U-Pb and Sm-Nd Isotope Geochemistry

Provenance analysis of lower Paleozoic cratonic quartz arenites of the North American midcontinent region: U-Pb and Sm-Nd isotope geochemistry Clark M. Johnson* Department of Geology and Geophysics, University of Wisconsin, Bryce L. Winter } 1215 West Dayton Street, Madison, Wisconsin 53706 ABSTRACT separates reflect mixing of the age groups and 1994), but few isotopic studies have focused on approximate relative proportions that are the quartz grains. Lower Paleozoic supermature quartz aren- identified from the single zircon results. The stratigraphy and physical sedimentology ites in Wisconsin and Michigan were derived All isotope data on quartz separates and U- of lower Paleozoic quartz arenites in the north- from several Proterozoic and Archean ter- Pb zircon data indicate that the detrital con- ern midcontinent region of North America have ranes. Single-grain, detrital zircon populations stituents (zircons and quartz framework been intensively studied; the provenance and from the Galesville and St. Peter Sandstones in grains) that compose the lower Paleozoic many aspects concerning the depositional his- Wisconsin yield very similar, closely concor- quartz arenites in Wisconsin were primarily tory of these extensive cratonic sheet sand- dant age distributions (Cambrian Galesville derived from the 2.7 Ga granite-greenstone stones have been the subject of considerable Sandstone: 1.1 Ga [n = 4], 1.4 Ga [n = 2], 1.8 Ga terrane of the southern Superior Province and debate since the early 1900s (Dake, 1921; Dott [n = 1], and 2.7 Ga [n = 2]; Ordovician St. Peter a 1.1 Ga terrane. The latter terrane is either and Byers, 1981; Dott et al., 1986). On the ba- Sandstone: 1.1 Ga [n = 2], 1.8 Ga [n = 1], and the silicic volcanic rocks associated with the sis of directional indicators of sediment trans- 2.7 Ga [n = 6]). In contrast, most of the nine de- Midcontinent rift system, or, more likely, the port (Potter and Pryor, 1961; Dott et al., 1986) trital zircons that were analyzed from the St. voluminous granitic rocks that are associated and the composition of heavy minerals (Tyler, Peter Sandstone in the Michigan basin have with the Grenville Province on the eastern 1936), it has been suggested that detrital grains high U contents (450–2500 ppm) and are margin of North America. The Middle Prot- that compose the cratonic sandstones in Wis- strongly discordant (60%–90%). Six zircons erozoic Grenville Province was the most im- consin were ultimately derived from Precam- from the Michigan basin that have ca. 1.0 Ga portant ultimate source of quartz and zircons brian felsic plutonic rocks that are exposed in 207Pb*/206Pb* ages define a regression line that to the St. Peter Sandstone in the Michigan the Lake Superior region. Precambrian bedrock has an upper intercept of about 1100 Ma and a basin; lesser amounts of material were con- of the northern midcontinent region of North lower intercept of 15 Ma; one zircon has a 2.7 tributed from the Archean Superior Province. America includes rocks that span >2.5 b.y. in Ga 207Pb*/206Pb* age. The zircon data indicate age (Fig. 1; e.g., Hoffman, 1989; Sims et al., that although a number of different terranes INTRODUCTION 1989), reflecting a diverse range of potential contributed detrital material to the Paleozoic sources for the Paleozoic cratonic sandstones in quartz arenites in Wisconsin, 1.1 and 2.7 Ga Supermature quartz arenites are common in Wisconsin and Michigan. These sources in- terranes were the dominant sources, and not the sedimentary record, particularly in the Prot- clude the >3.0 Ga southern gneiss terrane of the the local basement, which primarily consists of erozoic and lower Paleozoic sections of stable Superior Province; the ca. 2.7 Ga granite- the ≥2.7 Ga Marshfield terrane, the 1.8 Ga cratons, and usually are composed of >97% greenstone terrane of the Superior Province; Penokean orogen, and the 1.4 Ga Wolf River quartz and <1% heavy minerals (e.g., Tyler, the ≥2.7 Ga Archean inlier of the Marshfield batholith. A terrane that has a 1.1 Ga age is 1936; Dott et al., 1986). However, conventional gneiss terrane in central Wisconsin; the ca. probably the main source for the St. Peter petrographic (e.g., Dickinson and Suczek, 1979; 1.8–1.7 Ga Penokean orogen and postorogenic Sandstone in the Michigan basin. Folk, 1980) and bulk geochemical (e.g., Bhatia rocks; the ca. 1.4 Ga anorogenic granites of the Quartz separates were also analyzed for Pb- and Crook, 1986; Basu et al., 1990) approaches Wolf River batholith, eastern granite-rhyolite Pb and Sm-Nd isotope variations, and the data to provenance analysis of supermature quartz province, and parts of the Grenville Province; do not indicate significant source differences arenites generally yield few insights into these and the ca. 1.1 Ga Keweenawan rocks of the between the heavy mineral fraction (zircons) homogeneous rocks. The source ages of heavy Midcontinent rift system and the majority of and the quartz framework grains. Pb-Pb mineral components, such as zircons, have proven the Grenville Province (Fig. 1). isochrons and Sm-Nd isotope data for quartz to be an important aspect of provenance studies In this contribution we report the results of a (e.g., Ross and Parrish, 1991; Zhao et al., 1992; rare earth element (REE), Sm-Nd, Pb-Pb, and U- *E-mail: [email protected]. McLennan et al., 1993; Smith and Gehrels, Pb isotope study of quartz grains, as well as U-Pb Data Repository item 9990 contains additional material related to this article. GSA Bulletin; November 1999; v. 111; no. 11; p. 1723–1738; 11 figures; 6 tables. 1723 JOHNSON AND WINTER geochronology on single detrital zircons, from two lower Paleozoic supermature quartz sandstone units in south-central Wisconsin (Cambrian Galesville Sandstone and Ordovician St. Peter Sandstone), and one unit in the east-central Michigan basin (Ordovician St. Peter Sandstone) (Figs. 1 and 2). Both zircons and quartz were analyzed in an attempt to address possible differences in the sources of zircons and the primary quartz framework grains. The data highlight the great antiquity of most quartz and heavy minerals in the cratonic quartz arenites, and identify source terranes and possible sediment transport pathways. GEOLOGIC SETTING The lower Paleozoic stratigraphy of the North American midcontinent is characterized by 60– 90 m.y. (i.e., second order) cycles that consist of a basal quartz arenite and an upper carbonate unit, the former of which was deposited on wide- spread unconformities. These well-known, intensively studied strata were the basis for the devel- opment of the term orthoquartzite-carbonate Figure 1. Precambrian geologic basement map of the Great Lakes region (adapted from suite (Pettijohn, 1957), and the concept of uncon- Van Schmus, 1992). A major crustal boundary, the Great Lakes tectonic zone (GLTZ), divides formity-bounded lithologic sequences (Sloss, the southern Superior Province into a southern gneiss terrane (GT) of age 3.0–3.6 Ga and a 1963). These sandstones, which have exceptional northern granite-greenstone terrane (GGT) of age 2.7 Ga. The Niagara fault zone (NFZ) is the compositional and textural maturity, are com- southern boundary of the Superior Province and represents the suture zone with the magmatic posed of >97% unstrained, monocrystalline arcs of the ca. 1.8 Ga Penokean orogen. An inlier of 2.7 to >3.2 Ga Archean rocks in the Penokean quartz, and there is a distinct paucity of shale. orogen is defined as the Marshfield terrane (MT). Anorogenic plutons emplaced ca. 1.4 Ga per- They have a sheet-like geometry, and typically forate the Early Proterozoic crust throughout the midcontinent region; the Wolf River batholith have a thickness of 40–50 m over thousands of (WRB) in northeastern Wisconsin is a typical example. South of the Penokean orogen is the ca. square kilometers (Dott et al., 1986). 1.4 Ga eastern granite-rhyolite province (EGRP). The ca. 1.1 Ga Midcontinent rift system Most current sedimentologic models agree (MCRS) consists of an axial portion that is dominated by mafic igneous rocks (black), and postrift that the sheet geometry is largely the result of eo- flanking sedimentary basins (diagonal rules). The ca. 1.1 Ga Grenville Province (GP) comprises lian and fluvial processes that, following ex- the eastern margin of the North American craton. Additional data are from Sims et al. (1989). tended periods of subaerial erosion, distributed sand across large areas of the craton (Dott et al., 1986). Nonmarine deposition was followed by mented, and typically have only incipient quartz were a minor component in the ultimate source reworking and further deposition of sand in a overgrowths (<2 µm; Odom et al., 1976). Our en- region(s) (Tyler, 1936). shallow-marine environment. Paleocurrent indi- ergy-dispersive electron-microprobe analysis of The predominance of only the most durable cators from the subaerial facies of Cambrian and mineral inclusions in 38 quartz grains from these type of quartz (i.e., unstrained, monocrystalline) Ordovician quartz arenites are southwest di- two formations supports the detailed study of and composition of the framework heavy miner- rected, consistent with inferred deposition in the Tyler (1936), who physically separated and als in the detrital suites led Tyler (1936) to sug- paleo-trade wind belt. Paleocurrent indicators counted a statistically significant number of min- gest that the detrital grains have been through from the subaqueous facies are consistent with eral inclusions. Tyler’s results from the St. Peter multiple sedimentary cycles, and that the imme- other stratigraphic evidence that the Precambrian Sandstone indicate that zircon and apatite are diate source was a sedimentary terrane, rather craton of the Great Lakes region was a subtle dominant (70%–90% of heavy mineral inclu- than an igneous or metamorphic source.

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