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 40 Ar ⁄ 39 Ar, 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.A detailed timing history for the growth of a single garnet is recorded by a Sm-Nd garnet-whole rock age of 97.8 ± 8.1 Ma for the inmost garnet core (zone 1), Lu-Hf ages of 86.2 ± 0.2 Ma and 86.3 ± 0.2 Ma for overgrowth zones 2 and 3, a step-leach Sm-Nd age of 12 ± 37 Ma for zone 4, and growth of the garnet rim (zone 5) over the Alpine Fault mylonite foliation during the modern phase of oblique collision that began at c. 5-6 Ma.Plate convergence along the New Zealand portion of the Gondwana margin continued after c. 105 Ma, almost certainly culminating in the oblique collision of a large oceanic plateau (Hikurangi Plateau). The metamorphism of the Alpine Schist at c. 86 Ma is evidence of that hit. The mid-to lateCretaceous extension that is widespread elsewhere in the New Zealand region is attributed to upper plate extension and slab roll-back. The effects of the collision with the Hikurangi Plateau may have contributed to the changing plate motions in the region leading up to the opening of the Tasman Sea at c. 82 Ma.

Strain localisation within ductile shear zones beneath active faults: The Alpine Fault contrasted with the adjacent Otago fault system, New Zealand

“…Mylonitisation increases inhomogeneously westwards across the zone from amphibolite facies schist protolith in the east, to protomylonite, mylonite, and ultramylonite adjacent to the active fault trace. Mineral assemblages in the mylonites indicate deformation under amphibolite facies conditions at depths of 20-30 km (Holm et al, 1989;Grapes and Watanabe, 1994;Vry et al, 2004). We can definitely say that a narrow zone of faulting with a slip rate of around 25 mm/yr at the surface overlies a mylonite zone 1-2 km wide originally developed at a depth of 20-30 km.…”

Episodicity of Mesozoic terrane accretion along the Pacific margin of Gondwana: implications for superplume-plate interactions

“…In North Island, the situation was complicated by the approach of the Hikurangi Plateau (Vry et al 2004) and a possible ridge-trench collision (Luyendyk 1995).…”

“…This was followed by opening of the Tasman Sea (Vry et al 2004) and associated basins (Carter 1988), and onset of New Zealand-Antarctica rifting (Laird & Bradshaw 2004). …”

Lead isotope constraints on the origin of Cenozoic orogenic gold systems in the Southern Alps and northwestern Otago, South Island, New Zealand

“…The nature and timing of these younger metamorphic events are not well understood. Garnet grade metamorphic mineral assemblages in various portions of the Alpine Schist have given U-Pb, Sm-Nd and Lu-Hf ages that range from c. 100 to c. 70 Ma (Mortimer & Cooper 2004;Vry et al 2004). Volumetrically minor anatectic pegmatite dikes in the Alpine Schist have also given U-Pb ages of c. 70 Ma (Chamberlain et al 1995;Batt et al 1999).…”

“…Volumetrically minor anatectic pegmatite dikes in the Alpine Schist have also given U-Pb ages of c. 70 Ma (Chamberlain et al 1995;Batt et al 1999). This prolonged Middle to Late Cretaceous metamorphism and associated deformation is interpreted as related to the collision and subsequent underthrusting of the Hikurangi oceanic plateau (Vry et al 2004). Post-Miocene deformation becomes progressively more intense from faulting near the Main Divide, through upright folding in greenschist facies, to the development of new foliations in upper greenschist and amphibolite facies.…”