Contrasting metamorphic record of heat production anomalies in the Penokean orogen of northern Michigan

It is proposed that the contrasting metamorphic mineral assemblages of the isolated amphibolite facies metamorphic highs in the Penokean orogen of nor thern Michigan may be caused by different heat production rates in the Arch ean basement. This hypothesis is based on concentrations of K, U, and Th in the Archean basement gneisses and Paleo-proterozoic metasediments that ind icate significant contribution of radiogenic heating during Penokean metamo rphism. Heat production was anomalously high (similar to 10.6 mu Wm(-3)) wh ere andalusite-bearing mineral assemblages indicate that high temperatures were attained at shallow crustal levels (similar to 550 degrees-600 degrees C at similar to 3 kbar). In contrast, where exposed metamorphic rocks indi cate peak temperatures of 600 degrees-650 degrees C at 6-7 kbar, heat produ ction in the Archean basement was lower (similar to 3.7 mu Wm(-3)) The effe ct of heat production rates on the metamorphic pressure-temperature paths w as tested with numerical thermal models. The calculations show (1) that if the heat production rate, where andalusite-bearing assemblages formed, was significantly <6.0 mu Wm(-3), the estimated pressure at peak temperatures ( P-Tmax) would be much higher and lie in the sillimanite or kyanite stabilit y fields; and (2) differences between P-Tmax estimates for the metamorphic highs based on thermobarometry can be reproduced if thermal history involve d significant crustal thickening as well as moderate unroofing rates.