MELT INCLUSIONS IN GARNET FROM ULTRAMAFIC HP GRANULITES (DRONNING MAUD LAND, ANTARCTICA)

The Dronning Maud Land (DML) is the southern continuation of the Panafrican Mozambique Belt, formed by the collision of E and W Gondwana. In the DML, crystallized melt inclusions (MI) occur in garnet from ultramafic granulites hosted in orthogneisses of the Conrad-gebirge. The granulites contain an assemblage of Grt-Opx-Cpx, formed at c. 900°C and 10-15 kbar, and retrograde amphibole and Spl-Opx-Pl ± Ol symplectites around garnet.

Garnet porphyroblasts up to 2 cm contain two generations of MI. Type 1 is primary, and occurs in the inner part of the crystals. MI are as isolated, <10 mm, and generally with negative crystal shape. Tubular inclusions, <40 mm in length, are also present. MI contain kumdykolite (Kml), kokchetavite (Kok) and phlogopite, with quartz/cristobalite and zoisite as minor phases. Kml and Kok are metastable polymorphs of Ab and Kfs respectively. The second generation of MI (type 2) occurs as larger (up to 30 mm), pseudosecondary inclusions containing amphibole, feldspar, zoisite, minor magnesite, halite and orthopyroxene. A CO₂-rich fluid may be present in type 2 MI.

Primary MI in garnet indicate that melt was present during garnet growth under peak conditions. This would suggest that both melt and garnet are products of the partial melting of the ultramafic boudins, most likely through amphibole dehydration melting.

Fragments of garnet containing MI were heated in a piston cylinder at 950°C, 22-24 kbar for 24 h. The inclusions were successfully re-homogenized and subsequently analyzed by electron microprobe. Type 1 MI re-homogenized as a melt of trachytic composition with K/Na = 3.55 and H₂O ≈ 2.5 wt.%, while type 2 MI as a dacitic melt with K/Na= 0.56 and H₂O ≈ 12 wt.%.

The high-silica, K-rich melt of type 1 MI is unexpected. Such composition can be due to the participation of biotite as reactant in the melting reaction. Or it may result from the open-system percolation of K-rich melts formed in the adjacent orthogneisses during garnet growth. An exotic origin is even more likely in the case of the type 2 MI, which are clearly located within microfractures crosscutting garnet.

Despite the uncertainty on the petrogenetic significance of the melt preserved in the MI, this study is important because it documents for the first time the occurrence of MI in metamorphosed (ultra)mafic protoliths.