15:00 〜 15:15
[SCG58-07] 苦鉄質グラニュライト中の結晶化したメルト包有物とシュードセクション法による部分溶融過程の推定
キーワード：結晶化したメルト包有物, 苦鉄質グラニュライト, Lutzow-Holm岩体, 相平衡モデリング
We report here new petrological date of crystalized melt inclusions (CMIs) and phase equilibrium modeling of partially melted mafic granulite to evaluate the influence of partial melting to the phase relation from the Neoproterozoic - Cambrian Lutzow-Holm Complex (LHC), East Antarctica (Shiraishi et al., 1992). Previous petrological studies of the LHC suggest an increase in the metamorphic grade from northeast (amphibolite facies) to southwest (granulite facies) (Hiroi et al., 1991). CMIs are often reported from the pelitic and felsic granulites (e.g. Cesare et al., 2009). However they are relatively rare in mafic to ultramafic granulites. We thus attempt to investigate textures of the CMIs in the mafic to ultramafic granulites to discuss the partial melting process.
The examined mafic and ultramafic granulites occur as boudin or small blocks of several meters within psammitic and hornblende-biotite gneisses of the granulite-facies zone. Based on detailed microscopic observations, we found CMIs bearing mafic and ultramafic granulites from four different exposures within the LHC. The representative samples of mafic to ultramafic granulite are composed mainly of coarse-grained garnet, hornblende, orthopyroxene, clinopyroxene, plagioclase, and ilmenite. The garnet often contains CMIs. The CMIs consist of fine-grained quartz, orthopyroxene, biotite, K-feldspar, plagioclase, and ilmenite which size varies from 1 to 50μm. The size of CMI grains is up to 100μm, and they show negative crystal shapes of the host garnet. We subsequently calculated chemistry of the CMIs based on modal abundance and chemistry of the minerals for each CMI. The results are nearly equivalent to the compositions of andesitic to dacitic melt.
Occurrence of hornblende and biotite within garnet in the rock suggests dehydration melting of the hydrous minerals and formation of andesitic to dacitic melt during prograde stage. Phase equilibrium modeling in NCKFMASHTO system demonstrated that some mafic to ultramafic granulites experienced considerable amounts of melt loss (up to 6.5-7 wt. %) defined by the stability field of clinopyroxene and modal isopleth of clinopyroxene. Stability field of quartz expands toward lower pressure side with increase of melt amount in the phase diagram. Based on phase equilibrium modeling of melt-bulk interaction, the stability field of quartz and clinopyroxene is critical to estimate the P-T condition and amounts of melt extraction during partial melting. We estimated peak P-T condition of 900 ℃ and 10-11 kbar and clockwise P-T path for the rock based on the integrated bulk composition. Modal isopleth of the mineral also demonstrated that partial melting progressed through the following reactions. Hbl + Bt ➞ Liq + Cpx + Grt. Hbl + Pl ➞ Liq + Cpx + Grt. This study demonstrated that partial melting took place under plagioclase free field and plagioclase stable field.
The peak condition is comparable with previous estimations of 800-950 ℃ and 7-12 kbar (Yoshimura et al., 2004). Our results suggest that partial melting and melt loss are common processes even in mafic to ultramafic granulites from the LHC, and CMIs could preserve the composition of melt which has already been extracted from the system. Phase equilibrium modeling suggests that melt loss during prograde stage have critical influence on the mineral assemblage and stability field of the mineral of the examined samples.
References
Cesare, B., Ferreol, S., Salvioli, M.E., Pedron, D., Cacallo, A., 2009. Geology 37, 627-630.
Hiroi, Y., Shiraishi, K., Motoyoshi, Y., 1991. Geological Evolution of Antarctica, Cambridge University Press, Cambridge, 83-87.
Shiraishi, K., Hiroi, Y., Ellis, D.J., Fanning, C.M., Motoyoshi, Y., Nakai, Y., 1992. Recent Progress in Antarctic Earth Science. Terra, Tokyo, 67-73.
Yoshimura, Y., Motoyoshi, Y., Miyamoto, T., Grew, S. Edward., Carson, J. Christopher., Dunkley, J. Daniel., 2004. Polar Geoscience 17, 57-87.
The examined mafic and ultramafic granulites occur as boudin or small blocks of several meters within psammitic and hornblende-biotite gneisses of the granulite-facies zone. Based on detailed microscopic observations, we found CMIs bearing mafic and ultramafic granulites from four different exposures within the LHC. The representative samples of mafic to ultramafic granulite are composed mainly of coarse-grained garnet, hornblende, orthopyroxene, clinopyroxene, plagioclase, and ilmenite. The garnet often contains CMIs. The CMIs consist of fine-grained quartz, orthopyroxene, biotite, K-feldspar, plagioclase, and ilmenite which size varies from 1 to 50μm. The size of CMI grains is up to 100μm, and they show negative crystal shapes of the host garnet. We subsequently calculated chemistry of the CMIs based on modal abundance and chemistry of the minerals for each CMI. The results are nearly equivalent to the compositions of andesitic to dacitic melt.
Occurrence of hornblende and biotite within garnet in the rock suggests dehydration melting of the hydrous minerals and formation of andesitic to dacitic melt during prograde stage. Phase equilibrium modeling in NCKFMASHTO system demonstrated that some mafic to ultramafic granulites experienced considerable amounts of melt loss (up to 6.5-7 wt. %) defined by the stability field of clinopyroxene and modal isopleth of clinopyroxene. Stability field of quartz expands toward lower pressure side with increase of melt amount in the phase diagram. Based on phase equilibrium modeling of melt-bulk interaction, the stability field of quartz and clinopyroxene is critical to estimate the P-T condition and amounts of melt extraction during partial melting. We estimated peak P-T condition of 900 ℃ and 10-11 kbar and clockwise P-T path for the rock based on the integrated bulk composition. Modal isopleth of the mineral also demonstrated that partial melting progressed through the following reactions. Hbl + Bt ➞ Liq + Cpx + Grt. Hbl + Pl ➞ Liq + Cpx + Grt. This study demonstrated that partial melting took place under plagioclase free field and plagioclase stable field.
The peak condition is comparable with previous estimations of 800-950 ℃ and 7-12 kbar (Yoshimura et al., 2004). Our results suggest that partial melting and melt loss are common processes even in mafic to ultramafic granulites from the LHC, and CMIs could preserve the composition of melt which has already been extracted from the system. Phase equilibrium modeling suggests that melt loss during prograde stage have critical influence on the mineral assemblage and stability field of the mineral of the examined samples.
References
Cesare, B., Ferreol, S., Salvioli, M.E., Pedron, D., Cacallo, A., 2009. Geology 37, 627-630.
Hiroi, Y., Shiraishi, K., Motoyoshi, Y., 1991. Geological Evolution of Antarctica, Cambridge University Press, Cambridge, 83-87.
Shiraishi, K., Hiroi, Y., Ellis, D.J., Fanning, C.M., Motoyoshi, Y., Nakai, Y., 1992. Recent Progress in Antarctic Earth Science. Terra, Tokyo, 67-73.
Yoshimura, Y., Motoyoshi, Y., Miyamoto, T., Grew, S. Edward., Carson, J. Christopher., Dunkley, J. Daniel., 2004. Polar Geoscience 17, 57-87.