METAMORPHIC EVOLUTION OF THE SOUTHWESTERN PART OF THE MEGUMA TERRANE NOVA SCOTIA, CANADA

The Meguma terrane, which outcrops in southwestern Nova Scotia, is the most outboard terrane of the Appalachian orogen. Its metamorphic rocks may provide important constraints on tectonic processes operating during the <390 Ma Neoacadian Orogeny. Psammitic and pelitic sediments have been variably metamorphosed from lower greenschist to upper amphibolite facies. Additionally, there are numerous granitoid plutons that have U-Pb zircon, titanite, and monazite crystallization ages of 373 Ma. However, it is not currently known the extent to which the amphibolite facies metamorphism in the southwestern Meguma terrane was driven by the intrusion of these granitoid plutons or the result of regional metamorphism from micro-continent collisions.

Here, we present petrography and metamorphic pressure-temperature (P-T) estimates of rocks from the Pubnico Point area in the southwestern Meguma terrane, which is in the vicinity of the Devonian (373 Ma) Barrington Passage Tonalite (BPT). A combination of pseudosection modeling, Ti-in-biotite thermometry, GASP geobarometry, and garnet-biotite exchange thermometry consistently places peak P-T conditions at 550-600ºC and <4 kbar.

Microtextures and interpretations of metamorphic P-T conditions suggest that these rocks were already at amphibolite facies prior to the intrusion of the BPT. In one locality H₂O-rich fluid inclusions in garnet suggest that some of these rocks recrystallized in the presence of a fluid. It is possible that this fluid was expelled from the BPT during crystallization and it then infiltrated the surrounding metamorphic rocks. Thus, while the southwestern Meguma terrane was likely already at amphibolite facies prior to the 373 Ma intrusion of granitoid plutons, heat and/or fluid from the plutons may have locally driven metamorphic recrystallization.

These new P-T and microtextural data will be integrated with previous petrographic observations and P-T estimates of metamorphism across the Meguma terrane to explore different tectonic interpretations that may explain the distribution of metamorphic isograds and localized fluid-induced modification of the metamorphic rocks.