Possible polymetamorphism and brine infiltration recorded in the garnet–sillimanite gneiss, Skallevikshalsen, Lützow–Holm Complex, East Antarctica

Abstract

Chlorine–rich (>0.3 wt%Cl) biotite inclusions in the core of garnet porphyroblasts in the garnet–sillimanite (Grt–Sil) gneiss from Skallevikshalsen, Lützow–Holm Complex (LHC), East Antarctica is estimated to be stable under >1.2 GPa, 820–850 °C, coexisted with granitic melt as suggested by the nanogranite/felsite inclusions. Rare occurrence of matrix biotite, in spite of the common occurrence of biotite as inclusions in garnet, suggests almost complete consumption of pre–existed matrix biotite during the prograde to peak metamorphism. Brine infiltration during prograde to peak metamorphism in Skallevikshalsen is supported by Cl–rich scapolite described in previous studies. Brine infiltration and progress of continuous biotite–consuming melting reactions were probably responsible for elevating the Cl content of biotite in the studied sample.
 In situ electron microprobe U–Th–Pb dating of monazite and the in situ laser ablation inductively coupled plasma mass spectrometry (LA–ICPMS) U–Pb dating of zircon in the Grt–Sil gneiss revealed that both monazite and zircon has the ‘older age population’ with ~ 650–580 Ma and the ‘younger age population’ with ~ 560–500 Ma. The REE and trace element pattern of one of the P–rich patches in the garnet core is different from the P–rich garnet rim. The isotope mapping of the same patch by LA–ICPMS revealed that the patch is also observed as a domain depleted in ⁵¹V, ⁸⁹Y, ¹⁶⁵Ho, ¹⁶⁶Er, ¹⁶⁹Tm, ¹⁷²Yb, and ¹⁷⁵Lu. Clear difference in ⁵¹V concentration between the patch and the garnet rim suggests that this patch is not a continuous part from the garnet rim, but is likely a relic of preexisted garnet. Kyanite included in the patch suggests that the precursor rock was presumably a medium– to high–pressure type metamorphic rock. Presence of the older age population (~ 650–580 Ma) monazites in Skallevikshalsen and Skallen also suggest that rocks in these areas experienced polymetamorphism, and resetting by the ~ 560–500 Ma metamorphic event was incomplete in these areas. Taking into account the presence of Cl–rich biotite inclusions in garnet, infiltration of brine accompanied by partial melting is one probable event that took place at ~ 560–500 Ma in the Skallevikshalsen area, and part of the monazite possibly recrystallized by this brine infiltration.
 Detailed microstructural observation using trace element mapping combined with detailed petrography especially focusing on the Cl–bearing minerals as a tracer of brines would become a powerful tool for better interpreting the results of monazite and zircon dating and for investigating the fluid–related crustal processes.