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Petrogenesis of the Mesozoic Sistefjell syenite intrusion, Dronning Maud Land, Antarctica and surrounding low-¹⁸O lavas

Department of Geological Sciences, University of Cape Town, Rondebosch 7700, South Africa, Département de Géologie, Université Jean Monnet, 23 rue Paul Michelon, F-42023 Cédex 02, France, e-mail: chris.harris{at}univ-st-etienne.fr

Warren P Johnstone

Department of Geological Sciences, University of Cape Town, Rondebosch 7700, South Africa, e-mail: wjohnstone{at}gemcom.co.za

David Phillips

Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia, School of Earth Sciences, The University of Melbourne, VIC, 3010, Australia, e-mail: dphillip{at}unimelb.edu.au

The Sistefjell intrusion is approximately 10km in diameter and is the largest Mesozoic pluton in western Dronning Maud Land. The main intrusive phase is a syenite consisting of >78 modal % alkali feldspar, quartz, sodic-calcic amphibole, aegirine-augite, and minor fayalite, aegirine and aenigmatite. Associated sills and dykes include microsyenite, quartz porphyry and microgranite. The variation in chemical composition of these rocks is largely consistent with fractional crystallization. Oxygen isotope ratios of mineral separates from the quartz syenite indicate an original magma ¹⁸O value of 6.7, whereas the more evolved minor intrusions crystallized from magmas with slightly lower ¹⁸O values. The only exposed country rock, the undeformed late-Precambrian (?) Sistenup lavas, have whole-rock ¹⁸O values which range from +4.4 to –1.8 (mean +1.0, n=8), and D values which range from –96 to –167 (mean –129, n=7). The low ¹⁸O values of the lavas, coupled with an abundance of secondary hornblende, are consistent with alteration at elevated temperatures (450°C) and high water/rock ratios. Several lines of evidence suggest that fluid/rock interaction occurred prior to the intrusion of the syenite, and was possibly related to influx of meteoric water during regional metamorphism. The low D values of the lavas indicate interaction with water with abnormally low D value typically found at high latitudes and/or high altitude.

The freshest syenite sample gave a three point (feldspar, amphibole whole-rock) Rb-Sr isochron (MSWD = 0.12) equivalent to an age of 173 ± 2 Ma, whereas ⁴⁰Ar-³⁹Ar step-heating of amphibole from lavas close to the contact suggest a maximum age of intrusion of ~182 Ma. Older high-temperature ⁴⁰Ar-³⁹Ar ages are consistent with a much older age for the Sistenup lavas. Although the syenite and related minor intrusions have low Zr/Nb ratios (mean value 7.2) and high Ce/Y ratios, which are characteristic of rift zone magmas, their _Nd values of –1.8 to –4.2 indicate crustal contamination. Two stages of contamination are required involving, firstly, the dominant metamorphic rocks in the area and, secondly, the Sistenup lavas.

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				T. R. Riley, M. L. Curtis, P.T. Leat, and I. L. Millar The geochemistry of Middle Jurassic dykes associated with the Straumsvola-Tvora alkaline plutons, Dronning Maud Land, Antarctica and their association with the Karoo large igneous province Mineralogical Magazine, August 4, 2009; 73(2): 205 - 226. [Abstract] [Full Text] [PDF]

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