Geology of the Davis Mountains: The Buckhorn Caldera and the Gomez Tuff, Papers of Theatre

Download Geology of the Davis Mountains: The Buckhorn Caldera and the Gomez Tuff and more Papers Theatre in PDF only on Docsity! RESEARCH ARTICLE Large-scale silicic alkalic magmatism associated with the Buckhorn Caldera, Trans-Pecos Texas, USA: comparison with Pantelleria, Italy Don F. Parker & John C. White Received: 1 February 2005 /Accepted: 8 March 2007 / Published online: 20 June 2007 # Springer-Verlag 2007 Abstract Three major rhyolite systems in the northeastern Davis and adjacent Barrilla Mountains include lava units that bracketed a large pantelleritic ignimbrite (Gomez Tuff) in rapid eruptions spanning 300,000 years. Extensive silicic lavas formed the shields of the Star Mountain Formation (37.2 Ma-K/Ar; 36.84 Ma 39Ar/40Ar), and the Adobe Canyon Formation (37.1 Ma-K/Ar; 36.51-39Ar/40Ar). The Gomez Tuff (36.6 Ma-K/Ar; 36.74-39Ar/40Ar) blanketed a large region around the 18×24 km diameter Buckhorn caldera, within which it ponded, forming sections up to 500 m thick. Gomez eruption was preceded by pantelleritic rhyolite domes (36.87, 36.91 Ma-39Ar/40Ar), some of which blocked movement of Star Mountain lava flows. Following collapse, the Buckhorn caldera was filled by trachyte lava. Adobe Canyon rhyolite lavas then covered much of the region. Star Mountain Formation (~220 km3) is composed of multiple flows ranging from quartz trachyte to mildly peralkalic rhyolite; three major types form a total of at least six major flows in the northeastern Davis Mountains. Adobe Canyon Formation (~125 km3) contains fewer flows, some up to 180 m thick, of chemically homogenous, mildly peralkalic comendite, extending up to 40 km. Gomez Tuff (~220 km3) may represent the largest known pantellerite. It is typically less than 100 m thick in extra-caldera sections, where it shows a pyroclastic base and top, although interiors are commonly rheomorphic, containing flow banding and ramp structures. Most sections contain one cooling unit; two sections contain a smaller, upper cooling unit. Chemically, the tuff is fairly homoge- neous, but is more evolved than early pantelleritic domes. Overall, although Davis Mountains silicic units were generated through open system processes, the pantellerites appear to have evolved by processes dominated by extensive fractional crystallization from parental trachytes similar to that erupted in pre- and post-caldera lavas. Comparison with the Pantelleria volcano suggests that the most likely parental magma for the Buckhorn series is transitional basalt, similar to that erupted in minor, younger Basin and Range volcanism after about 24 Ma. Roughly contemporaneous mafic lavas associated with the Buckhorn caldera appear to have assimilated or mixed with crustal melts, and, generally, may not be regarded as mafic precursors of the Buckhorn silicic rocks, They thus form a false Daly Gap as opposed to the true basalt/trachyte Daly gap of Pantelleria. Keywords Pantellerite . Rheomorphic tuff . Caldera . Extensive silicic lava . Assimilation fractional crystallization Introduction Peralkalic silicic calderas are commonly small (<10 km diameter), often surmount trachytic shield volcanoes, and Bull Volcanol (2008) 70:403–415 DOI 10.1007/s00445-007-0145-2 This paper constitutes part of a special issue dedicated to Bill Bonnichsen on the petrogenesis and volcanology of anorogenic rhyolites. Editorial responsibility: E. Christiansen Electronic supplementary material The online version of this article (doi:10.1007/s00445-007-0145-2) contains supplementary material, which is available to authorized users. D. F. Parker (*) Department of Geology, Baylor University, Waco, TX 76798-7354, USA e-mail: Don_Parker@baylor.edu J. C. White Department of Earth Sciences, Eastern Kentucky University, Richmond, KY 40475-3102, USA erupt ignimbrites with total magmatic volumes of only a few km3 (Mahood 1984; Williams et al. 1984). The largest silicic peralkalic ignimbrites known to us are those of Gran Canaria (total volume >100 km3; Schmincke and Swanson 1967) and those associated with the McDermitt Caldera (three units with total volume ~1,000 km3; Conrad 1984). Peralkalic ignimbrites often are densely welded throughout sections, exhibit strongly rheomorphic structures as a result of flowage after emplacement, and may be easily confused with silicic lava or, in places, welded air fall deposits (Gibson 1970; Henry et al. 1989; Henry and Wolff 1992; Wolff and Wright 1981). We report on what may be largest strongly peralkalic ignimbrite, the pantelleritic Gomez Tuff (~220 km3), which was erupted ~36.7 Ma from the 18×24-km-diameter Buckhorn Caldera (Parker 1986; Henry et al. 1989, 1994) in the northern Davis Mountains of Trans-Pecos Texas. We also describe slightly older and slightly younger extensive alkalic silicic lava units, the Star Mountain and Adobe Canyon formations, respectively, which are spatially and temporally associated with the Buckhorn Caldera. We compare and contrast these rocks with the Pantelleria Volcano, the type locality for the peralkalic, iron-rich rhyolite named from this small island in the Strait of Sicily (Mahood and Hildreth 1986; Civetta et al. 1998). Trans-pecos volcanic field Magmatism in the Trans-Pecos volcanic field lasted from about 48 to 17 Ma (Henry and McDowell 1986). Early magmatism (~47 Ma) with continental arc affinities oc- curred in the El Paso area (Barnes et al. 1991); magmatism peaked during the period 38–32 Ma, during which time numerous calderas formed and extensive silicic lava units were erupted; from 32–28 Ma, silicic magmatism waned. The final episode from 24–17 Ma was dominated by widespread eruptions of minor alkalic basalt (preserved mostly as dikes and lava flows) associated with Basin and Range extension. Alkalic rocks occur throughout the magmatic intervals after 38 Ma (Barker 1987). Davis Mountains volcanic field The Davis Mountains volcanic field is the largest contig- uous segment of the Trans-Pecos volcanic field and was largely formed within the major episode of silicic eruption from 38–32 Ma (Parker and McDowell 1979; Henry et al. 1994). Field study has identified four calderas in the Davis Mountains (see Appendix Fig. 8); others are suspected (Parker et al. 1991). The Buckhorn Caldera, the oldest and largest of the four, is the subject of this report. Besides the calderas, extensive silicic lava units (Henry et al. 1988, 1989) such as the Star Mountain and Adobe Canyon formations of this report, occur, as well as mafic (mostly alkalic intermediate rocks) and trachytic lava units; volca- niclastic sedimentary rocks occur at several major horizons. Minor Basin and Range age alkalic basaltic dikes contain- ing mantle and lower crustal xenoliths also occur in the immediate region of the Buckhorn Caldera (Henry et al. 1989). These are undated, but are unlikely to be any older than 24 Ma, the age of the oldest Basin and Range basalts (Henry and McDowell 1986; Henry et al. 1991). Northeastern Davis Mountains The three major units of this report (Star Mountain Formation −36.85 Ma; Gomez Tuff −36.74 Ma; Adobe Canyon Formation −36.51 Ma) were rapidly erupted during the late Eocene (40Ar/39Ar ages from Henry et al. 1994). Following their eruption, igneous activity shifted to the southwest around the Paradise Mountain caldera, which produced tuffs and extensive silicic lavas of the Barrel Springs and Wild Cherry formations (35.61 and 35.35 Ma, respectively) that extended northeastward into the north- eastern Davis Mountains (Parker and McDowell 1979; Henry et al. 1994). Buckhorn Caldera Field Relations The Buckhorn Caldera (~18×24 km) formed during the eruption of the Gomez Tuff, which spread out as a relatively thin sheet (<100 m but commonly <20 m thick) around the caldera but accumulated to thickness up to 500 m within it (Parker 1986; Fig. 1). Failure of caldera walls formed of structurally weak upper Cretaceous rocks (limestone and shale) and Tertiary Huelster Formation (volcaniclastic sedimentary rock with interlayered mafic lava) was concurrent with collapse. Megabreccia blocks (Lipman 1976) of these units up to 100 m diameter occur within the tuff (Parker 1986; Henry et al. 1989). Within the caldera, the Gomez Tuff varies in thickness due to the topography over which it was emplaced (stop 2, Price et al. 1986). Much of this paleotopography was developed over volcanic domes within the Tertiary Huelster Formation, which overlies marine Cretaceous strata and underlies Gomez Tuff in the northern Davis Mountains and lava of the Star Mountain Formation in the Barrilla Mountains. Although the Huelster Formation, over its outcrop, is dominantly composed of volcaniclastic sedi- mentary rock with interlayered mafic lava, in the Buckhorn Caldera area it locally contains trachytic lava, peralkalic volcanic domes and their associated pyroclastic products and intrusive equivalents. We interpret these trachyte lavas and peralkalic volcanic domes as early samples of trachyte 404 Bull Volcanol (2008) 70:403–415 lated magma lumps that became drawn out to form ribbon- like structures during rheomorphic flow. Lithic inclusions of a variety of other rocks (limestone, sandstone, basalt) are present. A few enclaves of trachyte were discovered that may represent the product of magma mixing within the Buckhorn magma chamber. The tuff commonly overlies bedded tuffa- ceous sedimentary rock, but in one locality, it overlies an explosion breccia composed of green, flow-banded rhyolite. Air fall deposits are otherwise conspicuously absent. Star Mountain Formation Star Mountain Formation (SMF) is a widespread lava unit (~220 km3) in the eastern Davis Mountains and adjoining Barrilla Mountains (Gibbon 1967; Parker and McDowell 1979; Henry et al. 1989, 1994); it is the product of coalesced lava shields (Parker et al. 1991), and the term “flood rhyolite” has been proposed for it and similar units (Henry and Wolff 1992). Star Mountain Formation (SMF) is composed of multiple flows that range in composition from quartz trachyte to low-silica alkali rhyolite. Three units have been identified that consist of at least six flows; additional flows are suspected (Parker 1996). Flow units range up to 100 m thick and have clearly autobrecciated bottoms and tops and contain vesicles. Flow lobe fronts have been located where the flow shows abrupt lateral decrease to zero thickness, unlike pyroclastic flows that typically “feather” out. Despite intensive search for pyroclastic textures, none have been found in SMF units. The SMF units are broadly similar to other units in the Trans-Pecos region such as the Crossen Trachyte of the southern Davis Mountains (McAnulty 1955), lavas of the Decie Formation of the Paisano Volcano (Parker 1976, 1983) and the Bracks Rhyolite of the Sierra Vieja (Henry et al. 1990). They share many physical similarities to the extensive silicic lava flows of southwestern Idaho (Bonnichsen and Kauffman 1987), al- though the Davis Mountains units are lower temperature (see discussion below) than the Snake River Plain units (Andrews et al. 2007; Christiansen and McCurry 2007). “Low-silica” SMF is similar to trachyte lava present locally within the Buckhorn Caldera below the Gomez Tuff. Gomez Tuff overlies SMF in the Barrilla Mountains, although it is typically separated from the SMF by a few meters of sedimentary rock and/or mafic to intermediate lava. The SMF flows (37.2 Ma, 36.84 Ma) were blocked in their movement towards the center of the Buckhorn Caldera as they lapped upon the cones of pre- Gomez peralkalic rhyolite domes (36.87 Ma, 36.91 Ma). Adobe Canyon Formation Adobe Canyon Formation (ACF) is another extensive rhyolite lava unit (36.51 Ma) composed of thick flows of rhyolite (Parker 1986; Henry et al. 1994; Parker et al. 2000). It consists of only two flows of chemically homogeneous, mildly peralkalic comendite, one of which extends 40 km (Fig. 1). In the Buckhorn Caldera, the ACF is represented by a single, large flow (up to 180 m thick); immediately west of the caldera, two thick flows are present. Within the caldera, ACF overlies trachyte lava of the Fox Canyon Formation, which flooded the interior of the caldera after Gomez eruption, and where the Fox Canyon is absent, the Gomez Tuff, from which is separated by a few meters of sedimentary rock. In regional age dating, the Adobe Canyon Formation of the northern Davis Mountains was lumped with similar units (Sheep Pasture Formation; rhyolite of Tricky Gap; Rhyolite of Aquilla Creek) in the central Davis Mountains (Henry et al. 1994); in our report “ACF” will refer to the unit of the northern Davis Mountains and “other ACF” will refer to associated central Davis Mountains units, which are non-peralkalic and possess different trace element characteristics. Younger units The volcaniclastic Frazier Canyon Formation and “other ACF” silicic lava and silicic units of Barrel Springs Formation locally overlie Buckhorn Caldera rocks; in the nearby Barrilla Mountains, Frazier Canyon Formation (containing mafic lava flows as well as volcaniclastic sedimentary rock) and Barrel Springs Formation are widespread. The Barrel Springs units represent ignimbrite and extensive lava units that most likely were erupted from vents in the central Davis Mountains (Parker et al. 1991; Henry et al. 1994). Although metal- uminous, Barrel Springs units possess phenocryst assemb- lages and trace element characteristics similar to SMF silicic units (Henry et al. 1989; Parker et al. 1991). Basaltic dikes and other small mafic intrusions are present in the Northern Davis Mountains (Parker 1972; Henry et al. 1989). These were emplaced parallel to Basin and Range faults; some contain peridotite and granulite xenoliths. Our data set includes six basalts (undated) that probably belong to this younger suite; we compare these basalts with the mafic units associated with main phase volcanic section and with Pantelleria basalts in an attempt to constrain parental magmas for the silicic rocks. Pantelleria Pantelleria is a Pleistocene volcano located in the Strait of Sicily (Villari 1974; Mahood and Hildreth 1986); it is the type locality for the iron-rich, strongly peralkalic variety of rhyolite known as pantellerite (Washington 1913a, b, 1914). The island is dominated by the 6-km diameter Cinque Denti caldera (Monastero caldera of Cornette et al. 1983) formed during the eruption of the Green Tuff. The caldera contains Bull Volcanol (2008) 70:403–415 407 a trap-door structurally resurgent dome (Montagna Grande), and numerous younger domes and flows of pantellerite and pantelleritic trachyte and their associated pyroclastic cones (Mahood and Hildreth 1983, 1986; Civetta et al. 1984, 1998). Tephra from the Green Tuff eruption (45 ka) is a well-known marker bed in marine sediments of the Mediterranean (Keller et al. 1978). An older caldera may have formed 114 Ka, and is believed to have been filled by the eruption of three tuffs at 106, 94 and 79 Ka (Mahood and Hildreth 1986). Geochemistry and mineralogy Northern Davis Mountains rocks include basalt (transitional basalt and basanite) basaltic trachyandesite, trachyandesite, trachyte, and rhyolite (see Electronic supplementary material 1, 2 and 3; Fig. 4, this report). Pantelleria rocks are more bimodal, with abundant basalt and rhyolite and subordinate trachyte (Fig. 4, this report; Civetta et al. 1998). Davis Mountain rhyolites extend to higher silica and are less peralkaline, with many of the extensive silicic lava units of the Star Mountain and Adobe Canyon formations only borderline peralkaline (see Appendix, Fig. 9). In the Al2O3- FeO classification of peralkalic rhyolite of Macdonald (1974), the Pantelleria rocks are more iron-rich, with only one analysis plotting as comendite; in contrast, the Davis Mountains rocks are mostly comendite, with only Gomez Tuff and associated domes plotting as pantellerite, and Buckhorn Caldera trachyte and some SMF plotting as comenditic trachyte (see Appendix, Fig. 10). Geothermometry and geobarometry Phenocryst assemblages of NDM rocks do not readily yield data on intensive parameters, typically lacking two feldspars, two pyroxenes and two oxides (see Electronic supplemen- tary material 1; Electronic supplementary material 3). Only two northeastern Davis Mountains samples (SMF-85601; ACF-K34) contained FeTi oxides suitable for geothermo- metry; the Star Mountain sample yielded a temperature of 853°C and an fO2 of −13.97 and the Adobe Canyon sample yielded 888°C and −14.00 using the QUILF95 version of QUILF (Andersen et al. 1993). One sample of trachyte porphyry (DP142) contained two oxides, but the ilmenite was unsatisfactory; a trachyte porphyry (PP129) with similar mineralogy, and major and trace element chemistry from the Paisano volcano of the southern Davis Mountains (Parker 1976) yielded 900°C and −12.57 by QUILF. A two-oxide Barrel Springs metaluminous rhyolite lava (sample 87508) from the central Davis Mountains with a spider plot similar to Gomez Tuff yielded 815°C and log fO2=−15.83. Gomez Tuff analyses, when projected into the qz-ab-an-H2O system (with 4.5% acmite and 4.5% Na metasilicate) plot close to the minimum (750°C; Carmichael and MacKenzie 1963). When this temperature is applied to QUILF equilibria, four samples of Gomez Tuff yielded the following log fO2 values: −17.54, −17.24, −17.30, −17.78 (assuming P=2 kbar). These temperature results (trachyte to rhyolite, ~900–750°C) compare favorably with the ~990-700°C range determined for Pantelleria trachyte to pantellerite (White et al. 2005). Isotopic compositions Cameron et al. (1996) reported Nd, Pb and Sr isotopic compositions of Davis Mountains igneous rocks, including Buckhorn Caldera units, SMF and ACF, the Paisano Fig. 4 Davis Mountains (this report) and Pantelleria rocks (this report; Civetta et al. 1998) plotted in Total Alkali Silica classification scheme (LeMaitre 1989). Davis Mountains series extends to higher silica and contains basaltic trachyandesite and trachyandesite. Basalt in Davis Mountains is probably restricted to younger Basin and Range intrusions. B basalt; TB trachybasalt; BTA basaltic trachyandesite; TA trachyande- site; T trachyte; R rhyolite. Pantelleria analyses form a classic Daly gap between transitional basalt and trachyte (note: in following figures, only Baylor analyses are used to represent Pantelleria) 408 Bull Volcanol (2008) 70:403–415 Volcano and the younger, metaluminous units associated with the Paradise Mountain Caldera. Two mafic Buckhorn rocks had ENdt of 2.24 and 1.38; two Fox Canyon Formation trachytes had values of 0.50, samples of Gomez Tuff 0.07 and the Cherry Canyon dome −0.05. One sample of ACF was 0.30. Five SMF samples had ENdt ranging from −0.50 to −1.57. Units associated with the Paradise Mountain Caldera, and the Paisano Volcano, had ENdt values ranging from, respectively, 1.38 to −0.20 and from 1.62 to −1.63. The ENdt of Paradise Mountain Caldera rocks are thus similar to Buckhorn rocks and those of the Paisano volcano similar to SMF rocks. These values precluded the involvement of ancient silicic crust in the genesis of Davis Mountain silicic rocks and limited crustal sources to either Cenozoic silicic rocks or unmetamor- phosed mafic rocks of any age. A follow-up study of Grenvillian basement granulite samples from the Big Bend area of west Texas documented strong isotopic contrast between these samples and the Davis Mountains rocks (Cameron and Ward 1998). Forbidden Mountain (north- eastern Davis Mountains) Tertiary-age granulite samples studied by Cameron and Ward, however, had, with one exception, ENdt (3.26 to −0.55). ENd for two undated Basin and Range basalts of this report are 3.34 and 2.13 Fig. 5 Spider diagram for Buck- horn rocks: a (basalt and tra- chyandesite), b (trachyandesite, trachyte and 1 dome sample), and c (remaining dome samples and Gomez Tuff). Spider plots for Star Mountain and Adobe Canyon analyses (not shown) are similar. Normalization fac- tors to bulk silicate earth from McDonough and Sun (1995) Fig. 6 Spider diagram for Pan- telleria rocks (Baylor analyses): a (basalts) and b (trachytes and rhyolites). The two trachytes with smallest Eu anomaly (sam- ples 030503 and 030514) have the lowest SiO2 values. Normal- ization to bulk silicate earth of McDonough and Sun (1995) Bull Volcanol (2008) 70:403–415 409 episode. Six probable Basin and Range age Davis Moun- tains basaltic rocks have Mg numbers ranging from 66 to 41; the two Pantelleria analyses have 53 and 50. The older northern Davis Mountains mafic rocks occur mostly as lavas, have lower Mg numbers (45–12) and are quartz normative rather than transitional. These older mafic rocks (trachybasalt and trachyandesite) do not appear to be genetically related to the peralkaline silicic rocks and may have assimilated some unknown crustal component, as discussed above. Pantelleria exhibits complete gradation between trachyte and rhyolite, as does SMF of this report, reinforcing the widely held theory that peralkalic rhyolite may be closely related to trachyte by a fractional crystallization process (Weaver et al. 1972; Bizouard et al. 1980; Parker 1983; Civetta et al. 1998). In the Buckhorn caldera, however, a clear compositional gap existed between peralkalic rhyolite and underlying trachyte, suggesting that, although they may have been related by a fractional crystallization process, a large cupola of highly differentiated magma overlay less evolved trachyte in sharp discontinuity. Major element mass balance calculations suggest that 81wt% crystalliza- tion of a parental trachyte (DP142) could produce 19% Gomez Tuff pantellerite (97408). Many silicic systems have been shown to have been erupted from magma chambers in which silicic magma overlay less-evolved melt in sharp discontinuity (cf. Druitt and Bacon 1988; Macdonald 1987). As the Buck- horn caldera was not resurgent, and as the post-caldera eruptions were trachytes, the Buckhorn pluton was probably drained during Gomez eruption of most of its pantelleritic cap. This cap may have resembled the pancake- shaped magma bodies proposed for tuffs associated with large calderas such as the Lund Tuff (Christiansen 2004) and laccolith calderas (Henry et al. 1997). Side-wall crystallization in which evolved magma migrates up the flanks of plutons has been proposed for Kenya pantellerite volcanoes (Macdonald 1987). An addi- tional viable petrogenetic mechanism might be separation of evolved silicic magma from trachytic crystal mushes by filter pressing such as proposed for the Paisano volcano, which also has peralkalic rhyolite erupted before trachyte (Parker 1983). A similar mechanism has recently been forwarded for the production of crystal-poor rhyolite (with trace element contents suggestive of crystal fractionation) from granodioritic plutons by melt segregation from crystal mushes (Bachmann and Bergantz 2004). The slightly older SMF lavas and the slightly younger ACF lavas, both of which have some similarities to the Buckhorn magma series, suggest that the Buckhorn system, as well as those of other Davis Mountains units of the Paisano Volcano and the Barrel Springs Formation, were special cases of a more general process that led to creation of flood rhyolite units through open system processes involving a combination of mantle and crustal sources, crystal fractionation, and magma mixing. The exact nature of those mantle and crustal sources, however, remain poorly constrained. Conclusions 1. Pantellerite of the Buckhorn Caldera is present in pre- caldera volcanic domes and associated ejected blocks and in the 220 km3 Gomez Tuff. Other, roughly contempo- raneous silicic lava units include trachyte, quartz tra- chyte, alkali rhyolite and mildly peralkaline comendite. 2. The Buckhorn Caldera was flooded with trachyte lava following eruption of the Gomez Tuff, without any apparent resurgent doming. These lavas and trachyte enclaves within Gomez Tuff suggest that an evolved cap of pantellerite magma lying above a trachyte reservoir was tapped during Gomez eruptions. 3. Buckhorn magmas exhibit trace element patterns similar to units associated with the Pleistocene Pan- telleria Volcano, except for lower Nb, Th and La in the Buckhorn silicic units and higher incompatible element contents in Davis Mountain basalts. Silicic rocks of both series show prominent negative anomalies for Ba, Sr, P, Eu and Ti, all of which would be depleted by fractional crystallization of observed phenocrysts. 4. Nd isotopic values previously published preclude involvement of ancient silicic crust in the genesis of the Davis Mountains series, and limit crustal sources to Tertiary silicic rocks and mafic rocks of any age. Open system processes probably led to the observed differ- ences between silicic units, although evolution of Buckhorn Caldera magmas appears to have been dominated by fractional crystallization. 5. The Daly gap between basalt and trachyte on Pan- telleria is paralleled by a false gap between Davis Mountain mafic volcanic units and trachyte and rhyolite of the Buckhorn Caldera, to which they are not related. True basalt, including both alkali and transitional varieties, has only been found in the Davis Mountains as small dikes and plugs most likely related to a younger phase of Basin and Range extension. Acknowledgements We wish to thank the Department of Geology, Baylor University for support of this project, the University of Texas at Austin for access to their electron microprobe facility, and the landowners of the Davis Mountains for access to their land. We also thank Minghua Ren for analysis of a Pantelleria richterite. We thank Elizabeth Anthony and Ricardo Avanzinelli for critical reviews of the manuscript, as well as Daniel Barker for review of an earlier version. Chris Henry read and improved our ideas and supplied three analyses of Basin and Range basalts from the area. None of the reviewers are responsible for any errors that remain in the paper. 412 Bull Volcanol (2008) 70:403–415 Appendix Fig. 9 Agpaitic index vs. silica plot (this report). Pantelleria silicic rocks are more strongly peralkaline than Buckhorn rocks. Star Mountain and Adobe Canyon rocks have agpaitic index close to 1.0 Fig. 10 Silicic NE Davis Mountains and Pantelleria rocks (this report) plotted within the classification scheme of Macdonald (1974). Pantelleria rocks extend from comenditic trachytes (CT) through pantelleritic trachytes (PT) to pantellerite; Davis Mountains rocks contain less FeO; Buckhorn silicic rocks (domes and Gomez Tuff) are pantelleritic (P); Star Mountain and Adobe Canyon rhyolites are comenditic (C); Buckhorn trachyte and some Star Mountain trachyte plot as comenditic trachyte Fig. 8 The Davis Mountains volcanic field with major known eruptive centers (BC Buckhorn Caldera; MC El Muerto Caldera; PM Paradise Mountain Caldera; PV Paisano Volcano; BA Big Aguja Center, a major center for Star Mountain Formation (SM). Solid line with hatches encloses major outcrop of volcanic rocks; dashed lines outline approximate caldera boundaries; dashed double line indicates approximate areal extent of volcanic rocks associated with Paisano Volcano; Dashed line with single hatches indicates major outcrop of Star Mountain Formation. Cities: B Balmorhea; A Alpine; M Marfa Fig. 11 Zr/Th vs. Th. Pantelleria and Buckhorn caldera rocks (trachytes, domes, Gomez Tuff) plot roughly parallel to Th axis with low negative slope (trend 1). Mafic lavas and Basin and Range basalts plot from vicinity of Pantelleria basalts (Zr/Th ~65; low Th) extending downwards to low Zr/Th values (trend 2). Sample DP60 is a unique plagioclase-biotite rhyolite (see text). Adobe Canyon Formation samples extend from high Th end of Gomez plot arc downwards towards higher Th values (trend 3). SMF samples plot within and above the space outlined by the three trends Bull Volcanol (2008) 70:403–415 413 References Andersen DJ, Lindsley DH, Davidson PM (1993) QUILF: a Pascal program to access equilibria among Fe–Mg–Mn–Ti oxides, pyroxenes, olivine and quartz. Comput Geosci 19:1333–1350 Anderson JE (1969) Development of snowflake texture in a welded tuff, Davis Mountains, Texas. 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Bull Volcanol 33:438–474 Gibson IL (1970) A pantelleritic ash-flow tuff from the Ethiopian Rift Valley. Contrib Min Petrol 28:89–111 Henry CD, McDowell FW (1986) Geochronology of magmatism in the Tertiary volcanic field, Trans-Pecos Texas. In: Price JG, Henry CD, Parker DF, Barker DS (eds) Igneous geology of Trans-Pecos Texas. Bureau of Economic Geology Guidebook vol 23, The University of Texas at Austin, USA, pp 99–122 Henry CD, Wolff JA (1992) Distinguishing strongly rheomorphic tuffs from extensive silicic lavas. Bull Volcanol 54:171–186 Fig. 12 Zr/Nb vs. Nb. Buckhorn caldera rocks (trachytes, domes, Gomez Tuff) form a rough trend parallel to the Nb axis (trend 1) with a Zr/Nb of ~12. One Gomez analysis and one dome analysis fall well below this trend. Mafic lavas and Basin and Range basalts extend down towards low Zr/Nb (trend 2). Adobe Canyon and “other Adobe Canyon” analyses extend from Gomez values towards low Zr/Nb and high Nb values (trend 3). Star Mountain analyses again plot largely within the space enclosed by these three trends. Pantelleria analyses form a linear trend with slight positive slope at low Zr/Nb (note: Pantelleria analyses with Nb>300 not shown in order to show Buckhorn data more clearly) 414 Bull Volcanol (2008) 70:403–415