ISSN:
1432-0967
Source:
Springer Online Journal Archives 1860-2000
Topics:
Geosciences
Notes:
Abstract Four progressive mineral zones I (chloritoid+quartz), II (staurolite+chlorite+muscovite), III (kyanite+biotite) and IV (sillimanite + muscovite) have been mapped in a well-defined stratigraphic unit of ferruginous pelitic composition from north to south over a distance of about 15 km in the Unazuki area, Hida terrane, central Japan. Textural and chemical evidence indicates that a given metamorphic mineral assemblage was formed from the mineral assemblage that precedes it in a spatial sequence of mineral zones. An unusually large number of chloritoid-bearing mineral parageneses have been found within a small area in zone I, suggesting that they were equilibrated close to the seven-phase invariant point consisting of staurolite, chloritoid, garnet, chlorite, biotite, muscovite, and quartz in the five-component system Al2O3-FeO-MgO-K2O-SiO2 at a fixed $$P_{{\text{H}}_{\text{2}} {\text{O}}}$$ . Chloritoid also has been found as inclusions in garnet from zones II and III, but is never in direct contact with quartz in these zones. All garnets in the chloritoid-bearing rocks from zones I, II, and III are characteristically poor in Mn and Ca at the cores, suggesting, in conjunction with textural and modal evidence, that they were formed as the products of chloritoid-consuming reactions. Compositional and textural features of the garnets in the chloritoid-bearing rocks suggest that rocks in zones II and III bypassed the seven-phase invariant point on the high pressure side during prograde metamorphism. Thus the metamorphic geotherm based on the progressive mineral zones is not always identical with the P-T paths followed by rocks.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00399711
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