PALEOMAGNETIC AND ROCK MAGNETIC RESULTS

University of HelsinkiCorrespondence to:

Elisa Piispa - [email protected]

We thank K. Anderson, J. Havu, G. Lerner, R. Curganus, G. Bluth, C. Schepke, and J. Diehl for their

assistance with the field work. Support for this project was provided by the National Science

Foundation grant EAR-1149434 (to A. S.).

Fig. 1. Sampling sites of the BM dykes. Also included are the approximate

locations of BM dykes studied by Pesonen and Halls (1979). The inset shows the

sampling area (box) related to the Midcontinent Rift system (green shaded area).

PALEOMAGNETISM OF THE ~1.1 GA BARAGA-

MARQUETTE DYKES (MICHIGAN, USA)Elisa J. Piispa1, Marine S. Foucher1, Jeanine A. Chmielewski2,

Aleksey V. Smirnov1,2 and Lauri J. Pesonen3

The mean VGP poles calculated for the lower reversed section (MPlr1 and MPlr2),

the lower normal section (MPLn), the upper reversed section (MPUr), and the

upper normal (MPUn) section of the Mamainse Point lava flow sequence

(Swanson-Hysell et al., 2009). The other poles are from the Powder Mill basalts

(PM; Palmer and Halls, 1986), the Lower and Upper Osler Volcanics (OSr and

OSn; Halls, 1974); the Lower North Shore Volcanics (NS; Halls and Pesonen,

1982), the Marquette dike swarm (MQ; Pesonen and Halls, 1979), the Portage

Lake Volcanics (PLV; Halls and Pesonen, 1982); the Lake Shore Traps (LST;

Kulakov et al., 2013) and the Coldwell Complex (CCr and CCn; Kulakov et al.,

2014, in press).

1. Department of Geological and Mining Engineering and Sciences, Michigan Technological University, Houghton, USA

2. Department of Physics, Michigan Technological University, Houghton, USA

3. Department of Physics, Division of Geophysics and Astronomy, University of Helsinki, Helsinki, Finland

1. The paleomagnetic carrier is dominantly SD to PSD low-Ti subhedral

titanomagnetite with minor occurrences of pyrite, ilmenite, hematite

and maghemite (Fig. 3 and 4, see also poster GP43A-3624 by

Foucher et al.).

2. The majority of BM dykes yield steep reversed-polarity directions of

ChRM (Fig. 5a and 6a) indicating that they belong to the early stage

of MCR development.

3. The reversed dykes from Baraga and Marquette areas are

statistically distinguishable (Fig. 7a and b).

4. Two positive baked contact tests of the Marquette reversed dykes

support the primary origin of the magnetization (Fig. 5d).

5. Several dykes from Marquette area yield steep normal-polarity ChRM

directions (Fig. 5b and 6b), significantly different from the typical

direction exhibited by the other normally magnetized MCR

sequences.

6. In addition, a single mafic dyke from the Baraga basin with a recently

published U/Pb age of 1120±4 Ma (Dunlop, 2013) resulted in a

shallow normal ChRM direction (Fig. 5c). This direction is carried by

secondary hematite created by hydrothermal alteration (Fig. 4)

a) Steep reversed direction b) Steep normal direction

Fig. 5. Equal area and orthographic plots

representing the characteristic remanent

magnetizations (ChRMs) from the Baraga-

Marquette dykes. a) Typical reversed ChRM

directions. b) Steep normal ChRM direction,

c) shallow normal ChRM direction and d)

Two positive baked contact test for reversely

magnetized dykes. D: Declination, I:

Inclination, MAD: Maximum angular

deviation, Range: The temperature or AF

field range used for the ChRM calculation,

α95 : radius of confidence. On equal area

plots red/blue represent up/down directions.

L1F-1 Thermal

D = 124.1°I = -62.9°MAD = 1.5°

Range = 520°-560°C

L1F-2 AF

D = 122.7°I = -61.3°MAD =1.3°

Range = 20-60 mT

The development of the Midcontinent Rift System (MRS) is characterized by

multiple intrusions of diabase dyke swarms parallel to sub-parallel to the rift axis

(e.g. Green et al. 1987). The dykes are generally considered to be feeders to now

eroded lava flows once deposited on the flanks of the rift. We present new

detailed paleomagnetic and rock magnetic results from ~50 dykes exposed in the

Baraga-Marquette (BM) area of the Upper Peninsula of Michigan ,USA.

INTRODUCTION

GEOLOGY

Fig. 2. Field photos of BM

dykes. The older N-

polarity dykes are more

greenish diabase with a

salmon-colored

groundmass whereas the

younger R-polarity dykes

are dark grey diabase. a)

and b) N-polarity dykes

A11 and A7 on the shore

of Lake Superior in

Marquette, c) A positive

baked contact test site,

where a ~20 m wide R-

polarity dyke B16-A with

chilled margin has baked

the older B16-B dyke, d) A

narrow ~1m wide R-

polarity A3 dyke on Little

Presque Isle, Marquette,

e) A10 R-polarity dyke on

Sugar Loaf Mountain,

Marquette.

Fig. 7. Equal area plot showing combined mean paleomagnetic R- directions from

Baraga (a) and Marquette (b) dykes (circles – this study, triangles - Pesonen and

Halls (1979)). Dm, Im: mean declination and inclination, α95/A95: radius of

confidence for paleomagnetic direction/VGP, k: precision parameter, N: number of

dykes included in the mean calculation and S: angular dispersion of VGPs.

DISCUSSION AND CONCLUSIONS

The group mean directions do not share a common mean at the 99.98%

confidence level (McFadden and McElhinny, 1990). The corresponding

paleomagnetic pole plots close to the apex of the so called “Logan Loop”, a

segment of the Apparent Polar Wander Path (APWP) for the North American

continent for ~1000-1200 Ma (Fig. 8). The Baraga and Marquette dykes

represent two different emplacement episodes; the "paleomagnetic" age of the

Marquette dykes is likely to be around 1108-1105 Ma.

Fig. 8. Mean VGP

poles of the

Baraga (BR) and

Marquette (MQ)

dykes (open stars)

with selected

poles from the

MCR sequences.

The open/solid

symbols represent

reversed/normal

polarities,

respectively.

Baraga R dykes

Mean direction:

Dm = 105.7°Im = -77.7°α95 = 5.1°k = 90.14

N = 10

Mean Pole:

Plat = 47.69°Plong = 237.6°

A95 =9.03°S = 15 ± 2.4

Marquette R dykes

Mean direction:

Dm = 117.0°Im = -66.5°α95 = 2.8°k = 98.0

N = 27

Mean Pole:

Plat = 48.9°Plong = 210.8°

A95 = 4.1°S = 12.0 ± 2.6

ACKNOWLEDGEMENTSREFERENCES

The study area is located in the Upper Peninsula of Michigan in the Southern

Province of the Canadian Shield (Fig. 1). Three basins of metasedimentary and

metavolcanic rocks lie unconformably on a dominantly granitic basement of

Archean age. The E-W trending dykes vary in thickness from few centimeters to

40m (Fig.2). Cross-cutting relations suggest that there are at least two ages of

Keweenawan dykes in the study area.

GP43A-3638

Logan loop

Examples of the demagnetization behavior

A7B-1 Thermal

D = 23.3°I = 86.0°MAD = 3.2°

Range = 555°-580°C

Reflected light microscope images

BAR3-J Thermal

D = 300.3°I = 14.0°MAD = 4.7°

Range = 520°-550°C

c) Shallow normal direction

Site mean directions

Fig. 6. Equal area plot of the site-mean paleomagnetic directions calculated from

the BM a) steep reversed and b) steep normal dykes. The blue and red symbols

show down/up directions. Circles – this study, triangles - Pesonen and Halls

(1979). Dm, Im: mean declination and inclination, α95: radius of confidence for

paleomagnetic direction, N: number of dykes included in the mean calculation.

Fig. 4. Alteration

features in dykes that

carry a secondary

remanence. a)

Hematite radiance,

b), c) and d) examples

of altered

titanomagnetites.

Altered dykes

Fig. 3. Unaltered

titanomagnetite grains

in dykes that carry a

primary R-direction.

a), b) and c) Fresh

looking titano-

magnetites with

various exsolution

lamellaes, d) Skeletal

Ti-magnetite formed in

a chilled margin.

Fresh dykes

d) Baked contact tests

H3 dyke

D = 117.0°I = -68.9°α95 = 9.6°

H3 baked rocks

D = 126.5°I = -64.4°α95 = 4.3°

Half baked

Baked

Unbaked

No unbaked rocks available at this outcrop.

a) b)

Group Mean calculated

from the steep R directions

Dm = 115.2°Im = -69.6°α95 = 2.8°

N = 37

Group Mean calculated

from the steep N directions

Dm = 88.9°Im = 87.1°α95 = 7.0°

N = 7

• Green, J.C., Bornhorst, T.J., Chandler, V.W., Mudrey, M.G. Jr., Myers, P.E., Pesonen, L.J., Wilband, J.T., 1987. Keweenawan dikes of the Lake Superior region: evidence for evolution of the middle Proterozoic Midcontinent Rift of North America. In: Halls, H.C., Fahrig, W.F. (Eds.), Geol. Ass. of Canada, Spec. Paper 34, 289–302.

• Halls, H.C. (1974). A paleomagnetic reversal in the Osler volcanic group, northern Lake Superior. Canadian Journal of Earth Sciences, 11, 1200- 1207.

• Halls, H. C., and Pesonen L. J. (1982). Paleomagnetism of Keweenawan rocks, in Wold, R. J. and Hinze, W. J., eds., Geology and Tectonics of the Lake Superior Basin, Geological Society of America Memoirs, 156, 173–203.

• Kulakov, E.V., A.V. Smirnov, J.F. Diehl (2013). Paleomagnetism of ∼1.09 Ga Lake Shore Traps (Keweenaw Peninsula, Michigan): new results and implicationsCanadian Journal of Earth Sciences, 2013, 50, 1085-1096, 10.1139/cjes-2013-0003.

• McFadden P. L., and McElhinny, M. W. (1990). Classification of the reversal test in palaeomagnetism, Geophysical Journal International, 103, 725–729.

• Palmer K.C. and Halls H.C. (1986).Palmer, H.C., and Halls, H.C. 1986. Paleomagnetism of the Powder Mill Group, Michigan and Wisconsin: A re-assessment of the Logan Loop. Journal of Geophysical Research, 91, 11 571 - 11 580.

• Pesonen, L.J. and Halls, H.C., 1979. The paleomagnetism of Keweenawan dikes from Baraga and Marquette Counties, northern Michigan. Canadian Journal of Earth Sciences, 16, 2,136-2,149.

• Swanson-Hysell, N.L., Maloof, A.C., Evans, D.A.D. and Weiss, B.P. (2009). No asymmetry in geomagnetic reversals recorded by 1.1-billion-year-old Keweenawan basalts. Nature Geoscience, 2, 713-717, doi:10.1038/ngeo622.

B16 dyke

D = 112.3°I = -63.5°α95 = 5.4°