Drumduan Group of East Nelson, New Zealand: Plant‐bearing Jurassic arc rocks metamorphosed during terrane interaction

The Drumduan Group, consisting of the Marybank and Botanical Hill formations, forms a fault‐bounded block northeast of Nelson City, and comprises bedded pyroclastic and cpiclastic (including sedimentary) debris of probable terrestrial deposition. Lawsonitc is widespread, but not uniformly present. Plant macroibssils, including Mataia podocarpoides, Rissikia talbragarensis, Taenioptens thomsoniana, T . sp. cf. T. crassinervis, Cladophlehis sp. and Podozamites sp., in the Marybank Formation indicate a Jurassic age. The group is separated from the Brook Street Volcanics (Permian) by the Delaware Fault Zone, along which arc slivers of Wakapuaka Phyllonite (pumpcllyitc‐actinolite facies). In the west the group shows widespread hydrothermal alteration related to the adjacent Tasman Intrusives, but the Brook Street rocks are not similarly altered and are not metamorphosed beyond prehnite‐pumpellyite facies. We infer that during the Rangitata Orogeny the Drumduan Group, the Tasman Intrusives, and the underlying basement rocks, including the Pepin Group, were underthrust below the Brook Street Volcanics along the Delaware Fault Zone. The resulting depth of burial and confining nature of the overthrust plate on one side, and the Tasman Intrusives on the other, were responsible for the development of metamorphic rocks characterised by a relatively high P/T ratio and by the crystallisation of lawsonitc, locally in abundance. Fluid pressures were probably nowhere greater than lithostatic pressure. The Delaware Fault Zone appears to form a major suture between the rocks of east and west Nelson. The Drumduan Group, the Tasman Intrusives, and the Pepin Group are assigned to the Tuhua Orogen.