Compositional characterization of volcanic products from a primarily sedimentary record: The Oligocene Espinaso Formation, north-central New Mexico
DANIEL W. ERSKINE J- Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131 GARY A. SMITH
ABSTRACT Successive burial by younger extrusive rocks or uplift and erosion to plutonic levels generally inhibits detailed study of volcanic rocks rep- Direct evidence for the style and composition of volcanism associ- resenting the entire history of activity in an igneous province. Sedi- ated with magmatic centers of the Oligocene Ortiz porphyry belt, New ments derived from volcanic highlands commonly are well preserved; Mexico, has been largely removed by erosion. Careful study of volcan- however, this is especially true where they have been deposited in iclastic sediments of the coeval Espinaso Formation provides an indirect topographically low areas, which are less susceptible to erosion, and record of the composition of erupted materials and mode of eruptive where they are distal to the magmatic centers, and thus not subject to activity associated with the Cerrillos Hills and Ortiz Mountains eruptive obliterative effects of later intrusions. These sedimentary records, centers. Petrographic and major-element chemical analyses of conglom- with or without distal volcanic facies, may be critical to interpreting erate clasts fromnea r Cerrillos and at Espinaso Ridge confirm the pro- the evolution of volcanic terranes, because volcanoes themselves are gression from calc-alkaline to alkaline magmatism, as suggested by ear- seldom preserved or exposed (Smith and others, 1987, 1988). lier work on Ortiz intrusive rocks. Based on modal mineralogy, three Reconstructing the evolution of a volcanic region from a pre- distinct stratigraphic intervals are recognized at the Cerrillos locations. dominantly sedimentary record should include establishing the loca- The lowest is dominated by a phenocryst assemblage of hornblende + tion of volcanic edifices, the nature of eruptive products, the struc- clinopyroxene + plagioclase; in the second interval, clinopyroxene is tural setting and evolution of the volcanic terrane, and the almost completely absent. The stratigraphically highest interval con- composition of erupted materials (Smith and others, 1987, 1988). tains variable amounts of clinopyroxene with repeated trends from Studies by Parker and others (1988), Larue and Sampayo (1990), and hornblende-rich to clinopyroxene-rich compositions. Similar horn- Ingersoll and Cavazza (1991) stand as examples of the usefulness of blende-to-clinopyroxene trends can be seen in the lower part of the petrographic and geochemical investigations of volcaniclastic sedi- section at Espinaso Ridge, but these rocks are compositionally distinct ments to elucidate the general characteristics and tectonic setting of from those near Cerrillos. At a stratigraphic break high in the section, volcanic source areas. To date, however, there have not been any interpreted to be the calc-alkaline-to-alkaline transition, hornblende + attempts to trace temporal changes in the eruptive style and compo- clinopyroxene gives way to biotite + clinopyroxene. sition of erupted products from individual volcanic centers on the Msyor-element data on clasts fromEspinas o Ridge suggest that these basis of detailed stratigraphic and petrological study of derivative vol- clasts can be related by fractionation of plagioclase ± hornblende and/or caniclastic aprons. clinopyroxene and are consistent with the hypothesis that Espinaso Ridge This paper presents the results of a petrographic and geochem- material was derived from the Ortiz eruptive center. In contrast, Cer- ical study of a well-exposed volcaniclastic sequence in order to char- rillos-area clasts generally have less silica than those of Espinaso Ridge, acterize the evolution of volcanic activity at two different eruptive compatible with the observation that the Cerrillos section contains less centers. The volcanic petrology is compared to that previously de- pyroclastic debris. Cerrillos data show the effects of fractionation by an scribed for plutonic rocks now exposed in the source areas. Fe-Ti phase and, possibly, plagioclase. These rocks were probably de- rived fromth e Cerrillos Hills eruptive center. Differences between com- GEOLOGIC SETTING positions of volcanics fromth e two eruptive centers may be related to the depths of emplacement of their subjacent magma chambers. The min- Stocks, laccoliths, sills, and dikes of the Ortiz porphyry belt eralogy of the reworked pyroclastic material suggests that explosive (Fig. 1) mark vents of a 40-km-long chain of eroded volcanoes that eruptions originated from a deeper magma body than the ones that pro- were active between about 36 to 26 Ma (Baldridge and others, 1980; vided lavas that are represented by conglomerate clasts. Kautz and others, 1981; Maynard and others, 1990). The Oligocene Espinaso Formation represents erosional remnants of extensive INTRODUCTION aprons of volcaniclastic debris that accumulated adjacent to active volcanic centers. Numerous geochronological studies have left some Reconstructing the volcanic history of ancient magmatic systems ambiguity on the age of the Espinaso Formation, but the most recent is typically hampered by incompletely exposed volcanic sequences. assessments (for example, Maynard and others, 1990) indicate that
Data Repository item 9325 contains additional material related to this article.
Geological Society of America Bulletin, v. 105, p. 1214-1222, 9 figs., 1 table, September 1993.
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figure 1. Location map of north-central New Mex- ico, showing the distribution of the Espinaso Formation with respect to the Ortiz Mountains and Cerrillos Hills intrusive centers (northern end of the Ortiz porphyry belt). A, B, and C are locations of measured sections east of Cerrillos. T is the location of the Arroyo del Tuerto section at Espinaso Ridge.
I I Espinaso Formation
|i|| Oligocene Intrusions 14
calc-alkaline magmatism occurred between 36 Ma and 34 Ma, fol- bedded than at Espinaso Ridge. Primary pyroclastic rocks are rare, lowed by alkaline magmatism between 30 Ma and 26 Ma. These iso- but several fine-grained tuffs containing accretionary lapilli are scat- topic dates imply a hiatus in magmatism between intrusive episodes. tered through the upper part of the section. One lava flow is exposed The modern level of exposure is shallower northward, so that vol- at the westernmost section (A). These sections of the Espinaso For- canic rocks are restricted to the northern part of the belt. Espinaso mation are unconformably overlain by Santa Fe Group or younger Formation sedimentary and minor volcanic rocks are the only pre- gravel veneers on a high-level geomorphic surface (Bachman, 1975). served record of Ortiz porphyry belt volcanism (Stearns, 1953; Kautz and others, 1981; Smith and others, 1991). At Espinaso Ridge, the Espinaso Formation is —400 m thick. METHODS Here, as at most locations, it rests conformably on the Eocene Gal- isteo Formation, and the contact between the two units is gradational Samples for petrographic and geochemical study were collected (Stearns, 1953; Gorham and Ingersoll, 1979; Kautz and others, 1981). from three measured and correlated sections east of the town of Cer- Galisteo clastic rocks were derived from Laramide (early Tertiary) rillos (Fig. 1) and from the type section of the Espinaso Formation orogenic highlands, and the conformable relationship indicates con- along Arroyo del Tuerto at Espinaso Ridge (Fig. 1). tinuity of deposition in Laramide basins during the initial stages of Two types of samples were collected for petrographic and mid-Tertiaiy volcanism (Gorham and Ingersoll, 1979; Smith and oth- geochemical study. Highest priority was given to the few primary ers, 1991). The Espinaso Ridge section of the Espinaso Formation is volcanic units. These include lava flows, which are a very minor com- dominated by well-bedded fluvial sandstones and conglomerates, al- ponent of the Espinaso Formation, and pumice and juvenile-lithic though in some parts, particularly in the middle of the section, debris- clasts from pyroclastic-flow deposits. Most samples, however, are flow deposits are abundant. Smith and others (1991) interpreted the conglomerate clasts from sedimentary deposits. Because Espinaso Espinaso Ridge exposure to represent a braid plain draining the Ortiz strata at any horizon include clasts of more or less synchronously Mountains eruptive center. This locality also contains three non- erupted rock types as well as older materials, sampling was designed welded ignimbrites, three vitric ash-and-pumice-fall deposits, one to emphasize collection of newly erupted material in order to retain block-and-ash-flow deposit, and one lava flow. The Espinaso Forma- the stratigraphic and temporal significance of compositional varia- tion is unconformably overlain by the early Miocene and younger tions. This objective was accomplished by collecting samples in the Santa Fe Group sedimentary rocks and intercalated lavas (Baldridge following order of priority: and others, 1980; Kautz and others, 1981). (a) clasts that contain thermal-contraction joints, indicating in At measured sections east of the Cerrillos Hills eruptive center situ cooling from relatively high temperature (Fig. 2; Smith and oth- (Fig. 1), the Espinaso Formation forms an eastward-thinning and -fin- ers, 1991; Smith and Lowe, 1991); ing, debris-flow-dominated, alluvial-fan complex (Disbrow and Stoll, (b) clasts from monolithologic, or nearly monolithologic, debris- 1957; Smith and others, 1991). As at Espinaso Ridge, these rocks flow breccias whose restricted clast composition suggests derivation conformably overlie the Galisteo Formation with a gradational con- from recently generated volcanic debris (compare with Smith and tact. The section here, however, is much coarser and more poorly Lowe, 1991);
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tain hornblende or hornblende + clinopyroxene, whereas alkaline lithologies lack hornblende and contain clinopyroxene ± biotite. Calc-alkaline lithologies occur at both the Espinaso Ridge and Cer- rillos sites, whereas alkaline detritus is present only at Espinaso Ridge. The calc-alkaline group can be subdivided into three strati- graphically significant subgroups at Cerrillos based on relative abun- dances of ferromagnesian-silicate phenocrysts. Modal compositions are represented here by a "hornblende index," defined as the per- centage of hornblende in a sample divided by the combined percent- age of hornblende, augite, and biotite. This index permits easy dis- tinction of calc-alkaline and alkaline lithologies, and it also characterizes mineralogical variation within the calc-alkaline suite. Calc-alkaline compositions are highly porphyritic hornblende latites and augite-hornblende latites. Plagioclase phenocrysts and mi- crophenocrysts generally range from 20% to 40% of the sample vol- Figure 2. Prismaticaliy jointed Espinaso Formation clast in a de- ume, with hornblende and augite together accounting for another 5% bris-flow deposit at a location east of Cerrillos. Joints are a good indi- to 25%. Hornblende indices range from near 0 to 1.0. Larger augite cator that the clast cooled in situ from relatively high temperature. Joints phenocrysts display strong oscillatory zonation. Oscillatory zonation commonly form perpendicular to cooling surfaces, and, if cooling had in pyroxenes is common in calc-alkaline rocks at both locations, and occurred and joints had formed before emplacement, it is likely that the simple color zonation characterizes pyroxenes in the alkaline section clast would have disaggregated. at Espinaso Ridge. There is no significant variation in major-element composition across the zones and therefore must be a trace-element phenomenon. Biotite is rare and occurs only in pyroclastic-fall de- (c) most abundant clast lithology in polylithologic debris-flow or posits that, with one exception, are restricted to the lowest strati- sheetflood deposits that was not found in underlying unit and, hence, graphic interval. Based on uniformity of feldspar stain, the ground- interpreted to be recently introduced material. mass appears to be dominated by sanidine, although quartz and Samples of intrusive rocks from the Cerrillos Hills were also plagioclase are probably present. Sanidine also occurs as rare phe- collected and analyzed for comparison to the compositions of vol- nocrysts in stratigraphically high samples from the calc-alkaline part caniclastic material. We utilized compositional data in Coles (1990) to of the section. Nearly all clasts are holocrystalline and poorly vesic- characterize intrusive rocks at the Ortiz Mountains center. ular, suggesting derivation from lava flows or domes. The high phe- Seventy-two conglomerate clasts were point-counted, with 1,000 nocryst content (45%-50%) suggests viscous extrusion as domes points per thin section. Thin sections were stained for feldspar iden- (Marsh, 1981), which were then sources for block-and-ash flows and tification, and all mineral species were identified and texturally dis- avalanches of carapace debris that supplied lithic-pyroclastic and au- tinguished as phenocrysts (>0.5 mm in long dimension), microphe- toclastic fragments to the Espinaso volcaniclastic aprons. nocrysts (0.1-0.5 mm), and groundmass (<0.1 mm). Thirty-five Despite strong similarities between calc-alkaline rocks from the samples were analyzed for major-element composition by X-ray-flu- Cerrillos and Espinaso Ridge locations, there are some differences. orescence methods. Selection of samples for modal and geochemical For instance, Espinaso Ridge clasts contain between 0.2% and 3.0% analyses was based on degree of alteration and the order of preference Fe-Ti oxides as microphenocrysts. In contrast, Cerrillos samples con- given above for similar clasts. In addition, 48 sandstones were point- tain between 2.0% and 7.0%. Furthermore, there are significant tex- counted (300 points each) using the Gazzi-Dickinson method (Inger- tural differences between hornblende phenocrysts at the two loca- soll and others, 1984) to evaluate the relationship between sandstone tions. Oxide-replaced hornblende cores and rims are nearly and conglomerate-clast compositions. Point count data for sand- ubiquitous in Cerrillos samples (Fig. 3a). This texture is less common stones and conglomerate clasts can be found in Data Repository on Espinaso Ridge clasts, where hornblende is commonly rimmed by item 9325. optically continuous augite (Fig. 3b). Probable cognate inclusions Finally, microprobe analyses of pyroxenes in three samples were (glomeroporphyritic aggregates of clinopyroxene and clinopyroxene obtained using a JEOL 733 Superprobe in order to evaluate distinctive plus plagioclase) are found in clasts from both sites. Xenoliths of features of pyroxenes in both the calc-alkaline and alkaline lithologies. Precambrian metamorphic rocks are locally abundant in clasts from the Cerrillos sections. RESULTS Alkaline clasts at Espinaso Ridge are moderately to highly por- phyritic augite and augite-biotite latites with a phenocryst assemblage Modal Petrographic Analyses of plagioclase, augite, and/or biotite in a sanidine-dominated ground- mass. Plagioclase phenocrysts account for between 19% and 36% of Volcanic clasts, lava flows, and pyroclastic units of the Espinaso the total volume of each sample. Augite phenocrysts are present in Formation range from basalt and basaltic andesite to trachyte in com- amounts ranging from 3% to 27%. These display a color zonation from position, but the bulk of samples from all locations are latites. Samples pale green cores to yellow-brown rims. Clinopyroxene phenocrysts in can be divided into two general groups on the basis of ferromagnesian- the alkaline section at Espinaso Ridge are notably pleochroic, in con- silicate mineralogy. Based on comparison with descriptions of con- trast to those in the calc-alkaline section, and are richer in titania sanguineous intrusive rocks (Disbrow and Stoll, 1957; Sun and Bald- (l%-2%) and alumina (3%-6%). Euhedral biotite plates up to 1 mm win, 1958; Atkinson, 1961; Maynard and others, 1990; Coles, 1990), across generally account for less than 3% of any sample. Hornblende and as confirmed by chemical analyses, calc-alkaline lithologies con- is rare and shows features (partially replaced by very fine intergrowth
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Figure 3. Photomicrographs of Espinaso Formation calc-alkaline clasts. (a) Typical calc-alkaline augite-hornblende latite from the Cerrillos section, showing Fe-Ti oxide cores and rims on hornblende. This is a texture that can be found at the Espinaso Ridge location, but it is almost ubiquitous at the Cerrillos locations, (b) Calc-alkaline augite-hornblende latite from the Espinaso Ridge location. In this case, the hornblende in the center of the field of view is rimmed by optically continuous augite.
of dark brown pleochroic material), indicating a reaction relationship fan sequence containing these clasts thins to the east (Fig. 4). The to the melt. Sanidine phenocrysts are seen in a few thin sections, and second interval contains only hornblende-rich clasts and also thins sphene occurs as phenocrysts in two samples. eastward. The third interval is characterized by a wide range in the Three distinct stratigraphie intervals are defined by vertical vari- hornblende index, and there is a crude suggestion of repeated trends ations in the hornblende indices at the Cerrillos sections (Fig. 4). Lat- from hornblende-rich to augite-rich compositions. Only this third in- eral correlation of these intervals corroborate correlations based on terval contains clasts with intermediate (0.4-0.7) hornblende indices. lithologie markers (Smith and others, 1991) that define alluvial-fan At Espinaso Ridge, the hornblende index varies greatly within depositional surfaces. Early-deposited clasts of interval I contain a the calc-alkaline clasts. There are no obvious compositional correla- mixture of hornblende-rich and augite-rich latites. The augite-rich tions between the Arroyo del Tuerto section and the Cerrillos sec- clasts are glomeroporphyritic and distinctive in outcrop. The alluvial- tions. Trends from hornblende-rich to augite-rich compositions re-
EAST OF CERRILLOS ESPINASO RIDGE Canada de la Cueva (B) Arroyo del Tuerto (T) Anaya Springs Canyon (A) Windmill Valley (C) Cerrillos Hills Figure 4. Modal on hornb!ende/(hornblende CO cm • •• OÄ 400 + clinopyroxene+biotite) S S versus stratigraphie height. Filled circles represent Alkaline 300 conglomerate clast litholo- Calc-alkaline gies; open squares are vol- • canic sandstones. Correla- .8 tion lines between units I, 200 n, and m east of Cerrillos •n are based on walking out distinctive marker hori- zons. Arrows represent in- 100 terpreted hornblende-to- pyroxene-dominant trends. .0 0 0.5 1.0 0 0.5 1.0 0 0.5 1.0 0 0.5 1.0 HORNBLENDE INDEX hb/(hb+cpx+biot)
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semble trends in interval III at Cerrillos, but Cerrillos clasts are lower Miocene Santa Fe Group sediments that unconformably overlie compositionally distinct, based on petrographic characteristics men- the Espinaso Formation at Espinaso Ridge (Fig. 4, above 395 m). The tioned above and on major-element chemistry presented in the next volcaniclastic component in sands farther north has been interpreted section. An abrupt decrease in the hornblende index above 300 m in to have been derived from the San Juan Mountains of Colorado (In- the Espinaso Ridge section records the transition from calc-alkaline, gersoll and others, 1990). At Espinaso Ridge, however, paleocurrent hornblende-augite latite clasts below, to alkaline augite-biotite latite directions from cross-bedding indicate an eastern source. Because clasts in the upper part of the section. there are no early Miocene volcanic centers of appropriate compo- Sandstone compositions are concordant with the clast compo- sition in this region, we assume that these sands, and volcanic pebbles sitions (Fig. 4). This correspondence is most striking for the Canada within them, were eroded from extinct Ortiz porphyry belt volcanoes. de la Cueva section, and the calc-alkaline part of the section at Es- Based on isotopic dating by Baldridge and others (1980), the hiatus pinaso Ridge, which are represented by the most data. A major dif- represented by the unconformity atop the Espinaso Formation was ference occurs, however, in the oxidation state of iron in hornblende about 1 m.y. in duration. The paucity of alkaline detritus above the in conglomerate clasts and sandstones. The hornblende in all but one unconformity suggests that most or all of the alkaline volcanic rocks of the clast samples is oxyhornblende (pleochroic in shades of red and in the Ortiz Mountains were eroded and transported elsewhere. When brown), indicating a high Fe3+:Fe2+ ratio. Oxyhornblende is also extensional subsidence initiated early Miocene deposition in this area, present in all sandstone samples from the calc-alkaline part of the detritus was being delivered from the older calc-alkaline volcanic section; however, 76% of the sandstones contain green hornblende, rocks. signifying a lower Fe3+ content. Green hornblende composes as much as 47% (average 20%) of the hornblende in these sandstones Petrogenetic Relationships among Calc-alkaline Rocks (Lotosky, 1991). Calc-alkaline pyroclastic-fall and pyroclastic-flow deposits contain 1%-15% hornblende, comparable to conglomerate Pearce-element ratio plots (Pearce, 1968, 1987; Russell and clasts, but 40%-100% (average 77%) of this hornblende is green (Lo- Nichols, 1988) are useful in evaluating the mineralogical and compo- tosky, 1991). sitional differences between Cerrillos and Espinaso Ridge calc-alka- line samples (there are insufficient data for alkaline lithologies to eval- M^jor-EIement Analyses uate their petrogenetic relationships). In this analysis, major elements of interest are normalized to a conserved element (that is, an element The compositional range for the Espinaso Formation (Table 1; that did not take part in the process that led to the diversity of a rock Fig. 5) appears to be representative of the magmas recorded by Coles suite) (Pearce, 1968, 1987; Russell and Nichols, 1988). Phosphorus (1990) in the Ortiz Mountains intrusives. Intrusive rocks at Cerrillos was adopted as the element least likely to be incorporated as an es- also have similar compositions. Espinaso Ridge calc-alkaline samples sential species into major mineral phases of this rock suite because are, overall, more siliceous than are Cerrillos area calc-alkaline sam- apatite was rare to non-existent in the examined thin sections. The ples, which may account, in part, for the greater abundance of pri- Cerrillos clast and intrusive samples show a striking linear trend in mary pyroclastic debris in the Espinaso Ridge section. Furthermore, Fe/P versus Ti/P (Fig. 6) (correlation coefficient = 0.96) which we the Espinaso Ridge rocks increase in total alkali content (generally 5% interpret as evidence that these samples have a strong genetic rela- to 8%) with increasing silica (between 60% and 64%) in contrast to tionship to one another (for example, products of the same magma Cerrillos rocks that maintain relatively constant total alkalies (gener- chamber). In contrast, Espinaso Ridge clasts and Ortiz intrusions do ally 6%-7%) across a range of silica values (from 52%-63%). not bear as strong a petrogenetic relationship to one another and are distinct from Cerrillos clasts (Fig. 6) in spite of the general composi- DISCUSSION tional similarity of rocks from these locations. The high correlation for Cerrillos area clasts suggests that these samples are related by addi- Stratigraphic Relationships tion or removal of a phase containing iron and titanium (that is, py- roxene, amphibole, and/or iron-titanium oxides). A Pearce plot of Clasts exhibit systematic vertical mineralogical variations that, in Mg/P against Ti/P (Fig. 7a), however, shows a poorer correlation the case of the Cerrillos sections, are laterally correlative. These non- (0.65), and so pyroxene and amphibole played a minor role in the random variations indicate that the sampling strategy successfully differentiation of these rocks compared to the iron-titanium oxides. defined meaningful stratigraphy, which can be used to interpret tem- This conclusion is consistent with the abundance of Fe-Ti oxides. poral variations in the petrology of erupted materials. Correlation of Cerrillos area clasts show a linear trend in a plot of plagioclase indices stratigraphic intervals on the basis of hornblende index corroborates [(2Na+Al)/P versus Si/P, Fig. 7b], but the correlation (0.89) is weaker lithostratigraphic correlations between sections near Cerrillos, which than that for Fe-Ti oxides, and so the role that plagioclase played in permit definition of the morphology of alluvial-fan surfaces within the the differentiation of these rocks is less clear. sedimentary sequence. The calc-alkaline clasts from Espinaso Ridge also are related to Clasts and sandstones reflecting alkaline and calc-alkaline one another by addition or removal of specific phases. Figure 8a is a sources occupy distinct stratigraphic intervals at Espinaso Ridge, Pearce plot of plagioclase indices for Espinaso Ridge calc-alkaline with very little mixing (Fig. 4). Furthermore, field studies (Smith and clasts. If the rocks are related only by addition or removal of plagi- others, 1991) show that the alkaline clasts occur above an erosion oclase, then the slope of the regression line would be one. If samples surface with at least 15 m of relief. These stratigraphic relationships are related only by addition or removal of an Fe-Mg silicate, then the confirm inferences drawn from geochronology in the Ortiz porphyry slope would be zero. The slope of this regression line is 0.65, with a belt that suggest temporal separation of intrusion of melts represent- correlation coefficient of 0.98, implying that these rocks are related by ing the two suites. addition or removal of plagioclase and an Fe-Mg silicate. Some, but Three hornblende-rich sandstones were examined from the not all, of the calc-alkaline samples from Coles (1990) also fit this line.
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TABLE 1. WHOLE-ROCK XRF ANALYSES TABLE 1. (Continued)
C.AT C.AT C.AT C.AT C.AT C.WMV C.WMV C.CH C.CH A.CH 17.1 25.1 29.1 33.1 59.1 71.25.2 12.1 1 2 3
Si02 59.16 60.02 60.96 52.44 61.04 SÍO2 58.67 54.74 60.11 62.82 47.36 TiOj 0.93 0.54 0.63 1.06 0.55 Ti02 0.65 0.70 0.59 0.49 1.13 16.40 AIA 16.71 17.90 17.03 17.06 Ai203 17.54 18.97 17.28 16.02 15.77 FeO* 6.42 5.26 5.54 9.27 4.78 FeO* 6.35 6.95 5.82 4.84 10.66 MnO 0.16 0.14 0.11 0.13 0.14 MnO 0.16 0.23 0.07 0.16 0.20 MgO 2.10 1.78 1.41 3.23 1.70 MgO 1.80 2.67 1.64 1.46 4.86 CaO 5.69 5.09 5.35 7.13 5.20 CaO 6.58 6.31 4.86 3.55 9.93 Na20 3.57 4.14 3.88 3.14 4.37 Na20 3.78 3.59 4.27 4.21 2.12 KjO 2.07 3.58 2.94 2.00 3.15 K20 2.61 3.81 2.23 3.99 3.97 LOI 0.81 2.23 0.89 1.67 0.95 LOI 0.94 2.04 2.16 0.96 1.18 PA 0.35 0.23 0.23 0.25 0.19 P2O5 0.27 0.28 0.19 0.27 0.45 Total 97.65 99.73 99.84 97.35 99.14 Total 99.35 100.29 99.23 98.76 97.63 C.AT C.AT C.AT C.AT C.AT 63.1 87.1 99.1 106.1 119.1 Note: letter preceding sample indicates petrographic affinity. C = calc-alkaline, A = alkaline, I = inclusion in calc-alkaline rocks at Espinaso Ridge. AT in sample name indicates the Espinaso Ridge location. CC, WMV, and ASC are Cerrillos location designations. CH samples are Cerrillos Si0 62.52 60.18 59.31 57.62 59.23 2 area intrusive rocks and lava flows. Ti02 0.56 0.58 0.79 0.83 0.80 AI2O3 16.70 17.37 16.05 16.77 17.29 FeO* 4.93 4.82 5.69 6.07 5.53 MnO 0.12 0.14 0.20 0.17 0.16 MgO 1.36 1.30 2.07 2.35 2.15 CaO 4.88 5.29 5.50 6.08 5.50 A Pearce-element plot to test whether pyroxene is the sole Fe-Mg Na20 4.43 4.60 3.67 3.66 4.23 K20 3.24 3.26 2.21 1.93 2.14 silicate involved is linear, but the slope is less than one (Fig. 8b), LOI 0.56 0.69 1.74 3.02 2.16 PA 0.18 0.18 0.31 0.28 0.25 indicating that pyroxene alone is not responsible for this trend. We Total 99.49 98.41 97.54 98.77 99.45 conclude that the Fe-Mg silicate involved is amphibole or amphi- 1.AT A.AT A.AT A.AT A.AT bole + pyroxene. 122.2 124.2 127.1 129.2 131.1 The differences between calc-alkaline clasts from the Cerrillos SÌQ2 48.41 54.47 47.52 55.46 60.22 and Espinaso Ridge areas may result from differences in temperature Ti02 2.15 0.90 2.99 1.20 0.55 A1203 13.93 18.53 14.68 17.63 16.71 and water pressure of the magma (Fig. 9). If a magma in the stability FeO* 10.99 5.95 11.01 6.20 5.32 MnO 0.20 0.21 0.20 0.18 0.14 field of hornblende experiences a large loss of water pressure, it will MgO 6.31 2.07 4.37 1.93 1.77 CaO 11.09 5.75 9.52 5.62 5.02 Na20 3.02 4.21 2.71 4.28' 3.98 K20 2.10 4.32 3.15 4.87 3.55 LOI 0.52 2.16 2.10 0.55 0.90 Ortiz Mtns. Intrusions P2Os 0.49 0.26 0.80 0.31 0.29 Total 99.21 98.83 99.06 98.24 98.45 C.CC C.CC C.CC C.CC C.CC 14.1 30.1 34.1 39.3 39.6
Si02 59.57 55.15 57.90 55.82 55.99 TiOj 0.53 0.78 0.70 0.81 0.79 AI2O3 18.65 17.85 17.50 17.29 17.39 FeO* 5.40 7.71 6.81 7.53 7.54 MnÓ 0.11 0.11 0.16 0.16 0.18 MgO 1.15 2.19 2.08 2.05 1.98 CaO 5.66 6.69 6.22 6.96 6.51 Na20 4.04 3.34 3.91 3.42 3.57 K20 2.73 2.59 2.49 2.69 3.05 LOI 0.98 2.45 1.07 1.03 0.36 P2O5 0.18 0.24 0.24 0.31 0.37 Total 99.01 99.10 99.08 98.07 97.73 c.cc c.cc C.CC C.WMV C.WMV 43.1 43.6 65.1 49.4.2 71.3.1
SÌO2 53.35 54.45 61.55 60.12 51.16 Ti02 0.94 0.87 0.62 0.62 1.01 MA 17.53 17.61 16.97 17.04 17.74 8.22 5.86 FeO* 9.12 5.92 9.22 -i , 1 i MnO 0.14 0.15 0.14 0.10 0.10 MgO 3.09 2.20 1.42 1.17 2.83 CaO 7.51 6.98 5.32 4.83 6.50 40 50 60 70 Na20 3.19 3.32 3.72 3.70 3.39 K20 1.93 2.56 2.90 2.84 2.94 LOI 3.26 1.54 1.33 2.08 3.34 Si02 P2O5 0.22 0.32 0.21 0.21 0.21 open symbols - calc-alkaline (hb+cpx) Total 100.28 98.22 100.03 98.63 98.44 C.WMV C.WMV C.ASC C.ASC C.ASC closed symbols - alkaline (cpx+biot) 17.11.1 72.4.1 18.1 20.1 70.1 Figure 5. Total alkalies plotted against silica. Outlined fields are Si02 53.90 58.46 54.88 57.02 53.23 from Coles' (1990) data from the Ortiz intrusive center. Squares rep- Ti02 0.83 0.64 0.83 0.67 0.78 AIA 18.41 17.33 18.04 18.11 19.22 resent Espinaso Ridge clasts, circles are Cerrillos clasts, triangles are FeO* 8.28 7.03 8.27 6.84 7.89 MnO 0.13 0.10 0.11 0.11 0.16 Cerrillos intrusions, and hexagons are other clasts and lava flows.Ope n MgO 2.14 1.85 2.33 1.81 1.88 CaO 6.59 5.83 5.74 5.77 6.08 symbols indicate rocks with calc-alkaline mineralogy; closed symbols, Na20 4.02 3.20 3.43 3.44 3.98 rocks with alkaline mineralogy. The "i" indicates a mafic inclusion of K20 3.01 3.27 2.96 2.90 3.06 LOI 1.50 1.57 3.39 2.69 1.89 a type seen in several clasts in an Espinaso Ridge block-and-ash-flow. P2O5 0.23 0.23 0.20 0.20 0.29 Total 99.04 99.51 100.17 99.56 98.45 The alkaline/subalkaline dividing line is from Macdonald and Katsura (1964) for Hawaiian tholeiitic and alkaline rocks.
Geological Society of America Bulletin, September 1993 1219
Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/105/9/1214/3381943/i0016-7606-105-9-1214.pdf by guest on 28 September 2021 ERSKINE AND SMITH
• Espinaso Ridge Calc-alkaline Clasts 50- Q Cerrillos Clasts /\ Cerrillos Calc-alkaline Intrusions Figure 8. (a) (2 Na + Ortiz Mountains Intrusions Al)/P versus Si/P for Es- 40 _ Q> pinaso Ridge calc-alkaline • conglomerate clasts. (b) Clinopyroxene plus plagi- oclase indices[(2Ca+3Na) pass out of the hornblende stability field and into the magnetite sta- bility field. A smaller pressure loss brings it out of the hornblende field and into the clinopyroxene field. Cerrillos clasts, commonly contain- ing hornblende rimmed by Fe-Ti oxides, may record the first path, whereas hornblende rimmed by clinopyroxene from Espinaso Ridge samples is consistent with the second. Pressure loss can occur in one of two ways: eruption of volatile-rich magma at the top of a chamber, 1220 Geological Society of America Bulletin, September 1993 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/105/9/1214/3381943/i0016-7606-105-9-1214.pdf by guest on 28 September 2021 VOLCANIC PRODUCTS FROM SEDIMENTARY RECORD, NEW MEXICO sandstones, in contrast to oxyhomblende in the conglomerate clasts, ences between the two locations may be associated with different requires separate consideration, however. Cather and Folk (1991) levels of emplacement of subjacent magma chambers at the two lo- noted the greater abundance of green hornblende in andesitic sand- cations. Sandstones were derived, in part, from pyroclastic deposits stones, relative to conglomerate clasts containing oxyhomblende and and suggest that the sparseness of pyroclastic-fall and -flow deposits green hornblende with oxide rims. Their preferred interpretation in the Espinaso Formation is due more to reworking into pyroclastic (Cather and Folk, 1991, p. 223 and Fig. 16) attributed the difference sand than to a paucity of explosive eruptions. The mineralogy of the to abrasion of oxide rims on hornblende during fluvial transport. This reworked pyroclastic material suggests that explosive eruptions orig- explanation is implausible for the Espinaso Formation because green inated from a deeper magma body than the ones that provided lavas hornblende is, with only one exception, completely absent from lithol- that are represented by conglomerate clasts. ogies represented by conglomerate clasts. Green hornblende, there- Study of preserved volcaniclastic sedimentary sequences can fore, must have a separate source from the lava flows and domes that yield valuable information relating to the composition and petrogen- supplied conglomerate clasts. The abundance of green hornblende in esis of erupted materials in cases where volcanic edifices have been the few preserved primary pyroclastic deposits in the Espinaso For- eroded. Major-element data can be valuable in unraveling the petro- mation suggests that it was derived from fluvial reworking of similar genetic history of an eruptive center if sampling can be designed to pyroclastic deposits. This interpretation is supported by the common insure that samples collected are representative of coeval magmatism. observation that slow cooling of lava flows and high-level intrusions Clasts containing cooling fractures and clasts from monolithologic leads to the development of oxyhomblende, and that green volcanic units are best for evaluation of stratigraphic, and thus temporal, vari- hornblendes are restricted to quenched pyroclastic deposits or, ations in composition. Strong evidence relating clast samples to co- rarely, lava-flow margins (Kuno, 1950; Rose, 1973; Ujike, 1974; Ruth- eval volcanism could justify the additional steps of collecting trace- erford, 1993). element and isotopic information from these samples. Trace elements The abundance of green hornblende is important for two rea- would make the Pearce-element analysis more rigorous, and isotopic sons. First, as a volumetrically significant component of the Espinaso information could more tightly constrain magmatic processes. sandstones, green hornblende implies that preserved pyroclastic-fall and -flow deposits greatly underrepresent the original volume of py- ACKNOWLEDGMENTS roclastic material erupted by Ortiz porphyry belt volcanos. About 10% of the green hornblendes in both sandstones and primaiy pyro- This work was largely supported by the donors to the Petroleum clastic deposits exhibit oxide rims, suggesting that the rims are not as Research Fund of the American Chemical Society. Lac Minerals, susceptible to transport-abrasion as inferred by Cather and Folk U.S.A., Incorporated provided additional analytical support. Steve (1991). Abundant green hornblende in volcaniclastic sandstones may Taylor and Dan Larsen assisted with sample collection and strati- generally reflect pyroclastic sources, even when preserved primary graphic studies. John Husler performed the X-ray fluorescence anal- pyroclastic deposits are volumetrically minor components of a vol- yses. Albert Kudo provided helpful microprobe mineral analyses. caniclastic sequence. Second, the presence of fresh green horn- blendes in Espinaso Formation pyroclastic deposits and sandstones requires eruption of melts in which hornblende was a stable phase. REFERENCES CITED These magmas are different from those represented by the lavas con- Atkinson, W. 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J., 1993, Experimental petrology applied to volcanic processes: EOS (American Geo- physical Union Transactions), v. 74, p. 49-55. Smith, G. A., and Lowe, D. R., 1991, Lahars: Volcano-hydrologic events and deposition in the debris flow-hyperconcentrated flow continuum, in Fisher, R. V., and Smith, G. A., eds., Sedimentation MANUSCRIPT RECEIVED BY THE Society September 28,1992 in volcanic settings: Society of Economic Paleontologists and Mineralogists, The Society for REVISED MANUSCRIPT RECEIVED FEBRUARY 10,1993 Sedimentaiy Geology, Special Publication 45, p. 59-70. MANUSCRIPT ACCEPTED FEBRUARY 25, 1993 Printed in U.S.A. 1222 Geological Society of America Bulletin, September 1993 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/105/9/1214/3381943/i0016-7606-105-9-1214.pdf by guest on 28 September 2021