Neoproterozoic high-pressure/low-temperature metamorphic rocks in the Avalon terrane, southern New Brunswick, Canada

High‐P/low‐T metamorphic rocks of the Hammondvale metamorphic suite (HMS) are exposed in an area of 10 km² on the NW margin of the Caledonian (Avalon) terrane in southern New Brunswick, Canada. The HMS is in faulted contact on the SE with c. 560–550 Ma volcanic and sedimentary rocks and co‐magmatic plutonic units of the Caledonian terrane. The HMS consists of albite‐ and garnet‐porphyroblastic mica schist, with minor marble, calc‐silicate rocks and quartzite. Pressure and temperature estimates from metamorphic assemblages in the mica schist and calc‐silicate rocks using TWQ indicate that peak pressure conditions were 12.4 kbar at 430 °C. Peak temperature conditions were 580 °C at 9.0 kbar. ⁴⁰Ar/³⁹Ar muscovite ages from three samples range up to 618–615 Ma, a minimum age for high‐P/low‐T metamorphism in this unit. These ages indicate that the HMS is related to the c. 625–600 Ma subduction‐generated volcanic and plutonic units exposed to the SE in the Caledonian terrane. The ages are also similar to those obtained from detrital muscovite in a Neoproterozoic‐Cambrian sedimentary sequence in the Caledonian terrane, suggesting that the HMS was exposed by latest Neoproterozoic time and supplied detritus to the sedimentary units. The HMS is interpreted to represent a fragment of an accretionary complex, similar to the Sanbagawa Belt in Japan. It confirms the presence of a major cryptic suture between the Avalon terrane sensu stricto and the now‐adjacent Brookville terrane.