Hinged Alleghanian Uplift of the Bronson Hill terrane in ...

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 9104495 (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. Julies horn ages on a map Ages in Ma

Hornblende cooling ages 360 340 320 300 280 260 240 220 200 41 41.2 41.4

41.6 41.8 42 Latitude 42.2 42.4 42.6 42.8

Julies musc ages on a map Ages in Ma Muscovite Cooling Ages 360 340 320 300 280 260 240 220 200 41

41.2 41.4 41.6 41.8 42 Latitude 42.2 42.4

42.6 42.8 Ages in Ma Hornblende and Muscovite cooling ages 360 340 320 300 280 260 240 220

200 41 41.2 41.4 41.6 41.8 42 Latitude 42.2

42.4 42.6 42.8 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 Temperature in C Hornblende-Plagioclase 850 800 750 700

650 600 550 500 450 0 kb 5 kb 10 kb 15 kb 41 41.5 42

Latitude 42.5 43 Temperature in C Hornblende-Plagioclase at 5 kb 850 800 750 700 650 600 550

500 450 5 kb 41 41.5 42 Latitude 42.5 43

Garnet-Hornblende for Cores Temperature in C 700 650 600 550 500 450 41.2 41.4 41.6

41.8 Latitude 42 42.2 42.4 Garnet-Hornblende for Rims Temperature in C 700 650 600

550 500 450 41.2 41.4 41.6 41.8 Latitude 42 42.2

42.4 Garnet-Hornblende for Cores and Rims Temperature in C 700 650 600 core 550 rim

500 450 41.2 41.4 41.6 41.8 Latitude 42 42.2 42.4

Core vs Rim Temperatures for Garnet-Hornblende Core Temperatures in C 700 650 Retrograde 600 550 500 Prograde

450 450 500 550 600 Rim Temperatures in C 650 700 Hornblende-Plagioclase and Garnet-Hornblende

Temperature in C 850 750 Hb-Pl 650 Gar-Hb 550 450 41 41.25

41.5 41.75 42 Latitude 42.25 42.5 42.75 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.