Abstract

The Upper Cambrian Big Horse Limestone Member of the Orr Formation is contact metamorphosed by the Jurassic Notch Peak quartz monzonite in the central portion of the House Range, Utah. Two lithologic types were sampled from specific stratigraphic horizons within the member over a lateral distance of 6 km, at locations reflecting a range in metamorphic grades. The rocks were metamorphosed at ∼2 kbar, on the basis of estimates of stratigraphic overburden at the time of intrusion. The lithologies are relatively pure dolomitic limestones and impure argillaceous limestones (argillites). The progressive metamorphism has resulted in the successive appearance of talc, tremolite, scapolite, diopside, and forsterite in the meta-limestones and the appearance of biotite, tremolite, diopside, plagioclase, scapolite, vesuvianite, grossular, and wollastonite in the argillites. Mineral assemblages of the limestones compared with isobaric 2 kbar-phase equilibria in the CaO-MgO-SiO2-H2O-CO2 system suggest that: (1) fluid buffering by metamorphic reactions has occurred; (2) domains of equilibrium are small; and (3) the limestones behaved as relatively closed systems during metamorphism. Maximum temperatures and X(CO2) composition consistent with assemblages in the limestone are: (see pdf for table). Temperatures estimated by the calcite-dolomite geothermometer are consistent with these temperature estimates.

Temperatures estimated from limestone mineral assemblages are used to establish the sequence of prograde reactions in the multi-component argillite system. Compositions of fluids in equilibrium with low-grade argillites are poorly constrained but may have reached a maximum X(CO2) of ∼0.75. Fluid compositions in equilibrium with medium- to high-grade argillites were more H2O-rich than X(CO2) = 0.20 indicated primarily by the presence of wollastonite at temperatures between 475 and 600 °C. Therefore, the argillites were either initially more water-rich than limestones at the same grade, or they were more open to H2O-rich fluids during metamorphism.

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