The geology of the Greenhills ultramafic complex, Bluff peninsula, Southland, New Zealand

Abstract

The Greenhills Ultramafic Complex occupies 5 square miles on the north-west part of Bluff Peninsula. It consists of a deformed layered series of ultrabasic rocks of "early late" Permian age intruded into Lower Permian metasediments of the Southland Syncline. The main penetrative planar structures in the complex are primary layering and cleavage; both are inclined. Mesoscopic folds in primary layering are not uncommon. Widespread slumping in the layered series and the emplacement of the complex as a plug of hot rock bounded by a fault and ringed by a "lubricating" marginal gabbro indicate recurring conditions of instability during evolution of the complex. Emplacement of the Greenhills Complex was accompanied by the intrusion of innumerable dykes and by folding and faulting. Two main generations of dykes are recognized: 1) early ultrabasic dykes (in order of decreasing age: dunite, wehrlite and eucrite) are unchilled and preceded emplacement of the marginal gabbro; 2) late dykes (in general order of decreasing age: eucrite, anorthosite, trondhjemite, hornblende-andesite, dolerite, tremolite-picrite, hornblende-plagioclase pegmatite and aplite) are chilled and postdate the marginal gabbro. Emplacement of the Greenhills Complex effected contact metamorphism of country rock spilitic metasediments of the Greenhills Group. These rocks, dominantly volcanic microbreccia with intercalated bands of impure marble, increase from regional prehnite-pumpellyite facies through greenschist facies as seen at Mokomoko Inlet to hornblende-hornfels adjacent the Greenhills Complex.

Fossils including the gastropod Peruvispira aff. imbricata Waterhouse and the bivalve Atomodesma aff. marwicki indicate a late Lower Permian age for the middle part of the Greenhills Group. The coral Plerophyllum aff. timorense Gerth occurs in a lower horizon of the Greenhills Group. Bands of marble containing shell fragments and radiolarians are common in the upper part of the section. The layered series of the Greenhills Ultramafic Complex is stratigraphically divisible into: 1) an upper eucritic portion dominated by a 2000 ft thick Eucrite Zone which includes a 50 ft thick allivalite unit; 2) a lower ultramafic portion comprised essentially of a 500 ft thick Wehrlite Zone (which includes near its base a poikilitic peridotite unit less than 100 ft thick) and a basal Dunite Zone exceeding 2000 ft in thickness. A Transition Zone of feldspathic wehrlite occurs between the upper and lower portions of the complex and with the poikilitic peridotite unit is an important marker horizon. The layered series shows well developed accumulate structures and textures resembling cumulates in classic stratiform intrusions. Cryptic layering is shown by the range in composition of the essential primary minerals. These minerals in order of separation from the Greenhills magma are: olivine (Fo90-65), clinopyroxene (Ca41Mg54Fe5 to Ca44Mg47Fe9) and plagioclase (An92-88). Other primary minerals present in minor quantities include orthopyroxene (avg. En81), brown hornblende and chromite (Mg48) (Cr60Al22).

Optical and X-ray studies of the plagioclases suggest a discontinuity in the unit cell geometry between An90.5 and An93.0; this break probably represents the boundary between transitional and primitive anorthite structures.

Single crystal X-ray study of augite lamellae exsolved on (100) of orthopyroxene shows that the c and b-crystallographic directions of orthopyroxene host and included lamellae are coincident. The unit cell dimensions of the lamellae as determined after least squares refinement are: aA9.79, bA8.90. cA5.29 all ± 0.04 A, B10°614' ±1', V 442.62 ± 0.5 A3.

The Greenhills Ultramafic Complex is adequately explained by crystal settling during fractional crystallization of a basaltic magma. This magma is believed to have had affinities transitional between alkaline and tholeiitic magma types. Differentiation took place in a hydrous open system connected with the surface of the earth.