Echinus Granite and Pepin Group of Pepin Island, northeast Nelson, New Zealand: Drumduan Terrane basement or exotic fragment in the Median Tectonic Zone?

Abstract

On the northwest coast of Pepin Island, near Nelson City, New Zealand, the Carboniferous Echinus Granite cuts granitic mylonite and orthogneiss of the Platform Gneiss. Echinus Granite has typical arc‐related I‐type geochemistry and mineralogy. Platform Gneiss is a metaplutonic unit similar in composition to Echinus Granite and is not derived from the sedimentary Fall Formation as previously supposed. Contacts between Fall Formation and the granitoid rocks are poorly exposed and there is no thermal aureole, but the formation shares with the Echinus Granite the last period of local strong ductile deformation.

Echinus Granite, Platform Gneiss, and Fall Formation are overlain by Marybank Formation (Drumduan Terrane). Contacts are not exposed, but field relationships are suggestive of an unconformity or a thrust contact. An unconformable contact implies Echinus Granite and related rocks represent the basement to the Drumduan Terrane. A thrust contact suggests the Echinus Granite and related rocks may represent a terrane fragment incorporated 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 and are geo‐chemically and chronologically distinct. South of the Alpine Fault, however, Paleozoic rocks do occur within the MTZ and are pointers to a late Paleozoic arc that has been incorporated in the MTZ. Rocks of similar age occur in the magmatic arc(s) of Marie Byrd Land‐Thurston Island.

Keywords:

• Pepin Island
• Drumduan Terrane
• Median Tectonic Zone
• orthogneiss
• mylonite
• granodiorite