Unravelling a HP-HT prograde metamorphic history from quartz and rutile inclusions in garnet
Résumé
Trace-element compositions of mineral inclusions in
garnet (e.g., quartz, zircon and rutile) are commonly used to
provide thermo(baro)metric estimates. However, the
systematics of the thermometric equilibria in these minerals at
high pressure and temperature are not fully understood. To
investigate this, we analyzed in-situ quartz and rutile
inclusions in a Fe-Ti-rich eclogite from the Sveconorwegian
orogen in southern Sweden by SIMS. Abundant quartz, rutile
and zircon in distinct microstructural sites (garnet core, garnet
rims and matrix) make this sample particularly suitable for
this study.
A P–T path, peaking at 16.5–19 kbar and 850–900 °C has
been deduced for this rock by pseudosection modelling; input
pressures from this model were used for trace element
thermometry of each garnet microtextural domain. For garnet
cores, Zr-in-rutile yields 700–715 °C and Ti-in-quartz 620–
640 °C at 7 kbar. For garnet rims, temperature estimates are
760–790 °C (Zr-in-rutile) and 740–890 °C (Ti-in-quartz) at
12–18 kbar. Finally, matrix rutile records 775–800 °C and
locally ~900 °C, and quartz records temperatures up to ~890
°C. Alongside, direct combination of Ti in quartz and Zr in
rutile isopleths yields a prograde path that is nearly identical
to that 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 was completed at
~730 °C. Thereafter, rutile grains in the garnet rim and the
matrix grew larger by recrystallization. However, these rutile
grains generally do not record the peak-P temperatures, but
instead yield 775–815 °C. This is interpreted to reflect
recrystallization associated with a major prograde
dehydration reaction in the rock, involving continuous
breakdown of hornblende and production of clinopyroxene.
This study illustrates that Zr-in-rutile and Ti-in-quartz
thermobarometry can robustly constrain prograde P–T
conditions and also yield important insights on
recrystallization processes at high temperatures. The
combination of these methods and integration of the results
with pseudosection modelling provides a versatile tool for
investigating the petrologic history of high-grade rocks.