Geochemistry and petrogenesis of the newer basalts of Victoria and South Australia

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Abstract
The Pliocene‐Recent Newer Basalts province contains a large variety of magma types: quartz tholeiite, olivine tholeiite, olivine basalt, alkali olivine basalt, hawaiite, K‐rich hawaiite, nepheline basanite, transitional olivine analcimite and olivine nephelinite (normatively low‐K basanite to low‐K nepheline mugearite), nepheline hawaiite, K‐rich nepheline hawaiite, and nepheline mugearite. Despite some spatial restriction of the different magmas, there is no well‐defined compositional progression with time. Many of the alkaline lavas contain Cr‐diopside lherzolite‐series xenoliths and must be of direct upper‐mantle derivation. Based on systematics of Mg‐values (100 Mg/Mg + Fe2+ ratios) coupled with data for the olivine‐liquid equilibrium and the results of other experimental studies, it is deduced that most of the basanitic magmas are primary melts of garnet‐peridotite mantle, essentially unmodified by crystal fractionation. The nepheline hawaiites and nepheline mugearites are considered to be fractionated liquids derived from basanites by progressive removal of mainly olivine and kaersutite at mantle pressures. The hawaiites may be similarly related to alkali olivine‐basalt parental liquids. The olivine tholeites and olivine basalts cannot be primitive mantle melts and must have undergone fractionation of olivine and perhaps pyroxene. The quartz‐tholeiite and K‐rich hawaiitic magmas are probably products of low‐pressure fractionation processes. A synthesis of chemical data for Tertiary‐Quaternary basaltic lavas from throughout eastern Australia reveals that nepheline basanite is the most widespread mantle‐derived magma type, and that the high‐pressure lineage nepheline basanite ? nepheline hawaiite ? nepheline mugearite ? nepheline benmoreite is much more important than previously recognized.