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

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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 eastwards from about 20 Ma to 120 Ma. It is possible that these ages may also be a cooling age pattern, reftecting the continuation of uplift at a much slower rate throughout the late Cretaceous and Cenozoic, but it is more probable that the ages represent a section in the metamorphic sequence where only partial accumulation of argon has taken place while buried at depth.(3) In a narrow 10 km zone adjacent to the fault, the mica and total-rock ages are very young (12–30 Ma) but the coexisting hornblende and plagioclase ages are very widely discordant (40-1600 Ma).This indicates that excess argon is definitely present in the plagioclase and hornblende and possibly throughout the rocks as a wh oie. Amounts of excess argon in the high-grade schists are 1-4 nljg, and they decrease westwards to less than 0.5 nljg in the biotite zone greenschist facies and to zero amounts in the chlorite zone greenschist facies rocks.Graphical display of the K and 40Ar (radiogenic) contents of the K-poor and K-rich mineral phases suggests that the excess argon was uniformly distributed throughout the rocks during an isotopic rehomogenisation event 12 ± 5 Ma aga (Miocene-Pliocene). This young age must reftect the time of the last uplift of the Haast Schist near the Alpine Fault during the formation ofthe Southern Alps. It is suggested that this uplift phase during the Kaikoura (Miocene-Recent) orogeny was sufficiently rapid and localised to cause some heating during shearing, and consequent radiogenic argon loss from micas, and re-incorporation under a high argon partial pressure into coexisting K-poor phases such as plagioclase and hornblende.