, on- and off-craton VVP mantle portions show similar Archean/Proterozoic Re-Os ages, suggesting that the Marosticano mantle domain could be interpreted as the vestige of an Archean/Proterozoic cratonic keel, whose signature was not erased by the carbonatitic/CO2-rich silicatic metasomatism, whereas Lessini Mts. and Val d’Adige xenoliths are remnants of circum-cratonic domains compositionally rejuvenated by infiltration of asthenospheric-derived melts, which upwelling could be induced by the retreatment of the European slab after the Adria/Europe collision.

Petrological and geochronological study of magmatic products and mantle xenoliths from Cenozoic Southalpine Magmatic Province (North-East Italy)

BROMBIN, Valentina
2018

Abstract

The Cenozoic Southalpine Magmatic Province, known as Veneto Volcanic Province (VVP; NE Italy) is one of the widest magmatic areas within the Adria plate. It is formed by five main magmatic districts that are from north-west to south-east: Val d’Adige, Lessini Mts., Marosticano, Berici Hills, and Euganean Hills. Only in this last district volcanic and subvolcanic rocks range from subordinate basalts to prevalently acidic types, mostly quartz-trachytes and rhyolites, while for the rest of the VVP districts the outcropping volcanics are wholly undifferentiated lavas, ranging in composition from mela-nephelinites to quartz-normative tholeiites. Commonly alkaline basalts host mantle peridotites. Despite the extensive petrological studies on this province, the geochronological data of the related magmatic activities for each district are poorly defined or even totally missing. The combination of previous biostratigraphic data with new 40Ar/39Ar radioisotopic ages of VVP magmatic products allowed to reconstruct the temporal evolution of the magmatic activity, which was discontinuous and covers a time-span of about 30 Ma (from late Paleocene to early Miocene). In addition, i) new isotropic and anisotropic tomographies for the Alpine region and ii) recent numerical modelling revealing the rarity of the magmatism induced by slab breakoff allow to develop a new model for mechanism triggering the VVP magmatism. This involves the upwelling of toroidal/poloidal flow induced by the progressive retreat and steepness of the subducting European slab after the Adria-Europe continent collision. A likely explanation for the southeastward migration of the magmatism can be accounted for the overidding Adria plate moving faster than the European slab retreatment. The geochemistry of the VVP mafic samples shows a typical OIB signature, as well as positive Ba, Sr, and P anomalies, probably inherited by a carbonatite metasomatized mantle source. This last consideration seems to be confirmed by the geochemical features of the new occurrences of Marosticano mantle xenoliths that reveal an unexpected on-craton type mantle with fingerprint of carbonatitic/CO2-rich silicatic melt metasomatism. According to the previous studies Val d’Adige and Lessini Mts. mantle xenoliths exhibit characteristics of off-craton lithospheric mantle variably affected by Na-alkaline silicatic metasomatism. However, on- and off-craton VVP mantle portions show similar Archean/Proterozoic Re-Os ages, suggesting that the Marosticano mantle domain could be interpreted as the vestige of an Archean/Proterozoic cratonic keel, whose signature was not erased by the carbonatitic/CO2-rich silicatic metasomatism, whereas Lessini Mts. and Val d’Adige xenoliths are remnants of circum-cratonic domains compositionally rejuvenated by infiltration of asthenospheric-derived melts, which upwelling could be induced by the retreatment of the European slab after the Adria/Europe collision.
BONADIMAN, Costanza
COLTORTI, Massimo
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11392/2488173
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