Zoned (Cretaceous and Cenozoic) garnet and the timing of high grade metamorphism, Southern Alps, New Zealand

Abstract

New (garnet Sm-Nd and Lu-Hf) and existing (Rb-Sr, 40Ar/39Ar, U-Pb and Sm-Nd) ages and data on deformational fabrics and mineral compositions show for the first time that the garnet growth and ductile deformation in the Alpine Schist belt and Southern Alps orogen, New Zealand are diachronous and partly Cenozoic in age. The dominant metamorphic isograds in the Alpine Schist formed during crustal thickening at a previously unsuspected time, at c. 86 Ma, immediately prior to the opening of the Tasman Sea at c. 84-82 Ma. Obvious changes in the textures and compositional zoning patterns of garnet are not always reliable indicators of polymetamorphism, and fabric elements can be highly diachronous...