PERMIAN AND EARLY MESOZOIC EXHUMATION OF BRONSON HILL 
TERRANE ROCKS: SIGNIFICANCE FOR THE TECTONICS OF THE 
HARTFORD BASIN

	WINTSCH, Robert P., Dept. of Geological Sciences, Indiana 
		University, Bloomington, IN 47405.
	KUNK, MICHAEL J., U. S. Geological Survey, Reston, VA, 22092
	BOYD, Julie L., and HELLICKSON, Bradly P., Dept. of Geological 
		Sciences, Indiana University, Bloomington, IN 47405.

40Ar/ 39Ar age data on hornblendes and muscovites document the 
increasing effect of Alleghanian orogeny in rocks of the Bronson Hill 
terrane in Massachusetts and Connecticut.  Most hornblende ages 
from Massachusetts are Carboniferous, and can be explained as 
reflecting cooling from the Acadian orogeny.  In Connecticut, 
however, hornblende ages are reset to Permian, with a north to south 
age gradient from early to late Permian.  In contrast, muscovite ages 
are uniformly between 245 and 255 Ma, and show no obvious age 
gradients.
	The age gradient in Connecticut parallels an apparent gradient in 
metamorphic grade, with migmatites rare in northern Connecticut, 
becoming more common in the Middletown pegmatite-migmatite 
district, and ubiquitous in the Killingworth dome of the southern 
Bronson Hill.  Quantitative thermobarometry is not possible in most 
of these rocks because the assemblages are not appropriate. 
However, local assemblages containing hornblende-garnet-
plagioclase in the southern Killingworth dome yield temperatures of 
about 650°C and 4-5 kb pressure. Such assemblages probably yield 
minimum estimates of these conditions because of closure 
temperature problems. 
	Resulting P-T-t  modeling shows that a minimum of 15 km of rock 
was removed from the Bronson Hill terrane during the Permian and 
early Triassic.  Less than 5 km of Bronson Hill rocks were eroded 
during the late Triassic to early Jurassic deposition of the New Haven 
Arkose and the Portland Formation.  Thus during the Permian and 
early Triassic many cubic km of sediments were shed off the Bronson 
Hill terrane and passed over the area now occupied by the Hartford 
basin without being deposited.  It is thus very possible that the 
Triassic sediments in the present Hartford basin accumulated there 
before the area took on the topographic expression of a basin in the 
Early Jurassic.