Search

The mode of occurrence of borosilicates and the breakdown fronts of prograde tourmaline (tourmaline-out isograd) in three anatectic migmatite regions of the Ryoke metamorphic belt, SW Japan, are reported. The breakdown of tourmaline in the migmatite zones and release of boron into the melts, followed by the extraction of the boron-bearing melts from the migmatite zones occurred throughout the Ryoke metamorphic belt. Retrograde, magmatic tourmaline in interboudin partitions filled with leucosome is useful for calculating the degree of partial melting in the migmatites. Using boron contents in the leucosomes and pelitic schists, the degree of partial melting at the migmatite front of the Aoyama area is estimated to be 12 wt.%. Extraction of the boron-bearing melt is suggested by the boron-depleted nature of the migmatites. Connection of boudinage structures probably supplied the vertical pathways of the segregated melts, and major transport of the melts was accomplished by dyking. Irregularly shaped, amoeboid tourmaline locally occurs on the high-temperature side of the tourmaline-out isograds in the Yanai and Komagane areas, implying incomplete extraction of boron-bearing melts from those areas. Discriminating retrograde from prograde tourmaline enables correct recognition of the tourmaline-out isograd. The amount of retrograde tourmaline in migmatites can potentially be used as an indicator of the degree of melt extraction from them. [ABSTRACT FROM PUBLISHER]

Abstract: Multiple equilibrium stages were identified from garnet-rich gneiss at Ktiš in the Lhenice shear zone of the southern Bohemian Massif, Czech Republic. The matrix of the rock mainly comprises cordierite (Crd), sillimanite (Sil), biotite (Bt), garnet (Grt), quartz (Qtz), K-feldspar (Kfs) and plagioclase (Pl) along with accessory minerals such as spinel (Spl), apatite (Ap), monazite (Mnz) and zircon (Zrn). Bt, Sil, kyanite (Ky) and Pl occur as inclusion phases in Grt. Crd occurs only in the matrix both as isolated grains and as reaction coronas developed around Grt. Spl is closely correlated with the Crd corona. Some coarse-grained (>3mm) Grts show chemical heterogeneity both in major and trace elements. Grossular (Grs)-content is homogeneous and high {Xgrs=Ca/(Ca+Mg+Fe+Mn)=0.27} in a center of the grain and smoothly decreases towards the grain margin (Xgrs=0.02). However, pyrope (Prp)-content shows an inverse pattern against Grs-content; i.e., Prp-content is low and constant {Xprp=Mg/(Ca+Mg+Fe+Mn)=0.03} in the center of the grain and gradually increases towards the margin (up to Xprp=0.28). The contours of Grs- and Prp-contents show symmetrical hexagonal shapes. The distribution pattern of phosphorus, however, shows a striking contrast against Grs-content. The core of the grain is characterized by low-phosphorus content almost below the detection limit of the EPMA analysis but it is armored by the high-phosphorus rim accompanying with local development of phosphorus-poor outermost rim. The outline of phosphorus-poor core shows a hexagonal shape, which is symmetrical to those of Grs- and Prp-content contours, but it is located outside of higher-Grs (Xgrs=0.27)- and lower-Prp (Xprp=0.03)-content contours. These observations suggest that the outline of phosphorus-poor core should indicate the original shape of Grs-rich garnet developed during an early stage of the metamorphism. The zoning pattern of major/trace elements in garnet and the mode of occurrence of constituent minerals suggest that the studied rock experienced three equilibrium stages represented by the following mineral assemblages with excess of Bt, Pl, Qtz and Kfs: Stage 1: Grs-rich (Xgrs=0.24–0.27) and phosphorus-poor Grt inner core+Ky+Ap+Mnz Stage 2: Grs-poor (Xgrs=ca. 0.05) and phosphorus-rich Grt rim+Ky/Sil Stage 3: Grs-poor (Xgrs=0.02) and phosphorus-poor Grt outermost rim+Sil+Crd±Spl+Mnz The geothermobarometry gives the following P–T conditions for each stage: 1.5–2.3GPa at 700–900°C for Stage 1, 730–830°C and 1.0–1.3GPa for Stage 2, and 740–850°C and 0.6–0.8GPa for Stage 3. The P–T conditions for the Stage 2 are slightly higher than the peak P–T conditions for gneisses of the Varied/Monotonous Units in the literatures and the P-conditions for the Stage 1 are similar to those of HP-granulite in the Gföhl Unit. The inferred P–T conditions of the studied rock, a model petrogenetic grid, and a pseudosection analysis suggest that the studied rock experienced the isothermal decompression at least from the Grt rim stage (Stage 2, 1.0–1.3GPa) to the matrix stage (Stage 3, 0.6–0.8GPa). This decompression path would overstep the following dehydration melting reactions at different depths: Ms+Qtz=Grt+Bt+Sil+Kfs+liq at 1.0–1.2GPa and Bt+Sil+Qtz=Grt+Crd+Kfs+liq at 0.3–0.6GPa. The high-phosphorus Grt rim should be formed through these reactions; in other words, higher-phosphorus content of Grt can be used as an indicator of partial melting of the host rock. [Copyright &y& Elsevier]