Showing papers on "Incompatible element published in 1978"

Petrogenesis of Archaean ultrabasic and basic volcanics: Evidence from rare earth elements

Abstract: Rare earth element (REE) and major element data are presented on 44 Archaean samples which include spinifex textured ultramagnesian lavas (STPK) spinifex textured basalts (STB) and low MgO tholeiites. The samples come from the Yilgarn and Pilbara Blocks (W. Australia), Barberton (South Africa), Belingwe and Que Que (Rhodesia), Abitibi (Canada) and the 3.7 b.y. Isua Belt of Western Greenland. In addition REE data are given on three near primitive mid-ocean ridge basalts (MORB) and a glassy MORB-type basalt from Taiwan. We suggest that REE patterns, particularly the light REE and Eu, can be affected by metamorphism, but argue that the consistency of pattern from samples both within and between areas enables recognition of primary patterns. La/Sm ratios of 2.7 b.y. STPK are characterised by being lower than those of associated basalts. The 3.5 b.y. STPK Barberton material does not show this feature but instead displays significant heavy REE depletion. The separation of garnet from these liquids is suggested as a possible mechanism for the high CaO/Al2O3 ratios, (Al loss) and the heavy REE and Sc depletion. The REE data on Barberton material is equivocal on the derivation of the so-called basaltic komatiites from the peridotitic komatiites. However, REE analyses on STPK and high magnesian lavas from elsewhere suggests that crystal fractionation is not a viable mechanism to produce one from the other. We suggest instead, that varying amounts of partial melting of different sources is responsible for the spectrum of compositions. The STB appear to be an easily recognised rock type within the Archaean. They are characterised by quench (clinopyroxene) textures and a light REE enriched pattern. It is suggested that these are near primary melts and that their REE patterns mirror their mantle source. We propose a two stage model for the 2.7 b.y. mafic complexes, in which, prior to the generation of ultrabasic magmas, the source underwent a small amount of partial melting which resulted in the removal of a melt enriched in incompatible elements. The depletion process could be achieved either during mantle diapirism or by upward migration of interstitial melts into an Archaean low velocity zone. The spread of La/Sm ratios in STPK and STB is used as an argument that the Archaean mantle was chemically heterogeneous and that the degree of heterogeneity was similar to that observed in modern ocean volcanics. As a result, partial melting of the mantle under different P-T conditions produced a spectrum of magma types. The information presently available on Archaean mafic and silicic magmatism and the incompleteness of geochemical data on present day tectonic environments are two major obstacles in formulating Archaean tectonic models. In addition a comparison of present day and Archaean ultramafic and silicic rocks suggests that plate tectonic models as presently understood may not be suitable analogues for all Archaean tectonic environments.

Fractional crystallization and the origin of tin deposits in granitoids

Abstract: A comparison between tin-bearing granitoids in an anorogenic setting (Bushveld Complex) and an orogenic setting (Blue Tier Batholith, Tasmania) reveals a number of genetically important similarities. These include: in situ fractional crystallization characterised by marked decrease in Ba and Sr and increase in Rb; the accumulation of late melt in a sheet-like form near the roof zone; the association of barren pegmatites overlying the ore; and of aplites; and the occurrence of conformable tin-bearing sheets, often exhibiting greisenization. These features allow the formulation of the following genetic model. A crustally-derived granitoid magma is emplaced and undergoes fractional crystallization from the margins inwards, with bottom crystallization dominating. Disruption of earlier formed solids by rest liquid commonly occurs. Continued fractional crystallization causes enrichment in volatiles and incompatible elements in the late rest melts, which have a sheet-like habit. The efficiency of enrichment of incompatible elements is critically dependant on the degree of separation of melt from solids throughout crystallization. An early, tin-poor vapour may separate after initial water-saturation of the magma is achieved, and this collects under the roof, commonly forming an impermeable barrier to later tin-bearing fluids. Continued fractional crystallization on the floor further enriches incompatible elements, and at a very late stage a Sn-rich vapour separates within the intercumulus phase and becomes concentrated by progressive crystallization of the intercumulus melt. At a late stage of solidification, this vapour loses equilibrium with the earlier formed feldspars and greisenization ensues, accompanied by the crystallization of cassiterite and other ore minerals. The nature of the mineralization changes if through-going fractures tap the late fluids. This model predicts systematic changes in trace element geochemistry with crystallization which provide useful tools for assessing the tin potential of a granitoid, and for indicating the direction of crystallization of the magma, and hence the location of possible ore.

Late Cretaceous high-potassium volcanism in eastern Srednogorie, Bulgaria

Abstract: The Srednogorie zone of Bulgaria was the site of extensive Late Cretaceous calc-alkalic and shoshonitic magmatism. The calc-alkalic volcanism developed mainly in the central and western parts, whereas shoshonitic volcanism was the predominant magmatic feature of the eastern part. Leucitic basanites, limburgites, and picrites occur in minor amounts in eastern Srednogorie. The shoshonitic volcanic rocks are intermediate to silicic in composition and are characterized by high K 2 O, K 2 O/Na 2 O around unity, high Rb, Sr, and Ba, and low TiO 2 contents. They are associated with calc-alkalic rocks less strongly enriched in K and incompatible elements. Petrographically, the calc-alkalic rocks are characterized by the presence of hydrous mineral phenocrysts that are not present in the shoshonitic rocks. Shoshonitic and calc-alkalic rocks have some common chemical characteristics such as low TiO 2 , lack of absolute iron enrichment, and large overlap in many trace-element abundances. These are considered evidence for a genetic relationship between their primary magmas, which may have undergone different degrees of enrichment in incompatible elements. The eastward increase of K-rich volcanic rocks relative to calc-alkalic rocks in the Srednogorie zone is believed to be related to distension tectonics connected with the opening of the Black Sea.

Geochemistry and petrogenesis of the early Tertiary lava pile of the Isle of Mull, Scotland

Abstract: Major and trace element analyses and strontium isotope ratios are presented for twenty-four samples of lavas and plugs from the early Tertiary lava pile in Mull. The samples were selected on the basis of petrographic freshness from a large collection from outside the hydrothermally altered “zone of pneumatolysis” which occupies the central region of the volcanic complex. Most of the analyses yield normative hypersthene and we argue that these are essentially unaltered magmatic compositions. The analytical data indicate that the samples may be divided into three groups on the basis of major element chemistry, initial 87Sr/86Sr ratios and correlations between lithophile element contents. Group I comprises an alkaline series (basalt-hawaiite-mugearite) with extremely low initial 87Sr/86Sr ratios (≦0.7030) and generally low lithophile element contents. Apart from their alkalinity and high Sr and Zr contents these samples have affinities with abyssal tholeiites. Group II contains hypersthene normative basalts with more tholeiitic characteristics but (as in the case of the Skye Main Lava Series) the more evolved rocks are trachytes. This group is characterized by more normal levels of lithophile element concentrations and relatively high initial 87Sr/86Sr ratios of about 0.7055. Group III is less clearly defined and contains basalts that are generally sparsely olivine-phyric and in most chemical respects fall between Group I and Group II-including initial 87Sr/86Sr ratios (0.7033 to 0.7043). They may represent mixtures of Group I and Group II type sources or magmas. Groups I and II appear to be similar, respectively, to the relatively sodic iron-rich and the relatively potassic ironpoor silica enrichment trends distinguished in the Skye Main Lava Series. In the Group I magma series the behaviour of Y and Sr relative to other incompatible elements can only be explained by differential partial melting of a deep garnet-lherzolite mantle source. Fractional crystallization has undoubtedly occurred at some stage during the ascent of these magmas from the mantle, as indicated by the behaviour of Ni and Cr, but has not been a major factor in the production of evolved magma compositions. The Group II magmas appear to have originated from a source more enriched in lithophile elements, and a relatively shallow (< 50 km) plagioclase-lherzolite mantle source is suggested for these magmas because they have Sr/Ba ratios between one and two orders of magnitude lower than those characteristic of Group I. Rb-Sr systematics suggest that the vertical heterogeneity of the mantle which was largely responsible for the chemical differences between these three groups may have existed for a very long time prior to Tertiary magmatism.

Evolution of the tertiary volcanic rocks in the Izu-Mariana arc

Abstract: Petrological evolution of the Tertiary island arc in the Izu-Mariana region has been accompanied by the development of three different volcanic suites: 1) oceanridge basalt now exposed as the metamorphic basement on Yap; 2) island-arc tholeiites of Eocene to early Oligocene age characterized by low contents of incompatible elements at all levels of silica enrichment; and 3) calc-alkalic rocks of late Oligocene to early Miocene age showing higher contents of silica and incompatible elements. All these three suites have primitive, undifferentiated basalts or andesites (boninites) characterized by high Mg/Fe, Cr, and Ni, suggesting that they have been derived from an upper mantle peridotite at relatively high temperatures. The earliest volcanism appears to have occurred at a spreading ridge. Later, as subduction proceeded, the island-arc tholeiite magma may have been produced by the introduction of a smaller amount of water into the locus of fusion from the subducted oceanic crust. An increasingly larger amount of water introduced into the same region could have led to the development of the more siliceous, calc-alkalic magma, as represented typically by the boninite.

Komatiites and other high-magnesia lavas: some problems

Abstract: Criteria for the recognition of high-magnesia liquids, among which the absence of phenocrysts is the most important, are discussed. Some high-magnesia lava sequences are strongly porphyritic but it can be demonstrated that their character is not due to the accumulation of ferromagnesian phenocrysts in normal basaltic magmas. The term primitive porphyritic magma is introduced to describe the magmas from which such sequences crystallize. Possible origins of primitive porphyritic magmas include advanced crystallization of high-magnesia liquids without loss of phenocrysts. The occurrence of Phanerozoic high-magnesia lavas associated with continental break-up is described and comparison is made with Archaean komatiite vulcanism. Low levels of incompatible elements are characteristic of the Archaean rocks but high CaO/Al 2 O 3 is not a specifically Archaean feature. Phanerozoic liquids with MgO much above 20 % have yet to be identified but may possibly have existed. Major-element data for komatiites are discussed with a view to the constraints they put on the composition of the source material. Several interpretations, which also have widely varying implications for depth of origin and degree of melting, are presently possible. A new model involving the complete mobilization of source material after a comparatively low degree of partial melting is presented. Bulk compositions of magmas produced lie on a mixing line between the composition of the source and the composition of the liquid fraction at the moment of mobilization.

Geochemical zonation in the Sunda volcanic arc, and the origin of K-rich lavas

Abstract: The Sunda volcanic arc provides a good example of variation in the geochemistry of lavas across an island arc. In addition to the well-known correlation between K2O/SiO2 ratios and depths to Benioff Zone in Pleistocene ? Recent lavas of Java, there are well-defined relationships for 'incompatible' elements (Rb, Cs, Ba) and light rare earth elements. Estimated primary magma compositions for individual volcanic centres of Java indicate a progressive change in the conditions of primary basaltic magma production across the arc. The proportion of partial melting of peridotitic upper mantle appears to decrease from about 25% for olivine tholeiite magmas associated with tholeiitic lava series, to 5?10% for basanite primary magmas of high-K calc-alkaline lava series. The corresponding depths of final equilibration of magmas with crystalline residues increase from about 30 km to 60 km. These variations in conditions of magma production are probably superimposed upon an increase in the content of K and incompatible elements in the magma sources. The distinctive potassium-rich, strongly undersaturated Pleistocene lavas of northern Java probably originated from magmas produced at greater depth, in mantle enriched in phlogopite. Preliminary liquidus phase relationships for a primitive leucite basanite indicate a maximum depth of mantle origin of about 80 km. Such a composition could not be derived by melting of a lherzolite mantle in the presence of H2O alone, and the probable presence of CO2 in the mantle is indicated.