Determining the extent and nature of Mazatzal-related overprinting of the Penokean orogenic belt in the southern Lake Superior region, north–central USA

Abstract

Twenty-one hornblende and mica 40Ar/ 39Ar dates from central and northwest Wisconsin, USA, provide important information on the timing, spatial extent, and intensity of Mazatzal-age metamorphism and deformation which overprinted the Paleoproterozoic (1870–1820 Ma) Penokean orogenic belt in the southern Lake Superior region. 1760–1750 Ma mica plateau ages from bedrock beneath undeformed 1750–1630 Ma quartzites are interpreted as the time of lower temperature (∼300–350°C) cooling and crustal stabilization after the Penokean orogeny. Six mica ages from bedrock underlying deformed Paleoproterozoic quartzites cluster around 1600 Ma (1576–1614 Ma). The complete absence of 1760–1750 Ma mica ages beneath regions of deformed quartzites suggests widespread heating to temperatures above 300–350°C during Mazatzal-related deformation and metamorphism. The ∼1600 Ma mica ages are interpreted to date the cooling phase of this metamorphism. Two anomalously young biotite dates are interpreted to indicate partial resetting associated with Mesoproterozoic rifting at 1100 Ma. Ten hornblende 40Ar/ 39Ar dates obtained in this study address the higher-temperature overprinting effects in the southern Lake Superior region. One latest Archean age of 2503±18 Ma and two ages of 1853 and 1830 Ma are interpreted as remnant evidence of Archean and Penokean age amphibolite metamorphic events, respectively. The majority of hornblende ages are younger than the Penokean orogeny, scattering between 1796 and 1638 Ma. Microtextural analysis indicates that similar microstructures exist in samples yielding highly discordant hornblende ages. This suggests that shearing and recrystallization did not play an important role in the retention or loss of argon. The 1638 Ma hornblende age is concordant with the Mazatzal orogeny to the south and is interpreted as representing complete thermal or fluid-related resetting associated with that event. Six other post-Penokean ages scatter over a 70 million year interval (1796–1723 Ma) and probably reflect variable retention of radiogenic argon. They are interpreted to indicate variable degrees of partial intermediate-temperature (350–500°C) resetting of hornblende argon systematics at ∼1650–1630 Ma. Collectively, these data suggest that the effects of the Mazatzal orogeny in the southern Lake Superior region involved 350–500°C metamorphism and penetrative deformation.