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Isotopic and morphologic evidence for the age of the Fordham gneiss

Journal Article · · Am. J. Sci.; (United States)

The U-Th-Pb and Sm-Nd isotopic methods have been used to date a granitic gneiss in Member C of the Fordham Gneiss in southeastern New York. The protolith of the dated granitic gneiss may have been volcanic rock or, more likely from textural evidence, an intrusive sheet. Zircons from the gneiss are subdivided into two populations. Group (1) zircons are clear, euhedral, and contain between 660 and 930 ppm uranium. Group (2) zircons are dark, rounded, and contain between 1720 and 2040 ppm uranium. U-Th-Pb systematics from group (1) zircons indicate a minimum crystallization age of 1157 my for the protolith of the gneiss. Group (2) zircons are thought to have been inherited from an older crustal source, were reset in the late Precambrian, and suffered episodic lead loss in the middle Paleozoic. U-Pb dating of sphene from the same sample at 435 my indicates that the Taconian orogeny played a major role in disturbing the isotopic systematics. A Sm-Nd model age on a whole-rock split suggests that group (2) zircons were derived from a 1.5 by-old source. Experiments with zircon abrasion reveal: (1) the true age of the protolith of the gneiss is probably about 1170 my, (2) minor amounts of uranium can be leached from the abraded spheres, (3) coatings on abraded spheres derived from the adhesive in the abrasion mill contain common lead, and (4) spheres should be washed in very weak acid leaches. Because of the uncertainty in the origin of this rock, the age of 1170 my is regarded as a minimum age for the formation of the Fordham Gneiss.

Research Organization:
Geological Survey, Denver, CO
OSTI ID:
6757178
Journal Information:
Am. J. Sci.; (United States), Journal Name: Am. J. Sci.; (United States) Vol. 285:5; ISSN AJSCA
Country of Publication:
United States
Language:
English

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Related Subjects

510101* -- Environment
Terrestrial-- Basic Studies-- Radiometric Techniques-- (-1989)
54 ENVIRONMENTAL SCIENCES
58 GEOSCIENCES
580400 -- Geochemistry-- (-1989)
ACTINIDE ISOTOPES
ACTINIDE NUCLEI
AGE ESTIMATION
ALPHA DECAY RADIOISOTOPES
CHEMISTRY
EVEN-EVEN NUCLEI
EVEN-ODD NUCLEI
FEDERAL REGION II
GEOCHEMISTRY
GNEISSES
HEAVY NUCLEI
HOURS LIVING RADIOISOTOPES
INTERMEDIATE MASS NUCLEI
ISOMERIC TRANSITION ISOTOPES
ISOTOPE DATING
ISOTOPE RATIO
ISOTOPES
LEAD 204
LEAD 206
LEAD 207
LEAD 208
LEAD ISOTOPES
METAMORPHIC ROCKS
MINUTES LIVING RADIOISOTOPES
NEODYMIUM 143
NEODYMIUM 144
NEODYMIUM ISOTOPES
NEW YORK
NORTH AMERICA
NUCLEI
OROGENESIS
RADIOISOTOPES
RARE EARTH ISOTOPES
RARE EARTH NUCLEI
ROCKS
SAMARIUM 147
SAMARIUM ISOTOPES
SECONDS LIVING RADIOISOTOPES
STABLE ISOTOPES
TECTONICS
THORIUM 232
THORIUM ISOTOPES
URANIUM 235
URANIUM 238
URANIUM ISOTOPES
USA
YEARS LIVING RADIOISOTOPES