Granulite facies metapelites (khondalites) from Chittikara are characterized by the assemblage Grt + Sil + Bt + Crd + Spl + Kfs + Pl + Qtz ± graphite. The rocks show a series of mineral reactions such as: (1) Bt + Sil + Qtz = Grt + Crd ± Kfs, (2) Grt + Sil + Qtz = Crd, (3) Grt + Crd + Sil = Spl + Qtz , (4) Spl + Qtz = Crd, and (5) Grt + Sil = Spl + Crd. Spinel and quartz occur in direct grain contact and their stability considerations in appropriate reaction grids indicate > 950°C at 8 kbar. This, together with the ca. 1000°C temperatures derived from mesoperthites suggests ultrahigh temperature metamorphism in the khondalite belt. Electron microprobe data on the various minerals have been used to compute pressure-temperature estimates from mineral phase equilibria thermobaromtery. Temperature estimates from spinel-cordierite thermometry are around 900°C, while garnet-cordierite thermometer yields an upper estimate of 818°C. Estimates based on garnet-sillimanite-plagioclase-quartz barometer indicate pressures up to 8 kbar. Evaluation of the microstructures and reactions in the khondalites using available reaction grids for pelitic rocks provide evidence for peak metamorphism at high or ultrahigh temperatures followed by steep decompression from around 8 kbar down to 4 kbar. The P-T path attests to a rapid exhumation history probably associated with the extensional collapse following collision of continental blocks during the assembly of Gondwana in the late Pan-African.

抄録全体を表示

Orthopyroxene-sillimanite-quartz granulites of Ganguvarpatti (GVP) from the northern Madurai Block in southern India record ultrahigh-temperature (UHT) metamorphism and complex exhumation history. Orthopyroxenes in different textural setting in the GVP rocks shows marked variation in Al₂O₃ and X_Mg contents. Inclusions, cores of porphyroblasts and some of the discrete grains in the matrix possess the highest Al₂O₃ contents (8.5-9.4 wt%). Orthopyroxenes occurring as fine to medium grained symplectitic intergrowths with cordierite show Al₂O₃ values ranging from 7.6-8.2 wt%. Those which form symplectites with sapphirine have intermediate Al₂O₃ values (6.9-7.5 wt%) while orthopyroxenes associated with spinel possess values between 7.3-7.5 wt%. Late orthopyroxene rim on biotite shows the lowest Al₂O₃ range between 5.9-6.5 wt%. Orthopyroxenes from various textural associations define distinct P-T slots in the Al in orthopyroxene thermometry based on the X_Mg-X_Al isopleths. Thus, orthopyroxene porphyroblasts and inclusions yield temperatures around 1050°C and pressure above 10 kbar. Orthopyroxene composition in orthopyroxene-cordierite symplectite yields temperatures of 950-990°C at pressure above 8 kbar and those associated with sapphirine and spinel symplectites show 850-900°C at a pressure around 8 kbar. The late orthopyroxene rim around biotite yields temperatures around 850°C at 6 kbar. The UHT conditions attained by these rocks are also corroborated by garnet-orthopyroxene thermometry, which provides an upper estimate of 1050 ± 50°C. The GVP rocks preserve a series of mineral reactions indicating ultrahigh-temperature decompression. The reaction textures when evaluated in KFMASH petrogenetic grid indicate two stages of isobaric cooling and decompression, suggesting a complex exhumation history. The textures and mineral reactions observed in this study in conjunction with the available age data suggest that the multistage exhumation of the UHT rocks in Ganguvarpatti could be related to the orogenic events involving continent collision, crustal thickening and extensional collapse accompanying the step-wise suturing of Gondwana supercontinent during the late Neoproterozoic.

抄録全体を表示

We report a new occurrence of sapphirine- and corundum-bearing silica-deficient Mg-Al rocks from Paramati at the northern margin of the Palghat-Cauvery Shear System which define the Archaean-Proterozoic collisional boundary in southern India. Sapphirine here occurs as medium-grained euhedral to subhedral crystals enclosed within plagioclase and contains inclusions of spinel and sillimanite, and is locally intergrown with corundum, suggesting the reaction Spl + Sil → Spr + Crn. Temperatures estimated from sapphirine-spinel equilibria lie around 930-950°C, suggesting ultrahigh-temperature peak metamorphic conditions. Gedrite is also a common rock forming mineral in these rocks, and coexists with cordierite, corundum, and sillimanite. As gedrite is totally surrounded by cordierite, we infer a prograde reaction Ged + Sil → Crn + Crd + V. Lack of orthopyroxene in our samples suggests a decompressional P-T history following the prograde event. Our results suggest that the whole of northern Madurai Block as well as parts of the Palghat-Cauvery Shear System experienced ultrahigh temperature metamorphism followed by exhumation along a clockwise P-T path.

抄録全体を表示

Spinel in direct contact with quartz is newly found in the Grt-Sil leucogneiss from Skallevikshalsen, Lützow-Holm Complex, East Antarctica. It is included in the outermost core of the garnet porphyroblast and is characterized by low ZnO content (2.77-3.83 wt%) and very low Cr₂O₃ content (0-0.09 wt%). Incorporation of 2.77-3.39 wt% ZnO stabilized the Spl + Qtz assemblage to lower temperatures for about 50 degrees. As a result, their coexistence can be attained around the peak P-T conditions of Skallevikshalsen. The occurrence of relic staurolite, kyanite and breakdown texture of staurolite to spinel and kyanite included in the garnet core suggests that prograde stage is preserved in the garnet core and breakdown of staurolite itself was attained in the kyanite stability field. The Spl + Qtz assemblage is included at outermost part of the garnet core, suggesting that outermost core of the garnet was formed near the peak metamorphic conditions. Decrease in anorthite content of plagioclase included in the garnet core from that found in the rock matrix suggests that decompression took place during garnet formation. Attainment of these reaction sequences requires clockwise P-T path as previously proposed for the Lützow-Holm Complex.

抄録全体を表示

We report here the occurrence of osumilite as fine isolated inclusions in garnet porphyroblasts from a granulite facies rock in the southwestern part of the Highland Complex in Sri Lanka. Garnet porphyroblasts in the matrix are partly rimmed by sillimanite-biotite-K-feldspar assemblage as well as cordierite-K-feldspar-quartz assemblage. Spinel coexisting with quartz is also observed in the same sample. The pressure-temperature condition must be above 950°C at around 7 kbar pressure, on the basis of osumilite stability in KFMASH system. The cooling and decompression continued down to 850°C, into the biotite-sillimanite-K-feldspar-quartz stability field. During decompression f_O2 must have increased leading to the stability of Zn-rich-spinel with quartz after cordierite. Our findings provide direct evidence for ultrahigh-temperature (UHT) metamorphic conditions in southwestern Highland Complex, and also suggest the existence of a possible UHT zone within the high-grade terrane of Sri Lanka.

抄録全体を表示

The contact aureole at Fuka, Okayama, Japan is peculiar for the occurrence of extensive high-temperature skarn resulting from the intrusion of Mesozoic monzodiorite into Paleozoic marine limestone. The occurrence is also notable for the finding of ten new minerals, of which five are calcium-boron-bearing minerals, and scores of other rare minerals. Skarn formation at Fuka can be classified into three major types 1. Grossular-vesuvianite-wollastonite endoskarn, 2. Gehlenite-exoskarns, and 3. Spurrite-exoskarns. Grossular-vesuvianite-wollastonite endoskarn forms a narrow zone (few centimeter width) separating the exoskarn and the igneous intrusion. It is also found, developed independently, along contacts of the younger basic intrusive dykes and limestone in the region. The gehlenite-exoskarns, in most cases, are spatially associated with igneous intrusion and are extensive (decimeter to meter thick). However, exceptions of independent gehlenite dikes are also observed. Retrogression of the gehlenite endoskarns results in the formation of hydrogrossular and/or vesuvianite. Accessory phases include schrolomite and perovskite. The predominantly monomineralic spurrite-exoskarn was formed in the outer zone of the gehlenite-skarn parallel to the contact as well as independent veins, dikes and tongues. The spurrite-exoskarn may extend tens of meters. Spurrite coexists with tilleyite or rankinite, although larnite is absent. Idiomorphic gehlenite and vesuvianite are the most common accessory phases observed. Retrograde hydration of spurrite to foshagite, scawtite and hillebrandite is commonly observed as veins and alteration zones within the spurrite exoskarn.
    Petrogenetic grids were constructed using “THERMOCALC” for the observed mineral assemblage in the spurrite skarn. Mineral-fluid equilibrium in the CaO-SiO₂-CO₂ system was computed, considering the metasomatic input of aqueous silica. Phase diagram analysis in the form of T-X_CO2 grids with varying silica activity indicated that the stability field of spurrite is strongly controlled by the activity of silica in the fluid. Optimum silica concentration in the fluid was between 9.1 × 10⁻⁴ and 4.5 × 10⁻³ mol/liter, above which wollastonite becomes stable, whereas further reduced silica activity will generate larnite. Appropriate temperature condition for the formation of spurrite is between 850°C and 1080°C at an X_CO2 fluid composition of 0.05 to 0.42. At temperature conditions lower than 850°C, the spurrite stability field becomes narrow, with low CO₂ activity. The formation of extensive spurrite-exoskarn suggests that the silica activity, temperature and fluid composition remained within the spurrite stability field. Petrogenetic analysis of phase diagrams suggests that the exoskarn formation at Fuka contact aureole was robustly controlled by the activity of silica in the high temperature metasomatic fluid.

抄録全体を表示

A late Paleozoic crustal section of an intra-oceanic island arc occurs in the Middle Unit (M-Unit) of the Asago body from the Yakuno ophiolite, SW Japan. It consists of mafic metamorphic rocks and felsic to mafic later intrusions. The M-Unit is composed of three lithologic zones: the Lower migmatite zone (L-MZ), the Middle metagabbro zone (M-MZ), and the Upper amphibolite zone (U-AZ). The metamorphic grade of the lowest horizon of the M-Unit reaches granulite-facies conditions (P = 3.5-5.5 kbar at T = 850-860°C) and an assemblage of plagioclase + clinopyroxene + orthopyroxene (pseudomorph) + hornblende (tschermakite or magnesiohastingsite) appears in the mafic metamorphic rocks. Structural relationships of the migmatite and related rocks suggest that a small-scale leucosome (i.e. melt) was generated at the bottom of L-MZ and its upward melt segregation led to the formation of medium- to large-scale plutons and dykes of later intrusions into the M-MZ and U-AZ. The bulk rock chemistry of these rocks suggests that the later intrusions were derived from the partial melting of mafic metamorphic rocks, leaving melanosome as residuum. Major and trace element compositional variation of the later intrusions indicates that the leucosome (SiO₂ 48-71 wt%) may be a source of the plutons and dykes of medium-silica content (SiO₂ 52-67 wt%), whereas chemical compositions of the plutons and dykes of high-silica content (SiO₂ 72-76 wt%) and those of the leucosomes do not overlap. The fact indicate that the plutons and dykes of high-silica content may have been derived from either compositionally more differentiated magma or a completely exotic one.

抄録全体を表示