Browsing by Subject "Midcontinent Rift"

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    The effects of contact metamorphism by the Duluth complex on proterozoic footwall rocks in Northeastern Minnesota.
    (2010-06) Frost, Shelby Jeanne
    The Duluth Complex is composed of numerous mafic intrusions that were emplaced in northeastern Minnesota during formation of the Midcontinent Rift approximately 1.1 Ga (Miller et. al., 2002). When it intruded, the heat of this igneous body significantly affected the wall rocks around it and created a distinctive contact metamorphic aureole. Footwall rocks directly west of the Duluth Complex include the Animikie Group and the North Shore Volcanic Group. The purpose of this research is to understand crustal conditions associated with emplacement of a large mafic igneous massif such as the Duluth Complex, and develop a better idea of the thermal state of the crust during the time of rifting. To constrain crustal conditions we must determine the effects that intrusion of the Duluth Complex had on adjacent wall rocks, in particular the Ely’s Peak basalts of the North Shore Volcanic Group, and the Thomson and Virginia formations of the Animike Group. These effects include the extent and grade of metamorphism. At the time of intrusion, metamorphic pressure was approximately 2.5 kbar and temperature was 600-700°C, based on the presence of metamorphic minerals such as orthopyroxene and wollastonite. The contact metamorphic aureole extends from the Duluth Complex into the Animikie Group for approximately 200 m and into the Ely’s Peak basalts for approximately 100 m. This is consistent with estimates made by Severson (1995), Duchesne (2004) and Kilburg (1972). The criteria used to define the aureole include textures such as mortar texture and spotty slate texture in the Animikie Group, and granoblastic texture in the Ely’s Peak basalts. Porphyroblasts of metamorphic indicator minerals such as cordierite, wollastonite, garnet, and pyroxene also help define the aureole. A simple 1D thermal conduction model reproduces the conditions in the contact aureole determined by petrographic relations, and helps explain why the aureole is thin. Despite the high temperature of the intrusion, the aureole is thin because the wall rocks were fairly cool when the Duluth Complex intruded, having equilibrated over about 600 m.y. at shallow crustal levels to a typical continental geotherm. The thin contact aureole also indicates that multiple intrusions may have occurred instead of one large intrusion. This would have led to insulation of the younger intrusions and a thinner aureole. Liberation of fluids in the wall rocks by the intrusion may have also played a role in keeping the contact aureole relatively thin by lowering reaction temperatures.
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    Geologic and geochemical attributes of the Beaver River Diabase and Greenstone Flow: Testing a possible intrusive-volcanic link in the 1.1 Ga Midcontinent Rift
    (2016-02) Doyle, Michael
    Over the last century, numerous geological studies have reasonably resolved the overall tectonomagmatic evolution of the 1.1 Ga Midcontinent Rift (MCR). The structural complexity, duration of magmatic activity, and relative scarcity of continuous exposure, however, have thus far hindered efforts to correlate the numerous MCR flood basalts with their intrusive feeder systems. Providing such correlations is essential to furthering our understanding of the magmatic processes that operated during rifting. This study provides field, petrographic, and geochemical evidence of one such intrusive-volcanic correlation between the Beaver River diabase (BRD) and Greenstone Flow (GSF) – two of the largest igneous systems associated with the MCR. The BRD is an extensive, multiply-intrusive, composite dike and sill network and the most extensive intrusive phase of the Beaver Bay Complex (BBC) in northeastern Minnesota. The GSF is an enormous (> 2,000 km3), similarly composite flood basalt exposed on Isle Royale and the Keweenaw Peninsula in northern Michigan. The notion that these two systems might be related was first suggested by Miller and Chandler (1997) based on the recognition that very large (≤ 400m) inclusions of anorthosite, interpreted to be xenoliths derived from the lower crust, occur in BRD dikes and sills at hypabyssal depths. Because vertical conduits wide enough to accommodate such large xenoliths traversed the crust to shallow depths, these intrusions likely reached the Earth’s surface to erupt extraordinarily large lava flows. Likewise, the GSF may possibly be the product of this venting, as supported by several lines of circumstantial evidence: 1) overlapping U-Pb ages - 1094.0 ± 1.5 Ma for the GSF (Davis and Paces, 1990) and 1095.8 ± 1.2 Ma for the BRD (Paces and Miller, 1993); 2) similar ranges in lithologies – ophitic olivine diabase/basalt to ferromonzodiorite; and 3) similar enormous sizes. This study seeks to more rigorously evaluate a possible comagmatic link between these two systems by carefully comparing their field, petrographic, and geochemical attributes. The BRD is composed of a network of ophitic olivine diabase dikes and sills that intrude the medial section of the North Shore Volcanic Group and host numerous, smaller composite intrusions of variably fractionated lithologies. In the southern BBC near Silver Bay, MN, these composite intrusions occur as a series of singularly emplaced, concentrically-zoned bodies of vari-textured ferromonzodiorite cored by foliated ferrogabbro/diorite, which are collectively termed the Silver Bay Intrusions (SBI). In the northern BBC, composite intrusions in the BRD range from intergranular gabbro to ferrodiorite and have been interpreted by Miller and Chandler (1997) to have been emplaced in at least two intrusive pulses into BRD diabase dikes. The GSF is composed predominantly of ophitic to subophitic, olivine tholeiitic basalt which can be divided into upper and lower zones. The core of the GSF, here termed the Heterolithic Zone, separates the ophitic zones and contains a heterogeneous suite of evolved rocks ranging from intergranular gabbro to prismatic ferromonzodiorite. Field observations and petrographic data suggest that the core of the initial tholeiitic basalt flow was intruded, inflated, and partially displaced by one or possibly two intrusive pulses of evolved magma. Previous workers (e.g. Cornwall, 1951) have suggested that the occurrence of evolved lithologies in the core of the GSF resulted from in situ differentiation. However, evidence for composite emplacement of evolved magmas within the GSF, presented here, is given by: 1) abrupt changes in mineralogy, texture, mineral chemistry, and lithogeochemistry over centimeter to meter scale; 2) inclusion relationships between evolved and ophitic GSF lithologies; and 3) the occurrence of remnant blocks of initially crystallized GSF ophitic basalt interlayered with evolved lithologies within the Heterolithic Zone. Petrographic observations show that the comparable rock types in the BRD and GSF are nearly indistinguishable in terms of modal mineralogy and texture. Most notably are the similar occurrences of distinctive clustering of plagioclase, and the occurrence of coarse-grained plagioclase megacrysts in both the BRD ophitic diabase and GSF ophitic basalt. The presence of plagioclase megacrysts in the GSF ophitic basalt with similar anorthite contents (up to 81 mol%) as reported for the anorthosite inclusions in the BRD (An54-80; Morrison et al., 1983) strongly supports the interpretation that these crystals are xenoliths derived from a similar source as those in the BRD ophitic diabase. Geochemical data are also consistent with the interpretation that the various rock types in the BRD and GSF crystallized from chemically similar parental magmas. SEM-EDS analyses of mafic mineral compositions show considerable overlap in the mean and range of En’ contents in augite and Fo contents in olivine between comparable rock types in the BRD and GSF systems. Compositional overlap is also observed in whole rock analyses of trace element abundances and ratios between comparable rock types in each system. Concluding from the field, petrographic, and geochemical data that the BRD dike and sill network acted as the intrusive feeder system for the GSF lava flow implies that previous estimates of the volume of the GSF (White, 1960; Long, 1984) are grossly underestimated. Projecting the GSF westward under Lake Superior from exposures on Isle Royale and the Keweenaw Peninsula to the proposed feeder system on the Minnesota shore implies that the GSF has an areal extent of roughly 20,000 km2 and a total volume of at least 2,000 km3. These estimates indicate that the GSF is possibly the largest single lava flow on Earth.
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    Geology and Petrology of the Wilder Lake Intrusion, Duluth Complex, Northeastern Minnesota
    (2016-07) Leu, Adam
    Abstract The Wilder Lake Intrusion (WLI) is a well differentiated, northward-dipping, sheet-like mafic layered intrusion that is part of the 1.1 Ga Duluth Complex exposed in northeastern Minnesota. While several reconnaissance studies of the well exposed western quarter of the WLI have shown it to have several distinctive petrologic characteristics compared to other layered intrusions in the Duluth Complex, its relative inaccessibility within the Boundary Waters Canoes Area Wilderness precluded a detailed study of its field and petrologic attributes along its entire 10 km strike length. This changed in the Fall of 2011 when an intense forest fire (the Pagami Creek Burn) created easy access to excellent exposure of the WLI. Through field mapping, petrographic observations, and geochemical analysis, this study sought to document the igneous stratigraphy of the WLI along its entire strike-length with the goal of better understanding the petrogenesis of its unique petrologic attributes. These attributes, noted by others and confirmed here, include the up-section stratigraphic changes characterized by 1) a cumulus reversal from a olivine oxide gabbroic cumulate of Pl+Cpx+Ox+Ol to a troctolitic (Pl+Ol) cumulate; 2) a reversed cryptic variation of Fo in olivine and En’ in clinopyroxene; and 3) the cumulus arrival of Fe-Ti oxide before augite (Miller and Ripley, 1996). Detailed mapping conducted in 2012 and 2013 revealed that most cumulate units of the WLI can be followed along its entire strike length, but with some notable exceptions. Remapping in the western part of the WLI has confirmed that the 2 km-thick igneous stratigraphy exposed here starts with a basal unit of heterogeneous, intergranular olivine oxide gabbro that is in sharp contact with Anorthositic Series rocks. This marginal gabbro is overlain by a troctolitic unit of Pl+Ol cumulates, which can be subdivided into a heterogeneous subunit, a layered subunit and an anorthosite inclusion-rich subunit. The troctolite unit is overlain by a thin (20-100 m thick) oxide troctolite unit of Pl+Ol+ Ox cumulates that abruptly gives way to an olivine oxide gabbro unit of Pl+Cpx+Ox+Ol cumulates. This four-phase cumulate is abruptly overlain by another troctolitic cumulate unit demarking a cumulus reversal back to Pl+Ol cumulates. Mapping of the excellent exposures created by the Pagami Creek Burn reveal that the upper troctolite unit cross-cuts and locally scoured out the four-phase gabbro and part of the oxide troctolite unit. Thus it is interpreted as a recharge of more primitive magma into the upper part of the WLI chamber rather than a downward crystallizing roof zone unit as speculated by Miller (1986). Detailed mapping by overland traverses in the central and eastern extents of the WLI show it to thin from 700 m in the west to 500 m in the east. Moreover, several units pinch out in the eastern section of the intrusion. The oxide troctolite pinches out just east of center, but swells back to about 20 meters in stratigraphic thickness before pinching out again farther east. The lower gabbro also pinches out around the same place and is replaced by a taxitic unit that dominates at the eastern margin; a similar heterogeneous unit also can be found at the western margin. Petrographic study of 223 thin sections collected along three profiles across the WLI at its western, east-central and eastern extents helped to confirm and refine the stratigraphic variations in mineralogy and texture noted from field observations. In addition, olivine and pyroxene from many of the samples were analyzed by University of Minnesota Duluth’s SEM/EDS (Scanning Electron Microscope/Energy Dispersive X-Ray Spectroscopy) to document cryptic variation of mg# (=Mg/(Mg+Fe), cation %). This mineral chemical data was acquired to verify the reversed cryptic variation previously documented in the west and to determine if this variation persists along strike to the east. Reversed cryptic variation of upwardly increasing magnesium number (mg# =MgO/(MgO+FeO), mol %) in olivine (Fo) and pyroxene (En’) was confirmed in the west and shown to persist in the eastern profiles. However, the data also reveal that the mg# tends to decrease at a particular stratigraphic horizon from west to east. The reversed cryptic variation up-section is interpreted to reflect a trapped liquid shift within the oxide troctolite and olivine oxide gabbro units. Trapped liquid shift occurs where high mg# cumulus olivine re-equilibrates with low mg# intercumulus liquid. As evidenced by their strong foliation and the low abundance of augite in the oxide troctolite unit, these rocks are clearly adcumulates with very little postcumulus minerals (i.e., representing the trapped liquid component) and thus retain their high-mg#. The lateral decrease in mg# to the east, as well as the disappearance of the oxide troctolite unit, is thought to be caused by the thinning of the intrusion which in turn would result in the eastern portion of the intrusion to cool more rapidly than the west. This more rapid cooling would have trapped intercumulus liquid (and thus a stronger trapped liquid shift) and promoted oxide and pyroxene to crystallize more synchronously since their liquidus temperatures are not very different. Finally, whole rock geochemical analyses of the basal intergranular gabbro samples were evaluated to determine if they may be representative of a parental liquid composition. One piece of evidence that this is the case is that the WLI marginal gabbro composition is comparable to other tholeiitic magma compositions occurring in the North Shore Volcanic Group (NSVG). Another method used to evaluate the parental composition of the marginal gabbro is to apply its composition to a MELTS-based phase equilibrium program, Pele (Boudreau, 2006). This modelling indicates that the phases in equilibrium with the fractional crystallization of the marginal gabbro compositions can replicate the cumulate stratigraphy observed in the WLI with normal to reduced fO2 conditions (between QFM and 3 log units below QFM). These modeling results indicate that the cause of early oxide crystallization relative to augite was largely the result of an Fe-Ti enriched parental magma composition, and not elevated oxygen fugacity. References Joslin, G.D., and Miller, J.D., Jr., 2003, Stratiform Pd-Pt-Au mineralization in the Sonju Lake Intrusion, Minnesota. Geological Society of America Abstracts with Programs, v.34, no. 7, p. 101. Miller, J.D., Jr. 1999, Geochemical evaluation of platinum group element (PGE) mineralization in the Sonju Lake intrusion, Finland, Minnesota. Minnesota Geological Survey Information Circular 44, 32 p. Miller, J.D. Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.E., and Wahl, T.E., 2002, Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota. Minnesota Geological Survey Report of Investigations 58, 207p. w/ CD-ROM Phinney, W.C., 1972. Northwestern part of Duluth Complex. In: Sims, P.K. & Morey, G.B. (eds.) Geology of Minnesota -A centennial volume. Minnesota Geological Survey, p. 335-345
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    Petrographic and Geochemical Study of the Hybrid Rock Unit Associated with the Current Lake Intrusive Complex
    (2015-06) Chaffee, Matthew
    The Current Lake Intrusive Complex (CLIC) is one of several ultramafic to mafic intrusions associated with the Midcontinent Rift that host Ni-Cu-PGE deposits. Geophysical surveys and drilling have outlined a six kilometer-long mafic to ultramafic, chonolith-shaped intrusion that is composed primarily of wehrlite to dunite and has intruded into granitic (Current Lake Zone or CLZ) and metasedimentary rocks (Beaver Lake Zone or BLZ) of the Quetico Subprovince. Initial core logging conducted by Magma Metals (now Panoramic Resources Ltd.) noted the occurrence of a zone of texturally and mineralogically heterogeneous, intensely altered, inclusion-bearing, mafic to intermediate intrusive rocks located at the margins of the main mineralized body. These intrusive rocks were initially referred to as the Hybrid Rock unit (HRU), and due to subtle differences in color, were subsequently spilt into the "red hybrid"� and "gray hybrid"� units. Observations from core logging and petrography of the "red hybrid"� show it to be an intensely altered and hematized, locally poorly mineralized, vari-textured (prismatic to subophitic), fine- to medium-grained, quartz gabbro to lesser quartz ferrodiorite. The mineralogical, textural, and chemical characteristics of this rock type are similar in both the CLZ and BLZ, with the exception that the BLZ contains large, chlorite-mantled quartzite and granitic xenoliths and has more abundant interstitial quartz. Some of the "grey hybrid"� rocks were found to be very similar to "red hybrid"� rocks and were thus renamed the Heterolithic Quartz Gabbro (HQG) unit in both the CLZ and BLZ. Further petrographic investigations of the non-HQG "grey hybrid"� rocks indicated a mineralogical and textural resemblance to the CLIC rocks as they were moderately to intensely altered, fine- to medium-fine grained, intergranular to subpoikilitic quartz-bearing feldspathic wehrlite to melagabbro in the BLZ and gabbro to melagabbro in the CLZ. Consequently, the non-HQG "grey hybrid"� rock were renamed the Melagabbro unit in both the CLZ and BLZ. In both zones, the Melagabbro unit is observed as the marginal phase of the CLIC rocks. The principal goals of this study were to document the mineralogical, textural, chemical, and alteration attributes of the HQG and Melagabbro units, the nature of their lithologic heterogeneity, and transition into the CLIC rocks, as well as to establish the petrogenetic relationship, if any, between the HQG and Melagabbro units to the CLIC rocks. Evaluation of lithogeochemical data indicate that despite the intense felsic contamination and hydrothermal alteration, the trace element characteristics of the HQG unit are consistent with having similar parental magma as the CLIC rocks. These data also indicate that the granitic and metasedimentary country rocks of the Quetico Subprovince are a reasonable source of contamination. These results and the contact relationships between the HQG unit and the CLIC rocks are best explained by a two-stage model of emplacement. First, as the HQG unit was emplaced it incorporated abundant country rock xenoliths, becoming strongly contaminated, and extensively hydrothermally altered. Then, the CLIC mafic magma was emplaced into the HQG unit, likely by reaming out and inflating the semi-molten core of the HQG intrusion.
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    RI-69 Reexamination of the Minnesota River Valley Subprovince with Emphasis on Neoarchean and Paleoproterozoic Events
    (2014-03-06) Southwick, David
    The Minnesota River valley subprovince (MRV) is a fragment of Mesoarchean continental crust that was sutured to the southern margin of the Superior craton about 2,600 m.y. ago. The suturing event induced widespread regional metamorphism and local anatexis in a dominantly orthogneissic crust and ended with the emplacement of numerous granite plutons. In the Paleoproterozoic era the MRV was a tectonically rigid part of the cratonic foreland with respect to Penokean (geon 18), Yavapai (geon 17), and Mazatzal (geon 16) accretionary events. As such, it was affected by crustal extension and the emplacement of mafic dikes associated with the ca. 2,070 Ma opening of the pre-Penokean ocean. Subsequently, internal shear zones that had formed during Neoarchean docking of the MRV crustal block were reactivated in response to stresses applied during cycles of Paleoproterozoic stretching and subsequent compression from the south and southeast. Most of this reactivation is inferred to have taken place between 2,000 and 1,750 Ma. The Minnesota segment of the Great Lakes tectonic zone, the Neoarchean suture, was not significantly reactivated, whereas the Appleton shear zone and the Yellow Medicine shear zone both were. Six sets of mafic dikes were emplaced in the interval between 2,070 and ca. 1,750 Ma. Two sets that were emplaced early in the interval are the southwesternmost members of the pre-Penokean Kenora–Kabetogama/Fort Frances dike swarm. Two and perhaps four younger dike sets were emplaced during a period of vigorous crustal heating and magmatic activity that affected much of the MRV in early- to mid-geon 17. Numerous plugs and small plutons also were emplaced in early- to mid-geon 17. These intrusions range in composition from peridotite to granite and are comparable to rock types within and satellitic to the East-Central Minnesota batholith; they are most abundant in the eastern and southern parts of the MRV, relatively near the inferred Penokean and Yavapai tectonic fronts. Transtensional stress during the extensional stage of the Mazatzal orogenic cycle generated differential subsidence of crust south of the Yellow Medicine shear zone and produced en echelon fault-bounded depressions that became depocenters filled by supermature clastic sediment ancestral to the Sioux Quartzite. The Sioux Quartzite was deposited, lithified, and hydrothermally altered over a prolonged time interval that may have begun as early as ca. 1,730 and ended as recently as ca. 1,280 Ma.