hinged alleghanian uplift of the bronson hill terrane in connecticut and massachusetts elizabeth r....

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.