Ash-flow magmatism

Abstract

Calderas related to ash-flow sheets show a positive correlation between caldera area and ejecta volume; this correlation places constraints on magma drawdown during eruption and implies a systematic relationship between these parameters and magma volume of the chamber. Caldera areas range from 1 to 10 4km 2; the volume of ejecta from caldera sites ranges from 1 to 10 4km 3, and the volume of the related magma chambers is thought to range from 10 to 10 5km 3. A tentative correlation between ejecta volume and the time required to produce that volume reveals an approximate production rate of 10 −3km 3/yr. This seems to hold for relatively small eruptions, such as the ash-flow and related pyroclastic eruptions of as little as 10 −3km 3that are associated with central-vent volcanoes, even though volumes below about 1 to 10 km 3are not related to caldera formation. The correlation also holds for large eruptions up to the limits for cumulative ejecta volumes and composite batholith emplacements. Precaldera magma chambers probably all have physical and chemical gradients and measurable variations in chemistry and mineralogy. These variations are revealed in the pyroclastic deposits, if the eruption taps to the maximum eruptible level, and give insight into differentiation processes. Compositional contrasts are commonly greater in small-volume central-vent systems than in large-volume ring-fracture systems, but all systems tend to become more mafic with depth. Successive caldera-forming eruptions from the same system commonly become more mafic with time. This is possibly due to two effects: (1) decreasing thermal input and (2) progressive depletion in “residual” elements. However, new thermal inputs, probably in the form of mafic primitive magma from the mantle, take place intermittently throughout the volcanic life of the system and may extend, as a waning influence, into and beyond the crystallization stage of the pluton. Very strong resurgence of primitive magma may reactivate large-volume systems to begin new cycles of magmatic activity. In common stratovolcanic systems (Crater Lake, Oregon) that rarely fractionate to high-SiO 2rhyolite, alternating progression and regression of chemical trends with time is dominated by major-element variations. In high-SiO 2rhyolite magmas (Bandelier Tuff, New Mexico), minor elements may sometimes show striking alternating enrichments and depletions with successive eruptions from the most-fractionated boundary layer at the top of the magma chamber. In the Bandelier magma, many elements such as Nb, Ta, U, Th, Cs, Rb, Li, Sn, Be, B, W, Mo, F, CI, Pb, Zn, Sm, and the heavy rare-earth elements concentrated upward in the system, whereas other elements such as Ba, Sr, Eu, Ti, Cr, Co, Sc, Au, and Cu concentrated downward. For magmas in general, the direction and amount of concentration of both groups of elements are a function of both the initial composition of the parent magma and the effects of the dominant fractionation process operating at any given time. Thus the permissibility of mineralization by or ore formation involving those elements that are dependent on magmatic concentration may be directly related to specific stages in the volcanic history of any given system. Any ash-flow sheet holds basic clues for the behavior of certain elements in that system at a specific time and, seen in conjunction with the composition of magmas erupted from the system at other times, suggests concentration trends that provide insights into the controlling magmatic processes.