The garnet-albite-biotite zone and evidence for high P/T metamorphism of the Alpine Schist, McArthur Range, Southern Alps, New Zealand

Abstract

This thesis presents results obtained from a detailed investigation into the metamorphism of the Alpine Schist from the McArthur Range, which is located in the northern Southern Alps approximately 20km southeast of Hokitika. Results of petrography, structure, whole rock chemistry and mineral chemistry of the Alpine Schist are presented and the data are used to constrain the pressure and temperature conditions of garnet porphyroblast growth in greyschist from the study area. This was done using modern techniques of constructing pressure-temperature pseudosection phase diagrams using the computer programme Thermocalc, and also more traditional methods of geothermobarometry. The Alpine Schist in the McArthur Range grades from low biotite zone schist in the eastern field area ~3.5km southeast of the Alpine Fault, through to garnet-albite-oligoclase-biotite zone schist in the western field area ~2km southeast of the Alpine Fault. The garnet-in isograd precedes the first appearance of oligoclase in the field area and furthermore the garnet porphyroblasts grew mostly, if not entirely within the garnet-albite-biotite zone. Thus, the metamorphic history of the Alpine schist in the present study is unlike that of the well studied Franz Josef - Fox Glacier region. There the appearance of oligoclase precedes the growth of garnet and there is no garnet-albite-biotite zone. This is the first detailed study of the metamorphic history of the Otago Schist like garnet-albite-biotite zone rocks in the Alpine Schist. The deformational fabrics and textures of the Alpine Schist in the McArthur Range have preserved evidence for several stages of foliation development. The foliations have been successively deformed by younger foliations to produce a complex composite fabric. The dominant foliation is a consistently planar foliation that formed from the transposition of an earlier foliation, which is preserved only as microfold hooks in the matrix and as relict internal foliations within porphyroblasts. Porphyroblast nucleation and growth is inferred to have occurred prior to the final formation of the dominant foliation. The dominant foliation has subsequently been weakly deformed by a crenulation foliation; this crenulation foliation has reinforced rather than overprinted the dominant foliation. The early, dominant and crenulation foliations are respectively interpreted to be comparable with the S1, S2 and Alpine (S3) foliations that have been well documented in the Franz Josef - Fox glacier region in the central Southern Alps (Little et al., 2002a). Whole rock geochemistry analyses of the Alpine Schist from the present study area show that the rocks are derived from a source very similar to the Torlesse terrane but with a more continental signature. The samples show very little affinity with the Caples or Aspiring terranes. The Alpine Schist from the present study area are distinct in their geochemistry as they have a low calcium content (calculated on a LOI and silica free basis normalised to 100%) that is approximately half that of the Alpine Schist from the Franz Josef - Fox Glacier region. The garnets in the greyschist from the study area very small (<1 mm) and are characterised by Fe-Mn-Ca chemistry with minimal Mg. Three types of garnets were identified: Mn-rich garnets that are gradually but strongly zoned from core to rim (Xsps≈20 to 10, Xalm ≈10 to 22, Xgrs≈12 to 10) (type A); Fe-rich garnets that show very little zoning from core to rim (Xsps≈20, Xalm ≈10, Xgrs≈10) (type B); and Fe-rich garnets with core compositions very similar to type B garnets that also show two distinct changes in composition near and at the rims (type C). The first compositional change is a steady decrease in the Ca content near the rims and is interpreted to represent the introduction of oligoclase into the system; the second compositional change is a sharp increase in the Ca content as a thin overgrowth on the garnets that is attributed to the formation of the Alpine Fault mylonite zone (Upton et al., 1995; Vry et al., 2004). Detailed PT paths for garnet growth constructed using Thermocalc show that the nucleation and garnet growth occurred over a wide range of pressures and relatively narrow range of temperatures. The composite PT path with increasing temperature passes through successive points of 380, 410, 440, 480 and 500°C respectively at 2.1, 5.0, 7.0, 9.5 and 6.0kbars. The Pt path shows that garnet growth occurred during both increasing and decreasing pressure and that peak pressure (9.5kbars) and peak temperature (510°C) conditions occurred at different stages of garnet growth; peak temperatures were not reached until subsequent decreases in pressure of ranging from ~0.5kbars to 1.5kbars. The PT paths for garnet growth also show that high whole rock MnO contents in samples significantly decreases the temperature at which garnet nucleation begins (up to ~100°C lower compared to rocks with normal or low whole rock MnO contents). By analysing garnets in a set of rocks that represent a large range in the whole rock MnO content, detailed PT paths that represent a large portion of the garnet growth history (i.e. nucleation through to peak temperatures) can be constructed. This effect of MnO content on the temperature of garnet nucleation is interesting as it shows that the first appearances of garnet in the McArthur Range are actually the last samples to nucleate garnets clue to their low whole rock MnO contents. Garnet nucleation occurred in the garnet-albite-biotite zone and the growth for many garnets was entirely within this zone. Some garnet growth continued after the formation of the higher temperature garnet-albite-oligoclase-biotite zone. The introduction of oligoclase into the rock is recorded as a significant decrease in the Ca content of these garnets. A method is offered by which to obtain realistic PT conditions for garnet growth using the traditional methods of geothermobarometry in rocks traditionally unsuitable for such studies where garnet is in equilibrium with albite and not oligoclase. The use of An~10 plagioclase instead of higher calcium oligoclase has given results using garnet-biotite thermometry and garnet-biotite-plagioclase-muscovite barometry that are closely comparable to the Thermocalc results. The calculated 11-component (MnNCKFMASHTO) PT pseudosections and PT paths for garnet growth in the present study area show that the commonly accepted PT paths for the Alpine Schist, which are based on work conducted in the Franz Josef - Fox Glacier region can not be completely correct. The PT pseudosections from the present study area show that the typical PT paths for the Franz Josef - Fox Glacier region can not start at the surface, reach the reported peak pressure and temperature conditions and still cross the oligoclase-in isograd prior to the garnet-in isograd as reported. Thermocalc results from the present study area show that the PT paths for the Alpine Schist in the Franz Josef - Fox Glacier region instead are most likely to start at much higher temperatures and at some depth. The Alpine Schist from the McArthur Range shows several significant differences compared to the well documented Alpine Schist of the Franz Josef - Fox Glacier region. The appearance of garnet with albite before oligoclase, and thus the presence of the garnet-albite-biotite zone, and the steep pressure/temperature gradient (high P/T conditions) inferred for garnet growth are unlike elsewhere in the Alpine Schist. The metamorphic assemblages, mineral compositions, and PT conditions for garnet growth from the present study area are instead strikingly similar to those reported in the Otago Schist. There they are attributed to a high P/T metamorphic event that predates the high T/P metamorphic event that is associated with the Alpine Schist. The deformational fabrics, sequence of mineral isograds, mineral chemistry and PT conditions of garnet growth in the Alpine Schist from the McArthur Range appear to have preserved evidence of a high P/T metamorphic event that is very similar or the same as the metamorphic event that formed the Otago Schist. This has previously not been recognised elsewhere in the Alpine Schist. The cause of the isolation of the garnet-albite-biotite zone rocks of the McArthur Range from the similar rocks of the Otago Schist, some ~300km to the south is not yet understood. The original spatial relationship between the two areas and any mechanisms responsible for the possible relocation of the rocks in the McArthur Range has yet to be determined.