China
Africa
Explore chapters and articles related to this topic
Magmatism in the Context of the Present-Day Tectonic Settings
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, V.V. Yarmolyuk
Structures of hot-spot type, not related to rifting areas, show a similar pattern of evolution. The most typical is the compositional variability, due apparently to intra-chamber differentiation. This is reflected in a sequence of eruptions from older, moderately alkaline basalts to younger sialic and alkaline products. In Africa (Ahaggar and Djos plateaux), there are phonolite and trachyte stocks and domes capping basaltic plateaux (Tibesty), and in eastern Australia there are trachytes, trachyrhyolites, comendites and pantellerites generated during the final stages of the development of large shield volcanoes. In the Hawaiian Islands, incipient magmatism is alkaline-basaltic, and there are tholeiite basalts (shield volcano stage, the largest in volume), which, during their evolution, give way to alkaline and high alkaline rocks (Decker et al., 1987). Continental rift and hot-spot moderately alkaline basalts and nephelinites often contain mantle xenoliths: mainly spinel lherzolite and less commonly garnet lherzolite and websterite, which could be fragments of asthenospheric material.
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
Figure 80 is a peridotite consisting of olivine and clinopyroxene and can be classified (see Figure 79) as a lherzolite (as sufficient orthopyroxene is also present in areas of the rock). All the minerals show relatively high relief and little colour so it can be difficult to distinguish the pyroxenes. The olivine grains have higher relief in plane polarized light (Figure 80) and also have brighter interference colours in crossed polars (Figure 81). They show the fractures which are characteristic of olivine. The grain boundaries are very distinct due to minor alteration, probably to serpentine. Figure 82 & Figure 83 shows another peridotite containing olivine with some orthopyroxene, The olivine grains here are small and form a texturally equilibrated granoblastic texture. The orthopyroxene shows cleavage and first order grey interference colours (Figure 83) and is elongate (in right hand part of view), probably as a result of high temperature deformation. An opaque mineral is also present which is likely to be a Cr spinel.
Basaltic dykes and their xenoliths from the Gerroa–Kiama region, southern Sydney Basin, New South Wales: evidence for multiple intrusive episodes
Published in Australian Journal of Earth Sciences, 2022
S. Abu-Shamma, I. T. Graham, P. Lennox, G. Bann, A. Greig
The Gerringong Volcanic Complex (Figure 3), which is late Permian (265–263 Ma; Belica et al.,2017; Campbell et al.,2001), is a collection of flows and sills that consists of ten extrusive latite members and one tuff member that are shoshonitic in character and vary from basaltic andesite to basalt and andesite (Bann & Jones, 2001; Campbell et al.,2001; Carr, 1984). The lava flows significantly diminish in thickness to the west (Harper, 1915). Field evidence suggests that the flows that constitute the Bumbo and Blow Hole latite members flowed to the north and north-northwest, respectively (Campbell et al.,2001). The origin of these flows is most probably an emergent island volcano or volcanic archipelago that formed offshore from the current coastline a few tens of kilometres to the south-southeast of Kiama (Bann, 1999; Bann & Jones, 2000; Campbell et al.,2001; Harper, 1915). Sedimentological and geophysical data indicate that the Gerringong Volcanic Complex ranges for at least 200 km offshore to the northeast (Carr, 1984). Possible source compositions and trace-element modelling indicate that the magmas were produced by 10–15% partial melting of spinel lherzolite that had been formerly enriched in incompatible elements. The most basic rocks of the area were formed by 20–30% fractionation of early-produced olivine, spinel and pyroxene (Carr, 1984; Carr & Fardy, 1984).
Influence of host magma alkalinity on trachytic melts formed during incongruent orthopyroxene dissolution in mantle xenoliths
Published in New Zealand Journal of Geology and Geophysics, 2020
Andreas Auer, Marco Brenna, James M. Scott
The alkaline intraplate shield Dunedin Volcano (45°46′21.9″S 170°43′42.4″E) was active between c. 16 and 11 Ma and is the volumetrically largest portion of the Dunedin Volcanic Group (25–9 Ma) (Coombs et al. 2008; Scott et al. 2020). The Dunedin Volcano consists of the early submarine to subaerial phreatomagmatic tuff and breccia (Martin and White 2001), overlain and intercalated with lava flows, domes as well as pyroclastic and reworked volcaniclastic sediments (Price and Coombs 1975; Price et al. 2003; Scott et al. 2020). The peridotitic mantle beneath the Dunedin Volcanic Group is dominated by lherzolite although it also contains harzburgite, dunite and wehrlite (Scott, Hodgkinson, et al. 2014; Scott, Waight, et al. 2014; McCoy-West et al. 2015; Dalton et al. 2017). Many of the East Otago peridotites were metasomatised whilst in the mantle by HIMU-like CO2-bearing or carbonatitic melts, probably in the Cretaceous (Scott, Hodgkinson, et al. 2014; Scott, Waight, et al. 2014; Dalton et al. 2017; van der Meer et al. 2017).
An updated catalogue of New Zealand’s mantle peridotite and serpentinite
Published in New Zealand Journal of Geology and Geophysics, 2020
Three peridotite xenolith-bearing occurrences have been found in the Tokatoka area of Northland. The main xenolith location is in a c. 13 Ma (Hayward et al. 2001) nephelinite at Todds Quarry at Arapohue, where spinel peridotite xenoliths reach 10 cm in diameter but are more commonly c. 5 cm (Black and Brothers 1965; Rodgers et al. 1975). Modal analysis shows these xenoliths to range from lherzolite to dunite, with lherzolite and harzburgite most abundant (Rodgers et al. 1975). Mineral chemistries have been reported for only one lherzolite, which has an olivine Mg# of 85.7 (Rodgers and Brothers 1969); however, this composition was obtained by wet chemistry and the authors report that they had difficulty separating pure fractions, which may account for the Mg# being lower than common mantle (> 89; Pearson et al. 2014). Furthermore, such a low olivine Mg# is inconsistent with the bulk rock data of another lherzolite that has a Mg# of 90.9 (Rodgers et al. 1975).