The Rotoroa Complex located in NW Nelson, represents a remnant plutonic fragment of an extensive subduction system active between 140-130 Ma. The arc system has been subject to large scale transcurrent faulting during the Early Cretaceous. The result is a series of "dismembered Mesozoic volcanic and plutonic arc associations that are sandwiched between terranes of the Western and Eastern Provinces" (Kimbrough et al 1993). This association is collectively described as the Median Tectonic Zone (MTZ) by Bradshaw (1993), and Kimbrough et al (1993). The rocks encompassed by the term Rotoroa Complex range from rare hornblendite through gabbro to diorite and quartz diorite, all typically rich in hornblende. The complex has been metamorphosed in part by the Early Cretaceous emplacement of the Separation Point Batholith, resulting in widespread felsic sheets and dikes of Separation Point Suite affinity being present throughout the Rotoroa Complex. Lamprophyre camptonite dikes of unknown age are also intruded throughout the complex, which has been subjected to brittle deformation due to movements on the Alpine Fault since Late Oligocene - Early Miocene times. The Rotoroa Complex is broadly metaluminous, calc-alkaline, medium to high K in nature, and has island-arc subduction-related geochemical characteristics. The geochemical variation reflects primary igneous fractionation processes. The Rotoroa Complex probably correlates with the Darran Complex of Fiordland, based on similarities in rock type, geochemistry, structural position and magnetic anomalies. The geochemical trends obtained from analyses of the Rotoroa Complex are sufficiently similar to those described in the Darran Complex to support this notion.