Comparison Of The Petrology And Mineral Chemistry Of The Coromandel And Kiwitahi Volcanic Rocks, North Island, New Zealand

Abstract

This is thesis was done based on the textural, mineralogical and mineral chemistry studies of arc rocks from the North Island of New Zealand. The rocks in the area are of two main groups: the Kiwitahi and Coromandel groups. Both the Coromandel group and Kiwitahi andesites are typically porphyritic with a phenocryst assemblage (20–60%) consisting of plagioclase + pyroxene + Fe –Ti-oxide in a granular groundmass rich in plagioclase laths and pyroxene microcrystals. The Kiwitahi samples consist of andesite, dacite and basalt. The Kiwitahi samples which are from two of the centers namely the Miranda and Tahuna are all andesites. The Coromandel and Kiwitahi rocks are arc rocks and the arc volcanism has migrated south through the North Island over the last 25 Ma. The Colville Ridge is an oceanic arc, and the Coromandel and Kiwitahi are parallel continental arcs, all of which were active during the Miocene. The study was conducted to identify the different rock types in the study area based on their petrology and mineral chemistry and also to determine their state of equilibrium against their state of disequilibrium using the same dataset. The prepared thin sections were first studied under a petrographic microscope, based on which the petrographic description of the samples was done and photomicrographs of interesting features were taken. Afterwards, selected thin sections were taken to the microprobe laboratory for further studies. The samples were studied under the microprobe and BSE images of interesting features were taken. The Kiwitahi Tahuna andesite was found to be petrologically comparable to the Miranda andesite but lacks hornblende. Plagioclase is the dominant mineral in the samples and occurs as euhedral or subhedral crystals, up to 2-3 mm in size, some of which show complex internal structures. These internal structures which include oscillatory zoning, internal resorption surfaces and sieve textured cores are very common in the plagioclase crystals in the rock, and are indicative of disequilibrium. The pyroxene crystals also have reaction rims with Ti-magnetite inclusions which is also indicative of disequilibrium. The sieve texture in the plagioclase favours a magmatic interaction process. Thus the samples studied have undergone disequilibrium through magmatic interaction which can possibly be magma mixing due to the occurrence of the sieve and boxy-cellular textures which denotes that the undercooling of a more mafic magma occurred in contact with a cooler more evolved magma.