Crust–mantle interaction in the genesis of siliceous high magnesian basalts: evidence from the Early Proterozoic Dongargarh Supergroup, India
TL;DR: In this paper, the authors present chemical (XRF, INAA) and mineralogical data on a siliceous high magnesian basalts (SHMB) suite from the Early Proterozoic (2.1-2.5 Ga) Dongargarh Supergroup, Central India, the first of its kind reported from the Indian Precambrian.
About: This article is published in Chemical Geology.The article was published on 2002-07-01. It has received 27 citations till now. The article focuses on the topics: Incompatible element & Volcanic rock.
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TL;DR: Based on geochemical and petrological studies of the Meso- to Neoproterozoic basic-acid rocks, a preliminary model for the formation and evolution history of the western Jiangnan orogen in the area was put forward as discussed by the authors.
184 citations
TL;DR: The Huangshandong mafic-ultramafic intrusion is the largest in the eastern Tianshan Orogenic Belt of the CAOB and consists of a layered unit intruded by a massive unit.
67 citations
Cites background from "Crust–mantle interaction in the gen..."
..., 1989), and modification of magmas through crust–mantle interaction (Sensarma et al., 2002)....
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...SHMB was first reported from western Australia (Redman and Keays, 1985) and rocks derived from SHMB-like magmas have subsequently been described elsewhere in the world, mostly from the Archean–Proterozoic with a few in the Phanerozoic (Arndt and Jenner, 1986; Maier and Barnes, 2010; Seitz and Keays, 1997; Sensarma et al., 2002; Srivastava, 2006, 2008; Srivastava et al., 2010; Sun et al., 1989, 1991; Wang and Zhou, 2006)....
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TL;DR: In this article, the evolution of the late Archean Belingwe greenstone belt, Zimbabwe, is discussed in relation to the geochemistry of the ultramafic to mafic volcanic rocks.
Abstract: The evolution of the late Archean Belingwe greenstone belt, Zimbabwe, is discussed in relation to the geochemistry of the ultramafic to mafic volcanic rocks. Four volcanic types (komatiite, komatiitic basalt, D-basalt and E-basalt) are distinguished in the 2� 7Ga Ngezi volcanic sequence using a combination of petrography and geochemistry. The komatiites and D-basalts are rocks in which isotopic systems and trace elements are depleted. Chemical variations in komatiites and D-basalts can be explained by fractional crystallization from the parental komatiite. In contrast, komatiitic basalts and E-basalts are siliceous and display enriched isotopic and trace element compositions. Their chemical trends are best explained by assimilation with fractional crystallization (AFC) from the primary komatiite. AFC calculations indicate that the komatiitic basalts and E-basalts are derived from komatiites contaminated with � 20% and � 30% crustal material, respectively. The volcanic stratigraphy of the Ngezi sequence, which is based on field relationships and the trace element compositions of relict clinopyroxenes, shows that the least contaminated komatiite lies between highly contaminated komatiitic basalt flows, and has limited exposure near the base of the succession. Above these flows, D- and E-basalts alternate. The komatiite appears to have erupted on the surface only in the early stages, when plume activity was high. As activity decreased with time, komatiite magmas may have stagnated to form magma chambers within the continental crust. Subsequent komatiitic magmas underwent fractional crystallization and were contaminated with crust to form D-basalts or E-basalts.
65 citations
Cites background from "Crust–mantle interaction in the gen..."
...Komatiitic basalt and E-basalt Komatiitic basalts are enriched in both LREE and radiogenic isotopic compositions, and their chemical characteristics, including high SiO2 and MgO contents, are similar to those of siliceous high magnesian basalts (SHMB), which have been suggested to be derived by crustal contamination of komatiite magmas (e.g. Redman & Keays, 1985; Arndt & Jenner, 1986; Barley, 1986; Sensarma et al., 2002)....
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TL;DR: In this paper, a new tectonic classification of the Bastar craton into the Western Bastar Craton (WBC), EBC, and the intervening Central Bastar Orogen (CBO) is proposed.
57 citations
TL;DR: The Bastar craton has experienced many episodes of mafic magmatism during the Precambrian as discussed by the authors, and this is evidenced from a variety of precambrian volcanic and dyke formations exposed in all parts of the Bastar basin in the form of volcanics and dykes.
Abstract: The Bastar craton has experienced many episodes of mafic magmatism during the Precambrian. This is evidenced from a variety of Precambrian mafic rocks exposed in all parts of the Bastar craton in the form of volcanics and dykes. They include (i) three distinct mafic dyke swarms and a variety of mafic volcanic rocks of Precambrian age in the southern Bastar region; two sets of mafic dyke swarms are sub-alkaline tholeiitic in nature, whereas the third dyke swarm is high-Si, low-Ti and high-Mg in nature and documented as boninite-norite mafic rocks, (ii) mafic dykes of varying composition exposed in Bhanupratappur-Keskal area having dominantly high-Mg and high-Fe quartz tholeiitic compositions and rarely olivine and nepheline normative nature, (iii) four suites of Paleoproterozoic mafic dykes are recognized in and around the Chattisgarh basin comprising metadolerite, metagabbro, and metapyroxenite, Neoarchaean amphibolite dykes, Neoproterozoic younger fine-grained dolerite dykes, and Early Precambrian boninite dykes, and (iv) Dongargarh mafic volcanics, which are classified into three groups, viz. early Pitepani mafic volcanic rocks, later Sitagota and Mangikhuta mafic volcanics, and Pitepani siliceous high-magnesium basalts (SHMB). Available petrological and geochemical data on these distinct mafic rocks of the Bastar craton are summarized in this paper. Recently high precision U-Pb dates of 1891.1±0.9 Ma and 1883.0±1.4 Ma for two SE-trending mafic dykes from the BD2 (subalkaline) dyke swarm, from the southern Bastar craton have been reported. But more precise radiometric age determinations for a number of litho-units are required to establish discrete mafic magmatic episodes experienced by the craton. It is also important to note that very close geochemical similarity exist between boninite-norite suite exposed in the Bastar craton and many parts of the world. Spatial and temporal correlation suggests that such magmatism occurred globally during the Neoarchaean-Paleoproterozoic boundary. Many Archaean terrains were united as a supercontinent as Expanded Ur and Arctica at that time, and its rifting gave rise to numerous mafic dyke swarms, including boninitenorite, world-wide.
55 citations
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07 Nov 2000
TL;DR: A discussion on the origin of igneous rocks was held in Section C (Geology) of the British Association at York on September 7 as mentioned in this paper, where the authors sketched out a general scheme of petrogenesis in the light of our present knowledge concerning the geological history, structure and thermal condition of the earth's crust.
Abstract: A DISCUSSION on the origin of igneous rocks was held in Section C (Geology) of the British Association at York on September 7. The discussion was opened by Prof. Arthur Holmes, who sketched out a general scheme of petrogenesis in the light of our present knowledge concerning the geological history, structure and thermal condition of the earth's crust. In certain continental regions the sedimentary and granitic layers appear to be separated from a deeper ‘basaltic’ layer by one which is probably composed of both acid and basic materials. The ‘basaltic’ layer is itself composite, a tentative interpretation being that amphibolite (a potential source of over-saturated basalts) is succeeded in depth by more basic granulite (a potential source of undersaturated basalts). Beneath these layers peridotite probably comes into place, merging within a few tens of kilometres into the glassy state regarded as characteristic of the substratum. Given some such setting of materials, igneous activity could arise, either by the ascent into the crust of heat from the feebly radioactive substratum, or by the accumulation of heat in specially thickened belts of the more strongly radioactive crustal rocks. The latter process fails, however, to account for the flooding by plateau basalts of regions where the crust has been thinned by denudation, and also for the ascent of granitic magma during orogenesis instead of long afterwards. The hypothesis of refusion of the crustal layers by heat from the substratum, with successive production of peridotitic, basaltic and granitic magmas, therefore seems to be worthy of special consideration.
129 citations
TL;DR: The peridotitc and quench-textured micropyroxenite sills are found adjacent to, and are members of, the pyroxenitic marginal group as discussed by the authors.
Abstract: Two fundamental groups of marginal rocks occur along the basal contact of the eastern Bushveld Complex. The lower part of the complex, comprising the lower zone and the pyroxenite subzone of the critical zone, has a marginal group that is mostly made up of pyroxenites and norites, some of which are quench textured. The upper part of the complex, including the anorthositic portion of the critical zone and the main and upper zones, is bordered by two-pyroxene gabbros. All lithologies that make up the marginal facies of the complex have counterparts in the different groups of sills that intrude the underlying metasedimentary rocks of the Transvaal Sequence.Suites of peridotitc and quench-textured micropyroxenite sills are found adjacent to, and are members of, the pyroxenitic marginal group. The peridotite sills range from homogeneous harzburgites to differentiated dunite-harzburgite-orthopyroxenite bodies. Some have quenched and crescumulate members. Most peridotite sills are petrographically similar and consist of subhedral olivine grains (Fo (sub 83-90) ), poikilitically enclosed by orthopyroxene (En (sub 84-90) ) with accessory plagioclase (An (sub 59-76) ), chromite, clinopyroxene, and biotite. They have ubiquitously high contents of incompatible elements such as 10 to 45 ppm Zr, 5 to 30 ppm Rb, and 0.17 to 0.4 wt percent K 2 O. A family of quench-textured micropyroxenite sills and marginal rocks occurs both in close association with the ultramafic sills and deep below in the floor. These sills are characterized by orthopyroxene (En (sub 78-92) ), with rare olivine (Fo (sub 77-90) ) set in a groundmass of comb-textured plagioclase (An (sub 65-75) ), orthopyroxene, and devitrified glass. Crystallites of plagioclase and hollow, quenched olivine and orthopyroxene crystals occur in some samples. Clinopyroxene, magnetite, and chromite are common accessories; biotite is an essential constituent. The quench-textured micropyroxenites have high SiO 2 ( approximately 55 wt %) and MgO ( approximately 14 wt %) contents. Like the peridotite sills they are rich in incompatible elements and contain 70 to 230 ppm Zr, 20 to 60 ppm Rb, and 0.6 to 1.2 wt percent K 2 O. Members of the pyroxenitic marginal group and the peridotite sills together give an errorchron age of 2,015 + or - 211 m.y. and an initial 87 Sr/ 86 Sr isotope ratio of 0.7049. Olivine, or more commonly orthopyroxene, is the first mineral on the liquidi of melts of the quench-textured micropyroxenites at 1,250 degrees to 1,300 degrees C. Such melts have the requisite properties to have been parental to the lower zone, shown by experimental data and computer simulations of their one-atmosphere fractional crystallization sequences. Peridotite sills crystallized at f (sub O 2 ) conditions between nickel-nickel oxide (NNO) and quartz-fayalite-magnetite (QFM), but micropyroxenite sills crystallized under more reducing conditions, which caused the precipitation of abundant chromite to be suppressed and most Cr to enter orthopyroxene. The floor rocks carry metamorphic assemblages that equlibrated at 4 to 5.5 kb and 700 degrees to 800 degrees C.The compositions of olivine, orthopyroxene, chromite, and clinopyroxene in the peridotite sills are similar but are slightly more magnesian than minerals in harzburgites from the deepest exposed part of the lower zone. Compositional disequilibrium between rounded olivine grains and poikilitically enclosing orthopyroxene is common. Most peridotite sills are made up of a solitary cycle of dunite-harzburgite-bronzitite, but some are lithologically homogeneous. Rare contorted layering structures are indicative of flow banding. Such evidence suggests that many of the sills are solidified suspensions of olivine crystals in liquid. Simple mixing between an average micropyroxenite composition and olivine from the peridotite sills (with the trace element content calculated by olivine-liquid partition coefficients from a micropyroxenitic parent) reproduces the average peridotite sill composition for major, trace, and rare earth elements with approximately 60 percent olivine. The amount of plagioclase, clinopyroxene, and chromite in the sills was controlled by the dilution of parental liquid by olivine crystals. The mushes were derived from semisolid lower zone cumulates and their parental liquids, which were expelled from the Bushveld magma chamber at different periods (hence their range of compositions and stratigraphic levels) during tectonic disturbances, such as those that gave rise to the folding along the basal contact of the complex. A peridotite member of one such sill which has distinctive harrisitic textures originated by crescumulate growth, followed by homogenization of its bladed olivine crystals through subsolidus diffusion at high temperatures.
113 citations
TL;DR: In this paper, the high magnesian series basalts (HMSB) and the low Magnesian Series basalts were derived from similarly depleted source regions and it was argued that the main petrogenetic difference between these three volcanic suites was the degree of partial melting from which they were derived.
101 citations
TL;DR: In this article, it was suggested that many of the geochemical characteristics of continental tholeiites result from melting at the walls of dykes as primitive magmas ascend through the crust.
Abstract: Continental tholeiites have higher SiO2, K2O and light rare earth element contents and more evolved isotopic characteristics than their oceanic counterparts. These differences can be explained if the compositions of the parent magmas to both types of tholeiites are similar but if continental magmas assimilate significant amounts of continental crust en route to the surface. Although there is little doubt that most continental tholeiites have assimilated crustal material, the lcoation and mechanism of assimilation remain uncertain. Longhi (1981) has argued that magmas derived directly from the mantle should crystallize little orthopyroxene. The abundance of orthopyroxene in most continental layered intrusions suggests that they have crystallized from magmas which have assimilated continental crust. Since orthopyroxene is an early crystallizing phase in layered intrusions, this assimilation must occur early, before the magma enters the chamber. Assimilation can occur at the margins of the dykes which feed magma chambers, depending on the nature of the flow. If the flow is turbulent the high temperatures at the centre of the dyke will extend to the margins and the magma will erode the dyke walls. If the flow is laminar, a conductive profile develops at the margin and the flowing magma chills against the walls, protecting them from thermal erosion. The nature of flow in a dyke can be predicted from the Reynolds number, the criteria for turbulence. Reynolds number calculations suggest that the flow of primitive magmas in continental dykes will be fully turbulent and, if this is the case, assimilation of low melting point components in the walls of the dyke is inevitable. It is therefore suggested that many of the geochemical characteristics of continental tholeiites result from melting at the walls of dykes as primitive magmas ascend through the crust.
99 citations