Structural evolution of the semischist-schist transition in an accretionary complex: the Otago Schist section at Lake Hwea, New Zealand

摘要

Permo-Triassic, Rakaia Terrane, pelitic and psammitic schists at Lake Hwea, Otago, show progressive metamorphism from TZII, pumpellyite-actinolite facies to TZIV, greenschist facies. The sequence shows evidence of five phases of deformation. D-1 and D-2 are progressive synmetamorphic phases, forming macroscopic nappe-like folds, interpreted to have formed by internal deformation within the Rakaia-Caples accretionary wedge during terrane collision. Thick S-1 quartz veins are the form surface for definition of D-2 folding, intensification of which progressively destroys evidence of D-1 at higher textural and metamorphic grades. D3-5 are post metamorphic, less pervasive, kink folds (commonly conjugate) and brittle fault zones that predate the intrusion of lamprophyre magma and carbonatitic fluids associated with the Alpine Dike Swarm (c. 25Ma). They relate to tectonic relaxation and exhumation of the schist in Cretaceous-Paleogene times. Fluid flow has been ubiquitous throughout the deformation, resulting in extensive quartz veining with diffusion of silica from the immediate country rock.