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General Features of Magmatic Evolution Throughout the Earth’s History
Published in O.A. Bogatikov, R.F. Fursenko, G.V. Lazareva, E.A. Miloradovskaya, A. Ya, R.E. Sorkina, Magmatism and Geodynamics Terrestrial Magmatism Throughout the Earth’s History, 2020
O.A. Bogatikov, V.I. Kovalenko, E.V. Sharkov
In general, the origin of the spilites remains uncertain. They did not appear until 2 Ga ago, concurrent with the first Fe–Ti basitic rocks, and these two rock types have much in common in terms of their mineral composition and geochemistry. Spilites have declined up to the present day. The conventional view is that spilites were derived from the alteration of tholeiitic basalts as a result of their reaction with sea-water (Amshtutz, 1974; Wedepohl et al., 1983), but this is thought to be unlikely, because on the present-day ocean floor where this reaction should have proceeded on a large scale, spilites have not been observed. The hypothesis of their relationship with regional Na-type metamorphism, characteristic of the early stages of orogenic belts, still does not solve the problem. Together with high Na content, they possess all the other attributes of moderately alkaline Fe–Ti basalts, in both the composition of their relict igneous minerals (titanomagnetite, titan-augite, kaersutite, apatite, etc.) and by their major and rare elements. The latter are characterized by high concentrations of Ti, Fe, P, Y, Ba, Be, V, Sr, Zr, etc., contrasting sharply with those of MORB tholeiites.
Petrology and petrogenesis of an intraplate alkaline lamprophyre-phonolite-carbonatite association in the Alpine Dyke Swarm, New Zealand
Published in New Zealand Journal of Geology and Geophysics, 2020
The Alpine Dyke Swarm is composed predominantly of lamprophyres, both alkaline and ultramafic, that were generated by partial melting of refractory, but subsequently metasomatised, volatile-rich garnet-bearing spinel peridotite mantle at approximately 25 Ma. Fractionation of olivine, kaersutite, clinopyroxene and titano-magnetite drove the evolved magma to intersect the silicate melt-carbonatite miscibility gap, resulting in generation of conjugate phonolite and alkaline carbonatite magmas coexisting with a fluid phase. Intrusion from a subvolcanic magma chamber beneath Haast River resulted in diffusion of this fluid phase into the country rock Haast Schist forming metasomatic sodic fenites. Carbonatites have a wide range of compositions, fractionating to late-stage ferro- and REE-Ba-Sr-rich varieties. The ADS is enriched in volatiles and incompatible elements compared to the DVG of East Otago. This enrichment suggests derivation of the ADS by melting of an intensely metasomatised mantle source that contains a complex mixture of depleted components, some of which may be as old as Archean (Liu et al. 2015).
Intraplate volcanism on the Zealandia Eocene-Early Oligocene continental shelf: the Waiareka-Deborah Volcanic Field, North Otago
Published in New Zealand Journal of Geology and Geophysics, 2020
James M. Scott, James D. L. White, Petrus J. le Roux
Mantle xenolithic material occurs in five locations (Scott 2020). Very small (∼1 cm) spinel peridotite fragments occur in tuff beds above the intact pillow lava at Boatmans Harbour (Coombs et al. 1986; Moorhouse 2015) (Figure 4A). The second occurrence is in lapilli tuff ∼ 2.5 km east of Round Hill, which is packed with moderately altered spinel peridotite xenoliths up to ∼ 8 cm in diameter (Scott et al. 2014b). A third occurrence is in thin lapilli tuff and tuff breccia beds directly beneath Ototara Limestone at Alma. These beds contain spinel peridotite and garnet pyroxenite xenoliths, as well as garnet, kaersutite, anorthoclase and augite megacrysts (Reay et al. 2002; Scott et al. 2014b). The peridotite clasts from Alma are extremely altered, with the mantle orthopyroxene and olivine having been completely converted to clay (Scott et al. 2014b). It is not known whether spinel lherzolite-bearing basaltic boulders in the river at Five Forks originate from the Dunedin Volcanic Group or Waiareka-Deborah-Volcanic Field.
Age and tectonic significance of the Louth Volcanics: implications for the evolution of the Tasmanides of eastern Australia
Published in Australian Journal of Earth Sciences, 2018
R. C. Dwyer, W. J. Collins, A. C. Hack, R. Hegarty, H.-Q. Huang
Two distinct igneous rock groups can be recognised based on their petrographic characteristics: alkaline and calc-alkaline. Intrusives of the alkaline group are typically doleritic to gabbroic with a diagnostic alkaline phenocryst assemblage that includes titanaugite and kaersutite (Figure 3a, b) (MYRTLE-06, NBHT1-06 and PANGBH7-10). Textually, all gabbro and doleritic samples (including Getty Gabbro from GETTY6/7-1-01: Glen et al., 2013) have complex subophitic to ophitic intergrowths (Figure 3a, b), suggesting coprecipitation of titanaugite and kaersutite with plagioclase. Symplectic intergrowths of magnetite and biotite are also a common feature of the alkaline gabbros. A striking example of the mineralogy and preservation of the alkaline samples is from an ankaramite at the base of PANGBH5. This olivine-phyric ankaramite has intergrowths of plagioclase and titanaugite forming a weakly flow aligned, trachytic-like felty matrix dominated by plagioclase. Deformation and alteration are variable within the alkaline rocks but limited to fractures and localised shears associated with moderate chloritisation and less commonly pervasive sericitisation of feldspars with chlorite replacing biotite. Generally, mineral assemblages in the alkaline mafic rocks are well preserved to pristine. For example, the pristine preservation of the ankaramite from PANGBH5 is revealed by the presence of clear unaltered groundmass and fresh olivine.