Basic lavas of the Archean La Grande Greenstone belt: Products of polybaric fractionation and crustal contamination

Despite the fact that some greenstone belts preserve the record of contemporaneous komatiitic and tholeiitic volcanism, a genetic link between the two is not widely accepted. The significance of a compositional gap seperating these magma types and differences in their respective degree of light rare earth element (LREE) enrichment, cited as evidence against a derivative relationship, are complicated by the possibility of crustal assimilation by magmas of komatiitic affinity. In the Archean La Grande Greenstone belt of northern Quebec a succession of metamorphosed tholeiitic basalts and younger, high-Mg, LREE-enriched andesites are preserved. The tholeiites are differentiated basaltic rocks whose chemical compositions appear to have been controlled by low pressure, gabbroic fractional crystallization and are similar to Type 1 MORB. Parental magmas were probably high-Mg liquids of compositions similar to komatiitic basalts which also occur in the greenstone belt. These high-Mg liquids are believed to be themselves the product of high pressure, OLIV+OPX fractional crystallization of more magnesian primary liquids of komatiitic composition. The higher La/Sm ratios of komatiitic basalts and tholeiites relative to komatiites in this belt, can be explained by small degrees of crustal assimilation. In the central part of the belt, late-stage, mafic igneous rocks have chemical compositions similar to Archean examples of contaminated volcanic rocks (e.g., Kambalda, Australia). These late-stage lavas consist of basalts and andesites with high-Mg, Ni and Cr abundances, LREE-enriched profiles and low Ti abundances. They are believed to be the products of crustal assimilation and crystallization of OPX-PLAG-CPX from high-Mg liquids of komatiitic affinity. The volcanic stratigraphy records the progressive effects of crustal contamination through time. A light sialic crust may have initially acted as a density barrier, preventing the eruption of primary high-Mg liquids and forcing fractionation at depth which produced more buoyant compositions. With subsequent thinning of the crust, the density barrier presumably failed, and primary liquids migrated directly toward the surface. Reaction of these liquids with tonalitic crust produced contaminated differentiates.     

Xenolith evidence for lithospheric melting above anomalously hot mantle under the northern Canadian Cordillera

A comparison of mantle xenolith suites along the northern Canadian Cordillera reveals that the xenoliths from three suites exhibit bimodal populations whereas the xenoliths from the other four suites display unimodal populations. The bimodal suites contain both fertile lherzolite and refractory harzburgite, while the unimodal suites are dominated by fertile lherzolite xenoliths. The location of the three bimodal xenolith suites correlates with a newly discovered P-wave slowness anomaly in the upper mantle that is 200 km in width and extends to depths of 400–500 km (Frederiksen AW, Bostock MG, Van Decar JC, Cassidy J, submitted to Tectonophysics). This correlation suggests that the bimodal xenolith suites may either contain fragments of the anomalously hot asthenospheric mantle or that the lithospheric upper mantle has been affected by the anomalously hot mantle. The lherzolite xenoliths in the bimodal suites display similar major element compositions and trace element patterns to the lherzolite xenoliths in the unimodal suites, suggesting that the lherzolites represent the regional lithospheric upper mantle. In contrast, the harzburgite xenoliths are highly depleted in terms of major element composition, but their clinopyroxenes [Cpx] have much higher incompatible trace element contents than those in the lherzolite xenoliths. The major element and mildly incompatible trace element systematics of the harzburgite and lherzolite xenoliths indicate that they could be related by a partial melting process. The lack of textural and geochemical evidence for the former existence of garnet argues against the harzburgite xenoliths representing actual fragments of the deeper anomalous asthenospheric mantle. Furthermore, the calculated P-wave velocity difference between harzburgite and lherzolite end-members is only 0.8%, with the harzburgites having higher P-wave velocities. Therefore the 3% P-wave velocity difference detected teleseismically cannot be produced by the compositional difference between the lherzolite and harzburgite xenoliths. If temperature is responsible for the observed 3% P-wave velocity perturbation, the anomalous mantle is likely to be at least 200 °C higher than the surrounding mantle. Taken together these data indicate that the refractory harzburgite xenoliths represent the residue of 20–25% partial melting of a lherzolite lithospheric mantle. The incompatible trace element enrichment of the harzburgites suggests that this melting was accompanied by the ingress of fluids. The association of the bimodal xenolith suites with the mantle anomaly detected teleseismically suggests that anomalously hot asthenospheric mantle provided both the heat and volatiles responsible for the localized melting and enrichment of the lithospheric mantle. Received: 16 May 1997 / Accepted: 25 October 1997     

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.Despite the complications of mechanical sorting of olivine crystals, the virtual compositional reciprocity of olivine addition and olivine fractionation requires that the bulk compositions of picritic lavas provide compositional analogues of their primary magmas. A comparison of Phanerozoic picrite suites indicates that the Fe contents of terrestrial primary magmas of tholeiitic affinity have a restricted range from 6–9 Fe. Primary magmas associated with intra-plate volcanism appear to be distinctly more Fe-rich than those associated with inter-plate volcanism. The Al/Si ratios of Phanerozoic picrite suites could suggest that the primary magmas of MORB volcanism have equilibrated with relatively Fe-poor source regions at deeper levels in the Earth's mantle than those of other tholeiitic primary magmas.     

Unusual electrical resistivity effects associated with fast-growing trees

A series of electrical resistivity profiles have been acquired in eucalyptus groves during archaeological surveys performed in Portugal. The combination of the sandy soils in the region plus the presence of high growth eucalyptus trees yielded anomalous resistivity values which were extremely high and showed large variability. Other moderate and low growth trees did not show similar unusual resistivities. We conclude that extreme caution must be exercised when interpreting data from archaeological surveys associated with unusual soils or vegetation. © 1993 John Wiley & Sons, Inc.