The Origin of Alkaline Trachybasalts of the Manning Massif
(East Antarctica)

A. V. Andronikov Mineralogisch-Petrologisches Institut Göttingen, Goldschmidtstrasse 1, D-37077, Göttingen, Germany and VNIIOkeangeologia, I Angliysky ave., 190121, St.-Petersburg, Russia

A flow of Cenozoic K-rich (5 - 6% K2O, 4-5% Na2O) trachybasalts lies unconformably on a surface of Precambrian granulite terrain in the central part of the Manning Massif, and has been considered to be related to lamproite. The main rock-forming phases are olivine phenocrysts and microliths, sanidine, anorthoclase, augite microliths, "phantoms" of leucite phenocrysts, interstitial glass and titanomagnetite. Glass also occurs in leucocratic globules and as inclusions in olivine phenocrysts. Interstitial glass and glass in the globules is interpreted as representing the final stages of melt differentiation within the rock. Glass inclusions in Mg-rich olivine phenocrysts (Mg# ~80) are primitive and may be close to primary melt composition. They have low SiO2 (47.6%) and relatively low K2O (1.8%) but high Na2O (8.1%). The glass also has Mg# around 75 (5.7% FeO, 9.4% MgO), and KDFe/Mg between olivine of the source peridotite and the glass of 0.33, which may indicate equilibrium with mantle peridotite. Glass compositions are plotted in the CIPW normative tetrahedron (Fig. 1) with cotectics defined by experiments on Hawaiian pyrolite and Tinaquillo Iherzolite. The composition of primary melt falls to the left of clinopyroxene-out lines, and the liquid requires a Iherzolite residuum. The primitive glass inclusion falls on cotectics for I5 kbar for Hawaiian pyrolite (fertile mantle) and 14 kbar for Tinaquillo Iherzolite (depleted mantle). Pressure estimation of origin for the melt by the methos of Ballhaus (1995) gives 21 kbar, suggesting generation of the melt in spinel or spinel-garnet Iherzolite at depths of 60-65 km. The sporadic occurrence of spinel lherzolite xenoliths in some lava sheets confirms this suggestion. All features described suggest a Na-rich primary melt for the trachybasalts and not a K-rich melt similar to lamproites. The alkalinity of the primary melts is most likely due to involvement of an alkali-bearing phase (amphibole, possibly plus clinopyroxene) in their generation. It is plausible that initial melts leading to trachybasalts are due to melting of amphibole plus clinopyroxene-rich veins. The melting event later proceeded to incorporate surrounding wall-rocks of mainly spinel or spinel-garnet Iherzolite composition.

Fig. 1: CIPW normative projection from diopside apex onto the plane (Jd+CaTs) - Qz - Ol.