Petrogenesis and evolution of alkalic basaltic magmas in a continental intraplate setting: the South Auckland Volcanic Field, New Zealand
type: CreativeWork
description: The South Auckland volcanic field (SAVF) is situated in a continental intraplate tectonic setting and consists of silica-undersaturated alkalic basalts that erupted from about 100 centres between 1.59 and 0.51 Ma. The basalts range in composition from basanites to quartz tholeiites. Two basalt groups are distinguished based on their contrasting mineralogical and geochemical characteristics: group A, a less silica-undersaturated transitional basalt to quartz tholeiite suite; and group B, a strongly silica-undersaturated basanite to nepheline hawaiite suite. The petrogenesis and evolution of the SAVF basalts is examined using new data from 200 samples on mineral chemistry from electron microprobe analyses, major and trace elements from XRF, incompatible trace elements and REE from ICP-MS, and Sr, Nd, and Pb isotopic data from TIMS analyses. Forsteritic olivine is the dominant phenocryst phase in each rock group (group A: Fo₆₁ - Fo₈₂; group B: Fo₆₄ - Fo₉₂). Clinopyroxene is abundant and ranges from augite (Wo₃₅₋₄₇ En₃₆₋₅₀ Fs₁₁₋₂₃) in group A to diopside (Wo₄₂₋₅₂ En₃₂₋₄₆ Fs₁₀₋₂₀) in group B basalts. Plagioclase (An₄₀₋₇₀) is found as a phenocryst only in group A basalts. The group A basalts are distinguished by their low total alkalis (3.0 - 4.8 wt.%), Nb (9 - 29 ppm) and Zr (97 - 210 ppm) abundances, and low (La/Yb)N (3.4- 7.6), which contrast with group B that have large total alkalis (3.3 - 7.9 wt.%), Nb (35 - 102 ppm) and Zr (194 - 491 ppm), and (La/Yb)N (12 - 47). The incompatible trace elements of group A lavas have characteristics indicating derivation from an enriched upper mantle source but relatively depleted in Th, K, Nb, Ta, and LREE with small LREE/HREE values, whereas those in group B exhibit strong trace element affinities (i.e., large Th, Nb, Ta, and LREE abundances) with an OIB-like source. Petrogenetic modelling suggests that the group A and B lavas evolved as discrete lineages that do not appear to be related to a common parental magma or source. Parti…
abstract: The South Auckland volcanic field (SAVF) is situated in a continental intraplate tectonic setting and consists of silica-undersaturated alkalic basalts that erupted from about 100 centres between 1.59 and 0.51 Ma. The basalts range in composition from basanites to quartz tholeiites. Two basalt groups are distinguished based on their contrasting mineralogical and geochemical characteristics: group A, a less silica-undersaturated transitional basalt to quartz tholeiite suite; and group B, a strongly silica-undersaturated basanite to nepheline hawaiite suite. The petrogenesis and evolution of the SAVF basalts is examined using new data from 200 samples on mineral chemistry from electron microprobe analyses, major and trace elements from XRF, incompatible trace elements and REE from ICP-MS, and Sr, Nd, and Pb isotopic data from TIMS analyses. Forsteritic olivine is the dominant phenocryst phase in each rock group (group A: Fo₆₁ - Fo₈₂; group B: Fo₆₄ - Fo₉₂). Clinopyroxene is abundant and ranges from augite (Wo₃₅₋₄₇ En₃₆₋₅₀ Fs₁₁₋₂₃) in group A to diopside (Wo₄₂₋₅₂ En₃₂₋₄₆ Fs₁₀₋₂₀) in group B basalts. Plagioclase (An₄₀₋₇₀) is found as a phenocryst only in group A basalts. The group A basalts are distinguished by their low total alkalis (3.0 - 4.8 wt.%), Nb (9 - 29 ppm) and Zr (97 - 210 ppm) abundances, and low (La/Yb)N (3.4- 7.6), which contrast with group B that have large total alkalis (3.3 - 7.9 wt.%), Nb (35 - 102 ppm) and Zr (194 - 491 ppm), and (La/Yb)N (12 - 47). The incompatible trace elements of group A lavas have characteristics indicating derivation from an enriched upper mantle source but relatively depleted in Th, K, Nb, Ta, and LREE with small LREE/HREE values, whereas those in group B exhibit strong trace element affinities (i.e., large Th, Nb, Ta, and LREE abundances) with an OIB-like source. Petrogenetic modelling suggests that the group A and B lavas evolved as discrete lineages that do not appear to be related to a common parental magma or source. Parti…
author: Cook, Craig
license: https://researchcommons.waikato.ac.nz/bitstreams/2eb44d9c-f824-4192-8933-e92298fea439/download
date published: 2002
date modified: 2026-07-24T05:57:52Z
provider: University Waikato
publisher: Networked Digital Library of Theses and Dissertations
type: WebPage
source organization: The University of Waikato