Contact Metamorphism of Calc-Silicate Rocks in the Belmont Contact Aureole, Central Nevada

Author

Kelly Plummer, University of Tennessee - Knoxville

Date of Award

5-2006

Degree Type

Thesis

Degree Name

Master of Science

Major

Geology

Major Professor

Theodore Labotka

Committee Members

Claudia Mora, Kula Misra

Abstract

This study focuses on the contact metamorphism of the Ordovician Zanzibar and Toquima Formations by the Cretaceous Belmont Pluton in central Nevada. I mapped the distributions of tremolite and diopside to determine the location of isograds. These isograds are the result of the reactions: 8 quartz + 5 dolomite + H₂O = 3 calcite + tremolite + 7 CO₂ and tremolite + 2 quartz + 3 calcite = 5 diopside + 3 CO₂ + H₂O. Pressure of metamorphism is estimated at between 4 kbar and 1 kbar, but is not definite because of uncertainty in the amount of overburden at the time of metamorphism. Temperatures of metamorphism ranged from 400 to 560 °C for the tremolite isograd and 450 to 565 °C for the diopside isograd. Fluid compositions ranged from X(CO₂) = 0.25 to 0.98. Mass balance calculations show that change in volume decreased with increasing distance from the contact (98.9 cm³/kg to 5.2 cm³/kg) which corresponds to loss in CO₂. Toquima carbon and oxygen isotope values increase with increasing distance from the pluton which is indicative of contact metamorphism. The δ¹⁸O (SMOW) values for both calcite and dolomite in Zanzibar samples show no pattern of decrease with increasing metamorphic grade. Calcite values close to the pluton range from 15.7‰ to 21.2‰ and lower grade samples range from 15.0‰ to 18.6‰. The δ¹³C (PDB) values are lower (-8.9‰ to -5.2‰) in higher grade samples. Low grade samples range from -6.6‰ to -0.8‰. Carbon isotopic values in Zanzibar samples do not increase with increasing distance from the pluton, but these values are systematic with the extent of reaction. There is no relationship between Zanzibar δ¹⁸O values and change in volume or change in CO₂. This shows there is no correlation between Zanzibar oxygen isotopic values and metamorphic reactions resulting from the emplacement of the pluton. The best explanation for the altered oxygen isotopic values is Tertiary volcanic activity. This activity is thought to be responsible for the formation of gold and silver ore deposits in the area of Belmont Nevada.

Recommended Citation

Plummer, Kelly, "Contact Metamorphism of Calc-Silicate Rocks in the Belmont Contact Aureole, Central Nevada. " Master's Thesis, University of Tennessee, 2006.
https://trace.tennessee.edu/utk_gradthes/1765