Mantle xenoliths and metasomatism at Kattothyrst, North Otago

Hodgkinson, Alice

Ultramafic xenoliths represent fragments of the upper mantle brought to the Earth’s surface during mafic volcanism and offer a rare opportunity to study the mineralogy, textures and processes associated with the mantle. The present study was based at Kattothyrst, a remnant volcanic neck in the Kakanui Range of north Otago. The host alkaline basanite outcrop of the Waipiata Volcanic Group encloses an abundance of mantle xenoliths of spinel lherzolite, harzburgite, dunite and wehrlite composition whose mineralogy comprises combinations of olivine, Cr-enstatite ± Cr-diopside ± spinel. Reconnaissance electron backscatter diffraction patterns suggest that at least one of the samples has a high temperature fabric and strong lattice preferred orientation. Rare samples have been found to contain modal apatite, a metasomatic mineral introduced to the mantle beneath Otago by a mobile fluid or melt which has undergone incompatible element exchange with the surrounding mantle.
Thermobarometry of the xenoliths yielded relatively consistent temperatures of crystallisation of 878-990°C (for pressures of between 10 and 20 Kb). Chemical LA-ICP-MS analyses of clinopyroxene grains show the whole suite has been enriched in light rare earth elements (LREE). Harzburgites are extensively melt depleted and show M (middle) REE and H (heavy) REE depletion trends that probably pre-date LREE enrichment. The trace element study, coupled with the fluid inclusion and microprobe analysis, indicates the metasomatic agent was probably rich in fluorine as well as LREE, Ca and P. The metasomatic agent is inferred to have most likely been a carbonatite melt. The timing of metasomatism is not known but must predate volcanism of about 16 Ma. The temperature differences between specimens from Kattothyrst and nearby Waipiata Volcanics enable a broad Miocene mantle stratigraphy to be constructed beneath the Kakanui Range. Given the geochemical evidence for significant melt depletion, the mantle history recorded probably represents old Gondwana margin processes.

vii, 118 pages : illustrations (some colour), 30 cm.+ 1 CD-ROM.

2012Hodgkinson

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