Quick viewing(Text Mode)

Origin of the Trachyte-Quartz Trachyte-Peralkalic Rhyolite Suite of the Oligocene Paisano Volcano, Trans-Pecos Texas

Origin of the Trachyte-Quartz Trachyte-Peralkalic Rhyolite Suite of the Oligocene Paisano Volcano, Trans-Pecos Texas

Origin of the - trachyte-peralkalic suite of the Oligocene Paisano , Trans-Pecos Texas

DON F. PARKER Department of , Baylor University, Waco, Texas 76798

ABSTRACT nental rifting and suites from some oceanic islands. Alkalic rocks of the Paisano volcano may be related to mantle diapirism triggered Volcanic rocks of the Paisano volcano include trachyte, quartz by subduction processes. trachyte, and peralkalic rhyolite. rocks, hawaiite and mugearite, occur within units that stratigraphically underlie and INTRODUCTION overlie rocks of the volcano. Quartz trachyte, trachyte, and nephe- line trachyte occur as discordant plugs and dikes intruded into Mid-Cenozoic alkalic rocks of the Trans-Pecos magmatic strata of the volcano. province were emplaced in the foreland of voluminous calc-alkalic Central eruptions from swarms led to the formation of the volcanism in Mexico and the southwestern (Barker, shield complex of the volcano ~ 35 m.y. ago. The central dike com- 1977; McDowell and Clabaugh, 1979). This paper concerns the plex, a generalized eruptive sequence of rhyolite-quartz trachyte- petrologic evolution of one of the best-exposed centers of the trachyte within the eruptive products of the volcano, and the Trans-Pecos province, the Paisano volcano, which is located in the development of a 5-km-diameter suggest the presence southern Davis Mountains southwest of Alpine, Texas (Parker, beneath the volcano of one or more shallow plutonic bodies in 1976, 1979). Field, petrographic, whole-rock major- and trace- which differentiation may have occurred. element, and electron microprobe data are integrated to form a Fractionation calculations, using whole-rock analyses to petrogenetic model, which is used as a constraint upon various represent liquid compositions and electron-probe microanalyses of tectonic interpretations of the province. to represent compositions of fractionating phases, indicate -plus- control in the evolution of TERMINOLOGY trachyte from mugearite, and anorthoclase control in the evolution of rhyolite from trachyte. The fractionation model is sup- The classification system used in this report, summarized in ported by strong enrichment of Rb and Zr, and strong depletion of Table 1, is modified from Baker and others (1974). It utilizes the Sr in the series, and by a striking Eu anomaly in REE plots. A compositional gap between mugearite and trachyte, analogous to the "Daly Gap" of oceanic islands, disappears when oxides of major TABLE 1. CLASSIFICATION OF IGNEOUS ROCKS elements are plotted versus Zr. Zr, however, cannot be used as a strict index of fractionation because Zr concentrations were buf- (Differentiation Index* <30, normative plagioclase >An5o)t fered by the crystallization of with feldspar in the crucial HAWAIITE (D.I. 30-45, normative plagioclase Anjo-so) trachyte stage of magmatic evolution. In more advanced quartz MUGEARITE (D.I. 45-65, normative plagioclase< An30) BENMOREITE (D.I. 65-75) trachyte and rhyolite stages, the melts greatly increased in peralka- TRACHYTE (D.I. >75, normative quartz < 10%) linity and zircon did not crystallize. This effect produced Zr concen- QUARTZ TRACHYTE (D.I. >75, normative quartz 10% to 20%) trations as high as 2,500 ppm in highly fractionated rhyolite. RHYOLITE (D.I. >75, normative quartz»20%) Glomeroporphyritic clusters of feldspar, zoned from (peralkalic rhyolite)« ne TRACHYTE (-normative trachyte with ne< 10%) to calcic anorthoclase, along with , opaques, and, in some (nepheline-normative trachyte with ne> 10%) samples, olivine, are ubiquitous in mafic trachyte. These clusters have textures indicative of crystallization in intrusions. They sug- •Differentiation Index (D.I.) is the sum of normative percentages of gest a genetic relationship between mugearite and trachyte where quartz, , , nepheline, leucite, and kaliophilite (Thornton and residual trachytic liquids are segregated by filter pressing from crys- Tuttle, 1960). tallizing of over-all mugearitic composition in subvolcanic + Priority given to normative plagioclase composition when criteria do not coincide. chambers. ® All peralkalic rhyolite of the Paisano volcano is comendite, according Paisano igneous rocks closely resemble suites of volcanic rocks to the criteria of Macdonald and Bailey (1973). from Afro-Arabian central volcanoes associated with intraconti-

Supplementary data in the form of Tables A, B, C, and D may be secured free of charge by requesting Supplementary Material No. 83-6 from the Data Bank. Write Publications Secretary or call GSA Headquarters.

Geological Society of America Bulletin, v. 94, p. 614-629, 16 figs., 2 tables, May 1983.

614

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/94/5/614/3444647/i0016-7606-94-5-614.pdf by guest on 02 October 2021 PAISANO VOLCANO, TRANS-PECOS TEXAS 615

differentiation index of Thornton and Tuttle (1960) and normative line trachyte occurs in small plugs intruded into the volcanic strata. plagioclase content for classification of mafic rocks, and normative The nepheline trachyte intrusions are part of a regional system of quartz content for more rocks. It differs from the scheme silica-undersaturated intrusive rocks that strike diagonally north- proposed by Macdonald and Bailey (1973) primarily by using a 20% west-southeast across the Paisano area (Barker, 1977; Parker, normative quartz content rather than 10% normative quartz to 1976). On account of this regional distribution and because of the separate rhyolite from quartz trachyte. absence of nepheline-bearing or nepheline-normative from the Paisano strata, 1 believe that these rocks have no genetic relation to FIELD RELATIONS the silica-oversaturated rocks of the Paisano volcano and probably represent a separate line of liquid descent from mafic magma. Central eruptions led to the formation of the lava shield of the Estimation of volumes of different rock types within the Pai- Paisano volcano during the middle Oligocene, ~35 m.y. ago sano volcano is difficult because of the size of the volcano and its (Parker and McDowell, 1979). The shield measured about 30 x 20 degree of erosion, but it appears to be composed of about 15 km, elongate in a north-northwest direction, and had a total esti- volume percent rhyolite, 60 volume percent quartz trachyte, and 25 mated volume of -150 km3. The volcano provided a variety of volume percent trachyte. Although lava flows, erupted from a cen- eruptive products, ranging over the compositional spectrum from tral dike swarm, were the dominant product of the volcano, ash- peralkalic rhyolite (comendite) to mafic trachyte. Mafic rocks, flow , agglomeratic tuff, lahar deposits, and stream-channel hawaiite and mugearite flows, occur in stratigraphic units underly- deposits are also represented. A 5-km-diameter caldera developed ing and overlying the rock units assigned to the volcano, and nephe- late in the history of the volcano.

3CF22'3CR—

3CT15-

Figure 1. Geologic map of the Paisano area. LM = Lower Mafic Unit. Decie Formation members: RU = Rhyolite Unit; SI = Lower Shield Unit; S2 = Upper Shield Unit. U = Upper Mafic Unit. C = caldera collapse terrane. I = intrusions. Q = alluvium and colluvium. Heavy lines are faults with bars on downthrown sides, dotted where concealed. Thin lines are dikes. Geographic localities: P = Paisano Peak; T = Twin Sisters; L = Lizard Mountain; R = Ranger Peak; E = Paisano Baptist Encampment.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/94/5/614/3444647/i0016-7606-94-5-614.pdf by guest on 02 October 2021 616 D. F. PARKER

Lower Mafic Unit bedded yellow agglomeratic tuff that contains abundant angular fragments of spotted rhyolite from the RU. These may represent The Paisano volcano was constructed upon a package of mafic lahar deposits, or air-fall deposits close to vents. Most sections in lava flows, informally referred to as the Lower Mafic Unit (LM in the arc of outcrop southeast and east of Paisano Peak contain two Fig. 1) in this report. The upper part of the Lower Mafic Unit is thin ash-flow sheets. Large rock fragments, as much as 2 m in diam- exposed in the eastern portion of the Paisano area, where mafic eter, of rhyolite and dark brown trachyte occur within the tuff in are interlayered with a few trachyte flows. The Lower Mafic this outcrop belt. Away from the center of the Paisano area, inclu- Unit crops out over a large area of the southern Davis Mountains sions are smaller, and the tuff is less welded, suggesting that the tuff (Parker and McDowell, 1979); it is about 360 m thick in a section 10 was erupted from vent areas within the central outcrop area of the km south of Alpine, Texas (McAnulty, 1955). The mafic lavas are RU. The abundance of spotted rhyolite inclusions within the tuff largely hawaiite and mugearite, dated at 35.9 m.y. (Gilliland and supports this conclusion. Clark, 1979). The lava component of the SI unit is characterized by lava flows (as much as 100 m thick) of slightly quartz Decie Formation trachyte with alkali feldspar of variable but small size (1 to 2 mm diam). Platy fracturing, characteristic of all of the flows, The Decie Formation includes all volcanic strata believed to is subhorizontal in the central parts but steeper in upper parts of have been erupted from the Paisano volcano (Parker, 1976, 1979). flows. Thin basal-flow and thicker upper-flow breccias Previous workers have referred to the strata as the "Decie Member aided recognition of flow units. of the Duff Formation" (McAnulty, 1955; Council, 1972). I have The volume percent of phenocrysts within the flows generally raised the Decie to formational status because of its thickness, areal increases upward in the section, and laterally, as younger flows extent, and lithologic differences from the predominantly tuffa- overlapped older flows away from the center of the volcano. A few ceous Duff Formation. Three informal members of the Decie of the youngest flows in the SI unit have 25 volume percent alkali Formation, described below, record an evolution of the eruptive feldspar phenocrysts. products of the Paisano volcano from nearly aphyric peralkalic The eruption of SI lavas formed a lava shield that extended rhyolite (comendite) to highly porphyritic mafic trachyte. These northward into the Mitre Peak area (Gorski, 1970) and to the informal members are, in ascending order, the Rhyolite Unit, the southeast at least as far as Ranger Peak (R in Fig. 1). This shield Lower Shield Unit, and the Upper Shield Unit. Each of the shield was overlapped on the southwest by tuff and lava of the Upper units records an episode of violent pyroclastic activity, followed by Shield Unit. lava-flow eruptions. Upper Shield Unit. The Upper Shield Unit (S2 in Fig. 1) is Rhyolite Unit. The Rhyolite Unit (RU in Fig. 1) crops out in composed of markedly porphyritic tuff and lava. The tuff is proba- the central Paisano area and in several smaller, outlying areas bly of quartz trachytic composition, although no chemical analyses where the unit was upwarped by intrusions. The central area is were performed. The lava ranges in composition from quartz dominated by porcelaneous, white, nearly aphyric rhyolite with trachyte to mafic trachyte. The sequence of tuff followed by lava spongy groundmass sodic that gives the rock a blue- eruption, as in the SI unit, occurs in the S2 unit, although it is not spotted appearance; this is the paisanite of Osann (1896). The rhyo- as systematically developed. Bedded air-fall tuff, ash-flow tuff, and lite weathers to rounded, grass-covered hills; prominent ridges on stream-channel deposits occur between lava flows of the S2 unit— these hills are developed by differential erosion of the rhyolite and in places, abundantly so. dikes of the central swarm. Distal outcrops are characterized by The basal tuff component of the S2 unit ranges up to 100 m in flow-banded, brecciated rhyolite with sugary texture and, in places, thickness within sections 2 km southeast of Paisano Peak, where it bizarre weathering forms. These outlying outcrops contain appreci- consists almost entirely of slightly to densely welded, porphyritic able ash-flow tuff and agglomeratic tuff interlayered with rhyolite ash-flow tuff. The tuff is complex in its welding zonation and lava; the flows are about 50 m thick on the average. The base of the appears to represent many small eruptions. Inclusions as much as 1 RU is nowhere exposed; it is more than 200 m thick in the central m in diameter, of spotted rhyolite of the RU and ash-flow tuff of the area and is 144 m thick in an incomplete section 5.5 km southeast of SI unit, occur within the S2 tuff in these sections, indicating that Paisano Peak (P in Fig. 1). the S2 tuff was erupted through these older units or else picked up I interpret the RU to be a collection of volcanic domes, stubby boulders from the underlying surface during emplacement of the flows, agglomeratic tuff, and ash-flow tuff that accumulated above ash flows. and around vent areas centered north of Paisano Peak. Lava flows The lava component of the S2 unit has a maximum thickness pinched out laterally, their place in the section taken by bedded tuff of nearly 300 m in a section 2 km south of Paisano Peak. S2 lava and ash-flow tuff. The youngest flows are transitional in chemistry forms slabby, rugged outcrops, usually with hackly surfaces. Flows and appearance with platy brown quartz trachyte lava of the Lower are more numerous and thinner than in the SI unit. Many flows are Shield Unit. channel-shaped and overlie conglomerates; this suggests that they Lower Shield Unit. The Lower Shield Unit (SI in Fig. 1) is flowed down drainages developed on the flanks of the volcano. composed almost entirely of aphyric to sparsely porphyritic, peral- S2 lavas greatly resemble many of the dikes in the prominent kalic quartz trachyte tuff and lava. The lower part of the unit is swarms of the central vent area and probably were fed from these agglomeratic tuff and ash-flow tuff; it exceeds 100 m in thickness in dikes in fissure eruptions. The S2 lavas formed a new shield cen- several sections. The overlying lava-flow component has a maxi- tered a few kilometres to the southwest of the SI shield, overlapping mum thickness of 240 m in a section 1.5 km southeast of Paisano the older shield to a considerable degree. S2 lavas extend to the west Peak, where it consists of four flows. and southwest beyond the study area. The highly porphyritic lava The tuff component of the SI unit is characterized by crudely flows mapped as the Decie Member of the Duff Formation in the

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/94/5/614/3444647/i0016-7606-94-5-614.pdf by guest on 02 October 2021 PAISANO VOLCANO, TRANS-PECOS TEXAS 617

Cathedral Mountain quadrangle (McAnulty, 1955) are probably S2 bly controlled by fracture systems in Precambrian basement rocks lavas. S2 lavas also occur as erosional outliers on mesas in the Mitre (Parker, 1976). Peak area (Gorski, 1970). The plugs, largely trachyte, quartz trachyte, and nepheline trachyte, penetrated and locally deformed shield rocks. Because Caldera Collapse Terrane «e-normative lavas are absent from the Paisano volcano, the nephe- line trachyte presumably fed flows that have been removed by ero- A 5-km-diameter caldera developed in the vent area of the S2 sion or did not breach the surface. These silica-undersaturated unit during its eruption. The collapse terrane (C in Fig. 1) is cen- rocks may represent a line of liquid descent from mafic magma. tered 4 km southwest of Paisano Peak and is underlain by meg- abreccia (Lipman, 1976), agglomeratic tuff, lahar deposits, and PETROGRAPHY foundered shield rocks. The caldera subsided in trapdoor fashion; the largest displacement of 300 m occurred along its northeastern Petrographic descriptions of the principal rock types asso- flank. The timing of collapse is uncertain. The S1 unit in the terrane ciated with the Paisano volcano are given below. Brecciated and is more structurally disturbed than the S2 unit, suggesting that vitroclastic samples, other than their particular textures, are similar collapse began early during the eruption of the S2 unit and con- in phenocryst and groundmass to normal crystalline tinued through much of it. samples.

Upper Mafic Unit Basaltic Rocks

The Upper Mafic Unit (U in Fig. 1) occurs as remnants only a True are not present in the Paisano area and are rare in few metres thick in the southern Paisano area. It is composed of the entire Trans-Pecos province (Barker, 1977, 1979a). The basaltic several thin mugearite flows and at least one trachyte flow. Thicker rocks of the Lower and Upper Mafic Units are hawaiites and sections (several hundred metres thick) of mafic rocks that may mugearites, according to the classification scheme used in this correlate with the lava flows of the Upper Mafic Unit occur north report (Table 1). They are nearly aphyric, with microphenocrysts of and southeast of the Paisano area (Parker and McDowell, 1979). opaques, olivine, and set in'groundmasses composed of tra- Presumably, much of the Paisano area was mantled at one time chytoid plagioclase laths, interstitial clinopyroxene, opaques, and a with mafic lava flows that have since been removed by erosion. The variable amount of material. A few samples con- Upper Mafic Unit has not been dated by isotopic methods. tain sparse phenocrysts of plagioclase. Most samples are extensively altered and oxidized. Intrusions Trachyte Intrusive rocks are represented in the Paisano area by abun- dant dikes and several small, discordant plugs. Dikes are concen- Quartz-normative trachyte occurs in two varieties in the Pai- trated in a central swarm about 8 km in diameter (Fig. 1). These sano area: mafic two-feldspar trachyte and more-evolved one- dikes cut the RU and, to a lesser degree, shield rocks. The dikes feldspar trachyte. Two-feldspar trachyte occurs as holocrystalline, mirror the abundances and compositions of shield rocks and more rarely vitrophyric, strongly porphyritic rocks (10-40 volume represent their intrusive equivalents. Dike orientations were proba- percent phenocrysts) with a variable alkali feldspar/plagioclase ratio. Typically, fritted calcic anorthoclase mantles plagioclase cores. Pale green clinopyroxene, iddingsitized olivine, , ilmenite, and apatite occur as phenocrysts or microphenocrysts. All mafic minerals occur as discrete grains and in glomerocrystic aggre- gates with feldspar. These aggregates range as high as 10 cm in diameter and are extremely variable in texture. Typically, they are medium to fine grained with subhedral to anhedral interlocking grains of zoned feldspar (Fig. 2). Smaller mafic minerals, largely clinopyroxene and opaques, are concentrated along borders of grains, particularly at triple junctions of feldspar grains in glomero- crysts with mosaic texture, but they also occur inside feldspar. Groundmass textures of two-feldspar trachyte are variable from glassy to cryptocrystalline or crystalline. Crystalline groundmasses have stubby alkali feldspar laths, equant opaques, and pale brown clinopyroxene interstitial to alkali feldspar. A small amount of quartz was observed in the groundmass of several samples. One-feldspar trachyte lacks plagioclase, has a slightly lower color index than two-feldspar trachyte, and contains more quartz. Alkali feldspar is a ubiquitous phenocryst phase, occurring in Figure 2. Glomerocryst in mafic trachyte. Clear plagioclase amounts of 10 to 20 volume percent, usually partially in glomero- cores are mantled with fretted anorthoclase. Clinopyroxene magne- porphyritic clusters. Mafic microphenocrysts include pale green cli- tite, ilmenite, and apatite occur between feldspar grains (field of nopyroxene and opaques and, in some samples, olivine. Ground- view is 9 x 6 mm, plane light). masses contain stubby alkali feldspar laths, pale green or pale

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/94/5/614/3444647/i0016-7606-94-5-614.pdf by guest on 02 October 2021 618 D. F. PARKER

yellow clinopyroxene, and minor quartz interstitial to alkali feld- spar. One-feldspar trachyte and two-feldspar trachyte occur as lava flows in the Upper Shield Unit (S2) and in dikes.

Quartz Trachyte

Trachyte grades into quartz trachyte with increasing quartz in the groundmass. Quartz trachyte occurs as flows, dikes, and small plugs in the Paisano area. It is abundant in the Upper Shield Unit and makes up most of the Lower Shield Unit. S2 quartz trachyte is subalkalic; SI quartz trachyte is marginally peralkalic. Alkali feld- Figure 4. phenocryst and microphenocryst composi- spar is again the dominant phenocryst phase, but it is typically less tions plotted in terms of atomic percent Ca-Mg-(Fe2+ + Mn). Cir- abundant and smaller (1 -2 mm) in quartz trachyte than in trachyte. cles: pyroxene from trachyte and quartz trachyte. Squares: Microphenocrysts include pale green clinopyroxene, equant mag- nepheline trachyte. Dotted line separates analyses of one-feldspar netite, zircon, and rare . Groundmasses range from glassy to trachyte (right) from two-feldspar trachyte (left). Di = ; Hd crystalline. In crystalline groundmasses, variable development of = hedenbergite. Dashed line is Skaergaard trend of Wager and snowflake texture dominates, with anhedral, interstitial quartz poi- Brown (1967). kilitically surrounding subtrachytoid alkali feldspar laths (Ander- son, 1969). Green to dark brown pleochroic amphibole and green pleochroic clinopyroxene occur interstitial to alkali feldspar. amphibole (arfvedsonite), green sodic clinopyroxene, and, rarely, aenigmatite. The amphibole occurs as spongy grains enclosing Rhyolite groundmass alkali feldspar. A complete textural and mineralogical gradation exists between peralkalic rhyolite and quartz trachyte. Peralkalic rhyolite (comendite) is concentrated in the Rhyolite Unit, where it is generally aphyric, with rare microphenocrysts of Nepheline Trachyte alkali feldspar, green clinopyroxene, and magnetite. Groundmass textures are variable; many are orthophyric, with stubby alkali Nepheline-normative trachyte and occur as small plugs feldspar laths surrounded by anhedral quartz, which also occurs as and dikes intruded into shield rocks of the volcano. They contain patchy devoid of enclosed alkali feldspar. Some rocks con- sparse olivine, pale green clinopyroxene, and nepheline microphe- tain swarms of nearly equant alkali feldspar grains that give the nocrysts set in a groundmass composed of trachytoid alkali feldspar rock a sugary texture. Groundmass mafic minerals include sodic with interstitial green clinopyroxene, olive-green to dark brown pleochroic amphibole, aenigmatite, accessory apatite, and, in one An sample, (?). Mafic minerals rim nepheline; amphibole rims other mafic minerals as well as nepheline. In the Paisano area, the ne are petrographically and chemically homogeneous; no gradation into other rock types was observed.

ELECTRON-PROBE MICROANALYSIS

Microprobe investigations centered upon ten samples repre- senting major rock types in the Paisano area. The ARL-EMX probe system in the Department of Geological Sciences, University of Texas at Austin, was employed for all analyses. Correction procedures were those of Bence and Albee (1968) and Albee and Ray (1970). Petrographic study allowed correlation of samples selected for microprobe investigation with those analyzed for whole-rock major-element chemistry.

Feldspar

Feldspar ranges from intermediate plagioclase to anorthoclase (Fig. 3). In samples where two are present, alkali feldspar mantles plagioclase. Figure 3. Feldspar compositions, determined by electron-probe Microphenocrysts in mugearite range from An57 to An39, with microanalysis for Ca, Na, and K, plotted in terms of molecular An, no systematic zoning within the grains. Mafic two-feldspar trachyte Ab, and Or. Solid circles: mafic lava. Empty circles: two-feldspar contains glomeroporphyritic feldspar with cores of andesine and trachyte lava. Square: average of 11 point analyses from pheno- potassic rimmed by calcic anorthoclase and, in some crysts in one sample of one-feldspar trachyte. Crosses: feldspar samples, anorthoclase. Zoning is uniform from A1145 to An35 in analyses from several samples of intrusive syenite and extrusive plagioclase cores. Calcic anorthoclase rims on plagioclase cores one-feldspar trachyte. Triangles: intrusive nepheline trachyte. define a trend toward the Ab-Or join (Fig. 3). One sample showed

Arrows connect analyzed points within a single grain in two- continuous zoning from An25 Ab67 Org to Ani 5 Abss Or43 5. feldspar trachyte. Groundmass feldspar in mafic trachyte is anorthoclase.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/94/5/614/3444647/i0016-7606-94-5-614.pdf by guest on 02 October 2021 PAISANO VOLCANO, TRANS-PECOS TEXAS 619

Anorthoclase phenocrysts from one-feldspar trachyte show a No spread of points approaching the alkali feldspar minimum from the Ab corner, with a tight cluster near Or39. Groundmass feldspar from nepheline trachyte exhibits a spread from Or2i to Or39, parallel and adjacent to the Ab-Or join. These analyses were taken from cryptoperthitic grains; some of the spread of points is probably the result of fine-scaled exsolution during cooling of the intrusion.

Pyroxene

Clinopyroxene phenocrysts and microphenocrysts from Pai- sano rocks show a general -enrichment trend when plotted in terms of atomic percent Ca-Mg-(Fe2+ + Mn) (Table A1; Fig. 4). Terminology follows that of Poldervaart and Hess (1951). Mafic two-feldspar trachyte contains pale green augite and ferroaugite. One-feldspar trachyte and quartz trachyte have ferroaugite also, but with a higher Fe/ Mg ratio. Pyroxene phenocrysts in ne trachyte are salite and ferrosalite. In all clinopyroxene phenocrysts and microphenocrysts, the grains are variable from point to point, but no systematic core-to-rim zonation was observed, except where Figure 5. Pyroxene microprobe analyses plotted in terms of groundmass pyroxene mantled phenocryst grains in ne trachyte. atomic percent Na-Mg-(Fe2+ + Mn). Symbols as in Figure 4. Phenocryst pyroxene follows a trend of Fe2+/ Mg enrichment; groundmass pyroxene shows enrichment in Na/(Fe2+ + Mn) (Fig. 5). Some groundmass grains from peralkalic rhyolite approach F pure acmite in composition.

Other Mafic Minerals

Titanomagnetite is a widespread phenocryst and micropheno- cryst in Paisano rocks. Umenite occurs with magnetite in mafic two-feldspar trachyte and in an ejected block of two-feldspar syenite (Table B).2 The magnetite from this sample is inhomogene- ous, probably because of microscopic ilmenite lamellae, and there- fore was not used to calculate equilibration temperature and oxygen fugacity (Buddington and Lindsley, 1964; Spencer and Lindsley, 1981). Fayalite microphenocrysts from three rocks were analyzed. All have low MgO content, but fairly high MnO. PP269 fayalite repre- sents olivine crystallized from a vapor phase in cavities within the same intrusion as the microphenocrystic olivine of PP271. The vapor-phase olivine has a higher FeO/ MgO ratio. Two analyses of groundmass sodic amphibole, one from an intrusion and one from peralkalic rhyolite lava, are included in Table B. PP258, the rhyolite lava sample, contains arfvedsonite. PP3, nepheline trachyte, contains sodic amphibole that probably Figure 6. AFM plot of analyses of Paisano igneous rocks represents a solid solution between arfvedsonite and , reported in Table C. A = molecular Na20 + K2O; F = molecular judging from its MgO content. FeO + 0.9 (Fe203); M = molecular MgO. Aenigmatite occurs as a groundmass phase in intrusive nephe- line syenite and, more rarely, in peralkalic rhyolite and quartz Thornton-Tuttle differentiation index modified by the addition of trachyte (Table B). Aenigmatite of peralkalic rhyolite was not normative ac+ ns (Figs. 6, 7). Analyses used in construction of these analyzed. plots are given in Table C.3 They include samples of the Lower and Upper Mafic Units, the Decie Formation, and dikes and hypabyssal MAJOR-ELEMENT CHEMISTRY intrusions of the Paisano area and surrounding region.

Paisano igneous rocks fall into a bimodal distribution between Basaltic Rocks femic rocks (hawaiite and mugearite) and salic rocks (trachyte and rhyolite). This bimodality is evident from a conventional AFM plot The hawaiites and mugearites of the Lower and Upper Mafic and from variation diagrams of weight percent oxides versus the Units are, in general, highly oxidized, making the silica saturation of

2Tables A-D may be obtained from GSA upon request. 3 See footnote 1.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/94/5/614/3444647/i0016-7606-94-5-614.pdf by guest on 02 October 2021 Si02 CaO 80- io

70- • k 60- 6 •• • o o o o 4J ^50- • • •

40- Wl 30- 40 50 60 70 80 90 100 40 50 60 70 80 90 100

TiOg Ko0 4-

3-'

cd2- F M • * . ' *****

40 50 60 70 80 90 100

AI2O3

20-

16- •• • •• fO _ o 12- rvj 40 50 60 70 80 90 100 < 8^

Na20

40 50 60 70 80 90 100 7-j

6 FeO 14-

i 12- • • 10-

8- 3 -

" 6- • A »

4- 40 50 60 70 80 90 100

2- P 0 40 50 60 » 70 80 90 100 4- 2 5 (all Fe os FeO) 3 MgO

QI

• • 40 50 60 70 80 90 100 O DI + (ac + TIS)

40 ~50 ' 60 ' 70 ' 80 ' 90 100 DI + (ac +ns)

Figure 7. Weight percent oxides of major elements of Paisano rocks plotted versus the Thornton and Tuttle (1960) Differentiation Index, modified by the addition of normative acmite and metasilicate. Circles and squares: extrusive rocks. Triangles: intrusive rocks, x = PP3, nepheline trachyte (no other nepheline trachyte analyses plotted).

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/94/5/614/3444647/i0016-7606-94-5-614.pdf by guest on 02 October 2021 PAISANO VOLCANO, TRANS-PECOS TEXAS 621

the original difficult to estimate. Given the Fe20j/Fe0 q* ratios reported by the analysts, the lavas vary over a considerable range from qz to ol + hv normative. When these analyses are recal- culated with all Fe2C>3 recalculated as FeO, qz disappears but, sig- nificantly, some analyses remain ol + hy normative rather than becoming ne normative. This suggests that a range from ne to ol + hy normative existed in the original magmas, thus providing possi- ble parent magmas for the two magmatic suites in the Paisano area: the silica-oversaturated suite of the Paisano volcano and the silica- undersaturated suite represented by some of the younger intrusions.

Trachyte

Quartz-normative trachyte grades into quartz trachyte with increasing normative quartz but is separated from the mafic rocks by a silica gap of 8%. Most intrusive trachytes are peralkalic; all extrusive trachytes are subalkalic, but they may have suffered more alkali loss during crystallization than did intrusive equivalents.

abz ^or Quartz Trachyte and Rhyolite

Figure 8. Analyses of Paisano silica-oversaturated igneous Silicic rocks of the Paisano volcano are moderately peralkalic, rocks (D.I. >75) plotted in qz-ab-or system, m = feldspar minimum, the most evolved having normative acmite of 7.75%. The Thornton- = m' = quartz-feldspar minimum (PH2O 1 kbar). Tuttle differentiation index begins to decline as ac + ns increase in norms. This trend is displayed on the AFM diagram by the array of points approaching the A corner and then moving along the A-F to results of the Parker and Schneider method, except where the side of the diagram toward the F corner (Fig. 7) (Barker, 1978). Wright and Doherty results are specifically indicated. Five samples were chosen to represent the principal rock types Nepheline Trachyte of the Paisano volcano. Mass balance calculations were performed by adding analyses of rocks, representing daughter magmas, to Nepheline trachyte in the Paisano area is petrographically and mineral analyses, representing possible fractionating phases, to chemically homogeneous. Although the analyses plot on the major- create a calculated parent magma. This calculated parent magma element trend of the qz normative suite, no gradation into other was then compared to an analysis of a rock chosen to be a possible rock types occurs. Most nepheline trachyte analyses are peralkalic; parent magma. A solution was selected after 10 to 35 trial runs, in normative nepheline does not exceed 5%. which the weight fractions of minerals in the mix were varied to

PETROLOGY

The eruptive sequence, the central style of eruptions, and the caldera of the Paisano volcano combine to suggest the presence of one or more large intrusive bodies at shallow depth beneath the volcano in which differentiation might have occurred. A fractiona- tion model for variation within the Paisano silica-oversaturated suite is discussed next. This model is then evaluated in regard to available trace-element data from the rocks of the volcano.

Fractionation Model

Paisano silica-oversaturated analyses with differentiation indi- ces greater than 75 fall along the thermal valley between the feld- spar minimum on the alkali feldspar join and the cotectic minimum (P = I kbar) when projected into the qz-or-ab system (Tuttle and 0-40 Bowen, 1958; Morse, 1969) (Fig. 8). This suggests that alkali feld- spar fractionation was an important petrogenetic process, at least or =o for the more differentiated salic rocks. Analyses of extrusive rocks plotted in the same system illustrate the generalized eruptive Figure 9. Enlarged part of Figure 8 showing only analyses of sequence of the volcano: rhyolite-quartz trachyte-trachyte (Fig. 9). Paisano extrusive rocks. In ascending stratigraphic order: empty Mass balance calculations were performed, using an iterative circles, Rhyolite Unit; open squares, SI Unit; filled circles, S2 Unit; program (Parker and Schneider, 1977) and a program similar to star, trachyte in Upper Mafic Unit. Numbers next to points refer to that of Wright and Doherty (1970) written for the University of analyses in Table C (see footnote 1). Arrows show stratigraphic Texas at Austin system by James R. Garrison. The results of both sequence of samples from the Rhyolite Unit into the SI Unit. Sam- methods are comparable (Table 2). The following discussion refers ple 21 is an ejected block of rhyolite within SI tuff.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/94/5/614/3444647/i0016-7606-94-5-614.pdf by guest on 02 October 2021 622 D. F. PARKER

TABLE 2. MAJOR-ELEMENT FRACTIONATION CALCULATIONS

FRACTIONATION STEP 1. MUGEARITE (PP391)—MAFIC TRACHYTE (PP314)

2 METHOD D AN,6AB67ORI7 AN50 CPX MT AP IL FA53 -R A 0.543 0.035 0.197 0.020 0.004 0.042 0.040 0.119 0.88 B 0.372 0.205 0.185 -0.011 -0.007 0.047 0.053 0.157 1.48 FRACTIONATION STEP 2. MAFIC TRACHYTE (PP314) -TRACHYTE (RBI)

METHOD D AN 11AB70OR19 AN38 CPX MT AP V A 0.750 0.060 0.130 0.015 0.035 0.010 1.14 B 0.754 0.094 0.113 -0.009 0.042 0.006 1.43 FRACTIONATION STEP 3. TRACHYTE (RBI) — PERALKALIC QUARTZ TRACHYTE (PP92)

METHOD D AN,, AB70OR|9 OR43 CPX MT AP FA72 V A 0.300 0.170 0.470 0.020 0.027 0.003 0.010 0.24 B 0.242 0.191 0.500 0.013 0.022 0.003 0.029 0.19 FRACTIONATION STEP 4. PERALKALIC QUARTZ TRACHYTE (PP92) — PERALKALIC RHYOLITE (PAIS 1A) METHOD D OR39 CPX MT AP -R2 A 0.514 0.451 0.017 0.017 0.001 0.14 B 0.500 0.456 0.031 0.016 -0.002 0.59

Key: METHOD A (Parker and Schneider, 1977); METHOD B (Wright and Doherty, 1970). D = weight fraction daughter magma, AN = anorthite, AB = albite, OR = orthoclase, CPX = clinopyroxene, MT = magnetite, IL = ilmenite, AP = apatite, FA = fayalite.

improve the match between computed and hypothesized parent trachyte, resembling phenocrysts in mugearitic lava (Table D).4 magmas. All analyses used in calculations were normalized to These clusters are, thus, mineralogically out of place in trachyte. 100%, minus CO2 and H2O; all Fe2C>3 was recalculated as FeO Without the glomerocrysts, the mafic trachyte would be one- before normalization. feldspar trachyte. The mafic trachytes may not represent true liquid The calculations indicated that 6 parts by weight of peralkalic compositions, but, rather, mixtures of trachytic liquid and frag- rhyolite (or 3 parts by weight according to the Wright and Doherty ments of mugearitic intrusions. Possible relationships between calculations) may be produced by fractional crystallization of 100 mugearitic intrusions and the production of trachytic magma will parts of ol + hy mugearite. The fits between computed and hypoth- be explored in a later section of this paper. esized parents were good, the largest sum of the squares of resid- uals being 1.14 for the Parker and Schneider method. Each step Trace-Element Investigations utilized minerals actually observed as phenocrysts in the intervals under consideration and, for most minerals, actual compositions of Whole-rock analyses for Rb, Sr, and Zr for most Paisano phenocrysts determined from microprobe investigation. samples were performed by D. S. Barker, using the XRF system at The steps from trachyte to quartz trachyte and from quartz the University of Texas at Austin (Table C).5 REE analyses for two trachyte to rhyolite required removal of large amounts of alkali samples, PP391 (mugearite) and PP254 (comendite), representing feldspar, in accord with predictions from the qz-or-ab system. The extremes of composition within the volcano, were performed by R. fits in these two steps were extremely good, the sums of the squares Cullers of Kansas State University (Fig. 10). of residuals being 0.24 and 0.14, respectively, from the Parker and The REE analyses show a threefold to fourfold enrichment for Schneider method (Table 2). most elements in the comendite relative to mugearite, suggestive of The steps from mugearite to mafic trachyte and from mafic a high degree of fractionation. Sm, however, is enriched by a factor trachyte to trachyte deserve special scrutiny. Both steps utilized of only 1.6, whereas Eu shows a strong depletion in the comendite. removal of plagioclase and anorthoclase. In all Paisano samples, Overall, the REE analyses indicate ~ 70% crystallization to produce the sequence of crystallization was plagioclase followed by anorth- the comendite from the mugearite, but this figure must be regarded oclase. The fractionation model, therefore, is not in accord with as a minimum estimate because fractionation of apatite, common in petrographic observation. An attempt was made in additional cal- Paisano rocks and known to concentrate REE, has most likely culations to model the step from mugearite to mafic trachyte, utiliz- occurred. The Eu anomaly in the comendite sample is, of course, ing a less calcic plagioclase without anorthoclase. These calcula- very much in accord with the fractionation model requiring a large tions yielded higher residuals than did those that used anorthoclase degree of feldspar fractionation. in the mix. Zr has proved a useful fractionation index in many suites of The highly porphyritic and glomeroporphyritic nature of Pai- peralkalic rocks because of its highly residual behavior in peralkalic sano mafic trachyte provides a possible solution to this dilemma. magmas (Weaver and others, 1972; Barberi and others, 1975; Baker

Plagioclase (up to A1143) is rimmed by calcic anorthoclase within and others, 1977). The Paisano suite, however, is less peralkalic these glomeroporphyritic clusters; chemical analyses of these rocks indicate a normative plagioclase of only Anio-15. Clinopyroxene within these clusters is more magnesian than one would expect for a 4' 5See footnote 1.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/94/5/614/3444647/i0016-7606-94-5-614.pdf by guest on 02 October 2021 PAISANO VOLCANO, TRANS-PECOS TEXAS 623

1000 3000

2000 Zr

100

-o 1000 S * •C ** U * * _o a E o «/> 0.8 0.9 1.2 1 0 (Na20 + K20)/Al203

Figure 11. Zr versus (NajO + K^O) /AI2O3 plot of Paisano O PP254(35) analyses. Stars indicate samples in which zircon was observed as a microphenocryst phase. Arrow connects two samples of one dike • PP391(9) (see text). Zr + L line indicates Watson's (1979) limit for zircon stability in peralkalic melts.

shows the Paisano Zr analyses from trachyte, quartz trachyte, and rhyolite plotted against molecular (Na20 + K20)/Al203. The plot shows a gradual increase in Zr content up to the zircon stability La Ce Sm Eu Tb Yb Lu limit of Watson (1979), beyond which the slope of the curve defined by the analyses steepens considerably. Rocks in which zircon was Figure 10. Chondrite-normalized REE abundance for two Pai- identified as a microphenocryst phase all plot to the left of the sano rocks. PP254, peralkalic rhyolite; PP391, mugearite. Numbers stability limit of zircon. Loss of alkalis, particularly sodium, during in parentheses refer to analyses in Table C. crystallization has shifted some zircon-free to the left of this limit. This shift is illustrated by the arrow connecting two sam- than the African suites of the above reports. Zircon occurs as a ples, PP406 and PP9, of a large dike. PP406, taken from a chilled microphenocryst phase in Paisano trachyte and quartz trachyte, margin of the dike, is strongly peralkalic, whereas PP9, taken from limiting the use of Zr as a crystallization index in the rocks. the interior of the dike, is marginally subalkalic. The over-all trend Despite the above limitation, the Zr analyses help to interpret of Figure 11 can be interpreted as a result of fractionation of plagi- the fractionation history of Paisano silica-oversaturated rocks. Zr oclase and calcic anorthoclase, both subalkalic, followed by frac- was residual during the early and late stages of this series. Figure 11 tionation of stoichiometric alkali feldspar with an (Na20 +

1500

1000 \ L-UMU Sr

/ S2 500 [,• V/'

"L-UMU > S2 \

1000 2000 1000 2000 Zr

Figure 12. Rb and Sr versus Zr diagrams. All values in ppm. L-UMU: Lower and Upper Mafic Units. SI: Lower Shield Unit. S2: Upper Shield Unit. RU: Rhyolite Unit. Other points represent dikes and other intrusions.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/94/5/614/3444647/i0016-7606-94-5-614.pdf by guest on 02 October 2021 624 D. F. PARKER

K20)/A1203 ratio of one. This interpretation is completely in is a gentle, convex-upward curve with considerable scatter that may agreement with petrographic, mineralogic, and whole-rock chemi- reflect the fact that the samples were drawn from six different vol- cal evidence presented elsewhere in this paper. canoes. No buffering of Zr is evident. The Zr values and In (100) Plots of Rb versus Zr and of Sr versus Zr (Fig. 12) show (Rb/Sr) values increase together throughout the entire composi- extreme Rb enrichment and Sr depletion within the Paisano suite, tional spectrum from basalt to . again suggesting large amounts of feldspar fractionation, although there is considerable scatter in both plots. Major-element oxides THE DALY GAP (CaO and MgO) plotted against Zr show a rapid decrease in the transition from mugearite to trachyte without the prominent gap The bimodal distribution of Paisano igneous rocks between that characterized the major-element oxides versus D.I. + (ac + ns) mafic rocks (hawaiites and mugearites) and rocks (trachytes plots (Fig. 13). Zircon-bearing trachyte and quartz trachyte samples and rhyolites) is analogous to the Daly Gap between basalt and are scattered toward lower Zr values in these plots, particularly in trachyte on oceanic islands (Chayes, 1963). Many different mecha- the Zr range from 500 to 800 ppm, suggesting that the crystalliza- nisms have been invoked to explain this distribution; these include tion of zircon has buffered the Zr concentration in that range. sampling bias (I. Baker, 1968), remelting of early-formed salic rocks The effect of possible zircon fractionation can be evaluated by within volcanic piles (Cann, 1968), preferential eruption of basalt using a plot of In (100) (Rb/Sr) versus Zr (Fig. 14A). This particular and trachyte during formation of volcanic cones (Cox and others, plot was chosen because the Rb/Sr ratio should be an effective 1969), a "processing" model within salic cupolas above batholith- measure of feldspar fractionation. The natural logarithm was used sized bodies of basaltic magma where the volume of intermediate to smooth out analytical errors, which are magnified in ratios where magma at any one time is not great (Weaver and others, 1972), and samples have low Rb and high Sr, or high Rb and low Sr values. the failure of conventional indices of fractionation (SiC>2, differen- Paisano samples with detectable Sr values (>6 ppm) define an S- tiation index) as accurate measures of differentiation in the transi- shaped curve. Zr versus In (100) (Rb/Sr) shows a linear increase in tion from basalt to trachyte (Clague, 1978). mugearite samples where zircon was not observed. Mafic trachyte, Data in this paper support the conclusions of Clague. Trachyt- trachyte, and quartz trachyte samples plot along the middle of the ic liquid appears to have been produced by ~60 weight percent curve and show a dramatic increase in In (100) (Rb/Sr), with little crystallization of mugearitic magma, involving fractionation of or no increase in Zr. These samples contain zircon. Fractionation of plagioclase, olivine, anorthoclase, and other minerals. Mafic zircon with feldspar buffered Zr concentration. The most evolved trachyte appears to represent trachytic magma incompletely samples show strong increases in both In (100) (Rb/Sr) and Zr. segregated from fractionating phases, or trachytic magma contain- These samples for the most part lack zircon. A comparison with the ing fragments of mugearitic intrusions—or both. zircon-free suites of the African is illuminating. Figure 14B Carman and others (1975) have described in situ differentiation shows the analyses presented by Weaver and others (1972) in a plot of a relatively thin on Rattlesnake Mountain in Big Bend similar to Figure 14A. In the plot of the African samples, the trend National Park. This sill has an over-all composition of ne mugear- ite. Rifting of a mush when the intrusion was about 50% to 55% crystalline allowed segregation of residual syenitic liquid into pods, layers, and cylinders. The net result of this process was an intrusion containing about 10 volume percent syenite (including a few percent plagioclase syenite) and 90 volume percent "mon- zonite."

Wt% MgO

Figure 13. Varia- • • / • «' » Ml • • tions diagrams showing 1000 2000 weight percent MgO and Zr ppm CaO (filled circles) ver- sus Zr (ppm). Weight o percent plot of CaO (empty circles) versus • o Wt % 4 D.I. + (ac + ns) included CaO for comparison.

• • • »o I • —I— 60 70 80 90 D.I. + (ac+ns)

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/94/5/614/3444647/i0016-7606-94-5-614.pdf by guest on 02 October 2021 PAISANO VOLCANO, TRANS-PECOS TEXAS 625

9

8 B » * 7

6

acá » O 4 O

£ 3

2

1

1000 1500 2000 0 500 1000 1500 2000

Zr (ppm) Zr (ppm)

Figure 14. A. Plot of InlOO (Rb/Sr) versus Zr for Paisano rocks. Samples with observed zircon plotted as stars. B. Plot of InlOO (Rb/Sr) versus Zr for African rocks. Zircon not present. Analyses taken from Weaver and others (1972).

Such a mechanism may have operated in the production of within continental crust in the presence of alkali-bearing vapor. The trachyte from mugearitic magma under the Paisano volcano, Paisano are clearly related to comenditic trachytes and although documentation of the production of qz trachyte from thus do not conform to the findings of Bailey and Macdonald. mugearite is lacking in studies of Trans-Pecos intrusions (Barker, If peralkalinity developed in the trachyte stage of the Paisano 1979a). Perhaps this differentiation occurred at a level not exposed series, how did it come about? Fractionation of stoichiometric by erosion. No mafic trachyte was erupted from the Paisano vol- alkali feldspar could only promote alumina undersaturation, once it cano before development of the caldera. Caldera collapse may have had developed, but could not initiate it. Most likely, the alumina opened fractures at depth, facilitating the ascent of glomerocryst- undersaturation developed in the trachyte stage by fractionation of charged trachytic magma to the surface from a largely crystallized plagioclase and calcic anorthoclase, the plagioclase effect of Bowen mafic intrusion. The glomerocrysts are mineralogically out of place (1945). in trachyte, and, as previously described, they have textures sugges- tive of crystallization within intrusions. They may represent frag- EVALUATION OF CRYSTAL FRACTIONATION ments of mugearitic intrusions that yielded trachytic magma by a filter-pressing process similar to that which occurred in the Rattle- The above discussion suggests that fractional crystallization snake Mountain sill. was a major factor in the evolution of the Paisano suite of silica- oversaturated rocks. Studies of voluminous ash-flow sheets, inter- DEVELOPMENT OF PER ALKALINITY preted as the eruptive products of high-level, compositionally zoned magma chambers, have, however, cast considerable doubt on the Analyses of Paisano silica-oversaturated rocks with differenti- effectiveness of crystal fractionation as a petrogenetic process, at ation indices greater than 85 plot, for the most part, in the field of least for high-silica, calc-alkalic magma systems (Hildreth, 1979;

comendites and comenditic trachytes within the system Si02- Smith, 1979). Instead, liquid-state diffusion driven by thermal gra-

Al203-(Na20 + K20) of Bailey and Macdonald (1969) (Fig. 15). dients has been proposed as the dominant means of differentiation Points falling on the alumina-oversaturated side of the line of alum- of silicic magma (Hildreth, 1981). This section evaluates the physi- ina saturation are lavas and dikes; these are only marginally cal possibility of crystal fractionation in subjacent magma cham- alumina-oversaturated and may have been displaced across the line bers in the generation of the silicic magmas of the Paisano volcano. of alumina saturation by sodium loss during crystallization. All The central style of eruption, dike swarm, and caldera of the analyzed samples of silica-oversaturated intrusions (exclusive of Paisano volcano evince the presence of one or more shallow plu- dikes) in the Paisano area are peralkalic, whereas many lavas and tonic bodies beneath the Paisano complex. The generalized eruptive dikes are not. This implies that the shallow intrusions may have sequence of rhyolite to trachyte can be interpreted as a more or less suffered less sodium loss than did some lavas and dikes which crys- progressive tapping of less-differentiated magma with time during tallized at or near the surface. the evolution of the volcano. Did crystal fractionation produce Bailey and Macdonald (1970) maintained that continental compositional zonation in these hypothesized chambers? comendites, unlike oceanic comendites, were not associated with The early products of the volcano, represented by the Rhyolite comenditic trachytes. Partially on that basis, they postulated separ- Unit and the Lower Shield Unit, record a progressive change in ate origins for continental and oceanic varieties of comendite, the eruptive compositions from peralkalic rhyolite to peralkalic quartz oceanic comendites descending from a trachytic parent and the trachyte. The chamber that produced the magmas of this sequence continental comendites possibly originating by partial melting was probably emplaced at a depth of 3 km or more, judging from

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/94/5/614/3444647/i0016-7606-94-5-614.pdf by guest on 02 October 2021 626 D. F. PARKER

3 4 xenocrysts and of high-grade peraluminous metamorphic crystal (ps = 2.56 g/cm ) will settle about 1,000 m in 10 yr. Peral- minerals and rocks associated with these units (Parker, 1976). If kalic melts are believed to possess lower viscosities than do calc- fractionation of anorthoclase was important in the generation of alkalic melts. If the viscosity of the melt was lowered dramatically peralkalic rhyolite from peralkalic quartz trachyte, it would have to by the higher alkali and halogen content of peralkalic rhyolite melt, have occurred within this chamber. say, to 2.5 x 106 poises, the settling rate would increase greatly: a 4 Physical conditions inside this chamber during such hypothe- 1-cm-diameter anorthoclase crystal would settle about 10 km in 10 sized fractionation can be estimated from the above data and from yr. These results, again, are for a single anorthoclase crystal sus- results of experimental work in the system. Lithostatic pres- pended in an infinite liquid, an unlikely state in nature and one not sure at depths of ~3 km is ~ 1 kbar. Temperatures during crystalli- likely to produce much fractionation. Interaction among suspended zation of a quartz trachytic magma at this pressure would be in the particles in a liquid increases the effective viscosity of the liquid and range 900 to 850 °C (Tuttle and Bowen, 1958). The viscosity of the decreases the settling velocity. Walker and others (1976) modeled melt is more difficult to estimate. Bottinga and Weill (1972) empiri- this effect in their analysis of fractionation of lunar basalts by mul- cally modeled Newtonian viscosity for silicate magmas in the tiplying the settling velocities calculated for single crystals by a 4 65 temperature range 1,800 to 1,200 °C. Extrapolating their results to factor of (1 - ) , where 0 is the volume fraction of crystals. 900 °C, the Newtonian viscosity coefficient (n) would be approxi- Using this revised Stokes' law, when a magma contains 10% crys- mately equal to 107 poises. This value is close to that of Shaw tals, the settling velocity is 61% of the simple Stokes' law result; 20% (1963), who studied the viscosity of under similar crystals gives a settling velocity 35% of the single-crystal velocity; conditions. and 40% crystals, 9% of the single-crystal velocity, assuming that all crystals are of the same size and density. The above calculations are, of course, only as good as the assumptions upon which they rest. Magmas are believed to be non- Newtonian fluids with high-yield strengths (Murase and McBirney, 1973; Irvine, 1978). Stokes' law will not strictly apply to non- Newtonian fluids. Nonetheless, these estimates of the physical parameters indicate that the settling model is plausible for the more advanced stages of the evolution of the Paisano magmas. Rapid and efficient fractionation of anorthoclase could account for the lack of observed crystal cumulates of anorthoclase and the sparsely porphyritic or aphyric nature of most quartz trachyte and rhyolite in the Paisano volcano. Feldspar fractionation has been shown to be possible over the entire compositional spectrum from basalt to trachyte in a suite of lavas from the southern Gregory in (Baker and others, 1977). In this Kenya suite, the density of feldspar was invariably lower than the calculated liquid density at each stage of fractionation, with the nearest match of densities occurring in the stage. Figure 15. Analyses of Paisano silica-oversaturated rocks with This near match of feldspar and liquid densities in the ben-

D.I. >85, plotted in terms of molecular Si02-AI203-(Na20 + K20) moreite stage (more or less equivalent to the mafic trachyte stage in (Bailey and Macdonald, 1969). Dashed line is trace of the plane of the Paisano series) indicates that feldspar fractionation by gravita- alumina saturation. A = stoichiometric alkali feldspar. Circles: tional settling should be least efficient in the transition from extrusive rocks. Triangles: dikes and other intrusions. Small trian- mugearite from trachyte. Baker and others (1977) attributed the

gle in upper right shows portion (in black) of Si02-AI2C>3-(Na20 + highly porphyritic nature of the in the Gregory Rift

K20) system enlarged in figure. suite to this effect. The filter-pressing mechanism proposed in this paper for the separation of trachyte magma from largely crystalline mafic intrusions overcomes the density difficulty because it would Assuming Newtonian behavior for the melt, the settling veloc- be driven by the net density difference between the trachytic liquid ity (w) of anorthoclase crystals can be calculated by using Stokes' and the crystals of plagioclase, clinopyroxene, olivine, and opaques. law: in the remaining liquid would also play a part by helping to fracture the , allowing the liquid to escape. 2 P w = (ps-p)gd / 18n

where: ps = density solid MAGMATIC EVOLUTION OF THE VOLCANO p = density liquid g = acceleration of gravity About 35 m.y. ago, a salic pluton rose from depth and lodged d = diameter of spherical grains in Precambrian crust beneath the Paisano area (Fig. 16). Ejected fragments of metadiorite and kyanite gneiss attest to the Under conditions of 900 °C, magmatic viscosity of 2.5 x 107 poises, peraluminous nature of the Precambrian crust, which contrasts and magmatic density of 2.36 g/cm3, a 1-cm-diameter anorthoclase greatly with the peralkalic nature of the salic magmas. The size of

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/94/5/614/3444647/i0016-7606-94-5-614.pdf by guest on 02 October 2021 PAISANO VOLCANO, TRANS-PECOS TEXAS 627

8 KM- h H RU

Figure 16. Conceptual diagram illustrating possi- B ble magmatic evolution of Paisano volcano. A. Segre- gation of salie pluton of over-all trachytic composi- tion from large subjacent reservoir of mafic magma. B. Development of vertical compositional gradient with salic pluton; eruption of Rhyolite Unit (RU). C. Eruption of SI Unit, fol- lowed by emplacement' of second pluton, or aipophysis of initial pluton. D. Erup- tion of S2 Unit and asso- ciated caldera collapse.

CALDERA

this pluton was probably similar to the diameter (~8 km) of the Crystal fractionation dominated as a mechanism of differentia- central dike swarm of the volcano. The salic pluton became compo- tion in these plutons. Gravitational settling of anorthoclase was sitionally zoned from peralkalic rhyolite downward to peralkalic extremely efficient in the production of peralkalic rhyolite from quartz trachyte. It vented rapidly to the surface along fracture sys- quartz trachyte. Fractionation of feldspar and mafic minerals dom- tems in the Precambrian basement, producing the eruptive inated intermediate stages. Trachytic magma may have been pro- sequence rhyolite-quartz trachyte observed in the Rhyolite and duced by filter-pressing of residual liquid from large reservoirs of Lower Shield Units. 50% to 60% crystalline mugearitic magma at greater depths beneath Renewed activity of the volcano was centered slightly to the the volcanic complex. southwest of the early activity and produced the tuff and lava of the Upper Shield Unit. A 5-km-diameter caldera developed at the new TECTONIC IMPLICATIONS center, overlying either a second pluton or an apophysis of the earlier pluton (Fig. 16). The second pluton was also composition- The fractionation model for the Paisano series suggests that ally zoned, but it was not tapped as systematically during eruption mugearite was the immediate parent to the salic rocks of the Pai- of the quartz trachyte and trachyte of the Upper Shield Unit. Cal- sano volcano. It implies that large volumes of such mafic magma dera fractures allowed the ascent of glomerocryst-charged mafic may be required for the generation of similar trachytic and rhyolitic trachytic magma from deep-seated portions of this pluton. volcanic rocks of the Davis Mountains region. Mafic rocks, how-

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/94/5/614/3444647/i0016-7606-94-5-614.pdf by guest on 02 October 2021 628 D. F. PARKER

ever, make up only a fraction of the total volume of igneous rocks accountable for any flaws it contains. The work would not have in the Davis Mountains and in the Trans-Pecos magmatic province been possible without the cooperation of landowners in the area, in general, although periodic eruption of mafic lava throughout the especially that of the Mclntyre family. Special thanks are also due history of the province indicates that it was available in sizable to Arch Campbell, who served as field assistant, and to G. K. quantities at all times. Either there are large mafic intrusions at Hoops, analyst for the Department of Geological Sciences of the depth in the province, or the salic rocks are not derived from the University of Texas at Austin. basaltic rocks. The latter hypothesis is, of course, not supported by this paper. REFERENCES CITED Barker (1977) interpreted the Trans-Pecos magmatic province as an eroded analogue to the Gregory Rift of eastern Africa. The Abbott, M. J., 1969, Petrology of the Nandewar volcano, N.S.W., Austra- Paisano volcano fits well into Barker's model, because volcanoes lia: Contributions to Mineralogy and Petrology, v. 20, p. 115-134. similar to Paisano in morphology and chemistry occur in the Albee, A. L., and Ray, L., 1970, Correction factors for electron probe Gregory Rift (Weaver and others, 1972; Baker and others, 1977), the microanalysis of silicates, oxides, carbonates, phosphates, and sulfates: Ethiopian Rift (Gibson, 1969, 1972), the Afar triangle (Barberi and Analytical Chemistry, v. 42, p. 1408-1414. others, 1975; Bizouard and others, 1980), and southern Arabia Anderson, J. E., 1969, Development of snowflake texture in a welded tuff, Davis Mountains, Texas: Geological Society of America Bulletin, v. 80, (Gass and Mallick, 1968; Cox and others, 1969). Such similarities, p. 2075-2080. however, do not prove a rift origin for the Trans-Pecos province. Bailey, D. K., and Macdonald, R., 1969, Alkali-feldspar fractionation Many oceanic islands contain volcanoes similar to the Paisano vol- trends and the derivation of peralkaline liquids: American Journal of cano (P. E. Baker, 1973). In North America, comenditic shield Science, v. 267, p. 242-248. volcanoes of British Columbia have been related to intracontinental 1970, Petrochemical variations among mildly peralkaline (comendite) from the oceans and continents: Contributions to Mineralogy rifting or hot-spot activity (Bevier and others, 1979; Souther and and Petrology, v. 28, p. 340-351. Symons, 1974). Baker. B. H„ Goles, G. G„ Leeman, W. P., and Lindstrom, M. M„ 1977, Plate-tectonic models developed for Cenozoic southwestern and petrogenesis of a basalt-benmoreite-trachyte suite North America have become increasingly sophisticated and com- from the southern part of the Gregory rift, Kenya: Contributions to Mineralogy and Petrology, v. 64, p. 303-332. prehensive. These models, developed from plate-motion studies and Baker, I., 1968, Intermediate oceanic volcanic rocks and the 'Daly Gap': subduction geometries deduced from the chemistry and ages of Earth and Planetary Science Letters, v. 4, p. 103-106. volcanic rocks, have treated the Trans-Pecos province in different Baker, P. E., 1973, Islands of the South Atlantic, in Nairn, A.E.M., and ways. The province has been variously omitted from some studies Stehli, F. G., eds., Ocean basins and margins, Volume 1, The South (Cross and Pilger, 1978), interpreted as a subduction-related suite Atlantic: New York-London, Plenum Press, p. 493-553. Baker, P. E„ Buckley, F., and Holland, J. G„ 1974, Petrology of alkalic rocks that grades southwestward into calc-alkalic rocks and geochemistry of : Contributions to Mineralogy (Christiansen and Lipman, 1972; Keith, 1978), as intraplate vol- and Petrology, v. 44, p. 85-100. canic rocks totally disassociated from subduction (Gilluly, 1971; Barberi, F., Ferrara, G., Santacroce, R., Treuil, M., and Varet, J., Barker, 1977), and as alkalic rocks related to mantle diapirism trig- 1975, A transitional basalt-pantellerite sequence of fractional crystalli- gered by subduction processes (Barker, 1979; Parker and McDow- zation, the Boina center (Afar rift, Ethiopia): Journal of Petrology, v. 16, p. 22-56. ell, 1979). Direct relation to subduction is questionable for the Barker, D. S., 1977, Northern Trans-Pecos magmatic province: Introduc- Trans-Pecos province; all subduction models are based upon calc- tion and comparison with the Kenya rift: Geological Society of Amer- alkalic, or alkali-calcic, largely andesitic suites of igneous rocks. ica Bulletin, v. 88, p. 1421-1427. Ferroaugite rhyolites in Chihuahua, of the same age and chemically 1978, Magmatic trends of alkali-iron- diagrams: American similar to the alkalic rhyolites of the Trans-Pecos province, may be Mineralogist, v. 63, p. 531-534. 1979a, Magmatic evolution in the Trans-Pecos province, in Walton, unrelated to associated calc-alkalic rhyolites. The ferroaugite rhyo- A. W., and Henry, C. D., eds., Cenozoic geology of the Trans-Pecos lites have been interpreted as the result of westward expansion of of Texas: University of Texas at Austin Bureau of alkalic volcanism into Chihuahua from southwestern Texas Economic Geology Guidebook 19, p. 4-9. (Cameron and others, 1980). 1979b, Cenozoic magmatism in the Trans-Pecos province: Relation to the Rio Grande rift, in Riecker, R. E., ed.. Rio Grande rift: Tectonics and magmatism: Washington, D.C., American Geophysical Union, ACKNOWLEDGMENTS p. 382-392. Bence, A. E., and Albee, A. L., 1968, Empirical correction factors for the electron microanalysis of silicates and oxides: Journal of Field work for this project was supported by National Science Geology, v. 76, p. 382-403. Foundation Grants GA-32089 and EAR 75-22201 to D. S. Barker, Bevier, M. L., Armstrong, R. L., and Souther, J. G., 1979, Miocene peralka- a Penrose grant from the Geological Society of America, and the line volcanism in west-central British Columbia Its temporal and Department of Geological Sciences of the University of Texas at plate-tectonic setting: Geology, v. 7, p. 389-392. Austin. The Department also provided whole-rock chemical anal- Bizouard, H., Barberi, F., and Varet, J., 1980, Mineralogy and petrology of Erta Ale and Boina volcanic series, Afar rift, Ethiopia: Journal of yses, thin sections, and access to analytical equipment. D. S. Barker Petrology, v. 21, p. 401-436. generously supplied the trace-element data for Rb, Sr, and Zr. The Bottinga, Y., and Weill, D. F., 1972, The viscosity of magmatic silicate Department of Geology and the University Research Committee, liquids: A model for calculation: American Journal of Science, v. 272. Baylor University, helped defray cost of manuscript preparation p. 438-475. and publication. Bowen, N. L., 1945, Phase equilibria bearing on the origin and differentiation of the alkaline rocks: American Journal of Science, I am indebted to F. W. McDowell and D. S. Barker for their v. 243A, p. 75-89. critical reading of this manuscript, although they cannot be held Buddington, A. F., and Lindsley, D. H., 1964, Iron-titanium oxide minerals

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/94/5/614/3444647/i0016-7606-94-5-614.pdf by guest on 02 October 2021 PAISANO VOLCANO, TRANS-PECOS TEXAS 629

and their synthetic equivalents: Journal of Petrology, v. 5, p. 310-357. 440-N-l, 37 p. Cameron, K. L., Cameron, M., Bagby, W. C., Moll, E. J.,and Drake, R. E., McAnulty, W. N., 1955, Geology of Cathedral Mountain quadrangle, 1980, Petrologic characteristics of mid-Tertiary volcanic suites. Chi- Brewster County, Texas: Geological Society of America Bulletin, v. 66, huahua. Mexico: Geology, v. 8, p. 87-91. p. 531-578. Cann, J. R., 1968. Bimodal distribution of rocks from volcanic islands: McDowell, F. W.. and Clabaugh, S. E., 1979, of the Sierra Earth and Planetary Science Letters, v. 4, p. 479-480. Madre Occidental and their relation to the tectonic history of western Carman, M. F., Jr., Cameron, M., Gunn, B., Cameron. K. L., and Butler, Mexico, in Chapin, C. E., and Elston, W. E., eds., Ash-flow tuffs: J. C., 1975, Petrology of Rattlesnake Mountain sill, Big Bend Geological Society of America Special Paper 180, p. 113-124. National Park, Texas: Geological Society of America Bulletin, v. 86, Morse, S. A., 1969, : Carnegie Institution of Washington Year p. 177-193 Book 67, p. 112-120. Chayes, F., 1963, Relative abundance of intermediate members of the oce- Muir, I. D., and Tilley, C. E., 1961, Mugearites and their place in alkali anic basalt-trachyte association: Journal of Geophysical Research, series: Journal of Geology, v. 69, p. 186-203. v. 68, p. 1519-1633. Murase, T., and McBirney, A. R., 1973, Properties of some common igne- Christiansen, R. L., and Lipman, P. W., 1972, Cenozoic volcanism ous rocks and their melts at high temperatures: Geological Society of and plate-tectonic evolution of the western United States; II. Late America Bulletin, v. 84, p. 3563-3592. Cenozoic: Royal Society of London Philosophical Transactions, ser. A, Osann, A., 1896, Beitrage zur geologie und petrographie der Apache (Davis) v. 271, p. 249-284. mts., westtexas: Tschermaks Mineralogische und Petrographische Mit- Clague, D. A., 1978, The oceanic basalt-trachyte association: An explana- teilungen. v. 15, p. 394-456. tion of the Daly Gap: Journal of Geology, v. 86, p. 739-743. Parker, D. F., 1976, Petrology and eruptive history of an Oligocene tra- Council, K. K., 1972, The geology and petrography of Paisano Peak, Brew- chytic , near Alpine, Texas [Ph.D. dissert.]: Austin, ster County, Texas [M.S. thesis]: Fort Worth, Texas, Texas Christian Texas, University of Texas at Austin, 183 p. University, 59 p. 1979, The Paisano volcano: Stratigraphy, age and petrogenesis, in Wal- Cox, K. G., Gass, I. G., and Mallick, D.I.J., 1969, The evolution of the ton, A. W., and Henry, C. D., eds., Cenozoic geology of the Trans- volcanoes of Aden and Little Aden, South Arabia: Geological Society Pecos volcanic field of Texas: University of Texas at Austin Bureau of of London Quarterly Journal, v. 124, p. 283-308. Economic Geology Guidebook 19, p. 97-105. Cross, T. A., and Pilger, R. H., Jr., 1978, Constraints on absolute motion Parker, D. F., and McDowell, F. W., 1979, K-Ar of Oligo- and plate interaction inferred from Cenozoic igneous activity in the cene volcanic rocks, Davis and Barrilla Mountains, Texas: Geological western United States: American Journal of Science, v. 278, p. 865-902. Society of America Bulletin, Part 1, v. 90, p. 1100-1110. Finger, L. W., 1972, The uncertainty in the calculated ferric iron content of Parker, D. F., and Schneider, R. R., 1977, An interactive program for a microprobe analysis: Carnegie Institution of Washington Year Book solving petrologic mixing problems: Kansas Academy of Science 71, p. 600-603. Abstracts with Program, 109th Annual Meeting. Gass, P. W., and Mallick, D.I.J., 1968, Jebel Khariz: An upper Miocene Poldervaart, A., and Hess, H. H., 1951, in the crystallization of strato-volcano of comenditic affinity on the South Arabian coast: Bul- basaltic magma: Journal of Geology, v. 59, p. 472-489.

letin Volcanologique, v. 32, p. 33-88. Shaw, H. R., 1963, 0bsidian-H20 viscosities at 1000 and 2000 bars in the Gibson, I. L., 1969, The structure and volcanic geology of an axial portion temperature range 700-900 °C: Journal of Geophysical Research, v. 68, of the main Ethiopian rift: Tectonophysics, v. 8. p. 561-565. p. 6337-6343. 1972, Chemistry and petrogenesis of a suite of from the Smith, R. L., 1979, Ash-flow magmatism, in Chapin, C. E., and Elston, W. Ethiopian rift: Journal of Petrology, v. 13, p. 31-44. E., eds.. Ash-flow tuffs: Geological Society of America Special Paper Gilliland, M. W., and Clark, H. C., 1979, Paleomagnetism of early Tertiary 180, p. 5-27. volcanics of the Big Bend, Texas, in Walton, A. W., and Henry, C. D., Souther, J. G., and Symons, D.T.A., 1974, Stratigraphy and paleomagne- eds., Cenozoic geology of the Trans-Pecos volcanic field of Texas: tism of Mount Edziza volcanic complex, northwestern British University of Texas at Austin Bureau of Economic Geology Guidebook Columbia: Geological Survey of Canada Paper 73-32, 48 p. 19, p. 48-66. Spencer, K. J., and Lindsley, D. H., 1981, A solution model for coexisting Gilluly, J., 1971, Plate tectonics and magmatic evolution: Geological iron-titanium oxides: American Mineralogist, v. 66, p. 1189-1201. Society of America Bulletin, v. 82, p. 2383-2396. Thornton, C. P., and Tuttle, O. F., 1960, Chemistry of igneous rocks; 1. Goldich, S. S., and Elms. M. A., 1949, Stratigraphy and petrology of the Differentiation index: American Journal of Science, v. 258, p. 664-684. Buck Hill quadrangle, Texas: Geological Society of America Bulletin, Tuttle, O. F., and Bowen, N. L., 1958, Origin of granite in the light of

v. 60, p. 1133-1182. experimental studies in the system NaAlSi308-KAlSi108-Si02-H20: Gorski, D., 1970, Geology and trace transition element variation of the Geological Society of America Memoir 74, 153 p. Mitre Peak area, Trans-Pecos Texas [M.A. thesis]: Austin, Texas, Uni- Wager, L. R., and Brown, G. M., 1967, Layered igneous rocks: San Fran- versity of Texas at Austin, 201 p. cisco, California, W. H. Freeman and Company, 588 p. Hildreth. W., 1979, The : Evidence for the origin of composi- Walker, D„ Longhi, J., Kirkpatrick, R. J., and Hays, J. F„ 1976, Differen- tional zonation in silicic magma chambers, in Chapin, C. E., and tiation of an Apollo 12 picrite magma, in Lunar Science Conference, Elston, W. E., eds., Ash-flow tuffs: Geological Society of America 7th, Proceedings, p. 1365-1389. Special Paper 180, p. 43-75. Watson, E. B., 1979, Zircon saturation in felsic liquids: Experimental results 1981, Gradients in silicic magma chambers: Implications for lith- and applications to trace element geochemistry: Contributions to Min- ospheric magmatism: Journal of Geophysical Research, v. 86, eralogy and Petrology, v. 70, p. 407-419. p. 10153-10192. Weaver, S. D„ Sceal, J.S.C., and Gibson, I. L., 1972. Trace-element data Irvine, T. N., 1978, Density current structure and magmatic sedimentation: relevant to the origin of trachytic and pantelleritic lavas in the East Carnegie Institution of Washington Year Book 77, p. 717-725. African rift system: Contributions to Mineralogy and Petrology, v. 36, Keith. S. B., 1978, Paleosubduction geometries inferred from Cretaceous p. 181-194. and Tertiary magmatic patterns in southwestern North America: Geol- Wright, T. L., and Doherty, P. C., 1970, A linear programming and least ogy, v. 6, p. 516-521. squares computer method for solving petrologic mixing problems: Geo- Lipman, P. W., 1976, Caldera-collapse breccias in the western San logical Society of America Bulletin, v. 81. p. 1995-2008. Juan Mountains, Colorado: Geological Society of America Bulletin, v. 87, p. 1397-1410. MANUSCRIPT RECEIVED BY THE SOCIETY NOVEMBER 18, 1981 Macdonald, R., and Bailey, D. K., 1973, The chemistry of the peralkaline REVISED MANUSCRIPT RECEIVED MAY 31, 1982 oversaturated obsidians: U.S. Geological Survey Professional Paper MANUSCRIPT ACCEPTED JUNE 8, 1982

Printed in U.S.A.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/94/5/614/3444647/i0016-7606-94-5-614.pdf by guest on 02 October 2021

Top View