Petrogenetic constraints on the origin and evolution of the volcanic rocks on Solander Island, New Zealand
Abstract
Quaternary (150-400 ka) volcanic rocks from Solander Island and Little Solander Island, south of South Island, New Zealand are related to the onset of oblique subduction of the Eocene-Miocene crust of the Australian Plate beneath the Pacific Plate. The eruptive products are amphibole-bearing trachyandesite-andesite (SiO2 58.20-62.19 wt. %) host to mafic porphyritic and equigranular enclaves (SiO2 54.40-54.92 wt. %). The rocks have elevated Sr/Y (65.83-146.39), LaN/YbN (17-50) and depleted heavy rare earth elements (HREE) and Y (10.86-16.62 ppm) indicating geochemical affinities with modern adakites and Archaean trondhjemite-tonalite-granodiorite (TTG) suites. The phenocryst assemblage in the host rocks comprises plagioclase, amphibole and biotite with accessory Fe-Ti oxides, apatite and zircon. Clinopyroxene is ubiquitous in the Little Solander rocks and is believed to be antecrystic. Plagioclase zoning includes oscillatory and patchy zonation and resorbed textures. Amphibole phenocrysts show simple, multiple or patchy zoning. Phenocryst zoning patterns have been used in this study to identify open-system processes relating to magma evolution and mixing. Resorbed plagioclase and simple-zoned amphibole phenocrysts in the host reflect late stage mixing with a mafic recharge magma enriched in Sr and Ba. Plagioclase cores with broad diffuse zones are considered to reflect long residence times in the magma chamber. In contrast oscillatory-zoned outer rims with dissolution textures may reflect variations in temperature and/or water content during the final stages of crystallization. Clinopyroxene antecrysts (XMg ~90) with reaction or overgrowth rims indicate disequilibrium within the host rocks. The host rocks record average pre-eruptive temperatures of ~780oC, pressures of 1.7-3.3 kbar and fO2 of 0.4-0.9 log units above the nickel-nickel oxide (NNO) buffer. The calculated melt in equilibrium with the clinopyroxene is basaltic (XMg ~70) with arc like characteristics (depleted Nb, Ta and Ti). Furthermore, these melts show a strong adakitic affinity with high Sr/Y and low HREE and Y, indicating that the adakitic signature is primary and has not been derived through crystal fractionation processes in the crust. The 87Sr/86Sr (0.702834-0.703011) and 143Nd/144Nd (0.512999-0.513055) plot within the E-MORB field, providing no clear evidence for assimilation of a crustal component consistent with the adakitic signatures reflecting a primary source feature.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.V43A2553F
- Keywords:
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- 1031 GEOCHEMISTRY / Subduction zone processes;
- 1036 GEOCHEMISTRY / Magma chamber processes