Igneous evolution of a complex laccolith-caldera, the Solitario, Trans-Pecos Texas: Implications for calderas and subjacent plutons Christopher D. Henry* Nevada Bureau of Mines and Geology, University of Nevada, Reno, Nevada 89557 Michael J. Kunk U.S. Geological Survey, Reston, Virginia 20192 William R. Muehlberger Department of Geological Sciences, University of Texas, Austin, Texas 78712 W. C. McIntosh Department of Geoscience, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801 ABSTRACT the Solitario. The spatial and temporal rela- INTRODUCTION tion along with sparse geochemical data sug- The Solitario is a large, combination lac- gest that the peralkaline rhyolites are crustal Laccoliths and calderas are, respectively, two colith and caldera (herein termed “lacco- melts related to the magmatic-thermal flux of the most common and interesting intrusive and caldera”), with a 16-km-diameter dome over represented by the main pulse of Solitario extrusive expressions of magmatism. Otherwise, which developed a 6 × 2 km caldera. This lac- magmatism. they would seem to have little in common. Lac- cocaldera underwent a complex sequence of Current models of laccolith emplacement coliths and other concordant intrusions have been predoming sill, laccolith, and dike intrusion and evolution suggest a continuum from ini- intensely studied since the nineteenth century and concurrent volcanism; doming with em- tial sill emplacement through growth of the (e.g., Gilbert, 1887; Corry, 1988; Jackson and placement of a main laccolith; ash-flow erup- main laccolith. Although the Solitario lacco- Pollard, 1988; Roman-Berdiel et al., 1995). tion and caldera collapse; intracaldera sedi- caldera followed this sequence of events, our However, both the earliest and many recent stud- mentation and volcanism; and late intrusion. field and 40Ar/ 39Ar data demonstrate that it ies emphasized small, shallowly emplaced exam- Detailed geologic mapping and 40Ar/ 39Ar dat- developed through repeated, episodic magma ples such that the prevalent view may be that all ing reveal that the Solitario evolved over an injections, separated by 0.4 to 0.6 m.y. inter- laccoliths are small, shallow, short-lived, and not interval of approximately 1 m.y. in three dis- vals of little or no activity. This evolution re- representative of voluminous terrestrial magma tinct pulses at 36.0, 35.4, and 35.0 Ma. The quires a deep, long-lived magma source, well systems such as calderas and batholiths (Mudge, size, duration, and episodicity of Solitario below the main laccolith. 1968; Corry, 1988; Hanson and Glazner, 1995; magmatism are more typical of large ash-flow Laccoliths are commonly thought to be Roman-Berdiel et al., 1995). For example, the calderas than of most previously described small, shallow features that are not representa- Glossary of Geology (Bates and Jackson, 1987, laccoliths. tive of major, silicic magmatic systems such as p. 364) specifies that laccoliths are “generally . Small volumes of magma intruded as abun- calderas and batholiths. In contrast, we sug- less than five miles in diameter, and from a few dant rhyolitic to trachytic sills and small lacco- gest that magma chambers beneath many ash- feet to several hundred feet in thickness.” liths and extruded as lavas and tuffs during the flow calderas are tabular, floored intrusions, Calderas have also been intensely studied and first pulse at 36.0 Ma. Emplacement of the including laccoliths. Evidence for this conclu- are known to be among the largest and longest- main laccolith, doming, ash-flow eruption, and sion includes the following: (1) many large plu- lived volcanic systems, commonly undergoing caldera collapse occurred at 35.4 Ma during tons are recognized to be laccoliths or at least episodic activity over 1 m.y. or more (Smith and the most voluminous pulse. A complex se- tabular, (2) the Solitario and several larger Bailey, 1968; Lipman, 1984; Cas and Wright, quence of debris-flow and debris-avalanche de- calderas are known to have developed over lac- 1987). The association of calderas with underlying posits, megabreccia, trachyte lava, and minor coliths, and (3) magma chambers beneath plutons or batholiths is well established (Lipman, ash-flow tuff subsequently filled the caldera. calderas, which are as much as 80 km in diam- 1984). Considerable field and modeling research The final magmatic pulse at 35.0 Ma consisted eter, cannot be as deep as they are wide or has addressed the style of caldera collapse (Druitt of several small laccoliths or stocks and numer- some would extend into the upper mantle. The and Sparks, 1984; Lipman, 1984; Walker, 1984; ous dikes in caldera fill and along the ring frac- Solitario formed during a tectonically neutral Komuro, 1987; Scandone, 1990; Marti et al., ture. Solitario rocks appear to be part of a period following Laramide deformation and 1994; Branney, 1995). However, the shape of the broadly cogenetic, metaluminous suite. preceding Basin and Range extension. There- underlying magma body has been little discussed, Peralkaline rhyolite lava domes were em- fore, space for the main laccolith was made by in part because at most only the top of the former placed north and west of the Solitario at ap- uplift of its roof and possibly subsidence of the magma chamber is exposed in most calderas. The proximately 35.4 Ma, contemporaneous with floor, not by concurrent faulting. Laccolith- mechanism by which space is made for the caldera laccolith emplacement and the main pulse in type injection is probably a common way that magma chamber is an unresolved problem (Lip- space is made for magma bodies of appreciable man et al., 1984), and a problem for large intru- *E-mail: [email protected] areal extent in the upper crust. sions in general. Although mushroom-shaped or GSA Bulletin; August 1997; v. 109; no. 8; p. 1036–1054; 12 figures, 1 table. 1036 IGNEOUS EVOLUTION OF THE SOLITARIO, TRANS-PECOS TEXAS flat magma chambers have been inferred by some (Smith, 1979; Scandone, 1990), probably most ge- ologists picture spherical or deep cylindrical bod- ies beneath calderas, not laccoliths. It has been recognized that many plutons are tabular and include laccoliths (Hamilton and Myers, 1967; Bergantz, 1991; Brown, 1994; Vigneresse, 1995). Marsh (1989) stated that the nearer to the surface a magma chamber reaches, the more sheetlike it becomes, and sheetlike bod- ies are generally much more voluminous than tall bodies. Modeling by Clemens and Mawer (1992) indicates that granitic magmas should rise along fractures and generate laccolithic or thin, floored plutons of all sizes at any depth. Despite these studies, recent attempts to solve the space prob- lem—i.e., the way in which granite plutons make space in the upper or middle crust—have focused on the role of contemporaneous faulting (e.g., Hutton, 1988; Petford et al., 1993; Hanson and Glazner, 1995), although the importance of lac- coliths is beginning to be emphasized (Morgan and Law, 1996; Petford, 1996). Similarly, the possibility that caldera magma chambers are tab- ular has not received wide recognition, although laccolith-caldera systems have been described (Hildebrand, 1984; Henry and Price, 1989). In contrast, we suggest that the magma cham- bers of many calderas (and plutons in general) were laccoliths or otherwise thin, floored bodies. This view is based on detailed mapping and 40Ar/39Ar dating of the Solitario (Henry and Muehlberger, 1996; Henry and Kunk, 1996), a laccolith-caldera complex (herein termed “lacco- caldera”) in Trans-Pecos Texas (Figs. 1 and 2), and on published descriptions of numerous tabu- lar plutons and a few other laccocalderas. What is a Laccolith? Laccoliths were recognized by Gilbert (1877), who specified both an intrusive shape and mecha- nism of emplacement. Laccoliths are floored in- trusions over which host strata are domed. They form when magma rises vertically in a dike or other narrow conduit, spreads between horizontal strata as a sill, and then lifts the overlying strata so that they bend concordantly over the intrusion (see also Daly, 1933; Corry, 1988; Jackson and Pollard, 1988). Although attempts have been made to dis- Figure 1. Location and generalized geology of the Solitario, Trans-Pecos Texas. The Terneros tinguish laccoliths and sills on the basis of thick- Creek Rhyolite consists of peralkaline rhyolite lava domes that erupted at approximately ness/diameter ratios, most summaries have em- 35.4 Ma, contemporaneously with the main pulse of activity, laccolithic doming, ash-flow erup- phasized that there is complete gradation between tion, and caldera collapse in the Solitario. the two (Gilbert, 1877; Daly, 1933; Corry, 1988). A meaningful distinction may be that strata are distinctly arched over a laccolith and are only neg- must make space by lifting its roof and/or depress- Geologic Setting of the Solitario ligibly so over a sill. We think a multikilometer- ing its floor. In this report, we use “laccolith” thick concordant intrusion should be called a lac- where we are reasonably certain of intrusive shape The Solitario developed between 36 and colith, regardless of diameter and of curvature or and where cited authors use the term; where less 35 Ma, during the 38 to 32 Ma main phase of flatness of the roof, because such a thick body certain, we refer to “floored, tabular intrusions.” magmatism in Trans-Pecos Texas (Henry and Geological Society of America Bulletin, August 1997 1037 HENRY ET AL. more gentle, than other parts. Dips along the southwestern rim commonly reach 55°, which we interpret to indicate superposition upon the southwest-dipping Fresno-Terlingua monocline (Fig. 1). Dips along the southeastern rim are at most 28°. The southeastern rim is superposed upon the structurally high Terlingua uplift, the upthrown side of the monocline (Erdlac, 1990). The dome appears to be doubly hinged, simi- lar to host rocks of laccoliths of the Henry Moun- tains, Utah (Jackson and Pollard, 1988).