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The Compass: Earth Science Journal of Sigma Gamma Epsilon

Volume 85 Issue 3 Article 2

11-12-2013

Timing of Thermal Overprints in the Silvermines and Associated Intrusions, St. Francois Mountains, Missouri

Renee C. Rohs Northwest Missouri State University, [email protected]

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Recommended Citation Rohs, Renee C. (2013) "Timing of Thermal Overprints in the Silvermines Granite and Associated Diabase Intrusions, St. Francois Mountains, Missouri," The Compass: Earth Science Journal of Sigma Gamma Epsilon: Vol. 85: Iss. 3, Article 2. Available at: https://digitalcommons.csbsju.edu/compass/vol85/iss3/2

This Article is brought to you for free and open access by DigitalCommons@CSB/SJU. It has been accepted for inclusion in The Compass: Earth Science Journal of Sigma Gamma Epsilon by an authorized editor of DigitalCommons@CSB/SJU. For more information, please contact [email protected]. Timing of Thermal Overprints in the Silvermines Granite and Associated Diabase Intrusions, St. Francois Mountains, Missouri

Renee C. Rohs Department of Natural Sciences Northwest Missouri State University 800 University Drive Maryville, MO 64468 USA [email protected]

ABSTRACT Igneous rocks exposed in the St. Francois Mountains record the geologic history of volcanic activity and plutonic intrusions contributing to the growth and stabilization of the continental crust during the . These igneous rocks also contain evidence of thermal overprinting both in the isotope chemistry and the crystalline structures. This study presents new isotopic, and petrologic data to support the timing of emplacement and thermal overprinting in the host rock. The study area for this research is along the East Fork of the St. Francis River at the Silver Mines Recreation Area where diabase dikes intrude the surrounding granite. Samples of the Silvermines granite and diabase were collected for petrologic study and isotopic analyses. Thin section analyses revealed and textures that suggest magmatic segregation as well as thermal variation in the diabase intrusions. The surrounding granite exhibits thermal alteration at the contact with the diabase. X-ray diffraction patterns support the thin section analyses and provide clarification on opaque minerals. An x-ray diffraction pattern for the granite also indicates orthoclase, supporting existing data sets for structural resetting of K- in both the volcanic and plutonic rocks in the area. Mineral separates were prepared for granite samples at the contact with the diabase and at a distance of 10 m from the contact. Gas ages were determined using Ar-Ar methodologies for the diabase groundmass as well as K- feldspar, , and amphibole in the host granite. According to the Ar-Ar data collected, the total gas age for the diabase is 1167.1  7.9 Ma. Ar-Ar data from the granite is more complex indicating Precambrian timing with total gas ages of 904, 1066, 1119, and 1258 Ma. Based on this information, it is evident that a significant thermal event altered the Silvermines granite as early as 1258 Ma to as late as 904 Ma. This thermal event may be associated with the intrusion of the Silver Mines and its magmatic source.

KEY WORDS: Precambrian, isotope chemistry, Silver Mines Recreation area, diabase dikes, Eastern Granite and (EGR) province, Skrainka group, Silver Mines mafic group, Butler Hill Granite, Slabtown Granite

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INTRODUCTION 1981) and is exposed along the St. Francois The purpose of this study is to River at Silver Mines. Isotopic dating of the examine the relationship between the other ring plutons has yielded similar ages Silvermines Granite and the mafic dike near the overall age of 1470 Ma for the swarm that intrudes the Precambrian granite. province (Van Schmus et al., 1993). This relationship includes the nature and However, two other granite bodies within timing of the dike emplacement as well as the St. Francois Mountains, the Graniteville the effect on the surrounding granite. In and Munger plutons, have been dated at addressing the purpose of this study, there 1357 Ma (Van Schmus et al., 1996) and are three distinct goals. The first goal is to 1378 Ma (Thomas et al., 1984) respectively. establish the mineralogy and petrology of These plutons are similar in age to the the intruding dikes. The second goal is to Southern Granite and Rhyolite (SGR) establish the timing of intrusion for the province to the west at approximately 1370 dikes. With a firm understanding of the dike Ma (Van Schmus et al., 1987; 1993; 1996 intrusions, the third goal is to examine the and Barnes et al. 1999). thermal impact of the dike intrusions on the Mafic rocks in the St. Francois surrounding granite. Mountains occur as dikes and sills. Two Basement rock units exposed in the groups of mafic rocks, Silver Mines and St. Francois Mountains (fig. 1) are part of Skrainka, have been recognized based on the Eastern Granite and Rhyolite (EGR) similarities in geochemical signatures province, representing a major pulse of (Sylvester, 1984). A gabbroic within the continental growth during the Meso- Skrainka group has been dated at at approximately 1470 Ma (Van approximately 1316 Ma (Rämö and others, Schmus et al., 1987; 1993; and Lidiak et al., 1994) and therefore post-dates the silicic 1993). Exposures of the Precambrian magmatism in the area. Mafic intrusions of basement in the St. Francois Mountains, the Silver Mines group have been provide a window in the continental crust interpreted as occurring at approximately the showing composition and configuration same time as silicic volcanism and plutonic (Rohs, 2003). emplacement based on compositional Most of the volcanic units within the similarities with a mafic volcanic flow St. Francois Mountains have been attributed occurring between units exposed at to large-scale, caldera-type volcanism with Johnson’s Shut-Ins state park (Sylvester, several volcanic centers (Kisvarsanyi, 1981). 1984). Sm-Nd isotopic data also support Several different granitic plutons have been this interpretation with depleted mantle recognized as forming in rings where model ages of 1.43-1.54 Ga for diabase intruded after structural collapse of samples in the area and 1.52-1.55 Ga for the the large calderas (Kisvarsanyi, 1981). One Silvermines Granite and a trachyte within of these ring plutons, the Silvermines the province (Rohs and Van Schmus, 2007). granite, is dated at 1484 Ma (Bickford et al.,

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Figure 1. General geologic map of the Butler Hill Caldera in the St. Francois Mountains showing sample locations as modified from Meert and Stuckey, 2002.

METHODS Selected samples were chosen for thin Sample collection and preparation section analyses, X-ray diffraction, and Hand samples were collected from isotopic analyses. the large diabase dike intruding the Silvermines Granite along the St. Francois Thin Section Analysis River at Silver Mines (fig. 2). Several Several samples were selected for samples were collected across the width of thin section analysis to determine the the dike to study petrographic changes from mineral content and textural differences. the chill margin to the interior of the dike. The thin sections were examined using a Additional samples were collected at several petrographic microscope under plane other dike intrusions along the St. Francois polarized light (PPL) and cross polarized River between the Silver Mines recreation light (XPL) viewed at 40x and 100x area and the Tiemann Shut-Ins area magnification. Textural differences were upstream (Aldieri et al., 2010). Finally, most easily recognized in PPL due to the samples were collected from the granite at abundance of opaque minerals in the mafic the contact with the dike intrusion and at a dike samples. Individual mineral relief and distance of 30 m from the dike intrusions. textures were also apparent in PPL including

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euhedral to anhedral forms as well as crystal habit. In XPL, the extinction angles of were used to determine the range of composition as An content based on the Michel-Levy method for . Twinning patterns in K-feldspars were used to identify microcline apart from orthoclase. Interference colors and mineral cleavage or fracture aided in the identification of apart from quartz and .

Figure 2. Silvermines diabase dike at Silver Mines

Thin Section Analysis Interference colors and mineral cleavage or Several samples were selected for fracture aided in the identification of calcite thin section analysis to determine the apart from quartz and pyroxene. mineral content and textural differences. The thin sections were examined using a XRD Analysis petrographic microscope under plane A couple of the samples were polarized light (PPL) and cross polarized prepared for mineral analysis using x-ray light (XPL) viewed at 40x and 100x diffraction. The samples were crushed and magnification. Textural differences were sieved to a size smaller than 63µm. The most easily recognized in PPL due to the samples were analyzed using a Cu anode in abundance of opaque minerals in the mafic the Miniflex X-ray diffractometer with dike samples. Individual mineral relief and continuous collection between 2θ angles of textures were also apparent in PPL including 5 and 70°. Background noise and kα2 euhedral to anhedral crystal forms as well as interference were removed from the crystal habit. In XPL, the extinction angles diffraction patterns prior to peak analysis. of feldspars were used to determine the Peak locations and intensity were compared range of composition as An content based with powder diffraction files for known on the Michel-Levy method for plagioclase. minerals for mineral composition. Twinning patterns in K-feldspars were used to identify microcline apart from orthoclase.

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U-Pb Isotopic Analyses Silvermines granite in contact with the Prior to isotopic analysis, zircon diabase, and the granite at a distance of 30 m were separated into magnetic from the contact with the diabase. These fractions, chosen for clarity, and abraded. samples were prepared for groundmass and Individual crystals were placed in a solution mineral separates at the Nevada Isotope of HF/HNO3 followed by HCl for Geochronology Laboratory including K- dissolution. After dissolution a 205Pb/325U feldspar, amphibole, and biotite. The tracer was added prior to analysis. Analyses samples were run as conventional furnace were completed at the University of Kansas step heating analyses to produce an apparent Isotope Laboratory using a age spectrum. This age spectrum assumes VG Sector thermal ionization mass that the non-radiogenic argon is at spectrometer with ion-counting Daly atmospheric levels with 40Ar/36Ar = 295.5. multiplier for isotopic ratios in the samples. If the ratio between these two isotopes of Ar -9 The decay constant of 0.155125x10 /year is greater than 295.5, the excess 40Ar will was used for 238U for and 0.98485x10-9/year 235 produce an older age and manifest itself as a for U (Steiger and Jaiger, 1977). See Rohs U-shaped spectra shape. Total gas ages and Van Schmus (2007) for additional using 40Ar/ 39Ar are equivalent to K/Ar and information on procedures. represent the primary data used in this study

since isochrones were not obtained and the Ar-Ar Isotopic Analyses samples did not yield significant plateaus Three samples were selected for Ar- during analysis. Ar isotopic analyses including the large diabase intrusion at Silver Mines, the

RESULTS Mineralogy and Petrology intrusions, is . Magnetite typically The mineral content and textures in occurs as euhedral or subhedral crystals the diabase dikes are quite variable as shown within most thin sections of the diabase. in Figures 3a-c, 4. In nearly all of the Several minerals could be identified in thin diabase samples, the primary mineral was section including quartz, calcite, or plagioclase. In the large dike at Silver hematite, , and pyrite. Other minerals Mines, plagioclase occurs in lath-shaped occurred as fine crystals or as alteration crystals with a composition of An45-55 as products including serpentine, sericite, and determined by the Michel-Lévy method chlorite. using extinction angles for a low- Mineralogy of the diabase was also temperature (Tobi and Kroll, determined by XRD and is shown in Figure 1975). Plagioclase makes up approximately 5. Peak location and intensity were used to 60-70% of the minerals contained within the identify andesine as the plagioclase variety diabase. The next most abundant mineral present in the sample. Magnetite and calcite based on thin section analyses of the diabase have also been identified as specific peaks

The Compass: Earth Science Journal of Sigma Gamma Epsilon, v. 85(3), 2013 Page 83 within the powder diffraction pattern. Two several of the smaller dikes as well. They additional silicate minerals, low quartz and can occur alone or in clusters. Mineralogy antigorite serpentine, were indicated in the and texture within the spherical structures sample based on statistical matching of the tend to fall into three categories. The first diffraction pattern with known minerals. group has a distinct sphere of calcite in the Texture within the large diabase interior and anhedral sulfide bodies within exhibited a gradual increase in grain size the calcite. The sulfide bodies are opaque in from the chill margin to the center of the transmitted light but a brassy- color in dike as shown in Figure 3. Spherical reflected light. A second group of these structures ranging from 0.5-1.0 mm in spherical structures have silicates and oxides diameter occur within the diabase (Figure 4a as fine-grained aggregates within the and b) and are more prevalent in some areas spheres. Mineralogy within the second compared with others. The spherical group appears to be plagioclase and ilmenite structures are typically rimmed with or hematite. The third type of spherical euhedral crystals of magnetite. structure is an aggregate of magnetite The occurrence of these spherical crystals and appears opaque in thin section structures is somewhat variable. They are (Aldeiri et al., 2010). found throughout the large dike and in

a. b. c. Figure 3a-c. Photomicrographs from the chill margin on the left to the center of the dike on the right showing the change in grain size. All photomicrographs have a viewing field of 4.5 mm across.

Figure 4a and b. Spherical structures in diabase samples as shown in plane-polarized light. The extent of the viewing field is 4.5 mm. Photomicrograph 4b. from Barnes, et al., 2010.

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Figure 5. Interpreted XRD powder diffraction file for Silvermines diabase.

Isotopic Ages zircon fractions were used to construct a Isotopic ages were determined for concordia plot shown in Figure 6 with an several samples collected from the intercept age of 1474±7 Ma. Silvermines Granite and intrusive diabase Ar-Ar isotopic data varied depending dikes. The resulting data exhibits an age on the rock type, minerals analyzed and range from 1473 to 960 Ma depending on relative distance. A sample of the the isotopic system and minerals analyzed. groundmass from the large diabase dike at Isotopic data are given in Tables 1 and 2 (at Silver Mines was analyzed and produced a end of paper). Zircons collected from the moderately discordant age spectrum, with large diabase dike were analyzed for U-Pb ages that progressively increased from an ages. Three zircon fractions were analyzed initial step of ~713 Ma to a “plateau like” for U-Pb isotopic composition and ages segment (steps 5-8) at ~1200-1280 Ma, were calculated. Two of the zircon fractions followed by decreasing ages with the last resulted in 207*Pb/206*Pb ages of 1473 and ~20% gas released. The total gas age of the 1477 Ma. Using these two zircons, a diabase is 1167.3  7.9 Ma, and there were concordia plot was constructed as shown in no plateau or isochron ages defined for the Figure 6 with an intercept age of 1474±7 diabase as shown in Figure 7a. Ma. The other zircon fraction was reversely A sample of K-feldspar from the discordant and anomalous with a 206*Pb/238U Silvermines Granite at the contact with the age of 339 Ma. Therefore, the two previous diabase displays a very discordant age

The Compass: Earth Science Journal of Sigma Gamma Epsilon, v. 85(3), 2013 Page 85 spectrum, with numerous features which the amphibole in the granite at the contact is suggest the presence of alteration and excess 904.8  8.2 Ma (Figure 7c). argon. The age spectrum begins with high, Finally, both biotite and K-feldspar and variable ages, which decrease until were analyzed in a sample of the ~45% gas released. From ~45-80% gas Silvermines Granite at a distance of ~10 m released the sample exhibits progressively from the contact with the diabase. The increasing ages, after which the ages biotite sample yielded a highly disturbed, decrease and then increase dramatically for discordant age spectrum (Figure 7d). Ages the final ~15% gas released. The total gas decrease from an initial step of ~1098 Ma to age is 1259.0 8.6 Ma. There were no ~525 Ma for the second step, followed by plateau or isochron ages defined for this generally increasing ages for steps 3-9. The sample. The initial ~8 steps, all at total gas age of the biotite is 1066.0  7.8 temperatures <600 oC, comprise an Ma. Aside from evidence for excess argon unusually large amount of gas release in the first ~10% gas released for the K- (~29%) for a K-feldspar as shown in Figure feldspar, the sample produced an age 7b. In comparison, amphibole from the spectrum which is very likely to be useful same sample produced a mildly discordant for multi-domain modeling with a total gas (i.e. mostly flat) age spectrum with most age of 1119.3  7.5 Ma (Figure 7e). steps around 960 Ma. The total gas age for

Figure 6. Concordia plot for zircons from Silvermines diabase.

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Figures 7a-e. Age spectrums for Ar gas release for a. diabase, b. K-feldspar at the contact, c. amphibole, d. biotite, and e. K-feldspar at ~10 m from the contact.

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DISCUSSION liquid droplets that crystallized during the original emplacement of the diabase dike. The mineralogy of the diabase dikes The occurrence of magnetite around the supports the interpretation of mafic to outside of these structures implies that they ultramafic composition. Although the acted as a nucleation surface for the individual dikes vary in mineralogical mode, magnetite. The calcite within the the primary minerals are plagioclase and segregation vesicles is fine grained and does magnetite. There is significant evidence of not contain void spaces as expected during alteration with the presence of serpentine, secondary filling of void spaces. Opaque chlorite and sericite in the groundmass of minerals in the spherical structures include the diabase samples. The spherical features pyrite and ilmenite. For example, the pyrite are the most unusual features of these is found within the calcite while ilmenite samples. Information from the thin section occurs with fine plagioclase laths. The analyses supports the presence of calcite and ilmenite is concentrated within the spherical sulfide minerals within spherical structures. structures but does occur elsewhere in the Because these structures are separate from finer matrix between plagioclase laths in the calcite veins, they most likely represent mass of the rock. segregation vesicles. Based on the texture The large diabase dike at Silver and mineral layout within the segregation Mines has been previously recognized as vesicles, they are interpreted as immiscible part of the Silver Mines dike swarm

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(Sylvester, 1984, Walker, et al., 2002). However, the K-feldspar at a distance of ~10 These intrusive structures along with a mafic m from the diabase is much more reliable volcanic flow exposed along the Black River and indicates a total gas age of at Johnson’s Shut-Ins, have been linked to approximately 1119 Ma. This age is similar one another geochemically (Sylvester, to the total gas age of the diabase at 1167. 1984). As a result, the Silver Mines dike These ages roughly correspond to the swarm has been interpreted as forming beginning stages of the Midcontinent Rift shortly after the emplacement of the system (Van Schmus and Hinze, 1993) to Silvermines granite (Sylvester, 1984 and the north and west of the study area. The Walker, et al. 2002) at 1484 Ma (Van thermal upwelling associated with the Schmus, et al., 1993). Data from this study opening of the Midcontinent Rift may have supports this interpretation suggesting that been sufficient to thermally reset the rocks the diabase dikes were emplaced as early as in the area though this should be evident 1474 Ma. However, since zircons are not throughout plutonic and volcanic rocks of commonly found in diabase, it is possible the Eastern Granite and Rhyolite province if that the zircons are xenocrysts from the this is the case. surrounding granite. If that is the case, then In conclusion, there appears to be the Ar-Ar age for the diabase groundmass evidence of a mafic magmatic event in could be interpreted as the minimum age of which a number of diabase dikes intruded intrusion at 1167 Ma. the granite and rhyolite units exposed near Studies of the structural state of K- the Silver Mines Recreation area in feldspar in the St. Francois Mountains southeast Missouri. This mafic magmatic suggest a large-scale thermal event that reset event exhibits evidence of immiscible both the plutonic and volcanic felsic rocks enriched in magnetite, calcite, and within the area (Plymate, et al., 2001, 1992, sulfides. These mafic intrusions may have Collins and Rohs, 2004). Based on the Ar- been emplaced as early as 1474 Ma or as Ar data from the Silvermines Granite, there late as 1167 Ma. If the magmatism occurred is evidence for at least one event of thermal at the more recent age, it may have been disruption. Thermal resetting may have associated with the structural resetting of K- taken place around 1240 Ma as indicated by feldspar within the region. If the a plateau-like region of the diabase and the magmatism was earlier, then the diabase, K- total gas age of K-feldspar at the contact feldspar, and biotite all show evidence of a with the diabase. On a regional level, the major thermal event around 1120±50 Ma. Grenville Orogeny was taking place during In any case, the Ar-Ar data presented here this time to the east but the volcanic and supports the evidence of earlier studies plutonic units in the study area are suggesting a thermal overprint in the undeformed (Lidiak, et al., 1993). The igneous rocks of the St. Francois Mountains. highly discordant spectrum associated with the K-feldspar at the contact with the diabase provides limited interpretation.

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ACKNOWLEDGEMENTS Bickford, M.E., Harrower, K.L., Hoppe, This research was supported by a W.J., Nelson, B.K., Nusbaum, R. L., number of Applied Research Grants and and Thomas, J.J., 1981. Rb-Sr and Undergraduate Research Grants provided by U-Pb geochronology and distribution Northwest Missouri State University of rock types in the Precambrian through the Faculty Research Committee, basement of Missouri and Kansas. Dr. Charles McAdams – Dean of the Geological Society of American College of Arts and Science, and Dr. Bulletin. 92, 323-341. Gregory Haddock – Vice Provost. I would Collins, R. D. and Rohs, C.R., 2004, like to thank Dr. W.R. (Randy) Van Schmus Mineralogy of granite and rhyolite and the Isotope Geology Laboratory at the units in the St. Francois Mountains. University of Kansas for preparing and GSA Abstracts with Programs, v.36, analyzing zircon samples for U-Pb isotopic n. 2. data as well as informative discussions at Kisvarsanyi, E.B., 1981, Geology of the many points along the way. I would also Precambrian St. Francois terrane, like to thank Dr. Terry Spell and Kathleen Southeastern Missouri: Zanetti at the Nevada Isotope Contributions to Precambrian Geochronology Laboratory for preparing Geology, v. 64, n. 8, 58p. and analyzing mineral and rock samples for Lidiak, E.G., Bickford, M.E., and Ar-Ar isotopic data. Finally, a special Kisvarsanyi, E.B., 1993. Proterozoic thanks to all of those undergraduate students geology of the eastern midcontinent who have worked with me on this project region, in Reed, J.C., Jr., and six collecting, cutting, and crushing rock others, eds., Precambrian: samples including Rachael Collins, Allen Conterminous U.S.: Boulder, Anderson, and Kati Tomlin. Colorado, Geological Society of America, Geology of North America REFERENCES CITED v. C-2, pp. 270-281. Aldieri, M., Johnson, A. W. and Rohs, C. R., Meert, J.G. and Stuckey, W., 2002. 2010, Petrology of the non-felsic Revisiting the paleomagnetism of the rocks associated with the 1.476 Ga St.Francois Mountains Silvermines Granite. GSA Abstracts igneous province, Missouri. with Programs, Vol. 42, No. 2. Tectonics v. 21 n. 2, p. 1007 Barnes, M.A., Rohs, C.R., Anthony, E.Y., Plymate, T.G., Kendall, J.D., Shepard, L.M. Van Schmus, W.R., and Denison, and Clark, K.C., 2001. Structural R.E., 1999, Isotopic and elemental state of K-feldspar in the felsic chemistry of subsurface Precambrian volcanic rocks and ring pluton igneous rocks, west Texas and New of the Butler Hill Caldera, Mexico: Rocky Mountain Geology, St. Francois Mountains,southeastern v. 34, no. 2, p. 245-262. Missouri; Can Mineral v. 39, p. 73- 83

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Plymate, T.G., Daniel, C.G., and Cavaleri, Planetary Science Letters, v. 36, p. M.E., 1992. Structural state of K- 359-362. feldspar in the Butler Hill-Breadtray Sylvester, P.J., 1984, Geology, petrology, Granite, St. Francois Mountains, and tectonic setting of the mafic southeastern Missouri; Can Mineral rocks of the 1480 Ma old granite- v. 3, p. 367-376 rhyolite terrane of Missouri, USA; Rämö, O.T., Boyd, W.W., Vaaskoki, M., Thesis (Ph.D.) Washington Cameron, R.L., and Ryckman, D.A., University, Dept. of Earth and 1994, 1.3 Ga mafic magmatism of Planetary Sciences, 455 p. the St. Francois Mountains of SE Tobi, A.C. and Kroll, H., 1975, Optical Missouri., implications for mantle determination of the An-content of composition beneath mid-continental twinned by the USA: Miner. Mag. 58A, 754-755. Carlsbad-law: A revised chart: Rohs, C.R., 2001. Identifying American Journal of Science, v. 275, Paleoproterozoic and Meso- p. 731-736. proterozoic crustal domains within Thomas, J.J., Shuster, R.D., and Bickford, the Southern Granite and Rhyolite M.E., 1984. A terrane of 1350- to province, mid-continent North 1400-m.y.-old silicic volcanic and America, Ph.D. thesis, University of plutonic rocks in the buried Kansas. Proterozoic of the mid-continent and Rohs, C. R., 2003. Mafic intrusions in the in the Wet Mountains, Colorado. St. Francois Mountains of Southeast Geological Society of America Missouri: A study of the petrology, Bulletin. 95, 1150-1157. geochemistry, geochronology, and Van Schmus, W.R., Bickford, M.E., and tectonic emplacement. GSA Zetiz, I., 1987. Early and middle Abstracts with Programs, v. 35, n. 2 Proterozoic provinces in the Central Rohs, C.R. and W.R. Van Schmus, 2007. United States: Proterozoic Isotopic connections between Lithospheric Evolution AGU basement rocks exposed in the St. Geodynamics Series, v.17, pp. 43- Francois Mountains and the 68. Arbuckle Mountains, southern mid- Van Schmus, W.R., Bickford, M.E., Sims, continent, North America. P.K., Anderson, R.R., Shearer, C.K., International Journal of Earth and Treves, S.B., 1993. Proterozoic Sciences, v. 96, p. 599-611. geology of western midcontinent Steiger R.H. and Jaiger, E., 1977, basement, in Reed, J.C., Jr., and six Subcommission on geochronology others, eds., Precambrian: convention on the use of decay Conterminous U.S.: Boulder, constants in geo- and Colorado, Geological Society of cosmochromonology. Earth America, Geology of North America v. C-2, pp. 239-259.

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Van Schmus, W.R., and Hinze. 1993 of the east-central Midcontinent Midcontinent Rift System, in Reed, basement. GSA Special Paper 308, J.C., Jr., and six others, eds., 7-32. Precambrian: Conterminous U.S.: Walker, J.A., Pippin, C.G., Cameron, B.I., Boulder, Colorado, Geological and Patino, L., 2002, Tectonic Society of America, Geology of insights provided by Meso- North America v. C-2, pp. 292-303. proterozoic mafic rocks of the St. Van Schmus, W.R., Bickford, M.E., and Francois Mountains, southeastern Turek, A., 1996. Proterozoic geology Missouri.

Table 1. U-Pb Isotopic Data and Ages

Isoplot Size U Pb Pb206 Pb207* ±2σ Pb206* ±2σ Correl Data Pb204 U235 U238 . Coeff. Sample (mg) ppm ppm (obs.) (pct) (pct) (rho) SFM- 0.001 307 78 1361 2.9293 0.83 0.2302 0.69 0.864 1V3 8

SFM- 0.005 132 39 690 3.1933 1.10 0.2504 0.63 0.609 1V1a 3

SFM-1V4 0.001 870 48 305 0.2926 3.37 0.0540 1.42 0.497 2

Calculat Pb207* ±2σ Pb206 ±2σ Pb207* ±2σ Pb207* ±2σ ed Ages Pb206* * U235 Pb206* U238 Sample (pct) Age (Ma) Age (Ma Age (Ma) ) SFM- 0.09226 0.42 1336 9 1390 12 1473 8 1V3

SFM- 0.09248 0.87 1441 9 1456 16 1477 17 1V1a

SFM-1V4 0.03929 2.94 339 5 261 9 1 0

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Table 2: Ar-Ar isotopic data associated with spectrums shown in Figure 7a-e.

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