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Northeastern Section - 36th Annual Meeting (March 12-14, 2001)

Paper No. 18
Presentation Time: 8:30 AM-12:00 PM

207PB/206PB IN SITU ION MICROPROBE DATING OF MONAZITE FROM PRECAMBRIAN METAMORPHIC SUITES, TOBACCO ROOT MOUNTAINS, SOUTHWESTERN MONTANA


WEBB, Alexander1, CHENEY, John2, HARMS, Tekla1 and COATH, Chris3, (1)Dept. of Geology, Amherst College, Amherst, MA 01002, (2)Geology, Amherst College, AC# 811, Campus Center, Amherst College, Amherst, MA 01002, (3)Department of Earth and Space Sciences, UCLA, Los Angeles, CA 90095, aawebb@amherst.edu

The Precambrian rocks of the Tobacco Root Mountains are subdivided into three metamorphic suites: the Pony-Middle Mountain Metamorphic Suite (PMMMS), Indian Creek Metamorphic Suite (ICMS), and Spuhler Peak Metamorphic Suite (SPMS). The PMMMS and ICMS are dominated by quartzofeldspathic gneisses with sedimentary and igneous protoliths. The SPMS is dominated by various kinds of amphibolite and represents ocean crust. Mafic sills and dikes (MMDS) intruded the PMMMS and the ICMS at 2100-2000 Ma, but are absent in the SPMS, suggesting juxtaposition of the SPMS with the supracrustal suites after the intrusion of the MMDS. Rocks from all suites record similar pressures and temperatures of 7-9 kbars and 650-750°C. The ICMS and SPMS contain relict minerals and reaction textures indicating earlier metamorphism at P > 10 kbars. 207Pb/206Pb ages of monazites from 5 PMMMS, 8 ICMS, and 16 SPMS samples have been obtained from the UCLA ion microprobe. 2-10 spot ages were aquired from 1-5 monazite grains per sample. The SPMS monazites have an average age of 1775 Ma, whereas the PMMMS and ICMS monazites have a generally bimodal age distribution of ~2450 Ma and ~1775 Ma. Some ICMS and PMMMS monazite grains display age zoning. The occurance of ~1775 Ma ages in all three suites and presence of significantly older ages only in the PMMMS and ICMS are consistent with the assembly of the SPMS with the supracrustal suites during a ~1775 collisional event. One PMMMS sample and two ICMS samples record only the ~1775 Ma age; if these may be reinterpreted as displaced slices of the SPMS, then all ICMS and PMMMS rocks experienced the ~2450 Ma metamorphism. ICMS samples strongly reflect both the ~1775 Ma and ~2450 Ma events, whereas PMMMS samples contain very few ~1775 Ma ages. This study also examines age distributions on both grain and terrane scales, and relationships between the textural occurance of monazite grains and their ages.

Session No. 7--Booth# 35

T10. Undergraduate Research I (Sponsored by Geology Division, Council on Undergraduate Research) (Posters)
Monday, 12 March 2001: 8:30 AM-12:00 PM
Geological Society of America Abstracts with Programs. Vol. 33, No. 1, February 2001
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