Orbicular volcanic rocks of Cerro Panizos: Their origin and implications for orb formation
Orbs, which are a rare occurrence in granitoid rocks, are present in a 5-m-thick stratum within the Cerro Panizos Ignimbrite and in two post-ignimbrite lava flows. Orbs in volcanic host rocks are extremely rare and have not been described in detail previously. The orbs consist of two to five crystalline rings surrounding a xenolithic or orthopyroxene core. The rings alternate between bands of large, radially oriented plagioclase and orthopyroxene crystals and bands of small, tangentially or radially oriented biotite and ilmenite crystals. The ignimbrite orbs are associated with two types of pumice: (1) a coarsely porphyritic biotite-quartz-plagioclase dacite with 35%-40% crystals found throughout the ignimbrite and (2) a finely porphyritic biotite-plagioclase quartz dacite 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 overlying 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 (An81-92 and En60-77), coarsely porphyritic pumice and lavas have the most felsic compositions (mostly An45-63 and En42-50), and the outer rings of the orbs span the entire gap between the two groups (An67-77 and En50-66).

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 nuclei. As limited mixing with the surrounding coarsely porphyritic magma occurred, heterogeneous nucleation in the supercooled magma began, forming the abundant small crystals seen in the finely porphyritic pumice. Eruption of the orbicular dacite occurred when a ring vent conduit tapped the magma in the cupola. Similar processes may form orbs in plutonic rocks, with pressure release related to either eruption or intrusion to higher levels in the crust.