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