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Principles of Petrological Analysis of Igneous Rocks
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
The K–Na moderately alkaline series (alkali basalt) consists of high-Ti and low-Ti varieties. The first one is composed of moderately alkaline picrobasalt (ankaramite), olivine alkali basalt, alkali basalt, hawaiite, trachyte and moderately alkaline and alkaline dacite and rhyolite. Rocks of andesitic composition (trachyandesite, benmoreite) are subordinate, as in the tholeiitic series, resulting in a bimodality characteristic of the volcanic activity of ocean islands and continental rift zones. The low-Ti series is characteristic of active plate margins.
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.