U-Pb dating of garnets in Paleozoic leucogranites, southwestern New England Appalachians
- State Univ. of New York, Stony Brook, NY (United States). Dept. of Earth and Space Sciences
U-Pb dating of zircon and monazite is generally the most effective method of precisely dating crystallization in granitic rocks. It is, however, often difficult to precisely date leucogranites using these minerals because of persistent inheritance in zircon and complicated U-Pb systematics in some monazites. Garnet, a common igneous mineral in leucogranites, has a closure temperature greater than 800 C and has been shown to yield precise U-Pb ages in metamorphic rocks. The utility of U-Pb dating of garnet in leucogranites was evaluated in two samples of Acadian leucogranites with euhedral garnet phenocrysts. These leucogranites are exposed in the Bridgeport synform in southwestern Connecticut. Zircons from both samples have significant inheritance making precise age determinations difficult. A garnet separate from the Shelton muscovite granite yields a concordant age of 380 [+-] 3 Ma, in agreement with U-Pb monazite ages for the sample. A garnet separate from a transported dike of muscovite granite yields a concordant age of 390 [+-] 4 Ma. This dike lacks monazite. These results in combination with U-Pb ages for other leucogranites and metapelitic units in the area show that leucogranites were generated during and toward the end of Acadian metamorphism in southwestern Connecticut. The authors developed procedures to purify Pb and U and to eliminate discordance during sample preparation. The garnets contain 2.35--2.64 ppm U and 0.146--0.157 ppm radiogenic Pb of which 5.1--10.9% is radiogenic Pb-208. Measured Pb-206/Pb-204 ratios vary from 101--113. Thus, initial Pb correction using leached K-feldspars is advised. Blank corrected U-238/P-204 ratios are 1560--2076.
- OSTI ID:
- 5591220
- Report Number(s):
- CONF-921058-; CODEN: GAAPBC
- Journal Information:
- Geological Society of America, Abstracts with Programs; (United States), Vol. 24:7; Conference: 1992 annual meeting of the Geological Society of America (GSA), Cincinnati, OH (United States), 26-29 Oct 1992; ISSN 0016-7592
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
APPALACHIAN BASIN
AGE ESTIMATION
CONNECTICUT
GARNETS
ISOTOPE DATING
DIKES
LEAD
PALEOZOIC ERA
SAMPLE PREPARATION
URANIUM
ZIRCON
ACTINIDES
DEVELOPED COUNTRIES
ELEMENTS
GEOLOGIC AGES
GEOLOGIC STRUCTURES
METALS
MINERALS
NORTH AMERICA
OXYGEN COMPOUNDS
SEDIMENTARY BASINS
SILICATE MINERALS
SILICATES
SILICON COMPOUNDS
USA
580000* - Geosciences