Geochemistry of the Kaiwekite flow, East Otago, New Zealand

Gurney, Patricia.

The Kaiwekite is an inclusion-rich lava flow outcropping in cliffs on either side of the mouth of the Otago Harbour, and was erupted during the First Main Eruptive Phase (Benson, 1941) of the Dunedin Volcano. It contains a large variety of lithic inclusions and kaersutite megacrysts, many of which are associated with calcite.
The Kaiwekite contains phenocrysts of clinopyroxene, kaersutite, olivine, feldspar, and titanomagnetite, within a microcrystalline groundmass of feldspar, clinopyroxene and titanomagnetite. The phenocrysts commonly display zoning, rimming, and resorption textures. At least two chemically distinct groups can be recognised for each of the main phenocryst phases. These do not correlate- simply with .observable petrographical groups. The presence of features indicating strong resorption in clinopyroxene and feldspar phenocrysts within the Kaiwekite are thought to have arisen through decompression on rapid transport to the surface, or magma mixing. Disequilibrium textures suggest changing physical conditions in the magma chamber, and a possible high pressure origin for some of the clinopyroxene phenocrysts is suggested by their high Al203 contents, feldspars by their high Ca contents, and for some of the olivine phenocrysts by their high Mg contents.
The major element chemistry does not reflect the homogeneous appearance of the rock. It has a highly variable Mg-number (reflecting variations in the MgO and Fe203 contents of the rocks) which is suggestive of quite evolved rocks (benmoreite, mugearite, hawaiite, trachyandesite) as does its alkali content. The similarity of the Kaiwekite's trace element pattern to that of crustally fractionated rocks from the Dunedin Volcano, and modelling done in this stud}) suggest that low-pressure crystal fractionation may have influenced the composition of the rock. Additional evidence from the petrography and trace element data suggests that magma mixing, or simply phenocryst resorption (and/or decompression through rapid transport to the surface) may also have had an influence on the composition of the Kaiwekite.
The lithic suite is composed of lower crustal-derived cognate xenoliths (cumulates crys~(sed at depth early in the history of the host rock) and accidentally incorporated fragments of sedimentary rocks of the Dunedin Tertiary Sequence and Haast Schist.
The kaersutite megacrysts are also believed to be cognate to their host, having a crustal origin. They exhibit major element chemistries similar to kaersutite megacrysts occurring associated with alkali basaltic rocks from around the world, but exhibit significantly higher Ti contents than the other analysed megacrysts.
Major and trace element modelling carried out in this study suggests that crystal fractionation, in association with magma mixing and (possibly) ponding in several magma chambers at different levels in the crust, are the major processes influencing the composition of !he Kaiwekite.

144 p. : col. ill., maps ; 30 cm.

1993Gurney