China
Africa
Explore chapters and articles related to this topic
Igneous rocks
Published in W.S. MacKenzie, A.E. Adams, K.H. Brodie, Rocks and Minerals in Thin Section, 2017
W.S. MacKenzie, A.E. Adams, K.H. Brodie
Phonolite is the volcanic equivalent of a nepheline syenite so that its essential constituents are nepheline and alkali feldspar, generally with a small amount of alkali pyroxene. This rock (Figure 119, Figure 120) contains euhedral phenocrysts of nepheline in a groundmass of nepheline and lath-shaped alkali feldspar crystals. There are also microphenocrysts of nepheline and lath shaped, brownish-green pyroxenes. Some of the nepheline crystals are rectangular in outline and some are hexagonal. Those that are hexagonal are black or very nearly black in the view with crossed polars (Figure 120). Since the birefringence and refractive indices of nepheline and feldspar are very similar it is difficult to distinguish them but a few differences can be noted: nepheline does not have two perfect cleavages like feldspar, it does not form simple twins and it has straight extinction in all sections whereas alkali feldspars have straight extinction only in some sections.
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).
Petrogenetic links between the Dunedin Volcano and peripheral volcanics of the Karitane Suite
Published in New Zealand Journal of Geology and Geophysics, 2018
Oliver E. McLeod, James D. L. White
Feldspar compositions span from moderately calcic to strongly sodic plagioclase to alkali feldspar (Figure 8B). Groundmass microlites in basanite range from An46–62 and plot within the labradorite field. In trachyandesite, phenocrysts are zoned and range from An17–23 and plot just outside of the oligoclase field due to their potassic (Or8–15) compositions. Core compositions range from An19–27 and the first zone rims show variable, small fluctuations in An and Ab content. The second rims (outermost zones) show increased Or contents up to about 11 mol%. In phonolite, the main feldspar composition is sanidine (Or38–54). Trace element contents are illustrated in plots H and I of Figure 9. Strontium contents of feldspar (H) are highest in trachyandesite (0.30–0.63 wt%), slightly lower in basanite (0.18–0.5 wt%), and < 0.1 wt% in phonolite. One feldspar in phonolite has similar CaO and Sr content to those in trachyandesite. Barium contents of feldspar show similar variation across the three rock groups of up to at least 0.25 wt %. The highest contents are in trachyandesite (0.09–0.36 wt%) whereas most feldspars in basanite and phonolite contain ≤ 0.15 wt% BaO.