Fluid-rock interaction in the late Proterozoic Noonday Dolomite and Johnnie Formation, Panamint Mountains, California

Author

Peter Souza

Date of Award

8-1991

Degree Type

Thesis

Degree Name

Master of Science

Major

Geology

Major Professor

Theodore C. Labotka

Committee Members

K. R. Walker, L. A. Taylor

Abstract

The mineralogy, petrology, and stable isotopic composition of the late Proterozoic Redlands Member of the Noonday Dolomite and lower Johnnie Formation have been studied to determine the effects of fluid-rock interaction attending low-pressure regional metamorphism in the Panamint Mountains, California. Temperature, determined by calcite-dolomite thermometry was uniformly 475 ± 25 °C, and pressure was ~2.5 kbar, based on stratigraphic overburden. Diagnostic assemblages in the Redlands Member include Dol + Qtz + Cal + Tic, Dol + Qtz + Cal + Tic + Tr, and Dol + Qtz + Cal + Tr (all with ± Phi, Chi, PI, Scp, Py, and Gr), indicating that fluid compositions were buffered at X_CO2 values near 0.55. Assemblages indicate temperatures that are in good agreement with those determined by calcite-dolomite thermometry. Dolomite δ¹³C values range from -3.2 to +7.8%o PDB, and δ¹⁸O values range from 18.2 to 23.9%o SMOW. Despite fluid-buffering assemblages in the Redlands Member, the wide range in isotopic composition of these rocks indicates substantial infiltration of an isotopically depleted fluid. Lateral and vertical trends in isotopic composition indicate that fluids from the lower Johnnie Formation did not cross the Noonday Dolomite/Johnnie Formation contact. There is little relation between the progress of talc- and tremolite-forming reactions and isotopic composition of dolomite. Rocks from fault zones and rocks with altered tremolite grains show the greatest depletion in isotopic composition. Modeled isotopic exchange attending fluid infiltration indicates that rocks associated with faults interacted with 1 to 3 fluid/rock mass ratios of an externally-derived fluid. These fluids had X_CO2 values that ranged from 0.05 to 0.50, with the majority of samples indicating exchange with fluids having X_CO2 values near 0.05. The great change in isotopic composition of these rocks, coupled with the low-X_CO2 compositions of these fluids, indicate exchange with fluids flowing through the faults during retrograde metamorphism. Diagnostic assemblages in the lower Johnnie Formation include andalusite + staurolite + biotite, cordierite + chlorite + biotite, andalusite + chlorite + biotite, and andalusite + cordierite + biotite (all with quartz, muscovite, plagioclase, ilmenite, ± tourmaline). Inclusion-rich poikiloblasts of andalusite, and porphyroblasts of biotite, staurolite, and cordierite are common, and typically preserve evidence for progressive prograde reactions and polymetamorphism. Mineral compositions projected onto the plane (Al₂O_{3,/sub>-3K2O)-FeO-MgO display crossing tie-lines, indicating that not all samples were in equilibrium under the same conditions of p, T, and aH2O and/or the lack of approach to equilibrium. Rocks that contained the assemblage andalusite + cordierite + biotite are found in close proximity to rocks containing the assemblage cordierite + chlorite + biotite which supports the interpretation that p, T, and/or aH2O varied over the field area. In most samples, the compositions of phases included in andalusite are essentially constant as well as nearly the same as those found in the matrix. Only those grains in the matrix that are visibly altered have compositions that have also been altered. aH2O values determined for the lower Johnnie Formation range from 0.45 ± 0.15 to 0.59 ± 0.20. Although there is considerable error in the estimate of aH2O, the uniform compositions of prograde phases within samples suggest that crossing tie-lines are due to a spatial variation in aH2O rather than the lack of approach to equilibrium. Textures in some rocks indicate the reactions:}

staurolite + muscovite + quartz = andalusite + biotite + H₂O (i),

cordierite + muscovite = andalusite + biotite + quartz (ii),

and cordierite + andalusite + biotite + H2O = chlorite + muscovite + quartz (iii) occurred. Analyses of cordierite in a sample that contained the assemblage andalusite + cordierite + biotite, and that presently contains reaction textures that indicate the occurrence of reaction (ii), suggest the presence of volatile species within the "cage" site of the cordierite channel. This suggests that reaction (ii) may have been a devolatilization reaction similar to reaction (i). Textures indicate that reaction (iii), which requires the addition of H₂O to the rocks, occurred during the later stages of prograde metamorphism. Calculated a_H2O values indicate X_CO2 compositions in the lower Johnnie Formation to range from 0.60 to 0.72. These X_CO2 values are greater than those determined for the Redlands Member which suggests that fluids evolved from the Redlands Member may have infiltrated the lower Johnnie Formation causing reaction (iii) to occur. The progress of talc- and tremolite-forming reactions in the Redlands Member, however, indicate that relatively little fluid was produced. Based on mass balance considerations, therefore, it is unlikely that the small amount of fluid evolved from the Redlands Member influenced fluid composition within the lower Johnnie Formation. Devolatilization reactions occurring in rocks at depth within the metamorphosing pile, however, may have been the source for these fluids. In most rocks that contained the assemblage andalusite + cordierite + biotite, cordierite was completely removed by reaction (ii) or (iii). In these rocks, the original assemblage was replaced by either andalusite + staurolite + biotite or andalusite + chlorite + biotite. The replacing assemblages in rocks that contained andalusite + cordierite + biotite indicate equilibrium with rocks containing the assemblage cordierite + chlorite + biotite. This indicates that mineral and perhaps fluid equilibrium was attained within the lower Johnnie Formation as metamorphism progressed. Retrograde metamorphism produced pseudomorphed porphyroblasts and mineral armoring, and was accompanied by folding along north-northwest trending axes. Retrograde textures indicate that all retrograde reactions required the addition of H₂O to proceed. This indicates that retrograde metamorphism of the lower Johnnie Formation was accompanied by the influx of H₂O-rich fluids, which is in good agreement with the interpretations made from the petrologic and isotopic data obtained from the Redlands Member.

Recommended Citation

Souza, Peter, "Fluid-rock interaction in the late Proterozoic Noonday Dolomite and Johnnie Formation, Panamint Mountains, California. " Master's Thesis, University of Tennessee, 1991.
https://trace.tennessee.edu/utk_gradthes/12528