Janet Gabites

Potassium-argon age studies of a low-grade, progressively metamorphosed greywacke sequence, Dansey Pass, South Island, New Zealand

39 potassium-argon age determinations from a progressively metamorphosed greywacke-schist sequence in southern New Zealand (prehnite-pumpellyite to greenschist facies), range from 199 to 154 Ma (early to late Jurassic). They show an inverse correlation of age with metamorphic grade. Concordant ages from varied lithologies at all metamorphic grades show that degassing of detrital potassium-bearing minerals was completed during metamorphism. The lowest grade metamorphic rocks, which yield the oldest ages, about 200 Ma, have only just exceeded the argon closure temperature and hence their ages date closely their time of metamorphism in the earliest Jurassic. Thus it is shown that the Jurassic-Cretaceous, Rangitata Orogeny in New Zealand commenced earlier, is more complex, and of longer duration than previously supposed. The remaining ages, from pumpellyite-actinolite and lower greenschist rocks, range uniformly through the Jurassic and are interpreted as cooling ages. From these data, a long-term uplift rate of 0.23 mm/yr is calculated for the Jurassic-Cretaceous.

Age of metamorphism and uplift in the Haast Schist Group at Haast Pass, Lake Wanaka and Lake Hawea, South Island, New Zealand

Abstract Potassium-argon ages are reported for mineral and total-rock sampies from the progressively metamorphosed Haast Schist sequence in the Haast valley, Haast Pass, Lake Wanaka, and Lake Hawea areas of Otago, South Island. These rocks extend from low-grade (chlorite zone greenschist facies) near Lake Hawea, through medium-grade (biotite and gamet zone greenschist facies) in the Haast valley, to high-grade (amphibolite facies) near the Alpine Fault. The ages of total-rock and micas show a general eastwards decrease from the Alpine Fault in three zones. (I) More than 25 km east of the fault, the ages increase steadily from 120 to I 73 Ma near Lake Hawea. These ages are interpreted as a cooling age pattern resulting from continued slow uplift, which began after the main (Rangitata) metamorphism at least 173 Ma aga (mid Jurassic) and which continued until at least 120 Ma aga (mid Cretaceous). (2) In a transitional zone, between 15 and 25 km from the fault, the mineral and totalrock ages increase rapidly ...

Precambrian granite and manganese nodules dredged from southwestern Campbell Plateau, New Zealand

Abstract Dredging on the southwestern Campbell Plateau at 54°51.4′S, 165°13.5′E recovered 2 angular boulders of granite and rounded boulders of granite, augen gneiss, abundant manganese nodules, and limestone covered with a 5 ern thick deposit of manganese-iron oxides. K-Ar dating gave ages of 1170 ± 10 Ma for the angular granite boulder, 1040 ± 20 Ma for the augen gneiss, and 490 ± 10 Ma for the rounded granite boulders. The rounded boulders are obviously ice rafted, but there is some doubt about the angular granite. Analyses of the manganese nodules gave an average of 12 % Mn, 16.3% Fe, 0.45% Ni, 0.18% Cu, and 0.16% Co.

Lead isotope constraints on the origin of Cenozoic orogenic gold systems in the Southern Alps and northwestern Otago, South Island, New Zealand

Abstract Lead isotopic compositions were determined for sulphides from Pliocene-Pleistocene gold-bearing veins in the Alpine Schist and from Miocene gold-bearing veins and vein breccias from the Shotover-Macetown area in the northwest Otago Schist belt. The lead isotopic signatures are consistent with derivation of Pb in the vein minerals predominantly from metasedimentary rocks that underlie the region. Differences in Pb isotopic signatures between deposits are interpreted to result from lateral and vertical lithological variability within the source rock mass. The host rocks also contain metabasic rocks with N-MORB, E-MORB or within-plate basalt chemistry. However, the observed Pb isotopic signatures in the gold-bearing veins preclude incorporation of significant amounts of Pb from the metabasites. The Pb isotopic signatures of lamprophyre dikes that were intruded into the Otago Schist coeval with Miocene gold mineralisation are distinctly more radiogenic than those of the hydrothermal veins. Thus, although the lamprophyre dikes were emplaced into similar extensional structural sites to the gold-bearing veins, there was no genetic relationship between lamprophyre dikes and gold mineralisation.

Potassium-argon age of the Glenham Porphyry, Murihiku terrane, eastern Southland, New Zealand

Abstract The Glenham Porphyry is a suite of andesitic terrestrial flows and subvolcanic intrusions in an inlier of Murihiku Supergroup sediments in eastern Southland. Potassium-argon dating has been used to determine the age of the volcanics. Plagioclase ages are in the range 182–240 Ma, indicating a probable Late Triassic to Early Jurassic time of extrusion. Discordant younger ages of whole-rock samples indicate argon loss from the glassy matrix, possibly caused by low-grade regional metamorphism during the Rangitata Orogeny.