Retrograde alteration of chromian kyanite in metachert and amphibolite whiteschist from the Southern Alps, New Zealand, with implications for uplift on the Alpine Fault

Abstract

Chromian kyanites with a maximum content of 2.88 wt.% Cr2O3 occur in metachert and amphibolite from the Southern Alps, New Zealand. The presence of the whiteschist assemblage kyanite-talc, together with kyanite-zoisite assemblages in calc silicate bands imply high pressure metamorphism, with climactic conditions of approximately 10 kb at 650°–700° C. Mylonitization caused by a change to oblique-slip movements on the Alpine Fault is succeeded by retrograde alteration of kyanite-bearing assemblages. Kyanite is pseudomorphed by Cr-margarite-fuchsite-Cr-zoisite assemblages in metachert and by less chromian margarite and zoisite in amphibolite. Contemporaneously hornblende and phlogopite break down to chlorite. Subsequently the metachert pseudomorphs are mantled by muscovite and those in amphibolite by anorthite and chromite. The breakdown of margarite and zoisite to anorthite implies decompression under a low thermal gradient, compatible with almost isothermal uplift on the Alpine Fault. Late stage retrograde products include fibrous kyanite (probably forming by recrystallization of primary alluminosilicate) and scapolite (possibly orginating through interaction of Cl-bearing fluids in a geothermal system).