HR: 0830h
AN: ED41C-1177    [PDF]
TI: The Alta Stock, Utah: An Exemplar Rock Suite for Learning Metamorphic Concepts
AU: * Davidson, C
EM: cdavidso@carleton.edu
AF: Carleton College, Dept. of Geology One North College St, Northfield, MN 55057 United States
AU: Haileab, B
EM: bhaileab@carleton.edu
AF: Carleton College, Dept. of Geology One North College St, Northfield, MN 55057 United States
AB: Using rock suites in a discovery-based setting is a terrific way to learn important petrologic processes and concepts such as the role of fluids during metamorphism, phase equilibrium, and metastability to name just a few. A particularly effective rock suite we use in our petrology course is from the contact aureole of the Tertiary Alta stock in Utah. The intrusion of the Alta stock into Paleozoic dolostone containing chert nodules produced a well-defined contact metamorphic aureole that includes talc-, tremolite-, forsterite-, and periclase-in field isograds (Cook and Bowman, 2000). Students work in free-form groups with collaboration between groups to define the mineral assemblages using hand samples and thin sections from throughout the areole and then work out what they think are the important petrologic processes that worked together to form what they see. The main goals of the exercise include: 1) learning to identify mineral assemblages common to metamorphosed siliceous dolostones; 2) learning to infer metamorphic reactions from progressive changes in mineral assemblages; 3) being able to identify balanced metamorphic reactions responsible for isograds mapped in the field; 4) begin to develop an understanding of how bulk composition and fluid composition controls mineral assemblages; and 5) learning how to infer the temperature and fluid composition evolution of the Alta stock aureole based on published T-X(CO$_{2}$)diagrams. This suite is particularly effective because all the important minerals the students observe in thin section can be defined by the easy to plot and understand CaO-MgO-SiO$_{2}$ system. This offers the opportunity to introduce (or reinforce) the concept of plotting minerals in composition space, using the phase rule to define the variance of your system, using tie lines to define stable mineral assemblages, and the flipping of tie lines to produce univariant reactions. In addition, the students work out the topology of these reactions in P-T space by plotting the resulting Shreinemaker's bundles and (hopefully) drawing on their developing geologic intuition on which phases should fall on the high temperature side of the univariant reaction line. Other important processes that can be addressed by this suite include the role of fluids during metamorphism, nucleation and growth of metamorphic minerals, metastability (e.g. there are beautiful reaction rims of brucite around periclase), and the influence of bulk composition on stable mineral assemblages (e.g. wollastonite is present in some rocks). Reference: Cook, S.J., and Bowman, J.R. (2000) Mineralogical evidence for fluid-rock interaction accompanying prograde contact metamorphism of siliceous dolomites: Alta Stock Aureole, Utah, USA. Journal of Petrology, 41, 739-757.
DE: 3660 Metamorphic petrology
DE: 6605 Education
SC: Education and Human Resources [ED]
MN: 2003 Fall Meeting