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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 The Cretaceous Zarza Intrusive Complex, located in the Peninsular Ranges of Baja California Norte, Mexico, is perhaps the bestpreserved multiple-center, cone-sheet–bearing ring complex documented in North America. The 7 km2 elliptical complex hosts three nested, non-concentric intrusive centers that are successively younger to the south. The northern and central centers show the same evolutionary sequence of (1) intrusion of concentric gabbroic cone sheets, (2) intrusion of massive core gabbros, and (3) development of subvertical, ductile ring faults. Ring-fault kinematics indicate that both centers moved down relative to the surrounding country rocks, suggesting collapse into an underlying magma chamber. The southern center is composed of approximately equal proportions of gabbro and tonalite and lacks cone sheets. Aluminumin-hornblende barometry on the tonalite indicates a maximum emplacement depth of 2.3 ± 0.6 kbar. The Zarza Intrusive Complex is surrounded by a ductile deformation aureole, and bedding is inward dipping and inward younging around the entire complex. Excellent preservation of the intrusive history allowed us to evaluate the origin of the aureole, and the three most applicable models are (1) collapse of the complex into its underlying magma chamber, (2) sinking of the complex and its chamber after solidification, and (3) formation of the aureole prior to emplacement of the complex. The preserved structural and intrusive relationships provide information on the dynamic evolution of subvolcanic magma *E-mail: [email protected]. chambers and suggest that the complex may have been overlain by a caldera. INTRODUCTION Because much of Earth’s continental crust was formed and/or influenced by magmatic processes (e.g., Taylor and McLennan, 1985; Hamilton, 1989; Saleeby, 1990; Lipman, 1992; Yanagi and Yamashita, 1994; Brown and Rushmer, 1997), the temporal and spatial evolution of magma plumbing systems remains one of the outstanding problems in our search for a better understanding of how continents grow and evolve. Particularly interesting and important parts of these systems are the pathways taken by magma travelling from shallow magma chambers to volcanoes. Where the surface expression of magmatism is a caldera, the magma pathways commonly manifest themselves as ring complexes, which contain a wide variety of intrusive phases including cone sheets, ring dikes, and massive central intrusions (e.g., Richey, 1948; Smith and Bailey, 1968; Lipman, 1984). Because of their well-defined spatial and geologic context, ring complexes provide an unparalleled opportunity to evaluate the evolution of subvolcanic magmatic systems and upper-crustal magma-transfer zones in general. In this paper, we evaluate the intrusive history of the shallow (2.3 ± 0.6 kbar) Zarza Intrusive Complex, which we suggest may be the solidified remains of a magma-transfer zone between a caldera and its underlying magma chamber. The Zarza Intrusive Complex is located in the western Peninsular Ranges batholith of Baja California Norte, Mexico (Fig. 1), where it intruded calcalkalic volcanogenic rocks of the Alisitos Formation. The complex is relatively small and com- GSA Bulletin; April 1999; v. 111; no. 4; p. 607–619; 15 figures; 1 table. 607 pletely accessible, and structural patterns are well developed within the complex and surrounding country rocks. Thus, we were able to collect a detailed structural data set, and evaluate its intrusive history and emplacement mechanisms (a detailed petrological/geochemical study of the complex can be found in Tate et al., 1999). The complex consists of three nested intrusive centers, two of which contain some of the best-preserved cone sheets in North America. The Zarza Intrusive Complex differs significantly from most other cone-sheet–bearing intrusive complexes in that it is surrounded by an intense, concentric, ductile deformation aureole in the adjacent country rocks. This aureole is intriguing, and we focus particular attention on evaluating its formation. BACKGROUND AND DEFINITIONS In this paper we present evidence that the Zarza Intrusive Complex is a cone-sheet–bearing ring complex. Because ring complexes are relatively rarely described in recent literature, we provide the following definitions of relevant terms used in this paper. These definitions partially incorporate definitions and descriptions provided by Billings (1943), Jacobson et al. (1958), Walker (1975), and Bates and Jackson (1980). Ring Dike. A ring dike consists of a discordant intrusive body that can be circular, elliptical, polygonal, or arcuate in plan and has steeply dipping to subvertical contacts. Widths are variable, but can reach up to several thousand meters, and rock types are generally felsic. The first description of a ring dike in relation to a collapsed cauldron was made at Glen Coe by Clough et al. (1909), but the term “ring-dyke” was first used by Bailey (1914). Cone Sheet. A cone sheet is a discordant intru- 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 seldom 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 “conesheets” 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. Complexes 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 underlying 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 Well over 100 ring complexes have been described around the world, but most of them lack cone sheets and are defined as ring complexes on the basis of ring dikes and arcuate intrusions. Notable examples of cone-sheet–bearing ring complexes have been previously described in (1) the British Tertiary intrusive centers of Mull, Ardnamurchan, Skye, southern Arran, and Carlingford (e.g., Richey, 1932, 1948; Walker, 1975); (2) the Georgetown Inlier of Queensland, Australia (Branch, 1966); (3) the Baie-des-Moutons syenitic complex of Quebec, Canada (Lalonde and Martin, 1983); (4) the Mediterranean island of Corsica (Bonin, 1986); (5) the Younger Granite province of northern Nigeria (Jacobson et al., 1958); and (6) the Canary Islands (Schmincke, 1967). GEOLOGIC SETTING The Zarza Intrusive Complex occurs in the Jurassic to Cretaceous Peninsular Ranges batholith, which extends ~1600 km from Riverside, California, USA, to the southern tip of the Baja California peninsula, Mexico (Fig. 1; Todd et al., 1994). The batholith is divisible into western and eastern belts on the basis of several criteria summarized in Figure 1. The western belt is thought to represent a relatively static arc constructed on oceanic crust, whereas the younger eastern belt apparently developed as a continental-margin arc that migrated east with time 608 Figure 1. Reconnaissance geology of the Peninsular Ranges batholith in Baja California Norte, Mexico, between La Calentura and Bahia Camalu. The black rectangle in the lefthand box (see arrow) shows the location of the main map. The Zarza Intrusive Complex is located in the southcentral part of the main map, which shows Mesozoic plutons and intrusive complexes in the western and eastern belts. Differences between the belts, summarized above the map, are based on information from Gastil et al. (1975, 1990, 1991), Walawender et al. (1990), Gromet and Silver (1987), Silver and Chappell (1988), and Rothstein (1997). Geology after Gastil et al. (1975) and this study. (Gastil et al., 1981; Silver and Chappell, 1988; Todd et al., 1988; Walawender et al., 1990). A semicontinuous, well-exposed oblique section occurs across the batholith in Baja California Norte (Fig. 1), with shallow-level rocks exposed in the west and middle-crustal rocks from depths as great as ~20 km (Rothstein, 1997) exposed in the east. The Zarza Intrusive Complex is located in the western belt, ~25 km from the west coast (Fig. 1). ZARZA INTRUSIVE COMPLEX The Zarza Intrusive Complex consists of three discrete intrusive centers (Fig. 2), which we describe below in terms of rock types, crosscutting relationships, chronology, barometry, and structural patterns. Figures 2 through 4 show the geology, bedding and foliation data, and a trend analysis of the bedding and foliation in the complex and surrounding country rocks. Figures 5 and 6 show block diagrams along the cross-section lines in Figure 4. Geological Society of America Bulletin, April 1999 General Description and Rock Types The northern intrusive center is composed of variably distinct, concentric, fine- to mediumgrained cone sheets with anorthositic gabbro compositions (Streckeisen, 1976). The sheets, which intruded volcanogenic country rocks, vary in width from ~0.1–10 m, lack chilled margins, strike subparallel to the margins of the center, and have an average inward dip of ~65°. Individual sheets vary markedly in length, and some of the wider ones are continuous for at least several hundred meters around the center. Two coarsergrained units of modally similar anorthositic gabbro (G1 and G2, Fig. 2) intruded much of the ring zone. G1 commonly occurs as lenticular intrusions elongated subparallel to the margins of the center, whereas G2 forms a relatively large body in which isolated rafts and blocks of ring-zone rocks locally form more than 50% of the outcrop. Thus, the ring zone originally occurred throughout much of the currently exposed center, and stoping was an important material-transfer ZARZA INTRUSIVE COMPLEX, BAJA CALIFORNIA, MEXICO assemblages dominated by plagioclase and either two pyroxenes or hornblende; some of the gabbros of the northern intrusive center also contain minor olivine. Overall, they also have similar major-element oxide, trace element, and rare-earth element patterns that require a somewhat similar parent with high-alumina basalt characteristics. Cone sheets and hornblende diabase dikes in the northern and central intrusive centers approximate near-parental material, which probably required accumulation of the gabbros from structurally equivalent cone-sheet magmas in the underlying magma chamber (Tate et al., 1999). SHRIMP Geochronology Figure 2. Geologic map of the Zarza Intrusive Complex. The northern intrusive center is composed of the northern ring zone and gabbros G1 and G2. The central intrusive center is composed of the central ring zone and gabbro G3. The southern intrusive center is composed of gabbro G4 and tonalite T1. process during emplacement of G1 and G2. The ring zone and gabbros G1 and G2 are cut by volumetrically minor radial and concentric dikes of hornblende diabase, epidotite, tonalite, and aplite. The central intrusive center largely mimics the northern center and is nested in its southern half. The ring zone is composed almost entirely of cone sheets with anorthositic gabbro compositions, but the origin of some fine-grained rocks with feldspar phenocrysts is unclear; they are either rapidly cooled sheets or volcanogenic screens. The ring zone was intruded by coarsegrained anorthositic gabbro (G3; Fig. 2), which contains rafts and blocks of ring-zone rocks, the abundance of which drops off markedly from the ring zone’s margins to its center. Thus, it is unclear whether the entire intrusive center originally contained cone sheets. Late radial and concentric dikes as described in the northern center are rare. The southern intrusive center instead contains coarse-grained anorthositic gabbro (G4) cut by a slightly more abundant hornblende-biotite tonalite (T1). These intrusive rocks are more hornblende rich than those in the two preceding centers, and they cut ring-zone rocks that may represent the southern extent of those in the northern and/or central centers. G4 contains abundant rafts and blocks of ring-zone rocks, which suggests that they previously occupied much of the area intruded by G4. The ring-zone rocks are cut by volumetrically minor radial and concentric dikes as described above for the northern center, but tonalite and aplite dikes are more abundant. All of the mafic intrusive rocks in the Zarza Intrusive Complex have similar modal mineral Geological Society of America Bulletin, April 1999 Absolute ages were determined by U-Pb (zircon) geochronology of units G3 and T1 in the central and southern intrusive centers, respectively, and all data were collected with the Australian National University SHRIMP (sensitive high-resolution ion microprobe) II instrument. Only nonmagnetic zircons were analyzed, and all of them showed ubiquitous oscillatory zonation consistent with a magmatic origin. Figure 7 shows that most of the analyses plot close to concordia and must be dominated by radiogenic Pb. A weighted mean of all 206Pb/238U ratios determined for G3 yielded an age of ca. 116.2 Ma; excess scatter reflects analyses 7.1 and 22.1 (indicated in Fig. 7), which came from low-U growth zones that contain cracks and apparently lost radiogenic Pb. Eliminating these two analyses gave an age of 116.2 ± 0.9 Ma at the 2σ confidence level. T1 gave a more reliable age of 114.5 ± 0.9 Ma at the 2σ confidence level. Although both ages overlap slightly at the limits of analytical uncertainty, the data are consistent with the observed crosscutting relationships, which indicate that T1 is younger than G3. The oldest intrusive units exposed in the northern intrusive center were not dated, and so the above ages provide minimum emplacement estimates for the complex as a whole. Al-in-Hornblende Barometry Biotite and K-feldspar crystals are rare in T1 (<2 vol%), as reflected generally by wholerock compositions that contain extremely low K2O concentrations (<0.6 wt%). Cobaltinitrite staining revealed K-feldspar as both a groundmass and phenocryst phase in only the most potassic tonalites, which also contain titanite and have the most potential for Al-in-hornblende barometry (e.g., Hammarstrom and Zen, 1986; Johnson and Rutherford, 1989). We conducted single-mineral analyses of these samples by using the Cameca SX-50 scanning elec- 609 JOHNSON ET AL. tron microprobe at Macquarie University, which was calibrated with geologic standards and operated in wavelength-dispersive mode with a 20 nA beam current, a 15 kV accelerating voltage, a 2–5 µm spot diameter, and an integrated counting time of 40 s. Amphibole rims adjacent to the buffer assemblage are tremolitic hornblendes (mostly <7.5 Si and >1.6 Ca pfu [per formula unit], respectively; Mg# = (Mg/Mg + Fe), ~53) that provide a temperature of 705 °C after consideration of the intermediate (An26) plagioclase compositions (Blundy and Holland, 1990). In conjunction with the general absence of saussuritization and other manifestations of deuteric alteration throughout T1, this temperature value suggests that the critical mineral assemblage equilibrated entirely above a wet solidus (Anderson and Smith, 1995). Depending on the calibration curve employed, pressure estimates range widely between 1.7 and 2.9 kbar and overlap with the lower boundary applicable for the technique (Johnson and Rutherford, 1989). Thus, we estimate a maximum emplacement depth of 2.3 ± 0.6 kbar for the Zarza Intrusive Complex, which accounts for the low total K-feldspar content and assumes that the experiments of Schmidt (1992) most reliably reproduce the intratelluric oxidation state of T1. Although there is current debate regarding temperature controls on element-partitioning behavior in amphiboles, suitable temperature corrections did not reduce our pressure estimates beyond the analytical uncertainties involved (Ague and Brandon, 1996). The shallow emplacement that we infer is consistent with (1) the rare presence of miarolitic cavities and edenitic biotite compositions in T1 and (2) the ring-complex characteristics of the Zarza Intrusive Complex in general. Structures, Microstructures, and Kinematics Most intrusive rocks in the Zarza Intrusive Complex contain only one foliation, which is magmatic (hypersolidus to near-solidus conditions) and characterized by alignment of igneous plagioclase and/or hornblende crystals. The cone sheets commonly contain variably developed magmatic foliations oriented subparallel to sheet boundaries, and mineral elongation lineations that plunge approximately downdip. Microstructural analysis of the sheets showed only rare evidence for minor solid-state deformation (on the basis of the criteria of Paterson et al., 1989a), and we have found no field evidence of boudinaged sheets. G1, G2, and G3 generally contain moderate to strong magmatic foliations, but in G3, the foliation progressively weakens toward its central “bull’s-eye,” where it forms a basinal shape (Figs. 5 and 6). T1 610 Figure 3. Foliation and bedding attitudes in the Zarza Intrusive Complex. See Figure 4 for composite map of foliation and bedding trends and geology. and G4 generally contain weak magmatic foliations, and lineations are either absent or very difficult to recognize in all of the larger intrusive bodies. This observation may be due partly to a lack of appropriate foliation-parallel exposures, but in general we infer that these bodies do not contain a strong linear fabric. Country-rock screens that lie between sheets in the northern center commonly contain a welldeveloped solid-state foliation that strikes parallel to, but locally dips less steeply inward than, the magmatic foliations in adjacent sheets. They also commonly contain a well-developed mineral-elongation lineation that generally plunges approximately downdip. Many of these screens contain microstructures and mineral assemblages of either the hornblende hornfels or pyroxene hornfels facies and were locally intruded by narrow felsic and quartzo-feldspathic veins. Geological Society of America Bulletin, April 1999 Remarkably, foliations (Fig. 4) and lithologic contacts (Fig. 2) in the northern and central intrusive centers are abruptly truncated by the central and southern centers, respectively. Neither the contacts nor the foliations in the older centers are deflected as younger intrusive contacts are approached, and they both locally strike into these contacts at high angles (e.g., Figs. 3 and 4). Furthermore, no penetrative solid-state fabrics were observed near these margins, which indicates that at least the two younger intrusive centers were emplaced with little or no lateral expansion or shearing of preexisting markers. These observations imply that older centers were relatively solid before intrusion of each younger center. Spectacular kinematic indicators are present in the northern and central intrusive centers and consist primarily of subvertical, discrete, ductile shear zones that are heterogeneously ZARZA INTRUSIVE COMPLEX, BAJA CALIFORNIA, MEXICO Structures Figure 4. Composite map of foliation and bedding trends, folds, and geology in the Zarza Intrusive Complex. Block-diagram cross sections along the lines A–B and C–D are shown in Figures 5 and 6, respectively. spaced on the centimeter to decimeter scale (Fig. 8). Locally, in zones of relatively high strain, asymmetrical indicators are surrounded by a pervasively developed, subvertical foliation. These subvertical shear zones and locally pervasive foliations cut across the cone sheets and country-rock screens and are concentrated near the outer margins of each center in what we refer to as kinematic zones (Figs. 2, 5, and 6). The sense of shear in these zones always indicates that the area they enclose moved down relative to the area outside (Fig. 8), and so we interpret them as ductile equivalents of ring faults. Variably abundant felsic and quartzofeldspathic intrusive rocks are localized in these zones, which suggests that they may be incipient ring dikes. ADJACENT COUNTRY ROCKS Rock Types The Zarza Intrusive Complex lies entirely within the Cretaceous Alisitos Formation, which is thought to represent calc-alkalic volcanogenic rocks deposited mainly in a shallow-marine environment (Gastil, 1983; Beggs, 1984). Country rocks surrounding the complex are dominantly lithic and crystal-lithic tuffs; basalt, andesite, and dacite flows; and minor siltstones and sandstones. Bedding is generally defined by contacts between different units and by variably developed alignment of clasts. Locally present sedimentary structures consistently indicated upward younging. Geological Society of America Bulletin, April 1999 Regional structures near the Zarza Intrusive Complex are largely undocumented outside of the map area shown in Figures 2–4, but the following generalizations can be made on the basis of our reconnaissance mapping: (1) bedding regionally dips moderately to the southwest or northeast, (2) upright, macroscale folds have been observed with axial-plane foliations developed locally in their hinges, and (3) no major faults have been observed in the vicinity of the complex. Approaching the complex, bedding is progressively deflected and folded into steep inward-dipping attitudes around the entire complex (Figs. 3–6). On the west side, bedding is folded over into an anticline to attain the steep inward dips (Figs. 4 and 5). This fold is discontinuous and upright to moderately south plunging; it becomes progressively tighter and overturned to the south. It is not clear whether the fold resulted entirely from emplacement of the complex or from both emplacement-related and synemplacement regional deformation. Two other folds are present to the north and east of the complex. The northern fold is only locally developed, whereas the eastern fold is regionally extensive, continuing off the eastern edge of the map (Fig. 4). Bedding attitudes southeast of the complex indicate a possible syncline, but alternatively could indicate a vertical fan of bedding (Fig. 6); the structure is complicated owing to the unmapped intrusive body with a steeply outward–dipping contact in the southwest corner of Figures 2–4. In the country rock, a bedding-parallel foliation and downdip mineral-elongation lineation become well developed within a few hundred meters of the complex and intensify toward its margins to define a deformation aureole. Although we recognize three intrusive centers in the complex, there is only one aureole, which is spatially associated with the first and largest intrusive center. Kinematic indicators in this aureole were only locally observed and appear to vary from rare, discrete, steeply inward–dipping cleavage seams that crosscut bedding near the outer edge of the aureole, to asymmetrical clasts and zones of heterogeneously partitioned shear within the beddingparallel foliation of the inner aureole. All such structures indicated downward movement of the inner aureole relative to the outer aureole. Strain Analysis Ideally, we would like to quantify deformation in the aureole caused by both rigid rotation and ductile flow of units, which can be done by determining the shortening of bedding in roofs above 611 JOHNSON ET AL. chambers or the rigid rotation and internal strain at many individual locations along a single bedding horizon throughout the aureole (Schwerdtner, 1995). However, neither of these approaches can be applied to the Zarza Intrusive Complex because none of the roof is preserved and no appropriate bedding horizons are traceable throughout the aureole. We have previously noted that all bedding near the complex was rotated from regional orientations into consistently steep inward dips, which required that material was transported out of the exposed map plane (Figs. 5 and 6). For this reason, an exact value of emplacement-related host-rock strain can never be determined, but some useful information about strain can be obtained by evaluating (1) the shapes and orientations of ellipsoids and (2) the amount of bulk shortening along transects perpendicular to the margins of the complex (e.g., Bateman, 1984; Paterson and Fowler, 1993). Lithic tuffs around the complex provide good samples for strain analysis. Fine-grained siltstones and volcanic rocks are locally present between the sampled tuffs and are generally more highly strained, indicating that deformation was preferentially partitioned into them. Thus, our analysis probably underestimates the amount of shortening. Five suitable samples were collected along a southern transect, three from a western transect, and an additional sample from near the southwest margin of the complex (Fig. 2). The two transects were chosen because they lie in the regional “strain shadow” and are nearly perpendicular to the regional strike of folded bedding, respectively (Fig. 4). Three mutually perpendicular cuts were made through each sample, and the cut surfaces were sprayed with acrylic and labeled with a right-handed coordinate system. Axial ratios and orientations of 35 or more lithic clasts were measured in each face. The magnitude of cleavage deflection near clasts and the percentage of clasts versus matrix were used to estimate viscosity contrasts (usually 1, for no viscosity contrast) between clasts and matrix (Paterson et al., 1989b). Shimamoto and Ikeda (1976) and Wheeler (1986) noted that any population of elliptical markers with different ratios and orientations can be represented collectively by determining a single average ellipsoid. If this population is then deformed, the average ellipsoid defined by the deformed objects will reflect strain plus any original fabric (Dunnet and Siddans, 1971; Seymour and Boulter, 1979). We have used the techniques of Shimamoto and Ikeda (1976), Miller and Oertel (1979), and Wheeler (1986) to calculate final average ellipsoids for the deformed clasts, which were then corrected for the presence of primary fabrics by using the procedure outlined in Paterson and Yu (1994). These corrected ellipsoids (Table 1) approxi- 612 Figure 5. Block-diagram cross section along the line A–B in Figure 4. Displacements along kinematic zones stylized. See Figure 2 for geology legend. Vertical and horizontal scales are equal. Figure 6. Block-diagram cross section along the line C–D in Fig. 4. Displacements along kinematic zones stylized. See Figure 2 for geology legend. Vertical and horizontal scales are equal. mate the tectonic strain at each locality with uncertainties in the range of 10% to 20% strain at the 95% confidence level. Strain intensities of samples within the aureole range from moderate values (1.52) near the complex to low “regional” values near the outer margin of the aureole, and Lode’s parameters indicate that all but one of the samples yield oblate strain ellipsoids (Table 1, Fig. 9). Two of the samples (BC 220 and BC 221) came from immediately outside of the deformation aureole and have very low strains with x-y planes of the strain ellipsoid subparallel to moderately dipping bedding. Thus, we suggest that they reflect strain caused only by primary compaction plus regional deformation. Another sample (BC 413), which came from the fold hinge to the west of the Zarza Intrusive Complex, has a prolate strain ellipsoid with the x-axis of strain at a low angle to the fold axis. Thus, we suggest that Geological Society of America Bulletin, April 1999 this sample reflects heterogeneous strain in the fold hinge. All other samples came from within the deformation aureole. Figure 10 shows an equal-area plot of the x-y planes and x-axes of strain for the oriented samples. The average Zarza Intrusive Complex margin orientations and x-y planes of strain correspond well with one another, and so it is particularly useful to examine shortening along the z-axis (shortening approximately perpendicular to the complex margin). These values range from 71% near the complex contact to 18% at the margin of the aureole and 12% outside of the aureole (Table 1 and Fig. 11). From Figure 11 we calculated bulk shortening in the aureole perpendicular to the Zarza Intrusive Complex margin by integrating the area under the best-fit curve to all the strain data (the two transects having essentially identical shortening gradients). We defined this outer margin approximately at sample BC 220, corresponding to a re- ZARZA INTRUSIVE COMPLEX, BAJA CALIFORNIA, MEXICO Figure 7. Tera and Wasserburg (1972) U-Pb (zircon) concordia curves for gabbro G3 and tonalite T1 analyzed by SHRIMP. MSWD—mean square of weighted deviates. gional shortening strain of 18%. The bulk shortening obtained depends on how we treat this regional strain, and so two integrations were made: one in which the regional shortening strain of 18% was included in the integration and one in which the 18% value was used as the base of the curve. These integrations resulted in bulk shortening strains of 59% and 38%, respectively, and we make use of this information in a later section. PROPOSED INTRUSIVE HISTORY FOR THE ZARZA INTRUSIVE COMPLEX This study has revealed a six-stage sequential intrusive history of the Zarza Intrusive Complex that involves three distinct, south-migrating episodes (Fig. 12). The cross-sectional view shown is the same as that in Figure 6, which is reproduced by stage 6 of the sequence. We evaluate the surrounding deformation aureole in the following section. Stage 1. The northern intrusive center (Fig. 2) was initiated by emplacement of cone sheets derived from an underlying magma chamber. On the basis of a linear subsurface projection of the northern-center cone sheets, the underlying chamber was probably situated no more than 2–3 km below the present level of exposure. At some point in the development of this chamber, it presumably became overpressured and began to dome or expand upward, creating the conditions required to fracture the overlying country rocks and intrude the cone sheets (e.g., Anderson, 1936; Roberts, 1970; Phillips, 1974). Stage 2. After the cone sheets solidified, the ring zone was intruded and partially destroyed by gabbros G1 and G2. Rafts and blocks of ring-zone rocks are common in the gabbros, indicating that stoping was an important emplacement process. Formation of the northern-center kinematic zone, which appears to be a ductile ring fault, resulted from collapse of the enclosed area into the underlying magma chamber. Although we have no clear evidence for the relative timing between displacement along this zone and intrusion of G1 and G2, we suggest that collapse resulted from deflation of the chamber during intrusion of the cone sheets and gabbros G1 and G2. Stage 3. As with the northern center, the central center was initiated by intrusion of cone sheets, which formed a second ring zone contained within the southern half of the northern center. The diameter of the central center is smaller than the northern, but the cone sheets have approximately the same range of dips (Fig. 3). Conesheet dips are apparently related to the depth of the magma chamber below the exposure level, the shape of the chamber, and the position on the chamber margin from which the sheets were derived (e.g., Anderson, 1936; Phillips, 1974). Given the smaller diameter of the central center and the fact that the dips of the cone sheets are Geological Society of America Bulletin, April 1999 similar to those of the northern center, we suggest that the central-center chamber (or some portion of it) rose closer to the surface to 1–1.5 km below the present level of exposure (Figs. 5, 6, and 12). The gabbros and cone sheets of the northern center are cut sharply by the cone sheets of the central center, which suggests that the entire northern center was fully crystallized when the central-center sheets were emplaced. Stage 4. After the cone sheets solidified, they were intruded by gabbro G3, and the central center collapsed along a kinematic zone similar to the one that surrounds the northern center. Rafts and blocks of ring-zone rocks in G3 indicate that, again, stoping was an important emplacement process. As in the northern center, we have no clear evidence for the relative timing between the formation of the kinematic zone and G3 intrusion, but the basinal form of magmatic foliations in G3 may indicate backflow of magma, possibly in conjunction with displacement along the zone. In this situation, formation of the kinematic zone would postdate the initial intrusion of G3, but predate its complete crystallization, consistent with collapse in response to deflation of the magma chamber during intrusion of the cone sheets and G3. Stage 5. At this stage, the sequence of intrusion of cone sheets followed by massive gabbro appears to have changed, and initiation of the southern center was marked by intrusion of gabbro G4. Abundant rafts and blocks of ring-zone rocks in G4 indicate that stoping remained an important emplacement process. On the basis of geologic and structural patterns in Figures 2–4, we interpret the preserved ring-zone rocks in the southern center to be remnants of the northern and/or central centers. As with the central center, the southern center migrated to the south, but is contained within both previous centers. Stage 6. The intrusive history was completed (except for emplacement of minor radial and concentric dikes) by intrusion of T1, which cuts G4 at rare field localities. The central-center cone sheets and G3 are cut sharply by T1, which suggests that the entire central center was fully crystallized when T1 was emplaced. This interpretation is consistent with the SHRIMP geochronology discussed previously. ORIGIN OF THE HOST-ROCK DEFORMATION AUREOLE The most puzzling structural feature of the Zarza Intrusive Complex is the intense ductile deformation aureole in the adjacent country rocks. In this section, we present and evaluate five models of events that might have contributed to this aureole in the context of the intrusive history discussed above. Cone sheets of the northern and 613 JOHNSON ET AL. central centers show only rare evidence for solidstate deformation, and the ring zone around the outer margin of the complex is remarkably continuous. These observations indicate that even moderate lateral expansion of the complex during emplacement of internal gabbro units can be ruled out. Thus, we do not consider the popular models of chamber expansion (“ballooning”— Sylvester et al., 1978; Holder, 1981; Bateman, 1985) or diapirism (e.g., Cruden, 1990; Weinberg and Podladchikov, 1994) as mechanisms for aureole development. 1. Accumulated Shortening during ConeSheet Emplacement. Cone sheets of the northern and central intrusive centers appear to have originally occupied a large area of the Zarza Intrusive Complex, prior to being intruded by later gabbros and tonalite. Collectively, the sheets displaced a considerable area of preexisting rock, and any lateral component of this displacement could potentially have contributed to the deformation aureole. However, if progressive cone-sheet emplacement had caused lateral expansion of the complex, we would expect to see clear evidence for solid-state deformation in at least some of the northerncenter sheets, apart from that associated with the kinematic zone. Microstructural analysis of the sheets showed only rare evidence for minor solidstate deformation, and we have found no examples of boudinaged sheets. Furthermore, emplacement of the central-center sheets had no recognizable deformational effect on adjacent rocks of the northern center. For these reasons, we suggest that any lateral deformation caused by cone-sheet emplacement was accommodated mainly by country-rock screens between the sheets. These screens were metamorphosed under conditions of hornblende hornfels to pyroxene hornfels facies and thus reached temperatures at which they may have partially melted and could be relatively easily deformed. The downdip lineations in the screens suggest a large vertical component of material transfer during their deformation, and we also speculate that vertical displacements may have occurred during opening of the brittle fractures filled by cone sheets (e.g., Le Bas, 1971; Walker, 1975). 2. Synemplacement and Postemplacement Regional Shortening. Northeast-southwest shortening in the Peninsular Ranges batholith resulted in a regionally consistent pattern of northwest-trending bedding and folds and locally developed axial-plane foliations. If regional strain occurred in the aureole, it should have had two effects: (1) increased shortening gradients along the western transect compared to the southern transect, because the latter is in the regional strain shadow, and (2) prolate strain ellipsoids in the southern transect because regional and emplacement-related shortening would have occurred at 614 Figure 8. Example of subvertical, discrete, ductile shear zones found in the kinematic zones of the northern and central intrusive centers. Sense of shear in these zones consistently indicates downward displacement of the Zarza Intrusive Complex relative to surrounding country rocks. In this example, the shear zones and sheared rocks have been disrupted and intruded by quartzo-feldspathic melts. high angles to one another (e.g., Guglielmo, 1993, 1994). Figure 11 indicates that the two transects have essentially identical shortening gradients within the uncertainties of the method, and the only data yielding a prolate strain ellipsoid were from sample BC 413 from the western transect (Table 1 and Fig. 9). Thus, we conclude that the contribution of regional deformation to the aureole was effectively negligible. 3. Collapse of the Zarza Intrusive Complex into Its Underlying Chamber. Kinematic zones within the northern and central intrusive centers indicate collapse of their contained areas into the underlying magma chamber. We could not quantify displacements in these zones, but we speculate that they may have accommodated several hundred meters of collapse. Space for this collapse was likely achieved largely by removal of magma from the chamber to form the cone sheets Geological Society of America Bulletin, April 1999 and larger gabbros. During collapse, the entire complex may possibly have moved downward, the kinematic zones accommodating only part of the total displacement. Figure 13 shows collapse of the complex into its underlying chamber for two initial configurations: emplacement of the magmas (1) into host rocks with originally flatlying bedding or (2) into the western limb of the regional anticline whose axial plane is to the east (Figs. 4–6). Partitioned strains across the northern ring zone during collapse could also explain the solid-state deformation and inward dips of the country-rock screens. 4. Sinking of the Zarza Intrusive Complex and Its Chamber after Solidification. Glazner (1994) and Glazner and Miller (1997) noted that many plutons of intermediate and mafic composition reach a level in the crust that, after they crystallize, is at or above their level of neutral ZARZA INTRUSIVE COMPLEX, BAJA CALIFORNIA, MEXICO TABLE 1. STRAIN DATA FROM THE ZARZA INTRUSIVE COMPLEX (ZIC) Sample number BC 472 BC 209 BC 208 BC 506A BC 207 BC 220 BC 387 BC 413 BC 221 Axial ratios Elongations x y z 6.48 7.12 3.56 3.45 2.19 1.46 2.33 1.48 1.27 6.33 3.53 2.96 2.63 1.90 1.26 2.00 1.11 1.17 1 1 1 1 1 1 1 1 1 x 87.9 143.1 62.4 65.4 36.2 19.2 39.5 25.4 11.3 y 83.6 20.5 35.0 26.1 18.1 2.8 19.7 –5.9 2.5 Ellipsoids z –71.0 –65.9 –54.4 –52.1 –37.8 –18.4 –40.1 –15.3 –12.4 SI 1.52 1.41 0.97 0.92 0.59 0.27 0.64 0.29 0.17 LP 0.98 0.29 0.71 0.56 0.64 0.22 0.64 –0.47 0.31 Orientations x-y plane (strike/dip) 318/66 235/63 235/58 15/69 212/54 ————332/54 27/13 ————- Distance from ZIC x axis (trend/plunge) ————336/63 240/8 113/69 321/52 ————91/50 136/12 ————- (m) 55 75 190 220 330 495 680 720 785 Note: Samples ordered by distance from the complex (locations in Fig. 2). SI = strain intensity, defined by Hossack (1968) as Es = 1/3 [(e1 – e2)2 + (e2 – e3)2 + (e3 – e1)2]1/2, where e1, e2, and e3 are the principal natural strains. LP = Lodes parameter. buoyancy. In the process of crystallization, these plutons generally increase their density by as much as 10%, which causes an important density contrast between the plutons and surrounding host rocks. These authors have argued that, given country-rock temperatures sufficient for ductile deformation, such a density contrast may cause the solidified plutons to sink some finite distance through the crust after their initial emplacement to a higher level. Because the bulk composition of country rocks near the complex approximates andesite, substantial sinking of solidified intrusive rocks would effectively require them to be gabbros. Currently exposed intrusive rocks in the complex are 90% gabbro, and we also infer a mafic composition for the underlying chamber (Tate et al., 1999); thus, this model can be effectively applied to the complex (Fig. 14). Rigorous Figure 9. Strain intensity and Lode’s parameter plotted against distance from the Zarza Intrusive Complex (ZIC) margin. Strain intensity increases toward the margin, apparently accompanied by a weak increase in Lode’s parameter. The one sample (BC 413) with a negative Lode’s parameter comes from the hinge of the anticline that bounds the west side of the complex (Fig. 4). application of the model is complicated by poor constraints on several parameters that are important for the required calculations, including (1) the regional thermal gradient; (2) the lithological and density stratification of the crust adjacent to the complex and with increasing depth; and (3) the size, shape, and composition of the underlying magma chamber. Nevertheless, by using Figure 10. Equal-area plot of x-y planes and x-axes of strain for the oriented samples shown in Table 1 and located in Figure 2 (with BC prefixes). The x-y planes (solid great circles) and the average orientations of the Zarza Intrusive Complex margin (large dashes) are plotted as different line widths corresponding to each transect (see Fig. 2): BC 207–209 come from the southern transect, BC 387 and 506A come from the western transect, and BC 472 comes from the southwestern margin of the complex. BC 472 gave a Lode’s parameter of 0.98 (very oblate strain ellipsoid). Thus, the xaxis orientation is unconstrained within the uncertainties of the method and is not plotted. Sample BC 413 came from the hinge of the anticline that bounds the west side of the complex (Fig. 4), as reflected by the orientation and prolate shape (Table 1) of the strain ellipsoid. Geological Society of America Bulletin, April 1999 ranges of what we considered to be acceptable possible values for these and other parameters and the methodology described in Glazner (1994), we calculated strain rates varying over four orders of magnitude from 10–11 to 10–14 s–1, which corresponded to sinking rates of 680 km/m.y. to 680 m/m.y. Strain rates on the order of 10–11 to 10–13 s–1 lead to acceptable time scales for development of the deformation aureole. 5. Formation of the Aureole prior to Emplacement of the Zarza Intrusive Complex. Development of the complex involved multiple stages of collapse along subvertical kinematic zones. We suggest that the deformation aureole may have formed by a similar collapse episode prior to emplacement of what we recognize as the first intrusive rocks in the complex—the cone sheets of the northern intrusive center. In Figure 15, A and B, collapse occurs partly along subvertical ring faults and partly by “downsagging” (e.g., Walker, 1984). The overall collapse process results in distributed ductile shear zones at depth, within which the country rocks are deflected downward into a funnel shape. The required geometrical relationships can be produced if the complex is then emplaced over this preexisting structure (Fig. 15C). Discussion of the Aureole Although the processes described in all five of these models may have contributed to the deformation aureole around the Zarza Intrusive Complex, we suggest that models 1 and 2 played minor or negligible roles. Models 3–5 are all similar in that they require downward sinking or collapse of material inside the aureole (Figs. 13–15). We favor this general process for aureole formation, but we are unable to suggest, on the evidence at hand, which of these three models is most appropriate. We prefer to consider them as three end members that are not mutually exclusive; they could have acted together, or they could have acted separately at different stages of the intrusive complex’s history. If the complex did actu- 615 JOHNSON ET AL. ally collapse into, or sink with, its underlying chamber (models 3 and 4), it is possible that cone-sheet dips increased during downward displacement, effectively decreasing the dihedral angle of the cone, which might help explain the highly strained country-rock screens. However, if this process occurred, it remains puzzling to us why the outer cone sheets do not show more evidence for solid-state deformation. We previously calculated bulk shortening strains of 59% and 38% in the aureole, depending on how the background regional strain was treated. In the situations presented by models 3 and 4, if we assume an approximately symmetrical aureole (supported by field observations and Figs. 4 and 11) and symmetrical displacement of the complex (supported by the orientation data in Fig. 3), this bulk shortening can be used to constrain the amount of relative downward displacement. If we assume that the intrusive complex is a rigid cone with a specific dihedral angle, we can calculate how far this cone would have to sink to shorten the surrounding country rocks by the required amount. Cone sheets in the northern intrusive center have an average dip of ~65°, and so if a cone with a dihedral angle of 50° is considered, the country rocks would shorten by 47 m for every 100 m of sinking. Bulk shortening strains of 59% and 38% would correspond to lateral shortening of country rocks in the aureole by 710 and 300 m, respectively. These values could be entirely accounted for by sinking of the complex, either into or with its underlying chamber, ~1.5 km and 640 m, respectively. Figure 11. Shortening strain plotted against distance from the Zarza Intrusive Complex (ZIC) margin. We consider the strain value for sample BC 387 to be anomalous, occurring in a highstrain zone just off the hinge of the anticline that bounds the west side of the complex (Fig. 4). Figure 12. Sequential intrusive history for the Zarza Intrusive Complex. See text for discussion. 616 Geological Society of America Bulletin, April 1999 ZARZA INTRUSIVE COMPLEX, BAJA CALIFORNIA, MEXICO In the situation presented by model 5, if we assume that the aureole is essentially a vertical shear zone, we can also use the increasing axial ratios, or the change in foliation and bedding orientations, toward the complex to calculate the shear strain (γ). Equations summarized by Ramsay and Huber (1983, 1987) relate the amount of displacement in a simple-shear zone to shear strain and to the orientations and axial ratios of strain ellipsoids. This approach is more difficult to constrain than the one applied to models 3 and 4 because the original orientation of bedding has an important effect on the total required displacement. By using the initial bedding orientations shown in Figure 13, A and B, and an average aureole width of 500 m, we calculated total displacements of 550 m and 2.5 km, respectively. Both of these approaches overlap the 1.5 km to 650 m displacement range, and our preferred interpretation is that any such displacements probably amounted to no more than 1.5 km. DISCUSSION Figure 13. Model that requires collapse of the Zarza Intrusive Complex (ZIC) into its partially evacuated underlying chamber to explain the deformation aureole and inward-dipping and inward-younging bedding around the complex. There are two possible initial configurations: emplacement of the complex into (A) flat-lying bedding or (B) dipping bedding in the western limb of the regional anticline to the east of the complex. (C) The required final configuration. Sequence A followed by C would require synemplacement to postemplacement regional deformation to form the anticlines on the western and eastern sides of the complex, whereas sequence B to C adequately produces the anticlines during collapse. Multiple Processes for Host-Rock Material Transfer During Emplacement of the Zarza Intrusive Complex The Zarza Intrusive Complex preserves evidence for the following five processes of hostrock material transfer during its emplacement: (1) downward transport of country rocks in the deformation aureole relative to those outside the aureole, (2) stoping during ascent and emplacement of the four massive gabbros (G1 to G4), (3) collapse along the kinematic zones in the northern and central intrusive centers, (4) country-rock partial melting directly adjacent to the intrusive complex and in the country-rock screens, and (5) deformation of country-rock screens caused by cone-sheet emplacement. We are impressed by the magnitude of stoping by the Zarza massive gabbros, and we can imagine that if they had occupied more area at the present level of exposure, they could have completely removed evidence for some or all of the other material-transfer processes. Even though we recognize multiple processes, the vast majority of material transfer during emplacement of the Zarza Intrusive Complex was vertical; the upward movement of magma was accompanied by downward movement of country rocks and parts of the complex. Implications for Subvolcanic Magma-Chamber Dynamics Figure 14. Model that requires sinking of the Zarza Intrusive Complex (ZIC) and its underlying chamber to explain the deformation aureole and inward-dipping and inward-younging bedding around the complex. See Figure 13 caption for further explanation. Geological Society of America Bulletin, April 1999 The intrusion and collapse cycles we have documented in the Zarza Intrusive Complex pre- 617 JOHNSON ET AL. serve compelling evidence for how pulses of magma escape from some high-level chambers and how pathways toward the surface are created. The magma chamber under the complex became overpressured and, at some stage, exerted enough upward force to fracture and disrupt the overlying crust. These fractures were filled with magmas that crystallized to form the cone sheets; we do not know how vertically extensive these sheets were or whether they provided pathways for magma transport to the surface. The disruption of the overlying crust relaxed an important energy barrier to voluminous magma transport from the chamber. The resulting network of fractures and sheet contacts provided a vast array of potential magma pathways (e.g., Walker, 1986), and the massive core gabbros used them to stope their way upward through the cores of the ring zones and thus form central conduits that may have supplied mafic volcanic eruptions at the surface. Although we have no conclusive proof that the complex was overlain by a caldera, its ring-complex characteristics, evidence for calderas in the western Peninsular Ranges batholith (e.g., Gastil, 1990; Delgado-Argote et al., 1995; Fackler-Adams, 1997), and the common occurrence of mafic and bimodal calderas around the world (Walker, 1993), lead us to favor this interpretation. SUMMARY AND CONCLUSIONS 1. The Zarza Intrusive Complex preserves structural and intrusive relationships that suggest it may have been overlain by a caldera. 2. Cone sheets preserve evidence for a dense population of fractures that disrupted the crustal integrity above the complex’s magma chamber. Massive core gabbros stoped their way through this disrupted zone and formed a pathway for continued magma ascent. 3. The presence of both sheets and massive intrusive bodies and the evidence for both brittle and ductile host-rock deformation illustrate that multiple ascent and emplacement processes can act together to facilitate subvolcanic magma transfer. 4. Nested intrusive centers in the Zarza complex and in other ring complexes indicate that pathways are commonly reused, even though they may completely solidify between intrusive pulses. 5. The Zarza Intrusive Complex provides a rare example in which we can demonstrate that (1) the intense ductile deformation aureole was not formed by diapirism or lateral expansion of the massive intrusive rocks and (2) host-rock displacements were mainly vertical during ascent and emplacement of these intrusive rocks. Stoping played an important role in both the ascent and emplacement of the massive core gabbros, and a moderate increase in their areas at the cur- 618 Figure 15. Model that requires formation of the deformation aureole prior to emplacement of the intrusive units that define the Zarza Intrusive Complex. See Figure 13 caption for further explanation. rent exposure level could have destroyed crucial evidence for the complex’s ring zones. In this instance, we may have encountered a zoned mafic pluton (plus or minus tonalite) surrounded by a ductile deformation aureole, and we may not have been able to rule out lateral expansion and diapirism as mechanisms for aureole development. ACKNOWLEDGMENTS This project was supported by an Australian Research Council Large Grant A39700451 and Queen Elizabeth II Research Fellowship (to Johnson), grant 4311PT from the Consejo Nacional de Ciencia y Tecnologia (CONACyT) of Mexico (to Johnson), and a Macquarie University Research Fellowship (to Tate). We thank Mark Brandon (Associate Editor), Allen Glazner, and Brendon McNulty for constructive reviews, which led to important improvements to the manuscript. 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