Lithology, field relationships, and structure of the pounamu ultramafics from the whitcombe and hokitika rivers, westland, new zealand

Abstract

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 thrustslice 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 XC01 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. © 1983 Taylor & Francis Group, LLC.