Petrologic and stable isotopic studies of metamorphic fluid-rock interaction and igneous petrogenesis, Lone Mountain, Nevada

Author

Ian James Richards

Date of Award

8-1994

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Geology

Major Professor

Theodore C. Labotka

Committee Members

Larry Taylor, Steven Driese, David Cole, Kelsey Cook

Abstract

Late Proterozoic to Cagibrian carbonate, calc-silicate and pelitic rocks from Lone Mountain, Esmeralda County, Nevada, were regionally metamorphosed during Mesozoic time and then contact metamorphosed during intrusion of the Lone Mountain granitic pluton in Late Cretaceous time. The rocks were metamorphosed at a pressure of 3.5 ± 1 kbar and a temperature of 450 ± 70°C. Calc-silicate rocks were in equilibrium with H₂O-rich fluids (X_CO2 less than 0.05), while marbles were in equilibrium with more CO₂-rich fluids (X_CO2 greater than 0.2). Essentially pure marbles consist of either calcite or dolomite with limited calcite + dolomite, and minor quartz. The stable isotopic compositions of marbles can be divided into two groups, one with positive δ¹³C values and high δ¹⁸O values, and the other with negative δ¹³C values and low δ¹⁸O values. Quartz from calcite marbles is brown- and blue-luminescent, clear, and smooth-textured, with high δ¹⁸O values; whereas quartz from dolomite marbles is brown-luminescent, opaque, and rough-textured with extremely low δ¹⁸O values. Low Mn/Sr ratios and high δ¹³C and δ¹⁸O values in calcite marbles indicates that their original isotopic compositions have not been significantly altered; whereas, the negative correlation between Mn/Sr and δ¹⁸O in dolomite marbles indicates that they do not preserve their original oxygen isotopic compositions. The trace element compositions of dolomite marbles, loss of Sr and Na, and gain of Zn, Mn, and Fe, compared with calcite marbles, are consistent with their interaction with meteoric fluids during diagenesis. Post-metamorphic meteoric fluid-rock interaction is responsible for the negative δ¹³C and low δ¹⁸O values in marbles and the depleted δ¹⁸O values of quartz and the development of authigenic quartz overgrowths and cements in dolomite marbles. Differences in behavior between dolomite versus calcite marbles requires dolomite marbles to have been more permeable than the essentially impermeable calcite marbles both before and after metamorphism. Grossular calc-silicate rocks contain the diagnostic mineral assemblage grossular + wollastonite + diopside + albite + calcite + quartz, with or without K-feldspar, epidote, vesuvianite, and titanite. Hornblende calc-silicate rocks contain hornblende + epidote + K-feldspar + plagioclase + quartz and may also contain calcite, biotite, chlorite, titanite and magnetite. The stable isotopic composition of calcite and silicate minerals within calc-silicate rocks varies in relation to the modal abundance of the minerals, although calcite and silicate minerals approached oxygen isotopic equilibrium during metamorphism. The amount of volatiles lost by the rocks during metamorphism was calculated from mass balance relationships between the inferred protolith and the metamorphic rock. Grossular calc-silicate rocks lost significantly more CO₂ and slightly less H₂O than hornblende calc-silicate rocks during metamorphism. Differences between the amounts of fluid evolved by grossular and homblende calc-silicate rocks are the result of differences in the original protolith mineralogy. Grossular calc-silicate rocks originally contained greater amounts of calcite, and homblende calc-silicate rocks greater amounts of muscovite and chlorite. The volume of fluid evolved by the rocks during metamorphism resulted in a significant loss of the original protolith solid volume. The isotopic composition of carbon in calcite within the calc-silicate rocks can be explained by mostly fractional devolatilization processes; however, the oxygen isotope data requires infiltration of low ¹⁸O fluids to cause the observed isotopic depletions. The amount of fluid that infiltrated grossular calc-silicate rocks was an order of magnitude greater than that which infiltrated homblende calc-silicate rocks. Fluids in equilibrium with the most depleted calc-silicate rocks are probably of magmatic origin from the Lone Mountain pluton. During metamorphism, fluid flow occurred primarily through calc-silicate rocks; whereas marbles were essentially impermeable. Differences in the extent of fluid-rock interaction also occur within calc-silicate rocks, with grossular calc-silicate rocks interacting with significantly more fluid than homblende calc-silicate rocks. The Lone Mountain granitic pluton is dominated by granite with minor granodiorite and quartz monzonite. Temperatures of pluton emplacement based on garnet + biotite and hornblende + plagioclase geothermometry are between 650 and 750°C; whereas temperatures based on the oxygen isotopic compositions of quartz and feldspar are between 325 and 615 °C, indicating that some subsolidus re-equilibration of oxygen isotopes occurred after pluton emplacement. The pressure of pluton emplacement estimated from aluminum in hornblende geobarometry and from consideration of epidote + quartz and muscovite + quartz stability in conjunction with the H₂O-saturated granite solidus, is 3.5 kbar, consistent with estimates from the surrounding metamorphic rocks and is equivalent to a depth of 12 to 15 km. High whole-rock &delta:¹⁸O values together with an intermediate initial ⁸⁷Sr/⁸⁶Sr ratio indicate that sedimentary or volcanic rocks must have provided a significant component of the source rocks for the granite. Lone Mountain granitic rocks could have been derived from rocks that currently surround the pluton, although isotopic data and phase equilibrium considerations negate the generation of the peraluminous Lone Mountain granitic pluton solely by the partial melting of pelitic schists. Limited chemical variation within the pluton from the most primitive parental magma composition, hornblende granodiorite, to the most evolved magma composition, garnet granite, can occur by fractional crystallization of hornblende and plagioclase; however, the well-developed foliation along the margin of the pluton and the lack of any systematic geochemical variation from the core to the rim of the pluton requires replenishment of the magma chamber to have occurred during multiple intrusive events associated with the emplacement of the pluton.

Recommended Citation

Richards, Ian James, "Petrologic and stable isotopic studies of metamorphic fluid-rock interaction and igneous petrogenesis, Lone Mountain, Nevada. " PhD diss., University of Tennessee, 1994.
https://trace.tennessee.edu/utk_graddiss/10574