Young eyes on old rocks: Evaluating tectonic models for Neoarchean(?) basin formation in the metamorphic core of the Black Hills, South Dakota

Overview: The crystalline core of the Black Hills of western South Dakota exposes Neoarchean and Paleoproterozoic plutonic and metamorphic rocks that record processes related to the assembly of Laurentia (Whitmeyer and Karlstrom, 2007). It is generally accepted that the tectono-metamorphic evolution of the Black Hills crystalline core represents the youngest phase of the Trans-Hudson orogen, which involved E-W convergence and eventual suturing of the Superior craton in the east with the Wyoming craton in the west (e.g., Gosselin et al., 1988; Redden et al., 1990; Figure 1 inset). However, that simplistic view of cratonic suturing in the Black Hills is complicated by exposure of the ca. 2549 Ma Little Elk granite, interpreted to be a fragment of Wyoming craton (Gosselin et al., 1988), along the northeastern margin of the Black Hills crystalline core (Figure 1). Models attempting to explain the location of the Little Elk granite typically call upon a Neoarchean rifting event along the margin of the Wyoming Craton, which displaced the Little Elk granite from other blocks of Wyoming affinity to the west (e.g., the Bear Mountain domain; Figure 1). Although the rifting model appears to explain the first-order distribution of Neoarchean rocks in the crystalline core of the Black Hills (e.g., Redden and DeWitt, 2008), the timing and tectonic mechanisms for such a rifting event are debated. This four-student project will use geochronology and stuctural analysis to test various models for the tectonic history of these facinating rocks. 

When: July 17 – August 11, 2023

Where: Rapid City, South Dakota

Who: Four students and project leader Dr. Trevor Waldien, South Dakota Mines, trevor.waldien@sdsmt.edu

Recommended Courses: Mineralogy, Petrology, Structure/Tectonics.

Expectations and Obligations:

1. Participation in field and laboratory activities during the summer (July 17 – August 11, 2023)
2. Travel to the University of Arizona Laserchron Center for analytical work (December 10-13, 2023)
3. Write an abstract and present a poster at the Geological Society of America Cordilleran/Rocky Mountain section meeting in Spokane, WA on May 15-17, 2024.
4. Write a short contribution (4-6 pages text + figures) to be published in the Proceedings of the Keck Geology Consortium 2024 Volume (first draft due Mid-February).
5. Expected but not required: Use this work for the completion of a senior thesis (or equivalent) at your home institution.

Figure 1: Generalized geologic map of the Black Hills crystalline core with cities (black circles) and regionally important structures. The study areas and map units relevant to the proposed study are outlined in red. Inset: Regional map of Laurentian cratonic blocks and sutures between blocks relative to state borders. Note the location of the Black Hills within the Trans-Hudson domain. CPO–Central Plains Orogen; MBO–Medicine Bow Orogen. Modified from Allard and Portis, (2013).

PROJECT DESCRIPTION

Controversy surrounding the petrogenesis of ≥2480 Ma rocks in the Black Hills has generated competing tectonic models describing the development of a Paleoproterozoic ocean basin between the Wyoming and Superior cratons. One model proposes that Neoarchean granitoids in the Little Elk and Bear Mountain domains formed by subduction-related magmatism in a contractional arc along the southern margin of the Wyoming craton, and associated Neoarchean metasedimentary rocks hosted the intrusions (Karlstrom and Houston, 1984; Gosselin et al., 1988). In this first model, calc-alkaline metagabbros spatially associated with the Neoarchean granitoids and metasediments record arc magmatism along the southern edge of the Wyoming craton until ca. 2480 Ma, after which plume-related continental rupturing developed an ocean basin by ca. 2010 Ma (Van Boening and Nabelek, 2008). An alternative model proposes that the ca. 2480 Ma metagabbros in the Black Hills correlate with a suite of similarly aged mafic igneous suites spanning from the Black Hills to Sudbury, Ontario and are collectively interpreted to represent a continental rifting event to seafloor spreading at that time leading (Dahl et al., 2006). In this second model, Neoarchean(?) sediments would have filled the rift basin and would have been metamorphosed together with Paleoproterozoic strata during the Trans-Hudson orogeny.

The key to vetting models of ocean basin development along the margin of the Wyoming craton rely on establishing: 1) The field relationship between Neoarchean granitoids and associated metasedimentary assemblages, and 2) The pre-collisional structural evolution of Neoarchean granitoids. Our research team will use targeted geochronology and structural geology projects aimed at collecting data that bear on the tectonic models described above.

Geologic Context

Precambrian crystalline rocks in the Black Hills are exposed in the core of a doubly plunging Laramide anticline, wherein syn- and post-Laramide erosion cut through Phanerozoic platform strata into the Precambrian core (Lisenbee, 1988). Much of the metamorphic core of the range consists of Paleoproterozoic metasedimentary rocks, which represent an ocean basin that was closed during Proterozoic suturing of the Superior and Wyoming cratons during the Trans-Hudson orogeny (Redden and DeWitt, 2008). Four generations of structures have been documented to record deformation and metamorphism during closure of the ocean basin: (1) N-vergent tight-to-isoclinal, recumbent folding estimated to have taken place at ca. 1883-1775 Ma prior to regional metamorphism (Dahl et al., 2005a). Fabrics associated with this folding (S₁/F₁) may be locally preserved as inclusion trails in porphyroblasts (Dahl et al., 2005b) (2) N-NW trending upright, isoclinal folding (F₂) with a strongly developed axial planar cleavage (S₂) at ca. 1760-1747 Ma (Dahl and Frei, 1998); (3) Vertically plunging folds (F₃) and sinistral mylonite zones that deform S₂ at ca. 1736-1719 Ma (Morelli et al., 2010). Mineralization at the Homestake gold mine is genetically related to F₃ folding (Bergmann et al., 2021); (4) Doming and associated folding (F₄) related to emplacement of the Harney Peak granite, which is dated at 1717-1715 Ma (Redden et al., 1990). The earliest evidence of regional metamorphism is associated with the F₂folding event (Nabelek et al., 2006), which persisted until crustal melting resulted in emplacement of the Harney Peak granite at ca. 1715 Ma (Redden et al., 1990).

Two Neoarchean domains bound the Paleoproterozoic metasedimentary rocks in the crystalline core of the Black Hills. In the northeast, the Little Elk domain consists of the 2549 ± 11 Ma Little Elk granite and associated quartzofeldspathic gneisses, iron formations, and quartzites (Gosselin et al., 1988; Redden and DeWitt, 2008). The quartzites are intruded by a suite of 2480 ± 6 Ma metagabbro bodies (Dahl et al., (2006), which brackets the age of the metasedimentary rocks to be earliest Proterozoic or Archean. In the southwest, the Bear Mountain domain consists of the 2392 ± 230 Ma Bear Mountain trondhjemite and a suite of quartzofeldspathic gneisses, quartzites, and metagabbros, which are mapped as correlative as those in the Little Elk domain.

Potential Student Projects

The proposed student research efforts are designed to investigate the temporal and structural evolution of the two Neoarchean domains (Little Elk and Bear Mountain) in the Black Hills crystalline core. Two students will work on each Neoarchean domain: one will address the structural evolution and the other will focus on geochronology. This model will give each student two direct collaborators: one working on the same Neoarchean domain but using a different technique, the other working on the other Neoarchean domain but using the same technique. Detailed descriptions of the student projects are as follows:

1. What is the relationship between Neoarchean intrusive suites and Neoarchean metasediments? (2 students): The Little Elk granite and Bear Mountain trondhjemite are both associated with Neoarchean quartzofeldspathic gneisses and Paleoproterozoic(?) quartzites that are intruded by metagabbros dated as old as ca. 2480 Ma (Gosselin et al., 1988; Dahl et al., 2006; Redden and DeWitt, 2008). Although the map patterns show an intrusive relationship between the Bear Mountain- Little Elk intrusive suites and their associated quartzofeldspathic gneisses, recent work on one of the gneiss bodies near the Little Elk granite suggests that it is a strained version of the granite (Nicosia and Allard, 2013), which raises a question about whether the granitoids intrude, or are buried by, the associated quartzites.

Two students will be tasked with detailed cathodoluminescence (CL) imaging and U-Pb dating of zircon grains extracted from the Little Elk and Bear Mountain granitoids and the associated gneisses and quartzites. If zircon grains extracted from the granitoids contain xenocrystic cores with U-Pb dates similar to the quartzites, then it is likely that the quartzites did not form in the same marine basin as the Paleoproterozoic strata. Because the quartzites are intruded by the ca. 2480 Ma gabbros that are variably interpreted to record either arc magmatism or continental rifting, the relationship between the gneisses, quartzites, and granitoids is essential to understanding when the ocean basin could have opened.

Additionally, the spatial resolution afforded by new laser ablation U-Pb dating will shed light on discordance in the published dates on the Little Elk and Bear Mountain intrusions, which were both dated by multi-grain dissolution and Thermal Ionization Mass-Spectrometry (Gosselin et al., 1988). Assuming xenocrystic cores exist in either/both granite bodies, the ages of the cores will help reinterpret upper-intercept ages and sources of discordance for both bodies. Ages from co-magmatic overgrowth rims (or the bulk of the crystal if xenocrystic cores are not present) may also be useful for refining the crystallization date on the Little Elk granite and decreasing the age uncertainty (230 Ma) on the Bear Mountain trondhjemite.

2. What is the genesis of meso-scale shear zones in the Little Elk and Bear Mountain intrusive rocks? (2 students): Intrusive rocks in both Archean welts display spatially variable fabric development ranging from weakly foliated orthogneissic textures with localized shear zones to penetratively foliated banded gneiss (Gosselin et al., 1988; Nicosia and Allard, 2013). Although evidence of ductile shearing is present in both domains, the tectonic events associated with the deformation remain unclear. Allard and Portis (2013) document the existence of a NW-striking sinistral shear system in the northeastern domain of the crystalline core associated with steeply plunging F3 fold axes and propose that associated structures may be present in both Neoarchean domains. However, that study did not focus on the Neoarchean rocks.

Two students will be tasked with a combined field and petrographic study of shear systems in the Neoarchean intrusive bodies. Capitalizing on the advent of new digital mapping techniques such as Strabotools (e.g., Glazner and Walker, 2020) and Field Move, the students will be able to collect large structural datasets consisting of georeferenced photos, foliation and fold axis orientation measurements, quantitative fabric intensity, and color index. The large structural datasets will enable the students to perform statistically robust structural analysis of the various structure sets, which they will use to determine whether structures in the intrusive bodies are compatible with the regionally documented NW-striking sinistral shear zone. Oriented thin sections of key samples in the orthogniesses will allow the students to use microstructural observations to evaluate the temperature conditions of deformation and perform quantitative strain analysis using Electron Backscatter Diffraction (EBSD) at their home institution if facilities exist.

Because the F3 folding event and associated sinistral shear is the final regional ductile deformation event in the Black Hills, the structural analysis component of the project will be essential to determine which, and how many, deformation events are recorded in the Neoarchean rocks of the Black Hills. If structures in the intrusive rocks are compatible with regional NW-striking sinistral shear, then that information will better establish the regional extent of sinistral shearing. If the structures are not compatible with NW-striking sinistral shear, then the students will evaluate the potential compatibility with older ductile deformation phases: F2 isoclinal folding during regional metamorphism, or possible Neoarchean deformation related to pluton emplacement, regional convergence, or continental rifting. D1/F1 deformation is inferred to predate Trans-Hudson metamorphism and therefore likely would not develop ductile shear zones in the intrusive bodies.

PROJECT LOGISTICS

The field component of the project will take place over three weeks from July 17 through August 4, 2023 on public lands of the Black Hills National Forest. We will use a 12-passenger campus van from SD Mines to access the field sites and camp sites in the national forest. Safety concerns in the Black Hills include bears, cougars, rattlesnakes, busy roadways, excessive heat, and lightening/hail storms. Despite the relatively developed nature of the Black Hills National Forest, the field portion of the project will require students to hike for long hours over uneven ground, camp in tents, and often disconnect from internet/cell phone service. If needed, hiking and camping gear can be provided by the Keck Geology Consortium.

The week of August 7-11 following field work is allocated to rock crushing, mineral separation, and structural data organization on campus at SD Mines. During the on-campus research phase, student researchers will be hosted in SD Mines dormitories with access to the dining hall. Before using rock crushing facilities, students will attend a safety briefing where they will learn the proper use of the facilities and associated personal protective equipment.

Required activites during the 2023-2024 academic year

In December, our research group will travel to the University of Arizona to perform laser ablation U-Pb dating at the Arizona Laserchron Center.  Tentative dates, including travel, are December 10-13, 2023.  In the spring, we will will present our work (talks and/or posters) at the Geological Society of America Cordilleran/Rocky Mountain section meeting on May 15-17, 2024. All expenses associated with travel to Laserchron and the conference are covered by the Keck Geology Consortium.

PROFESSIONAL DEVELOPMENT

We will travel to the joint GSA Cordilleran/Rocky Mountain section meeting in Spokane, WA in May of 2024 to present our results. This conference is ideal because it is scheduled for the end of the project timeline, attracts geologists with interests in western North America bedrock geology, and typically has a designated undergraduate poster session. Each student will submit an abstract for a poster presentation on their research project. The process of writing an abstract with several coauthors (advisors and peer collaborators) will give you experience writing scientific documents as part of a group. The process of developing a poster presentation will also give you experience communicating your science verbally and using graphics.

All students are also required to complete a 4-6 page paper (short contribution) that will be published in the Proceedings of the Keck Geology Consortium 2024 Volume (see examples from previous years). The first draft of this paper will be reviewed by your research advisor at your home institution sometime in late February, 2024, with the revised version sent to the project directors by March 1. Final versions of your paper and figures will be submitted to the Keck Office in Mid-March.

References

Allard, S. T., & Portis, D. H. (2013). Paleoproterozoic transpressional shear zone, eastern Black Hills, South Dakota: Implications for the late tectonic history of the southern Trans-Hudson Orogen. Rocky Mountain Geology, 48(2), 73-99.

Bergmann R., Lentz, B., Allard, S., (2021) New Eyes on Old Rocks: A New Structural Model Reinvents a World-class Gold District – Black Hills, South Dakota, USA, Society of Economic Geologists

Dahl, P. S., & Frei, R. (1998). Step-leach Pb-Pb dating of inclusion-bearing garnet and staurolite, with implications for Early Proterozoic tectonism in the Black Hills collisional orogen, South Dakota, United States. Geology, 26(2), 111-114.

Dahl, P. S., Hamilton, M. A., Jercinovic, M. J., Terry, M. P., Williams, M. L., & Frei, R. (2005a). Comparative isotopic and chemical geochronometry of monazite, with implications for U-Th-Pb dating by electron microprobe: An example from metamorphic rocks of the eastern Wyoming Craton (USA). American Mineralogist, 90(4), 619-638.

Dahl, P. S., Hamilton, M. A., Wooden, J. L., Foland, K. A., Frei, R., McCombs, J. A., & Holm, D. K. (2006). 2480 Ma mafic magmatism in the northern Black Hills, South Dakota: a new link connecting the Wyoming and Superior cratons. Canadian Journal of Earth Sciences, 43(10), 1579-1600.

Dahl, P. S., Terry, M. P., Jercinovic, M. J., Williams, M. L., Hamilton, M. A., Foland, K. A., Clement, S.M., & Friberg, L. M. (2005b). Electron probe (Ultrachron) microchronometry of metamorphic monazite: Unraveling the timing of polyphase thermotectonism in the easternmost Wyoming Craton (Black Hills, South Dakota). American Mineralogist, 90(11-12), 1712-1728.

Glazner, A., & Walker, J. D. (2020). StraboTools: A mobile app for quantifying fabric in geology. GSA Today.

Gosselin, D. C., Papike, J. J., Zartman, R. E., Peterman, Z. E., & Laul, J. C. (1988). Archean rocks of the Black Hills, South Dakota: Reworked basement from the southern extension of the Trans-Hudson orogen. Geological Society of America Bulletin, 100(8), 1244-1259.

Karlstrom, K. E., & Houston, R. S. (1984). The Cheyenne belt: Analysis of a Proterozoic suture in southern Wyoming. Precambrian research, 25(4), 415-446.

Lisenbee, A. L. (1988). Tectonic history of the Black Hills uplift. AAPG Field trip guide to the Powder River Basin.

Morelli, R. M., Bell, C. C., Creaser, R. A., & Simonetti, A. (2010). Constraints on the genesis of gold mineralization at the Homestake Gold Deposit, Black Hills, South Dakota from rhenium–osmium sulfide geochronology. Mineralium Deposita, 45(5), 461-480.

Nabelek, P. I., Labotka, T. C., Helms, T., & Wilke, M. (2006). Fluid-mediated polymetamorphism related to Proterozoic collision of Archean Wyoming and Superior provinces in the Black Hills, South Dakota. American Mineralogist, 91(10), 1473-1487.

Nicosia, C. & Allard, S., (2014), Petrologic and Geochemical Characterization of Archean Gneisses in the Little Elk Terrane, Black Hills, South Dakota. Student Research and Creative Projects 2014-2015. 11.

Redden, J. A., & DeWitt, E. (2008). Maps Showing Geology Structure and Geophysics of the Central Black Hills South Dakota (Vol. 2777, pp. 44-p). US Geological Survey Scientific Investigations Map, 1:100,000 scale; 2 sheets.

Redden, J. A., Peterman, Z. E., Zartman, R. E., DeWitt, E., 1990, U-Th-Pb geochronology and preliminary interpretation of Precambrian tectonic events in the Black Hills, South Dakota, in Lewry, J. F., and Stauffer, M. R., eds., The Early Proterozoic Trans-Hudson Orogen of North America: Geological Association of Canada Special Paper 37, p. 229–251.

Van Boening, A. M., & Nabelek, P. I. (2008). Petrogenesis and tectonic implications of Paleoproterozoic mafic rocks in the Black Hills, South Dakota. Precambrian Research, 167(3-4), 363-376.

Whitmeyer, S. J., & Karlstrom, K. E. (2007). Tectonic model for the Proterozoic growth of North America. Geosphere, 3(4), 220-259.