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Characterization of the Alamo Creek Basalt, Big Bend National Park, Texas.

Abstract

Mapping, petrography and chemical analysis allow the characterization of the Tertiary Alamo Creek Basalt (TACB), which occurs at the base of a section of volcanoclastic and volcanic rocks in Big Bend National Park, as consisting of eight distinct units, at least one of which consists of multiple flows. The distribution and directions of thinning suggest that the units had multiple sources, one to the south or southwest from Mexico, another to the west in northernmost Mexico or southernmost Texas, and possibly, at least one in Big Bend National Park itself. Six of the eight units consist of alkaline olivine basalt, while the other two are basaltic trachyandesite and trachyandesite. Chemistry of the units suggests that in some instances there was magmatic evolution during their extrusion, and in other cases between extrusion of the related flows of the units. A question remains as to whether there are chemical characteristics by which different sources can be distinguished.

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The Tertiary Alamo Creek Basalt (TACB) consists of a series of lava flows that occur west of the Chisos Mountains in the western part of Big Bend National Park. Maxwell et al. (1967) described the TACB as a single unit composing the basal member of the Eocene Chisos Formation. However, detailed mapping by Stewart (1984) reveals the member to consist of at least eight distinct units. The rocks are mostly alkaline olivine basalt, typical of the magmatism that marked the early emplacement phase in the Trans-Pecos igneous province (Henry et al. 1991; James & Henry 1991).

This report describes the TACB, covering field occurrence, petrography and geochemistry, emphasizing the evidence for the different units and the possibility of more than one source for the units. The mapping and initial identification of the units was first described by Stewart (1984). Subsequent studies by the authors included further mapping, petrography and an extensive geochemical study. Although preliminary geochemical interpretations are presented, a more detailed discussion of geochemistry and petrogenesis is reserved for a subsequent publication.

The Tertiary rocks of west Texas underwent moderate folding followed by faulting, marking the transition from a convergent magmatic arc setting to that of Basin and Range extension. The age of dated samples of the TACB is essentially 47 my (Henry et al. 1986; 1989), placing them in the late convergent phase or early extension phase of the transition to Basin and Range tectonics.

Stewart (1984) summarized the regional setting, following Maxwell et al. (1967), as follows: "Deformed and eroded Paleozoic rocks, mostly of flysch and molasse facies, are unconformably overlain by Cretaceous rocks composed dominantly of shelf carbonate facies. The Late Cretaceous to Early Eocene Laramide Orogeny changed the depositional environment from marine to continental with the advent of clastic deposition which included fine volcanoclastic beds, precursors of extensive Tertiary volcanism. The orogeny produced belts of deformation forming topographic highs, between which coarse volcanoclastic and extrusive Tertiary rocks were unconformably deposited on eroded Cretaceous beds." A central folded belt oriented NNW-SSE through the center of the Chisos Mountains appears to have acted as a barrier to eastward deposition of the TACB and westward deposition of Paleocene and Lower and Middle Eocene clastic rocks that occur east of the Chisos Mountains. Post Upper Eocene deformation raised the Chisos Mountains a nd modified the central folded belt. Post Middle Eocene igneous activity, and post Early Oligocene deformation, largely of Basin and Range type faulting, further disturbed the Cretaceous and early Tertiary rocks, and erosion then became the dominant process affecting the Big Bend area. The deformations have left the TACB with gentle eastward and westward dips in different parts of it's outcrop area.

FIELD OCCURRENCE

Those interested in the detailed location of the TACB are referred to the Geologic Map of Big Bend National Park, provided as plate II in Geology of Big Bend National Park, Texas, Bureau of Economic Geology Publication 6711 (Maxwell et al 1967) and also supplied with the Bureau of Economic Geology Guidebook 7, The Big Bend of the Rio Grande (Maxwell 1968).

The TACB is exposed in Big Bend National Park around the western base of the Chisos Mountains from Dogie Mountain in the north to the Gauging Station in the south, and Lajitas Mesa, just outside the Park, on the west (Fig. 1). The member extends southwestward across the Rio Grande into Mexico, but it has not been studied there for this report. Maxwell et al. (1967) note that basalt flows within the Canoe Formation, which underlies the Chisos Formation within and on the eastern flank of the Chisos Mountains, might be of the same age as the TACB but these are poorly studied and have not been dated. Exposures of the TACB are discontinuous because of folding and faulting; and they range from a few hundred yards in extent to a maximum of about eight miles of continuous outcrop in the southeastern part of the mapped area. For this reason it was necessary to use a combination of hand specimen recognition in the field and petrographic details and chemical composition to fully distinguish different units.

At least one of the eight units of the TACB consists of three separate flows, as distinguished by mapping and petrography and illustrated in their chemistry. For most units such separate flows were not generally identifiable in the field because of the similarity of hand specimens and the discontinuous nature of the outcrops. At no place are all eight units superimposed, but at several localities two or three units occur together. There are sedimentary rocks separating volcanic units at only one locality, but several volcanic units show evidence of erosion, with channeled contacts between them. The thickness of units ranges from a few feet to a maximum of 200 feet (for one unit, which is a trachyandesite rather than basalt), with an average thickness between 10 and 30 feet. Units are designated A through F, from oldest to youngest, G and H (Fig. 2).

In all exposures within the National Park the TACB rests unconformably on the uppermost Cretaceous Javelina Formation, but west of the Park, at Lajitas Mesa (Fig. 3), it rests on a thin layer of the basal Tertiary Jeff Conglomerate which overlies the Cretaceous Pen formation. The units overlap each other so that no single unit is at the base of the TACB over the entire mapped area. The maps of Figures 4, 5 & 6 show the extent of the TACB units east of the Rio Grande, as estimated from the pattern of their outcrops.

The relative ages of Units A and B could not be determined in the mapping because they were not observed to occur together at any outcrop. In the narrow south-southeast trending exposure between Smoky Peak and Punta de la Sierra (Figs. 4-6) all units from A-D and F occur, but exposures are poor and the units jumbled by faulting, and their stratigraphic relations could not be discerned. Either Unit A or B is at the base of the TACB where well exposed elsewhere, and each is overlain by Unit C or D. However, as shown under Chemistry (p. 118), Unit B is more evolved than Unit A. On this basis Unit B is tentatively postulated to be younger than Unit A. Units G and E are in close proximity at Sierra Aguja (G) and Terlingua Abaja (E) but they were not found together anywhere. Accordingly, their relative ages are not certain; although, again, as shown under Chemistry (p. 118), Unit G appears to be more primitive than Unit E, and is tentatively considered the older. Unit H occurs only at Lajitas Mesa, far west of all other units, and its age relative to units to the east is not known.

Unit A is at the base of the TACB, resting unconformably on the Javelina clay from the north-central to the southernmost part of the area at the Gauging Station, where it is overlain by Unit C (Fig. 3). Unit B is found only in the southeastern part of the area and it forms the base of the TACB wherever it its base is exposed, also resting unconformably on Javelina clay. Except in the jumbled strip between Smoky Peak and Punta de la Sierra, its exposures all lie east of those of Unit A (Fig. 4), and apparently Unit B flowed along the east side of that presumed older unit. Unit B is overlain by Unit C at the locality south-southeast of Mule Ears Peak and at all localities to the south and east. Just north and south of the Road Cut, Unit D overlies Flow Unit B; but locally, at the end of the exposure north of the Road Cut, a few feet of clay, believed to be reworked Javelina material, separates the two units. Unit C is limited to the area east of Smoky Creek (Fig. 5). It always rests on Unit A or B, and it is ev erywhere overlain by Unit D. Unit D rests on Unit A west and north of Cerro Castellan, to the point one mile northwest of the Cerro where it ends in a blunt scoriaceous mass that buttresses and is overlapped by Unit F.

The relatively great thickness of Unit D in the center of the mapped area (200 feet at the Road Cut, Fig. 3) may be partly attributable to its more silicic composition; but the thickening of other units in this area may also indicate ponding in a depression. Unit E occurs at Terlingua Abaja, around the base of Dogie Mountain, and at the southwest base of Little Christmas Mountain (Fig. 5). It is thickest at Terlingua Abaja, where flow contacts can be distinguished in a jumble of at least three flows which have an aggregate thickness of about 60 feet (Fig. 3) and it thins eastward from Terlingua Abaja. Unit F is extensively exposed in the north central part of the mapped area, where it overlies all units A-E (Fig. 3), and appears across most of the area except in the westernmost and easternmost extremes (Fig. 6). It thins eastward from its thickest occurrence north of Cerro Castelan (Fig. 3); however, to the west, at Terlingua Abaja, it appears as a thin capping on a hill top, with an unknown amount eroded. Un it G comprises the entire outcrop of the TACB at Sierra Aguja (Figs. 3 & 5), where it is 20 feet thick around the base of the mountain, rests on Javelina clay, and is overlain by volcanoclastic beds and thin lava flows of the Chisos Formation. It does not seem to occur southeast of Sierra Aguja in the hills at Terlingua Abaja, which are composed of Unit E, and the contact between the units is hidden by alluvium in the swale between them. In mapping it was thought that perhaps Units G and E interfingered, but detailed petrographic comparison of thin sections and chemical analyses show all the rocks at Sierra Aguja (Unit G) to be distinct from those at Terlingua Abaja (Unit E). Accordingly, their stratigraphic relationship cannot be determined; but, as noted previously, Unit G appears to be chemically more primitive than Unit E and is tentatively considered the older. Unit H is found in the far west of the area at Lajitas Mesa, where it is faulted against higher members of the Chisos Formation at its eastern ex tremity and it pinches out abruptly in Chisos beds 1.5 miles to the west with little marked thinning along its exposure. It rests on about 10 feet of the Jeff Conglomerate, containing Cretaceous cobbles and a few basaltic clasts, which is the basal Tertiary unit west of the Park, and which in turn overlies the Cretaceous Pen Formation.

PETROGRAPHY

The eight units of the TACB include six of olivine basalt, one basaltic trachyandesite and one trachyandesite. Most of the basalts are porphyritic and textures range from very fine-grained to very coarse-grained, using these terms in the volcanic sense (Table 1), and include ophitic, subophitic, intergranular and intersertal varieties.

The essential minerals of the basalts are the expected plagioclase, augite, olivine and iron-titanium oxides, while minor amounts of apatite, ferroaugite, and analcite were detected in some units. The mineralogy of the different units is summarized in Table 2, which gives their average modal analyses. Their general petrology can be outlined as follows:

1. Unit A is a moderately to strongly weathered porphyritic basalt with twenty five to thirty five percent large plagioclase phenocrysts. Phenocryst [cores-An.sub.75-86]; [groundmass-An.sub.43-65]. (An values were optically determined unless otherwise stated)

2. Unit B. is a dense porphyritic basalt with about twenty percent of tabular plagioclase and minor augite and olivine phenocrysts. Phenocryst [cores-An.sub.47-60], [microprobe-An.sub.56-59] [groundmass-An.sub.44-63], [microprobe-An.sub.47].

3. Unit C is a slightly to strongly vesicular, sparsely porphyritic basalt, with less than five percent plagioclase phenocrysts. Phenocryst [cores-An.sub.74-86]; [groundmass.sub.48-62].

4. Unit D is a relatively fresh, mostly dense but locally scoriaceous, porphyritic trachyandesite with ten to fifteen percent large plagioclase xenocrysts and phenocrysts. Phenocryst cores-microprobe A[n.sub.33-34]; groundmass-optically-A[n.sub.21-24]

5. Unit E is a variable, sparsely to abundantly porphyritic basalt. Phenocryst cores-A[n.sub.63-81]; groundmass A[n.sub.52-62]. This unit appears to consist of multiple flows; although the flows were not mapped in the field, this is indicated by flow contacts, variety in texture and mineralogy (Table 2) and chemistry (p. 116).

6. Unit F is composed of essentially aphyric basalt of varied petrography and chemistry, which suggests that it consists of at least two flows, although they were not distinguished in the field. Rare phenocryst cores-A[n.sub.40-70]; groundmass-A[n.sub.65-68] (p. 116).

7. Unit G is composed of sparsely to moderately porphyritic basalt, with gabbroid xenoliths, plagioclase glomerocrysts and olivine microphenocrysts. Phenocryst cores-A[n.sub.72-83]; microprobe A[n.sub.81]; groundmass-A[n.sub.54-68], microprobe A[n.sub.48].

8. Unit H is an aphyric basaltic trachyandesite exposed as an 80 feet thick cliff-former along the southern flank of North Lajitas Mesa. A[n.sub.31-33]

OVERALL DESCRIPTION OF TACB

All units except F and H are distinctly porphyritic, with phyric plagioclase grains ranging from 0.1mm to 7mm, and showing a complex history of formation. They can be grouped into three types: (1) glomerocrysts of anhedral grains; (2) single, subhedral to anhedral broken crystals and (3) single, euhedral to subbedral crystals. Types 1 and 2 have moderate to strong zoning, with sharp rim zones; many with unconformities in their core and between core and an epitaxial outer rim. Inclusions are common to abundant, sometimes in concentric zones, concentrated in the outer part of the grain; they include augite, rare olivine, platy and equant opaque grains and rounded and boxy smectite-filled patches. Type 3 crystals have few or no inclusions and very thin or no rims. These appear to be true phenocrysts, while types 1 and 2 are clearly mostly xenocrysts. The core composition for all three types of crystals, as determined mainly by extinction angle methods, is bytownite for units A, C, E and G; labradorite for unit B and oligoclase for unit D. Optical determinations were checked by electron microprobe for Units B, D & G. Some of the glomerocrystic clusters are clearly xenoliths, composed of plagioclase intergrown with coarse-grained augite, olivine and iron oxide in a gabbroic texture. Units A, B, D and G contain common to sparse olivine phenocrysts (0.2 - 3.5 mm). The relatively coarse grain size of xenoliths and xenocrysts indicate origin from subjacent plutonic sources, and the gabbro of Christmas Mountain and syenitic rocks of Dominguez Mountain show that such plutons exist in the Big Bend region.

The most common groundmass texture is intergranular, with seriate, very fine- to coarse-grained, finely albite twinned and tapered labradorite laths, stubby augite prisms, subequant olivine grains and platy to equant opaque grains. The trachyandesite of Unit D is distinctive in that the groundmass plagioclase laths are oligoclase, simply carlsbad twinned and bluntly terminated, rather than tapered; and the groundmass is mostly intersertal, with pale brown to clear glass which is sometimes altered to brown smectite. All units contain very fine apatite needles, though they are rare in unit A. Pale green to brown smectite is the dominant alteration product in the TACB lavas, commonly pseudomorphing olivine and as interstitial patches and veinlets throughout the rocks. Flow structure, with flow-aligned plagioclase laths, is widespread in units C, E, F, G and H, but rare to absent in units A, B and D.

CHEMISTRY

Samples were analyzed for major and trace elements, including rare earth elements (REE), as indicated in Tables 3 & 4. Samples completed at the University of Houston were analyzed by ICP-AES, based on the sample preparation methods reported in Meaux (1989) and Lindholm (1990). Rare earths and Y and were analyzed after chromatographic separations. The precision and accuracy for each element measured were determined using USGS standards BHVO-1m, BCR-1, W-2 and Sco-1. Precisions for oxide determinations were within 2%, except for [P.sub.2][O.sub.5] (9%) and [K.sub.2]O (5%). Precisions for trace elements are 4% or lower, except for Zr (6%), and accuracies are 5% or lower.

Table 3 shows the overall chemical nature of the TACB units, and serves to demonstrate the general makeup and similarity of these rocks, as well as their characterization as alkaline olivine basalts. The chemistry of representative samples of the TACB can be shown to delineate the units and flows that were recognized in the field and in thin section. Though the overall compositions are broadly similar, distinctive differences in some elements, major and trace, serve to characterize the units. Of course Units D and H stand out with their high [SiO.sub.2], [Na.sub.2]O, [K.sub.2]O and low MgO and CaO. Among the basalts, such details as the wide range of [A1.sub.2][O.sub.3] in Unit E and its progressive drop from Unit A, through B and C can be explained by the difference in plagioclase phenocryst content in the various units (Table 2). Unit A is the most abundantly porphyritic basalt unit and its resultingly high content of bytownite is reflected in its high [A1.sub.2][O.sub.3] and CaO. The relatively high MgO co ntent of Unit G is explained by the fact that it is the most olivine-rich of the basalts, but it is unusual in being the highest in Si[O.sub.2] among them.

Among the major elements the most striking relationship, and the one which best illustrates the different units, is that shown by a plot of Ti[O.sub.2] vs [P.sub.2][O.sub.5] (Fig. 7). There the analyses of Units A, B, C and D each cluster in a relatively small and isolated area, with the clusters of A, B and C forming a linear trend in stratigraphic order, from low Ti[O.sub.2] [P.sub.2][O.sub.5] (A) to high [TiO.sub.2] [P.sub.2][O.sub.5] (C). The analyses of units F and E each form a discrete linear trend subparallel to that of A, B and C, and those of Unit E are clustered in three groups that match the three flows of Unit E. The analyses of Unit F show a spread similar to that of Unit E, but they form a more continuous line, rather than being clustered. It is to be recalled that it was not possible to distinguish separate flows for Unit F, either petrographically or in the field. Unit G lies in the same region of the diagram as Unit A, with low [TiO.sub.2] and [P.sub.2][O.sub.5], and their patterns overlap; but the two units are different in other aspects of their chemistry (e.g., G has higher MgO, Ave. 6.0 vs 4.4; and Si[O.sub.2] Ave. 48.3 vs 46.7; and lower GaO, Ave. 8.9 vs 11.4), texture and some details of mineralogy (e.g., Unit G is oniy slightly porphyritic and has analcite and ophitic augite, which A does not). As is to be expected, Units D and H are isolated from the basalts in this and virtually all other plots of major and minor elements.

Another informative plot is that of MgO vs [TiO.sub.2] (Fig. 8), which allows some conclusions and speculations concerning the possibility of multiple flows. One of the most interesting features of this diagram is suggestive trends in Units F and E, in which Unit F has a general increase of [TiO.sub.2] with the decrease of MgO, while Unit E shows an overall weak trend of decreasing [TiO.sub.2] with decreasing MgO. Units A, D and H also appear to have a slight decrease of [TiO.sub.2] with decreasing MgO. With Unit E, in the successive flows that are distinguished petrographically, [TiO.sub.2] drops from about 4.0 in flows E1 and E2 to about 3.5 in flow E3, while MgO drops from about 5.0 in flows E1 and E2 to about 4.2 in flow E3. A decreasing trend might be explained by magma fractionation of iron-titanium oxides from flows one and two to flow three. It is proposed that the source of Unit F, which has MgO mostly above 4.5, was fractionating olivine, thus decreasing MgO while raising the relative concentration of [TiO.sub.2] This suggests that Unit F may be composed of a succession of flows spaced through time, even though they were not identified in the field or in thin section.

Although as expected, there is significant scatter in the bulk chemical data, a plot of MgO vs [SiO.sub.2] of all samples (Fig. 9) appears compatible with early fractionation of olivine followed by plagioclase, olivine and probable oxide fractionation. For example, samples of Unit F, which show little increase in [SiO.sub.2] with decrease MgO, appear to have evolved along an olivine control trend. Subsequent fractionation of olivine and plagioclase leads to a trend of increasing [SiO.sub.2] with decreasing MgO. This is consistent with abundant plagioclase phenocrysts as well as minor olivine phenocrysts in the bulk of the samples with MgO < [approximately equal to] 5.2 wt %. The geochemical trends suggest clinopyroxene saturation was not achieved and samples mostly lack clinopyroxene phenocrysts.

Modeling.--The modeling program Ptl2lb of Leonid Danyuvshesky (2001) was used to test the hypothesis for fractional crystallization of Unit F. It was determined that, starting with the most magnesium-rich sample 103 (Figs. 10a-c) it is possible to generate a reasonably close approximation of most of the other individual samples of Unit F, with the exception of samples 174 and H86-92; the latter of which comes from well north of the area mapped, but is similar stratigraphically, in age, and broadly in chemistry. The modeling assumes crystallization of olivine and plagioclase, under anhydrous pressure of one kilobar, to a final liquid composition of 4.5% MgO. When a liquid of the composition of sample 103 has fractionated 4% olivine and 8% plagioclase the remaining 88% melt will have reached a composition very close to that of sample 222, as shown in figures 10a-c, which illustrate the values for MgO, [TiO.sub.2], [SiO.sub.2] and [P.sub.2][0.sub5].

Difference of source.--In REE (rare earth elements) diagrams (Fig. 11a) the TACB suite of rocks shows a successive increase in LREE (light rare earth elements), suggesting that, over all, it might be some sort of evolutionary sequence. Figures 11b, c & d, which cover all of the data for each unit, illustrate LREE enriched trends for the entire TACB, and show how clearly the volcanic units are defined and separated by their REE patterns. Interestingly, Units A and G have nearly coincident patterns, and are the most primitive; even though they appear to have different sources, as indicated in figures lie, where REE patterns cross over each other, and 12, where La/Yb values are widely separated. Units B and F are more evolved (Fig. 11g), with elevated LREE, which show an identical spread of values for the range of samples represented here, and Unit E has values that fall between those of Units B and F, but with a slightly steeper slope. Unit C is still more evolved, forming an upper boundary for the group of bas alts (Fig. 11b). Units H and D are successively even more evolved (Fig. 11 a), as would be expected for basaltic trachyandesite and trachyandesite respectively, and the crossover of patterns (Fig. I It) indicates different sources for them.

Overall REE abundances show that all of the samples are LREE enriched with La/Yb cn ratios ranging from [approximately equal to]7.8 to 13.8 (Figure 12), a spread that indicates multiple parental liquids. Fig. 11 shows the range of REE values for each of the units compared to the overall range of all the units sampled. The entire suite of units sampled shows significant variation in LREE ranging from [approximately equal to]100 to 270 x chondrite, and HREE (heavy rare earth elements) ranging from 8 to 23 x chondrite. The lack of stronger depletion in the HREE may indicate lack of a garnet source influence. The intra-unit range of abundances is rather narrow for each of the units described (Figs. 11b, c & d), suggesting a common parent within each unit. However, inter-unit comparisons show distinct differences, such as variable LREE/HREE ratios, crossing REE patterns and variable LREE to HREE ratios (Figs. 11e & 11f). The crossing patterns suggest complex and variable parental lineages, or more complex evolutio n of a common parent through AFC processes. Comparison of La/Yb en vs La cn (Fig. 12) shows a wide variation between units that tends to support this. Unit E shows the highest La/Yb en ratios ([approximately equal to]13.8) and appears distinct from the remainder of the units; for example, Units A, B, C and F show La/Yb en ratios from 9.5 to 12.5. The basaltic trachyandesites of Unit H fall at the lower part of this range (9.5), but have relatively high La cn (200-220); while the trachyandesites of Unit D appear at the upper part of the range for basalts (12.5) and have much higher La cn (260-270). Finally, Unit G basalts have the lowest La/Yb cn ratios, which are between 7.5 and 8.5.

In general, intra-unit comparisons of analyzed samples show relatively tightly clustered La/Yb ratios and La cn abundances, but inter-unit comparisons show strong differences in La/Yb cn and La cn contents. Whereas variation in La cn contents between 100 and 270 may develop as the result of crystal fractionation processes from a common source, this is unlikely because: (1) there are large differences in La/Yb cn between individual units suggesting various parental melts and (2) there is no systematic correlation between major element fractionation indices (e.g., MgO and Si[O.sub.2]) and incompatible trace element indices (e.g., La cn contents) (Fig. 13). Although various units with similar ranges of La/Yb cn ratios may be cogenetic (Fig. 12), the fact that major element fractionation indices cannot predict incompatible trace element abundances would tend to indicate otherwise. When comparing Zr/Y vs Zr (Fig. 14) there is likewise a wide range of ratios that indicate units are unlikely to have common parents. It is concluded from the trace element data presented that the units are likely to be derived from distinct parental magmas that have resulted from variable sources and extent of melting; or that AFC processes have significantly modified a parental melt upon ascension through the crust.

Location of sources.--A large dike occurs on the east flank of Dogie Mountain and extends south for 4.5 miles (Fig. 4 & Maxwell et al. 1967, Loc. 160). Dike 160 was sampled, analyzed and dated at 47 my by Laughlin et al. (1982, VCK82TX029), and has major element chemistry somewhat similar to Unit B (Table 3 & Figs. 7, 8 & 9), and perhaps it may be the source of this unit. Unit F has an REE (rare earth elements) pattern strikingly similar to Unit B (Fig. 11g), and is very close to Unit B in its trace element ratios (Figs. 12 & 13). With these facts in mind, and noting the distribution patterns on figures 4, 5 and 6, it is tempting to postulate that A, B, E and F all came from the dike and spread southward, ponding in a depression between Terlingua Abaja and Punta de la Sierra. However, the exposure at the Gauging Station on the Rio Grande consists of units A, C and D and the same section can be seen across the river in Mexico extending southwestward where they are exposed in the Sierra Ponce, suggesting that u nits A, C and D had a source(s) to the south or southwest. It is clear that the wider extent of the TACB across the Rio Grande needs to be studied, and a resolution of differences in chemistry between the units is necessary, before any conclusion can be drawn concerning the relation of the TACB to dike 160.

If dike 160 was not the source of any TACB units, and if one looks at the distribution patterns alone, the following postulations can be made: Units A, B, C and D may have had source(s) to the south or southwest; whereas, units E, F, G & H seem have come from the west or northwest.

Ten miles northwest of Dogie Mountain there is a 25 feet thick lava flow (H86-92, Table 3) near the base of the Devil's Graveyard Formation that is dated at 47 my (Henry et al. (1989) and those authors suggest the possibility of this correlating with the TACB. Petrographically and chemically sample H86-92 is closest to those of Unit F. However, there are notable differences between sample H86-92 and those of Unit F (Figs. 7 & 8); and, if it is correlated with TACB, perhaps it should be considered as a ninth unit, occurring only northwest of the park.

CONCLUSIONS

The results presented here demonstrate that the Alamo Creek Basalt is composed of a complex series of at least eight units which can be distinguished by hand specimen and mapping in the field and petrographic examination. The chemistry of the units suggests that each one appears to have had a separate source. The field relations indicate that three of the units designated A, B, and D are most extensive in a northerly-southerly direction and thin eastward, westward and generally northward. This is in keeping with the structural grain of the area, in which the trend of late Laramide fold axes is NNW-SSE, and the whole TACB may have poured into a synclinal valley. The south and southwestward extent of the TACB is unknown, but it can be seen to extend across the Rio Grande into Mexico. The conclusion is that some of the umts, such as A, C and D had a source to the south or southwest in Mexico. Unit B, however, may have its source in a dike on the east side of Dogie Mountain. Unit F is thickest in the north centra l area, thins northeastward and southeastward, and may have been ponded northwest of Unit D; it does not appear to have come from the south. The remaining three units, E, G and H lie in the northwestern part of the area, are petrographically and chemically distinct from Units A-D and F, and are believed to have a different source, or sources, from them, lying to the west or northwest in northernmost Mexico or southernmost Texas. The various units show signs of magmatic evolution of their sources, either during their extrusion (Units F and E), or between extrusion of related units, (Units A versus C). However, the chemistry of the volcanic units does not distinguish the sources that are based on field evidence.

The suite as a whole is very similar to an alkali basalt-trachyte suite from the Southern Gregory Rift of Kenya (Baker et al. 1977) and those of other continental intra-rift zones. These alkali basalts are also characterized by chemistry in which [Na.sub.2]O is greater than [K.sub.20] and trace element patterns show a similar range. Thus the Alamo Creek basalts are likely to have originated from intra-plate extension in the region, starting shortly after the major Laramide compressional event. It is intended that these relations will be discussed in a later paper.

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Table 1

Volcanic textural terminology.

ROCK DESCRIPTION GRAIN SIZE (MM)

Very coarse-grained >0.5
Coarse-grained 0.1-0.5
Medium-grained 0.05-0.1
Fine-grained 0.01-0.05
Very fine-grained <0.01

Table 2

Modal analyses of chemically analyzed representative samples of Units
A-H.

Unit A A A A B B B
Sample no. 31 73 145 167 111 119 121
Rock type Basalt Basalt Basalt Basalt Basalt Basalt Basalt



Plagioclase
 phenocrysts 26.0 32.5 29.2 21.5 21.1 7.3 20.7
Pyroxene
 phenocrysts 0.0 1.0 0.0 0.0 3.9 Tr. * 1.8
Olivine
 phenocrysts 1.1 2.6 1.9 2.1 5.1 0.0 2.1
Opaque
 phenocrysts 0.0 0.0 0.0 0.0 0.0 0.0 Tr. *
Plagioclase
 groundmass 32.3 22.5 29.1 30.7 28.1 42.9 32.4
Pyroxene
 groundmass 21.3 18.7 21.8 26.4 20.9 23.0 20.2
Olivine
 groundmass 9.6 4.6 4.2 7.6 7.0 9.8 5.5
Opaque
 groundmass 8.7 16.3 8.8 8.0 10.1 9.8 11.9
Apatite 0.1 Tr. * Tr. * 0.0 0.2 Tr. * 0.4
Smectite 0.5 0.4 3.2 3.7 3.0 5.7 5.0
Sodic Cpx 0.0 0.0 0.0 0.0 0.0 0.0 0.0
Other 0.4 1.4 1.8 Tr. * 0.0 1.5 0.0

 100.0 100.0 100.0 100.0 100.0 100.0 100.0

Unit C C C D D D
Sample no. 92 112 118 A B 8a
Rock type Basalt Basalt Basalt Trachy- Trachy- Trachy-
 andesite andesite andesite


Plagioclase
 phenocrysts 3.9 3.5 6.4 11.5 16.5 6.4
Pyroxene
 phenocrysts 0.0 0.0 0.0 1.1 0.7 1.4
Olivine
 phenocrysts 0.0 0.0 0.0 0.5 0.9 0.5
Opaque
 phenocrysts 0.3 0.5 0.7 2.0 1.0 1.5
Plagioclase
 groundmass 43.7 43.8 37.7 40.7 42.1 44.2
Pyroxene
 groundmass 25.5 29.5 29.5 8.8 12.8 13.5
Olivine
 groundmass 3.4 1.9 3.0 0.3 3.3 5.9
Opaque
 groundmass 22.3 17.9 21.0 6.2 7.6 9.9
Apatite Tr. * 1.2 0.4 0.5 0.7 1.3
Smectite 0.0 1.6 0.4 22.6 10.2 15.4
Sodic Cpx 0.0 0.0 0.0 1.5 0.6 Tr. *
Other 0.9 0.1 0.9 4.3 3.6 Tr. *

 100.0 100.0 100.0 100.0 100.0 100.0

Unit E1 E2 E3 F F F G
Sample no. 154 187 151 14 147 193 96
Rock type Basalt Basalt Basalt Basalt Basalt Basalt Basalt



Plagioclase
 phenocrysts 1.0 5.7 22.1 0.0 0.0 0.0 5.0
Pyroxene
 phenocrysts Tr. * 0.0 0.0 0.0 0.0 0.0 15.1
Olivine
 phenocrysts Tr. * 0.0 0.0 0.0 0.0 0.0 Tr. *
Opaque
 phenocrysts Tr. * 0.0 0.0 0.0 0.0 0.0 0.0
Plagioclase
 groundmass 51.8 46.1 42.0 46.9 51.7 45.3 59.5
Pyroxene
 groundmass 28.6 23.0 18.3 20.6 22.2 19.9 0.0
Olivine
 groundmass 6.1 4.5 6.4 10.1 8.3 12.5 11.3
Opaque
 groundmass 10.8 15.5 6.0 10.8 10.7 14.4 4.9
Apatite 1.0 0.9 1.0 0.5 1.5 0.1 0.5
Smectite 0.5 3.1 1.3 10.6 1.8 7.4 2.1
Sodic Cpx 0.0 0.0 0.0 0.1 3.8 0.4 0.0
Other 0.2 1.2 2.9 0.4 0.0 0.0 1.6

 100.0 100.0 100.0 100.0 100.0 100.0 100.0

Unit H H
Sample no. 2 6B
Rock type Basaltic Basaltic
 trachy- trachy-
 andes. andes.

Plagioclase
 phenocrysts 0.0 * 0.0 *
Pyroxene
 phenocrysts 0.0 * 0.0 *
Olivine
 phenocrysts 0.0 * 0.0 *
Opaque
 phenocrysts 0.0 * 0.0 *
Plagioclase
 groundmass 59.1 56.1
Pyroxene
 groundmass 11.6 13.2
Olivine
 groundmass 3.1 3.2
Opaque
 groundmass 10.2 5.8
Apatite 2.5 1.5
Smectite 7.8 6.7
Sodic Cpx 0.0 0.0
Other 5.4 13.5

 100.0 100.0

Unit A A A A B B B
Sample no. 31 73 145 167 111 119 121
Rock type Basalt Basalt Basalt Basalt Basalt Basalt Basalt


Tr. * <0.1



Other includes:
Calcite 1.5
Glass Tr. *
Analcite Tr.? Tr. *
Zeolite
Unidentified 0.4 1.4 1.8

Unit C C C D D D
Sample no. 92 112 118 A B 8a
Rock type Basalt Basalt Basalt Trachy- Trachy- Trachy-
 andesite andesite andesite

Tr. * <0.1



Other includes:
Calcite
Glass 4.3 3.6 Tr. *
Analcite 0.9 Tr. * 0.4
Zeolite
Unidentified 0.1 0.5

Unit E1 E2 E3 F F F
Sample no. 154 187 151 14 147 193
Rock type Basalt Basalt Basalt Basalt Basalt Basalt


Tr. * <0.1



Other includes:
Calcite Tr. * 0.4
Glass
Analcite 2.0
Zeolite
Unidentified 0.2 1.2 0.9

Unit G H H
Sample no. 96 2 6B
Rock type Basalt Basaltic Basaltic
 trachy- trachy-
 andes. andes.
Tr. * <0.1 (Unit
 mostly (Tr. * (Tr. *
 in patches) Microph.) Microph.)

Other includes:
Calcite
Glass 5.1 13.1
Analcite
Zeolite 1.3
Unidentified 0.3 0.3 0.4

Table 3

Chemical analyses of representative samples of Units A-H.


Unit A A
Sample 31-RS/MC 73-RS
Latitude 39[degress]09.3' 29[degress]02.1'
Longitude 103[degress]09.3' 103[degress]23.3'
Lithology Basalt Basalt

[SiO.sub.2]2 46.74 46.98
[TjO.sub.2]2 2.47 2.35
[A1.sub.2][O.sub.3] 18.80 19.58
[Fe.su.2][O.sub.3] 11.67 11.27
MnO 0.17 0.16
MgO 4.17 3.60
CaO 11.29 11.15
[Na.sub.2]O 3.24 3.39
[K.sub.2]O 1.07 0.96
[P.sub.2][O.sub.5] 0.50 0.56
Total 100.00 100.00


Unit A A
Sample 97-ML 137-RS
Latitude 29[degress]13' 29[degress]10.0'
Longitude 103[degress]29' 103[degress]29.8'
Lithology Basalt Basalt

[SiO.sub.2]2 46.57 46.15
[TjO.sub.2]2 2.39 2.56
[A1.sub.2][O.sub.3] 18.77 19.00
[Fe.su.2][O.sub.3] 12.15 11.54
MnO 0.19 0.15
MgO 3.82 4.24
CaO 11.31 12.08
[Na.sub.2]O 3.38 2.76
[K.sub.2]O 1.02 1.01
[P.sub.2][O.sub.5] 0.41 0.50
Total 100.00 100.00


Unit A A
Sample 145-RS/MC 167-RS/MC
Latitude 29[degress]13.2' 29[degress]15.4'
Longitude 103[degress]13.2' 103[degress]26.9'
Lithology Basalt Basalt

[SiO.sub.2]2 46.17 46.09
[TjO.sub.2]2 2.49 2.49
[A1.sub.2][O.sub.3] 18.66 18.75
[Fe.su.2][O.sub.3] 11.76 11.53
MnO 0.18 0.16
MgO 5.11 5.52
CaO 11.52 11.43
[Na.sub.2]O 2.65 2.56
[K.sub.2]O 1.07 1.04
[P.sub.2][O.sub.5] 0.51 0.51
Total 100.00 100.00


Unit A B
Sample 218-SG 111-RS
Latitude 29[degress]05.6' 29[degress]05.0'
Longitude 103[degress]22.6' 103[degress]16.0'
Lithology Basalt Basalt

[SiO.sub.2]2 48.06 46.66
[TjO.sub.2]2 2.39 3.07
[A1.sub.2][O.sub.3] 19.24 16.08
[Fe.su.2][O.sub.3] 10.75 13.35
MnO 0.14 0.22
MgO 4.16 4.88
CaO 11.09 10.83
[Na.sub.2]O 2.73 2.75
[K.sub.2]O 0.98 1.33
[P.sub.2][O.sub.5] 0.47 0.83
Total 100.00 100.00


Unit B B
Sample 119-RS/MC 121-Rs/MC
Latitude 29[degress]04.0' 29[degress]09.8'
Longitude 103[degress]22.6' 103[degress]27.2'
Lithology Basalt Basalt

[SiO.sub.2]2 47.66 48.15
[TjO.sub.2]2 3.52 3.18
[A1.sub.2][O.sub.3] 15.32 16.91
[Fe.su.2][O.sub.3] 13.37 11.42
MnO 0.28 0.29
MgO 4.22 3.12
CaO 9.82 11.20
[Na.sub.2]O 3.37 3.34
[K.sub.2]O 1.62 1.61
[P.sub.2][O.sub.5] 0.98 0.93
Total 100.00 100.00


Unit B B
Sample 192-RS/MC 216-SG
Latitude 29[degres]10.6' 29[degress]04.7'
Longitude 103[degress]26.8' 103.[degress]22.4'
Lithology Basalt Basalt

[SiO.sub.2]2 46.95 47.48
[TjO.sub.2]2 3.18 3.00
[A1.sub.2][O.sub.3] 16.42 16.75
[Fe.su.2][O.sub.3] 13.11 12.21
MnO 0.57 0.21
MgO 3.91 4.81
CaO 10.66 10.22
[Na.sub.2]O 3.10 3.20
[K.sub.2]O 1.51 1.34
[P.sub.2][O.sub.5] 0.83 0.77
Total 100.00 100.00


Unit B C
Sample VCK82TX029 92-RS
Latitude 29[degress]20.1' 29[degress]02.1'
Longitude 103.[degress]22.4' 103[degress]23.4'
Lithology Basalt Basalt

[SiO.sub.2]2 48.97 43.93
[TjO.sub.2]2 3.01 4.25
[A1.sub.2][O.sub.3] 17.37 14.47
[Fe.su.2][O.sub.3] 11.13 14.91
MnO 0.18 0.21
MgO 3.06 4.90
CaO 7.91 10.80
[Na.sub.2]O 4.75 3.82
[K.sub.2]O 2.55 1.14
[P.sub.2][O.sub.5] 1.07 1.56
Total 100.00 100.00


Unit C C
Sample 112-RS 118-RS/MC
Latitude 29[degress]05.9' 29[degress]07.1'
Longitude 103[degress]16.1' 103[degress]23.5'
Lithology Basalt Basalt

[SiO.sub.2]2 44.11 44.45
[TjO.sub.2]2 4.28 4.26
[A1.sub.2][O.sub.3] 14.43 14.57
[Fe.su.2][O.sub.3] 14.97 14.80
MnO 0.22 0.22
MgO 4.91 4.99
CaO 11.13 10.86
[Na.sub.2]O 3.38 3.49
[K.sub.2]O 1.03 1.02
[P.sub.2][O.sub.5] 1.56 1.48
Total 100.00 100.00


Unit C C
Sample 217-SG 219-Sg
Latitude 29[degress]05.6' 29[degress]05.6'
Longitude 103[degress]22.6' 103[degress]22.6'
Lithology Basalt Basalt

[SiO.sub.2]2 44.48 45.04
[TjO.sub.2]2 4.40 4.30
[A1.sub.2][O.sub.3] 14.51 15.10
[Fe.su.2][O.sub.3] 14.82 14.33
MnO 0.21 0.21
MgO 4.95 4.79
CaO 10.66 10.66
[Na.sub.2]O 3.39 3.11
[K.sub.2]O 1.12 1.08
[P.sub.2][O.sub.5] 1.46 1.36
Total 100.00 100.00


Unit D D
Sample A-RS/MC B-RS/MC
Latitude 29[degress]09.8' 29[degress]09.8'
Longitude 103[degress]26.7' 103[degress]26.7'
Lithology Trachyandes Trachyandes

[SiO.sub.2]2 55.69 54.11
[TjO.sub.2]2 1.94 2.02
[A1.sub.2][O.sub.3] 17.47 17.11
[Fe.su.2][O.sub.3] 8.27 9.27
MnO 0.22 0.19
MgO 2.11 2.91
CaO 4.58 4.82
[Na.sub.2]O 5.67 5.37
[K.sub.2]O 3.48 3.46
[P.sub.2][O.sub.5] 0.71 0.86
Total 100.00 100.00


Unit D D
Sample 559-KC/MC 8a-RS
Latitude 29[degress]09.8' 29[degress]08.6'
Longitude 103[degress]30.4' 103[degress]30.4'
Lithology Trachyandes Trachyandes

[SiO.sub.2]2 55.41 55.92
[TjO.sub.2]2 1.92 2.03
[A1.sub.2][O.sub.3] 17.65 17.04
[Fe.su.2][O.sub.3] 8.55 8.69
MnO 0.22 0.19
MgO 2.12 2.13
CaO 4.82 4.16
[Na.sub.2]O 5.51 5.13
[K.sub.2]O 3.22 3.97
[P.sub.2][O.sub.5] 0.73 0.74
Total 100.00 100.00


Unit D D
Sample 76a-RS/MC 116-RS
Latitude 29[degress]02.5' 29[degress]08.0'
Longitude 103[degress]23.4' 103[degress]23.3'
Lithology Trachyandes Trachyandes

[SiO.sub.2]2 55.08 54.72
[TjO.sub.2]2 2.01 2.06
[A1.sub.2][O.sub.3] 17.49 17.75
[Fe.su.2][O.sub.3] 8.54 8.78
MnO 0.23 0.21
MgO 2.33 2.27
CaO 4.68 4.16
[Na.sub.2]O 5.50 5.77
[K.sub.2]O 3.51 3.48
[P.sub.2][O.sub.5] 0.76 0.81
Total 100.00 100.00


Unit D D
Sample 123-Rs/MC 194-RS/MC
Latitude 29[degress]07.7' 29[degress]10.6'
Longitude 103[degress]26.2' 103[degress]27.2'
Lithology Trachyandes Trachyandes

[SiO.sub.2]2 56.73 56.73
[TjO.sub.2]2 2.07 2.07
[A1.sub.2][O.sub.3] 17.63 17.63
[Fe.su.2][O.sub.3] 7.56 7.56
MnO 0.13 0.13
MgO 2.30 2.30
CaO 3.24 3.24
[Na.sub.2]O 5.29 5.29
[K.sub.2]O 4.41 4.41
[P.sub.2][O.sub.5] 0.74 0.74
Total 100.00 100.00


Unit D E
Sample 220-SG 151-RS/MC
Latitude 29[degress]05.7' 29[degress]05.7'
Longitude 103[degress]22.7' 103[degress]35.7'
Lithology Trachyandes Basalt

[SiO.sub.2]2 56.40 45.59
[TjO.sub.2]2 1.79 3.47
[A1.sub.2][O.sub.3] 17.98 17.19
[Fe.su.2][O.sub.3] 7.32 12.48
MnO 0.19 0.17
MgO 1.66 4.76
CaO 4.56 11.49
[Na.sub.2]O 5.92 2.77
[K.sub.2]O 3.46 1.10
[P.sub.2][O.sub.5] 0.73 1.21
Total 100.00 100.00


Unit E E
Sample 154-RS/MC 161-RS
Latitude 29[degress]12.1' 29[degress]11.6'
Longitude 103[degress]35.8' 103[degress]35.6'
Lithology Basalt Basalt

[SiO.sub.2]2 44.50 45.65
[TjO.sub.2]2 4.18 4.22
[A1.sub.2][O.sub.3] 14.51 14.85
[Fe.su.2][O.sub.3] 14.39 13.66
MnO 0.21 0.21
MgO 5.34 4.74
CaO 10.42 10.41
[Na.sub.2]O 3.32 3.16
[K.sub.2]O 1.41 1.40
[P.sub.2][O.sub.5] 1.80 1.82
Total 100.00 100.00

 E E
Unit 162-RS/MC 187-RS/MC
Sample 29[degrees] 11.2' 29[degrees]21.9'
Latitude 103[degrees] 35.5' 103[degrees] 27.8'
Longitude Basalt Basalt
Lithology

[SiO.sub.2]2 45.45 45.23
[TjO.sub.2]2 3.88 3.99
[A1.sub.2][O.sub.3] 16.08 15.96
[Fe.su.2][O.sub.3] 12.88 12.87
MnO 0.18 0.37
MgO 5.01 4.99
CaO 10.96 11.01
[Na.sub.2]O 2.90 2.88
[K.sub.2]O 1.23 1.26
[P.sub.2][O.sub.5] 1.53 1.59
Total 100.00 100.00

 E E
Unit 188-RS DMI-SG
Sample 29[degrees] 21.9' 29[degrees]21.4'
Latitude 103[degrees] 27.8' 103[degrees]28.0'
Longitude Basalt Basalt
Lithology

[SiO.sub.2]2 45.16 47.28
[TjO.sub.2]2 3.58 3.44
[A1.sub.2][O.sub.3] 17.33 17.98
[Fe.su.2][O.sub.3] 12.51 10.44
MnO 0.18 0.20
MgO 4.61 4.15
CaO 10.27 11.19
[Na.sub.2]O 3.94 2.95
[K.sub.2]O 1.22 1.09
[P.sub.2][O.sub.5] 1.21 1.28
Total 100.00 100.00

 E F
Unit DM2-SG 14-RS/Mc
Sample 29[degrees]21.4' 29[degrees] 16.5'
Latitude 103[degrees]28.0' 103[degrees] 31.5'
Longitude Basalt Basalt
Lithology

[SiO.sub.2]2 47.03 45.16
[TjO.sub.2]2 3.59 3.74
[A1.sub.2][O.sub.3] 17.87 16.13
[Fe.su.2][O.sub.3] 10.83 15.24
MnO 0.25 0.20
MgO 3.91 5.11
CaO 11.40 9.77
[Na.sub.2]O 2.78 2.68
[K.sub.2]O 1.14 1.21
[P.sub.2][O.sub.5] 1.20 0.83
Total 100.00 100.00

 F F
Unit 103-ML 108-ML
Sample 29[degrees]08.6' 29[degrees]20.0'
Latitude 103[degrees]30.5' 103[degrees]27.3'
Longitude Basalt Basalt
Lithology

[SiO.sub.2]2 44.92 44.22
[TjO.sub.2]2 3.52 3.84
[A1.sub.2][O.sub.3] 15.21 16.51
[Fe.su.2][O.sub.3] 15.49 14.90
MnO 0.22 0.20
MgO 5.85 5.37
CaO 9.42 9.47
[Na.sub.2]O 3.32 3.20
[K.sub.2]O 1.29 1.46
[P.sub.2][O.sub.5] 0.76 0.83
Total 100.00 100.00

 F F
Unit 135-RS/MC 147-RS/MC
Sample 29[degrees]09.3' 29[degrees] 14.1'
Latitude 103[degrees]30.6' 103[degrees]30.7'
Longitude Basalt Basalt
Lithology

[SiO.sub.2]2 44.64 45.07
[TjO.sub.2]2 3.74 3.80
[A1.sub.2][O.sub.3] 15.84 15.82
[Fe.su.2][O.sub.3] 15.19 15.27
MnO 0.20 0.19
MgO 5.50 5.13
CaO 9.95 8.93
[Na.sub.2]O 2.97 3.85
[K.sub.2]O 1.27 1.20
[P.sub.2][O.sub.5] 0.83 0.83
Total 100.00 100.00

 F F
Unit 166-RS 174-RS/MC
Sample 29[degrees]11.5' 29[degrees]16.5'
Latitude 103[degrees]31.3' 103[degrees]32.2'
Longitude Basalt Basalt
Lithology

[SiO.sub.2]2 44.95 47.49
[TjO.sub.2]2 3.63 4.06
[A1.sub.2][O.sub.3] 15.84 17.53
[Fe.su.2][O.sub.3] 15.02 9.33
MnO 0.19 0.40
MgO 5.33 4.62
CaO 9.53 10.47
[Na.sub.2]O 3.32 3.41
[K.sub.2]O 1.37 2.01
[P.sub.2][O.sub.5] 0.82 0.91
Total 100.00 100.00

 F F
Unit 183-RS/MC 193-RS/MC
Sample 29[degrees]21.0' 29[degrees] 10.5'
Latitude 103[degrees]27.7' 103[degrees]27.1'
Longitude Basalt Basalt
Lithology

[SiO.sub.2]2 44.85 44.62
[TjO.sub.2]2 3.98 4.25
[A1.sub.2][O.sub.3] 16.66 14.99
[Fe.su.2][O.sub.3] 14.07 16.00
MnO 0.50 0.26
MgO 5.87 5.44
CaO 8.96 9.33
[Na.sub.2]O 2.43 2.74
[K.sub.2]O 2.12 1.46
[P.sub.2][O.sub.5] 0.86 0.95
Total 100.00 100.00

 F F
Unit 222-SG H86-92
Sample 29[degrees]05.5' 29[degrees]25.6'
Latitude 103[degrees]22.8' 103[degrees]33.9'
Longitude Basalt Basalt
Lithology

[SiO.sub.2]2 45.23 47.21
[TjO.sub.2]2 4.25 4.27
[A1.sub.2][O.sub.3] 15.36 14.03
[Fe.su.2][O.sub.3] 15.15 15.10
MnO 0.20 0.19
MgO 5.08 4.54
CaO 8.13 8.58
[Na.sub.2]O 4.31 3.58
[K.sub.2]O 1.33 1.69
[P.sub.2][O.sub.5] 0.96 0.81
Total 100.00 100.00

 G G
Unit 94-ML 96-RS/MC
Sample 29[degrees] 12.7' 29[degrees] 12.8'
Latitude 103[degrees] 37.5' 103[degrees]
Longitude Basalt Basalt
Lithology

[SiO.sub.2]2 48.08 48.38
[TjO.sub.2]2 2.34 2.41
[A1.sub.2][O.sub.3] 16.21 16.72
[Fe.su.2][O.sub.3] 12.34 12.29
MnO 0.20 0.18
MgO 5.84 6.12
CaO 8.68 9.04
[Na.sub.2]O 4.62 3.47
[K.sub.2]O 1.28 1.05
[P.sub.2][O.sub.5] 0.41 0.47
Total 100.00 100.00

 G H
Unit 171-RS/MC Laj.Mes.-1
Sample 29[degrees] 13.6' 29[degrees] 16.5'
Latitude 103[degrees]37.8' 103[degrees] 46.1'
Longitude Basalt Basalt
Lithology Trachyandes

[SiO.sub.2]2 48.46 52.63
[TjO.sub.2]2 2.39 2.50
[A1.sub.2][O.sub.3] 16.91 14.92
[Fe.su.2][O.sub.3] 12.22 12.24
MnO 0.17 0.15
MgO 6.13 2.24
CaO 9.00 7.37
[Na.sub.2]O 3.28 4.42
[K.sub.2]O 1.08 2.37
[P.sub.2][O.sub.5] 0.45 1.16
Total 100.00 100.00

 H H
Unit Laj.Mes.-2 Laj.Mes.-3
Sample 29[degrees]16.5' 29[degrees] 16.5'
Latitude 103[degrees] 46.3' 103[degrees] 46.3'
Longitude Basalt Basaltic
Lithology Trachyandes Trachyandes

[SiO.sub.2]2 53.42 53.94
[TjO.sub.2]2 2.41 2.37
[A1.sub.2][O.sub.3] 15.19 15.84
[Fe.su.2][O.sub.3] 11.46 1120
MnO 0.19 0.15
MgO 3.00 1.37
CaO 6.57 7.00
[Na.sub.2]O 4.37 4.32
[K.sub.2]O 2.31 2.69
[P.sub.2][O.sub.5] 1.08 1.12
Total 100.00 100.00

 H H
Unit Laj.Mes.-66 H91-62
Sample 29[degrees] 16.5' 29[degrees] 16.6'
Latitude 103[degrees] 46.5' 103[degrees] 46.2'
Longitude Basaltic Basaltic
Lithology Trachyandes Trachyandes

[SiO.sub.2]2 53.77 53.45
[TjO.sub.2]2 2.54 2.53
[A1.sub.2][O.sub.3] 14.70 15.07
[Fe.su.2][O.sub.3] 11.41 11.45
MnO 0.18 0.21
MgO 2.76 2.59
CaO 7.08 6.70
[Na.sub.2]O 4.13 4.53
[K.sub.2]O 2.17 2.31
[P.sub.2][O.sub.5] 1.26 1.16
Total 100.00 100.00

RS- XRF analysis by X-ray Assay Laboratories Ltd. Ontario, Canada

MC-XRF analysis by Maryellen Cameron @ School of Geol. & Gephys., Univ.
Okla, Norman, OK

KC-XRF analysis by Kennech Cameron @ Earth Sci. Bd. pf Studies, Univ.
Calif. @ Santa Cruz

SG-ICP analysis by Susan Gilbert, Geosc. Dept. University of Houston,
Houston, TX

Laj. Mes.-ICP analysis by Anne Mcguire (Maj. & Tr.) and Kegan Boyer
(REE), Geosc. Dept. University of Houston, Houston, TX

ML-Wet Chem. analysis, Maxwell et. al. (1967)

H86-92-Henry et. al. (1989)

H91-62-Henry & Davis (1996)

VCK82TX029-Laughlin, et. al., 1982

Table 4

Trace and REE elements of samples of Units A-H.

Unit A A A A A B B B B
Sample 31- 73-RS 145- 167- 218-SG 111-RS 119- 192- 216-SG
 RS/MC RS/MC RS/MC RS/MC RS/MC
(ppm)

Ti 14805 14086 14925 14925 14326 18402 21099 19061 17982
Y 24 24.6 24 23.4 25 31.1 37.7 35.5 34
Zr 227 232 191 200 195 290 260 276 253
La 25.82 26.6 25.15 25.43 26.23 33.15 39.18 39.07 34.79
Ce 55.82 56.81 54.61 55.02 55.04 72.41 86.14 83.59 73.89
Nd 30.23 31.51 29.44 30.26 29.7 39.46 48.61 48.02 41.28
Sm 6.43 6.34 5.99 6.27 6.07 7.87 10.2 10.06 8.506
Eu 2.08 2.23 1.91 1.94 2.14 2.12 3.2 3.25 2.82
Gd 5.68 5.64 5.46 5.46 5.71 7.11 9.03 9.18 8.118
Dy 4.53 4.59 4.35 4.48 4.52 5.67 7.28 7.08 6.339
Er 2.19 2.26 2.08 2.12 2.21 2.76 3.6 3.42 3.022
Yb 1.77 1.84 1.69 1.71 1.86 2.29 2.95 2.81 2.584
Lu 0.26 0.394

Unit C C C D D D D E
Sample 118- 217-SG 219-SG A/MFC- B/MFC- 559/KC- 220-SG 151-
 RS/MC RS/MC RS/MC RS/MC RS/MC
(ppm)

Ti 25535 26374 25775 11629 12108 11509 10729 20800
Y 39 40 38 41 43 41 43 29.4
Zr 217 206 199 534 542 534 503 170
La 43.35 43.54 42.36 65 64 62 64.36 38.13
Ce 97.8 96.47 92.59 130 129 133.4 126.9 84.22
Nd 57.98 58.19 55.62 59 56 59.22 60.37 47.42
Sm 12.38 11.99 11.58 11.14 11.37 9.81
Eu 3.96 4.08 3.75 3.49 3.84 3.08
Gd 10.7 11.06 10.64 9.117 9.98 8.05
Dy 7.85 7.769 7.421 8.148 8.02 5.8
Er 3.53 3.376 3.277 4.217 4.08 2.53
Yb 2.54 2.627 2.523 3.54 3.741 2
Lu 0.3726 0.3805 0.5681

Unit E E E F F F F F
Sample 188-RS DM/1-SG DM/S-SG 14- 166-RS 183- 193- 222-SG
 RS/MC RS/MC RS/MC
(ppm)

Ti 21459 20620 21519 22418 21759 23857 25475 25475
Y 32 33 26 28 29 36 34
Zr 196 197 216 209 213 281 262
La 39.99 39.06 39.48 54 33.23 36.59 39.43 36.53
Ce 87.7 84.16 85.39 112 72.91 78.76 85.65 77.25
Nd 50.74 47.97 50.25 44 39.73 43.85 47.59 43.76
Sm 10.75 9.68 10.07 7.97 8.98 10.11 8.904
Eu 3.6 3.19 3.59 2.31 3.05 3.25 2.87
Gd 9 8.896 9.033 6.86 8.03 9.1 8.349
Dy 6.52 6.166 6.319 5.24 6.16 7.28 6.272
Er 2.8 2.623 2.741 2.47 2.78 3.46 2.985
Yb 2.19 2.065 2.149 2.01 2.32 2.74 2.477
Lu 0.2985 0.2985 0.3726

Unit G G H H H H H
Sample 96- 171- La Mes La Mes La Mes La Mes (H91.62)
 RS/MC RS/MC 1 2 3 6B
(ppm)

Ti 1446 14326 14985 14446 14206 15225 15165
Y 26 27 48.5 48.8 47.7 48 49
Zr 208 209 340 336 343 329 345
La 24 24.26 48.5 48.8 47.7 48 51
Ce 57 53.42 109.2 109.5 109.5 109.2 106
Nd 29.1 28.78 58.6 59.1 57.8 59.8 54.4
Sm 7.17 6.46 12.9 12.9 12.4 12.9 13.1
Eu 2.29 2.06 3.3 3.4 3.3 3.4 3.97
Gd 10.1 6.03 10.4 10.4 10.3 10.7 10.6
Dy 5.06 8.9 8.9 9 9.2 9.9
Er 2.54 4.2 4.3 4.3 4.4 4.92
Yb 2.06 2.26 3.6 3.6 3.5 3.6 3.84
Lu 0.35 0.58


ACKNOWLEDGMENTS

The manuscript was read by Christopher Henry, Frank Dickson, Maryellen Cameron and Kenneth Cameron, whose observations and suggestions materially improved the final result.

LITERATURE CITED

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Danyushevsky, L. V. 2001. The effect of small amounts of [H.sub.2]O on crystallization of mid-ocean ridge and backarc basin magmas. Jour. Volcan. Geoth. Res. I 10, No. 3-4:265-280.

Henry, C. D., F. W. McDowell, J. G. Price & R. C. Smyth. 1986. Compilation of potassium-argon ages of Tertiary igneous rocks Trans-Pecos Texas. Geol. Circ. 86-2a, Bur. Econ. Geol., Univ. of Texas at Austin, 34 pp.

Henry, C. D., 3. G. Price & D. E. Miser. 1989. Geology & Tertiary igneous activity of the Hen Egg Mountain and Christmas Mountains Quadrangles, Big Bend Region, Trans-Pecos Texas. Rept. of Investig. 183, Bur. Econ. Geol., Univ. of Texas at Austin, 105 pp.

Henry, C. D., J. G. Price & E. W. James. 1991. Mid-Cenozoic stress evolution and magmatism in the southern cordillera, Texas and Mexico:Transition from continental arc to intraplate extension. Jour. Geoph. Res., 96(B8)L13,545-13,560.

Henry, C. D. & L. L. Davis. 1996. Tertiary volcanic, volcaniclastic, and intrusive rocks adjacent to the Solitario: in Geology of the Solitario Dome, Trans-Pecos Texas: Paleozoic, Mesozoic, and Cenozoic sedimentation, tectonism, and magmatism. Rept. of Investig. 240, Bur. Econ. Geol., Univ. of Texas at Austin: 81-105 & Tb1. 5:182

Laughlin, A. W., V. C. Kress & M. 3. Aldrich. 1982. K-Ar ages of dike rocks, Big Bend National Park, Texas. Isochron/West, No. 35:17-18.

Lindholm, R. M. 1990. Regional correlation, age, provenance, and tectonic significance of sandstone-mudstone sequences in the Humber Arm allochthon, western Newfoundland, Canada. Unpubl. Master's Thesis, Univ. of Houston, Central Campus, 328 pp.

James, E. W. & C. D. Henry. 1991. Compositional changes in Trans-Pecos Texas magmatism coincident with Cenozoic stress alignment. Jour. Geoph. Res., 96(B8):13,561-13,575.

Maxwell, R. A., 1. T. Lonsdale, R. T. Hazzard & 3. A. Wilson. 1967. Geology of Big Bend National Park, Brewster County, Texas. Publ. 6711, Bur. Econ. Geol., Univ. of Texas at Austin, 320 pp.

Maxwell, R. A. 1968. The Big Bend of the Rio Grande: A guide to the rocks, landscape, geologic history and settlers of the area of Big Bend National Park. Guidebook 7, Bur. Econ. Geol., Univ. of Texas at Austin, 138 pp.

Meaux, D. P. 1989. Geology and geochemistry of subophitic volcanic rocks in the Humber Arm allochthon, western Newfoundland, Canada. Unpubl. Master's Thesis, Univ. of Houston, Central Campus, 369 pp.

Stewart, D. C. & C. P. Thornton. 1975. Andesite in oceanic regions. Geology, 3(10):565-568.

Stewart, R. M. 1984. Stratigraphy and petrology of the Alamo Creek Basalt, Big Bend National Park, Brewster County, Texas. Unpubl. Master's Thesis, Univ. of Houston, Central Campus, 186 pp.

Turner, F. 3. & J. Verhoogen. 1960. Igneous and Metamorphic Petrology, 2nd Ed., International Ser. in Earth Sciences, McGraw-Hill, 694 pp.

MFC at: jfcasey@uh.edu

Randall M. Stewart *

Department of Geosciences, University of Houston Houston, Texas 77204-5007 and * 1278 Creek Haven Circle, Reno, Nevada 89509
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