The geology of Bluff Hill, Southland, New Zealand.

Abstract

The mineralogy and structure of two gabbronorite intrusions forming Bluff Hill, is described. They intrude volcanogenic metasediments of the Greenhills Group and together comprise a segment of Brook Street terrane (Coombs et al., 1976), the inferred products of a Permian island arc environment. The Bluff Hill Gabbronorite is of arc-tholeiitic affinity and appears to represent the middle zones of a small south-west dipping pluton, the upper levels now removed by erosion. Mineralogical differences and a fractionation trend revealed by microprobe analysis of co-existing pyroxenes provide information on the structure and orientation of the pluton. The original magma was rather hydrous, as evidenced by the abundance of hornblende and the low temperatures (850° ± 100° C) obtained from co-existing pyroxene geothermometry. A convection current origin is put forward to explain several strongly flow-banded, thin xenolithbearing zones within the main mass. A segment of upper level Ocean Beach Gabbronorite, also of tholeiitic affinity,is described from Bluff Hill's western coast, which appears to post-date the Bluff Hill Gabbronorite. Later intrusives include several thick basaltic andesite (?) dykes, innumerable thin hornblende microgabbro veins and dykes, occasional hornblende pegmatite veins and diorite dykes. A Tertiary strandline deposit, the Foveaux Formation (new formation) is described. Its rich brachiopod and warm-water benthic foraminifera fauna would suggest a Waitakian to Otaian age. The cement consists largely of the zeolites thomsonite, gonnardite, and chabaz:Lte together with minor well-crystallized smectite and gibbsite.