Structure and emplacement history of a multiple-center, cone-sheet–bearing ring complex: The Zarza Intrusive Complex, Baja California, Mexico S. E. Johnson* Department of Earth and Planetary Sciences, Macquarie University, Sydney, New South Wales 2109, Australia, and Departamento de Geología, CICESE, Km 107 Carratera, Ensenada-Tijuana, Baja California, México S. R. Paterson Department of Earth Sciences, University of Southern California, California 90089-0740 M. C. Tate Department of Earth and Planetary Sciences, Macquarie University, Sydney, New South Wales 2109, Australia ABSTRACT chambers and suggest that the complex may pletely accessible, and structural patterns are well have been overlain by a caldera. developed within the complex and surrounding The Cretaceous Zarza Intrusive Complex, country rocks. Thus, we were able to collect a de- located in the Peninsular Ranges of Baja Cali- INTRODUCTION tailed structural data set, and evaluate its intrusive fornia Norte, Mexico, is perhaps the best- history and emplacement mechanisms (a detailed preserved multiple-center, cone-sheet–bearing Because much of Earth’s continental crust was petrological/geochemical study of the complex ring complex documented in North America. formed and/or influenced by magmatic processes can be found in Tate et al., 1999). The complex The 7 km2 elliptical complex hosts three (e.g., Taylor and McLennan, 1985; Hamilton, consists of three nested intrusive centers, two of nested, non-concentric intrusive centers that 1989; Saleeby, 1990; Lipman, 1992; Yanagi and which contain some of the best-preserved cone are successively younger to the south. The Yamashita, 1994; Brown and Rushmer, 1997), sheets in North America. The Zarza Intrusive northern and central centers show the same the temporal and spatial evolution of magma Complex differs significantly from most other evolutionary sequence of (1) intrusion of con- plumbing systems remains one of the outstand- cone-sheet–bearing intrusive complexes in that it centric gabbroic cone sheets, (2) intrusion of ing problems in our search for a better under- is surrounded by an intense, concentric, ductile massive core gabbros, and (3) development of standing of how continents grow and evolve. deformation aureole in the adjacent country subvertical, ductile ring faults. Ring-fault kine- Particularly interesting and important parts of rocks. This aureole is intriguing, and we focus matics indicate that both centers moved down these systems are the pathways taken by magma particular attention on evaluating its formation. relative to the surrounding country rocks, sug- travelling from shallow magma chambers to vol- gesting collapse into an underlying magma canoes. Where the surface expression of magma- BACKGROUND AND DEFINITIONS chamber. The southern center is composed of tism is a caldera, the magma pathways com- approximately equal proportions of gabbro monly manifest themselves as ring complexes, In this paper we present evidence that the Zarza and tonalite and lacks cone sheets. Aluminum- which contain a wide variety of intrusive phases Intrusive Complex is a cone-sheet–bearing ring in-hornblende barometry on the tonalite indi- including cone sheets, ring dikes, and massive complex. Because ring complexes are relatively cates a maximum emplacement depth of 2.3 ± central intrusions (e.g., Richey, 1948; Smith and rarely described in recent literature, we provide 0.6 kbar. The Zarza Intrusive Complex is sur- Bailey, 1968; Lipman, 1984). Because of their the following definitions of relevant terms used in rounded by a ductile deformation aureole, and well-defined spatial and geologic context, ring this paper. These definitions partially incorporate bedding is inward dipping and inward young- complexes provide an unparalleled opportunity definitions and descriptions provided by Billings ing around the entire complex. Excellent to evaluate the evolution of subvolcanic mag- (1943), Jacobson et al. (1958), Walker (1975), and preservation of the intrusive history allowed us matic systems and upper-crustal magma-transfer Bates and Jackson (1980). to evaluate the origin of the aureole, and the zones in general. Ring Dike. A ring dike consists of a discor- three most applicable models are (1) collapse In this paper, we evaluate the intrusive history dant intrusive body that can be circular, elliptical, of the complex into its underlying magma of the shallow (2.3 ± 0.6 kbar) Zarza Intrusive polygonal, or arcuate in plan and has steeply dip- chamber, (2) sinking of the complex and its Complex, which we suggest may be the solidi- ping to subvertical contacts. Widths are variable, chamber after solidification, and (3) formation fied remains of a magma-transfer zone between a but can reach up to several thousand meters, and of the aureole prior to emplacement of the caldera and its underlying magma chamber. The rock types are generally felsic. The first descrip- complex. The preserved structural and intru- Zarza Intrusive Complex is located in the western tion of a ring dike in relation to a collapsed caul- sive relationships provide information on the Peninsular Ranges batholith of Baja California dron was made at Glen Coe by Clough et al. dynamic evolution of subvolcanic magma Norte, Mexico (Fig. 1), where it intruded calc- (1909), but the term “ring-dyke” was first used by alkalic volcanogenic rocks of the Alisitos Forma- Bailey (1914). *E-mail: [email protected]. tion. The complex is relatively small and com- Cone Sheet. A cone sheet is a discordant intru- GSA Bulletin; April 1999; v. 111; no. 4; p. 607–619; 15 figures; 1 table. 607 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/111/4/607/3383223/i0016-7606-111-4-607.pdf by guest on 30 September 2021 JOHNSON ET AL. sive body that is arcuate in plan and has variably inward-dipping contacts. Collectively, a swarm of cone sheets can be circular or elliptical in plan. Thicknesses are highly variable; mafic sheets sel- dom reach more than a few tens of meters, whereas felsic sheets can reach widths greater than 60 m. Cone sheets were first described in the Cuillin district of Skye by Harker (1904), who called them “inclined sheets.” The term “cone- sheets” was later introduced by Bailey et al. (1924), reflecting the fact that they are generally of conical form surrounding intrusive centers. Ring Complex. A ring complex is a general term used to describe an intrusive complex that contains cone sheets and/or ring dikes. Com- plexes containing only cone sheets or ring dikes are occasionally called cone-sheet complexes and ring-dike complexes, respectively. Ring complexes have long been thought to represent transitional links between calderas and their un- derlying magma chambers (e.g., Williams, 1941; Richey, 1948; Turner, 1963; Smith and Bailey, 1968; Oftedahl, 1978). Ring Zone. The part of a ring complex that contains cone sheets and/or ring dikes is termed the ring zone. Notable Occurrences of Ring Complexes Figure 1. Reconnaissance geology of the Peninsular Ranges batholith in Baja California Norte, Well over 100 ring complexes have been de- Mexico, between La Calentura and Bahia Camalu. The black rectangle in the lefthand box (see scribed around the world, but most of them lack arrow) shows the location of the main map. The Zarza Intrusive Complex is located in the south- cone sheets and are defined as ring complexes on central part of the main map, which shows Mesozoic plutons and intrusive complexes in the west- the basis of ring dikes and arcuate intrusions. No- ern and eastern belts. Differences between the belts, summarized above the map, are based on in- table examples of cone-sheet–bearing ring com- formation from Gastil et al. (1975, 1990, 1991), Walawender et al. (1990), Gromet and Silver plexes have been previously described in (1) the (1987), Silver and Chappell (1988), and Rothstein (1997). Geology after Gastil et al. (1975) and British Tertiary intrusive centers of Mull, Ardna- this study. murchan, Skye, southern Arran, and Carlingford (e.g., Richey, 1932, 1948; Walker, 1975); (2) the (Gastil et al., 1981; Silver and Chappell, 1988; General Description and Rock Types Georgetown Inlier of Queensland, Australia Todd et al., 1988; Walawender et al., 1990). A (Branch, 1966); (3) the Baie-des-Moutons syenitic semicontinuous, well-exposed oblique section The northern intrusive center is composed of complex of Quebec, Canada (Lalonde and Martin, occurs across the batholith in Baja California variably distinct, concentric, fine- to medium- 1983); (4) the Mediterranean island of Corsica Norte (Fig. 1), with shallow-level rocks exposed grained cone sheets with anorthositic gabbro (Bonin, 1986); (5) the Younger Granite province of in the west and middle-crustal rocks from compositions (Streckeisen, 1976). The sheets, northern Nigeria (Jacobson et al., 1958); and depths as great as ~20 km (Rothstein, 1997) ex- which intruded volcanogenic country rocks, vary (6) the Canary Islands (Schmincke, 1967). posed in the east. The Zarza Intrusive Complex in width from ~0.1–10 m, lack chilled margins, is located in the western belt, ~25 km from the strike subparallel to the margins of the center, and GEOLOGIC SETTING west coast (Fig. 1). have an average inward dip of ~65°. Individual sheets vary markedly in length, and some of the The Zarza Intrusive Complex occurs in the ZARZA INTRUSIVE COMPLEX wider ones are continuous for at least several hun- Jurassic to Cretaceous Peninsular Ranges bath- dred meters around the center. Two coarser- olith, which extends ~1600 km from Riverside, The Zarza Intrusive Complex consists of grained units of modally similar anorthositic gab- California, USA, to the southern tip of the Baja three discrete intrusive centers (Fig. 2), which bro (G1 and G2, Fig. 2) intruded much of the ring California peninsula, Mexico (Fig. 1; Todd we describe below in terms of rock types, zone. G1 commonly occurs as lenticular intru- et al., 1994). The batholith is divisible into west- crosscutting relationships, chronology, barom- sions elongated subparallel to the margins of the ern and eastern belts on the basis of several cri- etry, and structural patterns.