TRACKING THE P–T PATH OF PRECAMBRIAN ECLOGITE USING PSEUDOSECTION, Ti-IN-QUARTZ AND Zr-IN-RUTILE THERMOBAROMETRY

Lorraine Tual (1) , Charlotte Möller (1) , Pavel Pitra (2) , Martin Whitehouse (3)

1 Department of geology
2 GR - Géosciences Rennes
3 NRM - Swedish Museum of Natural History

Sveconorwegian eclogite occur as a nappe within the high-grade metamorphic region in southern Sweden, which constitutes a window into the deepest part of this Precambrian mountain belt. Distinct microstructural domains (i.e., garnet core, garnet rim, and matrix) in a Fe-Ti-rich eclogite variety contain abundant quartz, rutile and zircon. A pseudosection approach was first applied and compared to results from a combination of Zr-in-rutile and Ti-in-quartz. The pressure input used for both thermometers was first deduced for each microstructural domain from the pseudosection. For the garnet core, Zr-in-rutile yields temperatures of 700-715°C and Ti-inquartz ~ 635°C at 7 kbar. For the garnet rim, temperatures of 760-790°C (Zr-in-rutile) and 740-890°C (Ti-inquartz) at 12-18 kbar were calculated. Matrix rutile recorded temperatures of ~ 810°C, while quartz recorded temperatures up to ~ 890°C. Additionally, direct combination of Ti content in quartz and Zr content in rutile isopleths (i.e., independent from the pseudosection) yield a prograde path in nearly perfect agreement with the one deduced from the pseudosection. The pseudosection shows that rutile was produced by continuous breakdown of ilmenite during the early stages of prograde metamorphism, a reaction that ran to completion at ~ 730°C. Most rutile grains in garnet rim and matrix are interpreted to subsequently form by recrystallization of smaller matrix grains. However, they generally do not record the peak-P temperatures and instead range mostly between 775 and 815°C, interpreted as a result of more efficient recrystallization during a dehydration reaction (progressive replacement of hornblende by clinopyroxene). This study illustrates that both Zr-in-rutile and Ti-in-quartz thermometry cannot only robustly constrain a prograde evolution, but when combined with a pseudosection model can also yield information on recrystallization processes. In fact, the combination of these three methods provides an unrivalled tool for petrologic interpretation. The variation in Ti concentration in quartz is small regardless of crystal size. This P–T path reach very high temperatures (up to 875°C) with a high dP/dT ratio, both during prograde and retrograde histories. The steep P–T path, together with preservation of garnet growth zoning, symplectitic textures and the lack of significant Ti diffusion in quartz is consistent with a short residence time at high-temperature, implying unusual fast burial and exhumation of the eclogite-bearing nappe.

Domaines: Minéralogie

Lorraine Tual, Charlotte Möller, Pavel Pitra, Martin Whitehouse. TRACKING THE P–T PATH OF PRECAMBRIAN ECLOGITE USING PSEUDOSECTION, Ti-IN-QUARTZ AND Zr-IN-RUTILE THERMOBAROMETRY. 2nd European Mineralogical Conference, Sep 2016, Rimini, Italy. 2016. ⟨insu-01378348⟩