Deep subduction, melting, and fast cooling of metapelites from the Cima Lunga Unit, Central Alps

Piccoli, Francesca; Lanari, Pierre; Hermann, Jörg; Pettke, Thomas (2022). Deep subduction, melting, and fast cooling of metapelites from the Cima Lunga Unit, Central Alps. Journal of metamorphic geology, 40(1), pp. 121-143. Wiley 10.1111/jmg.12621

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The Cima Lunga unit in the Central Alps is dominated by quartzofeldspathic gneisses
with subordinate mafic, ultramafic, and metacarbonate rocks. Only mafic and ultramafic
lithologies were thought to preserve clear evidence of Alpine high-P
metamorphism.
This led to the questions of whether the different rock types were subducted
and exhumed as a coherent unit or underwent different pressure–temperature
(P–T)
histories. New petrological and geochemical data from a metapelite associated with
garnet peridotite from Cima di Gagnone (Cima Lunga unit, Switzerland) were obtained
using major and trace element mapping. Complex zoning patterns in garnet and
white mica are observed. In particular, high Ti content in phengite and increasing P,
Zr, and HREE contents in pyrope-rich
garnet indicate that this metapelite underwent
high-P
and high-T
(HP–HT)
metamorphism involving fluid-fluxed
partial melting.
A P–T
path is reconstructed by combining textural analysis with petrological–geochemical
data and thermodynamic simulations. We show that the mineral record
preserves an evolution from prograde to HP–HT
peak conditions (2.7 ± 0.1 GPa and
800℃) followed by near-isobaric
cooling (~2.5 GPa and 700–750℃)
prior to decompression
(1.0 GPa and ~620℃). The reconstructed P–T
path suggests that the studied
metapelites were subducted to depths where the slab gets heated by proximity to
asthenospheric mantle related to slab break-off.
This heating resulted in the dehydration
of chlorite-to
garnet peridotite and the liberated fluids triggered partial melting
in the associated metapelites, which might have favoured the fast exhumation of the
entire Cima Lunga unit. Metapelites and garnet peridotite from Cima di Gagnone
underwent a common prograde to peak and retrograde P–T
path without significant
tectonic pressure difference between the different lithologies, and deviation from
lithostatic pressure is excluded. Lastly, the peak metamorphic conditions of metapelite
from Cima di Gagnone are comparable with P–T
estimates of ultramafic lithologies
from the southern Adula nappe and the Dascio Bellinzona zone, thus opening
new scenarios for the geodynamic interpretation of the Central Alps.

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