Petrogenesis of the Mesozoic Sistefjell syenite intrusion, Dronning Maud Land, Antarctica and surrounding low-δ¹⁸O lavas

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 δ¹⁸O value of 6.7‰, whereas the more evolved minor intrusions crystallized from magmas with slightly lower δ¹⁸O values. The only exposed country rock, the undeformed late-Precambrian (?) Sistenup lavas, have whole-rock δ¹⁸O values which range from +4.4 to -1.80‰ (mean +1.0‰, n=8), and δD values which range from -96 to -167‰ (mean -129‰, n=7). The low δ¹⁸O values of the lavas, coupled with an abundance of secondary horblende, are consistent with alteration at elevated temperatures (450°C) and high water/rock ratios. Several lines of evidence suggest that fluid/rock interaction occured prior to the intrusion of the syenite, and was possibly related to influx of meteoric water during regional metamorphism. The low δD values of the lavas indicate interaction with water with abnormally low δD 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-temperattore 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 ε_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.