Craddock et al., 2006. 19th Annual Keck Symposium; http://keck.wooster.edu/publications

PRECAMBRIAN EVOLUTION OF

JOHN CRADDOCK, Macalester College KARL WIRTH, Macalester College CAMERON DAVIDSON, Carleton College

PROJECT SUMMARY Keweenaw Rhyolite (Dhiren Patel, Ryan Porter) This project was organized to address outstanding geologic problems in the mid- The Keweenaw rift province was magmatically continent region (Minnesota, and active for a brief period (1.1-1.06 Ga; Davis Michigan) using Macalester College as a and Green, 1997) as a back-arc continental base for field and lab studies June 14-July15, spreading center in-board of the convergent 2005. Following a regional field trip through Grenville orogen (1.4-0.9 Ga). The rift the geology of northern Minnesota, students produced a thick sequence of intrusive and selected projects on igneous geochemistry, extrusive igneous rocks, dominated by magnetic fabrics in igneous rocks, the (~70%) and rhyolite (~15%) and lesser bi-modal provenance of clastic sedimentary rocks using components including gabbro, granophyre, detrital zircons, or a combination of the above. anorthosite, and lamprophyre. The rhyolites are Following individual sample collection (Figs. found near the top of the igneous stratigraphy 1 & 2), all students processed 2-3 clastic and are <30 m in thickness; some preserve no sediments for their zircon populations at obvious layering while other flows contain Macalester. All students also used anisotropy contorted flow folding at the base and tops. of magnetic susceptibility (AMS) to measure Dhiren Patel and Ryan Porter selected four strain ( or quartzites) or a magmatic flows (Figs. 1 & 2) where oriented samples fabric (rhyolites, granites, ) at the could be collected from the base, middle and Institute for Rock Magnetism (IRM) at the top of a flow and ~20 oriented equant cores (or University of Minnesota. Magnetic separation cubes) could be extracted for magnetic analysis. of zircons continued in the fall at Macalester, and Prof. Davidson traveled to Pullman in Anisotropy of magnetic susceptibility (AMS) is December, 2005 to mount, polish, and scan a technique used to measure a magnetic fabric (SEM-EDS and CL images) the pucks for laser ellipsoid as a proxy for strain (sediments) or analysis. Five students traveled to Washington magmatic flow. Patel and Porter hypothesized State University, Pullman, WA in January, 2006 that the tops and bottoms of flows would with Profs. Wirth and Craddock to use the preserve a chaotic (random) AMS fabric and laser ablation-mass spectrometer (LA-ICPMS) the flow interiors would preserve a magnetic system to obtain U-Pb ages on igneous and fabric with Kmax parallel to layering and trending detrital zircon populations. One granite (n=20 NW-SE, normal to the rift axis. In thin section, zircons) and sixteen sedimentary units (n=2000 some of the rhyolites contain glass shards zircons) were analyzed over ~3 weeks of lab and tridymite pseudomorphs that are aligned time in Pullman. parallel to bedding and normal to the rift. The petrofabric and AMS results for the rhyolite

120 Craddock et al.,2006. 19th Annual Keck Symposium; http://keck.wooster.edu/publications d n g n h v a v r m u g m b d X W S d A C X X X n a m g g W a m C X u A i O O s C h t c O p a u X g O m a X v f g X m m h m s X w r W Y X v Y g l d p X p q s h Y v X Y g u X h v u C b d X p j X X X g Y h b d w v Y w i m g Y Y m p g X X X X v g q m w X X Y u m g C X w c w m v a a l Y Y a p w i w r X Y Y Y w m j Y X Y r i c X m Y s t X f p n Y g p g Y g g X g X n w A n X g g g g b X A g W W X E g c g X r s r s X f r X g 3 w X W t - n X o Y b s g i w g s

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c i o z o r e n a h P c i o z o r e t o r p o s e M c i o z o r e t o r p o e l a P n a e h c r A e e t n t i i a n n e t a g e i o e r i t e t i e z n i t e g t i h r d e t y i n n e a i t e r s t n i r e e c t n i o p o i i n l y n a n g g d z a i r n a o h m r M o e a n r n M i v t e e a r b p i i e a G t a i r e s l o G z r t i e . e n G e t d i l t n i t G t i h t l e w u a k o i r r i i G M P t r M n a r o e i n p o . n t e l t e n S e a n a n P e e e u n t s a o e z b t s o i t v n e u a c u a i a P i v i m o t i U i i l t o e r z o r y i l r g r s s s r r n l t - a e t n i e i h a n i p Q s t h t R a r g u k k c o o C a b a R i e G s p d t g i G e p a e r r d t r r c c h f n a a r d M b u i r z n e r l a u c o n n n i s n t s d b b n r i g l s o o o o e u m o e n t l a g E y r i n t r o b a o t l g r r t Q t e i e a b b a o e e i k t e a n a i g c h o d r a l a a Q a - i s e n g i a a t c k c c c d a B P R W W i n f i t y d o p o c h e i i i e p u e n r s s s s s F ------z s e - d g g r t t t t t r t e o i e l a i t h i i i i i o o i e a B N q S t s s s s s r l e t t n o a a a r i i b i i i B B B B B B b - - - - - s i e i h k p n n n n n n n n t t t i e a g a i r a r e e b e e e b e i v h R R R R R R g P P P P P a a a a a a a a e e e l i k t R h a i p u t n l u o l r r r r r r n n a n n n r r a o a m m m o o q p S P a A C H W W W W W W H W W W W W g l g g g g g g g g f g g g g g g c c i g t r g n p s a n t d g r r s r f n c s t d p a g n b r a b g e n s p p b p r a m g w i f g g g a g g g g g o n h m g g q g g g g g g s w g g h m m g g w g w w w w w g w X X X w w Y W X X X Y A X X X X X X X Y Y Y W W Y Y Y X X X X Y X Y Y Y Y Y W X Y Y o Y W t u s l n p s o i k a s s c t k u o s r c e r s t x n o s e x n n r t c i e o i n M i l i e o t t l i c i o s c p i i a e e i s p t t n t f u i d s i i u l m r s m a a n m t r u x n r n a o t n r g a o n i t e a g m i n r n l o a C r i i n C o i g p m c c c g s i i i d c T y i c u c c m h n n n n i i d i r a n a c t o o o o o n n n i t t t o g B n r n t o o C a I P c c - c a t t r c o e e e e c c s n h y e n a t t t t e t t i t - - t s e e v a v a i t t t t t u n u l o a g a g s s s e e n n l a t u e a e s o o o o r n y y a P L P e s p p p S s B D l g g n i r t d r t m n d p p n b l s g m g i g d g g g o p b Y X W Y A W X X X W Y W W M r e r b e m b e t e x i m l s e n s m l e o c e i t c a d y p s e i i c s m l n t i e c h i g n i c l m i o a r n i e d n c e - a i i n o y c o h a n l n k o s s c v y i n a h l i c c a c C d t n s o o c c c l r i c h i i a i t o d l - c c e a e o a r V o l l s i c i c t s n n n - i V n l o i t d v M c o o w p n i n m a a a k k i - c n u o a V r y k a r v u . a V V c c c e s e e n i i e v a a l l l o s a e t s c o o - c c m P a e c e e i a l c d r d i i o o o e e r l r . l r k r r e F b e c k n c o n v k k g r n n r l V V o c o o d G e C C a G U k u V a c a a a a a m y a o t r o C F V i t d n e e r r r r e c c L L v n h a b l l s c H o e e n a r m n s n k & e e e e l o i r i c c c t o o R r e e r w a o a i c i c n d d a s m i e e e p o p r R v v e a n g g r e g t t t t g n n o e C n n r l i i i d n i i e l u d u e l l l i c a a e p n t w a c c a i a a r b i a o c h s t t c i i a p c m l l i k f o o o i l g p e m c c r r r r l l d c s f i c l l s s l c e y y y a i i a f l l m a o h e i o i o o o a a m l a n a i a o o h h h u e e n P M M m o S P P P r r r t K K H C C v H f f E d B b b B v D a c i g v c v d r s v r r c a c v v v d l t f g l v v v g s k h e b i r h h t k p m c c b l b r p p m m m p r X d X Y Y X X X X Y W W X Y Y W Y X Y Y X Y X X Y X X X X X o W v e a p n t u o s e o s z r k k s t c s M G c c i c e o i i o r c n r i n u d a c n a q i c c i n l c a n l c n i c o a a l o t a c V o e l V c l c n V o a i o e v g n v k e n t a a r a l t a a t t a o a e L m a e h s c f m e w l a S m o g g b a d o h c t s a n a i t e t t f k r e V o r x c e i a o h o s o N m m m e P r C n v z m b v v v n l q m i c n m m p Y Y W Y X W W s M p u o r r e t s e n G a ) n b a r s y o e t n i e g i m t a i n n l o m n e e l i m a n o n h t o o u t p i n a o o e m t o c c a l m i n s i u i o t P y l r t n a t o n g o d ( r o m i a g o d a o s r h o n r t M n a t i m c e p t d F t e r o o a m a - s C m G . p d d r u n n a r e n n o i F p e d o C u S P s e e m p o - d o e a n o n o o t v . F r u n n n i m r o i i r t d - u n n m e F t k o n i r r F S o o o a i o m z s t d o - t U n i n i t o o a G r a v t a i o o t c i i h F s p t r t r d G r S n b n e d s o o d I n m g r i F a G d s s r e e d d u m n S a u o r C n t i i d o e G a I u e r e F m a r e n t e e n o l e a u n e I a a h s h h n m e l r d o l n u k i y - u r e s d c n n a H R e v m c g m c c a a i i i e i Q e s n c a e v F d n m c a k t o o G t I i n a r s s u i n a t l S s r p i n a i i s a r a o R o e m m l t o n F y s u n u e s n g o e t l z o p a s s i i n b e e i b a a t i t i l r f u i n o o w n r k o e a t t q p p c r a r i w e r r i - i i h o s i o r d d n n y e q n g n n e l t t i n g c m e v p p o e i c r n a i a u b n l c c l a e e v l n i a a u i a e e y r o o o e a h e o o i h t a i i n r a j n o r r u i i r y o o a r r M M O N P N I M v m M M m o S S A P C J C D F F C B q C T R b A C i b g n c c e a i f i i n u r s f w s h v q u h c j i f i s s m t a p c u u n c m i s q m f m d p m A o v c s g c b p m a m Y X Y X Y S Y X C m X O O Y Y Y O W W X C m X O X m X Y X X X X X X X W X i X W d s e s k s k s c c k i i c o c n r o d o r a . r e c d y l . y s r e r o s k M a a s a v t c t m a n o m n n t d r F e a a e e i r n n y g m c r e i m m i s s o n e i a v a v k d k s k i o t d d i c c o e t n o n e e y R n m d o s o d t m p a o s e i P r r a r r i o e e t a d t u n , h m a n n m d a y y y t O h e a o r i i a r r r t T r i a o n n v i o d R t i m e a e a a i m , c l l t e t t t n i c G r s f e F g i i d a i n d d n v n n i s o d n D z e v n e c o t n d e o e e n i F n u i m , o r i s u g t a a s d a e d a g m m m m u r n s G a L M h s i i i r l n r i i k g u l S i o a , c d d d O i a m e a u e O a & i k e V r d r Q y e R e e i r n L d - p c r e F r , t i o s s s a e l b e b g t x l f t h a e e e s d n a a a - d t a t e i k a u u t t t l v r p m o S n n w r n d t e h i n o o w e i e e t o o r p a i i i s o o a r i o c r m M L p e S R N L m m T s C W U C H F B i n a s r v u i i s v q r l i g m q m f n u s f u l l a h p w i n s m s a u m K m m m Y C X m X Y O X X O X W X X O O X W W X M

122 Craddock et al.,2006. 19th Annual Keck Symposium; http://keck.wooster.edu/publications flows suggest the rhyolite eruptive direction was Juda showed that the granite xenolith is the by- from the rift axis to the northwest. product of rift magmatism, with a U-Pb zircon age of 1094 ± 11 Ma, and not a floated piece Keweenaw Rift Granites (Natalie Juda, Matt of crust. The major and trace element Andring) geochemistry is similar to other rift granites and granophyres, and the lack of an AMS fabric The central portion of the rift, exposed along the in the host diabase, granite or basaltic is shore of in northeast Minnesota, representative of the chaos of the intrusion of is defined by the chaotic Beaver Bay igneous the Beaver Bay complex for a within-plate, low- complex (Fig. 1). Diabase is the dominant potassium granite. Andring determined that igneous rock that surrounds a host of granitic the radial granite dikes and adjacent diabase are and anorthositic xenoliths intruded by various also geochemically similar to other rift granites mafic dike swarms. We chose two end-member and granophyres and the regional diabase field relations, 1) a granitic xenolith (1 square composition. The AMS results for the granite km), cross-cut by a mafic dike surrounded dikes and diabase were also inconclusive, by diabase (Natalie Juda) and 2) diabase preserving random magmatic flow during surrounding and surrounded by a swarm of intrusion and crystallization. “circular” granitic dikes (Matt Andring).

[Northeast] Minnesota [E. Central] Wisconsin-Michigan

St. Peter Paleozoic Sandstones ~520 Ma Franconia Sandstone Regional Munising Sandstone

Bayfield Group Sandstones Post-Rift Sandstones Hinckley Sandstone Fondulac Sandstone (LF-B) Regional Unconformity ?? Jacobsville Sandstone

Keweenaw Rift Province Freda Sandstone Nonesuch Shale 1085 Ma Copper Harbor Conglomerate 1094 Ma Rift Rhyolites (RP, DP) & Regional Unconformity Interflow sediments Rift Basalts (granite xenolith; NJ & MA)

1110 Ma Puckwunge Sandstone (EW) Nopeming Sandstone (EW) Regional Unconformity Bessemer Sandstone

Rove Formation (LK) Thomson Formation (LK) Tyler Formation (BL)

~1800 Ma Pre-Penokean Orogen Animikie & Marquette Supergroups Re ~2000 Ma gional Unc onformity Denham Clastics & Volcanics (SV) Pokegama Quartzite Regional Unconformity Palms Formation (BL) >2700 Ma Archean Basement (Gneisses 3.6-3.0 Ga; Granite-Greenstone Terranes 2.7 Ga)

Figure 2: Generalized regional stratigraphic correlation diagram for the mid-continent region in Minnesota and Wisconsin. Student participants are identified by their initials next to their sample interval. Yellow stars indicate a detrital zircon sample completed by a student whereas white stars indicate a clastic unit that was processed for zircons but not reported here.

123 Craddock et al.,2006. 19th Annual Keck Symposium; http://keck.wooster.edu/publications

Detrital Zircon Ages and Provenance (Lee Underlying the rift is the Pre-Penokean Finley-Blasi, Erin Walker, Laura Kerber, Animike basin sequence with the correlative Becky Lundquist, Sarah Vorhies) Rove (northern MN), Thomson (Duluth) and Tyler (east, MI) Formations composed The southern is characterized of interbedded sand and shale (turbidites). by a series of NE-trending terranes with Laura Kerber studied the Thomson and Rove radiometric ages of ~2.7 Ga and a few Formations, both of which contain predominant protocontinental terranes with older (3.0- zircon populations of ~1850-2000 Ma and a 3.6 Ga) gneisses (Southwick and Chandler, variety of ages back to 3.5 Ga. The maximum 1996; Bickford et al., 2006). Deposited on, depositional age of the Thomson is 1841 Ma, and accreted to, the Archean craton are the and 1826 Ma for the Rove. foreland sediments and thrust sheets of the 1.8 Ga Penokean orogen, followed by the The Tyler Formation, east of the Keweenaw Yavapai (1.75 Ga), Mazatzal (1.65 Ga), and rift, was studied by Becky Lundquist, has a Grenville-Keweenaw (1.4-0.9 Ga) orogens maximum depositional age of ~1818 Ma, and (Karlstrom et al., 1999; Holm et al., 2005). We its zircon population is dominated by Penokean chose to address the age and provenance of ages and rocks from the underlying 2.7 Ga clastic sedimentary units in the adjacent and granite-greenstone belts. Lundquist also overlapping Penokean-Animikie and younger analyzed the Palms Formation at the base of the Keweenaw sequence where the regional Marquette Supergroup, which contains zircons stratigraphy is well-constrained but little is dating 2.7 Ga, a few at 3.4 Ga and the youngest known of the absolute depositional ages or zircon is 2594 Ma. provenance (Fig. 2; Ojakangas et al., 2001). In east-central Minnesota, overlying the Lee Finley-Blasi studied the post-Keweenaw Archean McGrath gneiss is the locally-exposed Fond du Lac and overlying Hinckley and enigmatic Denham Formation (Boerboom Sandstones. The older Fond du Lac Formation and Jirsa, 2001), which was deformed and (youngest grain: 1010 Ma) contains a significant metamorphosed to amphibolite facies during population of pre-rift zircons, presumably the (Holm and Selverstone, transported from the Grenville orogen to the 1990). Sarah Vorhies analyzed the basal, east, whereas the younger Hinckley sands metamorphosed arkosic sandstone and an upper (youngest grain: 1052 Ma) contain many older, arkose, finding zircon populations between 2.1- and locally-derived zircons. At the base of the 3.4 Ga and 2.7-3.4 Ga, respectively. Pillow Keweenaw rift lies the Nopeming (Duluth), basalts from the middle Denham Formation Puckwunge (N. Minnesota) and Bessemer (east yield an alkaline, rift-related geochemical side of the rift) Sandstones. signature.

Erin Walker analyzed the zircon populations of the Nopeming and Puckwunge Formations, References finding the former to contain a concentration Bickford, M.E., Wooden, J.L., and Bauer, R.L., 2006, of pre-rift (1.2 Ga) zircons, Penokean-aged SHRIMP study of zircons from Early Archean zircons (~1.8 Ga) and a group from 2.2-2.6 Ga. rocks in the Minnesota River Valley: Implications The Puckwunge contains many older, Archean for the tectonic history of the Superior Province: populations, but surprisingly no zircons younger Geological Society of America Bulletin, v. 118, no. than Penokean. 1/2, p. 94-108. Boerboom , T. J. and Jirsa, Mark A, 2001, Stratigraphy

124 Murphy, J. 2006. 19th Annual Keck Symposium; http://keck.wooster.edu/publications

of the Denham Formation; a continental margin assemblage of basalt, arkose, and dolomite:47th annual meeting, Institute on Lake Superior Geology, p. 6-7.

Cannon, W.F., Kress, T.H., Sutphin, D.M., Morey, G.B., Meints, J., and Barber-Delach, R., 1999, Deposits of the Lake Superior region: U.S. Geological Survey Open-File Report 97-0455.

Davis, D. W., and Green, J. C., 1997, Geochronology of the North American Midcontinent Rift in western Lake Superior and implications for its geodynamic evolution: Canadian Journal of Earth Sciences, v. 34, no. 4, p. 476-488.

Holm, D., and Selverstone, J., 1990, Rapid growth and strain rates inferred from synkinematic garnets, Penokean orogen, Minnesota: Geology, v. 18, p. 166–169, doi:10.1130/0091- 7613(1990)0182.3.CO;2.

Holm, D. K., Van Schmus, W. R., MacNeill, L. C., Boerboom, T. J., Schweitzer, D., and Schneider, D., 2005, U-Pb zircon geochronology of Paleoproterozoic plutons from the northern midcontinent, USA: Evidence for flip and continued convergence after geon 18 Penokean orogenesis: Geological Society of America Bulletin, v. 117, no. 3/4, p. 259-275.

Karlstrom, K.E., Harlan, S.S., Williams, M.L., McLellend, J., Geissman, J.W., and Ahall, K.I., 1999, Refining Rodinia: Geologic evidence for the Australia -Western U.S. connection for the : GSA Today, v. 9, no. 10, p. 1-7.

Southwick, D.L. and Chandler, V.M.,1996, Block and shear-zone architecture of the Minnesota River Valley sub-province: implications for late Archean accretionary tectonics: Canadian Journal of Earth Sciences 26, 2145-2158.

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