Metamorphosed ultramafites and associated formations near Milford Sound, New Zealand

Abstract

Harzburgitic dunite mylonite, with lesser blastomylonite, o:1ivine schist, talcose ultramafic schist, and pegmatite, 'form a vertical septum 400-3000 ft wide, at least 14 miles long, in steeply folded 'and mylonitised gneiss on the Fiordland caast near Milford Saund. The mylanites are faliaceous and parphyroclastic, the schists xenoblastic granulitic to schistose. Contacts with adjaining gneiss are faults; that at Poisan Bay is the site of a hydragen-methane gas seep prabably generated by serpentinisation reactions.Associlated Lower Paleozoic rocks are medium to' maderately high grade (kyanite zone) gneisses of the amphibolite facies 0'1' metamorphism, naw extensively mylonitised. Mapped as Thursa, Saint Anne, and Milfard Farmatians, they formerly consisted 0'1' a basic igneous assemblage (gabbro, pyraxenite, etc.) probably assaciated with the uitramafites, as weH as greywacke, tuffaceaus sandstone, mudstone, and limestane. Minor basic dikes invaded the gneisses at a late stage of the Upper Palea:ZiOic regional metamorphism.The assemblage hlas undergone three distinct 'phases of metamorphism and deformation, as revealed by microtextures, petrafabric relationships, mesascopic fold-types and relationships, and associated neamineralisation. The first phase, carrelated with the wider regianal metamorphism of Fiardland, may have shortly followed or even accompanied the relatively anhydraus emplacement of ultramafites, strongly folded the hast gneisses, and develaped the ultramafic schists. Analyses by electron microprobe indicate thatalivine (Fa95) recrysivallised with little ,if lany significant change in campositian. A diffuse olivine subfabric in parphyroclasts and a strongly cleaved olivine polymorph may be relict fram this phase or from the earlier plutonic iemplacement. The second phase, of more strongly dynamic metamorphism occurred under lawer P-T canditions of the amphibo;Jite facies, generated common diaphtharetic minemls in the gneisses, and widespread but sparse antigorite in the ultramafhes, deV'eloped flexural-slip falds, mylanitic textures, foliation, and fluxion banding in all rocks. The third phase, more obviously relJated to' deformation at shallow depth, locally generated sparse antigorite, and in the gneisses quartz and mica, accentuated fluxion banding, ribbon-textures, land undulose extinction in minerals, and developed minar kink foilds and distinct zones of f'ault movement.The twa later phases are correlated with early movements of the nearby Alpine Fault; elsewhere alang the fault a wide range of shaUower cataclasites were developed after third phase movements.

First report of kyanite-sillimanite association in New Zealand (note)

“…10). Localities not shown in Hattori's map for andalusite are Preservation Inlet (Benson & Bartrum 1935) and the Pikikiruna Range (Ghent 1968), and for kyanite are Stewart Island (Williams 1934), Pakawau (Bishop 1971), in the Pariwhakaoho River (Grindley 1971), near Milford Sound (Wood 1972), and Davis Creek. a distance of about 1·5 km.…”

“…Kyanite in place or in boulders has been reported (a) in kyanite-amphibole schists from Stewart Island (Williams 1934, p. 323) ; (b) in boulders of muscovite-kyanite schists in the alluvium of the Paringa River, Westland (Hutton 1950, footnote on p. 697; shown in Hattori 1967, fig. 10) ; (c) in a quartz-biotite-muscovite~garnet-staurolite kyanite rock near a small adamellite intrusion at Pakawau, north-west Nelson (Bishop 1971) ; (d) in quartz-kyanite veins in the Pariwhakaoho River near Takata (Grindley 1971) ; and (e) in gneisses with the associations plagioclase-quartz-muscovite-biotite-garnet-kyanite and quartz-plagioclasemuscovite-microcline-garnet-kyanite in the Saint Anne Formation near Milford Sound (Wood 1972). To the author's knowledge, there has been no previous report of the kyanite-sillimanite association in New Zealand.…”

Geology of the crystalline basement between Milford Sound and the Hollyford Valley, New Zealand

“…Following the earlier reconnaissance by B. L. Wood (1962;1972), several short field seasons since 1969 have been used to study the petrology and structure of northern Fiordland in more detail. North of Milford Sound the Alpine Fault and a more eastern fault system, Of which the Hollyford Fault is part, converge rapidly northward, defining a southern wedge of igneous and metamorphic basement of the South Island ( Fig.…”

“…Rock outcrops, although sometimes difficult of access, are excellent and provide a sound basis for petrological and geochemical studies. Practically unmetamorphosed and in part layered basic intrusives, mapped by Wood (1962) as "Darran Diorite", make up much of the Darran Mountains. In the western part of the area a relatively late (Rangitata Orogeny?)…”

“…period of amphibolite facies metamorphism tends to obscure details of earlier magma intrusion and granulite facies metamorphism. Thus, the "Milford Formation" of Wood (1962) that follows, basic intrusives on either side of the metamorphic boundary must be considered original parts of the same intrusive complex, to be called Darran Basic Complex (see Discussion). Figure 2 provides a topographic base and an indication of the density of coverage of the present work.…”