The Cenozoic volcanic rocks of Ross Island and vicinity, Antarctica, are surface flows, tuffs, breccias, and small intrusives. The prominent rock is basanitoid. Alkali-basalt magma reached the surface over an appreciable length of time and was differentiated to produce a rock series: basanitoid-. trachybasalt-, phonolite. The alkalic, silica-undersaturated rocks are part of a larger petrologic province composed of volcanic centers in a belt nearly 2,000 km long roughly parallel to the Transantarctic Mountains. Two trends of magmatic differentiation are apparent. At depth the fractional crystallization of olivine, clinopyroxene, and opaque oxide minerals, all of which are abundant as phenocrysts in the basanitoid flows, produced trachybasalt magma. At crustal levels, fractional crystallization of clinopyroxene, apatite, opaque oxides, kaersutite, plagioclase, and anorthoclase developed the phonolitic rocks. Alkali enrichment is marked, and the end-member phonolites contain 11% NazO, 5.6% K20, and 57% SiO2. The basanitoids contain relatively large amounts of Ba (~ 400 ppm), Sr (~ 1,000 ppm), and Rb (~ 30 ppm). The concentrations of these and other trace elements in the various rock types support the model of small degrees of partial melting of peridotitic mantle for the derivation of the basanitoid magma, and of magmatic differentiation for the subsequent development of the rock series.

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ABSTRACT. Major, minor, and trace elements were measured with electron and ion microprobes for eighteen anorthoclase megacrysts from five locations. Comparisons with concentration ranges tabulated by Smith (1974) show excellent agreement except for consistently lower values of Mg and B. The discrepancy for Mg probably reflects Mg-rich impurities contaminating samples analysed by bulk methods. The cause of the discrepancy for B is unknown and deserves further study. Samples from Mt. Erebus, Antarctica, contain glass inclusions from which the following crystal/liquid partition coefficients ensue: Mg, 0.008; P, 0.04; Ti, 0.08; Fe, 0.04; Ba, 5.2; Sr, 8; Rb, 0.27. BECAUSE of the paucity of trace-element analyses of anorthoclase megacrysts, electron and ion microprobe analyses were made of specimens from several localities. Mountain (1925) described anorthoclases from Mr. Erebus (Antarctica) and Mt. Kenya. The geochemistry of the Cenozoic volcanic rocks of Ross Island and vicinity was studied by Goldich et al. (1975), and the mineralogy and glass chemistry by Clocchiatti et al. (1976) and Kyle (1977). Chapman (1974Chapman ( , 1976 Counting times were 20 seconds for major elements and 100 seconds for minor elements. The beam diameter was 30/~m for feldspar and glass, 5/zm for olivine and pyroxene. Table I gives details of the standards. The samples were cleaned and gold coated before being loaded into the ion microprobe. The following instrumental settings were used: 160-primary beam, 10 nA beam current, 20 kV accelerating voltage, 20/~m beam diameter. Because molecular interferences are present at some mass numbers (e.g. 4~160+ at m/e = 56) the minor and trace elements were analysed in two sets: S6Fe, SSRb, and 8aSr were analysed at high mass resolution to avoid molecular interferences, and ~Li, 11B, 24Mg ' *9Ti, laaCs, and 13SBa, which are interference free, were analysed at low mass resolution. Several standard feldspars (Table I) were checked at intervals during each session to confirm instrumental stability. The standards were previously analysed by electron probe using the methods given above or, in the case of Li and B, were analysed by chemical methods. Count rates from the anorthoclases were compared to those from the standards and used to calculate trace element concentrations.

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A 5–7 m thick analcime phonolite sill occurs in the middle of the Proterozoic Gateway Formation in southwest Alberta. The sill consists of sanidine, aegirine–augite, magnesian hastingsite, melanite with hydrogrossular rims, titanite, and minor biotite, apatite, and opaque minerals. Mineralogical and chemical similarities to the analcime-rich phases of the Cretaceous Crowsnest Formation found some 20 km to the north suggest a genetic relationship. Major differences are the presence of amphibole and hydrogrossular, minerals not reported in the Crowsnest Formation. The presence of amphibole as a primary hydrous phase in the Rainy Ridge sill indicates crystallization from a hydrous magma. Microprobe studies indicate a progressive enrichment of sodium in amphiboles and pyroxenes. An apparent difference in chemical composition and alteration behavior of primary analcime phenocrysts and groundmass analcime is interpreted to reflect crystallization of analcime from a hydrous melt at depth, followed by rapid transport to a shallow depth, and crystallization of the groundmass analcime and hydrogrossular rims.