Pseudoleucite in a Tinguaite from the Bearpaw Mountains, Montana

The article presents chemical analyses of the pseudoleuciteand groundmass of a pseudoleucite tinguaite, chemical analysesof the principal minerals, chemical estimates of the mode ofthe groundmass, and both chemical and micrometric modes of thepseudoleucite. The principal minerals are hedenbergitic acmite,a nepheline containing 2.3 per cent Fe₂O₃ and 7.9 per cent K₂O,and K-feldspar containing a little BaO and very little Na₂O.The micrometric mode of the pseudoleucite, in satisfactory agreementwith the chemical mode, is nepheline 29.8 per cent, K-feldspar66.2 per cent, acmite 3.0 per cent; the chemical mode of thegroundmass is nepheline 26.8 per cent, K-feldspar 46.6 per cent,and acmite 26.7 per cent. A new analysis of a nepheline fromthe Bancroft area, made to test the analytical procedure, isalso included.     

On Estimating the Magnitude of the Hidden Zone and the Compositions of the Residual Liquids of the Skaergaard Layered Series

Wager's more general assumptions about the relations betweenthe layered series of Skaergaard and its parent magma form thebasis for a simple procedure by which, from the compositionof the proposed initial magma and the compositions and amountsof the exposed zones, the amount and composition of the hiddenzone and the compositions of liquids remaining after the consolidationof each zone are readily computed. Application of the procedure to the Skaergaard data—moreproperly, to the relevant summary statistics presented by Wager(1960) and by Wager & Brown (1968)—suggests that currentlyaccepted graphical solutions materially overestimate the extentof end-stage alkali enrichment and may materially understatethe magnitude of the hidden zone; the data are quite as compatiblewith numerical solutions yielding a considerably larger hiddenzone and virtually no perceptible end-stage alkali enrichment.Differences between numerical and graphical solutions seem tobe generated primarily by the zone weights used in the latter.     

Variance--Covariance Relations in Some Published Harker Diagrams of Volcanic Suites

Sample statistics for data upon which a number of publishedHarker diagrams are based are reviewed in detail. A convincingHarker diagram—i.e. one in which most of the data pointslie reasonably close to simple curves—seems to requirethat the variance of silica be at least as large as the sumof all other variances. For this situation it can be shown apriori that strong negative correlation is to be expected betweensilica and some or all other oxides making substantial contributionsto the total variance of the array. The data suggest that thelarger the relative contribution to total variance made by anyother such oxide, the stronger its negative correlation withsilica. An excess of silica variance sufficient to generate strong negativecorrelation of silica with oxides of Ca, Mg, Fe, and Al seemscharacteristic of Cenozoic examples of the basalt-andesite-daciteassociation, whether or not they include rhyolitic material,and even when they contain no dacite. It occurs also in abouthalf the examples of the oceanic basalt-trachyte associationso far inspected, but is not found either in the basalt-mugearitesubgroup of trachyte-bearing suites of this association or inoceanic basalt suites lacking salic derivatives. The strong negative correlations of the typical Harker diagramcould be generated by any process which would greatly enlargethe variance of silica relative to other oxides.