Evidence for constitutional supercooling from orthopyroxene - pigeonite - plagioclase relations in the Borgmassivet suite of western Dronning Maud Land, Antarctica

Magma replenishment, and the significance of poikilitic texlures, in the Lower Main Zone of the western Bushveld Complex, South Africa

“…They are also significantly smaller than the augite oikocrysts described by e.g. Mathison (1987), Tegner and Wilson (1995) and Krynauw and Wilson (1995). Mathison (1987) suggested that plagioclase chadacrysts might well be the remains of selfnucleated clusters resulting from supercooling in situ, followed by crystallization of the oikocrysts, also mostly in situ.…”

“…The noritic cumulates between the pyroxenites are also distinguished, especially from those in the upper part of the unit, by the fact that they are poikilitic, containing plagioclase chadacrysts. The presence of plagioclase-bearing pyroxene oikocrysts in cumulates has been related to entrainment of plagioclase from evolved resident liquid by magma freshly injected into the chamber (Eales et al, 1991;Maier and Eales, 1994b), or to constitutional supercooling of fresh influxes of magma (Mathison, 1987;Tegner and Wilson, 1995;Krynauw and Wilson, 1995). Regardless of whether the oikocrysts are due to entrainment of allochthonous plagioclase or to supercooling, both textural and compositional evidence would seem to support a model whereby the poikilitic norites between the pyroxenites in N-II, as well as the pyroxenites themselves, are the products of a single injection of magma.…”

“…In the lowermost 110 m of unit G-I in borehole SK2, large orthopyroxene oikocrysts (5 to 10 mm diameter), set in a finer-grained gabbroic matrix, impart the distinctive porphyritic texture of the PG. Poikilitic cumulates, in which pyroxene or olivine oikocrysts enclose plagioclase chadacrysts, have been described from several different intrusions (Wager and Brown, 1968;Cameron, 1969;McBirney and Noyes, 1979;Mathison, 1987;Eales et al, 1990aEales et al, , b, 1991Wilson, 1992;Maier and Eales, 1994b;Maier, 1995;Tegner and Wilson, 1995;Krynauw and Wilson, 1995;McBimey and Hunter, 1995). A comprehensive criticism of classical cumulate terminology by McBirney and Hunter (1995), in which the interpretation of poikilitic cumulates features prominently, is a contemporary reflection of doubts about the adequacy of the cumulate terminology that have arisen periodically over the past 25 years or more, often prompted specifically by poikilitic textures (Cameron, 1969;Campbell, 1978;Mathison, 1987).…”

“…Most commonly, the discussion of poikilitic cumulates refers to clinopyroxene (angite) oikocrysts enclosing plagioclase chadacrysts (Mathison, 1987;Tegner and Wilson, 1995), although olivine oikocrysts are also present in several intrusions (Mathison, 1987;Maier, 1995), as are orthopyroxene oikocrysts (Maier and Eales, 1994b). Poikilitic cumulates are commonly located at or near the base of demonstrable cyclic units, and various features of the rocks, including the small size of plagioclase chadacrysts relative to matrix plagioclase, and their high degree of mutual attachment, are cited as evidence for supercooling of the order of 20 to 45 ~ (Mathison, 1987;Tegner and Wilson, 1995;Krynauw and Wilson, 1995). The spatial distribu-tion of plagioclase chadacrysts has been used as an indication that the pyroxene oikocrysts crystallized within a plagioclase crystal mush containing of the order of 50% crystals (Mathison, 1987;Tegner and Wilson, 1995).…”

Crystal size distribution analysis of plagioclase from gabbro‐anorthosite suite of Kuliana, Orissa, eastern India: implications for textural coarsening in a static magma chamber

“…Although there are records of the gabbro‐anorthosite suites from the Archaean to Phanerozoic and, in particular, over the Palaeo‐ to Meso‐Proterozoic period of widespread cratonisation throughout the earth (Bogatikov et al ., ), the mechanism of crystallization in their static magma chamber is still an enigmatic issue. Analysis of textures in rocks from such intrusions resulted in various hypotheses of crystallization history in terms of several competing processes, including polybaric crystallization (Zhou et al ., ; Charlier et al ., ; Howarth et al ., 2014), in situ crystallization and supercooling (Mathison, ; Langmuir, ; Krynauw and Wilson, ; Tegner and Wilson, ), ad/orthocumulate growth mechanism (Wager and Brown, 1961; Cawthorn, ; Tait and Jaupart, ), and textural coarsening (Higgins, , ), albeit to a lesser extent. The present work is based on textures observed in the layered gabbro‐anorthosite suite of Kuliana (22°10′ 0″N and 86°32′ 0″E), Orissa, in the eastern Indian shield (Fig.…”