Local variations of metamorphic record from compositionally heterogeneous rocks (Cima di Gagnone, Central Alps): Inferences on exhumation processes of (U)HP–HT rocks

Corvò, Stefania and Maino, Matteo and Langone, Antonio and Schenker, Filippo and Piazolo, Sandra and Seno, Silvio (2021) Local variations of metamorphic record from compositionally heterogeneous rocks (Cima di Gagnone, Central Alps): Inferences on exhumation processes of (U)HP–HT rocks. Lithos, 390-39. p. 106126. ISSN 0024-4937

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Abstract

The record of metamorphic conditions is commonly highly heterogeneous in spatially close rocks of different composition and rheology. The well-studied Cima di Gagnone area in the Central Alps represents such an example, as relatively small, 1– to 100 s-meter scale, ultrahigh pressure and high–temperature ultramafic lenses are enveloped within amphibolite–facies metasediments. We present new field observations, microstructural and petrological analyses, and thermodynamic modelling results on these metasediments, showing that these rocks generally experienced medium pressure and medium temperature conditions of 1.0–1.2 GPa and 640– 700 °C, followed by a retrograde stage around 0.6–0.8 GPa and 600–675 °C. However, a fewsamples fromthe immediate proximity of the ultramafic lenses recordsignificantly higher P–T conditions of 1.3–1.7 GPa and 750–850 °C, approaching the high pressure and high temperature conditions of the ultramafic bodies (1.5–3.1 GPa, 650–850 °C). Mineral/bulk chemistry changes during growth of newmineral phases hint to localmelt/fluid interaction (i.e., metasomatism) between metasediments and ultramafics during the high temperature deformation. Preliminary U-Pb LA–ICP–MS dating suggests that zircon grains from the metasomatic reaction zone have been fully re–equilibrated during the early stage of Alpine exhumation (~36 Ma), while the large part of the metasediments records only pre–Alpine ages. We finally recast these new data into the regional P–T–t–D paths and discuss the consequences of these findings for understanding the exhumation processes of HP rocks. We found different local equilibria (ΔP up to 2 GPa; ΔT up to 160 °C) that cannot be explained simply by retrograde metamorphism or by tectonic amalgamation. Rather, our work suggests a significant role of deformation in producing variable pressure and temperature record within the rocks.

Item Type:	Scientific journal article, Newspaper article or Magazine article
Subjects:	Physical sciences > Geology Physical sciences > Geology > Earth science Physical sciences > Geology > Earth science > Geoscience
Department/unit:	Dipartimento ambiente costruzioni e design > Istituto scienze della Terra
Depositing User:	Filippo Schenker
Date Deposited:	03 Jan 2022 08:37
Last Modified:	03 Jan 2022 08:39
URI:	http://repository.supsi.ch/id/eprint/13132

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