Mineral zoning, P-T-X-M phase relations, and metamorphic evolution of some Adirondack granulites

Journal of Petrology 1997


by

Frank S. Spear

and

Jeffrey M. Markussen


Department of Earth and Environmental Sciences
Rensselaer Polytechnic Institute
Troy, New York 12180 U.S.A.



Frank S. Spear email: spearf@rpi.edu




Images available as tiff files

The images shown for Figures 2-8 are available for downloading as tiff files for viewing in NIH Image or other program. Download links are on the pages for the respective figures




Figure 1


Figure 1. Map of the Adirondack mountains, NY, showing location of samples examined in this study. Light gray shading delineates the outcrop exposure of the anorthosite (after McLelland and Chiarenzelli, 1990); surrounding rocks are ortho- and paragneisses of diverse affinities; darker gray is water. Contours show isotherms of temperature calculated from feldspar and oxide thermometry by Bohlen et al. (1980b, 1985).

Figures 2 & 3 color images: Sample AS-31 (click here)



Figures 4,5 & 6 color images: Sample ET-10 (click here)



Figures 7 & 8 color images: Sample SAR-1 (click here)



Figure 9


Figure 9. AFM and CFM diagrams showing compositions of pyroxenes and garnets from samples ET-10 and SAR-1. Tie lines have been drawn between pyroxene rim and garnet rim compositions.

Figure 10


Figure 10. Pressure-temperature diagrams showing contours for Fe/(Fe+Mg), Al and Na in clinopyroxene (a) and Fe/(Fe+Mg) and Al in orthopyroxene (b) for the assemblage clinopyroxene + orthopyroxene + plagioclase + quartz. Arrow depicts a P-T path that is consistent with the observed zoning in pyroxene.

Figure 11


Figure 11. Pressure-temperature diagrams showing contours for Fe/(Fe+Mg), XGrs and molar abundance in garnet (a), (b) and (c); Fe/(Fe+Mg) and Al in clinopyroxene and orthopyroxene (d), (e) and (g), (h), respectively; and XAn in plagioclase (f) for the assemblage garnet + clinopyroxene + orthopyroxene + plagioclase + quartz. The arrow shows a P-T vector that is consistent with the observed zoning in all minerals.

Figure 12


Figure 12. (a) Plot of volume per cent of minerals versus temperature along a cooling path of 6 bars/°C. Garnet, quartz and clinopyroxene are produced at the expense of plagioclase and orthopyroxene. The measured modal amount of garnet (approximately 5 volume per cent) requires a temperature decrease of approximately 60 °C. (b) Schematic AFM diagram showing the shift in garnet and pyroxene compositions along a cooling path of 6 bars/°C.


Figure 13


Figure 13. Plots of the pyroxene quadrilateral showing the integrated compositions of pyroxene megacrysts from three samples (AS-31, ET-10, SL-8). The simple normalization was used for these plots, so the temperatures of metamorphic pyroxenes are believed to be underestimated in these diagrams by 20-70 °C.

Figure 14


Figure 14. Plots of the pyroxene quadrilateral showing the compositions of coexisting metamorphic orthopyroxene + clinopyroxene from all samples. Solid symbols are core compositions, open symbols rims (cpx only). (a) Simple normalization assuming all Fe is Fe2+. (b) Simple normalization assuming Fe3+ calculated from stoichiometric constraints. (c) Compositions of coexisting pyroxene cores plotted using the thermodynamic projection scheme described in Appendix A assuming Fe3+ calculated from stoichiometric constraints.

Figure 15


Figure 15. Plot of calculated temperatures using Fe-Mg partitioning between garnet + pyroxene rims. Black rectangles: all Fe is Fe2+; Gray rectangles: Fe3+ calculated from stoichiometric constraints. Length of rectangles reflects error associated with analytical uncertainty. Numbers and horizontal lines are average temperatures. (a) Garnet + clinopyroxene rim calculated using calibration of Powell (1985). (b) Garnet + orthopyroxene rim calculated using calibration of Harley (1984a). Rectangle labeled "core" for sample AS-19E is temperature calculated using orthopyroxene core.

Figure 16


Figure 16. (a) Plot of calculated temperatures using Fe-Mg partitioning between garnet + hornblende (gray bars; calibration of Graham & Powell, 1984) and garnet + biotite (black bars; Calibration of Patiño-Douce, et al., 1993). Average garnet + hornblende temperature, excluding sample ET-10 is 723 °C. Average garnet-biotite temperature, excluding samples SL-8 and SR-18 is 817 °C. Length of boxes reflects error associated with analytical uncertainty (± 25 °C). (b) Plot of calculated temperature versus F content for samples shown in (a). The effect of F is to lower the calculated temperature substantially.

Figure 17


Figure 17. P-T diagram showing the results of garnet + plagioclase barometry. Solid lines are from garnet + plagioclase + clinopyroxene + quartz barometry, dashed lines are from garnet + plagioclase + orthopyroxene + quartz barometry (calibrations of Eckert et al., 1991), dotted lines are from garnet + plagioclase + hornblende + quartz barometry (calibration of Kohn & Spear, 1990). Gray box shows peak P-T conditions for central Adirondacks from Bohlen et al. (1985).

Figure 18


Figure 18. P-T diagram summarizing the results of this and other studies on the P-T evolution of the Adirondack highlands with the geochronologic constraints summarized by McLelland & Chiarenzelli (1990).

Figure A1


Figure A1. (a) AC(FM) diagram showing the graphical basis of the "thermodynamic projection" scheme used to project from non-quadrilateral components onto the pyroxene quadrilateral. Large dots are a representative opx + cpx assemblage. Schematic tie lines are shown to define the two-phase opx + cpx region. Points "c" are the intersection of the two-phase region with the Al-free diopside - enstatite join. (b) AC(FM) diagram showing the effect of different projection schemes on plotting positions of pyroxenes on the pyroxene quadrilateral (diopside - enstatite join). The original ortho and clinopyroxenes contain 0.08 and 0.14 cations/six oxygens of Al + Ti + Fe3+, respectively. Projection point "a" is from the simple normalization scheme. Projection point "b" projects along exchange vectors. Projection point "c" results from a thermodynamic projection (Spear, 1988). Temperatures shown reflect isotherms for coexisting ortho and clinopyroxene at Fe/(Fe+Mg) = 0.4 from Sack & Ghiorso (1994). Note that the temperatures inferred from the three projection schemes each differ by 125 °C. Di = diopside, En = enstatite, Cats = Ca-tschermaks, Mg-Tsch = Mg-tschermaks.