1. Middle Proterozoic and The Mid-continent Rift
Rodinia, the Proterozoic supercontinent, was finally assembled ~ Ga by collision with proto-South America along the eastern edge of present-day North America
Development of a mantle plume beneath the modern Lake Superior and the upper peninsula of present-day Michigan caused thermal doming of proto North America (Laurentia), thinning of the crust, and rifting ~ Ga
The rift failed, for reasons not well understood, but not before intrusions of mafic igneous rock (gabbro) and eruption of mafic volcanic rock (basalt) filled much of the rift.
Deposition of siliciclastic sediment filled the remainder of the rift, prior to uplift and erosion at the close of the Precambrian
Deformation at the eastern margin of Laurentia (N.America) produced compressional forces, which reactivated rift-margin faults and exposed occurreences of copper deposits

2. Proterozoic Geologic History

3. Reconstruction of the Rodinia Supercontinent at time of Grenville Orogeny

4. Gravity Map for U.S.
Note the Mid-continent rift, which extends from the lower peninsula of Michigan, through theSuperior Basin, and south into Oklahoma

5. Precambrian Bedrock in Minnesota

6. Rifted Continent to Ocean Basin

7. Map of Mid-continent Rift
~2500 km long and km wide

8. Plume Head and Mid-Continent Rift
Note that the northern arm of the triple junction of the rift did not fully evolve.
Continued evolution of the rift was aborted by collision of Laurentia with Amazonia to form the Grenville Mountains

9. Map of Mid-continent Rift, Showing Age of Basement Rocks and Distribution of Basalt Lava Flows and Sedimentary Rocks

10. Map of Rock Units Filling Mid-continent Rift

11. Mid-Ocean Rift in Iceland

12. Red Sea is a Modern Analogue for the Mid-continent Rift

13. Rifting and Magmatic Activity
enormous volume of flood basalt erupted in rift from Ma
basalt sequences are as much as 30 km thick and older crust was nearly completely separated during synvolcanic extension.
isotopic signature consistent with onset of a new mantle plume beneath the area at about 1110 Ma.
plume head provided a hot source for production of basaltic magma by decompression melting.
Deposition of ~8 km thick mostly alluvial fan, river and lake sediments continued for a few tens of millions of years after volcanism and extension ceased. Some of this deposition overlapped basalts in time

14. Mantle Plume

15. Structure of a Thick Basalt Lava Flow
Margins cool more rapidly, center cools more slowly. Note the well-developed columnar joints at the top and bottom margins of the flow

16. Map of the Superior Syncline

17. Cross Section of the Superior Syncline
Weight of pile of basalt lava flows bows the crust downward

18. Duluth Complex and North Shore Volcanic Group
Note mineralization along northern margin of the complex

19. Relationship of Duluth Complex to North Shore Volcanic Group

20. Mid-continent Rift in the Vicinity of the Twin Cities

21. Cut Face Creek Lava Flow and Sandstone
Lava flow overlies reddish-colored sandstone deposited in the mid-continent rift
Sandstone contains grains of volcanic sand derived from nearby volcanic rocks at the margins of the rift valley

22. Gooseberry Falls State Park, North Shore Volcanic Group
Cascades tumble over edges of successive lava flows

23. Basalt Lava Flows, North Shore Volcanic Group
Lava flows are tilted southward toward center of Superior Syncline

24. Columnar Joints, North Shore Volcanic Group
Form when lava cools and shrinks
Cracks are polygonal (many-sided) and form perpendicular to the cooling surface

25. Columnar Jointing in Basalt Flows, North Shore Volcanic Group

26. Pahoehoe in Oxidized Basalt Flows, Temperance River, North Shore Volcanic Group
Ropy pahoehoe structures indicate fluid lava

27. Flow Boundaries Often Marked by Oxidized Zones, Temperance River, North Shore Volcanic Group

28. Pipe Vesicles Concentrated at Top of Flow, Sugarloaf Cove, North Shore Volcanic Group

29. Amygdules in Basalt Flow, North Shore Volcanic Group
Amygdules are mineral fillings of gas holes (vesicles) - they are NOT large crystals that form early in the solidification of the rock

30. Lake Superior Agates
Form as silica fillings in amygdules of basalt lava flows
Eroded from basalt and washed/abraded in the surf zone of Lake Superior
Transported southward by glaciers and outwash
Often found in gravel pits along the Mississippi River in SE Minnesota

31. Basalt Lava Flows at the Dalles of the St. Croix
The lava flows were exposed by a catastrophic flood from Glacial Lake Duluth down the St. Croix River Valley

32. Basalt Flow in Potholes, Dalles of the St Croix River
Note concentration of vesicles at top of flow

33. Copper Deposits in the Duluth Complex
mantle plumes rich in magnesium, chromium, copper, nickel, platinum, and relatively low in sulfur.
copper-bearing minerals are generally sulfides
when mafic rocks, such as the Duluth Gabbro, are intruded into crustal rocks, sulfur from the crustal rocks can combine with copper in the mafic magma to be deposited as copper sulfides
overlying lava flows are then depleted in copper
to expose the copper deposits, uplift of the deeper intrusions along faults is needed
compression during the final assembly of Rodinia caused uplift along the faults associated with the rift, exposing the copper deposits.

34. Copper and Iron Sulfides at the Northern Margin of the Duluth Complex

35. Map of Copper Sulfide Deposits in the Lake Superior District