Possible polymetamorphism in the Bronson Hill terrane: A 100 M. Y. age gradient in [sup 40]Ar/[sup 39]Ar hornblende ages

Conference · Mon Mar 01 04:00:00 EST 1993 · Geological Society of America, Abstracts with Programs; (United States)

OSTI ID:5722759

Boyd, J; Wintsch, R P ^[1]; Kunk, M J ^[2]

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1. Indiana Univ., Bloomington, In (United States). Dept. of Geological Sciences
2. Geological Survey, Reston, VA (United States)

New [sup 40]Ar/[sup 39]Ar ages of 11 hornblende samples from the Bronson Hill terrane in central and southern New England show a southward decreasing age trend from 345 Ma in the Pellham dome (MA) in the north to 250 Ma in the Killingworth dome (CT). Two samples from the southern Killingworth dome and one from the Glastonbury dome yield plateau ages, whereas the other age spectra are discordant broad U-shaped patterns, suggesting extraneous argon. Isotope correlation analysis of the [sup 40]Ar/[sup 39]Ar age spectrum data from the disturbed samples confirms the inclusion of minor amounts of extraneous argon and allows the resolution of meaningful geological ages. Within the Glastonbury dome, seven amphiboles have a N-S age gradient from 345 to 270 Ma. In the Killingworth dome, four amphibole separates define a southwest-facing age gradient from 270 Ma to 250 Ma. Currently available [sup 40]Ar/[sup 39]Ar data from muscovite along the same trend are consistent with published data from the Middletown pegnatite district and have uniform ages of [approximately]250 Ma. The times and rates of cooling established by hornblende-muscovite pairs in the southern Bronson Hill terrane are indistinguishable from those in the Avalon terrane to the east, suggesting simultaneous exhumation of the Avalon and southern Bronson Hill terranes. In the northern Bronson Hill terrane, however, the early Carboniferous hornblende ages do not support this interpretation, they are more consistent with slow cooling from an Acadian (Early Devonian) peak metamorphism. This 100-m.y. age gradient is unique among New England terranes, suggesting that the metamorphic history in the Bronson Hill terrane is more complicated than in adjacent terranes. It is possible that uplift of much of the southern Bronson Hill terrane was delayed until Late Paleozoic Alleghanian underplating by the Avalon terrane.

• OSTI ID: 5722759
• Report Number(s): CONF-9303211--
• Conference Information: Journal Name: Geological Society of America, Abstracts with Programs; (United States) Journal Volume: 25:2
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
580000* -- Geosciences
AGE ESTIMATION
AMPHIBOLE
ARGON ISOTOPES
CONNECTICUT
DEVELOPED COUNTRIES
GEOLOGIC DEPOSITS
GEOLOGIC HISTORY
GEOLOGY
HORNBLENDE
IGNEOUS ROCKS
ISOTOPE DATING
ISOTOPES
MAINE
METAMORPHISM
MINERALS
NORTH AMERICA
PETROGENESIS
PETROLOGY
PLUTONIC ROCKS
ROCKS
SILICATE MINERALS
USA