Abstract
Microscale dating of distinct domains in minerals that contain relics of multiple metamorphic events is a key tool to characterize the polyphase evolution of complex metamorphic terranes. Zircon and allanite from five metasediments and five metaintrusive high-pressure (HP) rocks from the Eclogite Micaschist Complex of the Sesia Zone were dated by SIMS and LA-ICP-MS. In the metasediments, zircon systematically preserves detrital cores and one or two metamorphic overgrowths. An early Permian age is obtained for the first zircon rim in metasediments from the localities of Malone Valley, Chiusella Valley and Monte Mucrone (292 ± 11, 278.8 ± 3.6 and 285.9 ± 2.9 Ma, respectively). In the Malone Valley and Monte Mucrone samples, the early Permian ages are attributed to high-temperature metamorphism and coincide with the crystallization ages of associated mafic and felsic intrusions. This implies that magmatism and metamorphism were coeval and associated to the same tectono-metamorphic extensional event. In the Malone Valley, allanite from a metasediment is dated at 241.1 ± 6.1 Ma and this age is tentatively attributed to a metasomatic/metamorphic event during Permo-Triassic extension. Outer zircon rims with a late Cretaceous age (67.4 ± 1.9 Ma) are found only in the micaschist from Monte Mucrone. In metagabbro of the Ivozio Complex, zircon cores yield an intrusive age for the protolith of 340.7 ± 6.8 Ma, whereas Alpine allanite are dated at 62.9 ± 4.2 and 55.3 ± 7.3 Ma. The Cretaceous ages constrain the timing of the HP metamorphic stage. The presence of zircon overgrowth only in the central area of the Eclogite Micaschist Complex is attributed to local factors such as (1) multiple fluid pulses at HP that locally enhanced zircon dissolution and recrystallization, and (2) slightly higher temperatures reached in this area during HP metamorphism.
Highlights
In the Sesia Zone, three distinct magmatic and metamorphic events have been recorded and their age can be retrieved by in situ dating of zircon and allanite grains from various rock types
In the Ivozio and Cima di Bonze metagabbros, mineralogical or geochronological evidence of a Permian metamorphism are lacking, while they have been pervasively overprinted at HP conditions during Alpine subduction
This concurs with similar scenarios proposed for other closely related tectonic units, e.g. the Ivrea Zone and the Dent Blanche Tectonic System, indicating that a high-thermal regime with associated magmatism broadly affected this portion of the future Adriatic continental margin during the early Permian
Summary
The Sesia Zone (SZ) in the Italian Western Alps represents a slice of Adriatic continental crust that underwent blueschist to eclogite facies metamorphism during convergence between the African and European plates.Since more than 50 years, extensive structural and petrological studies have been focused on unravelling the pressure–temperature (P–T) evolution of the SZ during the Editorial handling: Paola Manzotti.Full list of author information is available at the end of the article subduction, exhumation and continental collision of the Alpine orogenic cycle (e.g. Dal Piaz et al 1972; Compagnoni 1977; Pognante et al 1980; Oberhänsli et al 1985; Zucali et al 2004; Regis et al 2014; Giuntoli et al 2018a).The SZ is composed by large sections of preAlpine basement that record evolution through the Variscan collision and post-Variscan extension and rifting, before being involved in the Alpine cycle (e.g. Lardeaux and Spalla 1991; Rubatto et al 1999; Giuntoli et al 2018a, 2018b). Since more than 50 years, extensive structural and petrological studies have been focused on unravelling the pressure–temperature (P–T) evolution of the SZ during the Editorial handling: Paola Manzotti. Full list of author information is available at the end of the article subduction, exhumation and continental collision of the Alpine orogenic cycle The SZ is composed by large sections of preAlpine basement that record evolution through the Variscan collision and post-Variscan extension and rifting, before being involved in the Alpine cycle (e.g. Lardeaux and Spalla 1991; Rubatto et al 1999; Giuntoli et al 2018a, 2018b). Dating of robust mineral relics that may survive multiple metamorphic events is a key tool to study terranes mmons.org/licenses/by/4.0/
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