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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 ,1986; Anderson, 1989; Conway and Silver, Allhgre. C.J. and Rousmu, D.. 1984. The growth of the continents through time studied by Nd isotopic analysis of shales: Earth and Planetary Science Lettas, v. 67. p. 19- the Tonto Basin were deposited in unrelated ntologic grounds. That these sequences are Anderson. P.. 1989, Stratigraphic frameworkvolcanic-plutonic evolution, and vertical deformation of the Proterozoic volcanic belts of central Arizona. in Jenney. J.P.. and Reynolds. S.J., eds, Geologic evolution of Arizona: Precambrian conglomerates of the central Arizona arch: Bendix Field Engineering Corporation Open-File Report GJBX-33(81) for National Uranium Resource Evaluation, (average La/Lu = 10.6, 7.5 and 4.9, respec- ilarity uniu' which can caused by In addition, intense deformation was occurring interval related to separate, but possibly region- crustal shortening during Proterozoic orogeny in central Arizona: Geological Society of America Bulletin. 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