PALEOMAGNETIC AND ROCK MAGNETIC RESULTS University of Helsinki Correspondence 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. Piispa 1 , Marine S. Foucher 1 , Jeanine A. Chmielewski 2 , Aleksey V. Smirnov 1,2 and Lauri J. Pesonen 3 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, α 95 : 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)). D m ,I m : 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 : D m = 105.7° I m = -77.7° α 95 = 5.1° k = 90.14 N = 10 Mean Pole: Plat = 47.69° Plong = 237.6° A 95 =9.03° S = 15 ± 2.4 Marquette R dykes Mean direction: D m = 117.0° I m = -66.5° α 95 = 2.8° k = 98.0 N = 27 Mean Pole : Plat = 48.9° Plong = 210.8° A 95 = 4.1° S = 12.0 ± 2.6 ACKNOWLEDGEMENTS REFERENCES 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). D m ,I m : mean declination and inclination, α 95 : 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° α 95 = 9.6° H3 baked rocks D = 126.5° I = -64.4° α 95 = 4.3° Half baked Baked Unbaked No unbaked rocks available at this outcrop. a) b) Group Mean calculated from the steep R directions D m = 115.2° I m = -69.6° α 95 = 2.8° N = 37 Group Mean calculated from the steep N directions D m = 88.9° I m = 87.1° α 95 = 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° α 95 = 5.4°