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

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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 DON F. PARKER Department of Geology, 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. Mafic 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 dike swarms led to the formation of the volcanism in Mexico and the southwestern United States (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 caldera 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. phenocryst minerals to represent compositions of fractionating phases, indicate plagioclase-plus-olivine control in the evolution of TERMINOLOGY trachyte from mugearite, and anorthoclase control in the evolution of rhyolite from trachyte. The feldspar 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- BASALT (Differentiation Index* <30, normative plagioclase >An5o)t fered by the crystallization of zircon 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 andesine COMENDITE (peralkalic rhyolite)« ne TRACHYTE (nepheline-normative trachyte with ne< 10%) to calcic anorthoclase, along with augite, opaques, and, in some PHONOLITE (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, orthoclase, albite, nepheline, leucite, and kaliophilite (Thornton and residual trachytic liquids are segregated by filter pressing from crys- Tuttle, 1960). tallizing magma 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 silicic 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 lava 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 tuff, 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 lavas 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 porphyritic quartz Decie Formation trachyte with alkali feldspar phenocrysts 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).
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