It is proposed that the contrasting metamorphic mineral assemblages of the isolated amphibolite facies metamorphic highs in the Penokean orogen of northern Michigan may be caused by different heat production rates in the Archean basement. This hypothesis is based on concentrations of K, U, and Th in the Archean basement gneisses and Paleo- proterozoic metasediments that indicate significant contribution of radiogenic heating during Penokean metamor- phism. Heat production was anomalously high (-10.6 p-Wm-3) where andalusite-bearing mineral assemblages indicate that high temperatures were attained at shallow crustal levels (-550°-600°C at -3 kbar). In contrast, where exposed metamorphic rocks indicate peak temperatures of 600°-650°C at 6-7 kbar, heat production in the Archean basement was lower approx 3.7 #mu# Wm!~(-3)). The effect of heat production rates on the metamorphic pressure-temperature paths was tested with numerical thermal models. The calculations show ( 1) that if the heat production rate, where andalusite- bearing assemblages formed, was significantly <6.0 p-Wm-3, the estimated pressure at peak temperatures (~m.J would be much higher and lie in the sillimanite or kyanite stability fields; and (2) differences between ~m.. estimates for the metamorphic highs based on thermobarometry can be reproduced if thermal history involved significant crustal thickening as well as moderate unroofing rates.
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