Stability Of Chloritoid Research Articles

Stability of chloritoid + biotite-bearing assemblages in some metapelites from the Palaeoproterozoic Singhbhum Shear Zone, eastern India and their implications

Abstract The arcuate Singhbhum Shear Zone (SSZ) of the East Indian Shield area, fringing the Archaean Singhbhum Craton, exposes a mélange-like ensemble of polymetamorphosed and highly tectonized siliciclastic sediments, mafic–ultramafic extrusive, tourmaline-rich rocks, magnetite–apatite rocks, granites and Cu–Fe–U sulphide ores of Meso- to Palaeoproterozoic age (1.5–1.77 Ga). Metapelites from two localities in the SSZ developed the enigmatic assemblage chloritoid–biotite–garnet–chlorite, which are the first reported from India and the eighth in world occurrence. Textural studies and algebraic analyses of the phase compositions in the KFMASH system indicate the operation of the reaction, chloritoid+biotite→garnet+chlorite+H 2 O and this reaction has a negative slope in pressure–temperature ( P – T ) space. Quantitative geothermobarometry and pseudosections in the NCMnKFMASHO system indicate that this reaction occurred during prograde metamorphism that culminated at 6.3±1 kbar and 490±40 °C. The stability of the chloritoid+biotite is also sensitive to the MnO content of the bulk rock, and to the chemical potential of H 2 O and oxygen ( μ H 2 O and μ O 2 , respectively), of the ambient fluid phase. Thrusting of continental crust in a collisional setting is invoked to explain the peak metamorphic temperatures and the clockwise P – T trajectory is construed from the petrological study of the chloritoid–biotite schist.

Chloritoid stability in very iron-rich altered pillow lavas

Chloritoid occurs in altered pillow lavas which overlie the Archean Helen iron formation near Wawa, Ontario, Canada. Whole-rock δ18O values of +11.20‰ and 10.53‰ indicate interaction with seawater at ≦200° C during which the rocks were modified to compositions appropriate for chloritoid growth during subsequent metamorphism. The observed assemblage chloritoid-chlorite-quartz-ilmenite indicates that moderate changes in FeO/(FeO+MgO) and Al2O3 are accommodated by chloritoid-chlorite modal variation, without gain or loss of any phase. For chloritoid to be stable, there is no upper limit on bulk (FeO+MgO)/Al2O3, but bulk MgO/(MgO+ FeO) is limited by the appropriateK D chloritoid/chlorite (Mg/ Fe) which is a function of temperature (and pressure).

The Stability of Chloritoid Below 10 kb PH2o

The Stability of Chloritoid Below 10 kb PH2o

Chloritoid stability in manganese rich low-grade metamorphic rocks, Venn-Stavelot Massif, Ardennes

The Southern Venn-Stavelot Massif is characterized by Ordovician and Devonian rocks very rich in manganese and aluminum, which are attacked by a low grade regional metamorphism. The assemblages 1 (phengite, paragonite, chlorite, chloritoid, garnet, quartz, hematite, rutile) and 2 (phengite, paragonite, chlorite, kaolinite (andalusite, pyrophyllite), garnet, quartz, hematite, rutile) are of basic interest for the formation of chloritoid. As the two rock types are isofaciell and quasi-identical in chemistry except for the iron oxides, there is clear evidence for the influence of $$f_{{\text{O}}_{\text{2}} }$$ on the chloritoid formation at its lower p-T stability limit. This can be shown by a discussion of the phase relations of chloritoid, garnet, kaolinite, chlorite and phengite in respect to the oxidation ratio mol 2 Fe2O3x 100/2 Fe2O3+ FeO of the host rocks. Especially chloritoid and chlorite change their chemistry in a characteristic way with rising oxidation ratio in getting richer and richer in manganese and magnesium (chloritoid) and magnesium (chlorite). A simultaneous increase in trivalent iron in these phases is supposed. At an oxidation ratio of 85–90 the $$f_{{\text{O}}_{\text{2}} }$$ stability limit of chloritoid is reached. The increasing substitution of manganese and magnesium up to this limit should have a stabilizing effect. In a rough estimate the oxygen partial pressure is supposed to be in the order of 10−10 atm at the stability limit of chloritoid assuming a temperature of metamorphism between 360–400° C. Rocks with oxidation ratios between 90 and 100 are characterized by the presence of kaolinite. If the oxidation ratio is still higher (all iron as Fe3+, parts of the manganese in the trivalent state), the rocks belong to assemblage 3 (phengite, paragonite, chlorite, viridine, (kaolinite), (garnet), quartz, hematite, braunite, rutile).

Upper stability of the association chloritoid + quartz

Knowledge about the stability of the association of chloritoid + quartz is of greater petrological significance than that about the stability of chloritoid on its own. The results of a series of experiments are described which define an upper breakdown temperature for this association at 1,000 bars water pressure. To obtain data on this sluggish reaction use was made of the single-crystal techniques introduced by Fyfe and Hollander (1964). Since no unambiguous reversal of the reaction at the breakdown-temperature of 445° at 1,000 atm H 2O-pressure can be proved, this must be considered to be an upper temperature limit only for the association of chloritoid + quartz at these rather modest water-pressures.

Chloritoid stability and related paragenesis ; theory, experiments, and applications

Chloritoid stability and related paragenesis ; theory, experiments, and applications