Download 24_1991_Cox et al_AGSD_Proterozoic argillites

are-earth element chemistry of Early Proterozoic argillites,
ntral Arizona: Constraints on stratigraphy
NADH COX Department of Geology, Stanford University, Stanford, CA 94305
RL E. KARLSTROM Department of Geology, Northern Arizona University, Flagstqf AZ 86011
BERT L. CULLERS Department of Geology, Kansas State University, Manhattan, KS 66506
docking events. kwever, Hthologic similarities and geo-
fnom argillites from several seconstrain proposed correlations.
k g near Prescott, idemtified with
$&pergroup by Anderson
others
rY similar to the Tern* Gulch
Was originally P ~ O W Bb.y~ Both differ from pietitic ~ c h b f
which have each beem correlated
Yavapai fipergrou~ and the l k m
vn. REE data from the
ctly different from the "%@@
rocks and do not suppIgrt the
'between these two sequences
Silver and Conway (1989). REE
Iso differ among the Mazatzal
KBflsmm
Bowring, this volume).
This paper examines the rare-earthelement (REE) chemical s
m of a a l~i k s from several sepuam in c m a
Arizona whme skatipphic interrelationships remain con.
troversial. Figures 1and 2 summarize stratigraphicrelationships amang the Proterozoic rocks of central. Arizona
~(ICarlstrom and Bowring, in press), and show the locations
analysis. w e samples fotm
0f m @ h sampled for
pa~~d
a t b a setwlkctedto evaluate the effects of recycling
on @
m
jt
Western United States
{cab
Cox and o&ers, 1991)
ms.
This is in accord with a
ronologic and sedimento1ogic
suggests that sedimentary basins
erozoic in central Arizona were
distinct and formed a t different
Complex d~framr*
Iimited U-Db. dataland an
absence of di&iaim marker units haw @ontributedto
numerous controo&& rega~dh~g
comlaEim uf Proterozoic
sequences in central A&x%na. This m t b n outl~esthe
stratigraphy of several supmcmstal aequeaces in central
Arizona and reviews some of the debatestes
.
The Yavapal Supergroup and
Texas Gulch Formation
s of the Transition Zone of central
be divided into tectonic blocks with
md smctucal hiitories (Karlstrom
unit have been made (Andamn anil $$my,
Anderson
and others, 1971), but s;tnrctYral w@e;Xiw and complex
press), which may represent discrete
volcanic liiofxies changesrender sigraphic assignments
K.E,, 1991, Proterozoio Geology and Ore Deposits of Arizona, Arizona Oeologbal Soojety'Di@st 10, p. 67-68,
EARLY PROTEROZOIC ARG-
MAZATZAL PROVINCE
60
COX AND OTHERS
ambiguous (Conway and others, 1987; Anderson, 1989). A
minimum age for parts of the Big Bug Group of the Yavapai
Supergroup is given by the 1.75 Ga Brady Butte Granodiorite (Anderson and others, 1971). Direct ages on volcanogenic rocks range from 1.76 to 1.74 Ga (Karlstrom and
others, 1987; Karlstrom and Bowring, this volume).
The Texas Gulch Formation unconformably overlies the
Yavapai Supergroup (Blacet, 1966) and contains sandy and
tuffaceous felsic detritus and purple to grey argillite. It rests
unconformably on volcanic rocks and on the 1.75 Ga Brady
Butte Granodiorite and contains 1.72 Ga detrital zircons
(Karlstrom and Bowring, this volume). There has been
some confusion in assigning siliciclastic sequences to either
the Yavapai Supergroup or the Texas Gulch Formation, as
discussed below.
The following sequences were sampled:
1) pelitic rocks from the Middleton Creek area near the
Crazy Basin Quartz Monzonite, which have been correlated
with both the Texas Gulch Formation (Karlstrom and
others, 1990) and with the Yavapai Supergroup (Anderson
and others, 1971);
2) the Grapevine Gulch Formation of the Ash Creek
Group of the Yavapai Supergroup near Jerome (Anderson
and others, 1971), which is intruded by the 1.74 Ga Cheny
batholith;
3) a section near Prescott which has been correlated with
both the Texas Gulch Formation (Krieger, 1965; Bergh and
Karlstrom, in press) and with the Yavapai Supergroup (Anderson and others, 1971);
4) the Texas Gulch Formation near Brady Butte, which
is younger than 1.72 Ga (Karlstrom and Bowring, this
volume).
The Alder Group
The Alder Group is a sequence of shale, greywacke, and
lithic and quartz arenite with interbedded intermediate and
felsic flows and pyroclastic rocks (Fig. 2; Conway, 1976;
Sherlock and Karlstrom, this volume; Wessels and Karlstrom, this volume). The sequence crops out extensively in
the Tonto Basin and Mazatzal Mountains areas and is assigned to the Tonto Basin Supergroup, which also includes
the Red Rock Group and the Mazatzal Group (Fig. 2).
Chemical and petrographic evidence suggests that the highsilica rhyolites of the Tonto Basin Supergroup are of
continental derivation, and the sedimentary units have been
interpreted as having been deposited in a continental margin
setting (Conway and Silver, 1989; Condie and others, in
press). An age of about 1.71 Ga from a volcanic unit in the
middle to upper Alder Group was reported by Ludwig
(1974). The Alder Group rocks are believed to stratigraphically overlie wackes and basalts of the East Verde River
Formation, rocks of the Payson ophiolite (Dann, 1990), and
a basement containing 1.75 Ga granitoids (Conway and
others, 1987; Karlstrom and others, 1987, 1990).
Rocks of the Alder Group are confined to the area southwest of the Moore Gulch shear zone (Fig. 1). Rocks of the
Yavapai Supergroup and the Texas Gulch Formation crop
out to the northwest. The Texas Gulch Formation is lithologically similar to parts of the Alder Group, especially in
the Mazatzal Mountains (Conway and Silver, 1989). Purple
slates and felsic tuffs of the lower Alder Group have been
correlated with the Texas Gulch Formation (Wilson. 1939;
Anderson and Creasy. 1958; Karlstrom and others.. 1987).
implying that the Texas Gulch Formation and the Alder
Group could be part of a widespread supracrustal sequence
(Conway and Karlstrom, 1986; Conway and Silver, 1989).
This would suggest that there was a depositional surface
stretching across central Arizona by about 1.71 Ga
Samples were collected from the Breadpan and Houdon
Formations of the Alder Group in the Mazatzal Mountains
and from the Houdon Formation in the Sierra Ancha (Fig. 1;
Table 1).
The Mazatzal Group, Hess Canyon Group,
and quartzites of Chino Valley
Several kilometer-thick, quartz-rich siliciclastic sequences
with associated rhyolites occur in central Arizona. The
quartzites and conglomerates of Chino Valley bear a strong
resemblance to the rocks of the Mazatzal Group, and the
lithologic successions in the Mazatzal Group and the Hess
Canyon Group are very similar. The sedimentary facies of
the three units, which include alluvial fan and braided fluvial
deposits in the Chino Valley area, mixed fluvial and shallow
subaqueous deposits in the Mazatzal Group, and shallow
subaqueous deposits in the Hess Canyon Group, have been
interpreted as representing a platform succession deposited
on a south-dipping paleoslope with an inferred east-west
shoreline in the Mazatzal Mountains area (Trevena, 1979;
Conway and Silver, 1989).
Rocks of the Mazatzal Group core the Mazatzal Mountains in central Arizona. The sequence is about 3 km thick,
and composed of two sandstone sequences, the Deadman
Quartzite and the Mazatzal Peak Quartzite, separated by the
Maverick Shale (Wilson, 1939; Trevena, 1979). The
uppermost unit which has been preserved is the Hopi Spring
Shale (Doe and Karlstrom, this volume). The sandstones are
generally red, although the upper part of the Mazatzal Peak
Quartzite is white. These strata were deposited on rhyolitic
volcanics and volcaniclastics of the Red Rock Group
(Karlstrom and others., 1987; Conway and Silver, 1989;
Fig. 1). and the basal conglomerates of the Mazatzal Group
are locally interbedded with 1.70 Ga rhyolitic ash flow tuffs
(Silver and others, 1986). Similar quartzites, correlated with
the Mazatzal Group, are present in the Tonto Basin, near
Young (Conway, 1976; Conway and Silver, 1989; Sherlock
and Karlstrom, this volume).
A 1.5 km thick section of quartzite, conglomerate and
argillite crops out in the Chino Valley area of central Arizona (Krieger, 1%5; Fig. 1). These rocks resemble those aF
the Mazatzal Group, with which they have been correlated
by previous workers (Wilson, 1939; Krieger, 1%5; Trevena,
1979; Karlstrom and others, 1987; Conway and Silver,
62
COX AND OTHERS
1989). The sequence appears to represent the deposits of
alluvial fans and braided rivers (Trevena, 1979; Bayne, 1987)
produced in response to unroofing of adjacent basement
blocks cored by the Yavapai Supergroup (Middleton, 1985).
Distinct populations of detrital zircons from the upper
conglomerate give ages of >1.7 Ga, 1.7 Ga and 1.65 Ga
respectively (Chamberlain and others, 1991). The maximum age for the deposition of the unit is therefore considered to be 1.65 Ga. This age implies that the quartzites of
Chino Valley do not correlate with the Mazatzal Group of
the Mazatzal Mountains, the base of which is 1.70 Ga in
age (Silver and others, 1986).
The Hess Canyon Group (Figs. 1, 2) consists of the
White Ledges Formation, the Yankee Joe Formation and the
Blackjack Formation. The sequence has a sandstoneshalesandstone stratigraphy similar to that of the Mazatzal Group.
Correlation between the Hess Canyon Group and the Mazatzal Group was proposed by Livingston (1969), Trevena
(1979) and Anderson and Wirth (1981). The group reaches a
thickness of 1.6 km in the Hess Canyon area, which is a
minimum thickness for the sequence because the top is
missing. The Hess Canyon Group was deposited on felsic
rocks of the Redmond Formation (Trevena, 1979; Conway
and Silver, 1989),which have recently been dated at 1.66 Ga
(Karlstrom and Bowring, this volume). This suggests that
the Hess Canyon Group is not correlative with the Mazatzal
Group, whose base is 1.70 Ga. It also implies that the
speculative correlation between the Hess Canyon Group and
the Houdon Formation of the Alder Group (Conway and
Silver, 1989) is unlikely, as the upper parts of the Alder
Group have been dated at 1.71 Ga (Ludwig, 1974).
THE REE AS A PROVENANCE TOOL
The rare-earth elements (REE) have several features that
make them useful for sediment provenance and correlation
applications. They are very insoluble in aqueous fluids, so
that their abundance in sediments reflects their abundance in
the source rocks (Taylor and McLennan, 1985). Once
released by breakdown of their host minerals in the parent
rock, the bulk of the REE are immediately re-precipitated,
either as compounds, by adsorption on mineral surfaces, or
in exchangeable cation sites in clays (Roaldset, 1978; Fleet,
1984; McLennan, 1989). Field studies of the behavior of
the REE during weathering have shown that although they
are mobilized by the action of acid weathering solutions they
are usually re-precipitated within the weathering profile as
the the pH of the weathering solutions increases during
hydrolysis of feldspars and other minerals (Nesbitt, 1979).
Some REE will be removed from the system in solution,
and some fractionation will occur during this process; chemical differences between the rare earths result in a trend of
increasing solubility with increasing atomic number, so that
the heavier REE are fractionated into the weathering solution
or transport medium. However, this effect is small relative
to the trends produced by igneous fractionation, so that the
REE signature of a parent rock can only be slightly altered
by weathering. The REE signatures of rocks do not appear
to be affected by diagenesis (Fleet, 1984). It is still unclear
whether the REE are generally mobile during metamorphism. Whereas studies in many areas have shown them to
be immobile at a variety of metamorphic grades, in other
cases they appear to have been redistributed (Humphris,
19W Grauch. 1989).
Mudrocks are the mod useful sediment type for rareearth
provenance studies. Clay minerals are quantitatively enriched in rare earths (Cullers and others., 1975; Roaldset,
1978), so that the REE are concentrated in this size fraction
in sediments (Cullers and others, 1979, 1988). Secondly.
clay-sized sediment is usually carried in suspension in rivers
and therefore tends to remain in transport for extended
periods of time (Garrels and MacKenzie, 1971). The clays
become mixed and homogenized during transport, so that
mudrock compositionsusually represent an average of several sources to the basin (AllBgre and Rousseau, 1984).
METHODS
The rare-earth element analyses were made by insuumental activation at Kansas State University by a method
adapted from Gordon and others (1%8). The REE values for
all samples, normalized to chondrites, are shown in Table 1.
The ratios La/Lu and EuIEu* are calculated using the
chondrite-normalized values, and are shown in Table 2. The
normalization factors used are those of Wakita and others
(1971). The standard analytical error for each element is
shown in Table 3.
The ratio Eu,Eu* (Table 2) is a measure of the size
the Europium anomaly, where Eu is the measured value
the sample, normalized to chondrites, and Eu* is
nary value which Eu would have if it were no
relative to its neighboring elements. Eu* is calculated
assuming a straight line between Sm and Tb on a graph
relative abundance versus atomic number (eg. fig), and
applying the equation for a geometric progression:
Eu* = T b . x 2
where x is the geometric multiplier, expressed as:
x =
'Jm
RESULTS
The Yavapai Supergroup and
Texas Gulch Formation
Argillites from the supracrustal sequences in central
older than about 1.71 Ga are generally characte
negligible or very small Eu anomalies, with
generally greater than 0.75 (Table 2). There is v
among these rocks however, and argillites from the
a~eahave lower Emu* values (average Eu/Eu* = 0
do the rocks from the Middleton Creek locality
EufEu* = 0.89). LaLu values also vary as a func
1
roup, Mazatzal Mountains
0'76
Group. Tonto Basin
location. Samples from the Middleton Creek area have the
steepest patterns, with an average La/Lu value of 13.1,
whereas the Prescott rocks have an average La/Lu of only
5.3. The sin@ sample of the Grapevine Gulch Formation
of the Yavapai Supergroup is distinct from the Middleton
Creek and the Prescott samples in having both a very welldeveloped negative Eu anomaly (EuIEu* = 0.66) and a very
flat REE pattem (Lab = 2.3) (Fig. 3).
The sample from the Texas Gulch Formation at Mule
Canyon is similar to the Rcseott argillites, with a welldeveloped Eu anomaly (EuBu* = 0.64) and a flat REE
distribution pattern (La/Lu = 4.8). This lends tentative
support to the correlation of the Prescott rocks with the
Texas Gulch Formation (Krieger, 1965; Bergh and
Karlstrom, in press) rather than the Yavapai Supergroup
(Anderson and others, 1971). However, data from a single
sample are clearly insufficient to constrain the association.
The Middleton Creek samples, with their steep REE
patterns and large EuEu* values (Table 2) form a homoge
neous array, and are quite different from the Ash Creek,
Prescott and Texas Gulch samples (Fig 3). This limited dataset does not support the proposed correlation of the MiddleMlddleton Creek $rglllltes
-
Prescottarglllltes
Gra~evlneGulch Fmn.
Texas Gulch Fmn.
zites at Chino Valley
La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
mation and for samples from the Middleton
Creek and Prescott sections. Individual analyses
are listed in Table 1.
1
COX AND OTHERS
ton Creek rocks with the Texas Gulch Formation (Karlstrom
and others, 1990). and points towards a connection with the
Yavapai Supergroup(Anderson and others, 1971).
The Alder Group
The Alder Group argillites in general have moderate Eu
anomalies, with Eu/Eu* generally less than 0.75. They are
light rare-earth enriched, with an average La/Lu of about 8
(Table 2). There appear to be regional differences within the
Alder Group (Fig. 4). The Houdon Formation rocks of the
Tonto Basin have larger Eu anomalies in general (average
EWu* = .66) than do argillites of the Breadpan and Houdon
Formations in the Mazatzal Mountains (average Eu/Eu* =
.72), and their REE patterns tend lo be less steep, with
La/Lu averaging 7.1 as opposed to 8.8 (Table 2).
Condie and others (in press) have recently compiled
chemical data from pelites of the Alder Group in the Tonto
Basin. Their data, normalized to Wakita and others (1971)
produce an average LaLu value for five samples from the
Houdon Formation of 6.5, and for twenty-nine samples from
the Breadpan Formation of 7.2. Their values for Eu/Eu* for
the samples from these units, recalculated for direct comparison with the data presented here, are 0.62 and 0.70 respectively. These data are in broad agreement with the data from
our analyses.
The Mazatzal Group, Hess Canyon Group
and quartzites of Chino Valley
The REE distribution patterns for the argillites of these
groups are dominated by well-developed Eu anomalies (Fig.
5). The average Eu/Eu* values for the three sequences range
from 0.57 to 0.63, and are statistically indistinguishable.
However, further examination of the data reveals differences
between the threesequences (Table 2). The La1.u ratios for
the Hess Canyon Group range from 2.9 to 7.8. The range
of values for the Mazatzal Group is 5.5 to 9.6; and argillites
from the sequence at Chino Valley have values between 9.8
and 11.1. The average of three Maverick Shale samples
from the Mazatzal Group of Condie and others (in press) has
a value for La/Lu of 7.2, and for Eu/Eu* of 0.64. Both
values represent the data of Condie and others (in press)
normalized to Wakita and others (1971), for direct comparison with our data set, with which they are in agreement.
There are very strong differences between Hess Canyon
Group rocks (Fig. 5) and rocks of the Houdon Formation of
the Alder Group (Fig. 4). This, in combination with the
recently documented difference in their ages (Ludwig, 1974;
Karlstrom and Bowring, this volume), makes it unlikely
that these two sequences are related as has been tentatively
suggested (Conway and Silver, 1989).
.
DISCUSSION
We emphasize that these interpretations are provisional
as they are based on sample sets which are not statistically
significant. The dam suggest, however, that chemical variability may be a useful provenance tool in this area, and that
more extensive sampling might help unravel the complicated stratigraphicproblems of the Proterozoic in Arizona.
The chemical dam presented here indicate that there may
be distinct provenance differences between argillitic rocks of
the Yavapai Supergroup, Texas Gulch Formation and other
controversial sequences at Rescott and Middleton Creek
Alder Group,
Mazatzal Mountains
Alder Group.
TontoBasfn
Figure 4. Range of REE values for Alder
Group argillites from the Mazatzal Mountains
and Tonto Basin. The Tonto Basin samples are
distinguished by uniformly flat heavy REE distributions.
Individual analyses are listed in
Table 1.
Ouartzltes at Chino Valley
Figure 5. Range of REE values for the
zatzal Group and Hess Canyon Group arg
samples. Three analyses of argillites from
quartzites of Chino Valley are also shown. D
values are listed in Table 1.
EARLY PROTEROZOIC ARGILLITES
ch Formation and the Alder Group (Conway
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Allhgre. C.J. and Rousmu, D.. 1984. The growth of the continents through time studied by Nd isotopic analysis of
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