Temporal and spatial scales of garnet growth associated with serpentinization: A case study from Belvidere Mountain, Vermont, USA

Abstract

Serpentinites and rodingites can inform scientists about the evolution of the Earth's upper mantle, including metamorphic processes that occur in subduction zones. Serpentinization and rodingitization are metasomatic processes that are hypothesized to be geochemically and temporally related. Garnets that form in rodingites have the potential to constrain the temporal evolution of their host lithologies. However, garnet is stable in rodingites over a wide pressure and temperature range, and rodingites can contain multiple generations of garnet. In a serpentinite-hosted rodingite zone from Belvidere Mountain, Vermont, USA, three generations of texturally and chemically distinct garnets occur within ∼1 cm of each other in a single sample. We present results from electron microprobe, whole-rock geochemistry, phase equilibria modelling, and garnet Sm-Nd ID TIMS analyses for each garnet generation from this locality. Massive garnets occur as large anhedral masses (>1 cm) along with interstitial diopside, titanite, and chlorite, and yielded a Sm-Nd isochron age of 477.3 ± 6.1 Ma (n = 6; MSWD = 3.1). Massive garnets are interpreted to have formed by rodingitization of mafic blocks in a subduction setting during the Taconic Orogeny and provide a minimum temporal estimate for serpentinization of the Belvidere Mountain ultramafic rocks. Fine-grained (∼100–200 μm) anhedral garnets define the dominant foliation in the rodingite, and are surrounded by fine-grained diopside, epidote, and chlorite. The fine-grained garnets yielded an imprecise isochron age of 426.1 ± 26.7 Ma (n = 4, MSWD = 0.81) which may be compromised by non-garnet inclusions and diffusive loss of Nd, precluding an accurate interpretation. Garnetite veins (∼1 cm) crosscut the foliation and yielded an isochron age of 410.0 ± 4.0 Ma (n = 4, MSWD = 1.9). Garnetite veins are interpreted to date post-Taconic, synextensional, low-temperature metasomatism. Results indicate that multiple generations of garnet can form in rodingites over centimeter spatial scales and ∼70 Myr timescales.