U-Pb evidence for Permian anatexis and faulting, south-central Connecticut: isotopic constraints on the age of accretion of the Avalon terrane
An orthogneiss unit in the Rope Ferry Gneiss member of the Waterford Group and a cross-cutting, unfoliated granite dike from near Essex, south-central Connecticut, have been dated by the U-Pb method. Isotopic data from zircon, monazite and sphene yield the following results: (1) 9 zircon fractions from both rocks form a discordia with intercepts at 618 /plus minus/ 5 Ma (age of the protolith of the gneiss) and 267 /plus minus/ 9 Ma (age of the granite); (2) sphene from the orthogneiss and monazite from the granite are concordant at 268 Ma; (3) zircons from the granite, which are cloudy, pitted and subhedral, have older Pb/U ages than the clear, euhedral zircons from the gneiss. The morphology of the granite zircons and the colinearity of the combined U-Pb data indicate that these zircons are xenocrysts, inherited via anatexis from a 620-Ma source. The agreement of the concordant monazite and sphene ages with the lower intercept of the zircon discordia confirms the Permian age of the granite dike and suggests that it intruded the gneiss when the gneiss was near to the closure temperature of sphene (/approx/550/degree/C). The age of the dike further constrains the age of the gneissosity in the host gneiss to >270 Ma and the age of the alter ductile faults which deform both the gneiss and the granite to <270 Ma. Thus, the Alleghanian orogeny in southern New England involved ductile faulting as well as anatexis. The colinearity of the nine zircon fractions from both rocks precludes Taconian and/or Acadian lead loss and indicates that these events had little or not effect on these rocks. Thus, this part of the Avalon terrane may have first accreted to New England in the Permian.
- Research Organization:
- Indiana Univ., Bloomington (USA)
- OSTI ID:
- 5811428
- Report Number(s):
- CONF-8510489-
- Journal Information:
- Geol. Soc. Am., Abstr. Programs; (United States), Journal Name: Geol. Soc. Am., Abstr. Programs; (United States) Vol. 17; ISSN GAAPB
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
580100* -- Geology & Hydrology-- (-1989)
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580400 -- Geochemistry-- (-1989)
ACTINIDE ISOTOPES
AGE ESTIMATION
CHEMICAL ANALYSIS
CHEMICAL COMPOSITION
CHEMISTRY
CONNECTICUT
FEDERAL REGION I
GEOCHEMISTRY
GEOLOGIC AGES
GEOLOGIC FAULTS
GEOLOGIC FORMATIONS
GEOLOGIC FRACTURES
GEOLOGIC HISTORY
GEOLOGIC STRUCTURES
GNEISSES
GRANITES
IGNEOUS ROCKS
ISOTOPE DATING
ISOTOPES
LEAD ISOTOPES
METAMORPHIC ROCKS
MINERALS
NORTH AMERICA
OROGENESIS
OXYGEN COMPOUNDS
PALEOZOIC ERA
PERMIAN PERIOD
PETROGENESIS
PLUTONIC ROCKS
QUANTITATIVE CHEMICAL ANALYSIS
ROCKS
SILICATE MINERALS
SILICATES
SILICON COMPOUNDS
TRANSITION ELEMENT COMPOUNDS
URANIUM ISOTOPES
USA
ZIRCON
ZIRCONIUM COMPOUNDS
ZIRCONIUM SILICATES