Generation of highly silicic magmas at ultra-high temperature conditions : evidence from melt inclusions in peritectic garnet

Abstract

In this study we use former melt inclusions found in peritectic garnet to investigate anatexis and fluid regime of metapelitic granulites in samples from the Rundvågshetta area, the thermal axis of the Lützow-Holm Complex (East Antarctica). Peak P-T estimates are 925-1039°C at 11.5-15 kbar. The studied rock is a coarse-grained heterogeneous metapelitic granulite with a predominant mafic residual domain and a relatively more felsic, melt-rich domain. The mineral association in the mafic domain typically contains orthopyroxene (Al 2 O 3 6-8.1 wt.%) + sillimanite + quartz + garnet (Prp 42-55 Alm 40-52 Grs 3-4 Sps 0.2-1 ; X Mg 0.5) + K-feldspar (Kfs) + cordierite (X Mg 0.86) + rutile ± sapphirine ±biotite (X Mg 0.75; TiO 2 3.7-5.8 wt.%) ±plagioclase (An 35-46 ). Interstitial Kfs and quartz with low dihedral angles are often present, in particular as thin films between sillimanite and quartz; these features are interpreted as evidence for the presence of former melt along the grain boundaries. In contrast, the more felsic, melt-rich domain is composed of mesoperthite + quartz + garnet + sillimanite + brown biotite (X Mg 0.7; TiO 2 3.7-5.4 wt.%) + rutile, but is free of orthopyroxene. Cores of garnet porphyroblasts (0.2-0.8 cm, Prp 54-57 Alm 39-42 Grs 3-4 Sps 0.2-0.6 , X Mg 0.57) in the melt-rich domains contain clusters of primary glassy inclusions (GI) and crystallized melt inclusions (nanogranitoids; NI) together with multiphase fluid inclusions (MFI) and accessory phases (mainly rutile and apatite).