PALEOMAGNETISM OF THE ~1.1 GA BARAGA- MARQUETTE … · Sampling sites of the BM dykes. Also...
Transcript of PALEOMAGNETISM OF THE ~1.1 GA BARAGA- MARQUETTE … · Sampling sites of the BM dykes. Also...
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°