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.
Key insights into the timing of tectonometamorphic events in a complex high-grade metamorphic terrane can be obtained by combining results from SHRIMP I1 ion microprobe studies of individual monazite grains with SHRIMP I1 studies and scanning electron microscope (SEM)-based cathodoluminescence (CL) imaging of zircons. Results from the Reynolds Range region, Arunta Block, Northern Territory, Australia, show that the final episode of regional metamorphism to high-T and low-P granulite facies conditions is most likely to have occurred at c. 1580 Ma, not at 1785-1775 Ma, as previously accepted. The previous interpretation was based on zircon studies of structurally controlled granitoids, without SEM-based CL imaging. Monazites in a 1806 & 6 Ma megacrystic granitoid preserve rare cores that are interpreted to be inherited magmatic monazite, but record no evidence of another high-T event prior to 1580 Ma. Most monazites from the region record only a single high-T metamorphic event at c. 1580 Ma. Zircon inheritance is very common. Zircons or narrow overgrowths of zircon dated at c. 1580 Ma have only been found in two types of rocks: rocks produced by metasomatic fluid flow at high temperatures (2 750 "C), and rocks that have undergone local partial melting. Previous explanations that attributed these 1580 Ma zircon ages to widespread hydrothermal fluid fluxing associated with post-tectonic pegmatite emplacement at amphibolite facies conditions are not supported by the available evidence including oxygen isotope data.The observed high regional metamorphic temperatures require the involvement of advective heating. However, contrary to a previous tectonic model for the formation of this and other low-P, high-T metamorphic belts, the granites that are exposed at the present structural level do not appear to be the source of that heat, unless some of the granites were emplaced at c. 1580 Ma.
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