Columnar jointing in non-welded Cerro Galan ignimbrite: Paycuqui, Argentina

Well-developed columnar joint sets are thought to occur most commonly in moderately to densely welded facies of ignimbrites. Columnar joints in welded deposits form when sintering between juvenile grains provides sufficient cohesion to allow thermal volume contraction to be accommodated in brittle fracture. However, well- developed columnar joints can also form in unwelded deposits, where cohesion is produced by high temperature devitrification of the deposit. Such is the case at Paycuqui, Argentina, in the Cerro Galan ignimbrite. The Cerro Galan ignimbrite is a very large volume rhyodacitic pyroclastic flow deposit (>1000 km3), which erupted ~2.3 Ma. At the Paycuqui locality, 29 km west of the caldera margin, the flow is valley-confined and is over 30 m thick. The deposit is massive, pumice-poor, lithic-poor, crystal-rich, and contains well-developed joint sets in the upper 15m. Remant magnetization analyses of incorporated lithics indicate high temperature emplacement. However, welding deformation and sintering textures are absent. Instead, textural differences between the nonjointed base and the jointed top of the ignimbrite are coincident with devitrification variations, inluding dominant cryptocrystalline sanidine and crystobalite that increase in abundance upwards in the section. The presence of a 1mm wide, more devitrified zone along columnar joint faces constrains the timing of high temperature devitrification. Devitrification must have begun prior to brittle fracture and continued after joints had formed. The morphology of joints at Paycuqui differs from well-described joint sets elsewhere. The average diameter of polygons is 0.75 m. Intersecting columnar joint surfaces at Paycuqui form polygons with fewer sides (mean of 4.5 sides) than on many lava flows or than predicted in models. These polygonal patterns vary within the flow due to the presence of vertical, rose-shaped, and radiating joints. Such complexities may result from fumarolic vapor rise.