Geochemistry of West-central Chihuahua Obsidian Nodules and Implications for the Derivation of Obsidian Artefacts

Journal of Archaeological Science (1998) 25, 1023–1038 Article No. as980287

Geochemistry of West-central Chihuahua Obsidian Nodules and Implications for the Derivation of Obsidian Artefacts Philip W. Fralick Department of Geology, Lakehead University, Thunder Bay, ON, P7B 5E1, Canada

Joe D. Stewart Department of Anthropology, Lakehead University, Thunder Bay, ON, P7B 5E1, Canada

A. C. MacWilliams Department of Anthropology University of Arizona, Tucson, AZ 85721, U.S.A. (Received 24 September 1997, revised manuscript accepted 17 February 1998) Obsidian nodules (marekanites or ‘‘Apache tears’’) weathered out of volcanic units west of Chihuahua were utilized by the precontact native population for the manufacture of small stone implements. We subjected 77 specimens, including both unworked nodules and bipolar percussion debitage, collected in a 16,000 sq km area, to standard ICP–AES, XRF glass bead and C-H-N combustion analyses. The chemical compositions of the samples form four groups related to differing fractional crystallization histories which developed in four discrete magma chambers. K2O and Nb/Y versus SiO2 diagrams define the volcanics as high-potassium, calc-alkalic rhyolites, similar to a regionally extensive sequence of rhyolites related to Farallon Plate subduction, and erupted between 36 and 27 Ma. Harker and ratio diagrams indicate that amphibole crystallization was a major control on the chemistries of the northern, southern and western volcanic sequences. The geochemical signature of the central area volcanic rocks was mainly the result of plagioclase and K-feldspar crystallization in the magma chamber. Attributing variance in the data set to differing fractional crystallization histories allows elements and element ratios to be selected for grouping techniques which maximize the effectiveness of these techniques. In this study, Sr/Y versus Ba/Zr is the best choice, reflecting the crystallization histories of plagioclase, amphibole, K-feldspar and possibly sphene, apatite and zircon. Some 90% of samples were collected in the area which matches their geochemistry. In the southern area the ratio plot is capable of distinguishing between the geochemical signatures of individual sites, which indicates that most procurement was within approximately 1 km of where the nodule was worked. Of 10 anomalous, apparently exotic, samples, six were probably transported a minimum of 25 km by humans, and the means of transport of four is unclear. The northern area had no exotic fragments, though a small sample size (N=11) makes this finding only suggestive.  1998 Academic Press

Keywords: OBSIDIAN, ARTEFACTS, CHIHUAHUA, GEOCHEMISTRY, ICP–AES.

Introduction

A

rchaeological sites in west-central Chihuahua (Figure 1) frequently contain obsidian and pitchstone debitage, flakes and tools. These materials represent the working of small (rarely larger than 8 cm diameter) obsidian nodules (marekanites or ‘‘Apache tears’’) which weather out of volcanic units in the region. The geochemistry of worked material and unworked nodules was investigated to gain insight into the provenance of the utilized obsidian.

The samples analysed were obtained during the 1990 and 1991 field seasons of the Proyecto Arqueologico Chihuahua (PAC). The purpose of this ongoing project is to acquire some understanding of this region of Chihuahua and integrate the study area into regional prehistory. One of the major goals of PAC was to study procurement and distribution of artefact materials within west-central Chihuahua, particularly during the Late Prehistoric Period. Flaked stone, often recognized as a marker of exchange or mobility (Sheets et al., 1990), is one avenue of study towards this goal.

1023 0305–4403/98/101023+16 $30.00/0

 1998 Academic Press

1024 P. W. Fralick et al.

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216 155

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San Jose de Bavícora

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156 112

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40 km

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CHIHUAHUA Laguna Bustillos

112 111

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Figure 1. Physiography of the study region with site locations where obsidian was collected for analysis.

Geochemical studies of west Chihuahuan obsidian are also a first step towards identifying sources of volcanic glass in Chihuahua.

The Study Area Three large regions were selected for partial survey by the PAC (Figure 1). These areas encompass enclosed basins of the Chihuahuan basin and range province, and portions of two of Chihuahua’s major river drainages, the Rı´o Santa Marı´a and Rı´o Santa Clara. Although within the Chihuahua basin and range province, the project area is well inside the range of mid-Tertiary volcanics which extend at least 100 km east of the high Sierra Madre Occidental (McDowell & Clabaugh, 1984: 201). The northern region includes the Babı´cora (or Bavı´cora) Basin which intermittently holds a shallow lake. The several dozen prehistoric pueblo sites known in this area tend to occur in clusters around the lake basin (Carey, 1931; Kelley

et al., in press). The central region of the study area includes the middle zone of the Rı´o Santa Marı´a which flows north to an inland terminus. Chihuahua culture pueblo sites, post-dating c.  1200, occur on bajada, terraces over the Rı´o Santa Marı´a, and at the mouths of tributaries to this river where they leave north trending mountain ranges. The southernmost zone is centred around Laguna Bustillos. This shallow lake is in a closed basin at 1990 m ASL. Hunter-gatherer sites are by far most abundant in Bustillos Basin. Ceramic Period sites here consist of small agricultural hamlets composed of scattered jacales and pithouses. Archaeology in the Santa Marı´a and Babı´cora regions is dominated by surface adobe roomblocks related to Chihuahua culture (Casas Grandes culture, the Casas Grandes interaction sphere, etc.). Vestiges of Chihuahua culture do not occur southeast of the Rı´o Santa Marı´a with the exception of a few sherds (Kelley et al., in press).

Geochemistry of West-central Chihuahua Obsidian Nodules 1025

Excavations up to 1993 were conducted in nine sites, eight of which are open Ceramic Period sites. The ninth site is a Late Archaic–Ceramic Period rock-shelter near Laguna Bustillos. Thirty-nine accepted radiocarbon dates have two-sigma calibrated age ranges falling between c. cal- 800–1450, with some possibility of considerably earlier and slightly later end members (Kelley et al., in press). In the Bustillos region, excavations focused on small plainware sites with dates concentrated in the range of cal- 800–1200. In the Santa Marı´a and Babı´cora regions, excavations were concentrated on Medio Period roomblocks with assemblages clearly belonging to classic Chihuahua culture. The Babı´cora–Santa Marı´a obsidian samples for this study are exclusively from surface collections taken on Ceramic Period sites with the exception of one extensive workshop at a secondary source of unknown age (site Ch-190). The Bustillos samples are from both mixes Hunter–Gatherer–Ceramic Period sites, and Ceramic Period sites. Obsidian tools and debitage were collected from more than 40 PAC sites. Nodular obsidian is locally available almost ubiquitously in west-central Chihuahua. Although widely used, it is never a major component of flaked assemblages, which are typically dominated by rhyolite and basalt. The overall proportional share of obsidian is greater for projectile points than for debitage and shatter. Little preceding work addresses stone resource utilization in western Chihuahua. Bandelier (1890: 515–516) makes passing mention of obsidian nodules in adjacent northeast Sonora. Brand (1933) and Sayles (1936) collected obsidian artefacts during their extensive surveys in Chihuahua but did not have the means to attempt provenance studies. Brand (1933: 96) commented that, ‘‘the stones used were those common within the area’’ referring to western Chihuahua in general. Brand (1933: 100) also remarked that puebloan arrowheads are small and commonly made of obsidian. Gerald (1983) suggests that obsidian at Paquime´ is from ‘‘nearby’’ but does not elaborate. Volumes 7 and 8 of the Joint Casas Grandes Project (JCGP) include analysis and discussion of obsidian from Paquime´ (Di Peso, Rinaldo & Fenner, 1974). No provenance studies were done although some of the obsidian is attributed to non-local trade (e.g. Rinaldo in Di Peso, Rinaldo & Fenner, Vol. 7: 337). Shackley (1995) mentions ongoing research in Sonora and Chihuahua. Several geologists also mention volcanic glass in exposures in west central Chihuahua (e.g. Deux, 1983; Wark, 1991). From this point of departure, analysis of worked obsidian involves determining if local obsidian exists and, if so, identifying obsidian chemistry that reflects regional natural distribution. Where this is possible relationships between material supply and lithic technology can be well understood. In this instance Apache tears preclude macro-blade technology and require that bipolar reduction normally occurs. Regional

chemical tendencies, even if not tied to specific sources, will define some scale to movement of obsidian by people.

Felsic Volcanic Geochemistry To understand the possible compositional variability which may occur in obsidian fragments of an area, the geochemistry trends in the volcanic rocks of the area must be understood (see, also, Hughes & Smith, 1993). The geochemistry of igneous rocks may be controlled by a number of factors. These factors can be subdivided into those operating in the area where partial melting is generating the magma (i.e. source area composition, degree of partial melting, pressure and type and amount of volatiles present) and factors affecting the magma as it moves upwards (i.e. assimilation of country rock, crystal settling, development of an immiscible liquid phase, magma mixing and volatile partitioning). The magma may be tapped at various times forming volcanic units on the surface, the geochemistry of which will reflect the evolutionary stage of the magma at depth. For a more complete discussion of these processes, see Hildreth (1981) or igneous petrology reference books, such as Best (1982) or Wilson (1989). The northern Sierra Madre Occidental is possibly the largest rhyolite field in the world (McDowell & Clabaugh, 1979). Volcanic glass suitable for prehistoric tool-making (pitchstone and obsidian) is widely and recurrently associated with these silicic extrusives. McDowell & Keizer (1977) and McDowell & Clabaugh (1984) divide the rocks into two major units, the upper of which is of concern here. Upper Volcanic series units are composed of an upward sequence from intermediate (andesitic) volcanism to rhyolite, and finally, basalt. Wark (1991) reports that rhyolites were extruded after crystal fractionation, with little to no crustal assimilation, and were erupted from shallow depths. He concludes that this observation, made in reference to the Tomochic Caldera (Figure 1), is probably valid more widely. Rhyolitic volcanism in the northern Sierra Madre Occidental and adjacent basin and range province of northwestern Mexico mostly occurred between 36 and 27 Ma (McDowell & Keizer, 1977; Bagby, 1979; McDowell & Clabaugh, 1979; Cameron et al., 1980; Keller, Bockoven & McDowell, 1982; Swanson & McDowell, 1984, 1985; Albrecht et al., 1990; McDowell & Mauger, 1994). It is preceded, especially in the west, by calc-alkalic, andesitic volcanism (Wark, Kempter & McDowell, 1990), and is overlain by 29 to 17 Ma alkalic basalts (Keller, Bockoven & McDowell, 1982; Gunderson, Cameron & Cameron, 1986) related to crustal extension (Gunderson, Cameron & Cameron, 1986). Subduction of the Farallon Plate under northwestern Mexico generated the andesitic melts which differentiated, via plagioclase dominated

1026 P. W. Fralick et al.

crystal fractionation, into calc-alkalic, moderate- and high-K rhyolites (Cameron, Bagby & Cameron, 1980; Cameron & Hanson, 1982; Wark, Kempter & McDowell, 1990). Ferroaugite rhyolites are associated with the calc-alkalic rhyolites in the basin and range province, though the two silicious suites may not be genetically related (Gunderson, Cameron & Cameron, 1986). The obsidian nodules studied have three possible sources: (1) calc-alkalic, moderate-K rhyolite; (2) calc-alkalic, high-K rhyolite; (3) alkalic, ferroaugite rhyolite. Each of these rock types, and intermediate units, have characteristic geochemistries.

Samples Approximately 150 sites were recorded by PAC members. Most of these sites contain some obsidian or pitchstone. PAC members attempted to locate primary obsidian sources by talking to local residents, checking arroyo gravels, and visiting locations on the geologic maps where ‘‘vidrio’’ (glass) was indicated. Only a few outcrops of poor quality glassy material were found in west central Chihuahua, and there is no surficial evidence that the outcrops were quarried or mined. The 77 obsidian specimens analysed here represent some 500 pieces of obsidian collected by PAC from archaeological sites and nearby drainages, which bring the material down from unknown upland source terrains. The collections include both unaltered, complete nodules (marekanites or ‘‘Apache tears’’) and artefacts (tools, split nodules and debitage) representing a bipolar pebble technology. Obsidian tools were not used in this destructive analysis. The samples were selected to represent the major physiographic and archaeological zones within the study area (except the Santa Clara valley in which the PAC had not yet worked when the obsidian study began). Table 1 lists the samples by sample number for purposes of this paper (e.g. A1, where A is site Ch-011 and 1 is obsidian sample 1 from that site) and gives the corresponding PAC catalogue numbers, site numbers and brief technological descriptors. Site locations appear in Figure 1.

Methods Table 2 gives the oxide data (%) for major elements and element data (ppm) for minor and trace elements in the samples. All elements except Si and Rb were determined by ICP–AES (inductively coupled plasma– atomic emission spectroscopy) using a Jarrell-Ash 9000 ICP multi-channel analyser. Si was determined by XRF (X-ray fluorescence), using a Philips 4280 Analyzer, and Rb by AAS (atomic absorption spectroscopy), using a Perkin Elmer 2380. Analytical error for SiO2 by AAS was within 7%. CO2 and H2O contents were determined to within 1% from C and H measurements by a CEC 240-XAC-H-N high temperature combustion analyser. CO2 ranged 0% to

0·48% and H2O ranged 0% to 0·81%, values which do not significantly affect the major element percentages. The data were obtained in two separate runs. Table 3 gives values for certified standard reference materials run with the samples. In both ICP runs, a rock (syenite) reference material, SY-2, was run (expected values from Faye, 1978: 19); obsidian standard NBS278 was included in the first run (expected values from Trahey, 1995: 83) but was not available for the second. Values obtained from blanks in each of the two ICP runs were subtracted from raw data for samples to obtain the corrected values in Tables 2 and 3.

Results Table 2 shows the concentrations of Ca, Na, Ti, Mn, Ba, Sr, Zr, Y, Zn and V in the samples to be highly variable. To a large extent this variation is systematic with distinct patterns emerging when the data are plotted against north–south distribution of the sample sites (see Figure 2 for examples). Four groups of data points are present (see Figure 5(a) for sites in each group). Three of the groups are well defined, corresponding to a northern (Babı´cora) area, a central (Santa Marı´a drainage) area, and a southern (Laguna Bustillos) area. The fourth, western, group is more difficult to distinguish, but does vary independently of the other groups. Samples from the Bustillos area are high in Ca and Ba, and low in Na, Ti, Mn, Zr and Y; the Santa Marı´a group is high in Na, Zr, Y and Zn, and low in Ca, Ba and Sr; the Babı´cora group has moderate concentrations of all 10 elements; and the tentative western group is high in Na and Sr, and low in Ca, Ba, Zr, Y and Zn. It is evident that Ca, Ba and Sr are covarying and are inversely related to the concentrations of Na, Ti, Mn, Zr, Y and Zn. In a felsic melt the former elements will partition into feldspars, whereas the latter elements will substitute into amphiboles and form minor mineral phases. Thus, the geochemically defined groups may reflect differences in fractional crystallization histories among the four areas. Melts which previously crystallized feldspar will have the evolved erupted liquid phase depleted in Ca, Ba and Sr. Whereas, melts which crystallized amphibolezircon, sphene and other minor phases will have a liquid phase depleted in the second element grouping. As previously stated the obsidian may be lowpotassium calc-alkalic, high-potassium calc-alkalic or ferroaugite rhyolite. K2O/SiO2 ratios indicate that all samples (for which SiO2 values exist) are highpotassium and Nb/Y versus SiO2 clearly demonstrates that all samples (for which both Nb and SiO2 values exist) are calc-alkalic (Figure 3). Although SiO2 and Nb data are missing for some samples, the tight clusterings in Figure 3 strongly suggest that samples come from the same suite of volcanics and, therefore, geochemical differences are not related to gross

Table 1. Obsidian samples from west central Chihuahua Site number Ch-011 Ch-011 Ch-011 Ch-011 Ch-011 Ch-011 Ch-011 Ch-102 Ch-102 Ch-102 Ch-102 Ch-102 Ch-102 Ch-102 Ch-102 Ch-102 Ch-102 Ch-104 Ch-111 Ch-111 Ch-111 Ch-112 Ch-114 Ch-114 Ch-114 Ch-114 Ch-114 Ch-114 Ch-114 Ch-114 Ch-114 Ch-151 Ch-151 Ch-151 Ch-151 Ch-151 Ch-152 Ch-152 Ch-152 Ch-155 Ch-155 Ch-155 Ch-155 Ch-155 Ch-155 Ch-155 Ch-156 Ch-156 Ch-156 Ch-156 Ch-156 Ch-156 Ch-156 Ch-156 Ch-156 Ch-156 Ch-159 Ch-159 Ch-159 Ch-159 Ch-190 Ch-190 Ch-190 Ch-190 Ch-190 Ch-190 Ch-204 Ch-216 Ch-221 Ch-221 Ch-221 Ch-221 Ch-221 Ch-221 Ch-221 Ch-221 Ch-221

Catalogue number

Sample number

Specimen descriptor

1173-02 1216-02 1202-01 1208-07 1208-08 1213-02 2221-08 1032-11 1032-12 1032-17 1032-18 1032-19 1032-13 1032-14 1032-15 1032-16 1030-20 1126-01 1105-10 1105-09 1108-02 1136-01 1162-24 1162-25 1162-26 1162-27 1162-28 1162-29 1162-30 1162-31 1162-32 1234-05 2066-03 2111-10 1234-06 2067-05 1229-10 1230-06 1231-18 1239-07 1239-08 1239-09 1239-06 1239-10 1239-11 1239-12 3000-15 3171-12 3174-03 3174-04 1227-02 3171-15 3171-16 3172-01 3175-01 3175-02 1249-01 1252-06 3162-08 3162-09 2060-01 2060-02 2060-03 2060-04 2060-05 2060-06 2191-06 2229-15 2272-10 2272-13 2272-14 2272-15 2272-17 2272-18 2272-21 2272-22 2272-25

A1 A2 A3 A4 A5 A6 A7 B1 B2 B3 B4 B5 B6 B7 B8 B9 B10 C1 D1 D2 D3 E1 F1 F2 F3 F4 F5 F6 F7 F8 F9 G1 G2 G3 G4 G5 H1 H2 H3 I1 I2 I3 I4 I5 I6 I7 J1 J2 J3 J4 J5 J6 J7 J8 J9 J10 K1 K2 K3 K4 L1 L2 L3 L4 L5 L6 M1 N1 O1 O2 O3 O4 O5 O6 O7 O8 O9

Unworked nodule Cortical flake Core (split nodule) Cortical flake, bifacially retouched Core (nodule fragment) Cortical flake Cortical flake Core (split nodule) Core (split nodule) Core (nodule fragment) Core (nodule fragment) Core (split nodule) Cortical flake Cortical flake Cortical flake Core (nodule fragment) Core (weathered surface, apparently not nodule cortex) Core (nodule fragment) Unworked nodule Flake (weathered, but apparently not nodule cortex) Cortical flake Non-cortical flake Core (nodule fragment) Core (nodule fragment) Cortical flake Cortical flake Cortical flake Flake (no observation as to cortex) Cortical flake Non-cortical flake Non-cortical flake Core (split nodule) Core (nodule fragment) Unworked nodule Cortical flake Cortical flake (retouched/utilized) Unworked nodule Split nodule (retouched edge?) Cortical flake (edge damage) Unworked nodule Core (nodule fragment) Cortical flake Cortical flake Non-cortical shatter Cortical shatter Cortical flake (note green banding) Unworked nodule Core (split nodule) Core (nodule fragment) Cortical shatter Non-cortical flake Non-cortical flake Non-cortical flake Flake (bipolar split nodule) Cortical flake Cortical shatter Core (bipolar) Unworked nodule Core (nodule fragment) Core (nodule fragment) Unworked nodule Unworked nodule Unworked nodule Unworked nodule Unworked nodule Unworked nodule Unworked nodule Core (split nodule) Core (split nodule) Core (split nodule) Core (split nodule) Core (split nodule) Core (split nodule) Core (split nodule) Core (nodule fragment) Core (nodule fragment) Cortical flake

SiO2 %

73·77 75·13 76·02 75·78 78·13 76·92 nd 71·76 75·80 71·34 77·58 75·88 nd 78·56 71·87 77·92 77·80 70·21 75·69 nd nd nd 78·99 76·13 nd nd nd 80·05 nd nd nd 76·53 74·30 73·60 nd nd nd 75·50 nd 73·71 73·80 71·24 75·59 nd nd nd 74·74 75·07 74·13 nd 71·91

Sample number

A1 A2 A3 A4 A5 A6 A7 B1* B2* B3* B4* B5* B6 B7 B8 B9 B10 C1* D1* D2 D3 E1 F1 F2 F3 F4 F5 F6 F7 F8 F9 G1* G2* G3* G4 G5 H1* H2* H3 I1* I2* I3* I4 I5 I6 I7 J1 J2* J3* J4* J5

12·38 12·49 12·05 12·33 12·51 12·45 13·81 12·15 12·46 12·80 13·25 12·58 9·98 12·64 11·76 10·81 12·48 12·79 12·50 12·18 12·30 12·41 12·52 11·99 12·43 12·12 12·72 12·71 12·71 12·79 11·61 12·69 13·24 14·01 13·20 12·13 13·00 12·50 12·90 13·00 14·25 12·42 12·98 12·26 12·18 12·50 12·82 13·24 13·16 13·45 12·87

Al2O3 %

0·42 0·40 0·38 0·37 0·38 1·23 0·43 0·84 0·84 0·40 1·29 0·88 0·65 0·39 0·80 0·75 0·83 1·19 0·83 1·04 1·05 0·78 0·83 0·80 0·80 0·77 0·79 0·82 0·83 0·84 0·12 0·39 1·17 1·26 1·20 1·19 0·41 0·37 0·38 0·40 0·47 0·42 0·41 1·19 0·43 0·21 0·37 0·43 0·41 0·67 0·39

CaO % 1·21 1·41 1·17 1·20 1·15 1·31 1·59 0·95 0·96 1·18 1·33 0·96 0·95 1·17 0·92 0·83 1·19 1·53 1·18 1·30 1·32 1·18 1·19 1·14 1·16 1·15 1·20 1·19 1·22 1·22 2·89 1·46 1·46 1·66 1·60 1·58 1·54 1·40 1·44 1·51 1·63 1·43 1·52 1·47 1·42 1·81 1·40 1·47 1·46 1·50 1·48

Fe2O3 % 4·22 4·38 4·29 4·47 4·44 4·04 4·79 4·12 4·31 4·54 4·29 4·38 3·63 4·46 4·18 3·82 4·49 4·39 4·53 4·12 4·25 4·75 4·46 4·45 4·50 4·64 5·40 4·55 4·54 4·57 4·76 4·70 4·31 4·66 4·22 4·40 4·77 4·56 4·70 4·75 5·08 4·40 4·70 4·01 4·26 4·79 4·74 4·73 4·83 4·76 4·72

K2O % 0·20 0·12 0·18 0·18 0·18 0·32 0·12 0·23 0·23 0·19 0·34 0·23 0·12 0·18 0·22 0·21 0·14 0·24 0·14 0·20 0·21 0·14 0·16 0·14 0·00 0·13 0·13 0·14 0·14 0·14 0·10 0·12 0·25 0·27 0·24 0·25 0·13 0·10 0·10 0·13 0·13 0·13 0·13 0·24 0·14 0·10 0·11 0·13 0·10 0·14 0·12

MgO %

Table 2. Geochemical data for west central Chihuahua obsidian

0·09 0·08 0·09 0·09 0·09 0·05 0·09 0·05 0·05 0·09 0·05 0·05 0·04 0·09 0·05 0·04 0·04 0·05 0·04 0·04 0·04 0·04 0·04 0·04 0·04 0·04 0·04 0·05 0·05 0·05 0·12 0·08 0·05 0·05 0·06 0·05 0·08 0·08 0·08 0·08 0·20 0·08 0·08 0·05 0·08 0·09 0·08 0·08 0·08 0·08 0·08

MnO % 4·16 4·31 4·20 4·25 4·23 3·30 4·77 3·48 3·59 4·40 3·59 3·69 2·88 4·35 3·48 3·07 3·76 3·71 3·77 3·63 3·61 3·74 3·75 3·54 3·86 3·63 3·51 3·88 3·85 3·99 5·39 4·66 3·76 3·94 3·41 3·78 4·76 4·23 4·41 4·75 4·91 4·23 4·77 3·30 4·42 4·92 4·25 4·61 4·56 4·68 4·67

Na2O % 0·02 0·02 0·02 0·02 0·01 0·04 0·02 0·11 0·02 0·01 0·03 0·02 0·02 0·02 0·02 0·02 0·02 0·04 0·02 0·03 0·03 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·01 0·02 0·04 0·04 0·03 0·04 0·02 0·01 0·02 0·02 0·02 0·02 0·02 0·04 0·02 0·01 0·01 0·03 0·02 0·03 0·02

P2O5 % 0·23 0·25 0·22 0·24 0·22 0·22 0·27 0·17 0·17 0·22 0·21 0·17 0·16 0·23 0·17 0·16 0·18 0·21 0·17 0·21 0·21 0·20 0·18 0·17 0·18 0·20 0·19 0·18 0·18 0·19 0·30 0·25 0·22 0·24 0·32 0·25 0·26 0·24 0·26 0·25 0·28 0·25 0·25 0·25 0·28 0·29 0·24 0·27 0·24 0·27 0·26

TiO2 % 0·18 0·18 0·36 0·27 0·36 0·45 0·28 0·27 0·18 0·27 0·18 0·90 0·18 0·18 0·27 0·09 0·45 0·18 0·18 0·54 0·27 0·72 0·36 0·27 0·45 0·63 0·27 0·27 0·18 0·18 0·54 0·18 0·27 0·27 0·27 0·09 0·18 0·18 0·18 0·18 0·27 0·27 0·27 0·27 0·72 0·45 0·54 0·09 0·54 0·27

0·26 0·08 0·33 0·40 0·00 0·15 0·07 0·00 0·29 0·29 0·48 0·14 0·26 0·00 0·37 0·00

H2O %

0·00 0·37 0·07 0·18 0·04 0·15 0·29 0·00 0·04 0·04 0·04 0·22 0·18 0·07 0·04 0·04 0·00 0·00 0·00 0·18 0·15 0·33 0·07 0·11 0·15 0·29 0·07 0·11 0·04 0·11 0·15 0·00 0·00 0·26

CO2 % 566 465 457 317 323 1031 808 1222 1241 325 1087 1252 812 332 1183 1062 1049 918 1053 1076 1113 1058 1040 989 1048 1031 1082 1087 1092 1068 6 510 1165 966 871 832 527 267 272 522 539 472 551 813 460 44 267 493 272 508 518

Ba ppm 73 243 156 290 182 314 810 93 85 99 97 137 278 250 183 143 168 93 62 412 344 504 240 186 bd 593 251 203 189 294 421 143 147 150 1642 436 178 161 516 74 101 223 169 622 750 334 217 308 134 275 198

Co ppm nd 11 nd nd nd nd 11 nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd 5 4 5 4 nd 11 11 nd 13 11 nd 5 11 13 11 12 11 12 nd

Nb ppm 153 164 146 146 153 188 173 150 157 153 195 157 bd 153 153 146 160 198 164 139 132 165 160 160 160 160 146 160 153 160 271 150 142 208 210 213 153 169 169 157 186 169 153 202 158 213 158 169 169 158 153

Rb ppm 9 9 8 3 2 156 11 109 109 2 166 115 64 4 102 95 96 139 97 119 123 97 95 91 bd 95 99 98 98 100 bd 9 128 141 129 110 10 3 3 9 11 10 11 121 11 274 3 8 3 13 10

Sr ppm 4 bd 3 3 3 9 1 6 6 3 8 6 5 3 7 6 5 8 5 8 8 6 6 6 5 6 5 5 6 6 5 3 7 6 bd 6 3 1 bd 2 1 1 3 5 bd 50 1 1 bd bd 3

V ppm 47 48 43 44 45 15 48 14 15 46 16 15 9 46 13 13 15 16 15 18 19 15 15 14 15 14 15 15 15 15 71 47 20 15 17 10 50 47 48 50 55 48 49 16 47 25 48 50 49 50 49

Y ppm 86 76 85 89 82 40 50 31 33 84 40 32 28 82 29 28 26 33 24 28 29 25 26 26 27 33 26 26 28 28 130 75 34 12 43 8 78 71 76 80 88 84 76 41 79 109 70 78 78 79 78

Zn ppm

207 202 210 200 190 139 228 111 125 199 144 116 89 196 111 105 122 155 119 117 119 117 114 119 111 132 132 117 120 135 592 233 119 156 136 155 233 185 196 230 234 200 231 135 198 32 187 231 216 233 230

Zr ppm

1028 P. W. Fralick et al.

74·06 nd nd 75·25 nd 73·78 74·36 74·47 77·43 74·38 74·83 75·05 74·58 75·87 74·22 76·82 73·79 74·10 77·56 75·41 81·43 76·52 76·84 76·54 76·10 nd

J6 J7 J8 J9 J10 K1* K2* K3* K4* L1* L2* L3* L4* L5* L6* M1* N1* O1* O2* O3* O4* O5* O6* O7* O8* O9*

13·60 13·59 13·01 13·58 13·32 12·50 12·24 13·07 13·50 12·92 13·20 13·02 13·17 13·04 13·93 12·62 13·56 13·28 13·04 13·11 12·17 13·13 13·12 13·19 13·29 12·87

Al2O3 %

0·46 0·45 0·43 0·91 0·78 0·74 0·67 0·69 0·75 0·69 0·72 0·72 0·70 0·58 0·58 0·82 0·64 0·84 0·86 0·83 0·34 0·85 0·86 0·86 1·05 0·86

CaO % 1·52 1·49 1·49 1·53 1·51 1·40 1·32 1·38 1·46 1·37 1·41 1·41 1·39 1·29 1·34 1·21 1·4 1·26 1·27 1·23 1·09 1·26 1·26 1·24 1·38 1·29

Fe2O3 % 4·66 4·81 4·62 4·87 4·75 4·83 4·78 4·92 5·30 4·90 4·93 4·79 4·92 4·95 5·13 4·68 5·19 4·90 4·62 4·78 4·53 4·69 4·72 4·69 4·44 4·62

K 2O % 0·14 0·10 0·12 0·13 0·14 0·19 0·17 0·17 0·19 0·17 0·18 0·18 0·17 0·13 0·12 0·15 0·15 0·15 0·15 0·15 0·05 0·15 0·15 0·15 0·23 0·17

MgO %

*Sample values are averages of two or more determinations. nd, not determined; bd, below detection limit.

SiO2 %

Sample number

Table 2. Continued

0·08 0·08 0·08 0·08 0·08 0·05 0·05 0·05 0·05 0·05 0·05 0·05 0·05 0·05 0·06 0·05 0·05 0·05 0·05 0·05 0·09 0·05 0·05 0·05 0·05 0·05

MnO % 4·64 4·59 4·54 4·80 4·65 3·77 3·66 3·84 3·91 3·65 3·82 3·83 3·88 3·93 4·08 3·48 4·14 3·71 3·74 3·70 4·09 3·80 3·76 3·79 3·77 3·80

Na2O % 0·03 0·02 0·02 0·03 0·03 0·04 0·03 0·03 0·03 0·03 0·03 0·03 0·03 0·02 0·02 0·02 0·03 0·02 0·02 0·02 0·01 0·02 0·02 0·02 0·04 0·03

P2O5 % 0·28 0·26 0·27 0·26 0·27 0·25 0·23 0·24 0·25 0·23 0·24 0·24 0·24 0·21 0·22 0·18 0·26 0·19 0·18 0·18 0·13 0·19 0·19 0·19 0·21 0·19

TiO2 % 0·44 0·33 0·18 0·15 0·55 0·00 0·00 0·07 0·18 0·11 0·18 0·11 0·04 0·07 0·07 0·13 0·22 0·22 0·37 0·04 0·18 0·33 0·26 0·22 0·29 0·37

CO2 % 0·36 0·45 0·36 0·18 0·45 0·36 0·00 0·81 0·18 0·36 0·36 0·18 0·27 0·81 0·81 0·14 0·18 0·27 0·45 0·27 0·36 0·18 0·45 0·54 0·45 0·45

H 2O % 533 281 496 511 512 801 800 820 861 827 870 863 840 581 593 1005 926 1048 1064 960 4 1064 1021 1049 983 1080

Ba ppm 642 729 471 93 351 104 84 119 135 17 21 51 18 17 29 122 66 147 70 74 149 159 208 120 119 261

Co ppm 12 11 11 12 11 nd nd 7 7 6 7 6 7 7 7 4 7 4 4 4 9 4 4 4 5 4

Nb ppm 160 154 169 158 158 151 154 191 191 175 197 180 175 180 191 166 175 169 180 169 275 169 169 169 147 164

Rb ppm 9 4 10 14 14 70 70 72 80 74 77 78 74 39 39 90 60 94 94 89 1 94 95 94 114 94

Sr ppm bd bd bd bd bd 10 8 7 7 6 7 7 6 4 5 4 6 5 4 5 1 4 5 5 6 6

V ppm

50 49 49 51 50 22 23 26 27 27 27 27 28 31 30 15 26 17 15 17 43 18 17 17 21 17

Y ppm

83 84 78 79 81 34 42 49 44 41 41 41 46 45 58 34 38 38 28 27 69 29 29 27 34 27

Zn ppm

229 213 224 237 235 163 163 162 174 153 155 156 159 158 172 108 199 116 117 113 120 118 118 117 111 114

Zr ppm

Geochemistry of West-central Chihuahua Obsidian Nodules 1029

1 1 1 1 na 1 1 1 1 2 2 2 2 2 2 1, 2 1, 2 na

NBS-278 NBS-278 NBS-278 average NBS-278 .. NBS-278 expected SY-2 SY-2 SY-2 SY-2 SY-2 SY-2 SY-2 SY-2 SY-2 SY-2 SY-2 average SY-2 .. SY-2 expected

70·11 nd na na 73·05 nd nd nd nd nd nd nd nd 60·50 nd nd nd 60·07

SiO2 %

12·73 13·00 12·87 0·19 14·15 11·47 11·60 11·98 11·93 11·08 12·27 12·35 12·04 11·67 12·00 11·84 0·39 12·15

Al2O3 % 0·97 0·98 0·98 0·01 0·98 7·96 7·88 7·94 8·03 7·67 7·94 8·31 8·05 8·10 8·36 8·02 0·20 8·03

CaO % 2·11 2·17 2·14 0·04 2·04 6·23 6·21 6·14 6·14 6·65 6·36 6·31 6·14 6·37 6·57 6·31 0·18 6·34

Fe2O3 %

na, not applicable; nd, not determined; ns, not specified. Bracketed [ ] values not certified.

Run no.

Standard

Table 3. Oxide/element values for two rock standards

3·78 3·85 3·82 0·05 4·16 4·21 4·27 4·35 4·32 4·19 4·54 4·46 4·32 4·17 4·17 4·30 0·13 4·52

K 2O % 0·23 0·24 0·24 0·01 [0·23] 2·57 2·61 2·68 2·66 2·37 2·71 2·81 2·73 2·67 2·74 2·66 0·12 2·66

MgO % 0·05 0·05 0·05 0·00 0·05 0·29 0·29 0·30 0·30 0·29 0·20 0·30 0·30 0·30 0·31 0·29 0·03 0·32

MnO % 4·38 4·49 4·44 0·08 4·84 4·54 4·63 4·43 4·35 3·87 4·41 4·48 4·44 4·21 4·39 4·38 0·21 4·37

Na2O % 0·04 0·04 0·04 0·00 0·04 0·42 0·42 0·46 0·44 0·44 0·49 0·49 0·49 0·45 0·46 0·46 0·03 0·44

P2O5 % 0·23 0·24 0·24 0·01 0·25 0·13 0·13 0·13 0·13 0·14 0·14 0·14 0·14 0·14 0·14 0·14 0·01 0·15

TiO2 % 809 832 821 16 [1140] 459 422 437 431 395 410 431 423 413 429 425 17 460

Ba ppm 4 4 4 0 ns 10 10 10 10 10 11 10 10 11 11 10 0 11

Co ppm nd nd nd nd ns nd nd nd nd 21 22 23 23 22 23 22 1 23

Nb ppm

200 13 220

105 105 105 0 128 188 188 183 190 202 213 212 213 213

Rb ppm

50 52 51 1 64 256 258 257 253 233 253 261 251 248 257 253 8 275

Sr ppm

8 8 8 0 ns 51 51 51 51 44 49 43 42 42 43 47 4 32

V ppm

28 29 29 1 ns 110 111 113 112 104 124 118 117 116 120 115 6 130

Y ppm

40 50 45 7 ns 241 235 243 245 240 235 245 235 239 250 241 5 250

Zn ppm

227 240 234 9 ns 266 280 274 263 228 253 273 272 250 261 262 15 280

Zr ppm

1030 P. W. Fralick et al.

Geochemistry of West-central Chihuahua Obsidian Nodules 1031 km

S 0

25

50

75

100

125

N

S

150

0

Zr ppm

CaO%

1.2 0.8 0.4

Y ppm

100 60

75

N 100

125

150

220 200 180 160 140 120 100

50 30

20

10

1200

0.28

800 400

0.24 0.20 190 159

216 055

151

Sites

011 2 15 156

S

20414 1 1 22 102

N

11211 1

190 159

216 055

151

011 2 15 156

20414 1 1 22 102

11211 1

S

50

70

TiO2 %

Ba ppm

Sr ppm

140

km 25

N

Sites

Figure 2. Concentrations of selected major and trace elements plotted against north–south position of collection sites. Samples from collection sites in the southern (Laguna Bustillos), central (Santa Marı´a drainage) and northern (Bavı´cora, or Babı´cora, Basin) areas form three distinct groupings. Four of the five samples from site 151 also form a discrete, western group.

5

(a)

(b) Ferroaugite rhyolite Gunderson et al. (1986)

High-K 1.5

3 Medium-K

Nb/Y

K2O (wt%)

4

1

Calc-alkalic rhyolite Gunderson et al. (1986)

Transitional rhyolite Gunderson et al. (1986)

2

0.5

Calc-alkalic rhyolite Cameron et al. (1980)

1 Low-K This study 0 70

75 80 SiO2 (wt%)

0 71

73

75 SiO2 (wt%)

77

Figure 3. (a) The samples are all high-K rhyolites. (b) Comparison with analyses of other rhyolites in northwestern Mexico indicates that all samples are from the regional calc-alkalic suite.

differences in the source area of the melts. Sample F9 is the only noticeable exception to this. Major and trace elements indicate that this sample is mildly peralkaline.

Harker and trace element ratio diagrams provide information on whether differences in crystallization histories can account for the variable geochemistry

1032 P. W. Fralick et al. 68

(a)

(c)

Y (ppm)

CaO (wt%)

1.2

0.8

48

28

0.4

0 72

74

76 SiO2 (wt%)

78

80

3

8 72

78

74

76 Ba/Sr

78

80

(d)

2.4

12

Zr/Y

FeO* (wt%)

76 SiO2 (wt%)

16 (b)

1.6

0.8

0 72

74

8

4

74

76 SiO2 (wt%)

78

80

0 72

80

Figure 4. Harker and ratio diagrams. (a) The central area samples exhibit an inverse relationship between CaO and SiO2. This indicates that Ca was removed from the melt as SiO2 increased through time. It is likely that the depletion was caused by plagioclase formation and settling of the crystals. Trends in the other three areas are less well defined. (b) Fe contents are similar in all four melts and decrease with time, probably the result of Fe–Ti oxide phases crystallizing and settling to the bottom of the magma chamber. (c) Y depletion, via incorporation in amphibolephosphate phases, effects both central and southern area rhyolites. Prior to the southern melt reaching 77% SiO2, the slope of the decrease is greater than in the central area. After 77% SiO2 Y depletion of the southern melt lessens. (d) Ratio plots indicate that the crystallization histories of the northern, southern and western melts are similar, with amphibolephosphates, and Zr forming important crystal phases. The felsic portion of the central melt’s evolution is dominated by feldspar crystallization, causing the variable Ba/Sr ratio. , northern area; , central area;  southern area; *, western area.

(Figure 4). Figure 4(d) demonstrates that the magmas in the north, south and west followed similar differentiation trends with respect to crystallizing phases bearing Ba, Sr, Zr and Y. The high Ca, Sr and Ba values in these melts indicate that plagioclase, the main mineral phase bearing these elements, did not form as a significant crystallizing component as in the central area melt which has low Ca, Sr and Ba values. Cameron, Bagby & Cameron (1980) found that though plagioclase was the dominant crystal phase which controlled fractionation of calc-alkalic rhyolites in the region, two possibilities developed at approximately 66% SiO2. Pyroxene could continue to be the second most common phase, leading to higher Y–Zr rhyolites, or amphibole could become an important crystallizing phase leading to Y–Zr depleted rhyolites (Cameron, Bagby & Cameron, 1980). Various ratios of amphibole to pyroxene would lead to various Y–Zr contents. Apatite, sphene and possibly zircon fractionation may

have also been important in depleting the amphibole crystallizing melts in Y and Zr (Gunderson, Cameron & Cameron, 1986). In the study area, the southern, western and northern area melts crystallized a substantial amount of amphibole, and possibly also apatite, sphene and zircon, depleting the melts in Y and Zr. Increasing Zr/Y ratios with increasing SiO2 in these samples indicates that this trend was continuing as the obsidian bearing volcanic sequences were erupted. The high contents of Y and Zr in the central area indicate that pyroxene was the dominant ferromagnesian phase crystallizing and the constant Zr/Y ratio with increasing SiO2 demonstrates that these two elements were being incorporated in similar amounts in crystalline phases as eruptions occurred. Fe and Ti trends on Harker diagrams (Figure 4(c)) are consistent with the Cameron, Bagby & Cameron (1980) contention that Fe–Ti oxides crystallized in both ferromagnesian

Geochemistry of West-central Chihuahua Obsidian Nodules 1033

(a)

152 156 155 011

120

80

114 102 111 112 204 221

Site 104 151

Western area

(b)

Site Southern area

190 159 216

160 Sr (ppm)

Site Northern Central area area

200

Northern area

40

Central area (c)

0

8

18

28

38 Y (ppm)

48

160

58

68

30 (b)

(c)

Western area 140 Sr (ppm)

Sr (ppm)

20 120

10 100 Southern area 80

8

12

16 Y (ppm)

0 40

20

45

50

55

Y (ppm)

Figure 5. Understanding the geochemical variation facilitates choosing elements for scatter plots. Sr and Y contents of the samples are controlled by plagioclase and amphibolephosphate formation in the melts, respectively. The different crystallization histories of these minerals in the four areas produce a good separation of the samples on the diagram. Sample F9 plots off this diagram, as well as off Figures 6 and 7. (b) and (c) are enlargements of areas on (a).

280 Central area 230 Zr (ppm)

fractionation trends, depleting the melts in these elements with increasing silica. Knowledge of the fractional crystallization trends controlling the geochemistries of eruptive units enables selection of the most appropriate elements for scatter plots. Melts crystallizing plagioclase are depleted in Sr, whereas the other melts, which were primarily crystallizing amphibole, are depleted in Y and Zr. Thus, a scatter plot of Sr versus Y should produce a good separation of obsidian from the four sources (Figure 5). Similar logic leads to plotting Zr versus Ba, as Zr was enriched in melts where amphibolezircon were not major crystallizing phases and Ba was enriched in melts which were forming little, if any, plagioclase and K-feldspar (Figure 6). These plots give reasonably good separation among obsidian samples from the four areas. The separations can be enhanced by using ratios rather than scatter plots (Figure 7). The Sr/Y ratio represents the inverse of the amount of plagioclase crystallized and removed from the melt/the amount of amphibole crystallized and removed from

Western area

180 130

Northern area

80 30

Southern area 0

200

400

600 Ba (ppm)

800

1000

1200

Figure 6. Zr and Ba concentrations are controlled by amphibolezircon and phagioclaseK-feldspar crystallization in the melts, respectively. The different crystallization histories of these minerals in the four magma chambers show good separation.

1034 P. W. Fralick et al. 12 (a) 151 Western area

10

104

Sr/Y

8

6

Southern area

4

159 216

2

0 19

Central area

Northern area

(b) 0

2

4

6 Ba/Zr

(b)

8

10

(c)

12

10

7.5 0.4

155

Sr/Y

Sr/Y

112 6.5

204 221

0.2 5.5

156

0

111

114

152 1 Ba/Zr

1 01

2

4.5

7

9 Ba/Zr

11

Figure 7. Ratio plots enhance the separation of the data points. Sr/Y represents the inverse of plagioclase crystallization/ amphibolephosphates crystallization and Ba/Zr represents the inverse of plagioclaseK-feldspar crystallization/amphibolezircon crystallization. Amphibole and plagioclase on both axes result in the positive correlation trend. This ratio plot produces the best separation of the four areas as it is controlled by mineral phases which have different crystallization histories in the four areas. Sites in the southern area, and to a lesser extent the central area also separate on this diagram. This is probably due to fractionation in the magma chamber as successive eruptions are occurring producing chemically distinct units up sequence. The three different groups in (c) would reflect erosion of stratigraphically different horizons.

the melt; the Ba/Zr ratio represents the inverse of the amount of K-feldspar+plagioclase crystallized and removed from the melt/the amount of amphibolezircon crystallized and removed from the melt. The ratios improve the separation among the samples from the four areas, as they consider crystallization of four minerals rather than two, as in the scatterplots. The ratio plot also separates individual sites in the southern area and partially separates sites in the central area, though a great deal of overlap occurs here. Some 90% of obsidian samples have geochemical signatures matching the area in which they were collected. This leaves seven samples (B3, B4, B7, G1, G2,

A6 and I5) which do not correspond to the areas where they were found (Figure 8). In addition, three samples do not match the geochemistries of any of the four areas. Samples I7 from site Ch-155, F9 from Ch-114, and O4 from Ch-221 are clearly exotic. Cluster analysis, using both group average and Ward’s algorithms on Euclidean distance measures between standardized variables, was applied to the data set to evaluate the effectiveness of this technique of exploratory data analysis. Analyses on the whole data set in Table 2 (excepting Si and Nb, for which values do not exist for some samples) tend to partition the samples into three of the basic groups, but with numerous mismatches and the western group does not

Geochemistry of West-central Chihuahua Obsidian Nodules 1035

N 90% of samples were non-exotic n = 11 Namiquipa

155 C

n = 27 011 1 1 151

1

N

concluded (Di Peso, Rinaldo & Fenner, 1974, Vol. 7: 339). Rinaldo (Di Peso, Rinaldo & Fenner, 1974, Vol. 7: 337) and Di Peso (Di Peso, Rinaldo & Fenner, 1974, Vol. 8: 189), citing personal communication from Michael Spence, suggest that three pieces of obsidian at Paquime´ derive from the state of Durango. However, no geochemistry was done and that assessment was based on macroscopic visual observations. In any case, obsidian clearly is widely available throughout western Chihuahua, perhaps in greater abundance and diversity than Di Peso and his co-workers had an opportunity to discover. The reduction technology used for obsidian at both PAC sites and Paquime´ is consistent with what is widely available locally.

2 1 S n=6

n = 33 1

102

W 0

25 km

Cuauhtemoc

Scale Figure 8. Proposed derivation of anomalous, probably exotic, obsidian in the four geochemically discrete areas. All exotics are fragments, i.e. artefacts, rather than unworked nodules. Implications of the diagram are discussed in the text. n, number of samples in each area; , sample sites with no exotic obsidian; 151, sample sites with exotic obsidian; <, derivation of exotic obsidian; 1, number of exotic fragments.

effectively separate (dendrograms not included here). The best grouping was obtained when only 10 of the more systematically variable elements (Ca, Mn, Na, Ti, Ba, Sr, V, Y, Zn and Zr) were analysed, especially using Ward’s algorithm (Figure 9). However the western group still remains poorly defined. Having established that obsidian is locally available in west-central Chihuahua, and that regional distribution patterns exist, there is a basis for discussing locally available obsidian, lithic technology and regional distribution of flaked obsidian. In PAC sites, obsidian consistently occurs only as small fragments, often with cortex. Bipolar reduction of Apache tears is often indicated by opposite-end secondary platforms, diffuse bulbs of force, and elongate spalls. Tool shaping flakes do occur but represent reduction subsequent to breaking nodules apart. This technology is indistinguishable from the JCGP obsidian assemblage (Di Peso, Rinaldo & Fenner, 1974). This is the Chihuahua site presumably most likely to have exotic (e.g. Mesoamerican) obsidian, although there is no available chemical evidence for this. The 38 obsidian cores reported by Rinaldo in Di Peso, Rinaldo & Fenner (1974, Vol. 7) have a mean weight of only 10·0 g. There are 18 obsidian nodules from Paquime´ with a mean weight of 21·3 g. These data reflect reduction of small nodules, as Rinaldo

Discussion The observations on magma evolution have a profound influence on interpretation of provenance. The existence of small, localized eruptive centres, feeding from melts with different crystallization histories, creates geochemically distinct obsidian nodules confined to geographically limited areas. This enables the identification of discrete source areas with obsidian geochemistries controlled, on the regional scale, by previous crystallization history and, on the local scale, by fractional crystallization in the upper level magma chambers during eruptions. This is the ideal situation for a provenance study of obsidian nodules. It is apparent that most worked obsidian was obtained from the area surrounding each site. This is particularly evident for the southern area, where there are separate chemical groupings for several sites immediately east of Laguna Bustillos (Figure 7(c)) indicating that obsidian was procured mostly very near the respective sites. This is consistent with the abundance of obsidian nodules in arroyo gravels. While 10 of the 77 specimens do appear to be anomalous relative to the localities where we collected them, seven of these do match one of the other defined groupings, while three are chemically exotic, based on present knowledge. Of the 10 anomalous samples, three artefacts (B3, B4 and B7) from site Ch-102 in the Bustillos area have the most straightforward geochemical similarities to other groups. B3 and B7 match the central (Santa Marı´a) group, while B4 matches the western group. All naturally transported material at Ch-102 originated in the upland area 2 km to the east. For B3, B4 and B7 to be local, these facts would require that outcrops containing volcaniclastic debris with obsidian nodules from western and central area eruptions would also have to be present in the hills immediately to the east of Ch-102, and only this site. This seems improbable and human transport appears the most likely delivery mechanism for these three worked, chemically anomalous pieces from Ch-102. The anomalous obsidian specimens at sites Ch-11 (A6), Ch-151 (G1 and G2), and Ch-155 (I7) may have been

1036 P. W. Fralick et al. F3 F5 F4 E1 F8 F7 F6 F2 F1 D1 B 10 O3 O7 O6 B6 O9 O5 O2 O1 M1 B8 B2 B9 B5 B1 N1 K1 L1 L2 L3 L4 K3 K4 K2 A7 L6 L5 O8 G2 D3 D2 G5 I5 G4 G3 C1 B4 A6 O4 I7 F9 J9 J 10 J4 J7 J3 H3 J1 H2 I2 A4 B3 B7 A5 I6 J6 J8 J2 I3 A2 I4 I1 G1 J5 H1 A3 A1

S S S S S S S S S S S S S S S S S S S S S S S S S N N N N N N N N N C N N S W S S W C W W W S C S C S C C C C C C C C C C S S C C C C C C C C C W C C C C

Figure 9. Cluster analysis of the data set using 10 of the more systematically variable elements (Ca, Mn, Na, Ti, Ba, Sr, V, Y, Zn and Zr). This produces groupings similar to the ratio plot, except that the western area does not form as distinct a group. This is because the western volcanic area exhibits similarities to both the northern and southern areas. The ratio plot circumvents this problem by using element ratios related to major mineral phases with different crystallization histories in all the areas.

transported by drainage systems with resultant secondary enrichments similar to those described by Shackley (1992). Western area volcanics may occur in the drainage areas of streams flowing near Ch-11 and Ch-155. Likewise southern and central area rocks may occur in the drainage basin of streams flowing proximal to western area site Ch-151. Not enough information is available to ascertain if these anomalous pieces were transported naturally or by humans. The three samples (F9, I7 and O4, from sites Ch-114, Ch-155 and Ch-221, respectively) which do not correspond to any of the

four geochemical patterns come from sites which are downstream from small, localized drainage areas. Thus, it is most likely that these specimens were transported into those regions by humans, though excessive transport distance is not required, as indicated by the relatively small areas of the four identified volcanic domains. However, the geochemical signature of F9, the only peralkaline sample, is similar to obsidian found in the Sierra to the north of the study area (Shackley, pers. comm.). This suggests that this sample may have been carried over 150 km, or there is

Geochemistry of West-central Chihuahua Obsidian Nodules 1037

an unknown source of similar material elsewhere outside the study region. The use of ratio plots requires an understanding of the processes which have created the variance in the data set. Modelling of these processes for the highpotassium, calc-alkalic rhyolites of the region had previously been accomplished (Cameron, Bagby & Cameron, 1980; Cameron & Hanson, 1982; Wark, Kempter & McDowell, 1990), and it was only necessary to determine the general crystallization histories of the four distinct melts. This approach identifies the elements which fractionate differently in the various melts and also those which have ratios that change with progressive eruptions from the same magma source. Using these elements to construct ratio diagrams provides the best separation of samples into discrete fields (compare Figures 7 and 9) and provides a firm scientific basis for the groupings. Clearly, exclusion of some element data from grouping techniques is desirable. The effectiveness of grouping techniques increases from cluster analysis performed indiscriminately on the whole data set (excluding Si and Nb), through cluster analysis performed on a selection of 10 of the somewhat systematically variable elements to a ratio plot using four selected elements. This is because the signal to noise ratio increases as elements with lesser degrees of systematic variation between the four melts are eliminated. In this study elements being removed from the melts by crystallizing phases were used; in other areas melt chemistry may be controlled by wall rock assimilation, magma chamber recharge or a number of other processes. The tenet stressed here is the use of the elements which have the highest variance, due to the dominant process, to define groupings using ratio plots. Our view on this matter seems similar to that of Shackley (1995). In summary, four separate magma chambers with different crystallization histories produced the obsidian nodules present in the study area. This resulted in nodules with different geochemical signatures occurring in different areas. This pattern of geographically limited, obsidian-bearing, volcanic rocks with different geochemistries should be present throughout a large region in northwestern Mexico and the American southwest. If so, this will allow augmentation and refinement of the detailed data base obsidian procurement that researchers have been developing in recent years (Shackley, 1988, 1995, and references cited therein). Most nodules in the study area were procured locally, although human transport of some nodules is possible in the Santa Marı´a (central) and Bustillos (southern) areas. The lack of transported nodules in the Babı´cora (northern) area may be a result of smaller sample size or greater local abundance. The widespread availability of nodules, albeit at low densities in most drainages, precludes needing complex exchange or direct acquisition practices to obtain faraway obsidian (cf. Di Peso, Rinaldo & Fenner,

1974). While technological evidence reflects use of small nodules for obsidian and pitchstone tools, chemical analysis reflects localized acquisition. Although chemical analyses on a comparable scale have not been undertaken for the other lithic materials, such as rhyolite and cherts, our impression is that local procurement is also the pattern for those materials. At a more general level, this acquisition practice is consistent with what we see as the generally decentralized nature of Chihuahua culture to which all the northern area and central area sites and site Ch-151 in the western area belong (Kelley et al., in press). The southern area sites do not belong to the Chihuahua culture, the available southern area radiocarbon chronology does not extend up into the Medio Period time frame and, in general, less is known about the ancient culture of that area. Further work is needed in west-central Chihuahua (and other parts of the state) not only to test the pattern of obsidian acquisition suggested here but to develop all other aspects of the archaeological record.

Acknowledgements The obsidian samples were collected during the 1990– 1992 field seasons of the PAC (Proyecto Arquelo´gico Chihuahua), funded by a research grant to Jane H. Kelley (Principal Investigator) and Joe D. Stewart (Co-investigator) from the Social Sciences and Humanities Research Council of Canada (SSHRC file no. 410-90-1070). The field work was conducted under permit from the Instituto Nacional de Antropologı´a e Historia (INAH, Consejo reference no. 401-36-039-90). We thank the staff members associated with the INAH Centro Regional, Chihuahua, for their help and collaboration. Although too numerous to name individually, we also appreciate the people in Chihuahua who helped us officially and unofficially in the field. The geochemical analyses were supported by grants to Philip Fralick from the Natural Sciences and Engineering Research Council (NSERC). The geochemical analyses were carried out by Lakehead University technicians, Eleanor Jensen (Department of Chemistry), and Ain Raitsakas and Keith Pringnitz (Lakehead University Instrumentation Laboratory). Sam Spivak drafted the figures. Christopher Stevenson kindly provided the NBS-278 obsidian standard that was run with the first sample set. We also thank M. Steven Shackley, Jane H. Kelley and an anonymous reviewer for their comments on previous drafts of the paper.

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