Eastern Kentucky University Encompass EKU Faculty and Staff choS larship 1-2008 Large-scale alkalic magmatism associated with the Buckhorn caldera, Trans-Pecos Texas, USA: Comparison with Pantelleria, Italy. Don Parker Baylor University John C. White Eastern Kentucky University, [email protected] Follow this and additional works at: http://encompass.eku.edu/fs_research Part of the Geochemistry Commons, Geology Commons, and the Volcanology Commons Recommended Citation Parker, D.F. and White, J.C., 2008, Large-scale alkalic magmatism associated with the Buckhorn caldera, Trans-Pecos Texas, USA: Comparison with Pantelleria, Italy. Bulletin of Volcanology, v. 70, p. 403-415. (doi: 10.1007/s00445-007-0145-2) This Article is brought to you for free and open access by Encompass. It has been accepted for inclusion in EKU Faculty and Staff choS larship by an authorized administrator of Encompass. For more information, please contact [email protected]. Bull Volcanol (2008) 70:403–415 DOI 10.1007/s00445-007-0145-2 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 Mountains. Adobe Canyon Formation (~125 km3) contains Davis and adjacent Barrilla Mountains include lava units fewer flows, some up to 180 m thick, of chemically that bracketed a large pantelleritic ignimbrite (Gomez Tuff) homogenous, mildly peralkalic comendite, extending up to in rapid eruptions spanning 300,000 years. Extensive silicic 40 km. Gomez Tuff (~220 km3) may represent the largest lavas formed the shields of the Star Mountain Formation known pantellerite. It is typically less than 100 m thick in (37.2 Ma-K/Ar; 36.84 Ma 39Ar/40Ar), and the Adobe extra-caldera sections, where it shows a pyroclastic base Canyon Formation (37.1 Ma-K/Ar; 36.51-39Ar/40Ar). The and top, although interiors are commonly rheomorphic, Gomez Tuff (36.6 Ma-K/Ar; 36.74-39Ar/40Ar) blanketed a containing flow banding and ramp structures. Most sections large region around the 18×24 km diameter Buckhorn contain one cooling unit; two sections contain a smaller, caldera, within which it ponded, forming sections up to upper cooling unit. Chemically, the tuff is fairly homoge- 500 m thick. Gomez eruption was preceded by pantelleritic neous, but is more evolved than early pantelleritic domes. rhyolite domes (36.87, 36.91 Ma-39Ar/40Ar), some of Overall, although Davis Mountains silicic units were which blocked movement of Star Mountain lava flows. generated through open system processes, the pantellerites Following collapse, the Buckhorn caldera was filled by appear to have evolved by processes dominated by trachyte lava. Adobe Canyon rhyolite lavas then covered extensive fractional crystallization from parental trachytes much of the region. Star Mountain Formation (~220 km3) similar to that erupted in pre- and post-caldera lavas. is composed of multiple flows ranging from quartz trachyte Comparison with the Pantelleria volcano suggests that the to mildly peralkalic rhyolite; three major types form a total most likely parental magma for the Buckhorn series is of at least six major flows in the northeastern Davis 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 This paper constitutes part of a special issue dedicated to Bill caldera appear to have assimilated or mixed with crustal Bonnichsen on the petrogenesis and volcanology of anorogenic melts, and, generally, may not be regarded as mafic rhyolites. precursors of the Buckhorn silicic rocks, They thus form a Editorial responsibility: E. Christiansen false Daly Gap as opposed to the true basalt/trachyte Daly gap of Pantelleria. 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. Keywords Pantellerite . Rheomorphic tuff . Caldera . Extensive silicic lava . Assimilation fractional crystallization D. F. Parker (*) Department of Geology, Baylor University, Waco, TX 76798-7354, USA e-mail: [email protected] Introduction J. C. White Department of Earth Sciences, Eastern Kentucky University, Peralkalic silicic calderas are commonly small (<10 km Richmond, KY 40475-3102, USA diameter), often surmount trachytic shield volcanoes, and 404 Bull Volcanol (2008) 70:403–415 erupt ignimbrites with total magmatic volumes of only a 1989) such as the Star Mountain and Adobe Canyon few km3 (Mahood 1984; Williams et al. 1984). The largest formations of this report, occur, as well as mafic (mostly silicic peralkalic ignimbrites known to us are those of Gran alkalic intermediate rocks) and trachytic lava units; volca- Canaria (total volume >100 km3; Schmincke and Swanson niclastic sedimentary rocks occur at several major horizons. 1967) and those associated with the McDermitt Caldera Minor Basin and Range age alkalic basaltic dikes contain- (three units with total volume ~1,000 km3; Conrad 1984). ing mantle and lower crustal xenoliths also occur in the Peralkalic ignimbrites often are densely welded throughout immediate region of the Buckhorn Caldera (Henry et al. sections, exhibit strongly rheomorphic structures as a result 1989). These are undated, but are unlikely to be any older of flowage after emplacement, and may be easily confused than 24 Ma, the age of the oldest Basin and Range basalts with silicic lava or, in places, welded air fall deposits (Henry and McDowell 1986; Henry et al. 1991). (Gibson 1970; Henry et al. 1989; Henry and Wolff 1992; Wolff and Wright 1981). Northeastern Davis Mountains We report on what may be largest strongly peralkalic ignimbrite, the pantelleritic Gomez Tuff (~220 km3), The three major units of this report (Star Mountain which was erupted ~36.7 Ma from the 18×24-km-diameter Formation −36.85 Ma; Gomez Tuff −36.74 Ma; Adobe Buckhorn Caldera (Parker 1986; Henry et al. 1989, 1994) Canyon Formation −36.51 Ma) were rapidly erupted during in the northern Davis Mountains of Trans-Pecos Texas. We the late Eocene (40Ar/39Ar ages from Henry et al. 1994). also describe slightly older and slightly younger extensive Following their eruption, igneous activity shifted to the alkalic silicic lava units, the Star Mountain and Adobe southwest around the Paradise Mountain caldera, which Canyon formations, respectively, which are spatially and produced tuffs and extensive silicic lavas of the Barrel temporally associated with the Buckhorn Caldera. We Springs and Wild Cherry formations (35.61 and 35.35 Ma, compare and contrast these rocks with the Pantelleria respectively) that extended northeastward into the north- Volcano, the type locality for the peralkalic, iron-rich eastern Davis Mountains (Parker and McDowell 1979; rhyolite named from this small island in the Strait of Sicily Henry et al. 1994). (Mahood and Hildreth 1986; Civetta et al. 1998). Buckhorn Caldera Field Relations Trans-pecos volcanic field The Buckhorn Caldera (~18×24 km) formed during the Magmatism in the Trans-Pecos volcanic field lasted from eruption of the Gomez Tuff, which spread out as a about 48 to 17 Ma (Henry and McDowell 1986). Early relatively thin sheet (<100 m but commonly <20 m thick) magmatism (~47 Ma) with continental arc affinities oc- around the caldera but accumulated to thickness up to curred in the El Paso area (Barnes et al. 1991); magmatism 500 m within it (Parker 1986; Fig. 1). Failure of caldera peaked during the period 38–32 Ma, during which time walls formed of structurally weak upper Cretaceous rocks numerous calderas formed and extensive silicic lava units (limestone and shale) and Tertiary Huelster Formation were erupted; from 32–28 Ma, silicic magmatism waned. (volcaniclastic sedimentary rock with interlayered mafic The final episode from 24–17 Ma was dominated by lava) was concurrent with collapse. Megabreccia blocks widespread eruptions of minor alkalic basalt (preserved (Lipman 1976) of these units up to 100 m diameter occur mostly as dikes and lava flows) associated with Basin and within the tuff (Parker 1986; Henry et al. 1989). Range extension. Alkalic rocks occur throughout the Within the caldera, the Gomez Tuff varies in thickness magmatic intervals after 38 Ma (Barker 1987). due to the topography over which it was emplaced (stop 2, Price et al. 1986). Much of this paleotopography was Davis Mountains volcanic field developed over volcanic domes within the Tertiary Huelster Formation, which overlies marine Cretaceous strata and The Davis Mountains volcanic field is the largest contig- underlies Gomez Tuff in the northern Davis Mountains and uous segment of the Trans-Pecos volcanic field and was lava of the Star Mountain Formation in the Barrilla largely formed within the major episode of silicic eruption Mountains. Although the Huelster Formation, over its from 38–32 Ma (Parker and McDowell 1979; Henry et al. outcrop, is dominantly composed of volcaniclastic sedi- 1994). Field study has identified four calderas in the Davis mentary rock with interlayered mafic lava, in the Buckhorn Mountains (see Appendix Fig. 8); others are suspected Caldera area it locally contains trachytic lava, peralkalic (Parker et al. 1991). The Buckhorn Caldera, the oldest volcanic domes and their associated pyroclastic products and largest of the four, is the subject of this report. Besides and intrusive equivalents. We interpret these trachyte lavas the calderas, extensive silicic lava units (Henry et al. 1988, and peralkalic volcanic domes as early samples of trachyte Bull Volcanol (2008) 70:403–415 405 Fig. 1 Distribution map of Gomez Tuff and Adobe Canyon Formation, Davis and Barrilla Mountains, Texas (after Parker 1986). Diamonds mark location of measured sections and thick- ness in m of Gomez Tuff. Heavy isopach lines (50, 100 and 300 m) for Gomez Tuff; lighter lines (100, 200 and 300 m) for Adobe Canyon Formation lava.