Structural state of K-feldspar in the younger granites of the western St. Francois Mountains, southeastern Missouri: further evidence for pervasive alteration of the Mesoproterozoic St. Francois Mountains complex.

Abstract: K-feldspar in the younger granites exposed in the western St. Francois Mountains of southeastern Missouri shows a high degree of structural order. The structural state parameter Z (defined to vary from 0 for perfectly disordered sanidine to 1 for perfectly ordered microcline) averages 0.94 for the Graniteville Granite and O. 87 for the Munger Granite Porphyry. Within the precision and scatter of the data, the structural state of the K-feldspar in these rock units is identical to that reported earlier for both the plutonic and volcanic rock units of the Butler Hill Caldera, the major structure that dominates the eastern portion of the St. Francois Mountains complex. There is no significant variation of K-feldspar structural state with present topographic elevation or with lateral distance from the Simms Mountain Fault, the structure bounding the St. Francois Mountains complex to the northeast. We interpret the homogeneity of the K-feldspar structural state throughout the St. Francois Mountains complex as evidence for a pervasive alteration event. We believe the most likely cause of this alteration is the heat and/or metasomatic fluid associated with the final major phase of Mesoproterozoic igneous activity involved in formation of the St. Francois Mountains complex.

Key Words: K-feldspar, structural state, St. Francois Mountains, Mesoproterozoic, Missouri, Graniteville Granite, Munger Granite Porphyry

Introduction

The St. Francois Mountains (SFM) of southeastern Missouri provide the only significant exposures of the Precambrian basement in the central mid-continent region. Erosion of the Paleozoic sedimentary rocks off the top of the Ozark Dome has exposed a series of felsic volcanic rocks and epizonal granitic plutons of the Mesoproterozoic Eastern Granite-Rhyolite Province (Pratt and others, 1979; Kisvarsanyi, 1981; Sims, 1990; Van Schmus and others, 1993).

The eastern portion of the St. Francois Mountains is dominated by a large, partially collapsed caldera structure, the Butler Hill Caldera (Sides and others, 1981; Lowell, 1991). This structure exposes a sequence of rhyolitic ash-flow tufts and other layered felsic volcanic rocks, a large zoned granitic subvolcanic massif (the Butler Hill-Breadtray Granite), and a series of granitic ring plutons (the Silvermines and Slabtown Granites), all formed by magmatic activity at 1.47 [+ or -] 0.03 Ga. (Lowell, 1991; Van Schmus and others, 1996). In an attempt to clarify the geometry of the Butler Hill Caldera, Plymate and others (1992) and Plymate and others (2001) determined the structural state of the K-feldspar in these rock units. They discovered that the K-feldspar is consistently highly ordered everywhere within the Butler Hill Caldera, regardless of rock type (volcanic versus plutonic) or inferred depth of crystallization. They concluded that the structural state of the K-feldspar must have been homogenized by a pervasive re-heating event coincident with or subsequent to the final episode of crystallization within this caldera.

The western portion of the St. Francois Mountains is dominated by felsic volcanic rocks formed at approximately the same time as those associated with the Butler Hill caldera to the east (Van Schmus and others, 1996; Menuge and others, 2002; M. E. Bickford, personal communication cited by Walker and others, 2002). A second caldera structure, the Taum Sauk Caldera, is inferred to exist beneath the western St. Francois Mountains (Sides and others, 1981; Pratt and others, 1979), but it is less deeply eroded than the Butler Hill Caldera and its internal structure is therefore less well exposed. Two felsic plutonic bodies, the Graniteville Granite and the Munger Granite Porphyry, are exposed in the western St. Francois Mountains. At 1.36 [+ or -] 0.03 and 1.38[+ or -] 0.06 Ga. respectively, these plutons are the youngest felsic units yet dated within the St. Francois Mountains complex (Van Schmus and others, 1996). The difference in age between these granites and the volcanic rocks they intrude suggests that formation of these plutons was not associated with collapse of the Taum Sauk Caldera, but rather that these plutons represent a second, separate phase of felsic magmatism, perhaps associated with formation of the adjacent 1.34-1.40 Ga Southern Granite-Rhyolite Province (Bickford and Van Schmus, 1985; Van Schmus and others, 1993; Menuge and others, 2002).

The purpose of this study was to determine whether the structural state of the K-feldspar in the younger granites of the western St. Francois Mountains has been reset to the same extent as in the older rocks of the Butler Hill Caldera and to thereby further constrain the extent and timing of the process(es) or event(s) responsible for resetting the K-feldspar structural state in the felsic rocks of the St. Francois Mountains.

Experimental Procedure

We analyzed six samples of the younger granites of the western St. Francois Mountains, three samples from the Graniteville Granite and three from the Munger Granite Porphyry. In collecting the samples, we took care to avoid dikes, veins, and other obvious contamination, but the exposure in much of the western St. Francois Mountains is not sufficiently good to yield completely fresh samples. Each sample was slabbed to approximately one centimeter in thickness, and any visible weathering rind and/or xenolith material was trimmed away. The remaining material was then crushed and sieved to <120 mesh.

K-feldspar-rich chips were initially separated by density in a centrifuge using acetylene tetrabromide diluted to [rho]=2.595 with dimethyl formamide. Material floating at this density was filtered, rinsed with acetone, dried, and pulverized in a tungsten carbide ball mill, and sieved to <325 mesh. K-feldspar was then concentrated from this powder by density in a centrifuge using acetylene tetrabromide diluted to [rho]=2.58.

The K-feldspar concentrates were analyzed by x-ray diffraction using CuK[alpha] radiation in a Scintag XDS-2000 diffraction system with a theta-theta goniometer and a germanium solid-state detector. Diffraction spectra were collected from 20[degrees] to 52[degrees] 2[theta] at a scan rate of 0.10[degrees]/minute. Before collecting each data spectrum, the goniometer was calibrated using finely powdered quartz (Fisher Reagent Grade Si[O.sub.2]) as an external standard.

For each sample we obtained a preliminary estimate of the K-feldspar structural state from the (060) and the (204) peaks using the method of Wright (1968). We then indexed the remaining K-feldspar peaks assuming a triclinic unit cell according to Appendix A11 of Ribbe (1983). Unit cell refinements were calculated by multiple regression according to the procedure of Appleman and Evans (1973) using the 14 K-feldspar singlet peaks that are independent of any potential quartz or albite interference.

Results

Table 1 lists the results of our K-feldspar unit cell refinements and the structural-state parameters calculated from those refined lattice parameters. The uncertainty listed for each lattice parameter is the average standard error for that parameter generated in the six unit-cell refinement regressions. Each of these values is listed with one digit beyond the first uncertain digit so they can be used in subsequent calculations without the introduction of round-off error (Bevington, 1969; Lyon, 1970). The uncertainty listed for each calculated structural-state parameter was determined by propagating the experimental uncertainties through the defining equations according to the techniques discussed by Bevington (1969) and Meyer (1975).

The two most common methods for representing K-feldspar structural data are (1) to plot the b versus c lattice dimensions (Wright and Stewart, 1968; Kroll and Ribbe, 1983), and (2) to calculate a composite structural state parameter Z from the percentage of A1 occupying each of the four unique tetrahedral sites in the unit cell (Thompson, 1969). Defined as Z [equivalent to] [t.sub.1]o + [t.sub.1]m - [t.sub.2]o - [t.sub.2]m, this Z parameter varies from 0 for completely disordered sanidine to 1 for completely ordered microcline. Kroll and Ribbe (1987) provide a method for estimating A1 site occupancies from the lattice parameters for a K-feldspar unit cell refinement.

The K-feldspar in the younger granites of the western St. Francois Mountains shows a consistently high degree of structural order. Figure 1 shows the variation in the b and c lattice parameters in our analyses of the K-feldspar in the Graniteville Granite and the Munger Granite Porphyry. For comparison, this figure also shows the b-c variation in the K-feldspar in all the major rock felsic units of the Butler Hill Caldera as analyzed by Plymate and others (1992) and Plymate and others (2001). Samples from each rock unit cluster near, but not coincident with, the microcline end-point (Figure 1).

[FIGURE 1 OMITTED]

The structural state parameter Z averages 0.94 for our three samples of the Graniteville Granite and 0.87 for our three samples of the Munger Granite Porphyry. Within the precision and scatter of the data, there is no significant difference between these averages. Figure 2 compares the average Z value for these two younger granite rock units of the western St. Francois Mountains to the corresponding values for each of the major felsic igneous rock units exposed in the Butler Hill Caldera. Again, within the precision and scatter of the data, there is no significant difference between these average Z values, either among the three different modes of crystallization represented (volcanic phenocryst versus volcanic matrix versus pluton) or between the two ages of plutons represented.

[FIGURE 2 OMITTED]

Discussion

Plymate and others (1992) showed that the structural state of the K-feldspar in the Butler Hill Breadtray Granite is highly ordered, very homogenous, and virtually independent of inferred depth of crystallization. Plymate and others (2001) extended that observation to include the ring plutons and both the phenocrysts and the matrix of the felsic volcanic rock units of the Butler Hill Caldera. By applying the same type of analysis to the younger granites of the western St. Francois Mountains, the present study extends that observation to include all the major felsic igneous rock units exposed in the St. Francois Mountains Complex.

Because microcline is the thermodynamically stable state for K-feldspar under ambient conditions, one might expect re-equilibration to a highly ordered structure to be the norm for ancient igneous complexes. However, in the absence of hydrothermal, metasomatic, or metamorphic conditions, the driving force for the orthoclase-to-microcline transition is very low (Brown and Parsons, 1989), and K-feldspar with intermediate to low structural order has been documented from a number of unaltered Precambrian igneous complexes (Martin and Falster, 1986; Schandl and others, 1986; Stevenson and Martin, 1988). Therefore, we interpret the homogeneity of the K-feldspar structural state across the entire St. Francois Mountains Complex regardless of inferred mode or depth of crystallization (volcanic phenocryst versus volcanic matrix versus hypabyssal ring pluton versus subvolcanic massif) or crystallization age (1.47 Ga versus 1.36-1.38 Ga) to be evidence for a single, pervasive alteration event subsequent to (or coincident with) the last major igneous event in the area.

Petrographic observation of the samples analyzed in the present study, as well as the samples studied by Plymate and others (1992) and Plymate and others (2001), reveals that the feldspars are significantly clouded by alteration to cryptocrystalline clays, mica, and/or hematite. Numerous other studies of the felsic rocks of the St. Francois Mountains Complex have reported this same type of alteration of the feldspars (Tolman and Robertson, 1969; Anderson, 1970; Sides and others, 1981; Brown, 1983, 1984; Lowell, 1991; Menuge and others, 2002). Plymate and others (2001) suggested that the homogenization of the structural state probably resulted from the same event or process responsible for this clouding of the feldspars, and they reasoned that any such event or process must have been coincident with or subsequent to the final major crystallization event in the Butler Hill Caldera. The present study suggests that this alteration event or process was not confined to the Butler Hill Caldera and must have occurred coincident with or subsequent to the final major felsic igneous event recorded in the St. Francois Mountains Complex.

A number of processes and events have been proposed to account for alteration in the Mesoproterozoic rocks of the St. Francois Mountains. Those processes or events that might also account for re-setting the K-feldspar structural state throughout the St. Francois Mountains Complex include ...

1. heat and/or metasomatic fluids associated with alkaline magmatism at approximately 1.36 Ga. which produced two-mica "tin-granite" or "HHP" (high heat production) granite plutons, including the Graniteville Granite (Kisvarsanyi, 1980, 1981; Kisvarsanyi and Kisvarsanyi, 1989).

2. heat and/or metasomatic fluids associated with emplacement of the Skrainka suite of mafic intrusive bodies at approximately 1.33 Ga (Amos & Desborough 1970; Honda and others, 1985; Ramo and others, 1994; Lowell & Ramo 1999; Lowell & Young 1999; Walker and others, 2002).

3. migration of fluids associated with development of the Simms Mountain Fault and other related northwest-striking transfer faults formed during the Late Proterozoic--Early Cambrian opening of the Reelfoot Rift (Clendenin and others, 1989; Lowell, 1991).

4. migration of fluids responsible for dolomitization of the Upper Cambrian Bonneterre Formation (Stein and Kish, 1991; Shelton and others, 1992).

5. migration of fluids responsible for formation of the "Mississippi Valley-type" Pb-Zn-Cu sulfide ore deposits within the Upper Cambrian strata (Hagni, 1989; Aleinikoff and others, 1993; Clendenin and others, 1994).

6. migration of fluids associated with reactivation of the Simms Mountain Fault and other related northwest-striking faults during the Middle-to-Late Paleozoic uplift of the Ozark Dome (Clendenin and others, 1989; Lowell, 1991).

Alteration in the Mesoproterozoic igneous rocks of the St. Francois Mountains complex caused primarily by vertical migration of fluids through the overlying Paleozoic sedimentary rocks (events #4 or #5 above) would be expected to vary vertically with present topographic elevation. Sutton and Maynard (1996) reported just such a relationship for alteration in the Paleozoic strata in five cores drilled around the perimeter of the St. Francois Mountains. However, K-feldspar structural state in the underlying igneous rocks shows no corresponding variation with present elevation (Figure 3A). We therefore conclude that the process responsible for homogenization of the K-feldspar structural state must have been something other than, and more pervasive than, vertical migration of fluids through the overlying sedimentary rocks.

[FIGURE 3 OMITTED]

Alteration in the Mesoproterozoic igneous rocks of the St. Francois Mountains complex caused primarily by lateral migration of fluids from the Simms Mountain Fault and related northwest-trending faults (events #3 and #6 above) would be expected to vary laterally with distance from those structures. Wenner and Taylor (1976) found that oxygen isotope ratios in most major igneous rock units exposed in the St. Francois Mountains have been reset by reaction with low-temperature (<200[degrees]C) hydrothermal fluids, and Lowell & Clendenin (1991) demonstrated that the extent of this isotopic disturbance varies proportionally with proximity to the Simms Mountain Fault and the parallel fault to the southwest, the Black Fault. However, K-feldspar structural state in these same igneous rock units shows no corresponding variation with distance from these faults (Figure 3B). We therefore conclude that the process responsible for homogenization of the K-feldspar structural state must have been something other than, and something more pervasive than, fault-controlled migration of low-temperature hydrothermal fluids.

We believe the most likely cause of the homogenization of the K-feldspar structural state throughout the St. Francois Mountains is the heat and/ or metasomatic fluid associated with the final major phase of Mesoproterozoic igneous activity recorded in the complex (events #1 and #2 above). Although rocks formed by this magmatic activity represent a very small fraction of the present surface exposure in the Francois Mountains, numerous lines of evidence suggest they are much more abundant in the subsurface. Kisvarsanyi (1980, 1981) determined that a significant fraction of the portion of the St. Francois terrane presently buried under Paleozoic sedimentary rocks consists of plutons of alkali-rich two-mica granite that are relatively enriched in Sn, U, and Th. The Graniteville Granite is the only example of these "tin granite" or "HHP" (high heat production) granite plutons exposed at the surface, but fourteen others have been identified from drill cores (Kisvarsanyi and Kisvarsanyi, 1989). Walker and others (2002) summarize geophysical data suggesting that mafic plutons may be common in the upper crust beneath the St. Francois Mountains. We suspect the Skrainka suite of mafic intrusive bodies may represent merely the "tips of some much larger icebergs." Therefore, we suggest that the homogenization of the K-feldspar structural state throughout the St. Francois Mountains complex reflects the pervasiveness of the 1.36-1.33 Ga igneous activity in this area.

TABLE 1. Unit-cell and structural state parameters for
K-feldspar in the younger SFM granites

Sample                    Northing *   Easting *   Elev.     a
                             (m)          (m)      (ft.)

Graniteville Granite

  G-3                      4170169      704289     1180    8.6040
  G-4                      4171434      703699     1140    8.6072
  G-5                      4168072      703506     1260    8.6012

Munger Granite Porphyry

  M-6                      4163058      691446     1280    8.6400
  M-7                      4156096      692108     1450    8.6081
  M-8                      4155133      691964     1350    8.6118

Uncertainty ([section])                                    0.0038

Sample                         b          c      [alpha]
                          (Angstroms)

Graniteville Granite

  G-3                       12.9742     7.2262   89.998
  G-4                       12.9683     7.2241   90.286
  G-5                       12.9750     7.2225   90.392

Munger Granite Porphyry

  M-6                       12.9628     7.2163   90.324
  M-7                       12.9735     7.2215   90.026
  M-8                       12.9817     7.2250   90.466

Uncertainty ([section])     0.0027      0.0013    0.038

Sample                     [beta]     [gamma]   [t.sub.1]o
                          (degrees)             ([dagger])

Graniteville Granite

  G-3                      115.869    88.038       0.92
  G-4                      116.016    87.940       0.92
  G-5                      116.033    87.846       0.93

Munger Granite Porphyry

  M-6                      116.016    88.860       0.72
  M-7                      116.130    88.021       0.87
  M-8                      115.943    87.921       0.92

Uncertainty ([section])     0.021      0.044       0.02

Sample                    [t.sub.1]m   Z ([double
                          ([dagger])   ([dagger])

Graniteville Granite

  G-3                        0.09         1.01
  G-4                        0.05         0.94
  G-5                        0.01         0.87

Munger Granite Porphyry

  M-6                        0.23         0.90
  M-7                        0.03         0.80
  M-8                        0.03         0.90

Uncertainty ([section])      0.02         0.03

* Universal Transverse Mercator Grid, Zone 15, NAD27

([dagger]) Kroll & Ribbe (1987) equations 6 and 12

([double dagger]) Z [equivalent to] [t.sub.1]o +
[t.sub.1]m - [t.sub.2]o - [t.sub.m] where [t.sub.2]o
= [t.sub.2]m = (1 - (t.sub.1]o + [t.sub.1]m))/2
(Kroll and Ribbe, 1983)

([section]) Uncertainties for a, b, c, [alpha], [beta],
and [gamma] are the averages of the standard errors for
those parameters from the six unit-cell refinements;
uncertainties for [t.sub.1]o, [t.sub.1]m, and Z were
determined by propagating the experimental uncertainties
through the defining equations (Bevington, 1969; Meyer,
1975)

Acknowledgments

We thank the U.S. National Science Foundation (Grant # EAR-9218906) and Southwest Missouri State University for providing funding for the x-ray diffraction system used in this study. We thank Robert F. Martin, McGill University, for providing the reference microcline sample used to confirm the accuracy of our experimental procedure. We also gratefully acknowledge helpful suggestions from Vernon M. Brown, Gary R. Lowell, Cheryl M. Seeger, and two anonymous reviewers.

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Cheryl M. Mathenia and Thomas G. Plymate Department of Geography, Geology, and Planning Southwest Missouri State University Springfield, MO 65804