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Rocky Mountain (53rd) and South-Central (35th) Sections, GSA, Joint Annual Meeting (April 29–May 2, 2001)

Paper No. 0
Presentation Time: 1:00 PM-5:00 PM

PROTEROZOIC ROCKS OF THE UNCOMPAHGRE PLATEAU, WESTERN COLORADO AND EASTERN UTAH


LIVACCARI, R. F.1, BOWRING, T. J.1, FARMER, E. T.2, GARHART, K. S.1, HOSACK, A. M.1, NAVARRE, A. K.1, PETERMAN, J. S.1, ROLLINS, S. M.1, WILLIAMS, C. A.3, KUNK, M.4, SCOTT, R. B.5 and UNRUH, D.4, (1)Department of Physical & Environmental Sciences, Mesa State College, 1100 North Avenue, Grand Junction, CO 81502, (2)Department of Physical & Environmental Sciences, Mesa State College, 1100 North Avenue, Grand Junction, CT 81502, Albania, (3)Department of Physical & Environmental Sciences, Mesa State College, 1100 North Avenue, Grand Junction, CO 81502, Albania, (4)USGS, Federal Center, Denver, CO 81502, (5)USGS, Federal Center, Denver, CT 81502, rlivacca@mesastate.edu

Preliminary geologic mapping and geochronology has identified both Early and Middle Proterozoic rocks in the Uncompahgre Plateau. Early Proterozoic rocks include 1) sillimanite-bearing, biotite quartzofeldspathic migmatite (meta-arkose, 1,741+11-Ma discordant U-Pb zircon and 1,355-Ma 40Ar/39Ar biotite) and 2) meta-quartz monzonite pluton (1,721+14-Ma discordant U-Pb zircon and 1,290 Ma-40Ar/39Ar biotite and alkali feldspar). Migmatitic gneisses contain pinch-and-swell, blocky boudinage (flattening fabric, S1). The S1 fabric forms tight to isoclinal folds (F2), plunging WNW to ESE. Structural analysis supports the presence of large-scale sheath folds (Type-1 interference folds) that are either symmetrical or flattened domes along a vertical, WNW-ESE-striking axial surface. Ambiguous zircon morphology leaves it unclear whether the 1,741-Ma date for the migmatite represents the age of migmatization or age of the protolith. The monzonite pluton contains a fabric defined by flattened xenoliths and aligned alkali-feldspar phenocrysts and biotite. The pluton intrudes migmatite, and pluton and migmatite fabrics are concordant. We interpreted the pluton intrusion to be synkinematic with final phases of folding of migmatite. It is not certain, however, if there is an inherited component to the 1,721-Ma zircon age of this pluton. Middle Proterozoic rocks include 1) megacrystic, quartz-monzonite pluton (Vernal Mesa Quartz Monzonite, 1,433+2.2-Ma concordant U-Pb zircon, 1,395-Ma 40Ar/39Ar biotite), 2) medium-grained, garnetiferous, 2-mica granite pluton ("Taylor Ranch granite", 1,395+3-Ma 40Ar/39Ar muscovite), 3) rare NW-SE-striking, biotite-hornblende dikes (1,366-Ma 40Ar/39Ar biotite), and 4) tourmaline-bearing pegmatite dikes. No fabrics are recognized in these plutons, and contacts with migmatite are unmapped. Biotite-hornblende dikes cut across older migmatite fabrics and are ductilely boudinaged. A circa 1.4-Ga regional thermal reset is recognized in all rocks from 40Ar/39Ar mica dates.

Session No. 14--Booth# 27

S5. Tectonics of the Rocky Mountain Region (Posters)
Tuesday, 1 May 2001: 1:00 PM-5:00 PM
Geological Society of America Abstracts with Programs. Vol. 33, No. 5, April 2001, p.44
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