Magnesian Lavas Research Articles

The Baffin Bay lavas and the value of picrites as analogues of primary magmas

The Baffin Bay picrites have been the focal point of a controversy concerning the MgO content of primary magmas derived from the upper mantle. A sample population of 48 lava chilled margins collected across the Baffin Bay volcanic succession at the northeastern tip of Padloping Island exhibits a prominent compositional mode between 14 and 16 weight percent MgO (19–22 Mg, cation units = Mg/100 cations). The petrography of these samples, however, requires that the Padloping magmas were mixtures of olivine crystals and liquid at their eruption. Olivine phenocrysts constituted 15 to 30 volume percent of these magmas and retain compositions requiring coexisting liquid compositions with only 10 to 13.5 weight percent MgO (14–18.5 Mg). However, highly magnesian, olivine xenocrysts (up to Fo 93) found in the most magnesian lavas require the former existence of liquids with at least 18 weight percent MgO (24 Mg). If these xenocrysts represent early cumulates, then the primary liquids of the Padloping suite must have been at least this MgO rich with temperatures greater than 1,425° C. Such primary liquids could have evolved by olivine crystallization to a ‘steady state’, equilibrated crystal — liquid mixtures in a shallow reservoir system prior to eruption. The compositions of the liquids of these mixtures appear to have been perched at the point of plagioclase saturation at approximately 1,275° C.

Geochemical features of some Archaean and post-Archaean high-magnesian-low-alkali liquids

High-magnesian-low-alkali liquids are found as mafic lavas ranging in age from Archaean to Gainozoic. The most magnesian lavas are represented by Archaean spinifex textured peridotitic komatiites, and in this study these liquids are used as a comparative base for younger, less magnesian liquids. The post-Archaean lavas fall into three categories: (1) the Gape Smith (Proterozoic) - Baffin Bay (Gainozoic) group, (2) the low-Ti ophiolitic basalts of Cyprus, which represent remelting of a sequentially depleted source, and (3) the boninite group, which are the products of (wet?) melting of a source that had previously experienced depletion and addition of incompatible element enriched phases. With the use of parameters such as Al 2 O 3 /TiO 2 , Sc/Zr, Ti/V, a comparison of Archaean komatiites with the younger high magnesian lavas indicates that the bulk of the variation seen in these rocks types can be interpreted in terms of the amount of partial melting and nature of residual phases. However, some of the variability that occurs within individual lava provinces (particularly among the light rare earth elements) is best explained by a heterogeneity superimposed on a previously homogeneous source. The abundance of high-magnesian liquids declines sharply after the Archaean as does the maximum MgO content achieved by the lavas.

Geochemistry of basaltic and gabbroic rocks from the West Mariana basin and the Mariana trench

This paper describes the chemistry of 33 basaltic rocks dredged from the West Mariana basin and from the Mariana trench during the R/V “Dmitry Mendeleev” 1976 cruise in the western Pacific. The shipboard investigations were carried out by an international working group of 66 earth scientists under the IGCP Project “Ophiolites” and sponsored by the U.S.S.R. Academy of Sciences, Moscow. The purpose of the expedition was to investigate the structure and composition of the oceanic crust of marginal basins, remnant island arcs and deep-sea trenches. Tholeiitic basalts and gabbros as well as ultramafic rocks in various stages of alteration were dredged from the central part of the West Mariana basin demonstrating the presence of oceanic crust. The Pacific slope of the Mariana trench yielded altered basaltic rocks of tholeiitic and alkalic (? trachybasaltic to shoshonitic) composition. The lower part of the island arc slope contains typical tholeiitic basalts, dolerites and gabbros as well as ultramafics associated with flysch-type sediments. This is strong evidence for the formation of an “ophiolite-schuppenzone”, probably due to subduction of Pacific oceanic crust. Associated with these rocks are amygdaloidal, highly magnesian lavas (similar to boninites), which have not been recognized previously in oceanic ridge basalts. These rocks (together with the dolerites) are interpreted as parts of the Mariana island arc and are thought to be the first stage of island arc development (an immature island arc).

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

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.

The petrogenesis of peridotitic komatiites: Evidence from high-pressure melting experiments

The compositions of liquidus olivines and orthopyroxenes of natural specimens of spinifex- and quench-textured peridotitic komatiites from the Belingwe Greenstone Belt, Rhodesia have been determined for pressures between 10 and 40 kbars. In conjunction with the chemical variation exhibited by these peridotitic komatiites it is concluded that the more magnesian lavas cannot be derived by concentration of olivine phenocrysts, fractional crystallisation or equilibrium partial melting. The peridotitic komatiites could evolve by polybaric assimilation and complete melting of garnet lherzolite into an initial liquid containing about 24% MgO.

The Mafic and Ultramafic Lavas of the Belingwe Greenstone Belt, Rhodesia

The Belingwe Greenstone Belt (2.8 × 109 yrs old) contains a 7 km succession of mafic and ultramafic lavas and high-level intrusions which overlie a thin sedimentary formation, itself unconformable on a granitic basement. The lavas range in composition from andesites (4 per cent MgO) to peridotitic komatiites (32 per cent MgO). The mineralogy and textures of the most magnesian lavas demonstrate that they were extruded in a completely liquid state. If the source mantle had an MgO content around 40 per cent, then partial melts in the range 35 per cent to 55 per cent would be required to produce the most magnesian liquids observed. Chemical constraints on the petrogenesis of the ultramafic lavas allow estimates of source mantle composition. In particular, if the source had an MgO content around 40 per cent, then the overall source composition would be similar to that of garnet Iherzolite nodules in kimberlites. The calculated REE contents of the source are close to chondritic. If all the ultramafic lavas were derived from the same source then the variation in liquid composition may have been controlled by orthopyroxene as well as olivine during partial melting at depth. The evolution of the less magnesian komatiites, basalts, and andesites can be explained by lower degrees of partial melting of a common source, and by high-level fractionation of parent liquids similar to those extruded as ultramafic lavas. Physical constraints on the origin of the lavas imply derivation from a depth of 150 km or more, at temperatures of 1600–2000 °C.