Progressive metamorphism of Flinton Group pelitic schists: Grenville Province, southeastern Ontario.

摘要

The following five reaction isograds were mapped though the study area: Garnet+Chlorite+Muscovite=Chlori toid+Biotite+Quartz+Water Chloritoid+Muscovite+Quartz=Garn et+Staurolite+Biotite+Water Chlorite+Staurolite+Muscovite=Ky anite+Biotite+Quartz+Water Kyanite=Sillimanite Staurolite+Muscovite+Quartz=Garn et+Sillimanite+Biotite+Water. ; The assemblage garnet-chlorite with or without chloritoid is characteristic of the lowest-grade pelites in the area. This assemblage is found near Madoc and Bishop Corners in two geographically discrete areas of greenschist facies metamorphism that are not connected. Garnet-chlorite pelites typically contain trace amounts of chloritoid, either as inclusions in garnet or as xenoblastic relicts armoured in matrix quartz grains. This evidence indicates that chloritoid is unstable in the presence of garnet and chlorite, possibly due to the following continuous reaction in the KFASH system: chloritoid+chamosite+2 quartz=2almandine+5water. ; Most of the porphyroblast growth associated with the reactions above took place after the development of the primary S1 schistosity, but prior to the later D2 and D3 deformational events. (S1 formed during the main deformational phase that produced the synclinal structures hosting the Flinton Group sediments). The isograds were initially relatively flat lying, and were refolded by later deformational events about northeast and northwest trending folds. The resulting dome and basin interference pattern formed two low-grade nodes in the study area, a greenschist facies node centred on Bishop Corners and a lower amphibolite facies node in the northern portion of the Clare River synclinorium near Kaladar.; Pelites on the margin of the Mooroton shear zone contain kyanite and staurolite that has been fractured and deformed, showing that the isograds have also been displaced by late-stage (post-peak metamorphic) movement along the shear.; A set of thermodynamic data for chloritoid (fctd, FeAl₂SiO ₅(OH)₂), magnesiochloritoid (mctd, MgAl₂SiO ₅(OH)₂), chamosite (iron chlorite - fchl, Fe₅Al ₂Si₃O₁₀(OH)₈) and magnesiostaurolite (mst, Mg₄Al₁₈Si_7.5O₄₄(OH) ₄) was derived using published experimental data and Fe-Mg partitioning data from natural assemblages, this data is consistent with the Berman (1988) database.; The thermodynamic data was used in conjunction with the TWO program (Berman, 1991) to estimate peak metamorphic temperature and pressure conditions for samples located throughout the study area. Calculated metamorphic temperatures range from 540°C in low-grade areas (Madoc and Bishop-Corners) to 650°C in the highest-grade portions of the study area. Metamorphic pressure data showed little variation across the map area, ranging from 5000 to 5500 bars. (Abstract shortened by UMI.)