Relationships between the brook street Terrane and Median Tectonic Zone (Median Batholith): Evidence from Jurassic conglomerates

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Abstract
U‐Pb zircon ages of 237–180 Ma and c. 280 Ma of seven granitoid clasts from the Rainy River Conglomerate which lies within the eastern Median Tectonic Zone (Median Batholith) in Nelson, and the Barretts Formation of the Brook Street Terrane in Southland, constrain the depositional ages of both units to be no older than c. 180–200 Ma (Early Jurassic). The minimum age of the Rainy River Conglomerate is constrained by the 147 +2 ‐1 Ma (latest Jurassic) emplacement age of the One Mile Gabbronorite (new name: previously western Buller Diorite). The ages and chemistry of five of the granitoid clasts are broadly compatible with derivation from rocks that are now represented by Triassic plutons of the Median Tectonic Zone (Median Batholith), although ages as young as 180 Ma are slightly outside the range of the latter as currently exposed in New Zealand. The age (273–290 Ma, 237 ± 3 Ma) and chemistry of the other two clasts (one each from Rainy River Conglomerate and Barretts Formation) suggest derivation from the Brook Street Terrane. Similarity in stratigraphic age, depositional characteristics, granitoid clast ages and composition between Rainy River Conglomerate and Barretts Formation suggests that they are broadly correlative and collectively overlapped a combined Brook Street Terrane ‐ Median Batholith (MTZ) before the Late Jurassic (147+2 ‐1 Ma). Sedimentary overlap may also have continued across to Middle Jurassic conglomeratic strata in the Murihiku Terrane to the east of the Brook Street Terrane. A U‐Pb zircon age of 261 ± 2 Ma is reported for Pourakino Trondhjemite of the Brook Street Terrane.