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

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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 associa- tion 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 ig- nimbrite 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. 1048 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/104/8/1048/3381512/i0016-7606-104-8-1048.pdf by guest on 29 September 2021 ORBICULAR VOLCANIC ROCKS, CERRO PANIZOS 1049 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 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/104/8/1048/3381512/i0016-7606-104-8-1048.pdf by guest on 29 September 2021 1050 ORT 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.
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