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Geochemistry and petrology of ultramafic lamprophyres from Schirmacher Oasis, East Antarctica

Geochemie und Petrologie von Ouachititen aus der Schirmacher Oase, Ostantarktika

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Summary

Petrographic and geochemical data are presented for some ouachitites from the Schirmacher Oasis. The studied dike intruded into the metamorphic basement of the East Antarctic craton. The ouachitite samples contain substantial amounts of zoned phlogopite and diopsidic to salitic pyroxene as phenocrysts. K-rich nepheline is the predominant phase of the matrix. The rocks are characterized by low SiO2 concentrations (up to 36.5 wt.%), low mg# and high abundances of Al2O3, alkalis and volatiles. Trace element compositions of the Schirmacher ouachitites differ from those of ultramafic lamprophyres from other regions in that they contain significantly lower Cr and Ni concentrations and relatively low enrichments in LILE and LREE. The studied rocks have low143Nd/144Nd and enhanced87Sr/86Sr ratios. Pb isotope compositions of the ouachitites closely resemble those of MORB. The Δ18O values range between 4.1 and 4.70/00 which may be due to interactions with meteoric water. Because the Schirmacher ouachitites are only represented by a few samples it is impossible to discuss the petrogenesis of this magma. However, according to trace element and isotopic characteristics, it can be ruled out that Schirmacher ouachitites and associated minettes (documented inHoch, 1997;Hoch andTobschall, 1998) were derived from the same mantle reservoir.

Zusammenfassung

In dieser Arbeit werden petrologische und geochemische Daten von Ouachititen aus der Schirmacher Oase vorgestellt. Der beprobte Gang intrudierte in das metamorphe Basement des ostantarktischen Kratons. Zonierte Phlogopite und diopsidische bis salitische Pyroxene sind die häufigsten mafischen Einsprenglinge. K-reiche Nepheline bilden die Hauptmineralphase der Grundmasse. Charakteristisch für die Ouachititproben sind niedrige SiO2-Konzentrationen (bis 36.5 Gew.%) und mg#, aber hohe Gehalte an Al2O3 Alkalien und volatilen Bestandteilen. Die Spurenelementzusammensetzung der Schirmacher Ouachitite unterscheidet sich von UML-Proben aus anderen Regionen durch ihre extrem geringen Cr- und Ni-Konzentrationen und die vergleichsweise niedrigen Anreicherungen an LILE und LREE. Die bearbeiteten Gesteine zeigen niedrige143Nd/144Nd- und erhöhte87Sr/86Sr-Verhältnisse. Die Pb-Isotopie der Ouachitite entspricht der von MORB. 6180-Werte variieren zwischen 4.1 und 4.70/00, was auf eine eventuelle Beeinflussung durch meteorische Wässer hindeutet. Da von den Schirmacher Ouachititen nur wenige Proben für eine Bearbeitung zur Verfügung standen, ist es nicht möglich, die Petrogenese des Magmas zu diskutieren. Aber aufgrund der Spurenelement- und Isotopensignaturen kann ein gemeinsames Mantelreservoir der Schirmacher Ouachitite und Minette (beschrieben inHoch, 1997;Hoch undTobschall, 1998) ausgeschlossen werden.

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  1. Institute of Geology and Mineralogy, University of ErlangenNuremberg, Schlossgarten 5, D-91054, Erlangen, Germany

    M. Hoch

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Hoch, M. Geochemistry and petrology of ultramafic lamprophyres from Schirmacher Oasis, East Antarctica. Mineralogy and Petrology 65, 51–67 (1999). https://doi.org/10.1007/BF01161576

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