Radiogenic argon and strontium diffusion parameters in biotite at low temperatures obtained from Alpine Fault uplift in New Zealand

Copland and Cook River Valleys provide a cross-section through the 25 km wide, pum. pellyite-actinolite to amphibolite facies Alpine schists along the Indian/Pacific plates' boundary in South Island, New Zealand. Five phases of deformation are reported: D 1, possible soft-sediment folding; D2, syn-first metamorphic isoclinal folds; D3, syn-second metamorphic regional folds; D., local upright folds; Ds, faulting. The D3 structures include vertically and laterally anastomosing late D3 domains of intense transposition (Paringa domains). The D3 structures formed during mid Tertiary to Late Miocene times in response to broadly. distributed dextral transpressive strain along the Indian/Pacific plates' boundary. This strain was localised later to the Alpine Fault. The D3 structural style changes south of Moeraki Valley, and reflects the change in dip of the plate boundary, from steeply east-dipping in the study area, to shallowly east-dipping south of Moeraki Valley.

Section: Geochronology Of D3 Structuressupporting

confidence: 75%

Structure and interpretation of the Alpine schists in Copland and Cook River Valleys, South Island, New Zealand

Findlay

1987

“…Although none of the granites near any of the dikes were dated, biotites from his nearest granites to the north and south had been reset from older ages, indicated by other minerals in the rocks, to ages of 110 -+-4 and 124 -+-3 m.y. The only two K-Ar ages for rocks west of the Alpine Fault are those given by Hurley et al (1962 is for a biotite from granite from Paringa River 100 km south-west of the area sampled and gives an old 286 -t-12 m.y. age, and the other (B3950) is for a biotite from the Tuhua Granite 9 km from samples 6-11 and 25 km from 69-12 and GA3464.…”

Section: Whole Rock K-ar Agesmentioning

confidence: 99%

“…East of the Alpine Fault the difference extends to the schist country rock where mica ages are lower than the whole rock age of the dikes; a muscovite from schist (M3957) gave an age of 76±4 m.y. (Hurley et al, 1962), although collected only 50 m and 2 km from dikes with whole rock ages of 110 -t-3 m.y. and 124 -t-2 m.y.…”

Section: Mineral K-ar Agesmentioning

confidence: 99%

Potassium-argon age of some New Zealand lamprophyre dikes near the Alpine Fault

Wellman

Cooper

1971

Lamprophyre dikes in north-west Otago and north Westland yield whole rock potassium-argon ages mainly lying between 136 and 107 m.y. These ages correspond roughly to the ages inferred by others for the wall rock, suggesting that the ages give the latest major cooling event and that the dikes were possibly intruded earlier.In the dikes, the mica and amphibole ages are younger than their host whole rock age, while the plagioclase ages are older. The mineral ages are best explained as due to loss of argon from micas and amphiboles, and absorption by plagioclase and possibly other tectosilicates, the whole rock remaining approximately a closed system. The redistribution of argon is thought to have occurred at slightly lower temperatures than that causing outgassing of the whole rock. The whole rock dates support an earlier suggestion that the dikes were intruded as a single belt that has since been separated into two parts by a 120 km displacement along the Alpine Fault. The Alpine Fault displacement, which is dextral, was probably in two major periods: one in the Early Cretaceous, and the other in the Late Cenozoic.

“…In South Otago, the Haast Schist grades into nonschistose sediments of Permian-Jurassic age ofthe Caples and Hokonui facies, wh ich are normally at or below prehnite-pumpelIyite metamorphic grade. Age studies ofthe metamorphie rocks within the schist are have shown that the mineral and totalrock ages range very widely from I to 200 Ma (Mason 1961;Hurley et al 1962;Harper & Landis 1967;Sheppard et al 1975;Adams 1981;Adams et al 1984). Harper & Landis (1967) first suggested that the ages followed two patterns: (I) more than 25 km east ofthe Alpine Fault, the ages ofthe Haast Schist and immediately adjacent rocks ranged from about 100 Ma to about 140 Ma (mid-early Cretaceous), and because they were inversely correlated with metamorphie grade, they must represent cooling ages of the schists during postorogenic (Rangitata) uplift; (2) in a narrow, 25 km zone immediately east of the Alpine Fault, the ages are very much younger, down to less than 10 Ma, and these are directly correlated with distance from the fault.…”

Section: Introductionmentioning

confidence: 99%

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

Adams

Gabites

1985

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