Hydrothermal metamorphism of the Whangakea Basalt, New Zealand

Abstract

The Cretaceous Whangakea Basalt has undergone two episodes of alteration. The earlier one pervasively affected part of the formation and produced greenschist mineral facies assemblages of the type quartz-albite-chlorite-epidote-actinolite-sphene. The later episode gave rise to zeolites, including stilbite, analcime, thomsonite, epistilbite, heulandite-clinoptilolite, and laumontite, none of which co-exist stably wth quartz. Products of the zeolitic alteration are widespread, but occur predominantly in and adjacent to veins. Calcite crystallised at a late stage in this episode. Greenschist alteration probably occurred at temperatures in excess of 320°c, whereas zeolite alteration is believed to have taken place below 165–180°c.

The Whangakea Basalt, together with an associated gabbro-peridotite body, constitute an ophiolite complex. The localised development of the greenschist alteration, the presence of relic magmatic phases, the lack of preferred orientation of the secondary minerals, and their common pseudomorphous habit suggest that this alteration resulted from hydrothermal metamorphism. Zeoli tic alteration may have occurred late in this metamorphism, or may have occurred while the basaltic rocks were buried beneath some 3000 m of Tertiary strata.