M A R C H 3, 18 4 9

U.S. DEPARTMENT OF THE INTERIOR

Paleomagnetic and 40Ar/39Ar Results from the

Grant Intrusive Breccia and Comparison to the 

Permian Downeys Bluff Sill—Evidence for

Permian Igneous Activity at Hicks Dome, 

Southern Illinois Basin

By Richard L. Reynolds, Martin B. Goldhaber, and Lawrence W. Snee

EVOLUTION OF SEDIMENTARY BASINS—ILLINOIS BASIN

Jennie L. Ridgley, Project Coordinator

U.S. GEOLOGICAL SURVEY BULLETIN 2094–G

A multidisciplinary approach to research studies of

sedimentary rocks and their constiutents and the 

evolution of sedimentary basins, both ancient and modern

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1997
For sale by U.S. Geological Survey, Information Services

Box 25286, Federal Center

Denver, CO 80225

Any use of trade, product, or firm names in this publication is for descriptive purposes only and

does not imply endorsement by the U.S. Government

Library of Congress Cataloging-in-Publication Data

U.S. DEPARTMENT OF THE INTERIOR

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY

Gordon P. Eaton, Director

Reynolds, Richard L.

Paleomagnetic and 40Ar/39Ar results from the Grant intrusive breccia and comparison 

to the Permian Downeys Bluff sill—evidence for Permian igneous activity at Hicks 

Dome, southern Illinois Basin / by Richard L. Reynolds, Martin B. Goldhaber, and 

Lawrence W. Snee.

p. cm. — (Evolution of sedimentary basins—Illinois Basin ; G)

(U.S. Geological Survey bulletin ; 2094)

Includes bibliographical references

1. Rocks, Igneous—Illinois—Hardin County. 2. Intrusions (Geology)—Illinois—

Hardin County. 3. Geology, Stratigraphic—Permian. 4. Paleomagnetism—

Illinois—Hardin County. I. Goldhaber, Martin B. II. Snee, Lawrence W. 

III. Title. IV. Series. V. Series : U.S. Geological Survey bulletin : 2094.

QE75.B9 no. 2094–G

[QE461]

557.3 s

[552′.1′0977398]—DC21 97-27552

CIP
III

CONTENTS

Abstract........................................................................................................................... 1

Introduction .................................................................................................................... 1

Geologic Setting ............................................................................................................. 1

Tectonic Setting of Igneous Rocks and Fluorspar Deposits in the

Southern Part of the Illinois Basin...................................................................... 1

Possible Role of Thermal Energy and Magmatic Volatiles

in Mineralization................................................................................................. 3

Disparate Isotopic Ages from Hicks Dome and Uncertain Relations

to Mineralization................................................................................................. 3

Rock Units Studied and Basis for Paleomagnetic Studies ............................................. 4

Methods .......................................................................................................................... 6

Results ............................................................................................................................ 6

Grant Intrusive Breccia........................................................................................... 6

Geochronology and Paleomagnetism ............................................................. 6

Magnetic Petrology......................................................................................... 8

Downeys Bluff Sill ................................................................................................. 9

Paleomagnetism.............................................................................................. 9

Magnetic Petrology......................................................................................... 10

Discussion and Conclusions ........................................................................................... 10

References Cited............................................................................................................. 15

FIGURES

1. Map showing locations of Hicks dome and the Downeys Bluff sill, igneous rocks, and distribution of

known fluorspar deposits ........................................................................................................................................... 2

2. Simplified geologic map of the Hicks dome area, showing location of the Grant breccia........................................ 3

3. 40Ar/39Ar age spectrum for the Grant intrusive breccia ............................................................................................ 8

4. Demagnetization diagrams for specimens of the Grant breccia ................................................................................ 9

5. Equal-area plot of mean remanent directions from the Grant intrusion, the Downeys Bluff sill,

and baked contact limestone ...................................................................................................................................... 10

6. Reflected-light photomicrograph of polished thin section of the Grant breccia........................................................ 11

7. Thermomagnetic curve of a specimen of the Grant intrusion showing change in normalized magnetization

as a function of heating and cooling .......................................................................................................................... 11

8. Demagnetization diagrams for specimens from the Downeys Bluff sill and baked limestone.................................. 12

9. Thermomagnetic curve of a specimen from the Downeys Bluff sill ......................................................................... 13

10. Reflected-light photomicrographs of polished thin sections, Downeys Bluff sill site............................................... 13

11. Apparent pole wander path for the North American craton....................................................................................... 14

TABLES

1. 40Ar/39Ar data for mineral samples from the Grant intrusive breccia, southern Illinois........................................... 4

2. Summary of paleomagnetic results, Grant intrusive breccia, Hicks dome, southern Illinois .................................... 7

3. Summary of paleomagnetic results, Downeys Bluff sill............................................................................................ 7

4. Paleomagnetic site-mean directions and virtual geomagnetic poles.......................................................................... 11
1

Paleomagnetic and 40Ar/39Ar Results from the 

Grant Intrusive Breccia and Comparison to the

Permian Downeys Bluff Sill—Evidence for

Permian Igneous Activity at Hicks Dome,

Southern Illinois Basin

By Richard L. Reynolds, Martin B. Goldhaber, and Lawrence W. Snee

ABSTRACT

Igneous processes at Hicks dome, a structural upwarp at

lat 37.5° N., long 88.4° W. in the southern part of the Illinois

Basin, may have thermally affected regional basinal fluid

flow and may have provided fluorine for the formation of the

Illinois-Kentucky Fluorspar district. The timing of both igneous activity and mineralization is poorly known. For this reason, we have dated an intrusive breccia at Hicks dome, the

Grant intrusion, using 40Ar/39Ar geochronometric and paleomagnetic methods. Concordant plateau dates, giving Permian

ages, were obtained from amphibole (272.1±0.7 [1σ] Ma)

and phlogopite (272.7±0.7 [1σ] Ma). After alternating-field

(AF) demagnetization, specimens that contain titanomagnetite-bearing igneous rock fragments give a mean remanent

direction of declination (D)=168.4°; inclination (I)=–8°;

α95=8.6°; number of specimens (N)=10; this direction yields

a virtual geomagnetic pole (VGP) at lat 54.8° N., long 119.0°

E., δp=4.4°, δm=8.7°, near the late Paleozoic part of the

North American apparent pole wander path. A nearly identical magnetization was found for the nearby Downeys Bluff

sill (previously dated at about 275±24 Ma by the Rb-Sr

method), in southern Illinois. Both AF and thermal demagnetization isolated shallow, southeasterly remanent directions

carried by magnetite in the sill and from pyrrhotite in the

baked contact of the Upper Mississippian Downeys Bluff

Limestone: D=158.6°; I=–11.8°; α95=3.8°; N=15, yielding a

VGP at lat 53.0° N., long 128.7° E., δp=2.0°, δm=3.9°. The

paleomagnetic results, isotopic dates, and petrographic evidence thus favor the acquisition of thermal remanent magnetization by the Grant breccia and the Downeys Bluff sill

during the Permian. The isotopic dates record rapid cooling

from temperatures greater than 550°C to less than 300°C (the

closure temperatures for diffusion of 40Ar in amphibole and

phlogopite, respectively) after emplacement during the Permian. The results further indicate that individual clasts of the

Grant breccia were emplaced at temperatures greater than

about 550°C, the magnetization-blocking temperature of the

titanomagnetite in the breccia, and that it cooled very rapidly,

within less than 1–2 m.y. After cooling, the breccia was not

affected by thermal perturbations greater than about 300°C.

INTRODUCTION

Studies of igneous rocks in southern Illinois contribute

to multidisciplinary investigations of the thermal activity,

fluid-flow history, hydrocarbon generation, and mineralization in and around the Illinois Basin. In this study, we apply

40Ar/39Ar dating methods, along with paleomagnetic and

rock magnetic techniques, to determine the age of emplacement and the cooling history of the Grant intrusive breccia at

Hicks dome (figs. 1 and 2). Additional goals of this study are

to estimate the emplacement temperature of the breccia and

to interpret its post-emplacement thermal history. We also

report paleomagnetic and rock magnetic results from a

nearby intrusion—the Downeys Bluff sill (fig. 1), previously

dated as Permian in age—for comparison to the results from

the Grant breccia. The overall objective of the study is to provide information on intrusive activity that may then be used

to test models linking thermal activity and mineralization in

the southern Illinois Basin.

GEOLOGIC SETTING

TECTONIC SETTING OF IGNEOUS ROCKS 

AND FLUORSPAR DEPOSITS IN THE

SOUTHERN PART OF THE ILLINOIS BASIN

The sampled intrusive bodies occur within the Illinois-Kentucky Fluorspar district in the southern part of the

Illinois Basin (fig. 1) (Baxter and others, 1989; Bradbury

and Baxter, 1992). The Fluorspar district is located at the
2 EVOLUTION OF SEDIMENTARY BASINS—ILLINOIS BASIN

intersection of the Reelfoot rift and the Rough Creek graben (fig. 1) (Nelson, 1991; Kolata and Nelson, 1991). The

rift and graben, part of the New Madrid rift system that

formed during late Precambrian–Cambrian extension

(Bond and others, 1984), are filled with about 7 km of

Paleozoic sedimentary rock in the vicinity of Hicks dome,

a cryptovolcanic feature that lies in the northwest part of

the Fluorspar district (Goldhaber, Potter, and Taylor, 1992;

Goldhaber, Taylor, and others, 1992). During the Late

Pennsylvanian and Early Permian plate collision that

assembled Pangea, the interior of the North American

plate was deformed to form the Tolu arch, a northwesttrending anticline, and a series of major north-northwesttrending faults. Many of these faults were intruded by

mafic dikes (Zartman and others, 1967; Nelson and Lumm,

1984; Bradbury and Baxter, 1992; Kolata and Nelson,

1991). Fluorspar mineralization was localized in Mississippian strata and postdated the Pennsylvanian–Early Permian

north-northwest-trending structures and intrusions. The

mineralized faults of the Fluorspar district define a series

of northeast-trending grabens that offset the axis of the

Tolu arch and crosscut the mafic dikes. The time span represented by such offsets poorly constrains the period of

mineralization between the Early Permian and the Late

Cretaceous. The structural relief at Hicks dome—as much

as 1,200 m that reveal Devonian and Lower Mississippian

sedimentary rocks through a cover of Pennsylvanian

beds—was related to the intrusion of alkalic igneous rocks

at depth accompanied by explosive emplacement of gascharged breccias at shallower levels (Bradbury and Baxter, 1992). The vertical displacement of the rocks and disruption of seismic reflectors underlying Hicks dome

extend into the crystalline basement (Goldhaber, Potter,

and Taylor, 1992). Hicks dome is itself underlain by an

intensely fluorite mineralized explosion breccia hosted by

Middle Ordovician rocks (Brown and others, 1954). As

summarized below, however, the timing of uplift and

intrusive activity at Hicks dome is also poorly known.

Figure 1. Map showing locations of Hicks dome and the Downeys Bluff sill, igneous rocks, and distribution of known fluorspar deposits. Modified from Baxter and others (1989).
PALEOMAGNETIC, 3 40Ar/39Ar RESULTS FROM THE GRANT INTRUSIVE BRECCIA, S. ILLINOIS BASIN

POSSIBLE ROLE OF THERMAL ENERGY

AND MAGMATIC VOLATILES IN

MINERALIZATION

The igneous activity in the southern Illinois Basin may

have influenced fluid temperatures and fluid compositions in

the southern Illinois Basin and the origins of Mississippi Valley-type (MVT) ore deposits at both the southern and northern margins of the basin. For example, Goldhaber, Potter, and

Taylor (1992), Goldhaber and others (1994), Taylor and others (1992, 1995), and Plumlee and others (1995) propose

genetic and temporal connections between thermal activity

provided by intrusions and mineralization at Hicks dome and

mineralization in the Illinois-Kentucky Fluorspar district.

Moreover, Rowan and Goldhaber (1995) suggest that formation of the Upper Mississippi Valley district in southern Wisconsin may have involved the transport of fluorine and other

ore constituents northward from the Hicks dome igneous

center. The mineral deposits of the two districts may be

related by a common, regional brine-flow regime. Evidence

for such a relation comes from several studies (Goldhaber

and others, 1994; Plumlee and others, 1995; Taylor and others, 1995), many of which illustrate the possible importance

of heat and magmatic volatiles generated at Hicks dome.

First, fluid-inclusion data show that Hicks dome was a thermal center and that fluid temperatures decreased away from

the uplift. Second, fluorine concentrations in carbonate rocks

above and below regional aquifers, such as the St. Peter Sandstone, show systematic decreases away from Hicks dome.

Third, petrographic studies reveal that fluorite distribution

and its associations with dolomite and K-feldspar are consistent with transport of hydrothermal brines northward, away

from Hicks dome. Finally, lead isotopic data imply a common

genesis for lead in deeply buried Cambrian-Ordovician carbonate rocks, in galena from overlying Mississippian carbonate rocks that host the Illinois-Kentucky fluorspar ores, in

galena from Upper Mississippi Valley district MVT deposits,

and in pyrite in non-mineralized beds that lie between the two

districts (Goldhaber and others, 1995).

Another proposed source of thermal energy to drive

fluid flow in the Illinois Basin involves the rapid convective

release of radioactive heat caused by the fracturing of basement granite (Spirakis and Heyl, 1995). Such locally derived

heat, and circulating basinal brines driven by this heat, might

alternatively explain the Upper Mississippi Valley district

MVT deposits without a connection to fluids from the Illinois-Kentucky Fluorspar district.

DISPARATE ISOTOPIC AGES FROM

HICKS DOME AND UNCERTAIN RELATIONS 

TO MINERALIZATION

Tests and refinement of different models for MVT ore

genesis and basinal fluid flow would benefit from a better

understanding of the timing of igneous activity in the southern Illinois Basin. In fact, the temporal relations among the

intrusion of dikes and sills in the southern Illinois Basin, the

emplacement of breccias at Hicks dome (including the F-,

Nb-, Be-, Y-, and REE-mineralized1 breccias), and the widespread Fluorspar district mineralization are a matter of dispute (Bradbury and Baxter, 1992). Part of the problem centers

on conflicting isotopic dates from rocks at Hicks dome that

lead to uncertainty about the timing of emplacement of the

mineralized breccias. K-Ar dating of xenocrystic mica

(258±13 Ma) and amphibole (281±14 Ma) from the Grant

breccia (Zartman and others, 1967; uncorrected for post-1977

decay constants) suggest explosive activity during the Permian. Other isotopic data suggest a younger age for brecciation. Heyl and Brock (1961) obtained a Pb-alpha age of

between 90 to 100 Ma on brockite from Hicks dome that they

took to imply influence from the reactivation of the New

Madrid rift during the Cretaceous. An alternative interpretation was proposed by Ruiz and others (1988) on the basis of

a comparison between strontium isotopes in biotite from

Permian alkalic dikes and in nearby fluorite. Similar Sr isotopic compositions were attributed to a Late Triassic or Early

Jurassic igneous event (at about 200 Ma) centered at Hicks

dome that reset the Rb-Sr system in the dikes. Ruiz and others

(1988) speculated that such thermal activity was the result of

rift reactivation associated with the breakup of Pangea (see

1REE, rare earth element.

Figure 2. Simplified geologic map of the Hicks dome area,

showing the location of the Grant breccia. Modified from

Baxter and others (1989).
4 EVOLUTION OF SEDIMENTARY BASINS—ILLINOIS BASIN

Table 1. 40Ar/39Ar data for mineral samples1 from the Grant intrusive breccia, southern Illinois.

[Footnotes appear on facing page]

Temperature Radiogenic K-derived 40ArK/

39ArK(3) 39Ar/37Ar(4) Radiogenic Percent Apparent age

(°C) 40Ar(2) 39Ar(2) yield (%) 39Ar and error5 (Ma)

PPib-Grant 1; AMPHIBOLE

Total-gas date: 270.6±0.7 Ma; plateau date: 272.1±0.7 Ma (1,050°–1,150°C); J=0.010257±.25; wt = 189.8 mg

500 0.1485 0.0156 9.51 0.55 13.6 0.2 168±3

600 0.3825 0.0283 13.51 0.11 43.6 0.4 234±3

700 1.2955 0.0872 14.86 0.09 57.7 1.4 255.9±1.0

750 1.2195 0.0784 15.56 0.1 85.9 1.2 267.1±1.0

800 2.4919 0.1592 15.66 3.2 92.7 2.5 268.6±0.8

850 1.7180 0.1108 15.51 4.3 96.3 1.7 266.4±1.2

900 2.0717 0.1336 15.50 3.0 96.9 2.1 266.2±0.9

950 4.0623 0.2597 15.64 0.76 97.8 4.1 268.5±0.7

1,000 31.750 2.0110 15.79 0.51 98.8 31.7 270.8±0.7

1,050P 31.210 1.9646 15.89 0.48 99.6 30.9 272.3±0.7

1,075P 10.968 0.6933 15.82 0.47 99.2 10.9 271.3±0.7

1,100P 6.3323 0.3975 15.93 0.40 99.4 6.3 273.0±0.7

1,150P 5.3005 0.3324 15.95 0.31 99.1 5.2 273.3±0.7

1,250 0.7156 0.0458 15.62 0.27 84.3 0.7 268±4

1,450 0.5294 0.0333 15.90 0.33 77.1 0.5 273±3

PPib-Grant 1; PHLOGOPITE

Total-gas date: 272.3±0.8 Ma; plateau date: 272.7±0.7 Ma (700°–1,150°C); J = 0.010216±.25; wt = 45.2 mg

500 0.0553 0.0073 7.58 4.9 21.3 0.1 134±15

600 0.3538 0.0251 14.12 0.78 88.8 0.5 243±3

650 0.8339 0.0529 15.77 1.8 67.1 1.1 269±2

700P 2.3073 0.1440 16.02 37 95.2 2.9 273.5±1.0

750P 4.2363 0.2654 15.96 265 98.1 5.4 272.5±0.7

800P 6.0702 0.3799 15.98 365 98.2 7.7 272.8±0.7

850P 7.0805 0.4430 15.99 404 97.6 9.0 273.0±0.7

900P 7.7166 0.4834 15.96 418 97.5 9.8 272.5±0.7

950P 7.0009 0.4397 15.92 380 97.9 8.9 271.9±0.7

1,000P 7.4618 0.4676 15.96 589 98.0 9.5 272.4±0.7

1,050P 11.875 0.7447 15.95 792 98.2 15.1 272.3±0.7

1,150P 23.476 1.4671 16.00 689 99.1 29.7 273.1±0.7

1,300 0.3227 0.0210 15.34 90 68.0 0.4 263±3

Bradbury and Baxter, 1992). An important consideration

comes from petrographic evidence that alteration of some

intrusions in the Illinois-Kentucky Fluorspar district was

caused by later MVT mineralization (Oesterling, 1952; Snee

and Hayes, 1992). Yet Chesley and others (1994) have

obtained a 17-point 147Sm/144Nd isochron of 277±15.6 Ma

from fluorite in the Fluorspar district. This date overlaps,

within analytical uncertainty, K-Ar dates from igneous rocks

in the southern Illinois Basin (Zartman and others, 1967).

Thus, the time span between intrusion and mineralization is

not well constrained. In another approach to the problem,

paleomagnetic methods may be used to date both intrusions

and ore minerals. For example, a paleomagnetic pole position

obtained from fluorspar ore has been interpreted to indicate a

Jurassic age of mineralization in the Illinois-Kentucky Fluorspar district (Symons, 1994). These results will be discussed

in more detail below in the context of our paleomagnetic data.

ROCK UNITS STUDIED AND BASIS 

FOR PALEOMAGNETIC STUDIES

The carbonatitic Grant breccia was targeted for additional isotopic dating and for new paleomagnetic work

because it is the least weathered outcrop of intrusive breccia

in the Hicks dome area and because it contains fragments of

igneous rock. Paleomagnetic tests for age, coherency of magnetization among clasts, and emplacement temperatures

were conducted. For rocks of this age, paleomagnetic dating

involves the measurement of a mean direction of remanence

in a rock, the calculation of the position of the paleomagnetic

pole from the mean direction, and the comparison of the pole

to the apparent pole wander path (APWP) for cratonic North

America (see Van der Voo, 1989). The pole positions from

one or a few igneous units, as in this study, represent the
PALEOMAGNETIC, 5 40Ar/39Ar RESULTS FROM THE GRANT INTRUSIVE BRECCIA, S. ILLINOIS BASIN

ancient magnetic pole position at a geologic instant when a

thermoremanent magnetization is acquired upon cooling.

Such pole positions are termed virtual geomagnetic poles, as

distinct from paleomagnetic poles that are meant to represent

geographic north after the short-term variations in magnetic

field directions have been averaged out. When it is shown

that magnetization is acquired during cooling of clasts in an

igneous conglomerate or breccia, and that later magnetic

overprints are lacking, the coherency of remanent directions

provides clues to the cooling history (Hoblitt and Kellogg,

1979). If cooling takes place after emplacement of all the

clasts, then directions of thermoremanent magnetization

should be similar. At the other extreme, cooling of clasts, for

example in the chimney of a diatreme, followed by emplacement will randomize directions. Paleomagnetic methods can

also be used to recognize deposits in which, before emplacement, the clasts cooled within the higher range of

temperatures over which remanent magnetization is

acquired, but then came to rest at a lower but still elevated

temperature for the remainder of their cooling. In such cases,

thermal demagnetization at low temperatures would uncover

coherent remanent directions and would reveal random

directions at higher temperatures (still less than the maximum blocking temperatures of the included magnetic minerals). A similar condition might arise if rocks with

randomized directions are subjected to low- to moderatetemperature thermal events to produce partial thermoviscous

remanence. It is clear from these concepts that magnetic minerals and their origins must be identified to interpret paleomagnetic results.

The mineralogic and geochemical composition of the

Grant breccia indicates an association with deep-seated alkalic igneous activity (Bradbury and Baxter, 1992). The

breccia consists of a calcite-dolomite matrix with fragments

1 1- to 3-mm size grains of brownish mica and amphibole

were hand-picked from a sample of the Grant intrusive breccia.

Based on microprobe analysis and petrographic characterization,

the amphibole is richterite-riebeckite and the mica is phlogopite.

Both samples were cleaned in 1.5-percent HCl, reagent-grade ethanol, acetone, and deionized water in an ultrasonic bath, air dried,

wrapped in aluminum packages, and sealed in silica vials along with

monitor minerals prior to irradiation. Samples were irradiated in the

TRIGA reactor at the U.S. Geological Survey in Denver, Colo., for

40 hours at 1 megawatt.

2 A Mass Analyzer Products 215 rare gas mass spectrometer

with a Faraday cup was used to measure argon isotope abundances.

Abundances of “Radiogenic 40Ar” and “K-derived 39Ar” are reported in volts. Conversion to moles can be made using 1.838×10–12

moles argon per volt of signal. Detection limit at the time of this experiment was 2×10–17 moles argon. Analytical data for “Radiogenic

40Ar” and “K-derived 39Ar” are calculated to 5 decimal places;

“40ArR/

39ArK” is calculated to 3 decimal places. “Radiogenic

40Ar,” “K-derived 39Ar,” and “40ArR/39ArK” are rounded to significant figures using analytical precisions. Apparent ages and

associated errors were calculated from unrounded analytical data

and then each was rounded using associated errors. All analyses

were done in the Argon Laboratory, U.S. Geological Survey, Denver, Colo. Decay constants are those of Steiger and Jäger (1977),

i.e., λε=0.581×10–10/yr, λβ=4.962×10–10/yr, and

λ=λε+λβ=5.543×10–10/yr. The irradiation monitor, hornblende

MMhb-1 with percent K=1.555, 40ArR=1.624×10–9 mole/g, and

K-Ar age = 520.4 Ma (Samson and Alexander, 1987), was used to

calculate J values for this experiment.

3 “40ArR/39ArK” has been corrected for all interfering isotopes including atmospheric argon. Mass discrimination in our

mass spectrometer was determined by measuring the 40Ar/36Ar ratio of atmospheric argon; our measured value is 298.9 during the

period of this experiment; the accepted atmospheric 40Ar/36Ar ratio

is 295.5. Abundances of interfering isotopes of argon from K and

Ca were calculated from reactor production ratios determined by irradiating and analyzing pure CaF2 and K2SO4 simultaneously with

these samples. The measured production ratios for these samples

are (40Ar/39Ar)K=1.181×10–2, (38Ar/39Ar)K=1.302×10–2,

(

37Ar/39Ar)K=1.74×10–4, (36Ar/37Ar)Ca=2.61×10–4,

(

39Ar/37Ar)Ca=8.25×10–4, and (38Ar/37Ar)Ca=5.90×10–5. Corrections were also made for additional interfering isotopes of argon

produced from irradiation of chlorine using the method of Roddick

(1983). The reproducibility of split gas fractions from each monitor

(0.05–0.25 percent, 1 σ) was used to calculate imprecisions in J. J

values for each sample were interpolated from adjacent monitors

and have similar uncertainties to the monitors. Uncertainties in calculations for the date of individual steps in a spectrum were calculated using modified equations of Dalrymple and others (1981).

4 To calculate apparent K/Ca ratios, divide the 39Ar/37Ar ratio value by 2.

5 1 σ error. P Fraction included in plateau date.
6 EVOLUTION OF SEDIMENTARY BASINS—ILLINOIS BASIN

and crystals of biotite and amphibole that are 1 cm or more

across, clasts of various igneous and sedimentary rocks, and

lesser amounts of apatite and pyroxene. Fragments of feldspathic rocks and grains of both phlogopite and amphibole are

commonly surrounded by “lapilli” rims that are interpreted to

represent crystallized melt. These clasts and grains thus

existed within the magma at depth before emplacement of the

breccia. Many original fragments and grains have been

replaced by serpentine and carbonate, mainly dolomite. The

relatively high contents of titanium (2.15 percent), Zn, and Sr

are characteristic of rocks in alkalic provinces (Heinrich,

1966; Erickson and Blade, 1963).

The lamprophyric Downeys Bluff sill (Baxter and others, 1989) was selected for paleomagnetic comparison to the

Grant breccia because (1) it likely contained thermoremanent

magnetization; (2) it is located in nearly undeformed strata

close to Hicks dome; and (3) its Permian isotopic date is close

to the K-Ar date from the Grant breccia (Zartman and others,

1967). The Downeys Bluff sill yields an apparent age of about

275±24 Ma by the Rb-Sr method when recalculated from

Zartman and others (1967) using post-1977 decay constants.

METHODS

For 40Ar/39Ar dating, mineral separates from a sample

of the Grant breccia were obtained by a sequence of crushing,

sieving, and hand picking. Mineral separates and neutron

flux monitors were wrapped in aluminum foil, placed in

quartz vials, and irradiated for 40 hours in the U.S. Geological Survey TRIGA reactor (table 1). The neutron flux was

monitored using standards adjacent to each sample. The

monitor used was hornblende MMHb-1, having an assigned

K-Ar age of 520.4 Ma (Samson and Alexander, 1987; Alexander and others, 1978; Dalrymple and others, 1981). Measured argon isotopes of K2SO4 and CaF2

 included in the

irradiated package were used to correct for interfering isotopes. The samples were heated in a double vacuum resistance furnace in a series of temperature steps for 20 min each

to a maximum temperature of 1,450°C. At each step, the

isotopes of 40Ar, 39Ar, 38 Ar, 37Ar, and 36Ar were measured

using a mass spectrometer, and isotopic compositions were

corrected for mass discrimination. The decay constants of

Steiger and Jäger (1977) were used to calculate the apparent

ages. The age plateaus were determined by inspection of contiguous gas fractions using the critical-value test of

Dalrymple and Lanphere (1969).

Paleomagnetic measurements of specimens from oriented samples drilled from the outcrop were made using a

cryogenic magnetometer, a 5-Hz spinner magnetometer, or a

90-Hz spinner magnetometer. After measurement of the natural remanent magnetization (NRM), each sample was

demagnetized either in progressive alternating fields

(AF)—typically in 10 steps from 5 mT to 80 mT peak induction—or by thermal techniques—typically in 8 steps from

100°C to 600°C. After we confirmed the presence of

ferrimagnetic pyrrhotite in contact metamorphosed rock

below the Downeys Bluff sill, two more samples were

demagnetized with additional steps at 330°C and 360°C to

better define the magnetization from pyrrhotite. Directions of

remanent magnetization, listed in tables 2 and 3, were determined by least-squares line fitting of demagnetization decay

paths (Zijderveld, 1967; Kirschvink, 1980). The identity of

magnetic minerals was inferred from the magnetization lost

in response to demagnetizing temperatures or alternating

fields. Other, more direct methods for identifying magnetic

minerals included thermomagnetic analysis and petrographic

observation. Thermomagnetic analysis—the measurement of

magnetization as a function of temperature—was used to

identify magnetic minerals by measurement of their Curie

temperatures. Magnetic iron oxide and iron sulfide minerals

have diagnostic Curie temperatures and characteristic shapes

of the thermomagnetic curve. Polished thin sections were

examined in oil under reflected light at magnifications of as

much as 1,000 ×. We examined six polished thin sections

made from the end pieces of paleomagnetic samples. NRM

directions and magnitudes along with magnetic susceptibility

values are given in tables 2 and 3 because of their possible

value for magnetic modeling of intrusions in the region. Magnetic susceptibility (MS) was measured using a frequency of

about 600 Hz in an induction of 0.1 millitesla (mT).

RESULTS

GRANT INTRUSIVE BRECCIA

GEOCHRONOLOGY AND PALEOMAGNETISM

Concordant 40Ar/39Ar plateau dates for amphibole

(272.1±0.7 [1σ] Ma) and phlogopite (272.7±0.7 [1σ] Ma)

were obtained (fig. 3; table 1). Neither the amphibole nor the

phlogopite age spectra show evidence for younger thermal

disturbances above ≈550°C and ≈300°C (the amphibole and

phlogopite argon closure temperatures, respectively). The

concordance of the two dates indicates very rapid cooling

from greater than about 550°C to less than 300°C.

Magnetic susceptibility varies greatly, from a low

value of 6.9×10–4 to nearly 0.1 volume SI (table 2), reflecting the heterogeneous distribution of strongly magnetic

fragments of igneous rock (see Bradbury and Baxter, 1992)

that contain magnetic oxide minerals. NRM magnitude similarly varies by a factor of nearly 100. NRM directions scatter broadly with shallow to steep positive inclinations,

suggesting variable degrees of overprinting in a normal

polarity field (table 2). The downward component of magnetization is removed by both AF and thermal demagnetization in 10 of 15 specimens, isolating directions with southsoutheast declination and nearly horizontal inclination

(table 2; figs. 4A, 5). When uncovered by AF treatment, this

direction is revealed over most of the demagnetization trajectory, although some specimens become unstable above

the 60-mT step. With a few exceptions (fig. 4B), thermal

demagnetization reveals the southerly, shallow direction in
PALEOMAGNETIC, 7 40Ar/39Ar RESULTS FROM THE GRANT INTRUSIVE BRECCIA, S. ILLINOIS BASIN

Table 2. Summary of paleomagnetic results, Grant intrusive breccia, Hicks dome, southern Illinois.

[Dn, declination in degrees clockwise from true north; In, inclination in degrees, positive downward; M, magnitude (amperes/meter) of natural remanent magnetization; MS, magnetic susceptibility, in volume SI; Demag, demagnetization method (T, thermal; A, alternating field); Range, demagnetization range over which declination (D) and inclination (I)

was calculated by principal component analysis (in degrees Celsius for thermal results, in milliTeslas for alternating-field results); ORG, origin of orthogonal demagnetization plot.

RangeP, DP, IP ; demagnetization range, D, I of inferred late Paleozoic components. RangeT, DT, IT, demagnetization range, D, I of inferred remagnetization components. NRM,

natural remanent magnetization]

Specimen Dn In M MS Demag RangeP DP IP RangeT DT IT

(P0-7-)

1B . . . . . . 26.9 . . . . 72.8 . . . 3.35×10–2 . .1.90×10–3. . . . A. . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . NRM-30 . . 34.1 . . . . 73.8

2 . . . . . . 277.2 . . . . 71.0 . . . 1.55×10–1 . .1.76×10–2. . . . .T. . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . 500–600. . 139.0 . . . –58.8

3A . . . . . 197.7 . . . . 11.4 . . . 9.58×10–2 . .9.72×10–2. . . . .T . . . . . 400–600 . . . . 164.2. . . –14.7

 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100–400 . . . . 337.3. . . . . 9.6

3B . . . . . 146.3 . . . . 50.6 . . . 4.20×10–2 . .4.42×10–3. . . . A. . . . . 15–40; ORG . 163.0. . . . –4.8

4A . . . . . . . 0.7 . . . . 53.4 . . . 1.53×10–1 . .1.29×10–2. . . . .T. . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . 100–600. . 315.4 . . . . 32.9

4B . . . . . 150.0 . . . . 45.1 . . . 1.58×10–2 . .1.09×10–3. . . . A. . . . . 20–40 . . . . . . 174.4. . . . –2.5

5A . . . . . . 58.4 . . . . 74.1 . . . 1.07×10–2 . .7.30×10–4. . . . .T . . . . . 400–550 . . . . 165.8. . . –22.4. . . . NRM-400 . 17.7 . . . . 64.2

 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . 550–660. . 152.5 . . . –63.6

5B . . . . . . 48.3 . . . . 28.0 . . . 5.80×10–1 . .1.03×10–2. . . . A. . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . 5-ORG . . . . 70.2 . . . . 67.2

6A . . . . . 314.4 . . . . 63.9 . . . 6.96×10–3 . .6.91×10–4. . . . .T. . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . 100–400. . . . 3.6 . . . . 72.2

6B . . . . . 335.4 . . . . 66.0 . . . 9.71×10–3 . .7.43×10–4. . . . .T . . . . . 500–600 . . . . 156.3. . . –13.3

7A . . . . . 244.1 . . . . 67.8 . . . 1.14×10–2 . .1.15×10–3. . . . .T . . . . . 400–550; ORG193.2. . . . . 2.9. . . . 550–660. . 187.6 . . . –72.5

8B . . . . . 167.7 . . . . 24.7 . . . 3.55×10–2 . .1.33×10–3. . . . A. . . . . 15–40 . . . . . . 164.3. . . –10.7

10A . . . . 191.6 . . . . 15.7 . . . 8.28×10–2 . .1.96×10–3. . . . .T . . . . . 400–600 . . . . 173.8. . . . –4.1

10B . . . . 202.7 . . . . 33.9 . . . 4.76×10–2 . .1.70×10–3. . . . A. . . . . 15–40 . . . . . . 160.5. . . . –1.3

10C . . . . 292.9 . . . . 10.8 . . . 2.52×10–2 . .6.51×10–3. . . . .T . . . . . 400–600 . . . . 165.6. . . . . 4.0

 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100–300 . . . . 333.0. . . . 11.0

Table 3. Summary of paleomagnetic results, Downeys Bluff sill.

[Sp., specimen number; S, sill; B, baked sedimentary rock. Range, demagnetization range over which declination (D) and inclination (I) was calculated by principal component

analysis (in degrees Celsius for thermal results, in milliTeslas for alternating-field results); ORG, origin of orthogonal demagnetization plot; Tub, unblocking temperature. Other

notations same as in table 2]

Sp. Rock Dn In M MS Demag Range D I Range D I

(P0-6-) type high Tub high Tub high Tub

1B S 151.9 19.3 1.63×10–1 1.13×10–2 A 20–ORG 167.4 –16.6

3A S 95.6 59.5 8.40×10–1 1.55×10–2 T 400–ORG 146.6 16.8

3B S 84.5 57.1 8.17 2.01×10–2 A 20–ORG 163.1 –1.1

4A S 98.2 35.5 6.99 1.12×10–1 T 550–ORG 152.6 –9.9

4B S 100.8 34.1 5.41 1.09×10–1 A 10–ORG 150.6 –15.0

6A S 111.9 32.6 2.97 8.76×10–2 T 550–ORG 155.8 –7.9

7A S 100.7 32.8 5.57 5.71×10–2 T 500–ORG 151.6 –4.7

11A S 81.0 –28.5 2.31 5.59×10–2 T 550–ORG 149.7 –14.3

13A B 158.7 –12.2 3.72×10–2 6.16×10–5 T 200–400 157.7 –11.9 400–ORG 158.0 –14.3

14A B 160.3 –12.4 3.34×10–2 4.04×10–5 T 100–360 161.5 –10.4 360–ORG 162.2 –15.2

15A B 158.2 –13.6 5.45×10–2 4.51×10–5 T 200–400 158.5 –13.8 400–ORG 159.6 –14.7

16A B 161.3 –12.9 1.07×10–1 4.60×10–5 T 200–400 160.0 –15.5 400–ORG 161.6 –15.7

16B B 159.6 –14.7 5.62×10–2 3.95×10–5 A 20–ORG 159.3 –14.6

17A B 155.6 –21.3 6.10×10–2 5.60×10–5 T 200–400 156.1 –25.2 400–ORG 157.5 –18.4

19A S 146.4 44.8 2.72×10–1 3.12×10–2 A 20–ORG 162.7 –10.3

20B S 106.9 45.2 2.37×10–1 2.39×10–2 T 550–ORG 154.8 –4.0

21A B 165.0 –13.3 4.92×10–2 3.83×10–5 A 20–ORG 164.8 –13.9

22A B 164.8 –11.0 8.85×10–2 2.79×10–5 T 200–360 164.8 –11.0 360–ORG 164.4 –12.4
8 EVOLUTION OF SEDIMENTARY BASINS—ILLINOIS BASIN

290

270

250

230

0 50 100

PERCENT 39ArK RELEASED

APPARENT AGE (Ma)

Phlogopite

Amphibole

more complicated demagnetization paths (fig. 4D) or in trajectories defined by only three heating steps (e.g., specimens 5A, 6B, 7A; table 2). Some specimens displayed

erratic magnetizations after cooling from temperatures of

600°C probably because of spurious magnetizations

acquired in small, ambient inductions by magnetite that

formed from pyrite during heating. Curiously, two specimens (3A and 10C; fig. 4D; table 2) contained a magnetization, revealed over low unblocking temperatures (less than

400°C), nearly opposite to the southerly, shallow direction

uncovered at higher unblocking temperatures.

The site-mean direction for the southeasterly magnetization, uncorrected for the attitude of the enclosing strata, is

declination (D)=168.4°; inclination (I)=–8.0°; α95=8.6 ̊;

number of specimens (N)=10 (table 4; fig. 5). This direction

yields a VGP at lat 54.8° N., long 119.0° E. (δp=4.4°,

δm=8.7°). Specimens were given equal weight, including

pairs from the same sample (samples P0-7-3 and -10) when

one of the pair was treated by AF and the other by thermal

demagnetization; thermally demagnetized specimen P0-7-

10C was omitted from the calculation of the mean direction

so that the mean does not include more than one specimen per

sample for a given treatment. Correction for the tilt of the Fort

Payne Formation (strike N. 54° W., dip 14° SW.) near the

sampling locality yields a direction of D=166.0°; I=–17.2°;

and a VGP at lat 58.6° N., long 119.0° E. (δp=4.6°, δm=8.9°).

In the other five specimens, steep inclinations prevailed

through the demagnetization tests, probably the manifestation of remagnetization; the age of remagnetization is indeterminate and may encompass late Mesozoic to present-day

magnetic fields. Cretaceous remagnetization of some specimens, particularly those with northwesterly or southeasterly

declinations, cannot be ruled out. Moreover, three of the

specimens having the southerly, shallow direction also contained distinct steep inclination components over different

ranges of demagnetization treatment. Demagnetization isolates normal polarity directions (northerly, steep positive

inclination not far from the present Earth-field direction) in

three specimens and reversed polarity directions (southerly,

steep negative inclination) in two specimens (fig. 5; table 2).

One specimen (P0-7-5A) contains both components—a normal one from demagnetization to 400°C, and a reversed one

between 550°C and 660°C. The isolation of the reversed

direction over this temperature range, mostly above the

580°C Curie temperature of pure magnetite, indicates that

hematite carries most of this magnetization. The reversed

component may be related to chemical alteration during surficial weathering that formed hematite. The normal overprints

isolated over low temperature ranges and low AF inductions

are typical of viscous remanent components in magnetite.

MAGNETIC PETROLOGY

The demagnetization paths—the loss of magnetization

with increasing temperature or AF induction—strongly suggests that magnetite is the main carrier of remanence, with

minor contributions from hematite. In thermal demagnetization, most magnetization is removed by 550°C, just below

the magnetite Curie temperature, and thus maximum blocking temperature, of 580°C. AF demagnetization to 80 mT

removes nearly all remanence.

Magnetite is confirmed by petrographic observation and

thermomagnetic results. The magnetite clearly contains or is

associated with Ti, reflecting an igneous origin, and it is thus

not a product of later diagenetic alteration. Many igneous

rock fragments contain magnetite with ilmenite, either in

composite grains or as intergrowths that resulted in the subdivision of magnetite by ilmenite lamellae (fig. 6). Magnetite

is also commonly associated with TiO2-rich alteration products, typical of high-temperature alteration during initial

cooling. The magnetite has Curie temperatures of about

550°C (fig. 7), reflecting the presence of Ti in the magnetite

in amounts that correspond to about 6 mol percent ulvospinel.

Iron sulfide minerals (pyrite, marcasite, and chalcopyrite),

some of which may have replaced magnetite, formed in the

matrix and rock fragments after emplacement of the breccia.

Figure 3. 40Ar/39Ar age spectrum for the Grant intrusive breccia. Plateau dates are 272.1±0.7 Ma for amphibole and 272.7±0.7 Ma for

phlogopite.
PALEOMAGNETIC, 9 40Ar/39Ar RESULTS FROM THE GRANT INTRUSIVE BRECCIA, S. ILLINOIS BASIN

DOWNEYS BLUFF SILL

PALEOMAGNETISM

Paleomagnetic samples from the site at the Downeys

Bluff sill were taken from the intrusion and from the baked

sedimentary rock of the Mississippian Downeys Bluff Limestone, within about 20 cm below the sill. Samples of the

intrusion have high values of MS (0.02–0.1 volume SI) and

NRM magnitude (0.2–8.2 A/m) (table 3). Samples of baked

limestone have low MS values (about 4×10–5 volume SI) but

moderately high values of NRM magnitude (0.03–0.1 A/m)

approaching those of the sill (table 3). NRM directions from

the sill plot in a diffuse band from the northeast to southeast,

with shallow to moderate, mostly positive, inclination. In

contrast, NRM directions from the baked limestone cluster

tightly in the southeast with shallow negative inclination

(table 3). At low inductions and temperatures, demagnetization of specimens from the sill removes northerly, downward

components to isolate directions close to those of the NRM

from the baked limestone (figs. 8A, 8B; table 3). Remanent

directions of the baked limestone show minimal change in

Figure 4. Demagnetization diagrams

for specimens of the Grant breccia. The

upper diagram in each set is an orthogonal demagnetization diagrams (Zijderveld, 1967) in which the closed symbols

represent projections of the magnetic

vector on the vertical plane, whereas the

open symbols represent such projections

on the horizontal plane. Remanent magnetizations are given in amperes/meter

(A/m). Demagnetization levels are given

in millitesla (mT) and degrees Celsius.

The lower diagram in each set illustrates

the decay of remanent magnetization,

normalized to NRM, in response demagnetization induction or temperature.
10 EVOLUTION OF SEDIMENTARY BASINS—ILLINOIS BASIN

response to AF demagnetization, but most of the remanence

is removed over the narrow temperature range between

300°C and 360°C. The small component of magnetization

removed over higher unblocking temperatures has directions

essentially identical to those removed between 300°C and

360°C (table 3). The site-mean direction, D=158.6°,

I=–11.8° (α95=3.8°, N=15) (table 3; fig. 5), that includes

components from both the sill and baked limestone and that

is uncorrected for the 2° or less dip of the beds, is statistically

identical to the tilt-corrected direction of the Grant breccia.

The direction from the sill plus baked limestone yields a

VGP at lat 53.0° N., long 128.7° E.; (δp=2.0°, δm=3.9°). The

magnetization direction from the baked limestone is close to

the direction from another Mississippian limestone, the

Salem Limestone at a site in Indiana, that was acquired

through chemical alteration during Permian-Pennsylvanian

time (fig. 5). These directions contrast with a magnetization

direction calculated from a mostly Mississippian–Early

Pennsylvanian pole position (fig. 5) (Van der Voo, 1990).

MAGNETIC PETROLOGY

The demagnetization behavior and petrologic results

indicate that the sill and baked limestone have different magnetic minerals that, nevertheless, recorded the same geomagnetic field direction. In some specimens from the sill,

demagnetization to 550°C eliminates all of the remanence,

indicating titanomagnetite, but, in other specimens, remanence is not completely removed until the 630°C step (fig.

8A). These latter specimens do not show sharply defined

maximum unblocking temperatures for magnetite or titanomagnetite, suggesting instead that thermally stable

maghemite carries the remanence. However, a thermomagnetic curve for one such specimen (P0-6-11; figs. 8A, 9)

defines a range of Curie temperatures between about 500°C

and 540°C, indicative of titanomagnetite. Such inconsistency may reflect a heterogeneous distribution of magnetic

oxides in the sill (the mass of the rock chip used for thermomagnetic analysis was about 10–4 that of the specimen). AF

demagnetization of specimens from the sill show behavior

consistent with the low coercivity of magnetite (fig. 8C). Petrographic evidence for Ti-bearing magnetite includes the

common occurrence of titanium dioxide grains intergrown

with magnetite (figs. 10A, 10B).

AF demagnetization of specimens from the baked limestone, in contrast, does not remove even half of the NRM

magnitude by 60 mT, thus indicating a high coercivity magnetic mineral (fig. 8D). In thermal treatment, the loss of most

remanence between 300°C and 360°C (fig. 8C) strongly suggested pyrrhotite as the dominant remanence carrier, an

inference confirmed by petrographic examination. Insufficient amounts of pyrrhotite could be isolated to make a thermomagnetic analysis. Petrographic examination did not

provide any clues to the origin (whether detrital or related to

oxidation of sulfide minerals) of the small amount of iron

oxide that carries a direction identical to that from pyrrhotite.

Figure 5. Equal-area plot of mean remanent directions from the

Grant intrusion (GIB), the Downeys Bluff sill (DBS, all), and baked

contact limestone (DBS, baked contact; the component carried by

pyrrhotite). Solid symbols represent projections on the lower hemisphere. Open symbols represent projections on the upper hemisphere. Solid square, direction recalculated from early and middle

Carboniferous paleomagnetic poles (Van der Voo, 1990) for geographic coordinates at Hicks dome. Solid circle, mean direction

from the Mississippian Salem Limestone that carries a remagnetization direction acquired during the Permian-Pennsylvanian (from

Van der Voo, 1989) recalculated for geographic coordinates at

Hicks dome. Solid and open diamonds are inferred to represent respective normal and reversed polarity overprint components in

specimens from the Grant intrusion. “X” represents present-day

field direction at Hicks dome.

DISCUSSION AND CONCLUSIONS

The concordant 40Ar/39Ar plateau dates (272.1 and

272.7 Ma for amphibole and phlogopite, respectively) provide a straightforward interpretation that cooling of the Grant

breccia through argon closure temperatures of about 550°C

for amphibole and about 300°C for phlogopite occurred during the Permian. Because the amphibole and phlogopite cooling ages are identical within analytical error, cooling was

essentially instantaneous; these apparent ages are likely identical to the age of emplacement. These results confirm the

Permian age for the Grant intrusion. The 40Ar/39Ar results

also indicate a lack of severe post-emplacement alteration at

elevated temperatures, greater than about 300°C. Thus, a

later, distinct igneous event associated with later fluorspar

mineralization at Hicks dome is not indicated. Paleomagnetic

results from thermal demagnetization indicate that the

breccia was emplaced at high temperatures, at least 550°C.
PALEOMAGNETIC, 11 40Ar/39Ar RESULTS FROM THE GRANT INTRUSIVE BRECCIA, S. ILLINOIS BASIN

This conclusion is based on the observation that the characteristic southeasterly, shallow paleomagnetic direction was

isolated in primary titanomagnetite at demagnetization temperatures to at least 550°C.

With regard to establishing the age of intrusion, the

paleomagnetic results are not as definitive as the isotopic

dates for several reasons. First, the VGP from any small

intrusion represents only a spot recording of the ancient geomagnetic field, which at any moment (a few hundred to a few

thousand years) may lie far from the average geomagnetic

pole position that approximates the geographic pole throughout several thousand years to many millions of years. As

seen in figure 11, the Grant breccia VGP lies close to the

Early Triassic part of the APWP. For the reasons above, the

lack of correspondence between the VGP’s and the APWP is

not unexpected and is within reasonable limits to be consistent with the Permian isotopic dates for both units. As an

example, the present Earth field position lies nearly 20° of

latitude from the geographic north pole (fig. 11).

The question about whether emplacement and cooling

preceded or postdated tilting of Hicks dome, or took place

during tilting, cannot be resolved from the paleomagnetic

results because of the lack of other sites in rocks of equivalent age distributed in different fold limbs. After correcting

the magnetization directions for the structural attitude of the

Fort Payne Formation near the sampling locality, the VGP

of the Grant intrusion moves to a position about midway

between the Early Triassic and Middle Jurassic. This VGP

position partly overlaps the two VGP’s from the Downeys

Bluff sill, one based on the pyrrhotite component and the

other from all specimens (fig. 11). Whether or not the

Figure 6. Reflected-light photomicrograph of polished thin section of the Grant breccia. Phenocryst of magnetite (M, gray), partly

replaced by hematite (H, white), showing evidence for the incipient

development of ilmenite oxidation lamellae (IL). Field of view is

310 mm in the long dimension.

Figure 7. Thermomagnetic curve of a specimen of the Grant

intrusion (sample P0-7-3) showing change in normalized

magnetization as a function of heating and cooling. The decay

of magnetization far below 580°C (the Curie temperature of

pure magnetite) indicates the dominance of titanomagnetite in

the specimen.

Table 4. Paleomagnetic site-mean directions and virtual geomagnetic poles.

[DBS, Downeys Bluff sill; GIB, Grant intrusive breccia; Po comp (pyrrhotite component) based on thermal demagnetization of baked sedimentary rocks in steps to 360°C or 400°C;

tilt correct, the resultant remanent directions and resulting pole position determined upon correction for the strike (N. 54° W.) and dip (14° S.) of strata that contain the breccia. N,

number of specimens; D, mean declination, in degrees, clockwise from true north. I, mean inclination, in degrees, positive downward. α95, semi-angle of the cone of 95 percent

confidence in degrees. k, precision parameter (Fisher, 1953). R, resultant vector. Plat and Plong, latitude and longitude of the virtual geomagnetic pole (VGP). δp and δm, semiminor and semi-major axes (in degrees), respectively, of the oval of 95 percent confidence surrounding the VGP]

Site Selection N D I α95 k R Plat Plong δp δm

(° N.) (° E.)

DBS all data 15 158.6 –11.8 3.8 103.4 14.86 53.0 128.7 2.0 3.9

DBS Po comp. 6 159.8 –14.6 5.2 167.8 5.97 54.8 128.1 2.7 5.3

GIB in situ 9 168.4 –8.0 8.6 36.5 8.78 54.8 112.1 4.4 8.7

GIB tilt correct 166.0 –17.2 58.6 119.0 4.6 8.9
12 EVOLUTION OF SEDIMENTARY BASINS—ILLINOIS BASIN

VGP’s are statistically equivalent is not at issue, inasmuch

as several thousand to several million years may separate

the cooling of the intrusions. That is, we cannot establish

their temporal equivalence to the extent that would make

such a comparison meaningful with respect to the timing of

tilting at Hicks dome.

A major question addressed in this study is the timing of

the igneous events in and near the study area relative to the

timing of Fluorspar district mineralization. A number of lines

of evidence are best reconciled with the interpretation that

the fluorspar mineralization occurred during a geologically

short time interval following the Permian igneous activity

Figure 8. Demagnetization diagrams for specimens from the Downeys Bluff sill and

baked limestone, as described for figure 4. Symbols as in figure 4.
PALEOMAGNETIC, 13 40Ar/39Ar RESULTS FROM THE GRANT INTRUSIVE BRECCIA, S. ILLINOIS BASIN

responsible for both the intrusive breccias near Hicks dome

and the other intrusive rocks of the area (fig. 1). The most

persuasive evidence for this timing is the direct dating of fluorite using isotopes of Nd and Sm, which occur within the

fluorite crystal lattice (Chesley and others, 1994, 1995). The

studies by Chesley and others included samples that represent the range of fluorite paragenesis in the district. A welldefined 17-point isochron corresponding to an age of

277.0±15.6 Ma was determined. This age overlaps the 39Ar/

40Ar ages from the Grant intrusive breccia reported here. 

Additional lines of evidence for links between igneous

activity and fluorspar mineralization are less direct. Fluidinclusion filling temperatures associated with Fluorspar district mineralization have been interpreted to reflect a thermal zonation around Hicks dome (Taylor and others, 1992).

Although this interpretation was questioned by Spry and

Fuhrman (1993), a reevaluation of all available data (Taylor and others, 1995) confirms the existence of this zonation, suggesting that residual igneous heat affected

fluorspar mineralization. Moreover, the possible involvement of magmatic volatiles in fluorspar mineralization is

indicated from geochemical modeling (Plumlee and others,

1995) and the presence of magmatic volatiles in fluoritehosted fluid inclusions from the district (Hofstra and others,

1995). Geochemical zonation around Hicks dome in mineral phases associated with Fluorspar district mineralization

has also been recognized. High Sb contents characterize

galena in the Illinois portion of the Fluorspar district near

Hicks dome with systematically lower values with distance

away from Hicks dome into Kentucky (Hall and Heyl,

1968). An expanded data set (Goldhaber and Mosier,

unpub. data) further confirms this zonation. Likewise, a Pb

isotope zonation around Hicks dome, reported first by Heyl

and others (1966), has been confirmed by additional measurements (Goldhaber and Zartman, unpub. data). Zonation

patterns of this type reflect an influence of Hicks dome on

Fluorspar district mineralization. 

Other attempts to date the Fluorspar district mineralization that yield ages other than Permian may be challenged on

a variety of grounds. The 200-Ma age of Ruiz and others

(1988) is a model age based on the questionable assumption

that all Sr in fluorite was inherited from the biotite of igneous

rocks. The 155-Ma paleomagnetic age of Symons (1994)

may not represent a direct date on fluorite because the origin

of the remanence carrier magnetite, whether coprecipitated

with fluorite or formed later in microfractures, is unknown.

Figure 9. Thermomagnetic curve of a specimen from the

Downeys Bluff sill (sample P0-6-11), indicating the presence of

titanomagnetite.

Figure 10. Reflected-light photomicrographs of polished thin

sections, Downeys Bluff sill site. A, phenocryst of partly dissolved

magnetite (gray), with concentration of TiO2 near grain margins,

from intrusion. B, Euhedral and subhedral clots of TiO2

 surrounded by relict magnetite in altered phenocrystic magnetite, from

intrusion. Field of view is 160 μm in the long dimension in each

photomicrograph.
14 EVOLUTION OF SEDIMENTARY BASINS—ILLINOIS BASIN

Another complication in paleomagnetic dating arises from

uncertainty in the location of the APWP. Because of conflicting interpretations about the Middle and Late Jurassic APWP

(reviewed by Van der Voo, 1992; Hagstrum, 1993; Courtillot

and others, 1994; Symons, 1994), that part of the path was

represented by Symons (1994) as a bulge, as shown in figure

11. The central issue is whether the path traces a low- or a

high-latitude track between the Early Jurassic and Late Cretaceous. It is important to consider this uncertainty because

the high-latitude paleomagnetic pole obtained by Symons

(1994) from fluorite deposits (pole “Bu,” fig. 11) was attributed to magnetite presumed to have precipitated within fluorspar ore (Symons, 1994).

Interestingly, Brannon and others (1993) have recently

used 238U/206Pb systematics to obtain an age of 156 Ma for

late (post-mineralization) calcite in the Upper Mississippi

Valley district at the north end of the Illinois Basin. This

result contrasts with the Permian age of mineralization itself

(Brannon and others, 1992). Brannon and others (1993) interpret the late calcite as having formed by meteoric water intrusion related to post-mineralization weathering events. The

similarity between the calcite age and the one derived from

paleomagnetic data from Fluorspar district mineralization

raises the question as to whether the latter is related to postmineralization incursion of meteoric water.

The results of this study (1) confirm a Permian age for

the Grant intrusive breccia at Hicks dome, (2) indicate

emplacement of the breccia at temperatures greater than

550°C, and (3) reveal the absence of later reheating above

300°C after rapid initial cooling. These results, complemented by additional information from the literature,

closely link in time the mineralization in the Fluorspar

district with igneous processes, including those identified

from Hicks dome. 

Acknowledgments.—We thank our colleagues, J. Eidel,

D. Kolata, and J. Baxter, of the Illinois State Geological Survey for their guidance and invaluable input during the course

of this and earlier studies. We are also deeply grateful to T.

Hayes for playing a major part in conceiving and carrying out

the study. Detailed technical reviews by S. Harlan and C.

Taylor are gratefully acknowledged; comments on the manuscript by D. Kolata, D. Harris, and J. Rupp, as well as discussions with D.T.A. Symons, are also appreciated. We thank

W. Rivers for the paleomagnetic measurements. Editing by

R. Scott greatly improved the manuscript and is deeply

appreciated. This study was supported by the Evolution of

Sedimentary Basins Program of the U.S. Geological Survey.

Figure 11. Apparent pole wander path for the North American craton from Symons (1994). Pole positions: Gu, Gc, Grant breccia—uncorrected and corrected, respectively—for structural attitude of nearby

Paleozoic strata (see text); DBpo, Downeys Bluff sill location, pyrrhotite component from baked limestone; DBa, Downeys Bluff sill, all samples. Bu, pole from magnetite in fluorspar ore (Symons, 1994);

PEMF, present Earth magnetic field. Ellipses of 95 percent confidence surround the Gc and DBpo poles.

M, Mississippian; , Pennsylvanian; P, Permian; , Triassic; eJ, Early Jurassic; mJ, Middle Jurassic; lJ,

Late Jurassic; K, Cretaceous; T, Tertiary.
PALEOMAGNETIC, 15 40Ar/39Ar RESULTS FROM THE GRANT INTRUSIVE BRECCIA, S. ILLINOIS BASIN

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Published in the Central Region, Denver, Colorado

Manuscript approved for publication May 5, 1997

Edited by Richard W. Scott, Jr.

Graphics prepared by Carol A. Quesenberry; use made of authordrafted material on figures 1, 2, 4, 5, 7–9, and 11

Photocomposition by Carol A. Quesenberry