Abstract Various modes of occurrence of talc were identified in piemontite‐quartz schists collected from schist and eclogite units in the Kotsu area of the Sanbagawa Belt, eastern Shikoku, Japan. They can be classified into the following types: (A) matrix and (B) pull‐apart talc. The matrix talc is associated with aegirineaugite or glaucophane in the eclogite unit and with albite or chlorite in the schist unit. The pull‐apart talc is developed at the pull‐apart of microboudin structures of Na‐amphibole, along with albite or chlorite in samples from both units, suggesting that the pull‐apart talc was formed by Na‐amphibole consuming reactions in both units. The talc–aegirineaugite–phengite association is found in a thin layer (a few millimetres thick), with higher Na2O/(Na2O + Al2O3 + MgO) ratio in the ANM (Al2O3–Na2O–MgO) diagram projected from phengite, epidote and other minerals, in the eclogite unit. Crystals of aegirineaugite have decreased jadeite content [= 100 × Al/(Na + Ca)] and increased aegirine content [= 100 × (Na – Al)/(Na + Ca)] from the core (ca Jd40Aeg40Di20) to the rim (ca Jd23Aeg53Di24), and are replaced by winchite and albite in varying degrees at the crystal margins. Na‐amphibole is glaucophane/crossite, commonly rimmed by Al‐poor crossite or winchite at the margin in the eclogite unit, although it is relatively homogeneous crossite in the schist unit. These textures suggest that the talc‐phengite‐(aegirineaugite or glaucophane) assemblage equilibrated during an early stage of metamorphism and the pull‐apart talc was formed at a later stage in the eclogite unit. A plausible petrogenetic grid in the NCKFe3+MASH system with excess piemontite (regarded as epidote), hematite, quartz and water, pseudosection analysis for the aegirineaugite‐bearing layer and the observed mineral assemblages suggest that the talc‐aegirineaugite‐phengite assemblage is stable under high pressure conditions (ca 560–580°C and 18–20 kbar). The pull‐apart talc was formed at ca 565–580°C and 9.5–10.5 kbar by the reaction of glaucophane/crossite + paragonite = talc + albite during the decompression stage, suggesting that the piemontite‐quartz schist in the eclogite unit experienced high‐pressure metamorphism at ca 50–60 km depth and was then exhumed to ca 30 km depth under nearly adiabatic conditions. 相似文献
Abstract. Whole-rock chemical compositions of the Besshi basic schist closely associated with the Besshi massive sulfide deposit from the Sanbagawa Belt are reported. Studied samples were collected from four outcrops around the Dozan-goe, central Shikoku. Common metamorphic mineral assemblage of the basic schist is albite + epidote + actinolite + chlorite ± muscovite ± quartz. Major element contents are similar to those of typical tholeiitic basalts. Trace element patterns of the basic schist normalized to normal mid-ocean ridge basalt (N-MORB) are generally flat, although concentrations of highly mobile large-ion lithophile elements are quite variable. Chondrite-normalized rare earth element (REE) patterns are flat to slightly light REE-depleted patterns. In the Hf-Th-Ta and Nb-Zr-Y discrimination diagrams, basic schist samples closely associated with the sulfide deposit are plotted within the N-MORB field. The Th/Nb ratios of the basic schist are also comparable to those of N-MORB. These geo-chemical lines of evidence indicate that the protolith of the Besshi basic schist is N-MORB and the Besshi sulfide deposit was formed by hydrothermal activity in conjunction with MOR volcanism. 相似文献
The frequency of occurrence of minerals in 1876 samples of Sanbagawa pelitic schist in central Shikoku is summarized on the basis of microscopic observation accompanied, in part, by use of an electron microprobe. All samples contain quartz, plagioclase, phengite, chlorite and graphite. More than 90% of samples contain clinozoisite, titanite and apatite. Garnet is present in 95% of samples from the garnet zone, and biotite is present in 64% of samples from the albite‐biotite zone. Calcite is found in about 40% of samples of the pelitic schist collected from outcrop, but occurs in 95% of the pelitic schist from drill cores. Calcite was apparently ubiquitous in the pelitic schist during the Sanbagawa metamorphism, but must have been dissolved recently by the action of surface or ground water. The mineral assemblages of the Sanbagawa pelitic schist have to be analyzed in the system with excess calcite, quartz, albite (or oligoclase), clinozoisite, graphite and fluid that is composed mainly of H2O, CO2 and CH4. In the presence of calcite, reactions that produce garnet, rutile, oligoclase, biotite and hornblende, some of which define isograds of the metamorphic belt, should be written as mixed volatile equilibria that tend to take place at lower temperature than the dehydration reactions that have been proposed. The presence of calcite in pelitic schist suggests that fluid composition is a variable as important in determining mineral assemblages as pressure and temperature. Thus Ca‐bearing phases must be taken into account to analyze the phase relations of calcite‐bearing pelitic schist, even if CaO content of Sanbagawa pelitic schist is low. As calcite is a common phase, the mineral assemblages of the biotite zone pelitic schist may contravene the mineralogical phase rule and warrant further study. 相似文献
The banded iron-formation in the southeastern Bababudan Hills display a macroscopic synformal bend gently plunging towards
WNW. The bedding planes in smaller individual sectors show a cylindrical or conical pattern of folding. The dominant set of
minor folds has WNW-ESE trending axial planes and the axes plunge towards WNW at gentle to moderate angles, though there is
considerable variation in orientation of both axes and axial planes. A later set of sporadically observed folds has N-S trending
axial planes. The macroscopic synformal bend within the study area forms the southeastern corner of a horseshoe shaped regional
synformal fold closure which encompasses the entire Bababudan range.
The minor folds are buckle folds modified to a varying extent by flattening. In some examples the quartzose layers appear
to be more competent than the ferruginous layers; in others the reverse is true. The folds are frequently noncylindrical and
the axes show curvature with branching and en echelon patterns. Such patterns are interpreted to be the result of complex
linking of progressively growing folds whose initiation is controlled by the presence of original perturbations in the layers.
Domes and basins have at places developed as a result of shortening along two perpendicular directions in a constrictional
type of strain. Development of folds at different stages of progressive deformation has given rise to nonparallelism of fold
axes and axial planes. The axes and axial planes of smaller folds developed on the limbs of a larger fold are often oriented
oblique to those of the latter. Progressive deformation has caused rotation and bending of axial planes of earlier formed
folds by those developed at later stages of the same deformational episode. Coaxial recumbent to nearly reclined fold locally
encountered on the N-S limb of the macroscopic fold may belong to an earlier episode of deformation or to the early stage
of the main deformation episode.
The E-W to ESE-WNW strike of axial plane of the regional fold system in the Bababudan belt contrasts with the N-S to NNW-SSE
strike of axial planes of the main fold system in the Chitradurga and other schist belts of Karnataka. 相似文献
The Sanbagawa metamorphic rocks in the Besshi district, central Shikoku, are grouped into eclogite and noneclogite units. Chloritoid and barroisite-bearing pelitic schists occur as interlayers within basic schist in an eclogite unit of the Seba area in the Sanbagawa metamorphic belt, central Shikoku, Japan. Major matrix phases of the schists are garnet, chlorite, barroisite, paragonite, phengite, and quartz. Eclogite facies phases including chloritoid and talc are preserved only as inclusions in garnet. P–T conditions for the eclogite facies stage estimated using equilibria among chloritoid, barroisite, chlorite, interlayered chlorite–talc, paragonite, and garnet are 1.8 GPa/520–550 °C. Zonal structures of garnet and matrix amphibole show discontinuous growth of minerals between their core and mantle parts, implying the following metamorphic stages: prograde eclogite facies stage→hydration reaction stage→prograde epidote–amphibolite stage. This metamorphic history suggests that the Seba eclogite lithologies were (1) juxtaposed with subducting noneclogite lithologies during exhumation and then (2) progressively recrystallized under the epidote–amphibolite facies together with the surrounding noneclogite lithologies.
The pelitic schists in the Seba eclogite unit contain paragonite of two generations: prograde phase of the eclogite facies included in garnet and matrix phase produced by local reequilibration of sodic pyroxene-bearing eclogite facies assemblages during exhumation. Paragonite is absent in the common Sanbagawa basic and pelitic schists, and is, however, reported from restricted schists from several localities near the proposed eclogite unit in the Besshi district. These paragonite-bearing schists could be lower-pressure equivalents of the former eclogite facies rocks and are also members of the eclogite unit. This idea implies that the eclogite unit is more widely distributed in the Besshi district than previously thought. 相似文献
Semi-detailed gravity investigations were carried out over an area of approximately 2750 sq km with maximum N-S and E-W extents
of 55 and 50 km respectively in the Gadag region in the Dharwar craton with a view to obtain a clearer perception of the structural
configuration of the region.
From qualitative analysis of the gravity data, several tectonic features are inferred: the high density Gadag schist belt
is characterized by a gravity high and occurs in two discontinuous segments — the main N-S trending segment, and its thinner
NW-SE trending extension, the two separated by a NE-SW trending deep seated fault. While the N-S trend of the Gadag schist
belt is bounded on its east by the NW-SE trending Chitradurga thrust fault and on its west by another major NNWSSE trending
fault, the NW-SE extension is likewise bounded by two other NW-SE major faults.
Quantitative evaluation from forward modeling/inversion of five profiles in the region, assuming a density contrast of 0.29gm/cc
of the anomalous schistose body with the gneissic host rocks indicated a synclinal structure plunging to the southeast along
its axis for the Gadag schist belt. The maximum width and depth from surface of the schist belt are 22 km and 5.6 km respectively. 相似文献