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Metapelitic granulites from Jetty Peninsula, east Antarctica: formation during a single event or by polymetamorphism?
Author(s) -
HAND M.,
SCRIMGEOUR I.,
POWELL R.,
STÜWE K.,
WILSON C. J. L.
Publication year - 1994
Publication title -
journal of metamorphic geology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.639
H-Index - 114
eISSN - 1525-1314
pISSN - 0263-4929
DOI - 10.1111/j.1525-1314.1994.tb00042.x
Subject(s) - sillimanite , granulite , cordierite , geology , geochemistry , metamorphism , metamorphic rock , biotite , ilmenite , kyanite , migmatite , gneiss , facies , geomorphology , paleontology , quartz , materials science , ceramic , structural basin , composite material
Granulite facies metasedimentary gneiss exposed on Jetty Peninsula, east Antarctica, contains assemblages involving garnet‐sillimanite‐biotite‐cordierite‐spinel‐ilmenite‐rutile and garnet‐orthopyroxene‐cordierite‐biotite, as well as quartz and K‐feldspar. Peak assemblages involve garnet + sillimanite + ilmenite (±rutile) and garnet + orthopyroxene. P ‐ T calculations suggest formation conditions of approximately 800d̀ C at 7‐7.5 kbar. Cooling from peak conditions is suggested by biotite + garnet (±sillimanite) overprinting some peak assemblages. A subsequent increase in temperature is inferred from the formation of cordierite + garnet + biotite + ilmenite, garnet + sillimanite + cordierite + ilmenite and cordierite + orthopyroxene assemblages during D2. In slightly zincian bulk compositions, hercynitic spinel + cordierite + sillimanite constitutes the peak D2 assemblage. Average pressure calculations indicate peak pressures of 5.9 ±0.4 kbar at 700d̀ C for the cordierite‐bearing D2 assemblages. Available radiometric data suggest that peak metamorphism occurred at c. 1000 Ma and D2 occurred after 940 ± 20 Ma. The following two possibilities exist for the metamorphic evolution. (1) The formation of the lower pressure cordierite‐bearing assemblages is associated with a separate metamorphic event (M2), unrelated to the peak assemblage (M1), and the lower pressure assemblages have no relevance in terms of a single tectonothermal event. (2) The cordierite‐bearing assemblages formed during a progression from peak conditions. In this case, the lower pressure assemblages reflect a broadly decompressional metamorphic evolution, during which temperatures fluctuated. Comparison with P ‐ T paths from granulites of similar age in adjacent areas suggests that the second possibility should be preferred. The cooling interval between peak conditions and the development of cordierite‐bearing coronas and symplectites suggests affinities with isobarically cooled granulites of similar age immediately to the west, and the low‐ P /high‐ T post‐peak conditions are similar to the later stages of decompressional paths recognized in much of east Antarctica.