Geoscience Frontiers (Sep 2018)
Fluid-induced high-temperature metasomatism at Rundvågshetta in the Lützow-Holm Complex, East Antarctica: Implications for the role of brine during granulite formation
Abstract
We report new petrological, phase equilibria modeling, and fluid inclusion data for pelitic and mafic granulites from Rundvågshetta in the highest-grade region of the Neoproterozoic Lützow-Holm Complex (LHC), East Antarctica, and provide unequivocal evidence for fluid-rock interaction and high-temperature metasomatism in the presence of brine fluid. The studied locality is composed dominantly of well-foliated pelitic granulite (K-feldspar + quartz + sillimanite + garnet + ilmenite) with foliation-parallel bands and/or layers of mafic granulite (plagioclase + orthopyroxene + garnet + ilmenite + quartz + biotite). The boundary between the two lithologies is defined by thin (about 1–20 cm in thick) garnet-rich layers with a common mineral assemblage of garnet + plagioclase + quartz + ilmenite + biotite ± orthopyroxene. Systematic increase of grossular and decrease of pyrope contents in garnet as well as decreasing Mg/(Fe + Mg) ratio of biotite from the pelitic granulite to garnet-rich rock and mafic granulite suggest that the garnet-rich layer was formed by metasomatic interaction between the two granulite lithologies. Phase equilibria modeling in the system NCKFMASHTO demonstrates that the metasomatism took place at 850–860 °C, which is slightly lower than the peak metamorphism of this region, and the modal abundance of garnet is the highest along the metapelite–metabasite boundary (up to 40%), which is consistent with the field and thin section observations. The occurrence of brine (7.0–10.9 wt.% NaCleq for ice melting or 25.1–25.5 wt.% NaCleq for hydrohalite melting) fluid inclusions as a primary phase trapped within plagioclase in the garnet-rich layer and the occurrence of Cl-rich biotite (Cl = 0.22–0.60 wt.%) in the metasomatic rock compared to that in pelitic (0.15–0.24 wt.%) and mafic (0.06–0.13 wt.%) granulites suggest infiltration of brine fluid could have given rise to the high-temperature metasomatism. The fluid might have been derived from external sources possibly related to the formation of major suture zones formed during the Gondwana amalgamation. Keywords: High-temperature metasomatism, Brine, Phase equilibrium modeling, Fluid inclusion, Gondwana, Lützow-Holm Complex