GARNET SM-ND GEOCHRONOLOGICAL INSIGHTS INTO THE METAMORPHISM OF THE NASHOBA TERRANE, MASSACHUSETTS, USA

The Nashoba Terrane is a fault-bounded, metamorphosed remnant arc in southeast New England of Ganderian affinity^1,2. It lies between the Avalon terrane to the southeast and the Merrimack terrane to the northwest. Three metamorphic events have been proposed for the Nashoba Terrane: an amphibolite grade event (M1) associated with early arc-metamorphism due to the subduction of the Acadian seaway, a migmatization event (M2) due to tectonic wedging during the Devonian collision of Avalonia, and a later retrograde greenschist faces event (M3). Published monazite geochronology in the Nashoba Terrane puts these metamorphic events at 435-400 Ma (M1), ca. 390 Ma (M2), and ca. 378-371 Ma (M3)³. No direct chronology of metamorphic index minerals – like garnet – has been reported to test this metamorphic timing.

Here, we present the first Sm-Nd geochronological results derived from garnet-bearing samples within the Nashoba Terrane. Our preliminary garnet age of 369.0 ± 2.1 Ma (MSWD = 1.2, n = 4) shows Neo-Acadian (400-350 Ma) affinity, and barely coincides with the youngest M3 generation of monazites³. Thermodynamic modeling of a nearby sample suggests peak conditions of ~700C and 8.5 kbar⁴. Flattened major element zonation in the dated garnets indicates prolonged high temperatures during and/or after garnet growth. Thus, these high temperatures must have persisted through 369.0 ± 2.1 Ma, challenging the interpretation that the Nashoba Terrane experienced only low P/T (greenschist) metamorphism during the Neo-Acadian M3 metamorphism³. Trace element profiles in garnet will be used to determine if our garnet ages reflect prograde growth or diffusional re-setting at high temperatures. By constraining the spatial distribution of metamorphic garnet ages oblique to strike, we will assess for any heterogeneity in the timing of peak conditions, and the veracity of previously proposed tectonic models.

^[1]Kay et al. (2017) ^[2]Loan, (2011) ^[3]Stroud et al. (2009) ^[4]Yao et al. (in press)