Middle Proterozoic and The Mid-continent Rift Rodinia, the Proterozoic supercontinent, was finally assembled ~ 1.3-1.0 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 ~1.2-1.1Ga 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

Proterozoic Geologic History

Reconstruction of the Rodinia Supercontinent at time of Grenville Orogeny

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

Precambrian Bedrock in Minnesota

Rifted Continent to Ocean Basin

Map of Mid-continent Rift ~2500 km long and 150-200 km wide

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

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

Map of Rock Units Filling Mid-continent Rift

Mid-Ocean Rift in Iceland

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

Rifting and Magmatic Activity enormous volume of flood basalt erupted in rift from1108-1094 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

Mantle Plume

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

Map of the Superior Syncline

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

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

Relationship of Duluth Complex to North Shore Volcanic Group

Mid-continent Rift in the Vicinity of the Twin Cities

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

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

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

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

Columnar Jointing in Basalt Flows, North Shore Volcanic Group

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

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

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

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

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

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

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

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.

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

Map of Copper Sulfide Deposits in the Lake Superior District