Petrochronology of the UHP Chasteiran Unit (northern Dora-Maira Massif)

The Chasteiran Unit in the northern Dora-Maira Massif reached UHP conditions in the chloritoid-coesite stability field. The chemical and isotopic behaviour of zircon, garnet, and rutile was explored in a metapelite in order to reconstruct a timeline for the metamorphic evolution of this Unit.

Zircon crystals display detrital cores and thin (< 5 mm) undatable metamorphic rims. The dominant zircon population consists of Late Neoproterozoic (⁓600 Ma) magmatic grains whereas the youngest zircon cluster is Ordovician in age (∼470 Ma).

Garnet records three main growth stages: initial growth during a prograde P and T increase in the quartz stability field (2.5‒2.7 GPa at 470‒500 °C, inner core ‒ stage 1), peak growth in the coesite stability field (2.7‒2.8 GPa at 510‒530 °C, outer core ‒ stage 2), and final growth of the garnet rim between 2.3 GPa 520 °C and 1.5 GPa 510 °C (stage 3), contemporaneously with lawsonite consumption coupled with fluid production. LA-ICP-MS U-Pb dating of garnet indicates two distinct stages of growth for garnet cores and rims at ∼61 Ma and ∼43 Ma, respectively. The time interval separating the growth of garnet core and rim is consistent with our thermodynamic modelling, which indicates the absence of garnet growth during the initial stage of exhumation, between 2.7. GPa and 2.3 GPa.

Rutile is found both as inclusions in garnet and in the matrix. Rare inclusions of jadeite and Si-rich muscovite constrain rutile growth during burial at a minimum P of 2.0 GPa. Inclusions of rutile in garnet are commonly surrounded by fracture and some crystals display ilmenite exsolution lamellae, suggesting that despite their mode of occurrence, they might have behaved as an open system during later events. Rutile consumption took place during exhumation, as suggested by the increase in Ti content in garnet and muscovite rims and thermodynamic modelling. Rutile in the matrix is partially replaced by ilmenite corona, which developed at P < 1.5 GPa, after garnet growth. SIMS U-Pb dating of rutile, irrespective of its petrographic mode of occurrence, yields a date of ∼37 Ma.

Our geochronological data puts new constraints on the metamorphic evolution of the Chasteiran Unit, which will be discussed in the context of published chronological data and P‒T estimates from the Dora-Maira Massif.