Metamorphic history of garnet-rich gneiss at Ktis in the Lhenice shear zone, Moldanubian Zone of the southern Bohemian Massif, inferred from inclusions and compositional zoning of garnet

摘要

Multiple equilibrium stages were identified from garnet-rich gneiss at Ktis 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 (PI) along with accessory minerals such as spinel (Spl), apatite (Ap), monazite (Mnz) and zircon (Zrn). Bt, Sil, kyanite (Ky) and PI 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 (>3 mm) 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, PI, 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: 1.5-2.3 GPa at 700-900 °C for Stage 1, 730-830 °C and 1.0-13 GPa for Stage 2, and 740-850 °C and 0.6-0.8 GPa for Stage 3. The P-Tconditions 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 Gfdhl 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 2,1.0-1.3 GPa) to the matrix stage (Stage 3,0.6-0.8 GPa). This decompression path would overstep the following dehydration melting reactions at different depths: Ms + Qtz = Grt+Bt+Sil + Kfs+liq at 1.0-1.2 GPa and Bt + Sil+Qtz=Grt+Crd + Kfs + liq at 03-0.6 GPa. 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.