The Penokean Collisions

The Penokean orogeny was a major early Proterozoic mountain building episode that began soon after the rifting episode 2.4 bya. We see the roots of this ancient line of mountains today as a zone of deformed Archean and early Proterozoic rocks along the southern edge of the Superior Province from Minnesota eastward through the north shore of Superior. The orogeny began with the passive margin sedimentation after a rifting event along the southern edge of the Superior province. These sediments include the Iron Formations, that were subsequently deformed by the collision of an island arc around 1.85 bya. A second crustal body collided into the island arc around 1.84 bya. The Penokean orogeny closed with some post tectonic magmatism. Accretion of these terranes to the Archean craton showed that plate tectonic styles had evolved further, and were now behaving in more of a modern Wilson cycle fashion. The Penokean orogeny assembled the last major crustal pieces in the Great Lakes region that have been subsequently modified by major events such as the Wolf River Batholith, and the Mid-continental Rift system.

The Penokean orogeny began with rifting of the southern edge of the Archean craton along an east west line roughly parallel to the GLTZ, which you may remember is the suture between the Minnesota River valley subprovince and the Superior province.

As the rifting sequence continued, the newly created southern margin of the Superior province subsided allowing seas to encroach, depositing sediments such as the Iron Formations, other sand and gravels, and smaller amounts of dolomite. Rifting created many sedimentary basins that collected these types of deposits. The largest basins are the Animikie series in Minnesota, The Marquette Range Super group in northern Michigan and Wisconsin, and Huronian Supergroup in eastern Ontario. These basins really are basins an gentle sloping continental shelves on the sea floor that have filled in with sediment in a particular way that even after the sediment is compressed, heated and twisted, you can see enough of the original characteristics to be quite certain of its origin. These Rock assemblage names and their exact interpretations have changed remarkably over the last 100 years. The following descriptions are quite general as to avoid confusing the reader with nomenclature.

The passive margin sediments in the Great Lakes region have two general starting times of deposition, indicating rifting started unevenly. The Huronian Supergroup to the east began early, around 2.5 bya, and continued for 650 million years until 1.85 bya. The Marquette and Animike groups started later around 2.2 bya, and also continued up till roughly 1.85 bya. The Huronian Supergroup is a general name given to a number of sedimentary assemblages that have been subdivided into an upper and lower sequence, between which lies a regional angular unconformity. These are generally marine deposited sediments showing cyclicity in depositional environment from conglomerates through silt stones and back, Iron Formations are also found in these strata. The lower sequence has been subdivided further into the Elliot Lake, Hourg Lake, and Quirke Lake groups. These groups are bounded by faults associated with the rifting event, and are some of the first sediments in the area. This is an example of one of the pieces of evidence used by geologists to compile the geologic history of this region. Another comes from a bed of volcanic rocks interlayered with these early sediments that contains zircons that give a direct date of approximately 2.45 bya. The date given by these volcanics is a lower limit, or the earliest general time of the opening of the rift.

The Marquette Range Supergroup has similar marine sediment characteristics, such as conglomerates, bedded silts, sands, turbity currents, and Iron formations. These sediments are divided into four assemblages as well, the oldest is the Chocolay, the next are the Menominee, Baraga, and the Painted River. The Chocolay group, in one locality, has at its base gravels and conglomerates that were originally interpreted to be glacial deposits, but in more recient studies have been interpreted to be from rifting. An unconformity lies on top of these gravels, separating them from the rest of the Chocolay, which shows the characteristics of a stable passive margin. Another unconformity lies between this and the Menominee group that contains a sub-formation called the Hemlock made up mostly of volcanic rocks that give a date of 1.91 bya.

The Animike series in Minnesota contains the Mille Lacs and North Range which are the oldest, and Animike Group which unconformably overlies the previous two, and extends farther to the north onto the Archean craton. These formations are very poorly exposed and only known in modest detail from geophysical methods such as seismic and magnetic, as well as drill cores. Until the mid 1970šs, the Animike, which holds a majority of the iron ore producing Iron Formations, was the only recognized group in the area. It seemed to be the beginning of Proterozoic sedimentation sitting unconformably on Archean rocks. Recent work has shown that there are earlier sequences, now called the Mille Lacs and North Range, that underlie the Animike. These sequences are very hard to interpret due to the poor exposure and the tectonic rearrangement during later Penokean events.

Sedimentation styles of the passive margin changed as they came to a close. The sedimentary environment we see recorded near the end in these rocks changed from deep water shales derived from Archean rocks, to coarser clastic rocks derived from a younger Proterozoic source. This change is interpreted to be from the Island Arc as it closed in on the passive margin from the south just before its collision with the passive margin. Sediments that were shedding off of the Island Arc piled on top of the previously deposited passive margin sequences in a style called forestepping, where the younger sediments are deposited farther and farther inland on the stable craton as sediments are still being shed from the craton.

This island arc was the first of the two collisions involved in the Penokean orogeny. This Island Arc, termed the Pembine-Wausau terrane, collided into the Archean Superior provinces passive margin around 1.85 bya. Geographically, the Pembine-Wausau terrane extends 75 km to the south of, and 175 km east west along the suture line between this terrane and the Superior province called the Niagra fault zone. The Pembine-Wausau terrane was produced by a southward subduction zone on top of oceanic crust, and is composed mainly of volcanic and plutonic rocks that are broken down into two intervals. The rocks in the northern part tend to be older, deposited around 1.9-1.86 bya , and draw an analogue to modern island arcs in their chemical composition and style of deposition. The rocks in the central and southern part have a range of compositions similar to back-arc basin volcanism, and are mostly younger with ages of 1.845-1.835 bya.

The older 1.9-1.86 bya interval consists of metamorphosed basalt, andesite, dacite flows. Early rocks in this sequence are typified by the tholeiitic basalts and andesites of the Quinnesec formation, which give a general date of 1.9 bya. This date is a minimum, and there quite possibly are older rocks, but they are obscured. These rocks are similar to present day island arcs of the western Pacific ocean. In general Island arc rocks come from magmas derived from oceanic plate material that has been reheated during subduction under another oceanic plate. Oceanic plates are created and destroyed quite quickly in geological terms, so the rocks island arcs are made are not very old. Island arcs also tend to be short lived bodies that accrete onto a larger continental body before long because they sit above subduction zones that sooner or later trys to subduct a continent. The older portion of the Pembine-Wausau terrane has these characteristics, it contains no Archean derived igneous rocks and because of the homogeneity in rock ages, probably developed less than 75 million years before its suturing onto the Superior Province.

One of the ways these rocks have been shown to resemble modern island arc and back arc basins, is by petrologic studies that show a low FeO and TiO enrichment, and low rare earth element depletion. What this is telling you is the relative amount of differentiation that the rock in question has gone through from the mantle, a basic igneous source. In general, when you reheat rocks, minerals that make up a majority of continental crust tend to melt first, allowing them to migrate away leaving their host behind. When this happens, it changes the relative amounts of elements that make up the minerals in the rocks. When you analyze these rocks, you see some elements increase in the rock, and others decrease. island arcs from magmas derived from the melting of subducted oceanic crust piling up on oceanic crust. Since oceanic crust is basically one step above the mantle, you end up with relatively low amounts of enrichment. In the case of our suspected island arc, it fits the model of low enrichment.

Later Plutons of tonalite and granodorite intruded the earlier rocks just before collision with the craton from 1.87 to 1.855 bya. Volcanic rocks from andesite to rhyolite continued to deposit onto the island arc. As the volcanic rocks get younger in this region, they become more differentiated. This is due in part to the absence of fresh magmas from the subducting oceanic plate, besides the normal differentiation of partially melted magmas.

The younger 1.855-1.835 bya interval of rocks are deposited in the central and southern portions of the Pembine-Wausau terrane. These rocks were bi-modal, in other words both rocks like basic basalts, and differentiated rocks like rhyolites, were being deposited at the same time. These volcanic rocks have a wide range, but are generally higher in the amount of silica ranging from andesite to rhyolite, and are moderately enriched in rare earth elements as well. Due to specific petrologic evidence in these rocks, this region is presently thought of as a back arc basin, which is complementary to the island arc. This assemblage of rocks has been subjected to a lower grade of metamorphism.

The style and amount of deformation that went on shows that tectonics had evolved further from the Archean, and the crust was now behaving as modern crust does. Modern styles involve large rigid bodies of continental crust colliding and producing particular patterns of folds, faults and rock types that could have only have been produced with the collision of large bodies of crust. The most obvious is a semicontinuous major Niagra fault zone that runs for hundreds of miles, from western Minnesota through Wisconsin into central upper Michigan. This continuous fault zone indicates large sections of continental crust were involved. This is significantly different to the small blocks of crust seen in the Superior province. The first and second collision deformed a large band of rocks along the line of the suture. The main Niagra fault zone plane dips shallowly to the south showing the Pembine-Wausau terrane was thrust up on to the craton from the south. As the two opposing blocks collided, the craton was partly subducted under the Pembine-Wasuau terrane causing, some of the sedimentary rocks of the passive margin to be folded, and compressed to the north up onto the craton along with rocks from the Pembine-Wasuau terrane. During the collision, large blocks of gneissic Archean rocks from the subducted edge of the craton were brought up in the turbulence, and can be seen to the south of the Niagra fault zone. The Niagra fault now is a zone of ductile deformation indicating what we see today was formed at a considerable depth, near the roots of the colliding terranes. The overlying 10 km of rocks have been eroded off since.

This orogeny lacks an ophiolite sequence, which is a captured piece of oceanic crust. This is an important piece of evidence in the case of modern subduction collisions. In modern collisions, we see a mangled wedge of oceanic crust sandwiched between the two opposing terranes. This happens as the colliding terrane buckles the oceanic crustal edge of the other terrane just before collision, and drives it up onto the subducting terrane during the rest of the collision. The Penokean orogeny lacks an ophiolite sequence. Some workers believe one exists as a narrow discontinuous band of highly metamorphosed rocks in the Niagra fault zone itself. This however is not an ophiolite sequence.

The second collision involved a mature microcontinent, containing Archean gneisses and early Proterozoic plutonic rocks, into the southern edge of the Pembine-Wausau terrane. The direction of the subduction zone is in some debate because there is little exposure of a suture zone between the Pembine-Wausau terranes, and the microcontinent. It is thought to most likely have been subducting to the north, due to the presence of later volcanic in the range of 1.845 to 1.835 bya.

The full extent of this microcontinent body is not known, but is suspected to reach into northern Illinois. This microcontinent is called the Marshfield terrane and shows a long history of development. The complexity of this history can be demonstrated by an exposure near Stevens Point Wisconsin. The first preserved rocks in this region are at least 3.0 billion year old Archean gneisses which are very minor on the exposed surface, but play a larger role in interacting with magmas at greater depths that produced later intrusions. These Archean rocks were made into gneisses by the first wave of plutonic intrusions around 2.8 bya. These rocks tend to be tonalites that have been deformed themselves three or four times since. Later emplacement of amphibolite dikes, probably during the Penokean orogeny, cut the earlier rocks, and are followed later by some minor diabase, apilite and pegmatites.

After Active plate motions ceased in the Great Lakes region by 1.84 bya, a post tectonic suite of mainly granitoid rocks intruded the cooling crust. These rocks have a high degree of differentiation, because they have a high iron content and are very sodic. These rocks intruded the Pembine-Wausau terrane mostly, but minor other intrusions exist elsewhere. This closes the Penokean orogeny, and leaves the area as a stable craton with eroding mountains, and a sea far to the south.