Hinged Alleghanian Uplift of the Bronson Hill terrane in Connecticut

and Massachusetts

Elizabeth R. Goeke, Julie L. Boyd, and Robert P. Wintsch

Indiana University, Bloomington, Indiana

Research support by:

• Geological Society of America Student Research grants awarded to Elizabeth R. Goeke and Julie L. Boyd

• National Science Foundation grants #EAR 91-04495 (R. P. Wintsch) and #EAR99-09410 (R. P. Wintsch)

• Department of Geological Sciences, Indiana University

Goals: To evaluate the Alleghanian overprint on rocks previously

metamorphosed in the Acadian Orogeny.

Why the Bronson Hill terrane?

• The entire terrane was metamorphosed during the Acadian Orogeny.

• The terrane extends from the Long Island Sound to the New Hampshire/Canadian border.

• Cooling ages from Connecticut in the Bronson Hill are indicative of cooling from Alleghanian events.

Julie’s horn ages on a map

Hornblende cooling ages

200220240260280300320340360

41 41.2 41.4 41.6 41.8 42 42.2 42.4 42.6 42.8

Latitude

Ages

in M

a

Julie’s musc ages on a map

Muscovite Cooling Ages

200220240260280300320340360

41 41.2 41.4 41.6 41.8 42 42.2 42.4 42.6 42.8

Latitude

Ages

in M

a

Hornblende and Muscovite cooling ages

200220240260280300320340360

41 41.2 41.4 41.6 41.8 42 42.2 42.4 42.6 42.8

Latitude

Ages

in M

a

Ponder the Difference:

• Hornblende ages are cooling from Alleghanian heating in the south and the Acadian Orogeny in the north.

• Muscovite ages are fairly uniform the length of the terrane and indicative of cooling from Permian heating.

Map of ERG samples

Thermometers and Barometers:

• Holland and Blundy, 1994

hornblende-plagioclase thermometer

• Graham and Powell, 1984

garnet-hornblende thermometer

• Kohn and Spear, 1989 and 1990

garnet-hornblende-quartz-plagioclase barometer

Hornblende-Plagioclase

450500550600650700750800850

41 41.5 42 42.5 43

Latitude

Tem

pera

ture

in C 0 kb

5 kb

10 kb

15 kb

Hornblende-Plagioclase at 5 kb

450500550600650700750800850

41 41.5 42 42.5 43

Latitude

Tem

pera

ture

in C

5 kb

Garnet-Hornblende for Cores

450

500

550

600

650

700

41.2 41.4 41.6 41.8 42 42.2 42.4

Latitude

Te

mp

era

ture

in

C

Garnet-Hornblende for Rims

450

500

550

600

650

700

41.2 41.4 41.6 41.8 42 42.2 42.4

Latitude

Te

mp

era

ture

in

C

Garnet-Hornblende for Cores and Rims

450

500

550

600

650

700

41.2 41.4 41.6 41.8 42 42.2 42.4

Latitude

Te

mp

era

ture

in

C

core

rim

Core vs Rim Temperatures for Garnet-Hornblende

450

500

550

600

650

700

450 500 550 600 650 700

Rim Temperatures in C

Cor

e Te

mpe

ratu

res

in C Retrograde

Prograde

Hornblende-Plagioclase and Garnet-Hornblende

450

550

650

750

850

41 41.25 41.5 41.75 42 42.25 42.5 42.75

Latitude

Tem

per

atu

re i

n C

Hb-Pl

Gar-Hb

Various P-T paths with gar-hb points, hb-pl points, and exhumation lines

Conclusions:

• The southern portions of the Bronson Hill were buried to greater depths during the Alleghanian Orogeny than those in northern Connecticut and Massachusetts.

• The Bronson Hill was exhumed quicker in the south than to the north.