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Magmatism and Magmatic Rocks
Published in Aurèle Parriaux, Geology, 2018
The ideal model of fractional crystallization in a homogenous and closed system is often disrupted in nature by the process of magmatic segregation, which modifies the physical and mineralogical evolution of the creation of magmatic rocks. Minerals that form first are rich in iron and calcium and are much denser than the residual melt. They thus have the tendency to settle to the bottom of the magma chamber while the fluid residue occupies the upper part. The concentration of these refractory minerals forms what are called cumulates; under certain cases they form metalliferous ore deposits. The fluid that is depleted of refractory minerals is called evolved. If the magma is expelled upwards, the liquid phase is separated from the solid phase. The two masses evolve separately. These final rocks are said to be differentiated from the source magma.
Magmatism and Magmatic Rocks
Published in Aurèle Parriaux, Geology, 2018
The ideal model of fractional crystallization in a homogenous and closed system is often disrupted in nature by the process of magmatic segregation, which modifies the physical and mineralogical evolution of the creation of magmatic rocks. Minerals that form first are rich in iron and calcium and are much denser than the residual melt. They thus have the tendency to settle to the bottom of the magma chamber while the fluid residue occupies the upper part. The concentration of these refractory minerals forms what are called cumulates; under certain cases they form metalliferous ore deposits. The fluid that is depleted of refractory minerals is called evolved. If the magma is expelled upwards, the liquid phase is separated from the solid phase. The two masses evolve separately. These final rocks are said to be differentiated from the source magma.
Igneous Petrology and the Nature of Magmas
Published in Dexter Perkins, Kevin R. Henke, Adam C. Simon, Lance D. Yarbrough, Earth Materials, 2019
Dexter Perkins, Kevin R. Henke, Adam C. Simon, Lance D. Yarbrough
Figure 5.42, a schematic diagram, shows the principles behind fractional crystallization of a magma. While cooling, a parental magma crystallizes some high-temperature minerals. These minerals eventually sink to the bottom of the magma chamber, leaving an evolved magma above. Because high-temperature minerals are mafic, the evolved melt is more felsic (less mafic) than the original parent magma. While this goes on, a cumulate rock forms at the bottom of the magma chamber, and the evolved magma may move upwards and become completely separated from the cumulate.
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
Most dykes lack xenoliths, apart from a spectacular prominent dyke south of Boat Harbour Reserve, Gerringong (SA-17-18, 14/05) that contains a wide range of xenoliths. As shown in Table S3, the most common xenolith type is a gabbroic xenolith (Figure 7h). Three types of gabbroic xenoliths were found in this dyke, including alkali-gabbro xenoliths that contain plagioclase, microcline, orthopyroxene, green clinopyroxene and platy ilmenite. Biotite-rich gabbro xenoliths show strongly deformed abundant biotite, plagioclase, clinopyroxene, orthopyroxene and minor apatite. A third gabbro-type xenolith with a cumulate texture comprises plagioclase, alkali feldspar, clinopyroxene, orthopyroxene and pyrite. The first two gabbroic xenoliths have strongly sutured grain boundaries and reaction rims, and the third has a sharp contact with the host basalt.
Triassic to Neogene tectono-magmatic events within Lorne Basin evolution, coastal New South Wales, eastern Australia
Published in Australian Journal of Earth Sciences, 2020
F. L. Sutherland, I. T. Graham, H. Zwingmann, D. J. Och, C. J. Gardner, R. E. Pogson, R. J. Griffiths, A. Lay
A hawaiite east of Mount Comboyne carries mafic inclusions with antecrystic components (Figure 1, Location 11, BR; Appendix 2, DR16688 a–f). Abundant plagioclase crystals and composites of subhedral, zoned, partly embayed crystals up to 3 mm across lie in a fine-grained fluidal groundmass (DR16888 a). Smaller isolated plagioclase crystals show strongly resorbed, sieve textured cores, some with later rimming by groundmass feldspar (DR 16888 b). In cumulate-textured feldspar–pyroxene composites (DR 16888 c, d), plagioclase encloses abundant poikilitic pale green to brown clinopyroxene crystals (0.2–0.7 mm in size). Rare, resorbed orthopyroxene crystals in the matrix have wide reaction coronas composed of radial, elongate prismatic clinopyroxene crystals (DR 16888 e). Opaque iron oxide-bearing mesostasis within clinopyroxene clusters has developed sites for intergranular Fe-rich chlorite alteration (DR 16888 f).
The “intraorogenic” Svecofennian Herräng mafic dyke swarm in east-central Sweden: age, geochemistry and tectonic significance
Published in GFF, 2020
Åke Johansson, Andreas Karlsson
These melts presumably gathered in a magma chamber at depth, perhaps at the crust-mantle boundary, where different magma pulses could mix and homogenize. Penecontemporaneous with mixing, the magma contained in this magma chamber underwent fractional crystallization, initially probably of Mg-rich olivine, and subsequently of clinopyroxene, which remained as cumulate rocks at depth, while the magma evolved from basaltic to andesitic in composition (cf. fractionation calculation for the Avesta-Östhammar gabbros and diorites in Johansson & Hålenius 2013). More or less well-mixed and fractionated magma then erupted in different portions through narrow fissures into the upper crust where it solidified, although some magma may possibly have reached all the way to the surface and erupted as now-eroded basaltic to andesitic lava.