Quantifying the tectono-metamorphic evolution of pelitic rocks from a wide range of tectonic settings: mineral compositions in equilibrium
Caddick, Mark ; Thompson, Alan
In: Contributions to Mineralogy and Petrology, 2008, vol. 156, no. 2, p. 177-195
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- Commonly used thermometer and barometer calibrations are sensitive to mineral assemblage and, thus, bulk-rock composition. Calculated mineral stabilities for an average pelitic rock over a pressure-temperature (PT) range appropriate for normal, thickened, heated and shallowly subducted continental crust (400-900°C at 0.1-3.0GPa) reveal more than one hundred possible assemblages. Individual phase compositions are dependent on the assemblage in which they belong and combining isopleth sets to represent $${\left({X^{\rm Mg} /X^{\rm Fe}} \right)}_{\rm garnet} /{\left({X^{{\rm Mg}} /X^{{\rm Fe}}} \right)}_{{{{\rm biotite}}}}$$ and $$X^{{\rm Ca}}_{{\rm garnet}} /X^{{\rm Ca}}_{{\rm plagioclase}}$$ reveals several PT-ranges where commonly used mineral thermobarometers are less effective. For example, the garnet-biotite thermometer becomes increasingly P dependent in the absence of muscovite in high T melt-bearing assemblages, and biotite and plagioclase are not stable at pressures appropriate for lower thickened continental crust. Compositional thermobarometers involving equilibration between alternative phases (namely garnet, phengite and omphacite) are presented. Although the equilibrium compositions of phases at any P and T may change significantly as a function of bulk-rock composition, compositional-ratio thermobarometers are typically insensitive to this, unless a pseudo-univariant reaction is crossed and the buffering assemblage is altered. Quantification of the limits of efficacy of various thermobarometers allows the mineralogy of metapelites to be used to precisely determine segments of PT paths and infer their likely tectonic controls