Petrology of an Andalusite-Type Regional Metamorphic Terrane, Panamint Mountains, California

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Abstract
The terrane in the Panamint Mountains, California, was regionally metamorphosed under low-pressure conditions and subsequently underwent retrograde metamorphism. Prograde metamorphic isograds that mark the stability of tremolite + calcite, diopside, and sillimanite indicate a westward increase in grade. The study was undertaken to determine the effects of the addition of Ca on the types of assemblages that may occur in pelitic schists, to contribute to the understanding of the stability limits in P − T − aH2O − XFe of the pelitic assemblage chlorite + muscovite + quartz, and to estimate the change in environment from prograde to retrograde metamorphism. Pelitic assemblages are characterized by andalusite + biotite + staurolite and andalusite + biotite + cordierite. Within a small change in grade, chlorite breaks down over nearly the entire range in Mg/(Mg + Fe) to biotite + aluminous mineral. Chlorite with Mg/(Mg + Fe) = 0.55 is stable to the highest grade, and the generalized terminal reaction is chlorite + muscovite + quartz = andalusite + biotite + cordierite + H2O. Calcic schists are characterized by the assemblage epidote + muscovite + quartz + chlorite + actinolite + biotite + calcite + plagioclase at low grades and by epidote + muscovite + quartz + garnet + hornblende + biotite + calcite + plagioclase at high grades. Epidote does not coexist with any AFM phase that is more aluminous than garnet or chlorite. Lithostatic pressure ranged from 2.3 kb to 3.0 kb. During prograde-metamorphism temperatures ranged from less than 400° to nearly 700°C, and XH2O (assuming PH2O + PCO3 = Ptotal) is estimated to be ∼0.25 in siliceous dolomite, ∼0.8 in pelitic schist, and ∼1.0 in calcic schist. Temperature during retrograde metamorphism was 450° ± 50°C, and all fluid were H2O-rich. A flux of H2O-rich fluid during folding is believed to have caused retrograde metamorphism. The petrogenetic grid of Albee (1965b) is modified to position the (A, Cd) invariant point relative to the aluminosilicate triple point, which allows the comparison of facies series that involve different chloritoid-reactions.

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