ECHINUS GRANITE AND PEPIN GROUP OF PEPIN ISLAND, NORTHEAST NELSON, NEW-ZEALAND - DRUMDUAN TERRANE BASEMENT OR EXOTIC FRAGMENT IN THE MEDIANTECTONIC ZONE

On the northwest coast of Pepin Island, near Nelson City, New Zealand, the Carboniferous Echinus Granite cuts granitic mylonite and orthogne iss of the Platform Gneiss. Echinus Granite has typical are-related I- type geochemistry and mineralogy. Platform Gneiss is a metaplutonic un it similar in composition to Echinus Granite and is not derived from t he sedimentary Fall Formation as previously supposed. Contacts between Fall Formation and the granitoid rocks are poorly exposed and there i s no thermal aureole, but the formation shares with the Echinus Granit e the last period of local strong ductile deformation. Echinus Granite , Platform Gneiss, and Fall Formation are overlain by Marybank Formati on (Drumduan Terrane). Contacts are not exposed, but field relationshi ps are suggestive of an unconformity or a thrust contact. An unconform able contact implies Echinus Granite and related rocks represent the b asement to the Drumduan Terrane. A thrust contact suggests the Echinus Granite and related rocks may represent a terrane fragment incorporat ed into the Median Tectonic Zone (MTZ). The nearest granitic rocks of Paleozoic age in Nelson lie 100 km to the west in the Buller Terrane a nd are geochemically and chronologically distinct. South of the Alpine Fault, however, Paleozoic rocks do occur within the MTZ and are point ers to a late Paleozoic are that has been incorporated in the MTZ. Roc ks of similar age occur in the magmatic arc(s) of Marie Byrd Land-Thur ston Island.