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A Permian ($265 Ma) intrusive complex which formed as a magmatic feeder reservoir to an immature island-arc volcano is fortuitously exposed in southern New Zealand. Known as the Greenhills Complex, this intrusion was emplaced at shallow crustal levels and consists of two layered bodies which were later intruded by a variety of dykes. Cumulates, which include dunite, olivine clinopyroxenite, olivine gabbro, and hornblende gabbro-norite, are related products of parent-magma fractionation. Both primary (magmatic) and secondary platinum-group minerals occur within dunite at one locality. Using the composition of cumulus minerals, mafic dykes and melt inclusions, we have determined that the parent magmas of the complex were hydrous, low-K island-arc tholeiites of ankaramitic affinities. Progressive magmatic differentiation of this parent magma generated fractionated melt of high-alumina basalt composition which is now preserved only as dykes which cut the Complex. Field evidence and cumulus mineral profiles reveal that the magma chambers experienced turbulent magmatic conditions during cumulate-rock formation. Recharge of the chambers by primitive magma is likely to have coincided with eruption of residual melt at the surface. Similar processes are inferred to account for volcanic-rock compositions in other parts of this arc terrane and in modern island-arc systems.

The Pounamu Ultramafics comprise a series of parallel or en echelon lenses and layers of ultramafic and mafic rocks defining a northeaststriking, 2 km wide zone (the Pounamu Ultramafic Belt -PUB) in garnet zone Haast Schists of North Westland.In their most coherent form, the Pounamu Ultramafics comprise serpentinite, gabbro, polymict igneous breccia, and metabasite. Gabbros are intruded by feldsparphyric dikes and unconformably overlain by breccia. Breccia or metabasite is overlain sequentially by a pelagic sequence including: interlaminated limestone and chert, phosphate lenses, and a massive porphyroblastic biotite schist; and a terrigenous series of thin-bedded, upward younging units of graded quartzofeldspathic sandstone and siltstone, similar to distal turbidite lithologies from the western Torlesse terrane. Similar upward grading quartzofeldspathic metasediments occur beneath the ultramafic unit. The Pounamu Ultramafics are interpreted as an ophiolite segment representing the basement to this part of the Torlesse terrane. The ophiolite, complete with sedimentary cover, has been tectonically emplaced as a thrust sheet within the Torlesse sequence prior to intense deformation and the regional metamorphic climax. Transposition during emplacement is probably responsible for the tectonic thinning and internal dismembering of the ophiolite.Pre-or synmetamorphic deformation is dominated by moderate to steeply plunging, second generation, isoclinal folds which collectively define an anticlinorium closely corresponding in areal extent to that ofthe PUB. The Pounamu Ultramafic thrust Received 15 September 1982, accepted 11 July 1983 slice is structurally repeated in the cores of individual isoclines where diapiric movement of serpentinite during folding has led to thickened hinge zones and local piercement structures.Subsequent metamorphism, probably of late Triassic-early Jurassic age, has reached garnet grade in the PUB, with the serpentinite partially dehydrated under low XC0 1 conditions to serpentinedunite. Extensive associated metasomatism has resulted in zones of serpentine-magnesite, talcmagnesite, talc, tremolite, and chlorite parallel to, and. replacing, the original serpentinite-metabasite contact.Postmetamorphic defonnation has produced kink folds associated with 3 generations of faults. The youngest fault episode has resulted in dextral strikeslip displacement of the PUB on east-striking structures probably related to the Marlborough Fault System.

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