An interpretation of the metamorphic history of the alpine schist Northern Westland: evidence from diachronous garnets, mineral chemistry and zoning, whole-rock analyses, stable isotopes and Sm-Nd dating of garnet growth

Abstract

This thesis presents results of an investigation into the metamorphism of a section of the Alpine Schist in North Westland, between the Crooked River in the north and Griffin Creek in the south. The Alpine Schist is a narrow belt of dominantly quartzofeldspathic schist that has been rapidly extruded along the Alpine Fault. The schist is at its highest grade (garnet-oligoclase) immediately east of the Alpine Fault and grades to biotite zone facies at a distance of c.3 km west of the fault in this study. Field mapping, optical microscopy, whole-rock analyses, mineral chemistry and zoning, geothermobarometry, stable isotopes and Sm-Nd age determinations for garnet growth zones have enabled a well constrained metamorphic history to be interpreted. As part of this an estimate for the timing and P-T conditions of the peak of Alpine Schist metamorphism in the study area has also been possible. Large polyphase garnet porphyroclasts sampled from mylonitised Alpine Schist at Macs Creek record up to five zones of garnet growth. Textures within these garnets indicate that at least some garnet growth predated the development of the alpine foliation and that garnet growth occurred at least sporadically through most of the Cenozoic, finishing with garnet rims overgrowing the developing mylonitic foliation. Garnet separates were prepared and sent to VIEPS Radiogenic Isotope Laboratory, La Trobe University, Melbourne, Australia for Sm-Nd garnet growth age determinations. For the core (zone 1), a two point garnet-whole rock age of 98 ± 8.1 Ma indicates garnet growth in the mid-Cretaceous, during a time of widespread extension with scattered plutonism and volcanism in the New Zealand region, which predates the opening of the Tasman Sea by c. 20 Ma. A second Sm-Nd garnet growth age was obtained from zone 4, just inside the syn-mylonitic outermost rim (zone 5) and shows that Alpine Schist garnet growth occurred in the Late Cenozoic. The broad age of 12 ± 37 Ma (MSWD = 0.89) for zone 4 is further constrained independent information about the tectonics of the region and by the timing of the growth of the syn-mylonitic rim zone 5. Based on a depth of formation estimate of 40 km (from geobarometry for the outer most edge) and uplift to the surface by the present day, zone 5 is interpreted to have formed prior to c. 6 Ma. The maximum age for zone 4 is probably less than 40 Ma, as this is when the Alpine Fault inception is thought to occur [Sutherland et al. 1999]. Arguably this age could be younger than 25 Ma, as movement between the Pacific and Australian plates was small from 40-25 Ma [Sutherland 1995, 1999]. Results obtained for these garnets indicate diachronous prograde garnet growth from the core to rim of the garnets. Further, these results indicate that the peak of Alpine Schist metamorphism for this section of the Southern Alps occurred at c. 595°C and c. 10.5 kbar, synchronous with development of the mylonitic foliation at c. 6 Ma. For other garnet bearing samples of this study, well constrained geothermobarometry and chemical zoning studies show garnet growth has occurred since the development of the alpine foliation, with garnets first forming in samples with high whole-rock MnO (>0.30%) at c. 400°C and c. 6 kbar. A separate episode of garnet growth that is interpreted based on textural evidence to predate the development of the alpine foliation is seen in rare (two) samples from the biotite zone of the Alpine Schist. These garnets record a moderately high P/T metamorphic signature, suggesting that they formed during a different event to that in which the cores of the large polyphase garnets that formed during extension at 98 Ma. Inclusion trails in biotite and plagioclase porphyroblasts suggest that growth of these moderately high P/T garnets occurred coeval with the growth of at least some biotite and plagioclase. Detailed whole-rock analyses identify the Aspiring Lithological Association, occurring with metabasites within 3 km of the Alpine Fault. This shows that the Aspiring Lithological Association exists next to the Alpine Fault in at least two places in the Alpine Schist (also Franz Joseph-Fox Glaciers area), and may suggest that it extends even further north. Results of preliminary study of stable isotope samples, including one sample from over 5 km east of the Alpine Fault, support previous interpretations that near the Alpine Fault a metamorphic fluid system is dominant. At greater distances from the Alpine Fault however, meteoric fluids have at least locally been dominant. Stable isotope evidence and textural evidence of retrograde biotite, chlorite and calcite in garnet fractures is consistent with a model like that suggested by Wannamaker et al. [2001]. This model predicts that at the brittle-ductile transition hydrostatic fluid pressure replaces lithostatic fluid pressure and fluids rise on an Alpine Fault-induced network of fractures.