Peak metamorphic temperature and thermal history of the Southern Alps (New Zealand)

Abstract

The Southern Alps of New Zealand result from late Cenozoic convergence between the IndoAustralian and Pacific plates, and are one of the most active mountain belts in the world. Metamorphic rocks carrying a polymetamorphic legacy, ranging from lowgreenschist to high-grade amphibolites, are exhumed in the hanging wall of the Alpine Fault. On a regional scale, the metamorphic grade has previously been described in terms of metamorphic zones and mineral isograds; application of quantitative petrology being severely limited owing to unfavourable quartzofeldspathic lithologies. This study quantifies peak metamorphic temperatures (T) in a 300 x 20 km area, based on samples forming 13 transects along-strike from Haast in the south to Hokitika in the north, using thermometry based on Raman spectroscopy of carbonaceous material (RSCM). Peak metamorphic T decreases across each transect from ≥ 640°C locally in the direct vicinity of the Alpine Fault to less than 330°C at the drainage divide 15-20 km southeast of the fault. Thermal field gradients exhibit a degree of similarity from southernmost to northernmost transects, are greater in low-grade semischist than high-grade schist, are affected by folding or discontinuous juxtaposition of metamorphic zones, and contain limited information on crustal-scale geothermal gradients. Temperatures derived by RSCM thermometry are slightly (≤ 50°C) higher than those derived by traditional quantitative petrology using garnet-biotite thermometry and THERMOCALC modeling. The age of RSCM T appears to be mostly pre-Cenozoic over most of the area except in central Southern Alps (Franz Josef-Fox area), where the amphibolite facies schists have T of likely Cenozoic age. The RSCM T data place some constraints on the mode of exhumation along the Alpine Fault and have implications for models of Southern Alps tectonics.

Contrasting degrees of recrystallization of carbonaceous material in the Nelson aureole, British Columbia and Ballachulish aureole, Scotland, with implications for thermometry based on Raman spectroscopy of carbonaceous material

“…Recrystallization of CM and graphitization in the Ballachulish aureole was delayed likely due to the pretexturation of CM during regional metamorphism with the result that the RSCM thermometry underestimates peak temperature in the aureole. Such a delay in graphitization has not been observed elsewhere in the case of two successive regional metamorphic events even if the first one reached high‐grade conditions: this was shown in the central part of the southern Alps of New Zealand (Beyssac, Cox, Vry, & Herman, ) or in some parts of the European Alps (Wiederkehr, Bousquet, Ziemann, Berger, & Schmid, ). Note that in these settings, the successive regional metamorphisms are mostly due to burial related to subduction and/or collision.…”

High‐ to ultrahigh‐temperature metamorphism in the lower crust: An example resulting from Hikurangi Plateau collision and slab rollback in New Zealand

“…The 40 Ar/ 39 Ar ages of ~110–90 Ma in the core represent exhumation‐driven cooling (Gray & Foster, ). Amphibolite facies conditions are preserved in the Alpine Schist adjacent to the Alpine Fault (Figure inset), where the highest grade rocks are estimated to have reached ~600°C and 1.0 GPa in the Late Cretaceous to Palaeocene (Beyssac, Cox, Vry, & Herman, ; Cooper, ; Cooper & Ireland, ; Grapes & Watanabe, ; Scott et al., ; Vry, et al., ).…”

Constraints on Alpine Fault (New Zealand) Mylonitization Temperatures and Geothermal Gradient from Ti-in-quartz Thermobarometry

“…Thermobarometric data for the Alpine Fault mylonites and Alpine Schists (Beyssac et al, 2016;Cooper, 1980;Grapes and Watanabe, 1992;Grapes, 1995;Vry et al, 2004) indicate that mylonitization began at about 600 °C and 11 kbar. Toy et al (2008) and Little et al (2016;2015) observed crystallographic preferred orientations in quartz that generally form at upper greenschist to amphibolite facies conditions (~500-600 °C).…”

“…Given the high shear strains experienced by the Alpine Fault mylonites (Norris and Cooper, 2003), these late-stage veins formed during the last few percent of penetrative mylonitic strain. The presence of chlorite in these veins constrains their emplacement temperatures and therefore the cessation of mylonitization to within the chlorite stability field, a relatively wide range of ~360-500 °C in 10 Alpine Fault rocks (Beyssac et al, 2016;Vry et al, 2007).…”