Laccoliths of the Ortiz porphyry belt, Santa Fe County, New Mexico Stephen R. Maynard, Consulting Geologist, 2132-A Central Ave. SE, #262, Albuquerque, NM 87106, [email protected] Abstract lithic (concordant) style of intrusion to the ding. The plane of the intrusion remains sen- later stock (discordant) style of intrusion in sibly parallel to the Earth’s surface at the Twelve distinct laccoliths can be identified in the area is discussed in the context of the time of the intrusion, although the floor of the Ortiz porphyry belt. The laccoliths are transition from Laramide to Rio Grande large intrusions may sag as the intrusion grows. c) In cross section, the ideal laccolith composed of plagioclase-orthoclase-horn- rift tectonics. blende-quartz porphyry, which can be classi- of Gilbert (Fig. 2A) has the shape of a plano- fied as quartz andesite by its modal assem- convex, or doubly convex, lens flattened in blage. Laccoliths from 33.2 to 36.2 Ma and Location and geologic setting the plane of the bedding of the invaded for- related sills in the Ortiz porphyry belt, which mation. The lens may be symmetrical or intrude the entire range of the approximate- The mountain group consisting of South asymmetrical in profile; circular, elliptical, or ly 3-km-thick (1.9-mi-thick) Phanerozoic Mountain, the San Pedro Mountains, the irregular in plan view. d) There is a complete stratigraphic section, predate mid-Tertiary Ortiz Mountains, and the Cerrillos Hills in gradation between sills and laccoliths, with movement on the Tijeras–Cañoncito fault Santa Fe County, New Mexico, composed no clearly defined point at which a sill system and 27.9–31.4 Ma stocks, dikes, and of mid-Tertiary intrusive rocks and their becomes a laccolith. e) A laccolith lifts its roof as a result of the forcible injection of magma. base- and precious-metal mineralization. hosts, comprise the Ortiz porphyry belt. The range of depth of formation of concor- Hunt and Mabey (1966) point out that the dant intrusions in the Ortiz porphyry belt is Rocks associated with the Ortiz porphyry presence of concordant roof pendants, which about three times that commonly observed belt are also exposed in the La Ciénega are borne upward from shallow depths, in other laccolith groups. Two hypotheses area (Fig. 1). Roadcut exposures of lacco- implies a floored, laccolithic intrusion. are proposed to explain this feature: (1) Pre lithic rocks can be viewed along NM–14, at Corry restricts the use of the term lacco- 34-Ma movement on the Tijeras–Cañoncito points 5.0 mi (8 km) and 7.4 mi (12 km) lith to bodies greater than 30 m (98 ft) thick fault system placed laccolith-hosting Penn- north of Golden; and at Devil’s Throne, 0.8 (Corry 1988, appendix A). Corry’s thick- sylvanian and Permian strata on the south- mi (1.3 km) west of Cerrillos on County ness criteria are used in this study. east side of the fault system to the same ele- Road 55. The Cerrillos Hills laccolith is vation as Mesozoic strata on the northwest well exposed on hiking trails in the Cerril- side, allowing the laccoliths and sills to Laccolith models los Hills Park in the southwestern part of intrude in a narrower depth range; and (2) Cross-sectional diagrams of various lacco- Growing volcanic edifices, contemporane- the Cerrillos Hills. All other outcrops are ous with the intrusion of laccoliths and sills, on private land. Land owner permission is lithic models are presented in Figure 2. caused the range of neutral buoyancy, in required to visit them. Gilbert’s (1877) original laccolithic model which concordant intrusive bodies would The roughly north-south trending Ortiz (Fig. 2A) was modified by Jackson and Pol- form, to rise over time. Both of these models porphyry belt occupies the structural high lard (1988a) and placed into a stratigraphic may have worked together. between the Hagan and Santo Domingo context (Fig. 2B). The Christmas Tree lacco- The transition from Laramide east-west- Basins and the Sandia uplift on the west, lith, a series of multiple concordant intru- directed compressional regional stress field and the southern part of the Española sions connected by narrow feeder dikes (with vertical least compressive stress) to the Basin and the Estancia Basin on the east. (Fig. 2C), and the punched laccolith (Fig. Rio Grande rift-related extensional stress 2D) may be considered to be end-member field (horizontal least compressive stress) is The porphyry belt is cut by strands of the constrained by the ages of the older lacco- Tijeras–Cañoncito fault system in its South cases of laccolithic forms (Corry 1988). liths and the younger stocks, and by their Mountain and San Pedro and Ortiz Moun- Feeders for laccoliths have been consid- field relations. Igneous rocks of the Ortiz tains portions. ered by most investigators to be dikes or porphyry belt straddle the transition. Lacco- narrow pipelike conduits situated central- liths intruded near the waning of the ly underneath the laccolithic mass (e.g., Laramide stress regime during the interval Definition Gilbert 1877; Jackson and Pollard 1988a). from 33.2 to 36.2 Ma and may have occupied Laboratory models of laccolithic formation Laramide structures such as folds and arch- A laccolith has been defined as “A concor- dant igneous intrusion with a convex-up generally start with the centrally located es. Post-laccolith movement on the Tijeras– dike or conduit as well (e.g., Kerr and Pol- Cañoncito fault system and subsequent roof and known or assumed flat floor” intrusion of 27.9–31.4 Ma discordant stocks (Glossary of Geology, 4th edition, 1997, J. A. lard 1998; Roman-Berdiel et al. 1995; and and dikes indicate a change in regional stress Jackson, ed.). Laccoliths were first Dixon and Simpson 1987). fields, constraining the timing of the transi- described by Gilbert (1877) in his studies of Hyndman and Alt (1987) documented tion to the period from 31.4 to 33.2 Ma. the Henry Mountains of Utah as being laccolith feeder dikes that are tangential formed by magma that “insinuated itself with respect to laccoliths in the Adel Introduction between two strata, and opened for itself a Mountains of central Montana. The feeder chamber by lifting all the superior beds” dikes are members of radial swarms that This report describes the stratigraphic and (Fig. 2A). Since Gilbert’s work, laccoliths apparently were controlled by an overly- structural settings, and field relationships have come to be viewed as having the fol- ing central volcano (Hyndman and Alt, of laccoliths of the Ortiz porphyry belt. lowing characteristics listed by Corry 1987, fig. 3). Theoretical concepts of laccolith formation (1988; after Daly 1933): Feeders as central stocks with laccoliths are reviewed, with attention paid to the a) Laccoliths are formed by forcible intrusion growing laterally as tongue-shaped mass- development of the concept in the Ortiz of magma and initially are entirely enclosed es have been posited in the Henry Moun- porphyry belt. Models to account for the by the invaded formations except along the tains (Hunt 1953, 1988). Jackson and Pol- anomalously large range of depths of relatively narrow feeding channel. b) Like lard (1988a,b) disputed Hunt’s interpreta- laccolith emplacement in the Ortiz por- sills, laccolith contacts commonly follow a tion, citing structural relations of the phyry belt are advanced and discussed. bedding plane, though many instances are overlying sedimentary rocks, and conclud- known where the intrusion cuts across bed- Finally, the transition from the early lacco- ed that the source of the laccoliths was February 2005, Volume 27, Number 1 NEW MEXICO GEOLOGY 3 FIGURE 1—Simplified geologic map of the Ortiz porphyry belt, showing laccoliths in orange, later intrusive rocks in red, and late dikes in green. Fold axial traces: orange = laccolith related (mid-Ter- tiary), magenta = Laramide, and lavender = Rio Grande rift related. Faults are black. 4 NEW MEXICO GEOLOGY February 2005, Volume 27, Number 1 FIGURE 2—Laccolith models. A) Gilbert’s schematic cross section of a laccolith (Gilbert 1877); B) model of Henry Mountain laccolithic structure (Jackson and Pollard 1988a); note depths of emplace- ment and flexural slip of overlying beds; C) “Christmas Tree” laccolith model (Corry 1988); D) Punched laccolith (Corry 1988). February 2005, Volume 27, Number 1 NEW MEXICO GEOLOGY 5 probably narrow dikes centrally located intrusion. Magma rises in cracks until the Mountains and emphasized the sill-like under the laccolithic masses. weight of the magma above the neutrally nature of much of the intrusive rock. Peter- buoyant elevation balances the magma son (1958) and McRae (1958) described Stress field driving pressure. Some mechanism must laccolithic intrusions in the Ortiz Moun- allow magma to spread horizontally, form- tains. In an isotropic medium, tabular intrusions ing “protolaccoliths” or sills. Recent quadrangle-scale and more form perpendicular to the axis of least Dixon and Simpson (1987) present a detailed geologic mapping allows for more stress sigma 3. Therefore laccoliths must four-stage model of laccolith formation, detailed descriptions of individual lacco- form in a stress environment characterized based on observations of centrifuge mod- liths and recognition of the traits common by a vertical sigma 3; vertical dikes form in eling using silicone putty: (1) The sill stage to all parts of the Ortiz porphyry belt (Fer- a regime of horizontal sigma 3. Magma is represented by the spread of putty from guson et al. 1999; Maynard 2000; Maynard may propagate along a plane that is per- a vertical conduit to form a concordant et al. 2001; Lisenbee and Maynard 2001; pendicular to the axis of least stress, sigma intrusion of low aspect ratio (ampli- Maynard et al.