Abstract
A geological map of the Dusky Sound area (~ 700 km2) is presented at a scale of 1:59 000. The North and South Dusky areas are of generally contrasting character and are described separately. North Dusky is dominated by Early Cretaceous intermediate orthogneisses which are concordantly intrusive into varied amphibolite, paragneiss, psammite, and a complex unit including metaserpentinite pods. North Dusky metamorphism is of high P, high T type ( ~ 8 kb, 670°c) but evidence for an earlier low-pressure (andalusite-sillimanite) metamorphism is locally preserved. Magnesium staurolite which crystallised at ~12 kb is described from a metatroctolite cobble of uncertain origin. Green chromian staurolite is described from a probable mafic metatuff.
Metasediments of the South Dusky area are described in three informal groups. The Fanny Bay group (five formations) is predominantly quartz-rich and/or pelitic. The Edgecumbe group comprises mainly meta- conglomerate, sandstone and basaltic material. These two groups lithocorrelate respectively with the Ordovician Western Sedimentary Belt and the Cambrian of the Central Sedimentary Belt, west Nelson. Edgecumbe group lies east of and apparently overlying Fanny Bay group across a high angle fault - the same structural relation as that between the Central and Western Sedimentary Belts in Nelson. This structure strongly supports the lithocorrelations, and argues against the hypothesis that the Central Sedimentary Belt is grossly allochthonous and derived from the south. A group of structurally complex· undifferentiated metasediments, mainly biotite quartzite, calcic psammite and minor marble, occupies a large area in the east of South Dusky.
Edgecumbe group and the undifferentiated metasediments are combined as Goodyear terrane, cf. Fanny terrane (=Fanny Bay group). The two terranes were juxtaposed prior to the dominant regional metamorphism of probable early Mesozoic age. Metamorphic grade as defined by pelites rises from biotite zone in the west through staurolite zone (with andalusite and cordierite) to sillimanite zone (and incipient melting locally) in the east. Pressure during this, the second episode of regional metamorphism, is estimated at 3 - 4 kb. Metamorphism was accompanied by deformation producing diverse structures varying markedly in style with metamorphic grade and gross lithology. A sequence of granitoid plutons intruded during this metamorphic episode, ranging from small early concordant S-type orthogneiss to the 117 km2 discordant I-type Lake Mike Granite of probable Jurassic age. Lake Mike Granite intruded after the metamorphic climax but while regional temperatures remained high. Minor basicintermediate plutonics and dyke rocks also intruded during regional metamorphism. An earlier metamorphism was of low - medium grade (biotite schist) in the Goodyear terrane but of very low grade (slate) in the Fanny terrane. During a later metamorphism of retrogressive character, andalusite porphyroblasts (and sillimanite of the Lake Mike Granite contact zone) were pseudomorphed by kyanite, staurolite and muscovite.
Microstructural study of porphyroblast inclusions and matrix of staurolite-zone pelites of the Fanny Formation (Fanny Bay group) demonstrates that biotite, staurolite and andalusite generally grew in succession in relatively brief discrete intervals during progressive crenulation of early slaty cleavage and development of a new foliation, S2. Cordierite crystallisation overlapped with biotite and porphyroblasts commonly record a substantial increase in finite strain. The metamorphism and strain are attributed to an influx of heat, possibly fluid-borne, into a segment of crust under mild laterally compressive stress. Bedding has generally been transposed or rotated as a passive marker by 60 - 70° relative to S2 in response to progressive inhomogeneous bulk shortening across S2 . Minor components of simple shear strain were transmitted from nearby massive quartzites which buckled in response to the same regional stress field. It is inferred that many of the examples of porphyroblast rotation described in the literature are "pseudorotations", the error arising from the common assumption that Si in porphyroblasts represents an early stage in the development of matrix foliation Se.
The normal yellow colour and pleochroism of staurolite isattributed to Fe2+ - Ti 4+ charge transfer, the Ti being located in the tetrahedral Fe site of the staurolite structure.
Qualitative and quantitative studies of sector-zoning of Fanny Formation staurolite porphyroblasts, especially in Ti, indicate that the degree of sector-zoning is a direct monotonic function of the radial growth rate of the crystals. To a first approximation, a radial growth rate in excess of a threshold rate causes the Ti contents of each sector to deviate from the equilibrium content in proportion to a constant factor for each sector and the pth root of the excess growth rate, where p is estimated to be about 5. This function allows study of the variations in the relative growth rate of a crystal during its period of growth. From a rough calibration of the time scale of staurolite-growth, maximum growth rates are inferred to be ~ 0.1 mm/yr. Information of this nature on the kinetics of crystal growth has not previously been available for natural metamorphic crystals.
The rates of crystallisation were generally reaction-controlled (or interface-controlled). Under conditions of reaction-controlled growth, surface processes are of paramount importance, and sector-zoning is closely associated with distinctive inclusion patterns and the formation of "exclusion domes" (previously referred to as mica arcs). Similar phenomena are observed in andalusite (= chiastolite) and garnet of the South Dusky area, and also in west Nelson.