Orbicular Volcanic Rocks of Cerro Panizos: Their Origin and Implications for Orb Formation

Orbicular volcanic rocks of Cerro Panizos: Their origin and implications for orb formation

MICHAEL H. ORT* Department of Geological Sciences, University of California, Santa Barbara, California 93106

ABSTRACT clei. As limited mixing with the surrounding Thompson and Giles, 1974, 1980; Enz and oth- coarsely porphyritic magma occurred, het- ers, 1979, 1980; Vernon, 1985). The discovery Orbs, which are a rare occurrence in granit- erogeneous nucleation in the supercooled and description of orbs in volcanic rocks at oid rocks, are present in a 5-m-thick stratum magma began, forming the abundant small Cerro Panizos resolve some of this debate, as the within the Cerro Panizos Ignimbrite and in crystals seen in the finely porphyritic pumice. relative timing of orb formation and the compo- two post-ignimbrite lava flows. Orbs in vol- Eruption of the orbicular dacite occurred sition of the magma from which they crystal- canic host rocks are extremely rare and have when a ring vent conduit tapped the magma lized can be determined. Volcanic orbs, which not been described in detail previously. The in the cupola. Similar processes may form have only been described from one other local- orbs consist of two to five crystalline rings orbs in plutonic rocks, with pressure release ity (Koide, 1951), provide information on surrounding a xenolithic or orthopyroxene related to either eruption or intrusion to poorly understood events that occur in large core. The rings alternate between bands of higher levels in the crust. magma bodies during eruption. This paper de- large, radially oriented plagioclase and or- scribes orbs from the crystal-rich dacitic ignim- thopyroxene crystals and bands of small, tan- INTRODUCTION brite and lavas of Cerro Panizos, a large gentially or radially oriented biotite and ignimbrite center in the central Andes Moun- ilmenite crystals. The ignimbrite orbs are as- Orbs in plutonic rocks have been studied in tains, and presents a model for their origin. sociated with two types of pumice: (1) a some detail in an effort to understand processes Orbs are enclaves composed of concentric coarsely porphyritic biotite-quartz-plagio- along the margins of plutons, but unanswered rings of mafic and felsic minerals (Fig. 1). Crys- clase dacite with 35%-40% crystals found questions remain concerning the location, tim- tals are oriented radially or tangentially within throughout the ignimbrite and (2) a finely ing, and processes involved in the formation of any individual ring, and crystal orientation may porphyritic biotite-plagioclase quartz dacite orbs (Moore and Lockwood, 1973a, 1973b; vary between rings in a single orb. Orb cores with 75%-80% crystals found only in association with the orbs. The major- and trace- element and isotopic compositions of the two pumice types are identical. Bronzite and quartz megacrysts are also found with the ignimbrite orbs and rarely occur in the overly- ing sequence. Plagioclase and orthopyroxene compositions in orb-associated rocks exhibit large variations. The orb inner rings and finely porphyritic pumice have the most mafic compositions (Angi.92 and En6o_77), coarsely porphyritic pumice and lavas have the most felsic compositions (mostly An45_63 and E°42-5o)> and the outer rings of the orbs span the entire gap between the two groups (An67_77 and Enso-^). The orbs formed in a water-rich cupola along the roof of the magma body, where the magma was superheated and most crystals were resorbed. Pressure release related to eruption caused exsolution of water, leading to large degrees of undercooling. Orbs formed rapidly around the few available nu-

*Present address: Department of Geology, Box 6030, Northern Arizona University, Flagstaff, Arizona 86011.

Geological Society of America Bulletin, v. 104, p. 1048-1058, 13 figs., August 1992.

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may consist of country-rock fragments, cognate tion occurs around any available nuclei, such as vents opened at the end of emplacement of the igneous rocks, crystals of refractory minerals, or wall-rock fragments and refractory crystals. lower cooling unit and existed throughout the pieces of broken orbs (Vernon, 1985). Contacts rest of the eruption. Most post-caldera lavas between orbs and their dioritic to granitic host ORBS IN THE CERRO PANIZOS were erupted from vents along the ring fracture rocks are typically abrupt (Elliston, 1984). VOLCANIC ROCKS of the caldera (Ort, 1991).

Origin of Plutonic Orbs Eruptive Sequence of Cerro Panizos Orb Occurrence

Several theories on the origin of orbs in igne- The late Miocene Cerro Panizos eruptive cen- Orbs at Cerro Panizos occur in restricted lo- ous rocks have been proposed. Most of these ter, located in the Puna Plateau of the central calities associated with eruption from ring vents ideas involve magmatic (Van Diver, 1970; Andes Mountains, produced at least two large- (Fig. 3). Orbs are <1% of the rock within a Lofgren and Donaldson, 1975; Enz and others, volume ignimbrites and many lava flows. Vol- 5-m-thick stratum at the top of the lower cool- 1979, 1980; Brigham, 1983; Vernon, 1985) or canism began ca. 7.9 Ma with the eruption of ing unit. This layer was deposited by the earliest metasomatic (Eskola, 1938; Thompson and the dacitic Cienago Ignimbrite (Fig. 2). Dacite pyroclastic flow associated with caldera collapse Giles, 1974, 1980) processes. Other hypotheses lava flows were erupted from around this time (Fig. 2) and contains as much as 7% angular postulate the formation of orbs and comb layer- until the caldera-forming Cerro Panizos Ignim- volcanic lithic fragments. This early ring-vent ing from aqueous fluids segregated from a brite was emplaced at 6.75 Ma. Post-ignimbrite deposit crops out around the entire volcanic cen- magma and concentrated along the margins of lava effusion continued until at least 6.1 Ma ter, but orbicular ignimbrite occurs only in the the chamber (Moore and Lockwood, 1973a, (Ort, 1991). The Cerro Panizos Ignimbrite is southeastern quadrant. The ignimbrite orbs are 1973b) or suggest that they are metamorphosed divided into two cooling units, with no evidence found in association with finely porphyritic hydrosilicate aggregates (Elliston, 1984). of erosion between their emplacement. Ring pumice and orthopyroxene and quartz meg- Vernon (1985), in summarizing the strong evidence for a magmatic origin of orbs, pointed out that (1) orbs are composed of the same min- Composite Stratigraphie Column, Cerro Panizos erals as the surrounding granitoid, although with different abundances and mineral compositions; (2) orb bulk compositions are similar to normal Q. Cusí Cusi granitoid rock types; (3) orbs typically occur in METERS small intrusive bodies; (4) some orbs have small Q. Cuevas septa of matrix material between the shells; H - (5) in some orbicular granitoids, plagioclase in II -270 Lavas the orbs varies regularly in composition, becom- Upper Cooling Unit, ing less calcic outwards; and (6) in some orbicu- Cerro Panizos Ignimbrite lar/ rocks, the compositions of plagioclases in the -240 orb rims are consistent between orbs. Evidence presented in this paper further buttresses these Upper Cooling Unit arguments for an igneous origin. -210 Cerro Panizos Ignimbrite Vernon (1985) proposed a model for the ig- H Jt H neous formation of orbs that involves superheat- it ji -ib w H ing of the magma followed by supercooling. • 180 /Surge Deposits Superheating, resulting from the intrusion of Lower Cooling Unit .u? í ir.'»;'! Cerro Panizos Ignimbrite A ""Orb-bearing Ignimbrite hotter magma or addition of water to the melt, ji- destroys framework silicate nuclei. Supercooling •150 Il occurs during a lag time between the end of superheating and the beginning of crystal nu- - 120 ti ;< cleation. During this brief period, crystal forma- Cienago Ignimbrite Lower Cooling Unit, ^H M Cerro Panizos Ignimbrite )( n~ -90 M H -f Figure 1. Photos of type 1 (left and upper Volcaniclastic Rocks -f- » <1 Jl center) and type 2 (lower center and right) -60 >•'•'»'•'.>'•' orbs. Type 1 orb has meta-pelite core and radially oriented plagioclases and orthopy- » il" ? IIa pi< Tuff of Cusi Cusi roxenes in rings separated by thin rings of bio- •30 H M h O., , » O'l tite and ilmenite. Outer ring is a mixture of all four minerals. Type 2 orb has relatively large Volcaniclastic Rocks bronzite core with slightly resorbed edges I- 0 and thinner orb rings than type 1 orbs. Quartz megacrysts (below scale) are found in Figure 2. Composite stratigraphie column of the volcanic rocks of Cerro Panizos in Quebra- association with orbs. das Cusi Cusi and Cuevas. See Figure 3 for locations.

Geological Society of America Bulletin, August 1992

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12?

16?

20°

24°

74? 70° 66° Figure 3. Geologic map of Cerro Panizos. (L) Main ignimbrite, associated with central vent, (U) main ignimbrite, associated with ring vents, (PC) pre-caldera lava domes, (D) post-caldera ring domes and lavas, (Tv) Tertiary volcanic and volcaniclastic rocks, (QC) Quebrada Cuevas, (QCC) Quebrada Cusi Cusi. Black areas are shallow lakes. Orbs are found at the L/U boundary in the southeast quadrant and in lavas that issued from vents along the east and north sectors of the ring fracture. Location map in inset (modified from de Silva, 1989).

acrysts. The other host rocks for orbs are two orbs at Cerro Panizos, although some orbs have roxene cores of orbs typically consist of several post-caldera lava flows erupted from vents along thin radial cracks. The outer rings of the orbs are large (5-10 mm diameter) crystals. Xenolithic the northeast sector of the caldera ring fracture. intact, with no signs of abrasion of the outer orb cores are commonly fragments of thermally layer. metamorphosed pelitic sedimentary rocks, prob- Orbs The orbs at Cerro Panizos are of two types ably derived from the Paleozoic pelitic rocks of (Fig. 1). Type 1 orbs have a selvage of randomly the region. They consist of equigranular plagio- The orbs of Cerro Panizos are holocrystalline oriented crystals around their cores, and the total clase, quartz, biotite, orthopyroxene, hercynite, oval enclaves with concentric rings of crystals. orb ring thickness is greater than the core radius. and ilmenite crystals, with larger crystals near Ignimbrite and lava matrix textures wrap Type 2 orbs lack the selvage and have a greater the xenolith margin. around the orbs, with glass and elongate crystals core radius than their total ring thickness. Type The core-ring contact in type 1 orbs is some- paralleling the edges of the orbs. Contacts be- 1 orbs are somewhat more abundant in the ig- what diffuse, with some core crystals extending tween the orbs and groundmass are abrupt, and nimbrites, whereas type 2 orbs predominate in slightly into the first ring. A selvage that is as the material immediately surrounding the orbs is the lavas. much as 3 mm thick and that contains randomly indistinguishable from matrix material meters Type 1 orbs have as many as five rings oriented plagioclase and orthopyroxene crystals away. The orbs contain no glass or matrix crys- around a 1- to 4-cm orthopyroxene or xenolith surrounds the core. The first thick ring consists tals as inclusions, even at orb-matrix boundaries. core (Fig. 1). Individual rings, which range from of radially oriented plagioclase and orthopyrox- "Breakouts," in which rings are ductilely de- 1 mm to 8 mm wide, alternate between rela- ene crystals. In some orbs, several elongate, ra- formed or intruded by magma (Enz and others, tively wide bands of plagioclase and bronzite, dially oriented orthopyroxenes extend from the 1979,1980; Elliston, 1984), do not occur in the and thin rings of biotite and ilmenite. Orthopy- core-ring contact through the selvage and the

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innermost orb ring. Subsequent rings alternate between narrow (< 1 mm) bands of tangentially oriented biotite and ilmenite crystals, and thicker rings of radially oriented plagioclase and orthopyroxene (as much as 8 mm thick). Minor apatite is distributed throughout the orbs. Plagi- oclase and orthopyroxene crystals are commonly optically continuous across the biotite/ ilmenite rings. Rarely, biotites rim single crystal faces of radial orthopyroxenes. Individual biotite crystals are oriented radially or tangentially, but whole biotite clots have a radial orientation (Fig. 4). One orb consists entirely of radially oriented orthopyroxene crystals around an orthopyroxene core. Radially oriented plagioclase crystals are commonly twinned on both the 010 and 001 planes, creating twinning surfaces that form a V that is concave outward toward the orb rims (Fig. 4). Type 2 orbs, more common in the post- caldera lavas, consist of one or two 0.2- to 2-mm-thick rings around a 0.5- to 2-cm orthopyroxene crystal cluster. The contact between randomly oriented core orthopyroxenes and radially oriented plagioclase and orthopyroxene crystals is sharp (Fig. 5). The innermost ring is <3 mm thick, and a second, similar ring is commonly present, separated from the first by a single crystal band of tangentially or randomly oriented biotite and ilmenite.

Pumice in Orb-Bearing Rocks

Two types of pumice occur in orb-bearing outcrops. A light gray, coarsely porphyritic pumice is -20% of the orbicular ignimbrite. This pumice type, characteristic of the Cerro Panizos Ignimbrite, contains abundant, 1- to 2-mm crystals of plagioclase (15%-20%), quartz (10%- 15%), and biotite (10%-15%), with rare orthopyroxene, Fe-Ti oxides, apatite, and zircon (Figs. 6 and 7a). The second pumice type is dark gray and finely porphyritic, with as much as 70%-80% crystals (Figs. 6 and 7b). It makes up — 1% of the orbicular ignimbrite horizon. Plagio- clase, quartz, and biotite crystals are abundant (25%, 30%, and 25%, respectively), whereas or- Figure 4. Photomicrographs of orb textures. Field of view is 4 mm wide. Plane light (a) and thopyroxene is rare. Part of the rock (20%-30%) polarized light (b) views of orb textures. Tangentially oriented biotite (dark crystals in a) in is a glass matrix with as much as 40% vesicles radially oriented clots. Note 010 and 001 twinning in plagioclase in b, possibly related to rapid (Fig. 7c). Unzoned 0.1- to 0.4-mm-long plagio- growth. Up in photomicrographs is toward orb rim. clase laths and biotite crystals are poikilitically enclosed in 1.5- to 2.0-mm quartz crystals (Fig. 7b). Rare zoned and resorbed plagioclase crys- abundance of large crystals. Some pumice frag- ignimbrite sequence that overlies the orb- tals are similar in size to those in the coarsely ments are gradational in texture between the bearing outcrops and absent in the underlying porphyritic pumice. Some pumice fragments two main pumice types. rocks. Orthopyroxene crystals, similar in ap- contain both coarsely and finely porphyritic pearance to the cores of some orbs, are euhedral, Megacrysts in Orb-Bearing Rocks pumice material, with subparallel bands of each equant, and as much as 2 cm in size. Quartz magma type (Fig. 6). In these fragments, the Orbs occur in association with quartz and or- crystals are elongate, doubly terminated, euhed- finely porphyritic material contains a greater thopyroxene megacrysts, which are rare in the ral, and as much as 3 cm long and 1 cm wide.

Geological Society of America Bulletin, August 1992

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ered by automated Induced Neutron Activa- tion Analysis (INAA) at Los Alamos National Laboratory as described in Minor and others (1981). Sr, Nd, and Pb isotopic ratios were determined at the University of California, Santa Bar- bara, on a Finnegan MAT 261 multicollector mass spectrometer run in static mode. Blanks are 0.08 ng for Sr, 0.1 ng for Nd, and 0.2 ng for Pb, and are insignificant. Corrections for in situ decay of 87Rb were made using Rb and Sr concentrations from X-ray fluorescence analysis. In situ decay of Sm and Pb was insignificant, based upon elemental ratios from representative samples.

Mineral Compositions

Microprobe analyses of plagioclase and orthopyroxene compositions in the orbs, both pumice types, and lavas indicate a large, consistent variation in compositions within the suite. Orthopyroxenes range from bronzite to ferrohy- Figure 5. Photomicrograph of core-orb contact in type 2 orb. Note resorption of edge of persthene, and plagioclases range from anorthite bronzite core (upper right). Field of view is 4 mm wide. to andesine. Individual pyroxene crystals are chemically unzoned, and no chemical trends exist within Although the quartz megacrysts are associated were made by the ZAF method. Relative analyt- individual orb rings. On an FeO versus MgO with the orbs, the orbs themselves contain no ical precision for Mg and Fe in pyroxenes is 3% plot (Fig. 8), fieldso f orthopyroxenes in the orb quartz crystals. Both megacryst types rarely and 1.5%, respectively. Relative errors for Ca cores, orb inner rings, finely porphyritic pumice, occur as mono-mineralic aggregates. and Na in plagioclase analyses are 1%. Complete and magnesian pyroxenes in the coarsely por- analytical data are presented in Ort (1991). phyritic pumice overlap and are very magnesian, GEOCHEMISTRY OF ORBS AND Major- and some trace-element (Ba, Co, Cr, whereas coarsely porphyritic pumice and lava RELATED ROCKS Cu, Mn, Ni, Pb, Rb, Sr, V, Y, Zn, Zr, and Nb) orthopyroxene phenocrysts have relatively high compositions were analyzed using a Philips FeO/MgO ratios. Orb-rim orthopyroxene com- Major, trace-element, and mineral composi- 1400 X-ray fluorescence spectrometer at the positions cover the entire range between the tions, as well as Sr, Nd, and Pb isotopic ratios, University of British Columbia. Major elements fields of the orb inner ring and the coarsely por- were determined for the orbs, both pumice were analyzed on fused rock powders, and trace phyritic pumice and lava. types, post-caldera lavas, and holocrystalline xen- elements were analyzed on pressed powder oliths to constrain their origins and their rela- disks. Additional trace-element data were gath- tions to each other. Whole-rock compositions Figure 6. Coarsely porphy- were not determined for the orbs due to dra- ritic pumice (upper lefit) and matic modal variations from core to ring and finely porphyritic pumice (up- between rings. These variations imply that their per right) samples, collected whole-rock composition does not directly cor- within 1 m of each other. respond to the composition of the liquid from Both are crystal rich, but which they crystallized. Mineral compositions their textures are distinct. were used for comparisons between the orbs and Coarsely porphyritic pumice other eruptive products. is equigranular and about 40% crystals. Finely porphy- Analytical Techniques ritic pumice contains 80% crystals and contains biotite Mineral compositions were analyzed on a clots and large quartz crys- three spectrometer ARL EMX-SM 120000 mi- tals. Lighter color of coarsely croprobe retrofitted with a Tracor-Northem TN- porphyritic pumice is due to 200/400 energy-dispersive analysis system at greater abundance of glass in the University of California, Santa Barbara. the matrix. Mixed pumice Energy dispersive spectroscopy was used for all (bottom), composed of both elements except Na, which was analyzed using types of pumice, shows mix- wavelength spectrometers. Matrix corrections ing and mingling textures.

Geological Society of America Bulletin, August 1992

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Figure 7. Photomicrographs of coarsely and finely porphyritic pumice. Field of view is 4 mm wide. a. Coarsely porphyritic pumice, crossed nicols. Note abundant large equigranular crystals, resorbed quartz, zoned plagioclases, and paucity of vesicles, b. Finely porphyritic pumice, crossed nicols. Note very abundant and small crystals poikilitically enclosed by large quartz, crystals. Plagioclase and biotite crystals are nearly all of the same size. c. Finely porphyritic pumice, plane light. Note highly vesicular matrix glass.

with the ground mass arc abrupt, (c) the material surrounding the orbs shows no Evidence of al- teration, and (d) orbs did not engulf matrix crystals during growth.

Occurrence of Orb Assemblage The fact that the orb assemblage (orbs, finely Complex zonation is common in the pumice both pumice types and the lavas (Fig. 12A). Pb porphyritic pumice, and quartz and bronzite and lava plagioclases but is not observed in the isotopic ratios do not vary significantly in the megacrysts) occurs only in deposits associated orb plagioclases. Relationships similar to those ignimbrite and are identical to the values for the with the onset of eruption from ring vents sug- seen in the orthopyroxene data are evident for orbs (Figs. 12B and 12C). gests a genetic link between the two. The orb the plagioclase compositions (Fig. 9), although assemblage is a volumetrically minor part of the they are not as clearly defined. Plagioclases from DISCUSSION Cerro Panizos eruptive products. A caldera ring the inner orb ringsar e very calcic, whereas those vent erupts material near the roof or wall of the in orb rims trend toward higher Na20/Ca0 The characteristics of the source for the orbs, magma chamber that may have been some dis- values, approaching plagioclase rim composi- finely porphyritic pumice, and megacrysts at tance away from, and relatively unaffected by, tions of the coarsely porphyritic pumice. Cerro Panizos are constrained by their restricted any central vent conduit (Spera, 1984). The orbs spatial association, widely ranging mineral com- and finely porphyritic pumice were concen- Whole-Rock Geochemistry positions, and similar whole-rock geochemical trated in a small area along the chamber margin and isotopic compositions. and close to the base of the early ring-vent con- Whole-rock compositions were determined The orbs at Cerro Panizos are not xenoliths of duit, as they were erupted among the first prod- for coarsely and finely porphyritic pumice sam- wall-rock material, because (1) no xenolithic ucts of the new vent. Similar orb concentrations ples to determine whether their strikingly differ- material adheres to any orb, (2) there is no evi- along magma-body margins are described in or- ent appearances are compositional. Differences dence of resorption around the rims of the orbs, bicular granitoid rocks (for example, Moore and between the two pumice types in major- and (3) the mineral assemblages of the orbs and Lockwood, 1973a, 1973b; Elliston, 1984). trace-element abundances are small (Fig. 10). pumice are similar, and (4) the strong composi- Rare-earth-element compositions for both pum- tional zoning within the orbs indicates that they Pétrographie Textures ice types and the lavas are nearly identical (Fig. were not re-equilibrated during a contact meta- 11). Isotopically, the orbs are indistinguishable morphism event. The orbs grew before the The components of the orb assemblage are from the ignimbrite. Orb Sr and Nd isotopic emplacement of the ignimbrite, because (a) ma- rarely found without the whole assemblage. The values are well within the range of values for trix textures wrap around the orbs, (b) contacts compositions of mineral phases in the orbs and

Geological Society of America Bulletin, August 1992

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34 • Coarsely-Porphyritic Pumice • Coarsely-Porphyritic Pumice 32 a • Finely-Porphyritic Pumice . en • Finely-Porphyritic Pumice 30 o Lava ° Lava

28 - + Inner Orb Rings + Inner Orb Rings Outer Orb Ring 26 - 0 \„ 0 Outer Orb Ring

O 24 - CD O • • CM LL 22 «

18 - 16 v\ r *A V 14 7 12 1 , 1 1 i , i , r , 12 14 16 20 22 24 26 28 30 1 1 1 1 , i . i 10 12 14 16 MgO CaO Figure 8. FeO versus MgO (wt%) in orthopyroxenes in orbs, pumice, and lava from Cerro Panizos. Orb-core pyroxenes and some pum- Figure 9. Na20 versus CaO (wt%) in plagioclases in orbs, pumice, ice pyroxenes are distinctly more magnesian than lava and most and lava from Cerro Panizos. As in the pyroxene diagram, the pumice pyroxenes, and orb-rim pyroxenes span the compositional gap orb-outer-rim plagioclases trend toward a more evolved composition. between them. Pumice plagioclases have a wide range of compositions and do not resolve into distinct fields. They also show strong compositional within-crystal zonation. 1000-g O P87-78A

• P87-78B 100CH 100- n P87-8C P87-78A M • P87-28 P87-78B m 0 P87-74B P87-8C er P87-28 10-a te. O • AMO-16 •a•c 10CH P87-74B ^ v AMO-22C c AMO-16 o •C AMO-22C o O o 1-5 S cc o o . ..• IT 10-= 0.1 -s 14

0.01- -r— —I—I—I—I—I—I— I I I I I I I I —I 1— Sr Rb BaTh Ta Nb Ce Zr Hf Sm V Yb Sc Cr Ni La Ce Nd Sm Eu Tb Dy Yb Lu

Figure 10. Trace-element spider diagram for both pumice types and Figure 11. Rare-earth-element diagram for Cerro Panizos pumice lavas from Cerro Panizos, normalized to average crustal lamprophyre. and lavas. Note the strong similarities between all of the magmatic There is little compositional difference between the samples. P87-78A types from Cerro Panizos. Same samples as in Figure 10. and P87-78B = coarsely porphyritic and finely porphyrinic pumice, respectively, from same outcrop; P87-8C = pumice, base of main ignimbrite; P87-28 = ring lava; P87-74B = pumice, uppermost ignim- formed during short-lived changes in magmatic brite; AMO-16 = pumice, middle of main ignimbrite; AMO-22C = conditions. pumice, precursor ignimbrite. Some orb cores show slight resorption at their edges, but the core pyroxenes are very similar in composition to the inner-ring pyroxenes, and so in finely porphyritic pumice also indicate a thopyroxene crystals grew radially (optimal for no resorption should have occurred if the crys- common source, and they suggest that the orbs cation diffusion and space) around refractory tallization process was continuous. The orbs en- and finely porphyritic pumice formed in the nuclei. Moderate undercooling predominated close no non-oriented crystals, implying that same setting. The conclusions below are based during the formation of the non-skeletal crystals there were few crystals in the magma during largely on evidence from the ignimbrite orbs, as that dominate the orbs, but the skeletal bronzite their rapid growth. A superheating episode be- fewer constraints exist for the lava orbs. crystals in some orbs imply large degrees of un- tween the time of core formation and the onset The orbicular texture is related to rapid crys- dercooling during the earliest stages of orb for- of orb crystallization can explain these features. tallization (Vernon, 1985). Plagioclase and or- mation. Rings of biotite and ilmenite apparently The two pumice types are very similar in bulk

Geological Society of America Bulletin, August 1992

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to 15.69 • Coarsely Porphyritic Pumice 5 • Finely Porphyritic Pumice • • Lithic Fragments o Orbs 15.68 0 - "O X2 H • co gC L -5 ^ 15.67 I .O CL -10 r^ o CM 1 i.i, -15 15.66 0.70 0.71 0.72 0.73 0.74

87Sr/86Sr 15.65 Figure 12. Isotopic data for orbs, pumice, and tonalité and sedimentary fragments found in Cerro Panizos Ignimbrite. A. ^Sr/^Sr ver- 39.2 sus eM. Note wide range of values for pumice. Orbs are within range for pumice. Most tonalité and sedimentary fragments overlap with 39.1 X> Cerro Panizos magmatic ratios. CL CoM 39.0 j2 composition, and, apart from variations in min- Q. 38.9 CO o eral compositions discussed below, the main dif- CM ference between them is textural. The large 38.8 euhedral crystals of the coarsely porphyritic pumice, including multiply zoned plagioclases, 38.7 imply a growth history with a relatively slow 38.6 cooling rate (Swanson and Fenn, 1986). The 18.7 18.8 18.9 19.0 19.1 large number of uniformly small, non-skeletal, plagioclase and biotite crystals in the finely por- 206pb/204pb phyritic pumice implies a high rate of heterogeneous nucleation (Lofgren, 1983) and moder- Figure 12. (Continued). B. 207Pb/204Pb versus 206Pb/2

Geological Society of America Bulletin, August 1992

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compositional fields (Figs. 8 and 9) and modal orbs at Cerro Panizos can be explained by oscil- Cerro Panizos Ignimbrite caused a large drop in assemblages imply varying crystallization condi- lating water contents in the magma surrounding magmatic pressure. Water exsolved as a separate tions. The orb-rim orthopyroxenes and plagio- the orbs. phase from the cupola magma, rapidly lowering clases are compositionally distinct from the Some process must have concentrated water the magmatic water content (Fig. 13C). inner-ring phases and crystallized under mag- in the magma in significant quantities to induce As the water content dropped, the liquidus matic conditions that were changing toward superheating. Water in rhyolitic magmas can be rose quickly, and the magma became strongly those of the main magma body. concentrated near the roof of the chamber, pro- undercooled. Orb formation occurred after the ducing a crystal-poor cap (Hildreth, 1977), but magma became supercooled. Very efficient re- Conditions of Orb Assemblage there is no crystal-poor juvenile material in the sorption of sub-microscopic crystals (Lofgren, Crystallization Cerro Panizos Ignimbrite. Water is concentrated 1983) left wall-rock fragments and bronzite by a thermal gradient along the margins of a crystals as the only available nuclei, and orbs Two plausible explanations that can be made magma body (Kennedy, 1955; Fridrich and formed rapidly around them. The quartz meg- for the variations in the compositions of the Mahood, 1984) and may reach levels high acrysts may have grown at this time. No quartz mineral phases are (1) mixing of two magmas of enough to cause superheating, especially in cu- crystals are present in the orbs, possibly due to different temperatures and (2) a change in the polas. Another possibility is that water exsolved nucleation site incompatibilities. magmatic conditions during orb crystallization, as a separate phase during early phases of the Eventually, heterogeneous nucleation began so that the cores and inner rings crystallized eruption (Hildreth, 1981) and rose and concen- in the undercooled magma, possibly aided by from a magma relatively close to its liquidus. trated in cupolas near the top of the magma mixing with the main magma body. Rapid nu- The orb outer ring and both pumice types crys- body. cleation of plagioclase and biotite formed many tallized under conditions farther below the Another source for water in the magma is the small crystals (Fig. 13D). As nucleation pro- liquidus. roof or walls of the chamber, but Taylor (1974, gressed, the degree of undercooling dropped rap- Apart from the arguments about rates of heat 1977) and Taylor and Forester (1979) con- idly for plagioclase and biotite. Quartz took diffusion presented above, mixing of two mag- cluded that water does not infiltrate into magma longer to nucleate, but after it was initiated, it mas is unlikely because the coarsely and finely in large quantities from the wall rocks under crystallized to form the oikacrysts now found in porphyritic pumices are virtually identical in normal conditions. Taylor (1977) and Hildreth the finely porphyritic pumice. bulk composition. Differences in their mineral and others (1984) proposed that stoping and While the finely porphyritic pumice magma phases must be related to magmatic conditions assimilation of roof rocks can locally saturate a was crystallizing, the lesser degree of undercool- other than composition. It is difficult to envisage magma with respect to water and modify Sr and ing was also recorded in the outermost ring on a magma that is this similar to the coarsely por- O isotopic ratios of subsequent eruptive prod- the orbs. The compositions of the plagioclase phyritic magma composition ascending through ucts. An avalanche of water-saturated wall rock and orthopyroxene in the outer ring are similar thick crust and arriving at magma-chamber into the magma chamber at Cerro Panizos could to those of the coarsely porphyritic pumice, and depths with a significantly different temperature. have released water into a restricted area. There biotite and ilmenite are in the same ring with the The hotter magma body would then have to are no hydrothermally altered lithic fragments plagioclase and orthopyroxene, consistent with ascend through the magma chamber to the level associated with the orbs, however. The existence crystallization from a magma that was rapidly of erupted material without losing either its in- of xenolithic orb cores means that there were approaching the crystallization conditions of the tegrity or significant amounts of heat. some wall-rock fragments in the magma at the coarsely porphyritic pumice. The second model, in which superheating and time that orbs began to crystallize, but they were The orbs, finely porphyritic pumice, and supercooling occur due to variations in water scarce. Diffusion of water along a thermal gra- megacrysts were concentrated near the chamber content, is more viable. The liquidi of orthopy- dient is the more likely mode of water enrich- walls, close to where an early ring-vent conduit ment in the magma. roxene and plagioclase vary with PH2O> AS well tapped the chamber. This conduit tapped an as with composition and temperature. The effect area in the chamber that was far from the central of changes in water pressure on the composi- ORB CRYSTALLIZATION MODEL vent conduit (Fig. 13E). Orbs do not occur tions of plagioclase is similar to that of tempera- elsewhere in the ignimbrite, because they ture (Yoder and others, 1957). Higher water The model for orb crystallization developed formed in a cupola or cupolas not tapped by contents depress the liquidus, causing more cal- here is modified from that postulated by Vernon other conduits. cic plagioclase to crystallize. Because the liqui- (1985). The Cerro Panizos magma body con- The abundance of coarsely porphyritic pum- dus slants from high-temperature magnesian sisted of thousands of cubic kilometers of ice in orbicular ignimbrite outcrops at Cerro Pa- pyroxenes to lower-temperature ferric composi- crystal-rich dacitic magma. Material from the nizos implies that the orb assemblage did not tions (Huebner, 1980) and increasing water con- upper part of this magma chamber is form in the conduit itself. Orb formation within tent lowers the liquidus, higher PH2O also represented by the coarsely porphyritic pum- the conduit should result in far more abundant, produces more magnesian crystals. ice common throughout the Cerro Panizos finely porphyritic pumice than is the case at According to Naney's (1983) experimental Ignimbrite. Cerro Panizos, as virtually all of the magma in phase relations for synthetic granodiorite com- Water was concentrated in a cupola near the the conduit would experience the superheating positions at 200 MPa, orthopyroxene and plagio- top of the magma chamber due to either a diffu- and undercooling episode. Orb formation within clase crystallize between 800 and 900 °C at sion gradient or pressure release during early a conduit could potentially occur in other between 2 and 4 wt% water. If crystallization phases of the eruption (Figs. 13A and 13B). This systems. occurs at <~825 °C, a small increase in water lowered the liquidus of the magma in the cupola The lava orbs occur only in ring-dome lavas. content will initiate biotite crystallization. The area, causing resorption of crystals (Fig. 13B). The lavas are very similar in composition to the different mineral assemblages in the rings of the Evacuation of magma during eruption of the coarsely porphyritic pumice, and it is not known

Geological Society of America Bulletin, August 1992

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Figure 13. Proposed model for orb formation in Cerro Panizos magma chamber. A. Cartoon of magma chamber showing schematic location of orb formation. Box marks location of diagrams B-E.

if the lava and ignimbrite orbs formed at different times, or if the orbs formed at the same time but were extruded by different eruptions. Water would have re-established a concentration gradient after the ignimbrite eruption, and, although total water contents were probably Figure 13. (Continued). B. Water is concentrated in a cupola along the chamber margin, lower following the vapor release associated superheating the magma in affected area and resorbing crystals. C. Eruption from central vent with the eruption of the Cerro Panizos Ignim- lowers magmatic pressure, causing water to exsolve from the magma as a separate phase. As brite, contents may locally have been high water bubbles leave magma, magma becomes supercooled, well below its liquidus but with few enough to repeat the orb-forming conditions. nuclei around which to crystallize. Orbs form around xenoliths and refractory crystals that The events described above could have oc- were not resorbed during the superheating episode. D. Heterogeneous nucleation begins in the curred in association with the main eruption at supercooled magma, forming many fine-grained crystals, whereas orb crystallization stops as Cerro Panizos. Water concentrated in the cu- the degree of supercooling decreases. Outermost orb ring records the end of orb formation. pola either over a long period of time due to E. Ring venting begins, erupting orbicular magma from the area around the conduit base. diffusion or quickly due to exsolution of water "Normal," coarsely porphyritic magma is erupted along with the orb/megacryst/finely por- following an early phase of the eruption. Re- phyritic pumice association. sorption of crystals occurs quickly even with very small degrees of superheating. Lofgren (1983) has shown that heating plagioclase crys- reasonable eruptive period if an average magma needed to explain the presence of orbs in the tals in basalts 10 °C above the liquidus causes discharge rate of 105-106 m3/s (Spera, 1984) is rocks at Cerro Panizos, and eruption is the most complete resorption in a period of hours. Exso- assumed for the >500-km3 Cerro Panizos likely cause. lution of water as a separate phase due to pres- Ignimbrite. sure release is nearly instantaneous. The resul- Several parts of the model presented above COMPARISON WITH OTHER tant cooling would rapidly create a supercooled are best explained if the orb formation process ORBICULAR ROCKS magma, and orb growth rates are high. Crystal occurred during eruption. The Cerro Panizos growth rates of the bronzite and quartz meg- ignimbrite-forming eruption did not result in There is only one published description of acrysts may have been moderately high. Plagio- caldera collapse until two-thirds to three-fourths another orbicular volcanic rock, a single orb in clase and biotite nucleation and subsequent of the material was erupted. The downsagging an andesite flow at Akagi volcano in Japan growth of quartz oikacrysts were also rapid, pos- that occurred in association with the major part (Koide, 1951). The host rock is a hypersthene sibly on the order of days. Brandéis and Jaupart of the eruption is much smaller than the erupted augite andesite with ~30%-35% crystals (esti- (1987) suggested that peak growth rates are in volume. Volatiles must have exsolved from the mated from photos in Koide, 1951) and plagio- the range of 10 7 cm/s and that peak crystal magma into a separate phase when the amount clase compositions of An62-8o- The orb consists 2 2 nucleation rates can be as high as 10~ to 10 of magma that was erupted exceeded the vol- of a core of metamorphosed country rock and 3 cm s '. Orb formation could have occurred ume of wall rock that replaced it by collapse. two rings with radially to non-oriented plagio- during an eruption that lasted a week or more, a Significant changes in magmatic pressure are clase (Ang¡¡ 94) and subordinate orthopyroxene,

Geological Society of America Bulletin, August 1992

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magnetite, and olivine (reacting to orthopyrox- rarity of occurrence of orbs in volcanic rocks Eskola, P., 1938, On the esboitic crystallization of orbicular rocks: Journal of Geology, v. 46, p. 448-485. ene). Although it is from a poorly known local- probably is due to several factors. First, most Fridrich, C. J., and Mahood, G. A., 1984, Reverse zoning in the resurgent intrusions of the Grizzly Peak cauldron, Sawatch Range, Colorado: ity, this orb appears to be similar in many volcanic rocks are rather poorly crystalline. Al- Geological Society of America Bulletin, v. 95, p. 779-787. respects to those of Cerro Panizos. The core is though little additional water would be needed Hildreth, E. W., 1977, The magma chamber of the Bishop Tuff: Gradients in temperature, pressure, and composition [Ph.D. thesis]: Berkeley, Cali- xenolithic, and the orb minerals are more mafic to cause superheating in such magmas, a tre- fornia, University of California, 328 p. 1981, Gradients in silicic magma chambers: Implications for litho- than the crystal-rich volcanic host rock. As with mendous amount of vapor loss would be re- spheric magmatism: Journal of Geophysical Research, v. 86, the lava orbs at Cerro Panizos, there is no sepa- quired to cause significant undercooling. In p. 10153-10192. Hildreth, W„ Christiansen, R. L, and O'Neill, J. R„ 1984, Catastrophic iso- rate pumice type with which to correlate the addition, even if a magma chamber does de- topic modification of rhyolitic magma at times of caldera subsidence, Yellowstone Plateau volcanic field: Journal of Geophysical Research, orb. velop orbs, they may not be erupted. Orbs prob- v. 89, p. 8339-8369. ably form in isolated pockets where sufficient Huebner, J. S., 1980, Pyroxene phase equilibria at low pressure, in Prewit, Despite some differences in detail, the general C. T., ed., Pyroxenes: Mineraiogical Society of America Reviews in similarities of orbicular volcanic rocks at Cerro water is available to cause superheating, and Mineralogy, v. 7, p. 213-288. Kennedy, G. C., 1955, Some aspects of the role of water in rock melts: Geologi- Panizos with orbicular granitoids elsewhere sug- eruption conduits may not tap that section of the cal Society of America Special Paper 62, p. 487-504. magma chamber. Finally, even if orbs form and Koide, H., 1951, An orbicular rock in andesite from Akagi volcano, Japan: gest that the basic processes of orb formation are Geological Survey of Japan Report 139, p. 1-15. similar. Elliston (1984) and Vernon (1985) con- are erupted, they are likely to be dispersed over Leveson, D. J., 1963, Orbicular rocks of the Lonesome Mountain area. Bear- tooth Mountains, Montana and Wyoming: Geological Society of Amer- cluded that orbs are generally found in hydrous a wide area in a large volume of erupted mate- ica Bulletin, v. 74, p. 1015-1040. rial. In contrast, orbs found in plutonic rocks are 1966, Orbicular rocks: A review: Geological Society of America Bul- parts of plutons, commonly near the pluton letin, v. 77, p. 409-426. margins. Moore and Lockwood (1973a and generally concentrated in relatively small areas. 1973, Origin of comb layering and orbicular structure, Sierra Nevada Batholith, California: Discussion: Geological Society of America Bul- 1973b) presented evidence that orbs and comb Magmas with high crystal contents, such as letin, v. 84, p. 4005-4006. Lofgren, G. E., 1983, Effect of heterogeneous nucleation on basaltic textures: A layering occur along the edges of plutons, espe- those at Cerro Panizos, are more likely to con- dynamic crystallization study: Journal of Petrology, v. 24, p. 229-255. cially along inward-sloping walls and cupolas Lofgren, G. E., and Donaldson, C. H., 1975, Curved branching crystals and tain orbs than are crystal-poor magmas. Crystal- differentiation in comb-layered rocks: Contributions to Mineralogy and where water is concentrated, but Brigham rich ignimbrites were erupted from magmas that Petrology, v. 49, p. 309-319. Mahood, G. A., 1981, A summary of the geology and petrology of the Sierra (1983) has shown that they do not crystallize had dropped well below their liquidus. The drop La Primavera, Jalisco, Mexico: Journal of Geophysical Research, v. 86, from a separate aqueous phase. p. 10137-10152. in total pressure needed to exsolve water and Minor, M. M„ Hensley, W. K, Denton, M. M„ and Garcia, S. R, 1981, An significantly undercool the magma is much automated activation analysis system: Radioanalylical Chemistry, v. 70, Orbs typically occur near the borders of plu- p. 459. ! tons. Published descriptions and mapped rela- smaller for crystal-rich than for crystal-poor Moore, J. G., 1981, Geologic map of the Mount Whitney quadrangle, Inyo and Tulare counties, California: U.S. Geological Survey Map GQ-1545. tions (Van Diver, 1970; Moore and Lockwood, magmas and thus is more likely to occur in na- Moore, J. G., and Lockwood, J. P., 1973a, Origin of comb layering and 1973a and 1973b; Moore, 1981; Moore and Sis- ture. Orbs and comb layering are far more likely orbicular structure. Sierra Nevada Batholith, California: Geological So- ciety of America Bulletin, v. 84, p. 1-20. son, 1987) indicate that orbs also form near con- to occur in crystal-rich rocks, such as plutons, 1973b, Origin of comb layering and orbicular structure, Sierra Nevada Batholith, California: Reply: Geological Society of America Bulletin, tacts between dark and light or coarsely and than in crystal-poor ignimbrites. v. 84, p. 4007-4009. finely porphyritic phases of the same pluton, as Moore, J. G., and Sisson, T. W„ 1987, Geologic map of the Triple Divide Peak quadrangle, Tulare county, California: U.S. Geological Survey Map well as in granitoids containing blocks of differ- ACKNOWLEDGMENTS GQ-1636. Naney, M. T., 1983, Phase equilibria of rock-forming ferromagnesian silicates ent compositions. The magmas in which these in granitic systems: American Journal of Science, v. 283, p. 993-1033. orbs formed may have been superheated by Ort, M. H., 1991, Eruptive dynamics and magmatic processes of Cerro Panizos, I thank D. Dellinger, R. V. Fisher, C. Hop- central Andes [Ph.D. thesis]: Santa Barbara, California, University of complex water fluctuations that occurred late in son, J. Moore, C. Oldenburg, and N. Riggs for California, 474 p. Philpotts, A. R., 1990, Principles of igneous and metamorphic petrology: En- emplacement (Moore and Sisson, 1987). Subse- helpful reviews of earlier versions of the manu- glewood Cliffs, New Jersey, Prentice-Hall, 498 p. quent undercooling could result from pressure Spera, F. J., 1984, Some numerical experiments on the withdrawal of magma script. G. Lofgren and T. Thompson provided from crustal reservoirs: Journal of Geophysical Research, v. 89, release associated with dike emplacement. perceptive formal reviews of this manuscript. p. 8222-8236. Swanson, S. E., 1977, Relation of nucleation and crystal-growth rate to the An important difference between the orbs at J. Mattinson graciously allowed me use of his development of granitic textures: American Mineralogist, v. 62, p. 966-978. Cerro Panizos and those in plutonic rocks is the isotope laboratory. Field assistance by P. Guer- Swanson, S. E., and Fenn, P. M., 1986, Quartz crystallization in igneous rocks: absence of hornblende in the volcanic orbs. The stein, A. Sanguinetti, R. Lencina, and A. Benve- American Mineralogist, v. 71, p. 331-342. Taylor, H. P., 1974, Oxygen and hydrogen isotope evidence for large-scale mineral assemblage of granitic orbs commonly, nutto is gratefully acknowledged. This project circulation and interaction between ground waters and igneous intrusions, with particular reference to the San Juan volcanic field, Colorado, although not always, contains hornblende. was supported by Institute for Geology and in Hofmann, A. W„ Giletti, B. J., Yoder, H. S„ Jr., and Yund, R. A., Naney (1983) has shown that hornblende forms Planetary Physics Grant 165 (to R. V. Fisher) eds., Geochemical transport and kinetics: Washington, D.C., Carnegie Institute of Washington, p. 299-324. in granodioritic magmas with more than 4 wt% and by Geological Society of America Graduate 1977, Water/rock interactions and the origin of H20 in granitic batho- liths: Journal of the Geological Society of London, v. 133, p. 509-558. water at 200 MPa. Matrices of some orbicular Research Grants in 1986,1987, and 1988. Taylor, H. P., and Forester, R. W., 1979, An oxygen and hydrogen isotope rocks have pegmatitic textures (Leveson, 1963, study of the Skaergaard Intrusion and its country rocks: A description of a 55-M.Y. old fossil hydrothermal system: Journal of Petrology, v. 20, 1966, 1973; Elliston, 1984), indicative of high REFERENCES CITED p. 355-419. Thompson, T. B., and Giles, D. L., 1974, Orbicular rocks of the Sandia Moun- water contents. Undercooling at Cerro Panizos Brandéis, G-, and Jaupart, C., 1987, Crystal sizes in intrusions of different tains, New Mexico: Geological Society of America Bulletin, v. 85, was a result of a dramatic drop in vapor pressure dimensions: Constraints on the cooling regime and the crystallization p. 911-916. kinetic», in Mysen, B. O., ed., Magmatic processes". Physicochemical 1980, Igneous origin of the orbicular rocks of the Sandia Mountains, related to eruption. Vapor pressure drops due to principles: Geochemical Society Special Publication 1, p. 307-318. New Mexico: Discussion: Geological Society of America, v. 91, part I, Brigham, R. H., 1983, A fluid dynamic appraisal of a model for the origin of p. 245-246. intrusive events would probably be smaller, and comb layering and orbicular structure: Journal of Geology, v. 91, Van Diver, B. B., 1970, Origin of biotite orbicules in "Bullseye Granite" of so the magma would have to be nearly water p. 720-724. Craftsbury, Vermont: American Journal of Science, v. 268, p. 322-340. de Silva, S. L„ 1989, Correlation of large ignimbrites—Two case studies from Vernon, R. H., 1985, Possible role of superheated magma in the formation of saturated and probably in the amphibole stabil- the central Andes of northern Chile: Journal of Volcanology and orbicular granitoids: Geology, v. 13, p. 843-845. Geothermal Research, v. 37, p. 133-149. Yoder, H. S„ Stewart, D. B., and Smith, J. R„ 1957, Ternary feldspars: Carne- ity field to exsolve a separate aqueous phase. Elliston, J. N., 1984, Orbicules: An indication of the crystallisation of hydrosili- gie Institution of Washington Yearbook, v. 55, p. 206-214. cales, I: Earth-Science Reviews, v. 20, p. 265-344. The rare occurrence of orbs in plutonic rocks Enz, R., Kudo, A. M., and Brookins, D. G., 1979, Igneous origin of the apparently reflects the scarcity of magmatic sit- orbicular rocks of the Sandia Mountains, New Mexico: Summary: Geo- logical Society of America Bulletin, v. 90, part I, p. 138-140. uations in which superheating is followed by 1980, Igneous origin of the orbicular rocks of the Sandia Mountains, MANUSCRIPT RECEIVED BY THE SOCIETY JULY 8, 1991 New Mexico: Reply: Geological Society of America Bulletin, v. 91, REVISED MANUSCRIPT RECEIVED DECEMBER 11, 1991 large degrees of undercooling. The even greater part I, p. 246-247. MANUSCRIPT ACCEPTED JANUARY 18, 1992

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Geological Society of America Bulletin, August 1992

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