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[SCG58-07] Crystalized melt inclusions in mafic granulite: investigation of partial melting process based on pseudosection
Keywords:crystalized melt inclusions, mafic granulite, Lutzow-Holm Complex, phase equilibrium modeling
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.
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