The Zarza Intrusive Complex, Baja California, Mexico

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-

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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 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 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. Figures 2 through center, whereas G2 forms a relatively large body teria summarized in Figure 1. The western belt 4 show the geology, bedding and foliation data, in which isolated rafts and blocks of ring-zone is thought to represent a relatively static arc con- and a trend analysis of the bedding and folia- rocks locally form more than 50% of the outcrop. structed on oceanic crust, whereas the younger tion in the complex and surrounding country Thus, the ring zone originally occurred through- eastern belt apparently developed as a continen- rocks. Figures 5 and 6 show block diagrams out much of the currently exposed center, and tal-margin arc that migrated east with time along the cross-section lines in Figure 4. stoping was an important material-transfer

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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 some- what 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

Absolute ages were determined by U-Pb (zir-

con) geochronology of units G3 and T1 in the central and southern intrusive centers, respectively, and all data were collected with the Aus- tralian 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 deter-

mined for G3 yielded an age of ca. 116.2 Ma; ex- cess 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 Figure 2. Geologic map of the Zarza Intrusive Complex. The northern intrusive center is com- Ma at the 2σ confidence level. Although both

posed of the northern ring zone and gabbros G1 and G2. The central intrusive center is composed ages overlap slightly at the limits of analytical un- of the central ring zone and gabbro G3. The southern intrusive center is composed of gabbro G4 certainty, the data are consistent with the ob- and tonalite T1. served crosscutting relationships, which indicate that T1 is younger than G3. The oldest intrusive units exposed in the northern intrusive center

process during emplacement of G1 and G2. The dikes as described in the northern center are rare. were not dated, and so the above ages provide ring zone and gabbros G1 and G2 are cut by volu- The southern intrusive center instead contains minimum emplacement estimates for the com- metrically minor radial and concentric dikes of coarse-grained anorthositic gabbro (G4) cut by a plex as a whole. hornblende diabase, epidotite, tonalite, and aplite. slightly more abundant hornblende-biotite

The central intrusive center largely mimics the tonalite (T1). These intrusive rocks are more Al-in-Hornblende Barometry northern center and is nested in its southern half. hornblende rich than those in the two preceding

The ring zone is composed almost entirely of centers, and they cut ring-zone rocks that may Biotite and K-feldspar crystals are rare in T1 cone sheets with anorthositic gabbro composi- represent the southern extent of those in the (<2 vol%), as reflected generally by whole-

tions, but the origin of some fine-grained rocks northern and/or central centers. G4 contains rock compositions that contain extremely low with feldspar phenocrysts is unclear; they are ei- abundant rafts and blocks of ring-zone rocks, K2O concentrations (<0.6 wt%). Cobaltinitrite ther rapidly cooled sheets or volcanogenic which suggests that they previously occupied staining revealed K-feldspar as both a ground-

screens. The ring zone was intruded by coarse- much of the area intruded by G4. The ring-zone mass and phenocryst phase in only the most grained anorthositic gabbro (G3; Fig. 2), which rocks are cut by volumetrically minor radial and potassic tonalites, which also contain titanite contains rafts and blocks of ring-zone rocks, the concentric dikes as described above for the north- and have the most potential for Al-in-horn- abundance of which drops off markedly from the ern center, but tonalite and aplite dikes are more blende barometry (e.g., Hammarstrom and Zen, ring zone’s margins to its center. Thus, it is un- abundant. 1986; Johnson and Rutherford, 1989). We con- clear whether the entire intrusive center originally All of the mafic intrusive rocks in the Zarza ducted single-mineral analyses of these sam- contained cone sheets. Late radial and concentric Intrusive Complex have similar modal mineral ples by using the Cameca SX-50 scanning elec-

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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 accelerat- ing voltage, a 2–5 µm spot diameter, and an in- tegrated 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). Depend- ing 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 as- sumes that the experiments of Schmidt (1992) most reliably reproduce the intratelluric oxida-

tion state of T1. Although there is current de- bate 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 mi- arolitic cavities and edenitic biotite composi-

tions in T1 and (2) the ring-complex character- Figure 3. Foliation and bedding attitudes in the Zarza Intrusive Complex. See Figure 4 for istics of the Zarza Intrusive Complex in composite map of foliation and bedding trends and geology. general.

Structures, Microstructures, and Kinematics and G4 generally contain weak magmatic folia- Remarkably, foliations (Fig. 4) and lithologic tions, and lineations are either absent or very dif- contacts (Fig. 2) in the northern and central intru- Most intrusive rocks in the Zarza Intrusive ficult to recognize in all of the larger intrusive sive centers are abruptly truncated by the central Complex contain only one foliation, which is bodies. This observation may be due partly to a and southern centers, respectively. Neither the magmatic (hypersolidus to near-solidus condi- lack of appropriate foliation-parallel exposures, contacts nor the foliations in the older centers are tions) and characterized by alignment of igneous but in general we infer that these bodies do not deflected as younger intrusive contacts are ap- plagioclase and/or hornblende crystals. The cone contain a strong linear fabric. proached, and they both locally strike into these sheets commonly contain variably developed Country-rock screens that lie between sheets contacts at high angles (e.g., Figs. 3 and 4). Fur- magmatic foliations oriented subparallel to sheet in the northern center commonly contain a well- thermore, no penetrative solid-state fabrics were boundaries, and mineral elongation lineations that developed solid-state foliation that strikes paral- observed near these margins, which indicates that plunge approximately downdip. Microstructural lel to, but locally dips less steeply inward than, at least the two younger intrusive centers were analysis of the sheets showed only rare evidence the magmatic foliations in adjacent sheets. They emplaced with little or no lateral expansion or for minor solid-state deformation (on the basis of also commonly contain a well-developed min- shearing of preexisting markers. These observa- the criteria of Paterson et al., 1989a), and we have eral-elongation lineation that generally plunges tions imply that older centers were relatively

found no field evidence of boudinaged sheets. G1, approximately downdip. Many of these screens solid before intrusion of each younger center. G2, and G3 generally contain moderate to strong contain microstructures and mineral assemblages Spectacular kinematic indicators are present magmatic foliations, but in G3, the foliation proof either the hornblende hornfels or pyroxene in the northern and central intrusive centers and gressively weakens toward its central “bull’s-eye,” hornfels facies and were locally intruded by nar- consist primarily of subvertical, discrete, duc-

where it forms a basinal shape (Figs. 5 and 6). T1 row felsic and quartzo-feldspathic veins. tile shear zones that are heterogeneously

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Structures

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 discon- tinuous and upright to moderately south plung- ing; it becomes progressively tighter and over- turned 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 alter- natively could indicate a vertical fan of bedding (Fig. 6); the structure is complicated owing to the unmapped intrusive body with a steeply out- ward–dipping contact in the southwest corner of Figures 2–4. In the country rock, a bedding-parallel foliation and downdip mineral-elongation lineation be- Figure 4. Composite map of foliation and bedding trends, folds, and geology in the Zarza In- come well developed within a few hundred me- trusive Complex. Block-diagram cross sections along the lines A–B and C–D are shown in Fig- ters of the complex and intensify toward its mar- ures 5 and 6, respectively. gins to define a deformation aureole. Although we recognize three intrusive centers in the complex, there is only one aureole, which is spatially asso- spaced on the centimeter to decimeter scale ADJACENT COUNTRY ROCKS ciated with the first and largest intrusive center. (Fig. 8). Locally, in zones of relatively high Kinematic indicators in this aureole were only lo- strain, asymmetrical indicators are surrounded Rock Types cally observed and appear to vary from rare, dis- by a pervasively developed, subvertical folia- crete, steeply inward–dipping cleavage seams that tion. These subvertical shear zones and locally The Zarza Intrusive Complex lies entirely crosscut bedding near the outer edge of the aure- pervasive foliations cut across the cone sheets within the Cretaceous Alisitos Formation, which ole, to asymmetrical clasts and zones of heteroge- and country-rock screens and are concentrated is thought to represent calc-alkalic volcanogenic neously partitioned shear within the bedding- near the outer margins of each center in what rocks deposited mainly in a shallow-marine envi- parallel foliation of the inner aureole. All such we refer to as kinematic zones (Figs. 2, 5, and ronment (Gastil, 1983; Beggs, 1984). Country structures indicated downward movement of the 6). The sense of shear in these zones always in- rocks surrounding the complex are dominantly inner aureole relative to the outer aureole. dicates that the area they enclose moved down lithic and crystal-lithic tuffs; basalt, andesite, and relative to the area outside (Fig. 8), and so we dacite flows; and minor siltstones and sand- Strain Analysis interpret them as ductile equivalents of ring stones. Bedding is generally defined by contacts faults. Variably abundant felsic and quartzo- between different units and by variably devel- Ideally, we would like to quantify deformation feldspathic intrusive rocks are localized in oped alignment of clasts. Locally present sedi- in the aureole caused by both rigid rotation and these zones, which suggests that they may be mentary structures consistently indicated upward ductile flow of units, which can be done by deter- incipient ring dikes. younging. mining the shortening of bedding in roofs above

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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 Figure 5. Block-diagram cross section along the line A–B in Figure 4. Displacements along (e.g., Bateman, 1984; Paterson and Fowler, 1993). kinematic zones stylized. See Figure 2 for geology legend. Vertical and horizontal scales are equal. 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 ra- Figure 6. Block-diagram cross section along the line C–D in Fig. 4. Displacements along kine- tios and orientations of 35 or more lithic clasts matic zones stylized. See Figure 2 for geology legend. Vertical and horizontal scales are equal. were measured in each face. The magnitude of cleavage deflection near clasts and the percentage of clasts versus matrix were used to estimate vis- mate the tectonic strain at each locality with un- this sample reflects heterogeneous strain in the cosity contrasts (usually 1, for no viscosity con- certainties in the range of 10% to 20% strain at fold hinge. All other samples came from within trast) between clasts and matrix (Paterson et al., the 95% confidence level. Strain intensities of the deformation aureole. Figure 10 shows an 1989b). samples within the aureole range from moder- equal-area plot of the x-y planes and x-axes of Shimamoto and Ikeda (1976) and Wheeler ate values (1.52) near the complex to low “re- strain for the oriented samples. The average (1986) noted that any population of elliptical gional” values near the outer margin of the au- Zarza Intrusive Complex margin orientations and markers with different ratios and orientations reole, and Lode’s parameters indicate that all x-y planes of strain correspond well with one an- can be represented collectively by determining but one of the samples yield oblate strain ellip- other, and so it is particularly useful to examine a single average ellipsoid. If this population is soids (Table 1, Fig. 9). shortening along the z-axis (shortening approxi- then deformed, the average ellipsoid defined by Two of the samples (BC 220 and BC 221) mately perpendicular to the complex margin). the deformed objects will reflect strain plus any came from immediately outside of the deforma- These values range from 71% near the complex original fabric (Dunnet and Siddans, 1971; tion aureole and have very low strains with x-y contact to 18% at the margin of the aureole and Seymour and Boulter, 1979). We have used the planes of the strain ellipsoid subparallel to mod- 12% outside of the aureole (Table 1 and Fig. 11). techniques of Shimamoto and Ikeda (1976), erately dipping bedding. Thus, we suggest that From Figure 11 we calculated bulk shortening Miller and Oertel (1979), and Wheeler (1986) they reflect strain caused only by primary com- in the aureole perpendicular to the Zarza Intru- to calculate final average ellipsoids for the de- paction plus regional deformation. Another sam- sive Complex margin by integrating the area un- formed clasts, which were then corrected for ple (BC 413), which came from the fold hinge to der the best-fit curve to all the strain data (the two the presence of primary fabrics by using the the west of the Zarza Intrusive Complex, has a transects having essentially identical shortening procedure outlined in Paterson and Yu (1994). prolate strain ellipsoid with the x-axis of strain at gradients). We defined this outer margin approx- These corrected ellipsoids (Table 1) approxi- a low angle to the fold axis. Thus, we suggest that imately at sample BC 220, corresponding to a re-

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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

Figure 7. Tera and Wasserburg (1972) U-Pb (zircon) concordia curves for gabbro G3 and deflation of the magma chamber during intru- tonalite T1 analyzed by SHRIMP. MSWD—mean square of weighted deviates. sion of the cone sheets and G3. Stage 5. At this stage, the sequence of intrusion of cone sheets followed by massive gabbro gional shortening strain of 18%. The bulk short- required to fracture the overlying country rocks appears to have changed, and initiation of the ening obtained depends on how we treat this re- and intrude the cone sheets (e.g., Anderson, southern center was marked by intrusion of gab-

gional strain, and so two integrations were made: 1936; Roberts, 1970; Phillips, 1974). bro G4. Abundant rafts and blocks of ring-zone one in which the regional shortening strain of Stage 2. After the cone sheets solidified, the rocks in G4 indicate that stoping remained an im- 18% was included in the integration and one in ring zone was intruded and partially destroyed by portant emplacement process. On the basis of ge-

which the 18% value was used as the base of the gabbros G1 and G2. Rafts and blocks of ring-zone ologic and structural patterns in Figures 2–4, we curve. These integrations resulted in bulk short- rocks are common in the gabbros, indicating that interpret the preserved ring-zone rocks in the ening strains of 59% and 38%, respectively, and stoping was an important emplacement process. southern center to be remnants of the northern we make use of this information in a later section. Formation of the northern-center kinematic zone, and/or central centers. As with the central center, which appears to be a ductile ring fault, resulted the southern center migrated to the south, but is PROPOSED INTRUSIVE HISTORY FOR from collapse of the enclosed area into the under- contained within both previous centers. THE ZARZA INTRUSIVE COMPLEX lying magma chamber. Although we have no Stage 6. The intrusive history was completed clear evidence for the relative timing between (except for emplacement of minor radial and con-

This study has revealed a six-stage sequential displacement along this zone and intrusion of G1 centric dikes) by intrusion of T1, which cuts G4 at intrusive history of the Zarza Intrusive Complex and G2, we suggest that collapse resulted from rare field localities. The central-center cone sheets that involves three distinct, south-migrating epi- deflation of the chamber during intrusion of the and G3 are cut sharply by T1, which suggests that sodes (Fig. 12). The cross-sectional view shown cone sheets and gabbros G1 and G2. the entire central center was fully crystallized is the same as that in Figure 6, which is repro- Stage 3. As with the northern center, the central when T1 was emplaced. This interpretation is con- duced by stage 6 of the sequence. We evaluate the center was initiated by intrusion of cone sheets, sistent with the SHRIMP geochronology dis- surrounding deformation aureole in the following which formed a second ring zone contained cussed previously. section. within the southern half of the northern center. Stage 1. The northern intrusive center (Fig. 2) The diameter of the central center is smaller than ORIGIN OF THE HOST-ROCK was initiated by emplacement of cone sheets de- the northern, but the cone sheets have approxi- DEFORMATION AUREOLE rived from an underlying magma chamber. On mately the same range of dips (Fig. 3). Cone- the basis of a linear subsurface projection of the sheet dips are apparently related to the depth of The most puzzling structural feature of the northern-center cone sheets, the underlying the magma chamber below the exposure level, the Zarza Intrusive Complex is the intense ductile de- chamber was probably situated no more than shape of the chamber, and the position on the formation aureole in the adjacent country rocks. 2–3 km below the present level of exposure. At chamber margin from which the sheets were de- In this section, we present and evaluate five mod- some point in the development of this chamber, it rived (e.g., Anderson, 1936; Phillips, 1974). els of events that might have contributed to this presumably became overpressured and began to Given the smaller diameter of the central center aureole in the context of the intrusive history dis- dome or expand upward, creating the conditions and the fact that the dips of the cone sheets are cussed above. Cone sheets of the northern and

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central centers show only rare evidence for solid- state 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 Cone- Sheet 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 poten- tially 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 northern- center sheets, apart from that associated with the kinematic zone. Microstructural analysis of the sheets showed only rare evidence for minor solid- state 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 Figure 8. Example of subvertical, discrete, ductile shear zones found in the kinematic zones of mainly by country-rock screens between the the northern and central intrusive centers. Sense of shear in these zones consistently indicates sheets. These screens were metamorphosed under downward displacement of the Zarza Intrusive Complex relative to surrounding country rocks. conditions of hornblende hornfels to pyroxene In this example, the shear zones and sheared rocks have been disrupted and intruded by hornfels facies and thus reached temperatures at quartzo-feldspathic melts. which they may have partially melted and could be relatively easily deformed. The downdip lineations in the screens suggest a large vertical com- high angles to one another (e.g., Guglielmo, and larger gabbros. During collapse, the entire ponent of material transfer during their deforma- 1993, 1994). Figure 11 indicates that the two complex may possibly have moved downward, tion, and we also speculate that vertical transects have essentially identical shortening the kinematic zones accommodating only part of displacements may have occurred during opening gradients within the uncertainties of the method, the total displacement. Figure 13 shows collapse of the brittle fractures filled by cone sheets (e.g., and the only data yielding a prolate strain ellip- of the complex into its underlying chamber for Le Bas, 1971; Walker, 1975). soid were from sample BC 413 from the western two initial configurations: emplacement of the 2. Synemplacement and Postemplacement transect (Table 1 and Fig. 9). Thus, we conclude magmas (1) into host rocks with originally flat- Regional Shortening. Northeast-southwest that the contribution of regional deformation to lying bedding or (2) into the western limb of the shortening in the Peninsular Ranges batholith re- the aureole was effectively negligible. regional anticline whose axial plane is to the east sulted in a regionally consistent pattern of north- 3. Collapse of the Zarza Intrusive Complex (Figs. 4–6). Partitioned strains across the north- west-trending bedding and folds and locally de- into Its Underlying Chamber. Kinematic zones ern ring zone during collapse could also explain veloped axial-plane foliations. If regional strain within the northern and central intrusive centers the solid-state deformation and inward dips of the occurred in the aureole, it should have had two indicate collapse of their contained areas into the country-rock screens. effects: (1) increased shortening gradients along underlying magma chamber. We could not quan- 4. Sinking of the Zarza Intrusive Complex the western transect compared to the southern tify displacements in these zones, but we specu- and Its Chamber after Solidification. Glazner transect, because the latter is in the regional strain late that they may have accommodated several (1994) and Glazner and Miller (1997) noted that shadow, and (2) prolate strain ellipsoids in the hundred meters of collapse. Space for this col- many plutons of intermediate and mafic compo- southern transect because regional and emplace- lapse was likely achieved largely by removal of sition reach a level in the crust that, after they ment-related shortening would have occurred at magma from the chamber to form the cone sheets crystallize, is at or above their level of neutral

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TABLE 1. STRAIN DATA FROM THE ZARZA INTRUSIVE COMPLEX (ZIC) Sample Axial ratios Elongations Ellipsoids Orientations Distance number from ZIC xyzxy z SI LP x-y plane x axis (m) (strike/dip) (trend/plunge) BC 472 6.48 6.33 1 87.9 83.6 –71.0 1.52 0.98 318/66 ————- 55 BC 209 7.12 3.53 1 143.1 20.5 –65.9 1.41 0.29 235/63 336/63 75 BC 208 3.56 2.96 1 62.4 35.0 –54.4 0.97 0.71 235/58 240/8 190 BC 506A 3.45 2.63 1 65.4 26.1 –52.1 0.92 0.56 15/69 113/69 220 BC 207 2.19 1.90 1 36.2 18.1 –37.8 0.59 0.64 212/54 321/52 330 BC 220 1.46 1.26 1 19.2 2.8 –18.4 0.27 0.22 ————- ————- 495 BC 387 2.33 2.00 1 39.5 19.7 –40.1 0.64 0.64 332/54 91/50 680 BC 413 1.48 1.11 1 25.4 –5.9 –15.3 0.29 –0.47 27/13 136/12 720 BC 221 1.27 1.17 1 11.3 2.5 –12.4 0.17 0.31 ————- ————- 785 2 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 2 1/2 + (e2 – e3) + (e3 – e1) ] , where e1, e2, and e3 are the principal natural strains. LP = Lodes parameter.

buoyancy. In the process of crystallization, these application of the model is complicated by poor ranges of what we considered to be acceptable plutons generally increase their density by as constraints on several parameters that are impor- possible values for these and other parameters much as 10%, which causes an important density tant for the required calculations, including (1) and the methodology described in Glazner contrast between the plutons and surrounding the regional thermal gradient; (2) the lithological (1994), we calculated strain rates varying over host rocks. These authors have argued that, given and density stratification of the crust adjacent to four orders of magnitude from 10–11 to 10–14 s–1, country-rock temperatures sufficient for ductile the complex and with increasing depth; and (3) which corresponded to sinking rates of 680 deformation, such a density contrast may cause the size, shape, and composition of the underly- km/m.y. to 680 m/m.y. Strain rates on the order of the solidified plutons to sink some finite distance ing magma chamber. Nevertheless, by using 10–11 to 10–13 s–1 lead to acceptable time scales through the crust after their initial emplacement for development of the deformation aureole. to a higher level. Because the bulk composition 5. Formation of the Aureole prior to Em- of country rocks near the complex approximates placement of the Zarza Intrusive Complex. andesite, substantial sinking of solidified intru- Development of the complex involved multiple sive rocks would effectively require them to be stages of collapse along subvertical kinematic gabbros. Currently exposed intrusive rocks in the zones. We suggest that the deformation aureole complex are 90% gabbro, and we also infer a may have formed by a similar collapse episode mafic composition for the underlying chamber prior to emplacement of what we recognize as (Tate et al., 1999); thus, this model can be effec- the first intrusive rocks in the complex—the cone tively applied to the complex (Fig. 14). Rigorous 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 geo- metrical relationships can be produced if the complex is then emplaced over this preexisting Figure 10. Equal-area plot of x-y planes and structure (Fig. 15C). x-axes of strain for the oriented samples shown in Table 1 and located in Figure 2 (with BC Discussion of the Aureole prefixes). The x-y planes (solid great circles) and the average orientations of the Zarza In- Although the processes described in all five of trusive Complex margin (large dashes) are these models may have contributed to the defor- plotted as different line widths corresponding mation aureole around the Zarza Intrusive Com- to each transect (see Fig. 2): BC 207–209 come plex, we suggest that models 1 and 2 played mi- from the southern transect, BC 387 and 506A nor or negligible roles. Models 3–5 are all similar come from the western transect, and BC 472 in that they require downward sinking or collapse Figure 9. Strain intensity and Lode’s comes from the southwestern margin of the of material inside the aureole (Figs. 13–15). We parameter plotted against distance from the complex. BC 472 gave a Lode’s parameter of favor this general process for aureole formation, Zarza Intrusive Complex (ZIC) margin. 0.98 (very oblate strain ellipsoid). Thus, the x- but we are unable to suggest, on the evidence at Strain intensity increases toward the margin, axis orientation is unconstrained within the hand, which of these three models is most appro- apparently accompanied by a weak increase uncertainties of the method and is not plotted. priate. We prefer to consider them as three end in Lode’s parameter. The one sample (BC 413) Sample BC 413 came from the hinge of the an- members that are not mutually exclusive; they with a negative Lode’s parameter comes from ticline that bounds the west side of the complex could have acted together, or they could have the hinge of the anticline that bounds the west (Fig. 4), as reflected by the orientation and acted separately at different stages of the intru- side of the complex (Fig. 4). prolate shape (Table 1) of the strain ellipsoid. sive complex’s history. If the complex did actu-

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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 Figure 11. Shortening strain plotted against distance from the Zarza Intrusive Complex (ZIC) of 59% and 38% would correspond to lateral margin. We consider the strain value for sample BC 387 to be anomalous, occurring in a high- shortening of country rocks in the aureole by 710 strain zone just off the hinge of the anticline that bounds the west side of the complex (Fig. 4). 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 12. Sequential intrusive history for the Zarza Intrusive Complex. See text for discussion.

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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 par- Multiple Processes for Host-Rock tially evacuated underlying chamber to explain the deformation aureole and inward-dipping Material Transfer During Emplacement and inward-younging bedding around the complex. There are two possible initial configura- of the Zarza Intrusive Complex tions: 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 configu- The Zarza Intrusive Complex preserves evi- ration. Sequence A followed by C would require synemplacement to postemplacement regional dence for the following five processes of host- deformation to form the anticlines on the western and eastern sides of the complex, whereas se- rock material transfer during its emplacement: quence B to C adequately produces the anticlines during collapse. (1) downward transport of country rocks in the deformation aureole relative to those outside the aureole, (2) stoping during ascent and em-

placement 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 The intrusion and collapse cycles we have bedding around the complex. See Figure 13 caption for further explanation. documented in the Zarza Intrusive Complex pre-

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serve compelling evidence for how pulses of magma escape from some high-level chambers and how pathways toward the surface are cre- ated. The magma chamber under the complex became overpressured and, at some stage, ex- erted enough upward force to fracture and dis- rupt 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 volumi- nous magma transport from the chamber. The re- sulting 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 Figure 15. Model that requires formation of the deformation aureole prior to emplacement of Peninsular Ranges batholith (e.g., Gastil, 1990; the intrusive units that define the Zarza Intrusive Complex. See Figure 13 caption for further Delgado-Argote et al., 1995; Fackler-Adams, explanation. 1997), and the common occurrence of mafic and

bimodal calderas around the world (Walker, can Mineralogist, v. 80, p. 549–559. 1993), lead us to favor this interpretation. rent exposure level could have destroyed crucial Bailey, E. B., 1914, Summary of progress for 1913: Geological evidence for the complex’s ring zones. In this in- Survey of Scotland Memoir, p. 51. Bailey, E. B., Clough, C. T., Wright, W. B., Richey, J. E., and SUMMARY AND CONCLUSIONS stance, we may have encountered a zoned mafic Wilson, G. V., 1924, The Tertiary and post-Tertiary geol- pluton (plus or minus tonalite) surrounded by a ogy of Mull, Loch Aline and Oban: Geological Survey of 1. The Zarza Intrusive Complex preserves ductile deformation aureole, and we may not have Scotland Memoir, 445 p. Bateman, R., 1984, On the role of diapirism in the segregation, structural and intrusive relationships that suggest been able to rule out lateral expansion and di- ascent and final emplacement of granitoids: Tectono- it may have been overlain by a caldera. apirism as mechanisms for aureole development. physics, v. 110, p. 211–231. Bateman, R., 1985, Aureole deformation by flattening around a 2. Cone sheets preserve evidence for a dense diapir during in-situ ballooning: The Cannibal Creek population of fractures that disrupted the crustal ACKNOWLEDGMENTS granite: Journal of Geology, v. 93, p. 293–310. integrity above the complex’s magma chamber. Bates, R. L., and Jackson, J. A., 1980, Glossary of geology (2nd edition): Falls Church, Virginia, American Geological In- Massive core gabbros stoped their way through This project was supported by an Australian stitute, 751 p. this disrupted zone and formed a pathway for Research Council Large Grant A39700451 and Beggs, J. M., 1984, Volcaniclastic rocks of the Alisitos Group, continued magma ascent. Queen Elizabeth II Research Fellowship (to Baja California, Mexico: Society of Economic Paleontol- ogists and Mineralogists, Pacific Section, Field Trip 3. The presence of both sheets and massive in- Johnson), grant 4311PT from the Consejo Na- Guidebook, v. 39, p. 43–52. trusive bodies and the evidence for both brittle and cional de Ciencia y Tecnologia (CONACyT) of Billings, M. P., 1943, Ring-dikes and their origin: New York Academy of Sciences Transactions, ser. II, v. 5, p. 131–144. ductile host-rock deformation illustrate that mul- Mexico (to Johnson), and a Macquarie Univer- Blundy, J. D., and Holland, T. J. B., 1990, Calcic amphibole tiple ascent and emplacement processes can act sity Research Fellowship (to Tate). We thank equilibria and a new amphibole-plagioclase geother- together to facilitate subvolcanic magma transfer. Mark Brandon (Associate Editor), Allen mometer: Contributions to Mineralogy and Petrology, v. 104, p. 208–224. 4. Nested intrusive centers in the Zarza com- Glazner, and Brendon McNulty for constructive Bonin, B., 1986, Ring complex granites and anorogenic mag- plex and in other ring complexes indicate that reviews, which led to important improvements matism: Studies in geology: New York, Elsevier, 188 p. pathways are commonly reused, even though they to the manuscript. Branch, C. D., 1966, Volcanic cauldrons, ring-complexes and associated granites of the Georgetown inlier, Queens- may completely solidify between intrusive pulses. land: Australian Bureau of Mineral Resources Bulletin 5. The Zarza Intrusive Complex provides a rare 76, 158 p. REFERENCES CITED Brown, M., and Rushmer, T., 1997, The role of deformation in example in which we can demonstrate that (1) the the movement of granitic melt: Views from the labora- intense ductile deformation aureole was not Ague, J. J., and Brandon, M. T., 1996, Regional tilt of the tory and the field, in Holness, M. B., ed., Deformation- formed by diapirism or lateral expansion of the Mount Stuart batholith, Washington, determined using enhanced fluid transport in the earth’s crust and mantle: aluminum-in-hornblende barometry: Implications for London, Chapman and Hall, p. 111–144. massive intrusive rocks and (2) host-rock dis- northward translation of Baja British Columbia: Geolog- Clough, C. T., Maufe, H. B., and Bailey, E. B., 1909, The caul- placements were mainly vertical during ascent ical Society of America Bulletin, v. 108, p. 471–488. dron-subsidence of Glen Coe and the associated igneous and emplacement of these intrusive rocks. Stop- Anderson, E. M., 1936, Dynamics of formation of cone-sheets, phenomena: Geological Society of London Quarterly ring-dikes, and cauldron subsidences: Royal Society of Journal, v. 65, p. 611–678. ing played an important role in both the ascent Edinburgh Proceedings, v. 56, p. 128–157. Cruden, A., 1990, Flow and fabric development during the and emplacement of the massive core gabbros, Anderson, J. L., and Smith, D. R., 1995, The effects of temper- diapiric rise of magma: Journal of Geology, v. 98, ature and f on the Al-in-hornblende barometer: Ameri- p. 681–698. and a moderate increase in their areas at the cur- O2

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