Introduction Directory UMM :Data Elmu:jurnal:P:Precambrian Research:Vol104.Issue1-2.2000:

1. Introduction

The 1870 – 1820 Ma Penokean orogenic rocks of the North American midcontinent are part of a vast belt of juvenile crust accreted onto the south- ern margin of Laurentia – Baltica during the late Paleoproterozoic inset, Fig. 1. This belt formed the source region for subsequent generation of transcontinental Mesoproterozoic crustal-melt granites, some of the oldest of which are pre- served within the Penokean province. In particu- lar, the voluminous 1470 Ma Wolf river batholith and its associated plutons are little deformed and much of it is apparently minimally eroded Allen and Hinze, 1992; Holm and Lux, 1998. The tectonic history of the central Penokean province from ca. 1800 – 1400 Ma, therefore, provides in- sight, ultimately, into the cratonization of conti- nental crust. The purpose of this research has been to con- strain better the timing, nature, and extent of post-Penokean metamorphism and deformation in the southern Lake Superior region. Northwest Wisconsin is recognized here as a critical region for study of the post-orogenic history of this Paleoproterozoic accretionary orogen. This is the only part of the orogenic belt where undeformed Paleoproterozoic cratonic quartzites are preserved proximal to correlatable, but deformed, post-oro- genic quartzite. This region also contains sparse bedrock thermochronologic age data, although considerable data exist to the east in northeast Wisconsin and to the west in central Minne- sota. Many earlier studies have attributed post- Penokean metamorphism to an ‘enigmatic,’ low-grade but widespread event at ca. 1630 Ma Van Schmus and Woolsey, 1975; Van Schmus et al., 1975; Sims and Peterman, 1980; Peterman et al., 1985; Peterman and Sims, 1988. This study provides evidence for an intermediate temperature 350 – 500°C disturbance at that time and assigns its cause to orogenic activity along the southern Fig. 1. Proposed distribution of Paleoproterozoic crustal provinces in the United States prior to formation of the Mesoproterozoic igneous and tectonic provinces modified after Holm et al., 1998b. Cross-hatchure pattern depicts region of inner accretionary belt and Penokean belt deformed during main ca. 1650 Ma tectonism of outer tectonic belt after Van Schmus et al., 1993; Holm et al., 1998b. Ba, Barron quartzite; Bb, Baraboo quartzite; S, Sioux quartzite; Mv, Mojave province; Mz, Mazatzal province; Y, Yavapai province; CB, Cheyenne belt. Inset shows map of Paleoproterozoic belts along the reconstructed southern margin of Laurentia – Baltica after Kerr et al., 1997. Fig. 2. Summary map of localities dated in this study. margin of Laurentia Holm et al., 1998b, com- patible with models for a long-lived orogen along southern Laurentia Karlstrom et al., 1999.

2. Geologic setting