TY - JOUR

T1 - Age and petrogenetic constraints on the Lower Glassy Ignimbrite of the Mount Somers Volcanic Group, New Zealand

AU - van der Meer, Quinten

AU - Waight, Tod Earle

AU - Whitehouse, Martin

AU - Andersen, Tom

PY - 2017

Y1 - 2017

N2 - The Mount Somers Volcanic Group (MSVG) forms a large (~18000 km2) calc-alkaline volcanic complex on New Zealand’s Eastern Province. U-Pb SIMS spot ages on zircon from the lower glassy ignimbrite in Rakaia Gorge reveal a bimodal distribution of 99.0 ± 0.5 and 96.3 ± 0.5 Ma (2σ). These ages are within error of previous (unpublished) zircon SHRIMP ages but have a precision that enables identification of two distinct episodes of zircon crystallisation, indicating magmatic activity over a period of at least 2.5 Ma. The younger age is interpreted to represent the emplacement age of the ignimbrite. The older age may originate from the assimilation of underlying ignimbrites or prolonged magma chamber processes. Combined O and Hf isotopes in zircon indicate that melt compositions for the ~99 Ma and ~96 Ma magmatic episodes were very similar and intermediate between the local upper crust and mantle derived melts, supporting formation through hybridisation of mantle and crustal-derived melts. MSVG magmatism preceded, coexisted with and was succeeded by local mafic alkaline HIMU-type magmatism (97.0 ± 0.5 Ma and younger) on the nearby Pahau Terrane. In contrast, the mafic component of the MSVG was derived from a depleted and potentially subduction-modified source associated with the Rakaia Terrane.

AB - The Mount Somers Volcanic Group (MSVG) forms a large (~18000 km2) calc-alkaline volcanic complex on New Zealand’s Eastern Province. U-Pb SIMS spot ages on zircon from the lower glassy ignimbrite in Rakaia Gorge reveal a bimodal distribution of 99.0 ± 0.5 and 96.3 ± 0.5 Ma (2σ). These ages are within error of previous (unpublished) zircon SHRIMP ages but have a precision that enables identification of two distinct episodes of zircon crystallisation, indicating magmatic activity over a period of at least 2.5 Ma. The younger age is interpreted to represent the emplacement age of the ignimbrite. The older age may originate from the assimilation of underlying ignimbrites or prolonged magma chamber processes. Combined O and Hf isotopes in zircon indicate that melt compositions for the ~99 Ma and ~96 Ma magmatic episodes were very similar and intermediate between the local upper crust and mantle derived melts, supporting formation through hybridisation of mantle and crustal-derived melts. MSVG magmatism preceded, coexisted with and was succeeded by local mafic alkaline HIMU-type magmatism (97.0 ± 0.5 Ma and younger) on the nearby Pahau Terrane. In contrast, the mafic component of the MSVG was derived from a depleted and potentially subduction-modified source associated with the Rakaia Terrane.

U2 - 10.1080/00288306.2017.1307232

DO - 10.1080/00288306.2017.1307232

M3 - Journal article

VL - 60

SP - 209

EP - 219

JO - New Zealand Journal of Geology, and Geophysics

JF - New Zealand Journal of Geology, and Geophysics

SN - 0028-8306

IS - 3

ER -