Basalt Fractionation Research Articles

Flotation and remelting of plagioclase in the Kalka Intrusion, central Australia: Petrological implications for anorthosite genesis

Flotation of early-formed plagioclase in the layered gabbroic Kalka Intrusion of central Australia was accompanied by gravitational settling of co-crystallizing pyroxenes. Remelting of the rising plagioclase upset the normal gravitational crystal-extractive processes which control classic basaltic fractionation trends. Together with high pressures of crystallization which cause orthopyroxene rather than olivine to be the first phase to precipitate, this may help to explain the eventual production of at least 800 m of plagioclase cumulates (anorthosites) as late-stage products of fractionation in Kalka. Addition or removal of melts at various stages of fractionation are further processes affecting the differentiation trends of layered intrusions. Variations in the occurrence of such processes may explain many of the differences between anorthosite suites throughout the world.

K and Rb relations in basalts and other mantle derived materials. Is phlogopite the key?

Existing hypotheses to account for K and Rb relations in mantle derived materials are reviewed and found to be inadequate. A model is developed which assumes all mantle K and Rb are contained in phlogopite. This model is used to test the compatibility of various proposed source compositions and proposed degrees of melting with the K and Rb contents of a wide variety of mantle derived magmas. The model indicates that the K content and K/Rb values of such magmas can adequately be accounted for by their sequential derivation from a phlogopite bearing source having K and Rb contents most closely similar to chondritic meteorites. This model implies; 1. (1) that in oceanic areas alkali basalts with K/Rb < chondrites must be preceded by and rest upon basalts with K/Rb ⪢ chondrites; 2. (2) that in island arc sequences a trend of increasing K and decreasing K/Rb values requires a parallel trend of decreasing volume of products; 3. (3) that if the continental crusts have evolved by island arc accretion their K/Rb values must be 3–4 times higher than generally assumed, implying an intracrustal redistribution due to metamorphism and anatexis has taken place, enriching upper crustal levels in Rb more so than K; 4. (4) that ultrapotassic volcanics and kimberlitic rocks are derived by extremely small degrees of melting from a phlogopite bearing garnet lherzolite which represents residual mantle material depleted of its basaltic fraction.

The rare Earth geochemistry of the Troodos Ophiolite Complex

New chemical analyses of rocks from the Troodos ophiolite complex offer no clear evidence that it is a piece of oceanic crust. Ratios of selected nonlabile elements (REE, Zr, Ti) in Troodos lavas clearly distinguish them from the calc-alkaline lavas of evolved island arcs and the oceanic island basalts of Hawaiian and Icelandic type. However, these ratios are equivocal in assigning Troodos lavas to either oceanic ridge, small ocean basin, or young island arc magma series. Gabbros and quartz diorites from the Troodos plutonic complex appear to be derived by low-pressure crystal settling from magmas with the same REE pattern as the overlying lavas and dike rocks. The acidic plutonic rocks have large negative Eu anomalies that clearly distinguish them from silicified lavas in the complex, which have only small Eu anomalies. Harzburgite, the lowest plutonic unit, has a very low REE content and probably is residual mantle from which the basalt fraction has been removed.

A Comparison of the Geology of the Papuan and New Caledonian Ultramafic Belts

The Papuan and New Caledonian Ultramafic Belts lie 1,770 km apart on the southern branch of Glassener's inner Melanesian arc; a site of a former Mesozoic geosyncline. Both belts show strong affinities with the alpine-type ultramafic association. In both instances a dominant, noncumulate harzburgite forms a substrate on which cumulates of peridotite and gabbros appear to have formed. Overthrusting from the northeast of the complexes onto the sialic crust of the islands was preceded by eruption of voluminous tholeiitic basalts in the Cretaceous or Eocene. Intrusion of felsic stocks into both basement and overthrust sheets succeeded emplacement. The belts differ in their tectonic relationship to Tertiary blueschist metainorphism and to the tholeiitic basalts; in the greater variety and abundance of cumulate rocks in Papua; in the widespread occurrence of chromitites in New Caledonia; and in the relative abundance of clino-pyroxenites in the non-cumulate substrate. An upper mantle origin is indicated for the non-cumulates which perhaps represent depleted pyrolite following removal of the basaltic fraction.

Crystallization and Fractionation Trends in the System Andesite-H2O-CO2-O2 at Pressures to 10 Kb

Phase relations of a Mount Hood andesite, which has the composition of an average orogenic andesite, have been determined as a function of O 2 fugacity at 1 atm and of H 2 O fugacity to pressures of 10 kb, at O 2 fugacities of the quartz-fayalite-magnetite (QFM) buffer. All runs contained either a H 2 O or H 2 O–CO 2 fluid phase; melts in runs with a H 2 O–CO 2 fluid phase were H 2 O undersaturated. The H 2 O contents of the melts and H 2 O fugacities were calculated from NaAlSi 3 O 8 –H 2 O thermo-dynamic data on the assumption of ideal mixing in the system H 2 O–CO 2 . One-atmosphere runs show that melting relations of silicates are little affected by f o 2 but that both ilmenite- and magnetite-out temperatures are raised by higher f o 2 . Ilmenite precipitates at higher temperature than magnetite. In these runs and in all runs at high pressure with H 2 O and H 2 O–CO 2 fluid phases, oxides were not stable at temperatures of the silicate liquidus. Oxides might be stable on the silicate liquidus if f o 2 rose two or more log units above the Ni–NiO (NNO) buffer. However, calculations indicate that in natural magmas, those processes which might change f o 2 —crystal-liquid equilibria or exchange of H 2 , or H 2 and H 2 O with the wall rocks—cannot raise f o 2 by that magnitude. Because differentiation of basalt melts to andesite must involve iron-rich oxide phase subtraction, such fractionation models appear unreasonable. For the Mount Hood andesite composition, plagioclase is the liquidus phase under H 2 O–saturated conditions to 5 kb and under H 2 O–undersaturated conditions at 10 kb when the H 2 O content of the melt is less than 4.7 wt percent. For higher H 2 O contents, either orthopyroxene or, at H 2 O saturation at pressure greater than 8 kb, amphibole assumes the liquidus. In all cases, clinopyroxene crystallizes at lower temperature than orthopyroxene. Melting curves in the H 2 O–under-saturated region may be contoured either as percent H 2 O in melt or as P e H 2 O ; in either case, the topology of the various silicate melting curves is different from the case of H 2 O–saturated melting. Therefore, melting relations determined at H 2 O–saturated conditions cannot be used successfully to predict melting relations in the H 2 O–undersaturated region. Amphibole melting relations were studied isobarically at 5 kb as a function of temperature and fluid-phase composition. Amphibole has a maximum stability temperature of 940 ± 15°C for fluid compositions of 100 to 44 mole percent H 2 O; for fluids containing more CO 2 than 56 percent (or, equivalently, less than 4.4 wt percent H 2 O in melt), the melting temperature is lower. The same relations would be seen if CO 2 were not present and the melt were H 2 O undersaturated. These rather low melting temperatures, relative to other silicate phases, preclude andesite generation by basalt fractionation involving amphibole at pressures less than 10 kb.

Solid-earth geophysics in Canada

Over the past fifteen years the original data on the kimberlites of South Africa have been supplemented with a wealth of information on the kimberlites of the U.S.S.R., the U.S.A., and various parts of Africa. From their distribution pattern, it can now be shown that kimberlites are virtually confined to the old Precambrian cratons and that the diamond-bearing kimberlites exist on the older cratonic nuclei that have not been deformed for the past 2,500 m years. The kimberlite dykes infill major, deep-seated fractures that cross the cratons on a geometrical pattern. The kimberlite that infilled these fractures was a hot fluid that penetrated to within 2–3 km of the surface before there was explosive breakthrough to the surface with subsequent formation and infilling of the high-level diatremes by a gas-solid fluidisation process. The kimberlites can vary widely texturally, from the fragmental variety found in the diatremes, to massive varieties found within the hypabyssal dykes or skills.Both petrographically and chemically, kimberlite is a hybrid rock, formed by incorporation of crystals mainly derived from fragmentation of upper mantle garnet lherzolite into a matrix that has strong affinities with carbonatite.Kimberlites have carried out extensive sampling of the upper mantle during their ascent to the surface and from these it is possible to propose the structure of the upper mantle under the craton. Although garnet lherzolite and to a lesser extent, eclogite are the main upper mantle xenolith types, numerous variants of these two major rock-types point to both major and minor inhomogeneities within the upper mantle.There are at present two main hypotheses to explain the genesis of kimberlite. The “residual” hypothesis regards kimberlite as the end product of high-pressure fractionation of piprite basalt, that results itself from melting of four-phase garnet lherzolite. The alternative “incipient melting” hypothesis proposes that kimberlite is formed by incipient melting of a parental garnet lherzoline in which phlogopite is an additional phase; the initial melt liquid, plus fragments of the parental five-phase garnet lherzolite, is kimberlite. The latter hypothesis appear to explain more adequately various aspects of kimberlite petrology and geochemistry. However, although the recognition of phlogopite as an additional phase in the deeper parts of the upper mantle where kimberlite originates, explains the source of elements such as water, fluorine, potassium, and rubidium, there is still no adequate explanation for the source of carbon, sulphur, phosphorus and nitrogen within the upper mantle.

Advances in kimberlite geology

Over the past fifteen years the original data on the kimberlites of South Africa have been supplemented with a wealth of information on the kimberlites of the U.S.S.R., the U.S.A., and various parts of Africa. From their distribution pattern, it can now be shown that kimberlites are virtually confined to the old Precambrian cratons and that the diamond-bearing kimberlites exist on the older cratonic nuclei that have not been deformed for the past 2,500 m years. The kimberlite dykes infill major, deep-seated fractures that cross the cratons on a geometrical pattern. The kimberlite that infilled these fractures was a hot fluid that penetrated to within 2–3 km of the surface before there was explosive breakthrough to the surface with subsequent formation and infilling of the high-level diatremes by a gas-solid fluidisation process. The kimberlites can vary widely texturally, from the fragmental variety found in the diatremes, to massive varieties found within the hypabyssal dykes or skills. Both petrographically and chemically, kimberlite is a hybrid rock, formed by incorporation of crystals mainly derived from fragmentation of upper mantle garnet lherzolite into a matrix that has strong affinities with carbonatite. Kimberlites have carried out extensive sampling of the upper mantle during their ascent to the surface and from these it is possible to propose the structure of the upper mantle under the craton. Although garnet lherzolite and to a lesser extent, eclogite are the main upper mantle xenolith types, numerous variants of these two major rock-types point to both major and minor inhomogeneities within the upper mantle. There are at present two main hypotheses to explain the genesis of kimberlite. The “residual” hypothesis regards kimberlite as the end product of high-pressure fractionation of piprite basalt, that results itself from melting of four-phase garnet lherzolite. The alternative “incipient melting” hypothesis proposes that kimberlite is formed by incipient melting of a parental garnet lherzoline in which phlogopite is an additional phase; the initial melt liquid, plus fragments of the parental five-phase garnet lherzolite, is kimberlite. The latter hypothesis appear to explain more adequately various aspects of kimberlite petrology and geochemistry. However, although the recognition of phlogopite as an additional phase in the deeper parts of the upper mantle where kimberlite originates, explains the source of elements such as water, fluorine, potassium, and rubidium, there is still no adequate explanation for the source of carbon, sulphur, phosphorus and nitrogen within the upper mantle.

Rare earth element distribution in lavas and ultramafic xenoliths from the Comores Archipelago, Western Indian Ocean

Lavas and included xenoliths from the Comores Archipelago have been analysed for the rare earth elements (REE) La-Lu. Among basaltic lava types fractionation of REE rock/chondrite distribution patterns is more extreme with greater SiO2 undersaturation and contents of incompatible elements. Enrichment and slight fractionation of REE in the rock series basanite-phonolite is considered compatible with a model of fractional crystallisation at low pressures involving mainly olivine and clinopyroxene, and to a much lesser extent, plagioclase. Apatite is probably effective in curtailing further enrichment of REE. High level fractional crystallisation and eclogite fractionation at depth appear unlikely causes for the relative enrichment of light REE (La-Eu) in the undersaturated basalts. This effect is more probably due to mineralogical control during partial melting in the upper mantle. Lherzolite xenoliths are poor in REE, exhibiting a slight relative depletion in the light REE. These patterns are interpreted as those of possible mantle material subjected to small degrees of partial melting, although not necessarily related to those melts erupted as lava flows at the surface.

Temperature and distribution of radioactive matter in the upper mantle

Temperatures of the Earth’s upper mantle, derived from Gutenberg’s seismic velocities, have been revised, using some recent determinations of the clastic constants of dunites. For depths greater than about 50 km the temperature obtained is sufficient to melt the basaltic fraction of a periodotitic mantle (if this low-melting fraction still exists at those depths). The distribution of the radioactive heat sources appears to be fairly uniform down to about 110 km; below this level the radioactive matter is probably absent. This is what would be expected for the upper mantle below the oceans; therefore, Gutenberg’s velocities seem to correspond to oceanic areas, rather than to the mantle below a continental shield.

The bearing of phase equilibria studies in synthetic and natural systems on the origin and evolution of basic and ultrabasic rocks

Recent phase equilibria studies, and the concepts of basic magma evolution to which they have given rise, are reviewed. The variations with pressure of liquid compositions in equilibrium with peridotite are summarised, and their bearing on the origins of peridotite nodules in basalt and upon upper mantle mineralogy are discussed.The origin of observed concentrations of the elements K, Ti, P from source rocks containing only very low concentrations of these elements is readily explained by ca. 15% partial melting combined with variable eclogite fractionation and up to 50% further extraction of olivine, yielding optional concentration factors from 8 to 70. Orthopyroxene fractionation cannot explain these effects. There is no to 50% further extraction of olivine, yielding optional concentration factors of from 8–70. Orthopyroxene fractionation cannot explain these effects. There is no need for wall-rock reaction hypotheses which are difficult to envisage operating throughout the long history of a particular volcano.A revised closed system model of basalt magma evolution is presented, based upon these controlling factors; depth of partial melting, extent of partial melting, speed of movement towards the surface. The source rock of basic magmas at depths greater than ca. 60 km is probably a garnet-two pyroxenes-peridotite (garnet-lherzolite), which on partial melting yields and garnet-enstatite-peridotite (garnet-harzburgite), not two pyroxenes-peridotite (lherzolite) residua. The pyroxenite stage of basalt evolution, like the eclogite stage, is located at temperatures below the garnet- or spinel-lherzolite solidus and only affects slow moving magma batches.Six hypotheses for the origin of the calc-alkali series are reviewed. The fractionation of water-enriched high-alumina basalt at relatively high partial pressures of oxygen is concluded to be the mechanism in best accord with available facts.Fig.4–11 and their captions contain material and arguments not duplicated in the text. The figures, together with the conclusions section summarise the main themes of this paper.

Basalts from the deep ocean floor

SummaryBasalts dredged from the floor of the deep ocean show general tholeiitic affinities. Some samples are rich in aluminium and of very similar composition to the Warner high-alumina basalt from California. Both olivine tholeiite and high-alumina basalt have been found in the form of glass in dredgings from the Mid-Atlantic Ridge, indicating that liquid magmas of both compositions have been erupted on to the sea bed in this area. In explanation of this association a tentative hypothesis of fractional melting of hydrated upper mantle material is proposed. It is suggested that under the mid-ocean ridges the ‘basaltic fraction’ of the mantle is locally in the form of an amphibole. Fractional incongruent melting of this amphibole appears to be a possible explanation of the range of composition encountered in dredged glass samples.

Petrology of the Igneous Rocks from the West Gir Forest, Kathiawar, India

Previous articleNext article No AccessPetrology of the Igneous Rocks from the West Gir Forest, Kathiawar, IndiaS. K. ChatterjeeS. K. Chatterjee Search for more articles by this author PDFPDF PLUS Add to favoritesDownload CitationTrack CitationsPermissionsReprints Share onFacebookTwitterLinkedInRedditEmail SectionsMoreDetailsFiguresReferencesCited by The Journal of Geology Volume 40, Number 2Feb. - Mar., 1932 Article DOIhttps://doi.org/10.1086/623930 Views: 1Total views on this site Citations: 13Citations are reported from Crossref PDF download Crossref reports the following articles citing this article:Rohit Pandey, N V Chalapathi Rao, Mahendra K Singh, Debojit Talukdar Alkaline rocks from the Deccan Large Igneous Province: Time–space distribution, petrology, geochemistry and economic aspects, Journal of Earth System Science 131, no.22 (Apr 2022).https://doi.org/10.1007/s12040-022-01852-xCiro Cucciniello, Riccardo Avanzinelli, Hetu Sheth, Martina Casalini Mantle and Crustal Contributions to the Mount Girnar Alkaline Plutonic Complex and the Circum-Girnar Mafic-Silicic Intrusions of Saurashtra, Northwestern Deccan Traps, Journal of Petrology 63, no.33 (Feb 2022).https://doi.org/10.1093/petrology/egac007P. Krishnamurthy The Deccan Volcanic Province (DVP), India: A Review, Journal of the Geological Society of India 96, no.11 (Jul 2020): 9–35.https://doi.org/10.1007/s12594-020-1501-5Ciro Cucciniello, Ashwini Kumar Choudhary, Kanchan Pande, Hetu Sheth Mineralogy, geochemistry and 40Ar–39Ar geochronology of the Barda and Alech complexes, Saurashtra, northwestern Deccan Traps: early silicic magmas derived by flood basalt fractionation, Geological Magazine 156, no.1010 (Jan 2019): 1668–1690.https://doi.org/10.1017/S0016756818000924Hetu C. Sheth, Georg F. Zellmer, Pooja V. Kshirsagar, Ciro Cucciniello Geochemistry of the Palitana flood basalt sequence and the Eastern Saurashtra dykes, Deccan Traps: clues to petrogenesis, dyke–flow relationships, and regional lava stratigraphy, Bulletin of Volcanology 75, no.44 (Mar 2013).https://doi.org/10.1007/s00445-013-0701-xHetu C. Sheth, Ashwini Kumar Choudhary, Sudeshna Bhattacharyya, Ciro Cucciniello, Ramesh Laishram, Trupti Gurav The Chogat-Chamardi subvolcanic complex, Saurashtra, northwestern Deccan Traps: Geology, petrochemistry, and petrogenetic evolution, Journal of Asian Earth Sciences 41, no.33 (May 2011): 307–324.https://doi.org/10.1016/j.jseaes.2011.02.012 Address of the chairman (prof. S. C. Chatterjee, Ujjain) session: Petrology and mineralogy, Bulletin Volcanologique 35, no.44 (Dec 1971): 817–823.https://doi.org/10.1007/BF02596847S. Subba Rao Minor intrusions towards the east of the Girnar hills, Bulletin Volcanologique 35, no.44 (Dec 1971): 975–982.https://doi.org/10.1007/BF02596859A. C. Chatterjee A comparative study of the petrochemistry of the tholeiitic rocks of India, Bulletin Volcanologique 35, no.44 (Dec 1971): 1037–1050.https://doi.org/10.1007/BF02596864S. Balakrishna Geological and geophysical investigations on Deccan Traps, Bulletin of Volcanology 26, no.11 (Dec 1963): 273–296.https://doi.org/10.1007/BF02597292M. S. Krishnan Deccan Trap volcanism, Bulletin of Volcanology 26, no.11 (Dec 1963).https://doi.org/10.1007/BF02597300M. S. Krishnan Volcanic rocks of India, Bulletin Volcanologique 18, no.11 (Dec 1956): 125–132.https://doi.org/10.1007/BF02596615N. A. Vemban Differentiation trends in the Deccan traps, Proceedings of the Indian Academy of Sciences - Section A 25, no.22 (Feb 1947).https://doi.org/10.1007/BF03171387