Volume 38: January - December 1997

Issue 9: September 1997

Abstract

Petrogenesis of a phonolite-trachyte succession at Mount Sidley, Marie Byrd Land, Antarctica

KS. Panter, PR. Kyle¹ and JL. Smellie²
¹Department of Earth and Environmental Science, New Mexico Institute of Mining and Technology, Socorro, NM 87801, USA and ²British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UKCorresponding author
ABSTRACT
The 1.5 Ma evolution of the Late Pliocene (5.7 to 4.2 Ma) Mt Sidley volcano, Marie Byrd Land, is examined using major and trace elements, Sr, Nd, O and Pb isotopic data. A large (5 km x 5 km) breached caldera exposes lavas and tephras, deep within Mt Sidley, and allows its magmatic evolution to be elucidated. Two alkaline rock series are distinguished: (a) a strongly silica-under-saturated basanite to phonolite series; (b) a more silica-saturated to -oversaturated alkali basalt to trachyte series. Rock compositions in both series fall within a narrow range of ⁷⁷Sr/⁸⁶Sri (0.7028-0.7032), ¹⁴³Nd/¹⁴⁴Ndi (0.51285-0.51290) and [delta]¹⁸O (5.0-6.0%_o), and with ²⁰⁶Pb/²⁰⁴Pb (>19.5), suggest an asthenospheric source containing a strong mantle plume component. Partial melting models require <= 2% melting to produce primary basanite and <= 5% melting to produce alkali basalt from the same mantle source. The differentiation of the phonolitic series is modeled by fractionation of diopside, olivine, plagioclase, titaniferous magnetite, nepheline and/or apatite from basanite to derive 35% mugearite, 25% benmoreite and 20% phonolite as residual liquids. Fractional crystallization of a similar mineral assemblage from alkali basalt is modeled for compositions in the trachyte series. However, many trachytes have variable ⁸⁷Sr/⁸⁶Sri (0.7033-0.7042), low ¹⁴³Nd/¹⁴⁴Ndi (0.51280-0.51283), high [delta]¹⁸O (6.5-8.4%_o) and are silica oversaturated, suggesting they are contaminated by crust. The trachytes evolved by a two-step assimilation-fractional crystallization process (AFC). The first step involved contamination of alkali basalt by calc-alkaline granitoids within the middle crust where high assimilation to crystallization rates (high-r AFC) produced trachytic magmas characterized by depletions in Ta and Nb relative to K and Rb. The second step involved further fractionation of these magmas by low-r AFC within the upper crust to produce another suite of trachytes showing extreme incompatible element enrichment (e.g. Zr>1000 p.p.m/ and Th>100 p.p.m.).

Keywords: alkaline magmas; AFC; magma commingling; Marie Byrd Land; volcanism

Pages: 1225 - 1253

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