Tectonic insights provided by Mesoproterozoic mafic rocks of the St. Francois Mountains,...

Tectonic insights provided by Mesoproterozoic mafic rocks ofthe St. Francois Mountains, southeastern Missouri

James A. Walker a,*, Charles G. Pippin b,1, B.I. Cameron c,2, Lina Patino d,3

a Department of Geology and Environmental Geosciences, Northern Illinois University, DeKalb, IL 60115 2854, USAb North Carolina Department of Environment and Natural Resources, Groundwater Section, Mooresville, NC 28115, USA

c Department of Geosciences, University of Wisconsin-Milwaukee, Milwaukee, WI 53201, USAd Department of Geological Sciences, Michigan State University, East Lansing, MI 48824-1115, USA

Received 17 July 2001; accepted 28 June 2002

Abstract

Although Mesoproterozoic silicic rocks are predominant in the St. Francois Mountains of southeastern Missouri,

basalts, basaltic andesites and their plutonic equivalents are not uncommon. These mafic rocks fall into two distinct

petrologic suites as first discerned by Sylvester (Ph.D. thesis (1984) 588). One, the Silver Mines suite, consists of mafic

rocks formed contemporaneously with the voluminous silicic rocks. The second suite, the Skrainka suite, originated

from mafic magmatism that may have postdated silicic activity. The rocks of both suites have a number of incompatible

element indices typically associated with subduction zone environments. This suggests that the voluminous silicic

magmatism of the St. Francois Mountains, and contemporaneous portions of the Granite�/Rhyolite Provinces,

originated during subduction along an active continental margin or during post-subduction (orogenic) extensional

collapse. We favor the former tectonic setting, but an active margin in which extensional stresses were prevalent in the

overriding plate. # 2002 Elsevier Science B.V. All rights reserved.

Keywords: Missouri; Saint Francois Mountains; Mafic composition; Igneous rocks; Basalts; Chemical composition

1. Introduction

The Precambrian basement of North America

includes an extensive belt of Mesoproterozoic

igneous rocks that stretches from the southwestern

US to southern Michigan (Fig. 1) and perhaps on

into eastern Canada and Scandinavia (Van Bree-

men and Davidson, 1988; Windley, 1993; Rivers,

1997; Ahall and Connelly, 1998; Karlstrom et al.,

2001). Silicic igneous rocks appear to dominate the

Midcontinent portion of the belt, which has often

been referred to as the Granite�/Rhyolite Terrane

(Thomas et al., 1984). Van Schmus et al. (1993)

have subdivided the Granite�/Rhyolite Terrane

into two separate Mesoproterozoic provinces: (1)

the approximately 1470 Ma Eastern Granite�/

* Corresponding author. Fax: �/1-815-753-1945

E-mail addresses: [email protected] (J.A. Walker),

[email protected] (C.G. Pippin), [email protected]

(B.I. Cameron), [email protected] (L. Patino).1 Fax: �/1-704-663-6040.2 Fax: �/1-414-229-5452.3 Fax: �/1-517-353-8787.

Precambrian Research 117 (2002) 251�/268

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Rhyolite Province; and (2) the approximately 1370

Ma Southern Granite�/Rhyolite Province (Fig. 1).

The tectonic setting of these vast Granite�/Rhyo-

lite provinces remains enigmatic (e.g. Bickford and

Anderson, 1993). The geochemistry of the silicic

rocks is evocative of both intra-plate and subduc-

tion zone settings, although dominantly the former

(Shuster et al., 1992; Bickford and Anderson,

1993; Lidiak, 1996; Frost and Frost, 1997; Stal-

lings et al., 1998). Although volumetrically sub-

ordinate, mafic igneous rocks are present within

these Mesoproterozoic provinces (e.g. Van Schmus

et al., 1996). Here we report on the geochemistry

of mafic igneous rocks from the St. Francois

Mountains, hosts of the only surface exposures

of the Eastern Granite�/Rhyolite Province, and

discuss the tectonic implications of the results.

2. Magmatic history of the St. Francois Mountains

The St. Francois Mountains are located in

southeastern Missouri near the southwestern limits

of the Eastern Granite�/Rhyolite Province (Fig. 1).

Rhyolitic ash-flow tuffs and granites are the

preponderant Mesoproterozoic lithologies of the

St. Francois Mountains (Tolman and Robertson,

1969; Anderson, 1970; Kisvarsanyi, 1972; Berryand Bickford, 1972; Sides et al., 1981; Bickford et

al., 1981; Brown, 1983; Lowell, 1991). In general,

granites are the dominant lithology in the north-

eastern portions of the St. Francis Mountains,

whereas rhyolites are much more abundant in the

southwest (Sides et al., 1981). This skewed dis-

tribution has been attributed to southwestward

tilting and subsequent erosional beveling sometimeafter the cessation of magmatic activity (Bickford

et al., 1977; Sides, 1978; Sides et al., 1981).

Most of the granites and rhyolites of the St.

Francois Mountains have yielded U�/Pb zirco-

nages of 14709/30 Ma (Bickford and Mose, 1975;

Van Schmus et al., 1996). The coeval ages of many

of the igneous rocks suggest that there was one

main magmatic stage centered around 1470 Ma.Brown (1983) and Brown (1989) provides geologic

evidence that this main magmatic stage was

initiated in the southern St. Francois Mountains.

Magmatism then became focused in the eastern St.

Francois Mountains, culminating with the forma-

tion of the Butler Hill caldera (Sides et al., 1981;

Lowell, 1991). Magmatic activity then shifted to

the west as summarized by Sides et al. (1981).Two granite plutons in the western St. Francois

Mountains, the Munger and Graniteville granites,

have yielded ages of approximately 1370 Ma

(Bickford and Mose, 1975; Van Schmus et al.,

1996). These could represent isolated, outlying

magmatism associated with the Southern Gran-

ite�/Rhyolite Province (Bickford and Anderson,

1993; Van Schmus et al., 1996), or alternatively,indicate that some of the magmatic activity in the

western St. Francois Mountains, perhaps includ-

ing production of the hypothesized Taum Sauk

caldera, is actually considerably younger than

magmatism to the east and southeast (Anderson

et al., 1969). However, an unpublished date of

approximately 1480 for a rhyolite from the western

Fig. 1. Major geologic features of the central Midcontinent

region (modified after Van Schmus et al., 1996). G, Grenville

front; OF, Ouachita front; MC, Midcontinent rift; MGL,

Missouri gravity low; RR, Reelfoot rift; SFM, St. Francois

Mountains. Dashed line is inferred eastern limit of pre-1600 Ma

continental crust as discussed in Van Schmus et al. (1996).

J.A. Walker et al. / Precambrian Research 117 (2002) 251�/268252

St. Francois Mountains casts doubt on the latter

possibility (M.E. Bickford, pers. comm.).

The minor mafic igneous rocks of the St.

Francois Mountains occur as hypabyssal dikes

and sills, small stocks, and lava flows (Amos and

Desborough, 1970; Sylvester, 1984; Pippin, 1996).

The mafic dikes consistently intrude coexisting

granites and rhyolites suggesting they are at least

somewhat younger. As support, Ramo et al. (1994)

report U�/Pb badellyite ages of 13169/6 and

13009/10 Ma for two of the mafic plutons of the

St. Francois Mountains. There are, however, good

reasons for believing that mafic magmas were

present during the entire magmatic history of the

St. Francois Mountains. First, gravity and mag-

netic anomaly data suggest that mafic plutons may

be common in the upper crust below the St.

Francois Mountains (Hildenbrand et al., 1996).

Second, Van Schmus et al. (1996) have recently

obtained Sm�/Nd isochron ages for mafic plutons

in the subsurface of Missouri which overlap those

of main phase granites and rhyolites. Third, mafic

lavas have been found intercalated with main

phase silicic volcanics at Johnson Shut-Ins and

Marble Creek (Satterfield, 1966; Brown, 1983;

Pippin, 1996). Fourth, mafic enclaves and mafic

Fig. 2. Sample locations of mafic rocks of the St. Francois Mountains analyzed in this study. Extent of Precambrian rock outcrops

after Kisvarsanyi and Hebrank (1993).

J.A. Walker et al. / Precambrian Research 117 (2002) 251�/268 253

lithics are common in main phase granites andrhyolitic ash-flow tuffs, respectively (Sides, 1978;

Nusbaum, 1980; Sides et al., 1981; Lowell and

Young, 1999). In addition, Lowell and Young

(1999) have argued that most of the mafic enclaves

in the Silver mine Granite, although intermediate

in composition, originated through the hybridiza-

tion of mafic and silicic magmas. Lastly, mantle-

derived basaltic magmas likely floored and pro-vided heat to produce and sustain such a large

silicic magmatic system (Hildreth, 1981; Kay et al.,

1989; Bickford and Anderson, 1993; Frost and

Frost, 1997).

3. Sampling and analytical procedures

Sampling of mafic volcanic and plutonic rocks

was carried out throughout the St. Francois

Mountains using previous reports and maps as

guides (i.e. Amos and Desborough, 1970; Sides,

1978; Sides et al., 1981; Brown, 1983; Sylvester,

1984). Sample locations are shown in Fig. 2 and

detailed in Table 1. Whereas fresh samples were

collected for analysis, samples of visibly alteredrock were also taken at a few localities to assess

major and trace element mobility (see below).

Overall, 14 lava, 9 dike, and 8 pluton samples

were selected for chemical analysis. Multiple

samples of some lavas, dikes and plutons were

taken to evaluate chemical heterogeneity or in an

attempt to identify distinct units. Major and trace

elements were determined by X-ray fluorescence atNorthern Illinois University. Selected samples also

underwent trace element analysis by inductively

coupled plasma-mass spectrometry (ICP-MS) at

Rutgers University and Michigan State University.

Walker et al. (2000) provide analytical particulars

of XRF and Rutgers ICP-MS analyses. At Michi-

gan State, trace element concentrations were

determined via laser ablation-inductively coupledplasma-mass spectrometry (LA-ICP-MS) on glass

discs utilizing a Cetac LSX-200�/ laser and

Micromass Platform ICP-MS. Strontium was

used as an internal standard. Concentrations of

unknowns were calculated on the basis of linear

regression of international standards. Before data

acquisition, the ICP-MS was optimized whileablating the NIST-612 glass standard. For data

acquisition, the trace elements were divided into

three groups: light REE (La�/Tb), heavy REE

(Dy�/Lu) and other trace elements. Data acquisi-

tion lasted 0.7 min for each group. Precision and

accuracy of LA-ICP-MS was evaluated by repeti-

tive analyses of standard JB-1a. Reproducibility

for all elements was less than 5% and accuracy wasless than 10%. Complete analyses of St. Francois

mafic rocks are given in Table 2.

4. Alteration

The mafic rocks of the St. Francois Mountains

have experienced variable low-grade alteration/metamorphism (Amos and Desborough, 1970;

Wenner and Taylor, 1976; Sylvester, 1984; Pippin,

1996). Many of the rocks, however, exhibit primary

igneous textures and mineral composition (Amos

and Desborough, 1970; Sylvester, 1984; Pippin,

1996). Nevertheless, all of the mafic compositions

were run through an ‘alteration filter’ following

Beswick and Soucie (1978). As a result, six fresh-looking mafic rocks were deemed altered (Pippin,

1996) and are excluded from the figures and

discussion that follows. Comparisons of visibly

fresh and altered samples from the same outcrops

indicate that MgO, CaO, Na2O, K2O, Sr, Ba, Rb and

Cu are the most mobile elements in the mafic rocks

(Shaw and Kirchner, 1993; Pippin, 1996) and should

be used with caution in petrogenetic discussions. Indetail, Na2O is consistently lost, while K2O is

consistently gained, during alteration/metamorph-

ism (Lowell, 1991; Shaw and Kirchner, 1993; Pippin,

1996), suggesting variation diagrams employing

total alkali contents can only be used selectively with

tenuous confidence.

5. The mafic rocks of the St. Francois Mountains

Bearing in mind the aforementioned caveat, the

mafic rocks of the St. Francois Mountains can be

classified as subalkaline to alkaline basalts to

basaltic andesites and their plutonic equivalents

(Fig. 3). Their primary minerals are olivine (except


in the Silver Mines rocks), clinopyroxene, plagio-

clase and opaques (Sylvester, 1984; Pippin, 1996).

Sylvester (1984) subdivided the mafic rocks of the

St. Francois Mountains into two contrasting suites

based on variable geologic, mineralogic and che-

mical characteristics: (1) the Silver Mines suite;

and (2) the Skrainka suite. Our studies have

confirmed his subdivision. The Silver Mines suite

includes basaltic lava flows and dikes, while the

Skrainka suite is entirely plutonic, consisting of

fine-grained dikes and coarser, gabbroic intrusions

(Table 1; Sylvester, 1984; Pippin, 1996). The Silver

Mines rocks are considered to be older, which is

clearly the case at Johnson Shut-Ins, where a

Skrainka dike intrudes a Silver Mines lava flow

(Pippin, 1996). The Silver Mines rocks are taken to

represent mafic magmas contemporaneous with

approximately 1470 Ma silicic magmas, as the

lavas intercalated with main phase silicic volcanics

fall into this chemical group (Sylvester, 1984;

Pippin, 1996). The Skrainka rocks are believed to

represent the youngest magmatic activity in the

region, perhaps postdating all silicic activity,

consistent with the U�/Pb badellyite ages from

Ramo et al. (1994). Another distinction between

the groups is that Silver Mines dikes and sills tend

to have different preferred orientations than their

Skrainka counterparts (Sylvester, 1984). In addi-

Table 1

Samples and locations

Sample Type Group Section: township, range Location

10144 Lava Silver Mines 16: 33N, 02E Johnson Shut-Ins

JCB1 Lava Silver Mines 16: 33N, 02E Johnson Shut-Ins






JCB 11 Lava Silver Mines 16: 33N, 02E Johnson Shut-Ins

JCB 11a Lava Silver Mines 16: 33N, 02E Johnson Shut-Ins

JCB 13 Lava Silver Mines 16: 33N, 02E Johnson Shut-Ins

JCB17a Lava Silver Mines 16: 33N, 02E Johnson Shut-Ins

MCA-1 Lava Silver Mines 19: 32N, 05E Marble Creek Campground1

MCA-2 Lava Silver Mines 19: 32N, 05E Marble Creek Campground



72SR2 Dike Silver Mines 04: 33N, 05E St. Francois River

GMD3 Dike Silver Mines 11: 33N, 06E Plum Creek

MM1 Dike Silver Mines 29: 34N, 05E Brewers Creek

SMB2 Dike Silver Mines 10: 33N, 05E Silver Mine

SMB3 Dike Silver Mines 10: 33N, 05E Silver Mine

JJ1 Dike Silver Mines 10: 33N, 04E Highway JJ

GMD1 Dike Skrainka 11: 33N, 06E Plum Creek

GMD2 Dike Skrainka 11: 33N, 06E Plum Creek

JCB 19 Dike Skrainka 16: 33N, 02E Johnson Shut-Ins

JCB 20 Dike Skrainka 16: 33N, 02E Johnson Shut-Ins

AA2 Pluton Skrainka 27: 33N, 03E Hogan

GMD8 Pluton Skrainka 04: 33N, 06E Oak Grove

GMD9 Pluton Skrainka 04: 33N, 06E Oak Grove

O1 Pluton Skrainka 27: 34N, 02E Imboden Fork

R72D1 Pluton Skrainka 04: 33N, 06E Oak Grove

TSG1 Pluton Skrainka 08: 33N, 03E Devils Tollgate

TSG2 Pluton Skrainka 08: 33N, 03E Devils Tollgate

Mcpeg Pluton Skrainka 34: 34N, 06E Vogel Hill


Table 2

Major and trace element compositions of mafic igneous rocks of the St. Francois Mountains

Sample 10144 JCB1 JCB2 JCB3 JCB4 JCB5 JCB10 JCB11

Group SM SM SM SM SM SM SM SM

SiO2a 49.70 48.11 49.97 49.21 48.88 48.09 49.88 49.09

TiO2 1.77 2.06 1.90 1.78 0.98 0.96 1.67 1.78

Al2O3 15.33 15.36 15.85 15.36 16.50 16.06 14.73 15.52

FeOT 10.99 11.38 10.10 11.20 10.07 10.43 11.40 10.82

MgO 7.28 7.26 7.41 7.59 6.54 5.77 6.25 7.41

MnO 0.22 0.20 0.25 0.27 0.15 0.23 0.23 0.19

CaO 6.93 6.40 6.60 6.93 8.89 8.73 6.52 6.21

Na2O 3.67 4.34 3.89 3.71 3.05 3.07 3.80 4.32

K2O 0.99 0.75 0.88 0.74 1.49 1.81 1.03 0.75

P2O5 0.26 0.34 0.31 0.28 0.24 0.25 0.25 0.28

LOI 1.80 1.79 1.81 1.36 1.35 1.65 1.38 2.10

Nb 6.9 7.7 8.1 6.4 5.6 3.3 6.7 6.3

Zr 100 118 107 105 62 61 99 98

Y 31.4 38.1 34.0 33.2 22.0 20.6 29.6 31.4

Hf n.d.b 3.02 n.d. 2.64 n.d. 1.52 n.d. 2.45

Ta n.d. 0.48 n.d. 0.41 n.d. 0.21 n.d. 0.41

Sr 288 180 337 274 484 347 315 317

Rb 38.1 27.0 40.0 31.7 25 72 37.6 29.4

Ba 306 247 161 208 325 659 379 254

Th n.d. 1.54 n.d. 1.43 n.d. 0.96 n.d. 1.49

Pb n.d. 7.19 n.d. 7.21 n.d. 9.33 n.d. 7.26

U n.d. 0.49 n.d. 0.41 n.d. 0.29 n.d. 0.42

Ni 75 98 83 84 52 49 78 81

La n.d. 13.5 n.d. 16.1 n.d. 11.1 n.d. 13.6

Ce n.d. 33.4 n.d. 35.9 n.d. 24.6 n.d. 31.3

Nd n.d. 22.9 n.d. 22.0 n.d. 16.4 n.d. 19.8

Sm n.d. 5.61 n.d. 5.19 n.d. 3.51 n.d. 4.56

Eu n.d. 1.63 n.d. 1.72 n.d. 1.20 n.d. 1.43

Gd n.d. 5.85 n.d. 5.56 n.d. 3.65 n.d. 4.79

Dy n.d. 6.40 n.d. 5.43 n.d. 3.48 n.d. 5.26

Er n.d. 3.79 n.d. 3.24 n.d. 1.94 n.d. 3.10

Yb n.d. 3.26 n.d. 2.99 n.d. 1.88 n.d. 2.75

Sample JCB11a JCB13 JCB17a MCA1 MCA2 MCA3 MCA4 72SR2


SiO2a 48.73 48.85 48.08 52.80 52.08 51.83 51.52 51.48

TiO2 1.77 1.91 0.99 0.74 0.75 1.04 1.03 1.65

Al2O3 15.51 15.66 16.54 17.10 17.18 15.63 15.60 15.50

FeOT 11.15 11.49 10.71 9.41 9.30 11.16 11.21 10.55

MgO 7.46 8.01 6.52 4.85 5.07 4.83 4.70 5.54

MnO 0.18 0.24 0.13 0.15 0.15 0.19 0.19 0.23

CaO 6.05 5.33 8.67 8.12 8.12 8.19 7.65 6.27

Na2O 4.06 4.49 2.97 3.14 3.18 3.43 3.76 3.22

K2O 0.76 0.88 1.81 1.37 1.25 0.91 1.22 2.24

P2O5 0.26 0.29 0.26 0.18 0.18 0.24 0.25 0.64

LOI 2.27 1.54 1.87 1.21 1.46 0.88 n.d. 1.17

Nb 6.8 6.5 2.9 0.6 n.d. n.d. 3.5 9.2

Zr 101 110 65 82 83 86 83 128


Table 2 (Continued )

Sample JCB11a JCB13 JCB17a MCA1 MCA2 MCA3 MCA4 72SR2


Y 30.9 33.0 21.2 27.0 26.6 31.2 31.2 30.2

Hf n.d. n.d. 1.60 n.d. n.d. n.d. 2.18 n.d.

Ta n.d. n.d. 0.26 n.d. n.d. n.d. 0.28 n.d.

Sr 318 305 446 383 389 342 383 610

Rb 25.1 36.5 41.9 39.4 36.5 20.8 33.1 125

Ba 224 129 498 410 367 352 412 564

Th n.d. n.d. 0.90 n.d. n.d. n.d. 1.62 n.d.

Pb n.d. n.d. 5.20 n.d. n.d. n.d. 3.67 n.d.

U n.d. n.d. 0.29 n.d. n.d. n.d. 0.48 n.d.

Ni 80 81 55 33 33 30 27 58

La n.d. n.d. 12.1 n.d. n.d. n.d. 13.8 n.d.

Ce n.d. n.d. 26.9 n.d. n.d. n.d. 31.3 n.d.

Nd n.d. n.d. 16.0 n.d. n.d. n.d. 19.3 n.d.

Sm n.d. n.d. 3.77 n.d. n.d. n.d. 4.65 n.d.

Eu n.d. n.d. 1.30 n.d. n.d. n.d. 1.36 n.d.

Gd n.d. n.d. 3.84 n.d. n.d. n.d. 4.71 n.d.

Dy n.d. n.d. 3.46 n.d. n.d. n.d. 4.67 n.d.

Er n.d. n.d. 2.02 n.d. n.d. n.d. 2.94 n.d.

Yb n.d. n.d. 1.88 n.d. n.d. n.d 2.86 n.d.

Sample GMD3 MM1 SMB2 SMB3 JJ1 GMD1 GMD2 JCB19

Group SM SM SM SM SM SK SK SK

SiO2a 51.89 54.23 48.09 47.41 53.96 46.42 46.39 45.59

TiO2 1.16 1.28 1.97 1.95 1.16 1.60 1.76 2.74

Al2O3 15.05 15.18 14.73 13.95 15.11 17.13 16.51 15.22

FeOT 10.04 9.27 12.31 12.92 11.75 11.50 12.16 14.12

MgO 5.97 4.64 4.92 5.03 4.01 7.42 7.06 5.99

MnO 0.24 0.16 0.17 0.17 0.21 0.15 0.14 0.25

CaO 4.59 6.37 8.22 8.05 5.08 7.97 8.13 8.26

Na2O 4.12 3.39 2.54 2.53 4.50 2.65 2.59 2.62

K2O 2.94 1.95 0.87 0.91 1.41 1.27 1.16 1.11

P2O5 0.45 0.51 0.56 0.59 0.25 0.24 0.27 0.52

LOI 2.27 1.70 3.79 4.00 1.45 2.52 2.25 n.d.

Nb 6.2 10.2 4.8 4.2 4.5 3.0 3.4 6.5

Zr 176 168 49 51 103 108 122 155

Y 35.9 36.2 23.0 22.0 31.3 30.8 34.4 37.3

Hf 4.21 n.d. n.d. n.d. 2.77 2.77 2.95 4.05

Ta 0.32 n.d. n.d. n.d. 0.30 0.19 0.22 0.52

Sr 324 589 643 629 482 254 261 406

Rb 142 73.4 50.0 52.0 62.1 122 100 29.3

Ba 573 594 225 233 392 204 185 316

Th 3.42 n.d. n.d. n.d. 2.42 0.28 0.30 1.22

Pb 7.89 n.d. n.d. n.d 10.1 2.52 2.13 5.15

U 1.08 n.d. n.d. n.d. 0.73 0.12 0.13 0.38

Ni 51 43 62 62 10 100 87 106

La 18.1 n.d. n.d. n.d. 15.9 7.79 8.56 15.6

Ce 40.7 n.d. n.d. n.d. 34.9 19.9 21.5 39.4

Nd 25.4 n.d. n.d. n.d. 21.6 16.3 17.6 27.6

Sm 5.54 n.d. n.d. n.d. 4.67 4.09 4.62 7.01


tion, Silver Mines rocks lack olivine, which is

common in Skrainka rocks (Sylvester, 1984; Pip-

pin, 1996). Chemically, the Silver Mines rocks

have generally higher SiO2 contents (Fig. 3) and

show calc-alkaline affinities, whereas the Skrainka

rocks are more basic and tholeiitic (Fig. 4;

Sylvester, 1984; Pippin, 1996). The two groups

are also characterized by some different incompa-

tible element ratios, such as La/Sm, Zr/Y (Fig. 5)

and La/Hf (Sylvester, 1984; Pippin, 1996). The

Table 2 (Continued )

Sample GMD3 MM1 SMB2 SMB3 JJ1 GMD1 GMD2 JCB19

Group SM SM SM SM SM SK SK SK

Eu 1.41 n.d. n.d. n.d. 1.35 1.49 1.62 2.36

Gd 5.89 n.d. n.d. n.d. 5.04 4.68 4.95 7.23

Dy 5.71 n.d. n.d. n.d. 4.93 5.10 5.78 6.28

Er 3.51 n.d. n.d. n.d. 3.11 2.96 3.32 3.39

Yb 3.13 n.d. n.d. n.d. 2.92 2.64 2.80 2.92

Sample JCB20 AA2 GMD8 GMD9 O1 R72D1 TSG1 TSG2 Mcpeg

Group SK SK SK SK SK SK SK SK SK

SiO2a 46.11 47.00 45.57 46.54 46.83 46.46 46.92 45.05 45.99

TiO2 2.74 1.88 1.74 1.74 1.96 1.79 2.08 2.13 1.53

Al2O3 15.37 16.65 16.57 16.88 16.54 16.54 15.86 15.18 17.89

FeOT 14.20 12.17 12.31 12.10 12.38 12.05 13.13 12.78 10.18

MgO 6.07 6.70 7.22 7.16 7.25 7.34 7.92 6.53 5.84

MnO 0.28 0.17 0.17 0.17 0.18 0.17 0.19 0.28 0.15

CaO 8.15 8.68 8.64 8.79 9.13 8.71 7.50 7.71 9.39

Na2O 2.50 3.33 2.96 3.16 3.11 3.11 3.08 2.72 2.55

K2O 1.24 0.69 0.75 0.70 0.60 0.67 1.26 1.14 1.29

P2O5 0.53 0.31 0.28 0.29 0.27 0.28 0.33 0.35 0.28

LOI 0.67 0.50 1.04 0.15 0.03 0.04 0.38 1.50 3.03

Nb 4.8 5.3 3.8 4.1 4.1 5.3 4.8 5.8 4.0

Zr 159 111 106 117 107 99 131 128 109

Y 33.7 24.9 21.9 24.5 24.5 23.0 28.0 27.4 26.2

Hf n.d. n.d. n.d. 2.85 2.68 n.d. 3.23 n.d. n.d.

Ta n.d. n.d. n.d. 0.30 0.32 n.d. 0.36 n.d. n.d.

Sr 406 531 531 553 519 545 493 404 414

Rb 30.0 11.8 9.3 9.8 8.6 9.0 20.8 86.6 43.5

Ba 348 254 234 265 245 218 324 152 390

Th n.d. n.d. n.d. 0.86 0.81 n.d. 0.97 n.d. n.d.

Pb n.d. n.d. n.d. 2.66 3.11 n.d. 2.86 n.d. n.d.

U n.d. n.d. n.d. 0.27 0.26 n.d. 0.29 n.d. n.d.

Ni 105 146 153 140 163 155 201 127 54

La n.d. n.d. n.d. 11.0 10.0 n.d. 12.0 n.d. n.d.

Ce n.d. n.d. n.d. 26.2 23.5 n.d. 28.4 n.d. n.d.

Nd n.d. n.d. n.d. 18.9 17.0 n.d. 20.5 n.d. n.d.

Sm n.d. n.d. n.d. 4.51 4.06 n.d. 4.88 n.d. n.d.

Eu n.d. n.d. n.d. 1.59 1.49 n.d. 1.67 n.d. n.d.

Gd n.d. n.d. n.d. 4.51 4.12 n.d. 4.85 n.d. n.d.

Dy n.d. n.d. n.d. 4.42 4.25 n.d. 5.03 n.d. n.d.

Er n.d. n.d. n.d. 2.29 2.27 n.d. 2.67 n.d. n.d.

Yb n.d. n.d. n.d. 1.87 1.92 n.d. 2.16 n.d. n.d.

SM, Silver Mines group; SK, Skrainka group.a Major elements are in wt.% and trace elements are in ppm.b n.d., not determined.


latter distinctions indicate that their parental

magmas originated from distinct sources (Fig. 5;

Sylvester, 1984; Pippin, 1996).

Nearly all of the mafic rocks from both groups,

however, have La/Nb�/2 and La/Ta�/30 (Fig. 6),

and most have Ce/PbB/10, Nb/UB/20, Ba/Ta�/

450 and Ba/La�/15 (Sylvester, 1984; Pippin,

1996). These trace element ratios are significant

because they are all diagnostic signatures of

subduction zone basalts (Gill, 1981; Miller et al.,

1994; Hofmann, 1997) or continental intra-plate

basalts that have emanated, at least in part, from

subduction-modified lithosphere or have experi-

enced substantial crustal contamination (Arndt

and Christensen, 1992; Sylvester et al., 1997;

Lassiter and DePaolo, 1997).

6. Tectonic implications

Two contrasting tectonic environments havebeen proposed for the Mesoproterozoic Granite�/

Rhyolite Provinces (e.g. Bickford and Anderson,

1993). The first is an anorogenic, rift-related

environment (Emslie, 1978; Bickford et al., 1986;

Van Schmus et al., 1987; Bickford, 1988; Hoffman,

1989; Anderson and Bender, 1989; Windley, 1989,

Fig. 4. AFM diagram for the mafic rocks of the St. Francois

Mountains, including data of Sylvester (1984).

Fig. 5. La versus Sm (A) and Zr versus Y (b) for the mafic

rocks of the St. Francois Mountains. Data from Sylvester

(1984) is included in (A). Note the distinct ratios for rocks of

the Silver Mines and Skrainka suites. In both (A) and (B) the

more incompatible element (during mantle melting) is on the

abscissa. Hence, the ‘reversed’ positions of the two suites

between (A) and (B) indicate their parental magmas emanate

from distinct mantle source regions.

Fig. 3. Chemical classification of mafic rocks of the St.

Francois Mountains (modified after Le Bas et al., 1986).

Includes data from Sylvester (1984).


1993; Bickford and Anderson, 1993; Lidiak, 1996).

The second is an active continental margin (Van

Schmus and Bickford, 1981; Thomas et al., 1984;

Van Breemen and Davidson, 1988; Nyman et al.,

1994; Rivers and Corrigan, 2000; Karlstrom et al.,

2001). Contemporaneous basalts from the Gran-

ite�/Rhyolite provinces should provide the clearest

geochemical evidence for their original tectonic

setting (e.g. Condie, 1989). Hence, for the St.

Francois Mountains, we focus on the Silver Mines

rocks, although the Skrainka rocks share many of

their geochemical attributes (e.g. Fig. 6).

As pointed out above, the trace element geo-

chemistry of the Silver Mines basalts is strongly

suggestive of subduction zone or continental

affinities. The low Th contents and Th/La

(B/0.2) ratios of the Silver Mines rocks, however,

argue that their parental magmas experienced

minimal contamination from continental crust

(Fig. 7; Shirey et al., 1994; Sylvester et al., 1997).Therefore, we conclude that many of the impor-

tant trace element signatures of the Silver Mines

(and Skrainka) rocks from the St. Francois

Mountains emanate from their mantle source

which had been enriched at some time via subduc-

tion. To bolster this conclusion, all of the mafic

rock compositions of the St. Francois Mountains

have been passed through the trace element/tectonic screens developed by Condie (1989) for

Precambrian basalts and andesites (Fig. 8). As

shown in Fig. 8, the Silver Mines (and Skrainka)

basalts clearly fall within the subduction zone bins,

and ultimately are suggestive of an origin within

an active continental margin (Fig. 8C). Never-

theless, early-erupted mafic lavas from the adja-

Fig. 6. La versus Nb (A), and La versus Ta (B) for the mafic

rocks of the St. Francois Mountains. Data from this study. Arc

basalt fields enclose values for mafic (�/5 wt.% MgO) arc

basalts from: Elliott et al. (1997), Woodhead et al. (1998),

Ewart et al. (1998), Peate and Pearce (1998), and Taylor and

Nesbitt (1998). Field for basalts from Central America, an

active continental margin, encloses values for basalts (B/52

wt.% SiO2) from the Central American volcanic front, exclud-

ing data for the very alkaline basalts from Costa Rica and for

the MORB-like basalts of Nicaragua (Patino et al., 2000).

Concentrations of average N-type and E-type MORB from Sun

and McDonough (1989).

Fig. 7. Ta/La versus Th/La for mafic rocks of the St. Francois

Mountains (modified after Shirey et al., 1994). Data from this

study. Fields for mafic arc basalts and Central American basalts

as in Fig. 5. Ta/La and Th/La ratios for Yellowstone basalts are

shown as representative of plume-related continental basalts to

contrast with subduction zone basalts (data from Hildreth et al.

(1991)). CC, average 1.6�/1.8 continental crust (restoration

model) from Condie (1993). Note that some Central American

basalts define a trend toward continental crust consistent with

previous evidence of locally significant crustal contamination

(Carr et al., 1990). Dotted field encloses data for early-erupted

basalts from the MCR, specifically the lower part of the

Mamainse Point Formation, Ontario (data from Shirey et al.,

1994). Concentrations of average N-type and E-type MORB

from Sun and McDonough (1989).


cent Neoproterozoic Midcontinent Rift (MCR)show many geochemical similarities to the St.

Francois basalts and also ultimately fall into the

subduction zone fields in Fig. 7 and Fig. 8. The

early-erupted MCR basalts are part of a major

flood basalt province linked to prodigious melting

of a mantle plume, called by some the Keweenaw

plume (Hutchinson et al., 1990). Trace element

and isotopic evidence suggest that subduction-modified lithospheric mantle contributed signifi-

cantly to the mantle source of these earliest

erupted MRS lavas (Shirey et al., 1994; Nicholson

et al., 1997; Shirey, 1997). Hence, the incompatible

element geochemistry of the Silver Mines mafic

rocks of the St. Francois Mountains is compatible

with formation within either an active continental

margin or an anorogenic continental rift, as longas subduction-modified lithosphere significantly

contributes to magma generation in the latter.

Subduction-modified continental lithosphere also

is believed to play an important role in the

petrogenesis of the mafic rocks of the roughly

contemporaneous (1426 Ma) Michael Gabbro

from central Labrador (Emslie et al., 1997).

The eNd(t) values determined by Van Schmus etal. (1996) for ‘main phase’ mafic plutons from the

subsurface of Missouri (2.4�/4.5) provide impor-

tant constraints on any proposed tectonic model,

as they limit the involvement of Paleoproterozoic

source rocks, suggesting that concurrent or re-

cently concluded subduction (i.e. Mesoprotero-

zoic) was responsible for their subduction zone

signature. The reported Nd model ages for thesilicic igneous rocks of the St. Francois Mountains

and of the southeastern portions of the Granite�/

Rhyolite provinces are consistent with this con-

straint, as they indicate derivation from Mesopro-

terozoic source rocks (Van Schmus et al., 1996).

Many authors have discussed the pros and cons

of subduction versus rifting origins for the Meso-

proterozoic Granite�/Rhyolite Provinces (e.g.Bickford and Anderson, 1993). The possible cons

of a direct subduction zone origin for the Meso-

proterozoic Granite�/Rhyolite Provinces are: the

preponderance of silicic, as opposed to intermedi-

ate or basic, magmatism; and the apparent paucity

of significant, contemporaneous metamorphism

and compressive deformation in the rocks (e.g.

Fig. 8. Mafic rock compositions from the St. Francois Moun-

tains plotted within three successive trace element/tectonic

screens after Condie (1989). Data from this study. ARCB,

subduction zone basalts; NMORB, normal, depleted mid-ocean

ridge basalts; WPB, oceanic within-plate basalts; MORB,

transitional and enriched mid-ocean ridge basalts; IAB, island

arc tholeiitic basalts; CABI, calc-alkaline basalts from island

arcs; CABC, calc-alkaline basalts from active continental

margins (see Condie, 1989 for further details). Dotted fields

enclose lower Mamainse Point Formation lavas as in Fig. 7.


Van Schmus and Bickford, 1981). There are,however, geologically recent examples of active

continental margins dominated by silicic volcan-

ism and extensional tectonism. The first is the 200-

km-long Taupo volcanic zone of New Zealand

(e.g. Hochstein et al., 1993). Here rhyolitic volcan-

ism has dominated the eruptive activity over the

past 1.6 Ma (Hochstein et al., 1993; Wilson et al.,

1995; Graham et al., 1995; Wilson, 1996). Rhyo-litic magmas are believed to result largely from

combined assimilation-fractional crystallization

(AFC) of ultimately mantle-derived parents

(McCulloch et al., 1994; Graham et al., 1995).

The immense production of rhyolitic magmas in

the Taupo volcanic zone occurs within thinned

crust that is undergoing rapid extension/deforma-

tion and is anomalously hot, approximately seventimes as hot as other modern subduction zones

(Hochstein et al., 1993; Hochstein, 1995; Bibby et

al., 1995; Darby and Meertens, 1995; Wilson,

1996). Another example of a silicic-dominated

active continental margin is the roughly 200-km-

long Altiplano-Puna volcanic complex of the

Central Andes (e.g. de Silva, 1989). During the

late Miocene to early Pliocene, volcanism in theCentral Andean subduction zone was dominated

by voluminous silicic ignimbrite production (de

Silva, 1989). The silicic parent magmas are be-

lieved to have originated by extensive melting of

the lower crust synchronously with considerable

crustal thickening and convective thinning, or

delamination, of the lithospheric mantle of the

overriding plate (Hawkesworth et al., 1982; Isacks,1988; de Silva, 1989; Kay and Kay, 1993).

Although surrounded by widespread compressive

deformation, the high plateau region that hosts the

Altiplano-Puna volcanic complex is characterized

by neutral or extensional stresses (e.g. Froidevaux

and Isacks, 1984). A final example of rhyolite

domination in a recently active continental mar-

gin, and one closer in scale to the Granite�/

Rhyolite Provinces, is the over 1000-km-long,

mid-Tertiary Sierra Madre Occidental volcanic

province of Mexico (McDowell and Clabaugh,

1979; Wark et al., 1990; Ruiz, 1994; Nieto-

Samaniego et al., 1999). Here again, silicic volcan-

ism was accompanied by widespread extension

(Nieto-Samaniego et al., 1999) and the prodigious

formation of rhyolitic magmas has been chieflyattributed to extensive AFC of mafic mantle melts

(Cameron et al., 1980; Wark, 1991). Extension and

silicic volcanism during mid-Tertiary subduction

have been variously attributed to a ‘retreating

subduction boundary’ (Nieto-Samaniego et al.,

1999), a dramatic steepening of the angle of

subduction (e.g. Coney and Reynolds, 1977), or

just a general waning of subduction (Wark et al.,1990). Although not as recent an analogue, the

Jurassic rhyolites of southern South America and

western Antarctica define a large silicic igneous

province on the scale of the Mesoproterozoic

Granite�/Rhyolite Provinces (e.g. Kay et al.,

1989). Pankhurst et al. (1998) have recently con-

cluded that the origin of the Jurassic rhyolites is

linked to changes in contemporaneous subductiondynamics. In sum, therefore, we feel a direct

subduction origin for the Silver Mines basalts

and the Granite�/Rhyolite Provinces remains a

strong possibility. Precambrian orogenic belts may

have been even hotter and structurally weaker

(Grambling, 1981; Marshak, 1999), and hence

even more fertile locales of widespread rhyolite

production. Moreover, mantle convection waslikely more vigorous in the Precambrian (e.g.

Campbell and Jarvis, 1984), making the continen-

tal lithosphere more susceptible to convective

removal, increasing the likelihood of the close

juxtaposition of hot asthenosphere and fusible

continental crust (Sandiford, 1989) as has been

postulated for the Central Andes (e.g. Isacks,

1988).An anorogenic, rift-related environment, on the

other hand, is more compatible with the apparent

lack of compressive deformation and moderate- to

high-grade metamorphism throughout the Gran-

ite�/Rhyolite Provinces (e.g. Van Schmus and

Bickford, 1981). Suggested causes of widespread

rifting include: (1) a mantle plume (Emslie, 1978;

Anderson and Bender, 1989); (2) a mantle super-swell that developed beneath a stationary super-

continent (Hoffman, 1989); and (3) extensional

collapse following convergent margin orogenesis,

arc accretion, and crustal thickening (Bickford et

al., 1986; Bickford, 1988; Kay et al., 1989;

Bickford and Anderson, 1993), perhaps of a 1.5

Ga supercontinent (Hoffman, 1989; Windley,


1993; Lidiak, 1996). A mantle plume or superswellwould be a consummate heat source for wide-

spread crustal fusion and silicic magma produc-

tion. However, a large plume head or superswell

would also likely yield at least a comparable

volume of allied mafic igneous rocks, even if

overlain by thick continental lithosphere (Kent et

al., 1992; Puffer, 2001). Judging by the Silver

Mines and Skrainka rocks of the St. FrancoisMountains, mafic igneous rocks are only a minor

component of the Granite�/Rhyolite Provinces. A

prodigious volume of basic igneous rocks does,

however, characterize the considerably younger,

yet adjacent, MCR (Fig. 1) produced by the

Keweenaw plume. Following Kent et al. (1992),

it might be possible to link widespread Granite�/

Rhyolite formation to extended incubation of theKeweenaw plume. This would nicely explain the

trace element similarities of Silver Mine and early-

erupted MCR basic magmas (Fig. 8), but would

require plume incubation for some 350 million

years. Unfortunately, documented examples of

plume incubation only seem to occur on order-

of-magnitude smaller time scales (Kent et al.,

1992). Also, extended survival of thick continentallithosphere when juxtaposed to hot asthenosphere

may be even more unreasonable for the Precam-

brian (Sandiford, 1989). One plume scenario

compatible with surface/near surface dominance

of silicic magmatism would be a subcontinental

plume tail, a la Yellowstone (e.g. Christiansen,

2001). However, there is no evidence for a

systematic age progression along the Granite�/

Rhyolite Provinces, as along the Yellowstone-

Snake River volcanic system (e.g. Smith and

Braile, 1993), nor would such a scenario be

consistent with the two separate periods of silicic

magmatism recorded in the St. Francois Moun-

tains (Van Schmus et al., 1996). Hence, we feel a

plume/superswell environment for the Mesopro-

terozoic Granite�/Rhyolite Provinces is verydoubtful. In contrast, an environment where

voluminous silicic magmatism accompanies post-

orogenic lithospheric thinning at a previously

active continental margin is a less troublesome

anorogenic model for Mesoproterozoic Midconti-

nent magmatism, although such a model might be

better characterized as post-orogenic. Convergence

along an active margin may lead to crustal, andconcomitant lithospheric, thickening. With en-

ough thickening, the lithosphere, or part of it,

becomes gravitationally unstable (Houseman et

al., 1981), and susceptible to removal by delamina-

tion (Bird, 1979), convection (Houseman et al.,

1981), or Rayleigh�/Taylor instability (deblobbing)

(Houseman and Molnar, 1997). Lithospheric thin-

ning brings hot asthenosphere into closer proxi-mity to the lower crust and eventually compels

widespread production of silicic magmas with

anorogenic affinities (e.g. Turner et al., 1992).

The relevant recent analogue may be the mid-

Tertiary ‘ignimbrite flareup’ in the western United

States (Coney, 1978). This significant silicic mag-

matic event and its associated extension have been

attributed by some to lithospheric thinning on theheels of lithospheric and crustal thickening

brought about by convergent tectonics (Sonder et

al., 1987; Harry et al., 1993; Dilek and Moores,

1999). There is indeed considerable geophysical

and geochemical evidence for Tertiary lithospheric

thinning in portions of the western United States

(Jones et al., 1992; Daley and DePaolo, 1992;

Beghoul et al., 1993; Hawkesworth et al., 1995;Zandt et al., 1995). The mid-Tertiary silicic

magmas are believed to form by extensive con-

tamination and crystallization of mantle-derived

basalts in the lower crust (Gans et al., 1989;

Johnson, 1991). Bickford et al. (1986), Bickford

(1988), Kay et al. (1989), and Bickford and

Anderson (1993) have all espoused such a scenario

for the Midcontinent in the Mesoproterozoic,proposing that the 1.37�/1.47 Ga Granite�/Rhyo-

lite provinces formed in extensional environments

following the earlier, 1.6�/1.8 Ga Central Plains

orogen (e.g. Van Schmus et al., 1996). Similarly,

Puura and Floden (1999) have concluded that 1.5�/

1.65 Ga rapakivi magmatism in the Fennoscan-

dian province was related to extension and crustal

thinning that developed 150�/300 Ma after a majororogeny. One possible flaw in these Precambrian

adaptations of orogenic collapse is the lengthy

time lags required between convergent tectonics

and subsequent extension (i.e. H/150 Ma) (Hoff-

man, 1989). Available calculations suggest instead

that lithospheric thinning or removal should occur

within approximately 100 Ma after the end of


compression (Houseman et al., 1981; Sonder et al.,1987; Turner et al., 1992; Platt and England,

1993). This flaw would vanish if the geochemical

boundary subdividing the Granite�/Rhyolite pro-

vinces as identified by Van Schmus et al. (1996)

(Fig. 1) represents a tectonic boundary between

Mesoproterozoic orogenic and Mesoproterozoic

post-orogenic rocks. However, there is currently

no additional evidence from these rocks to supportsuch a tectonic bifurcation of the Granite�/Rhyo-

lite terrains.

The geochemical boundary identified by Van

Schmus et al. (1996) is of great pertinence for

deciding between subduction and post-orogenic

origins for the Granite�/Rhyolite Provinces. This

northeast�/southwest striking geochemical bound-

ary separates Mesoproterozoic granites and rhyo-lites to the northwest with Nd crustal residence

ages indicating older crustal sources (TDME/1550

Ma) from those to the southeast with Nd crustal

residence ages indicating Mesoproterozoic crustal

sources (TDM0/1550 Ma) (Fig. 1). Importantly,

the absolute ages of granites and rhyolites are

similar on both sides of the boundary (Van

Schmus et al., 1996). We believe that a consistentinboard change in Nd crustal residence ages for

contemporaneous igneous rocks is more compa-

tible with an active subduction zone origin for the

Silver Mines basalts and the coeval portions of the

Granite�/Rhyolite Provinces, as juvenile crustal

additions during post-orogenic extension need

not be concentrated nearest the paleo-plate bound-

ary. The cause of the approximately 1370 Masilicic magmatism in the St. Francois Mountains

and the Southern Granite�/Rhyolite Province

remains an intriguing problem, but given its

petrologic similarity to 1470 Ma magmatism (e.g.

Bickford et al., 1981), an analogous association

with subduction is likely.

Given their apparent age, structural and geo-

chemical distinctions from the Silver Mines group,the Skrainka mafic rocks may have formed in a

contrasting tectonic regime. It is tempting to

attribute Skrainka magmatism to the initial ther-

mal effects of the Keweenaw plume head, although

the earliest magmatic rocks within the MCR

System are only 1108 Ma (Nicholson et al.,

1997). Otherwise Skrainka magmatism would

represent small scale mobilization of continentallithosphere, probably in a localized extensional

environment. Sylvester (1984) and Honda et al.

(1985) tentatively correlated the Skrainka basalts

with the 1.27�/1.22 Ga Harp and Seal Lake mafic

rocks of Labrador. Rivers and Corrigan (2000)

suggest these rocks formed in a back-arc environ-

ment.

7. Conclusions

Mafic magmatism accompanied the voluminous

silicic Mesoproterozoic magmatism of the St.

Francois Mountains and the larger Granite�/

Rhyolite Provinces of which the St. Francois

Mountains are but a small part. As first docu-mented by Sylvester (1984), there are two geo-

chemically, and probably chronologically, distinct

groups of mafic rocks in the St. Francois Moun-

tains. One group was likely contemporaneous with

the voluminous silicic magmatism that dominates

the Mesoproterozoic of the Midcontinent region.

The mafic rocks of this group have geochemical

affinities with modern subduction zone basalts,suggesting they, and their coeval silicic rocks, were

formed in an active continental margin or during

post-orogenic lithospheric thinning (i.e. exten-

sional collapse) at a previously active continental

margin. Although we cannot completely rule out

the latter tectonic environment, we favor the

former. The second group of mafic rocks may

substantially postdate silicic magmatism and re-presents localized extension, perhaps in a back-arc

environment. Further detailed study of the rocks

of the Granite�/Rhyolite Provinces, their Lauren-

tian and Scandinavian correlatives (e.g. Ahall and

Connelly, 1998), and their buried associates (e.g.

McBride and Kolata, 1999) should produce more

definitive answers to the tectonic setting for 1.5�/

1.3 Ga Midcontinent magmatism.

Acknowledgements

The authors gratefully thank the Missouri

Department of Resources for their support for

this project. Essential analytical and graphic


assistance was provided by Neil Dickey and MarkHowland, respectively. Thorough reviews by Kent

Condie and Pat Bickford greatly improved the

manuscript.

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