Petrography and petrology of a basanitoid flow from Hut Point Peninsula, Antarctica

Includes bibliographical references.Includes illustrations and maps.Basaltic rocks of the Hut Point Peninsula, Ross Island, Antarctica, are a part of a linear trend of volcanic occur­rences which parallel the Transantarctic Mountains for 2000 km. Structural relations suggest the tensional nature of the region, a feature also indicated by the abundance of volcanic occurrences. The basanitoid (silica-undersaturated, alkali-rich oli­vine basalt lacking phenocrystal nepheline but with greater than 5% normative nepheline) of this study is of interest be­cause it carries an abundant suite of ultramafic inclusions and therefore provides a look at upper mantle mechanisms of generation and transport of basaltic magmas. The Twin Crater-Helo Pad flow extends for 1350 m with an average width of 175 m. The flow is massive to scoriaceous, moderately to extremely vesicular, and has a minimum thickness of 3.6 m. The intergranular matrix holds porphyritic mega- crysts of olivine, titanaugite, and kaersutite. Xenolithic material is abundant and includes dunitic or peridotitic nod­ules, gabbroic or granulitic crustal fragments, and metasedimentary basement inclusions. The flow exhibits broad variation within narrow limits, but most variation has been ob­scured by turbulence produced by the rapid and violent erup­tion of this basanitoid. Two other basanitoid occurrences of the immediate area may be included in a petrogenetic model for the Twin Crater flow, but neither can be correlated with the flow. Chemical analyses indicate that the magma originated by low degrees of partial melting of primitive peridotitic (pre­viously undepleted) mantle material at substantial depth. Rare earth element anomalies suggest that garnet was a residual phase, thus placing the melting zone at a depth greater than 100 km. Ultramafic inclusions further support an origin at this depth. Primary crystallization occurred at depths of 45 to 75 km probably under conditions of low water content and low initial oxygen fugacity and involved olivine, titan- augite, and titanomagnetite as major phases. Secondary crystallization at shallow levels was insignificant. The suite of ultramafic xenoliths and xenocrysts of kaersutite and two clinopyroxenes complicated definition of the cognate solidus assemblage for comparison to experimental results but had no apparent effect on the chemistry of the melt. The magma ascended rapidly from depth, as indicated by abundant mantle-origin xenoliths and a lack of primary plagioclase crystallization. Eruption and cooling were violent and tapid. Weathering processes have not affected the flow to any great degree, and alteration is restricted to xenolithic material reacting to magmatic or near-surface conditions. Secondary mineralization or hydrothermal alteration have had no signi­ficant effect on the Twin Crater-Helo Pad flow.M.S. (Master of Science