Igneous nepheline-bearing rocks of the Haliburton-Bancroft province of Ontario

Assemblages of magmatic origin—hypersolvus nepheline syenites,theralites, and theralitic canadites—developed in a beltof nepheline rocks extending through Monmouth and Glamorgantownships of Haliburton County, Ontario, are described and theirsignificance in the problems of origin of the Haliburton–Bancroftalkali province briefly disocussed.     

Origin of Basalt Magmas: An Experimental Study of Natural and Synthetic Rock Systems

Natural basalts and eclogites were investigated experimentallyat a series of temperatures in the pressure range 1 atm to 40kb and with water pressures 1 to 10 kb. Some runs were alsomade on related synthetic systems at 10 and 33 kb. The two principal magma types recognized by field investigators—tholeiiteand alkali basalt types-appear to be separated by equilibriumthermal divides at 1 atm. The principal divides were found byexperiment at elevated pressures to give way to a new set ofequilibrium thermal divides resulting from a new mineralogy.The change of the equilibrium thermal divides with pressureleads to the derivation of the two principal magma trends fromthe same bulk composition. The melting behavior of basalts and eclogites indicates thatboth are the partial melting products of a more primitive rock(e.g. garnet peridotite). In the region of magma generation(below 60 km) the parental material, presumed to be garnet peridotite,yields an eclogitic magma and its fractionation depends on thegarnet and omphacite of the eclogite, not on plagioclase andclinopyroxene of a basaltic magma. Increase of the garnet constituentsin the magma at high pressure by effective removal of omphaciteor shift of the garnet-omphacite boundary ‘surface’will give rise to a tholeiite-type magma at low pressure. Similarly,increase of the omphacite constituents in the magma at highpressure by physical or physicochemical means will give riseto an alkali basalt-type magma at low pressure. In general,alkali basalt-type magmas are to be expected to be generatedat greater depths than tholeiite-type magmas from the same primarysource rock. Establishment of the two major basalt series takes place inthe region of generation; additional minor diversification ofeach series may come about after emplacement in or on the crustby crystal settling, oxidation or reduction, gas fluxing, contamination,and other processes. The derivative magmas are greatly restrictedby the course of liquid thermal descent imposed at generation. Pressure-temperature limits established experimentally suggestthat the basalt-eclogite transformation may be responsible forthe Mohorovii discontinuity under the continents, but not underthe oceans. The field of stability of basalt is drastically reduced in thepresence of water, and amphibolite is produced. The meltingof amphibolite takes place over a much greater range of temperaturethan basalt. At 10 kb water pressure the beginning of meltingof amphibolite closely approaches that of granite. Partial meltingof amphibolite may yield anorthositic liquids having a relativelylow anorthite content at exceptionally low temperatures. Eclogiteitself is not stable in the presence of water and gives placeto amphibolite or pyroxene hornblendite. Magmas which crystallizeto basalt, gabbro, or eclogite must have had a low water-contentat the time of crystallization. Fifteen rock and twenty-three mineral analyses as well as numerouspartial chemical analyses of experimental products were madeby J. H. Scoon in the course of the investigation. These chemicalanalyses bear on many mineralogical and petrological problems.     

Differentiation of Hawaiian Basalts: Some Variants in Lava Suites of Dated Kilauean Eruptions

The paper discusses briefly the trend of fractionation exhibitedby basaltic lavas of three dated Kilauea eruptions, the suiteof 1921 in Kilauea Caldera and the suite of the 1840 and 1955flank eruptions of Kilauea in the east rift zone. The trendrevealed is graphically indicated in terms of iron/magnesiumand alkali enrichment.