Al zoning in pyroxene and plagioclase: Window on late prograde to early retrograde P-T paths in granulite terranes
- Univ. of Arizona, Tucson (United States)
Clino-and orthopyroxenes in granulites commonly show significant Al zoning typified by Al-rich cores and Al-poor rims. Zoning-relaxation calculations suggest that relaxation of Al growth zoning in pyroxenes and plagioclases during peak metamorphism is minor. Thus, Al-zoning data may be used to examine that part of the granulite P-T path along which pyroxene grew. The slope of the log K curves for the reaction CaTs + quartz = anorthite is negative below 20 kbar whereas that for jadeite + quartz = albite is positive, and the two may be combined to yield both pressure and temperature. A subregular quaternary activity model for the system CaTs-jadeite-diopside-hedenbergite was fitted for these calculations. Data for four granulite terranes were examined. Calculations for the Grenville Province, Ontario, yield decreasing pressures and temperatures and an average dP/dT of 54 bars/{degree}C. The average rim results (10.1 kbar, 860C) agree well with peak P-T values (10 kbar, 800-850C) obtained by other methods. Average core results (12.8 kbar, 910C) are significantly higher and suggest prepeak unloading. Data from the Furua Complex, Tanzania, are similar to those from the Grenville (core = 13.7 kbar, 930C; rim = < 11.6 kbar, < 845C), but yield only limits for the rim P-T because plagioclase rim compositions were not obtained. In the Fiordland complex, both Fe-Mg and Al zoning (core = 6.7 kbar, 650C; rim = 14.8 kbar, 810C) are in reasonable agreement and suggest an increase in P. Results from the Pikwitonei area, Manitoba, yield retrograde P-T paths (core = 6.7 kbar, 830C; rim = 3.3 kbar, 670C; 21 bars/{degree}C), and zoning patterns in the feldspar suggest that plagioclase may have grown during retrogression.
- DOE Contract Number:
- FG02-90ER14115
- OSTI ID:
- 5669384
- Journal Information:
- American Mineralogist; (United States), Journal Name: American Mineralogist; (United States) Vol. 76:7-8; ISSN AMMIA; ISSN 0003-004X
- Country of Publication:
- United States
- Language:
- English
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152002* -- Geothermal Data & Theory-- Properties of Minerals & Rocks
58 GEOSCIENCES
580000 -- Geosciences
ALUMINIUM
ANORTHOSITES
CANADA
CHEMISTRY
COOLING
DATA
DEVELOPED COUNTRIES
DIFFUSION
DISTRIBUTION
ELEMENTS
GABBROS
GEOCHEMISTRY
GEOLOGIC FORMATIONS
GEOLOGIC HISTORY
GRANULITES
IGNEOUS ROCKS
INFORMATION
METALS
METAMORPHIC ROCKS
METAMORPHISM
MINERALS
NORTH AMERICA
ONTARIO
OXYGEN COMPOUNDS
PLUTONIC ROCKS
PYROXENES
RESERVOIR PRESSURE
RESERVOIR TEMPERATURE
ROCKS
SILICATES
SILICON COMPOUNDS
TECTONICS