Metasomatised mantle xenoliths from lamprophyre dikes of the Fish River, New Zealand
Minerals, textures and bulk chemistries of mantle xenoliths from the Fish River, Haast Pass record a history of successive crystallisation, deformation, multiple metasomatism and lamprophyre melt generation. Outcropping in the upper Fish River are numerous dikes of alkaline lamprophyre, ultramafic lamprophyre, tinguaite and carbonatite which intrude the TZ2 to TZ4 Haast Schists. Fenitised schist and networks of very thin ankeritic veins are associated with the intrusions. Ultramafic lamprophyre dikes commonly contain rounded Cr-diopside group peridotites, Al-augite group cumulates, amphibole-apatite, gabbro and schist xenoliths and amphibole megacrysts. Many of the xenoliths and xenocrysts exceed 30 cm in diameter. The Cr-diopside group xenoliths are dominantly low-Al harzburgites. Whole rock chemistry suggests they represent compatible element-rich mantle residues formed from the extraction of a 20% partial melt at subsolidus temperatures. Undulose extinction in harzburgite olivine and bent exsolution lamellae in orthopyroxene indicate that the mantle experienced a period, or several periods, of subsolidus deformation. Thin orthopyroxenite layers and prevalent chromiteorthopyroxene- olivine-clinopyroxene symplectites also formed during mantle deformation events. Subsequent to deformation, the mantle experienced millimeter scale metasomatism by two distinct agents. The first was a low viscosity, grain boundary infiltrating melt which reacted with the host peridotite to produce high Mg#, high-Cr, mica, diopside and pargasitic amphibole along with LREE-enriched apatite and carbonate. Chromite in symplectite intergrowths was particularly reactive. Geochemical evidence including low Ti/ Eu and elevated Ca/ AI ratios suggests this melt was carbonatitic in character. Fe, K and Ti-rich, hydrous melts produced during alkaline magma genesis infiltrated the surrounding peridotite and crystallised as veins of Ti-rich phlogopite, amphibole, clinopyroxene and titanomagnetite. Metasomatism associated with this episode of melt intrusion is restricted to <1 0 mm wide zones along the margins of the veins as fluids expelled from the crystallising veins reacted with and recrystallised the host peridotite. Host orthopyroxene was much more reactive than host olivine. Among the mantle xenolith suite of the Fish River are rare chromite-dunite metacumulate xenoliths that consist almost entirely of coarse olivine porphyroclasts which poikilitically enclose spherical chromite grains. Many chromite grains are recrystallised and surrounded by a halo of fine-grained metasomatic minerals. The peculiar halo zone mineralogy of one of these xenoliths, including the growth ofrichteritic amphibole indicates the influence of a further, Narich metasomatic agent with a high Si/ AI ratio. The presence of chromite-dunite xenoliths and orthopyroxenite layers argues for a compositionally zoned mantle beneath the Fish River. Small degree, lithospheric melting of this volatile and LREE-enriched mantle produced a lamprophyre magma, which sampled and entrained fragments of the wall rock peridotite as it rose quickly to the surface. Segregation of the volatile-rich magma from the xenoliths near the surface produced dikes ofxenolith-rich magma residue.
195 leaves : col. ill., maps ; 30 cm.
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POLYGON ((169.062799728155198 -44.271413393530317,169.089291660416052 -43.929500731082406,169.440295555088682 -43.944920812282227,169.427151069625808 -44.088324124468734,169.219848049939827 -44.277917344763722,169.062799728155198 -44.271413393530317))
Norrie, Brendan Harwood, 1976-, “Metasomatised mantle xenoliths from lamprophyre dikes of the Fish River, New Zealand,” Otago Geology Theses, accessed April 16, 2021, http://theses.otagogeology.org.nz/items/show/368.