[SMP32-P02] Microstructures and pressure-temperature condition in pelitic gneisses from the Lützow-Holm Complex at Akarui Point, East Antarctica
Keywords:pressure-temperature condition, phase relation, Lützow-Holm Complex
The mineral assemblages of the studied gneisses are garnet, biotite, plagioclase, K-feldspar, quartz with and without sillimanite. Garnet grains in the Sil-bg. sample show round shape and contain large amount of fine quartz inclusions, whereas those in the Sil-free sample represent irregular shape and sporadically include coarse-grained plagioclase.
Garnet in both samples have chemically heterogeneity such that XCa (=Ca/Fe+Mg+Ca+Mn) and Mg/Fe decrease from the core to the rim. Furthermore, garnet in contact with biotite represents lower Mg/Fe than that in contact with other minerals. Garnet in the Sil-bg. sample is low in XCa and Mg/Fe as compared with garnet in the Sil-free sample. Phosphorus in garnet of the Sil-bg. sample represents euhedral shape of chemical contour in contrast to the garnet in the Sil-free sample where phosphorus shows no zoning. Biotite in the Sil-bg. sample shows Al-rich and low Mg/Fe as compared with that in the Sil-free sample. No systematic difference in Ti content in biotite is recognized. The Sil-free sample contains plagioclase with higher anorthite content than the Sil-bg. sample.
In the AFM approximation, garnet and biotite coexisting with sillimanite are expected to show higher Mg/Fe than those free from sillimanite. However, the present observations disagree with the prediction. Garnet contains significant amount of Ca, requiring addition of CaO to the chemical system. A calculated garnet composition diagram with respect to Ca-Mg-Fe shows that garnet coexisting with sillimanite represents a univariant line lower Ca and Mg/Fe than that free from sillimanite, and the present data are consistent with it. This suggests that two samples were not necessarily equilibrated at different P-T condition.
Geothermobarometry, in contrast, indicates significant difference in P-T condition. That is, garnet-biotite geothermometer (Ferry and Spear., 1978) and garnet-biotite-plagioclase-quartz geobarometer (Hoisch., 1990) show 685-718 ℃ at 6.4-6.6 kbar for the Sil-free sample and 828-855 ℃ at 7.2-7.8 kbar for the Sil-bg. sample.
It is puzzling feature that different samples from the same outcrop provide different P-T estimations using the same geothermobarometers. The Sil-bg. sample records higher temperature than the Sil-free sample, as described before. Preservation of euhedral chemical contour of phosphorus in the Sil-bg. sample would indicate that the garnet grew maintaining surface equilibrium, at high temperatures. In contrast, absence of such euhedral contour and highly-rough surface of garnet in the Sil-free sample suggest relatively high growth rate, at low temperatures.
The P-T path of Akarui Point contains pressure decrease during temperature decrease (Iwamura et al., 2013). Considering this P-T path, it is expected that the garnet of Sil-free sample didn’t exist at high temperatures and formed at low pressures and temperatures. The difference in estimated P-T conditions in the same outcrop can be attributed to not only the difference in the method but also the difference in the timing of the formation of minerals.