Mamainse Point Research Articles

Temporal evolution of mantle temperature and lithospheric thickness beneath the ∼1.1 Ga Midcontinent Rift, North America: Implications for rapid motion of Laurentia

The production of the magma volumes necessary for the formation of continental flood basalts is thought to result from extension-related decompression melting of thermo-chemically anomalous mantle. The relative importance of elevated mantle temperature or decompression associated with lithospheric thinning remains difficult to constrain because the temporal relationship between the initiation of extension and continental flood basalt is often ambiguous. The ∼1.1 Ga Midcontinent Rift (MCR) in North America provides an opportunity to probe these magma generation factors because the mantle thermo-chemical anomaly (Keweenaw plume) thought responsible for the continental flood basalts of the Keweenaw Large Igneous Province (LIP) intersected an existing rift. The role of elevated mantle temperatures in Keweenaw LIP magmatism remains ambiguous because a temporal decrease in MgO content in primitive lavas may reflect decreasing melting depths with time due to progressive lithospheric thinning, a temporal decrease in mantle temperature, or both. Furthermore, paleomagnetic studies show that Laurentia was moving rapidly during Keweenaw LIP magmatism, raising the question of how the plume remained connected to the MCR during 25 Myr of volcanism. Using samples from the most complete volcanic section preserved at Mamainse Point, Ontario, Canada, we constrain mantle potential temperature and lithospheric thickness by combining three major element thermobarometry models with forward modeling of rare earth element compositions. We find that the earliest stage of Keweenaw LIP magmatism (Early phase; 1109-1104 Ma) was governed by initially deeper melting (>85-100 km thick lithosphere) and high mantle potential temperatures (>1480-1630°C) related to the plume. Potential temperatures decreased to near-ambient conditions (∼1400-1445°C) during the latter portion of the Early phase, with these lower temperatures persisting throughout the Latent (1104-1098 Ma) and Main (1098-1090 Ma) phases. Major element-derived pressures of melt equilibration place broad constraints on lithospheric thickness, which decreased from ∼60-110 km at the onset of magmatism to 45-65 km by the Main phase. The decrease in primitive MgO contents thus reflects thinning lithosphere and decreasing mantle potential temperatures. We argue that the temporal decrease in mantle potential temperature reflects the severing of the connection between the MCR and Keweenaw plume after the Early phase of magmatism by the rapid plate motion. Our constraints on mantle potential temperature resolve the relationship between the MCR, the plume, Keweenaw LIP magmatism, and rapid plate motion, and indicate that an alternative mechanism, such as enhanced small-scale convection due to rapid plate motion and plume-lithosphere interaction, was required to generate the voluminous Main magmatic phase.

Mesoproterozoic porphyry copper mineralization at Mamainse Point, Ontario, Canada in the context of Midcontinent rift metallogeny

The Jogran copper prospect in southern Ontario, Canada, is located on the northeastern shoulder of the North American Midcontinent rift system where it is centered on two small, closely spaced quartz monzonite porphyry intrusions emplaced into Neoarchean mafic metavolcanic rocks. Notwithstanding an atypical alteration zoning pattern, the distinctive alteration styles and veinlet types confirm its porphyry copper affiliation. Two samples of molybdenite from chalcopyrite-bearing quartz veinlets in the two centers returned Re-Os ages of 1101 ± 5 and 1094 ± 5 Ma, showing that the mineralization took place during the main stage of extension and magmatic activity that gave rise to the Midcontinent rift and associated large igneous province (LIP). The nearby copper-bearing Tribag breccia pipes and Coppercorp veins have been shown previously to also be intrusion related, and likely formed penecontemporaneously with Jogran. The Jogran and nearby copper mineralization overlapped temporally with generation of low-grade magmatic copper-nickel-PGE deposits in the Duluth intrusive complex but predated the strata-bound native copper and chalcocite deposits in the Keweenaw Peninsula, which are believed to have formed from basinal fluids during rift inversion. Although such magmatic and strata-bound copper deposits typify intracontinental rift metallogeny, porphyry copper deposits are normally confined to convergent margin magmatic arcs, thereby making the rift-LIP setting of Jogran exceptional and possibly unique.

Significance of the metasomatized lithospheric mantle in the formation of early basalts and Cu – platinum group element sulfide mineralization in the Coldwell Complex, Midcontinent Rift, Canada

A diverse suite of tholeiitic to alkaline basalt and gabbroic intrusions located in the Coldwell Complex on the northern margin of the Midcontinent Rift exhibit unusual trace element signatures that show enriched large ion lithophile elements and light rare earth elements with negative Nb and Zr anomalies. These features are not typical of magmas derived by partial melting within or above a rising mantle plume, as might be expected in an early Midcontinent Rift magmatic event. In this paper, we provide a detailed geochemical study of a 500 m thick sequence of metabasalt that represents the earliest stage of magmatism in the Coldwell Complex. We show that contamination or crystallization processes or subsequent metasomatism cannot explain the trace element variations. Instead, we propose partial melting in a metasomatized Subcontinental Lithospheric Mantle source to explain the decoupled behavior of large ion lithophile elements from light rare earth elements and heavy rare earth elements and rare earth elements from high field strength elements and the enriched Nd isotope signature of metabasalt. Similar features occur in unit 5b of the Mamainse Point Volcanic Group located at the northern margin of the Rift. An objective of this paper is to relate Two Duck Lake gabbro, host rock for low-sulfur, high precious metal sulfide mineralization at the Marathon deposit, to the metabasalt sequence. The excellent match of trace element abundances in Two Duck Lake gabbro to metabasalt unit 3 confirms an early Coldwell Complex age for metabasalt and a Subcontinental Lithospheric Mantle source for Cu – platinum group element mineralized gabbros.

Geochemical Stratigraphy of the Keweenawan Midcontinent Rift Volcanic Rocks with Regional Implications for the Genesis of Associated Ni, Cu, Co, and Platinum Group Element Sulfide Mineralization

The development of the North American Midcontinent rift can be understood in the context of a tectonic and magmatic event in which the far-field effects of continental drift influenced magmatism produced by a mantle plume. We report and interpret new data for samples collected from stratigraphically controlled sequences of basaltic rocks in the Osler Volcanic Group and the Mamainse Point Volcanic Group in Ontario, Canada. We confirm that the earliest phase of rift development involved primitive melts that produced basalts in the Lower Formation of the Osler Volcanic Group with Fo74–80 olivine (1,490–2,280 ppm Ni). The earliest basalts are primitive in both the Mamainse Point Volcanic Group and Osler Volcanic Group sections, with elevated MgO, Cr, and Ni; these rocks also have high TiO2 and high Gd/Yb, which are features that indicate low degrees of melting of a garnet-bearing lherzolite mantle. The Central Formation A and B basalts of the Osler Volcanic Group that overlie the Lower Formation lavas record an upward increase and then decrease in the degree of contamination of basaltic rocks with normal Ni and platinum group element (PGE) abundance levels that require no sulfide saturation event to accompany contamination. The Upper Formation of the Osler Volcanic Group is characterized by a limited number of PGE-depleted flows; although these basalts are highly fractionated and typically develop textures with stellate to laminated plagioclase feldspar, they are usually not significantly contaminated. The Lower and Upper formations of the Mamainse Point Volcanic Group show a diversity in geochemistry consistent with contributions from different mantle sources, but contamination is restricted to a small number of flows with normal Ni and PGE abundance levels. There is no indication of a regional metal depletion signature on the scale of the Nadezhdinsky Formation at Noril’sk, but there is evidence that local S saturation events were triggered in the unexposed conduits feeding some of the upper sequence flows in response to fractionation of the magma. Massive to heavy disseminated Ni-Cu-PGE sulfide mineralization in the Midcontinent rift is hosted by small primitive intrusions that were formed early in the evolution of the rift and are typically associated with transtensional structures in the cratonic blocks adjacent to the main rift. Examples include the Eagle deposit in Michigan, the Tamarack mineralization in Minnesota, and the Current Lake Complex in Ontario. The American examples are associated with splays of the Great Lakes tectonic zone and the Canadian example is associated with the Quetico fault zone. The intrusions share the geochemical fingerprints of the more primitive lowermost volcanic rocks of the Midcontinent rift and they are broadly contemporaneous with the development of the lower stratigraphy of the Osler Volcanic Group and Mamainse Point Volcanic Group. Although the low-grade but large-tonnage Cu-Ni-PGE deposits of the Duluth were formed significantly later than the small primitive intrusions, their host rocks share the same lithophile trace element signature as these early intrusions. The parental magmas that produced all of these intrusions were derived from deeper mantle depths, where garnet was a stable phase, than the magmas that produced the Central Formation basalts and some of the Upper Formation basalts investigated in this study.

Paleomagnetism of the ~1.1 Ga Coldwell Complex (Ontario, Canada): Implications for Proterozoic geomagnetic field morphology and plate velocities

AbstractWe report new paleomagnetic data from the ~1108 Ma intrusive Coldwell Complex (Ontario, Canada) to investigate the apparent reversal asymmetry observed in some Midcontinent Rift (MCR) rocks. The rocks of eastern and central part of the complex are reversely magnetized with a group mean direction of D = 114.8°, I = −63.7° (α95 = 3.6°, N = 30). The corresponding paleomagnetic pole at Plat = 47.2°N, Plong = 206.5°E (A95 = 4.8°) is located close to the paleomagnetic poles from nearly coeval reversely magnetized rocks of the MCR system, including the lower lava flows at Mamainse Point. The rocks of western part of the complex are normally magnetized with a group mean direction (D = 298.0°, I = 56.9°, α95 = 5.8°, N = 10) that passes the reversal test with respect to the reversed group mean direction. Our results do not support the previous model in which the complex was emplaced during two periods of reversed geomagnetic field polarity separated by a period of normal polarity and hence encompasses two geomagnetic reversals. Instead, our new data indicate that the Coldwell Complex records only two polarity intervals separated by a symmetrical reversal at ~1102–1105 Ma. This reversal is likely equivalent to the lowermost reversal recorded at Mamainse Point and provides further evidence that the apparent reversal asymmetry reflects a plate motion rather than a persistent nondipole field geometry. Together with a high‐quality data from the ~1098 Ma North Shore Volcanics, our data indicate a rapid velocity of Laurentia at ~25 ± 4 cm/yr. The fast plate motion may reflect a decreased mantle drag due to vigorous mantle indicated by widespread intraplate magmatism at ~1.1 Ga.

Magmatic activity and plate motion during the latent stage of Midcontinent Rift development

Research Article| June 01, 2014 Magmatic activity and plate motion during the latent stage of Midcontinent Rift development Nicholas L. Swanson-Hysell; Nicholas L. Swanson-Hysell 1Department of Earth and Planetary Science, University of California–Berkeley, Berkeley, California 94720, USA Search for other works by this author on: GSW Google Scholar Seth D. Burgess; Seth D. Burgess 2Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA Search for other works by this author on: GSW Google Scholar Adam C. Maloof; Adam C. Maloof 3Department of Geosciences, Princeton University, Princeton, New Jersey 08544, USA Search for other works by this author on: GSW Google Scholar Samuel A. Bowring Samuel A. Bowring 2Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA Search for other works by this author on: GSW Google Scholar Geology (2014) 42 (6): 475–478. https://doi.org/10.1130/G35271.1 Article history received: 06 Nov 2013 rev-recd: 10 Feb 2014 accepted: 12 Feb 2014 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Nicholas L. Swanson-Hysell, Seth D. Burgess, Adam C. Maloof, Samuel A. Bowring; Magmatic activity and plate motion during the latent stage of Midcontinent Rift development. Geology 2014;; 42 (6): 475–478. doi: https://doi.org/10.1130/G35271.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The Keweenawan Midcontinent Rift of North America records significant continental rifting between ca. 1110 and 1085 Ma, and preserves the most detailed paleomagnetic record of plate motion of any continent in Precambrian time. U/Pb dates from extrusive and intrusive rocks of the western Lake Superior Basin suggest a latent stage of reduced magmatic activity from ca. 1106 to 1100 Ma that places constraints on the dynamics of rift development and the record of plate motion. However, it has remained unclear whether this stage is a feature of the entire >2500-km-long rift. The succession of picritic and basaltic lava flows at Mamainse Point in the eastern Lake Superior Basin may be the most continuous and best exposed record of rift-related volcanism and magnetic reversals, but its age and duration relative to the latent stage has been uncertain due to a lack of radioisotopic dates. We present a weighted mean 206Pb/238U date of 1100.36 ± 0.25 Ma on zircon crystals isolated from a newly discovered tuff within the upper reversed polarity portion of the stratigraphy below the Great Conglomerate. This date indicates that eruptive activity at Mamainse Point continued during the interval of diminished magmatic activity in the western Lake Superior Basin. This result strengthens the chronostratigraphic framework of rift development while explaining the preservation of additional geomagnetic reversals at Mamainse Point and the record of progressively decreasing paleomagnetic inclination that is indicative of rapid paleogeographic change. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Self-reversed magnetization held by martite in basalt flows from the 1.1-billion-year-old Keweenawan rift, Canada

In some basalt flows of the 1.1-billion-year-old Keweenawan Rift exposed at Mamainse Point, Ontario, there is a magnetic phase that holds a remanence antiparallel to the populations of magnetite and hematite that are typical of flows in the succession. The paleomagnetic and geological context of this component demonstrates that it is not a chemical overprint whose remanence is attributable to a subsequently reversed geomagnetic field, but that the component is a self-reversal of the primary magnetization. Here we use rock magnetic experiments and Mössbauer spectroscopy to show that this phase occurs in the most oxidized flows and is held by a fine-grained population of hematite that acquired its self-reversed remanence through interactions with a phase of lower blocking temperature. We propose that this self-reversed hematite is a manifestation of the self-reversal phenomena that has been observed to occur experimentally during the transformation of maghemite to hematite and that has been attributed to negative exchange coupling across the crystal lattices. We suggest that hematite formed in association with iron-silicates in the basalt flows carries a remanence reflective of the field in which it formed, but that martite (hematite pseudomorphed after magnetite) which formed through the progressive oxidation of magnetite to maghemite with subsequent inversion to hematite carries the self-reversed remanence. This study marks the second time that a naturally occurring self-reversed magnetization has been attributed to this mechanism and marks the oldest reported instance of any type of self-reversed remanence. The martite that forms through this process has the ability to retain a record of the negative exchange coupling from the time of its formation for hundreds of millions of years.

Chemostratigraphy of the Neoproterozoic Alona Bay lavas, Ontario

A basal sequence of flood basalt lavas associated with the Neoproterozoic Midcontinent rift system crops out in Alona Bay along the southeastern shore of Lake Superior in Ontario. The Alona Bay lava succession is about 1200 m thick and lies just north of the well-studied, contemporaneous Mamainse Point Formation. Detailed chemostratigraphy of the Alona Bay lavas suggests they are grossly correlative with the basal portion of the Mamainse Point Formation. For instance, like the basal part of Mamainse Point Formation, the Alona Bay section contains numerous high-MgO lavas and can be subdivided into 4–5 groups with distinct chemical characteristics. Chemical variations within the Alona Bay groups are largely the result of fractional crystallization, likely at moderate pressures. One small group of Alona Bay lavas also carries the compositional imprint of crustal contamination. The remaining inter-group chemical distinctions at Alona Bay are the consequence of temporal changes in partial melting and source character. With time and development of the Midcontinent rift, degrees of melting increased; mean pressures of melting decreased, reducing garnet control; and lithospheric source contributions waned. Similar temporal variations during flood basalt evolution have been documented elsewhere.

Magnetism of the Keweenawan Age Chengwatana Lava Flows, northwest Wisconsin

The Chengwatana Volcanics are subaerial plateau basalts which appear to be stratigraphically above the North Shore Volcanic Group of the Lake Superior Region. Sixty five oriented cores were collected at six sites near St. Croix Falls, Wisconsin. Magnetic cleaning of the samples was primarily step‐wise alternating field (A.F.) demagnetization to 100 mT. Thermal demagnetization and hysteresis property measurements indicate the dominant magnetic mineralogy is multidomain magnetite. However, a stable remanence is present which we attribute to grains with single domain behavior. The mean remanent direction is D=287.5°, I=45.8°, α‐95=8.0°, with a mean pole at 186.1°E., 30.9°N. This result is consistent with the 1.1 Ga. apparent pole for the North American craton. Two sites mapped as stratigraphically equivalent show distinct reversed directions. Both A.F. and thermal demagnetization isolate the same reversed direction which is interpreted as being primary. These particular flows record a short reversal in an otherwise normal sequence. This strengthens the evidence for more than one reversal in the Middle Keweenawan, and may be the same short reversal noted at Mamainse Point.

Re-Os isotopic compositions of Midcontinent rift system picrites: implications for plume – lithosphere interaction and enriched mantle sources

Picrites and tholeiites from the Mamainse Point Formation, a 5.3 km thick section of Keweenawan (1100 Ma) volcanic and sedimentary fill on the eastern flank of the central portion of the Midcontinent rift system, contain a nearly continuous record of rift magmatic activity. Picrites occur primarily in the lowermost two units of the formation. In this study, they are compared to rarely exposed, slightly older Keweenawan basalts from the North Shore Volcanic Group and the Powder Mill Group to constrain mantle source compositions during early phases of rift magmatic activity. The most primitive picrites analyzed have low Re content (0.069–0.18 ppb), high Os content (0.8–2.1 ppb), and low 187Re/188Os (0.28–1.18). A Re–Os isochron with an age of 1128 ± 54 Ma and an initial 187Os/188Os of 0.1267 ± 0.0013 (γOs = +5.7) was obtained from a 24-point isochron on all but two analyzed samples. The Re–Os data, regressed separately for the older basalts, and the groups 1 and 2 samples from the Mamainse Point Formation, have barely resolvable initial 187Os/188Os that decrease up-stratigraphy from initial γOs(1100) of +12.2 to +6.2 and +4.2, respectively, and couple with changes in initial Nd isotopic composition. These data can be explained by mixing of melts of an enriched mantle plume and unradiogenic continental lithospheric mantle. A radiogenic initial Os isotopic composition (γOs of +8 or higher) for the Keweenawan plume marks the first known appearance of demonstrably radiogenic plume-derived magmas on Earth. Plume-derived magmas with radiogenic Os signatures are more common later. The radiogenic Os signatures of Keweenawan plume magmas may mark the appearance of melts derived from mantle containing recycled slab components from late Archean subduction.

Rift-wide correlation of 1.1 Ga Midcontinent rift system basalts: implications for multiple mantle sources during rift development

Magmatism that accompanied the 1.1 Ga Midcontinent rift system (MRS) is attributed to the upwelling and decompression melting of a mantle plume beneath North America. Five distinctive flood-basalt compositions are recognized in the rift-related basalt succession along the south shore of western Lake Superior, based on stratigraphically correlated major element, trace element, and Nd isotopic analyses. These distinctive compositions can be correlated with equivalent basalt types in comparable stratigraphic positions in other MRS localities around western Lake Superior. Four of these compositions are also recognized at Mamainse Point more than 200 km away in eastern Lake Superior. These regionally correlative basalt compositions provide the basis for determining the sequential contribution of various mantle sources to flood-basalt magmatism during rift development, extending a model originally developed for eastern Lake Superior. In this refined model, the earliest basalts were derived from small degrees of partial melting at great depth of an enriched, ocean-island-type plume mantle source (εNd(1100) value of about 0), followed by magmas representing melts from this plume source and interaction with another mantle source, most likely continental lithospheric mantle (εNd(1100) < 0). The relative contribution of this second mantle source diminished with time as larger degree partial melts of the plume became the dominant source for the voluminous younger basalts (εNd(1100) value of about 0). Towards the end of magmatism, mixtures of melts from the plume and a depleted asthenospheric mantle source became dominant (εNd(1100) = 0 to +3).

Temporal changes in the sources of flood basalts: Isotopic and trace element evidence from the 1100 Ma old Keweenawan Mamainse Point Formation, Ontario, Canada

The Keweenawan Mamainse Point Formation, located on the eastern flank of a major 1100 Ma continental-rift/flood basalt province in the midcontinent of North America, consists of 5.3 km of continuously exposed picrite and basalt. The more than 350 lava flows exposed span nearly the entire duration of Keweenawan igneous activity and can be divided into eight groups on the basis of stratigraphic correlation of major, trace element and neodymium isotopic compositions. Flows from the lower part of the section result from a mantle plume interacting with old subcontinental lithospheric mantle based on their ϵ Nd(1100) ranging from −5 to −0.5 and on their enriched incompatible trace element abundances with distinctive depletions in Ta and Nb. Flows from the middle of the section are dominated by crustal assimilation as evidenced by their depletions in P and Ti, enrichments in Ba, Th, La and Ce and low ϵ Nd(1100) of −9. Voluminous flow groups from the upper part of the section derive from a mantle source that is a mixture of plume and depleted mantle based on their consistently more radiogenic ϵ Nd(1100) of 0 to +3.5 and major and trace element compositions closest to the Proterozoic equivalent of MORB. A volumetrically minor flow group from the uppermost part of the section has very high incompatible element contents and ϵ Nd(1100) values between −1 and 0, showing renewed late-stage melting at low extents from the mantle plume. The progression of magmatic sources from plume+lithospheric mantle to plume+depleted mantle at this one locality parallels the magmatic evolution of other flood basalt provinces which are usually only developed over a much wider area.

Post-Keweenawan compressional faults in the eastern Lake Superior region and their tectonic significance

GLIMPCE aeromagnetic data in eastern Lake Superior are characterized by a series of strong easterly- and northeasterly-oriented gradients that relate to mapped post-Keweenawan faults occurring along the eastern shore. The reversed nature of three of the faults is established through field observations and potential field modelling. Middle Keweenawan volcanic rocks at Mamainse Point are in fault contact on their south side with upper Keweenawan sandstone of Bayfield–Jacobsville type. Gravity modelling suggests that the fault is a low angle thrust dipping to the north. Field observations and high-resolution aeromagnetic data show that it extends inland along the southern margin of the Batchawana Greenstone Belt for at least 17 km. To the west, the Mamainse Point fault may extend across eastern Lake Superior to the Keweenaw Peninsula, linking several offsets in the seismic data that are consistent with the same attitude and sense of displacement. Along the south side of Batchawana Bay at Havilland, sandstones of Bayfield–Jacobsville type are isoclinally folded against a package of upthrust older rocks that include drag-folded middle Keweenawan volcanics. At Grindstone Point, north of Cape Gargantua, a reverse fault separating isoclinally-folded upper Keweenawan sandstones from Archean basement may, on aeromagnetic evidence, be an eastward extension of the Michipicoten Island fault.These faults mark a significant change in the style of late compressional tectonism observed within the Midcontinent Rift. All cut Keweenawan rocks across strike. The inference is that broad north–south or northwest–southeast compression, consistent in timing and orientation with the Grenville Orogeny, led to a reversal of movement along the major graben faults in western Lake Superior and was taken up in the eastern region by reverse faults oriented normal to the extensional axis of the rift.

Petrology of the Keweenawan Mamainse Point lavas, Ontario: Petrogenesis and continental rift evolution

The Mamainse Point Formation forms a 5250‐m‐thick sequence of flood basalts, ranging from high‐MgO picrites and basalts to basaltic andesites, that formed in the Keweenawan (1100 Ma) Midcontinent Rift of North America. This section is the most continuous and complete sequence of Keweenawan lavas and records almost the entire igneous history of the rift. The section consists of numerous stratigraphic groups of lavas that have distinct chemical characteristics. A variety of petrogenetic processes including partial melting of different sources at decreasing depth, periodically replenished, tapped, and fractionated magma chambers, crustal contamination, and polybaric fractionation were responsible for the evolution of the magmas. The dominant petrogenetic process changes with stratigraphy, which suggests that the physical evolution of the rift controlled which process operated at any time. The initial fracturing of the crust presumably allowed high‐MgO liquids generated at depth to be erupted. The early stage of rifting was apparently slow and allowed the formation of long‐lived magma chambers in which complex processes such replenishment/mixing and assimilation/contamination could occur. The late stage of rifting, however, was apparently vigorous and allowed voluminous magmas from throughout the crust to be erupted. These conclusions from the chemostratigraphy agree with the physical stratigraphy, which indicates a change in tectonic style from an early stage of broad subsidence to a late stage of active graben formation.

Geochemistry of the Mamainse Point volcanics, Ontario, and implications for the Keweenawan paleomagnetic record

The Keweenawan (1100 Ma) Mamainse Point volcanics, located along the eastern shore of Lake Superior in Ontario, formed in the Midcontinent Rift of North America. They are a 5250 m thick sequence of over 350 predominantly basaltic lava flows. The Mamainse Point section is the most continuous Keweenawan volcanic sequence and spans nearly the entire igneous history of the rift. The lower part of the section consists of high-MgO picrites and basalts, but the upper part of the section is composed of olivine tholeiites intercalated with numerous conglomerate layers. Major- and trace-element data reveal that the section consists of numerous stratigraphically constrained, geochemically distinct groups of lava flows. The comprehensive geochemical data on the entire sequence indicate that the section has no repetition due to faulting, as has been suggested by other workers on the basis of paleomagnetic studies. Evidently, the three paleomagnetic reversals previously found in the Mamainse Point section are real, and therefore there were multiple paleomagnetic reversals during Keweenawan time.

Evidence for Cu-(Ag) mineralization by magmatic-meteoric fluid mixing in Keweenawan fissure veins, Mamainse Point, Ontario

Locally rich (up to 40% Cu) Cu-(Fe) sulfide mineralization with minor Ag occurs in fault-related fissure veins cutting, but probably penecontemporaneous with, Keweenawan plateau basalt lava flows at Mamainse Point, Ontario. Paragenetic studies of samples from the Coppercorp mine define four stages of mineralization: (1) early pyrite-(chalcopyrite) with strong silicification and sericitic alteration; (2) a transitional stage of chalcopyrite-hematite and bornite-hematite assemblages, with propylitic alteration; (3) main-stage chalcocite-hematite, often replacing earlier sulfides, with propylitic alteration; and (4) late-stage, low-temperature calcite deposition, with local Cu degrees , Ag degrees , Cu arsenides, and cuprite. This paragenetic sequence suggests an increase in f (sub O 2 ) , perhaps accompanied by an increase in pH and a decrease in total sulfur activity.The mineralized fissures crosscut but are coeval with regional-scale low-grade (zeolitic to local epidotic) alteration in the plateau basalt sequence; their mineralogy, particularly the early sericitic wall-rock alteration, is at variance with the regional alteration mineralogy. This contrast is supported by fluid inclusion and light stable isotope data. Temperatures of the regional alteration reached little higher than 300 degrees C at the base of the lava pile, and fluid salinities were generally below approximately 2 equiv wt percent NaCl, suggesting the dominant presence of meteoric water. However, in the fissure veins, fluids varied from or = 450 degrees C and approximately 22 equiv wt percent NaCl. Fluid inclusions representing the hot saline fluids were only found in the presence of copper mineralization, and it is therefore concluded that these fluids were responsible for metal transport. Analysis of decrepitation residues from fluid inclusions in chalcocite indicate that the fluids were dominantly NaCl-CaCl 2 -H 2 O solutions; suppression of the critical curve by the presence of Ca (super +2) permits the interpretation that these saline, high-temperature fluids were near their critical point at approximately 450 degrees C. Their high temperatures compared to the ambient fluids suggest that they were derived from below the Keweenawan lava pile and that they may have been associated with intrusive magmatic activity. Approximately positively correlated arrays of homogenization temperature vs. salinity data from fluid inclusions in ore samples suggest that mixing between the hot saline fluid and the cooler meteoric ground waters was an important mechanism for deposition of Cu-(Fe) sulfides. The combined effects of rapid cooling, dilution, neutralization, and oxidation resulting from this process would cause a significant drop in copper solubility with consequent deposition of ore; this process would also explain the observed paragenetic sequence of reduced to oxidized assemblages.The delta 13 C values of calcite from the fissure veins average -3.9 + or - 0.9 per mil (n = 12), and fluid delta 18 O ratios calculated from seven samples of quartz and epidote intergrown with sulfides range from 10.7 to 2.5 per mil (+ or -2ppm due to temperature uncertainties). These isotopic ratios overlap the ranges for magmatic carbon and water, respectively. Eight samples of late calcite from mineralized veins indicate fluid delta 18 O values ranging from 4.8 to -3.1 per mil (+ or -0.5-4.5ppm). Calcite from barren veins (n = 3) and from veinlets associated with the regional alteration (n = 4) extend this range down to -6.9 per mil; delta 13 C ratios of between -2.7 and +0.7 per mil were obtained from these last four samples. Negative fluid delta 18 O values and delta 13 C values of carbonates near zero per mil suggest the dominant presence of meteoric waters and atmospheric carbon, respectively, in the regional system and the dominance of meteoric water in barren veins in the fissures; this interpretation is in agreement with the fluid inclusion data. The range of delta 18 O from magmatic to lighter values in the mineralized veins suggests that meteoric water was also present during ore deposition and was mixed in varying proportions with a magmatic fluid. The isotopic data, therefore, support a model of meteoric ground water and magmatic (or highly exchanged) fluid interaction in the fissure veins.Comparisons are made to the nearby Keweenawan Jogran Cu-(Mo) porphyry, and the Tribag Cu-(Mo) breccia pipes. Similarities between these deposits and the Mamainse Point fissure veins in mineralogy, fluid properties, and light stable isotope compositions suggest a genetic link. It is proposed that the Coppercorp, Tribag, and Jogran deposits represent successively deeper levels of exposure in Keweenawan porphyry-related hydrothermal systems. Richest ore deposition occurred in the high-level fissure veins at Coppercorp, due to focused mixing of hot metalliferous fluids with meteoric ground waters.

High-MgO lavas from the Keweenawan midcontinent rift near Mamainse Point, Ontario

Detailed study of the basal 500 m of the Keweenawan Mamainse Point volcanics in Ontario, Canada, has revealed that the section is dominated by basaltic to picritic lavas with up to 23 wt% MgO. Two stratigraphic suites with distinct chemical characteristics are separated by four flows of high-TiO 2 ferrobasalt containing large and abundant plagioclase spherulites. The focus of this study is on the lower suite that contains the most MgO-rich compositions and that generally has lower TiO 2 content. Although most of the flows are olivine-phyric, the olivine phenocrysts are skeletal, suggesting in situ crystallization from magmas with high normative-olivine content. Furthermore, the compositional variation of the lavas does not fall along an olivine-only control line. Taken together, the data indicate that the compositional variation of the lavas represents fractionation of high-MgO liquids rather than accumulation of olivine in basaltic liquids. Most of the fractionation apparently involved crystallization of olivine, clinopyroxene, and Cr-spinel, but later fractionation involved olivine, clinopyroxene, and plagioclase. The occurrence of picritic lavas low in the overall volcanic section is similar to relations in other flood-basalt provinces, and should eventually yield important tectonic and magmatic constraints for the development of the Keweenawan midcontinent rift of North America.

Paleomagnetism and UPb geochronology of volcanic rocks from michipicoten island, lake superior, canada: precise calibration of the keweenawan polar wander track

Paleomagnetic poles have been determined for normally magnetized samples of late Proterozoic volcanic rocks from 23 sites on Michipicoten Island, Lake Superior, Canada. The results are in general accord with an earlier, less precise, study by one of the authors and indicate a small polar shift between rocks of the Mamainse Point Formation (186°E, 38°N) and the Michipicoten Island Formation (175°E, 26°N). A quartz-feldspar porphyry intruding the lower Mamainse Point Formation but unconformably overlain by the Michipicoten Island Formation was sampled for UPb zircon dating. The zircons define an age of emplacement of 1086.5+1.3/−3.0 Ma . This age and the pole positions are consistent with, and supplement ages previously determined for the Keweenawan paleomagnetic polar wander curve. These data define an apparent polar wander rate of 14 cm a −1 . This age, combined with previously published UPb ages, indicates a time span of ∼ 22 Ma for most Keweenawan igneous activity. In addition, the time of the Keweenawan paleomagnetic reversal is constrained to be between 1086.5+1.3/−3.0 and 1097.6 ± 3.6 Ma , the age of the upper part of the reversely magnetized Osler Group. The zircon population contained an inherited component from a source 1403±50 Ma old. This suggests that the porphyry magma was at least partly derived by melting of an anorogenic pluton emplaced during the mid-Proterozoic.

Magma genesis in a late proterozoic proto-oceanic rift: REE and other trace-element data from the Keweenawan Mamainse Point Formation, Ontario, Canada

The Keweenawan Mamainse Point Formation of Ontario consists of a sequence of tholeiitic lavas and transitional basalt dykes, with trace-element characteristics similar to other proto-oceanic rift sequences. Stratigraphic variation of geochemistry allows the subdivision of the sequence into five series, each of which has suffered shallow-level fractional crystallization. Differences in incompatible element ratios and REE-patterns, however, can only be explained by variations in the melting process. REE-modelling suggests that parental magmas may have been derived from a heterogeneous mantle with flat chondritic and LREE-depleted sources. However, the systematic evolution of REE-patterns and other trace-elements are better explained by dynamic partial melting within two subsequent rising mantle-diapirs.

Late Precambrian Keweenawan Asymmetric Polarities as analyzed by axial offset dipole geomagnetic models

The Great Logan Paleomagnetic Loop has been interpreted to be a paleomagnetic signature of the North American plate motion during the Keweenawan igneous activity (1200–1000 Ma). However, the presence of three successive asymmetric polarities, along with new geochemical data from the Mamainse Point area, makes this interpretation untenable. A new interpretation is presented according to which the asymmetric polarities represent long‐term zonal nondipole geomagnetic field components. Wilson's single‐dipole offset model can in general explain the inclinational asymmetries, but it yields intensity ratios (reversed/normal) significantly lower than those observed. A coaxial two‐dipole model is proposed that better takes into account the observed intensity data. This model consists of a geocentric dipole and an offset dipole located at the core‐mantle boundary. The offset dipole simulates the long‐term zonal average of the spherical harmonic nondipole field. If the geocentric dipole reverses while the offset dipole retains its constant polarity, nonantiparallel reversal results. A method is presented by which the depth of the offset dipole and the ratio of the dipole strengths can be determined using only the observed inclinations. The two‐dipole model suggests that inclinational asymmetries are due to a different nondipole/dipole ratio during the normal and the reversed polarity. The most probable variant of the two‐dipole models predicts that this ratio is 35% and 49% for the normal and the reversed polarity, respectively. A new interpretation for the evolution of the Logan Loop is described according to which at least 50% of the hairpin‐shaped loop is produced by oscillation of the dipole strength ratio. Apparent polar wander (APW) is still evident, but the shape of the loop and the APW speed along the loop is reduced. The two‐dipole model is tested by worldwide paleomagnetic data of Keweenawan age.