MAGMA CHAMBERS AS DEPOSITIONAL SYSTEMS

Field evidence and consideration of material transport phenomena suggest that magma chambers are depositional systems. Felsic magmas, commonly viewed as sluggish, have viscosities low enough to permit effective transport. Both growth of crystals and the recharging required to form large intrusions induce gravitational instability; downward transport and deposition of denser materials (individual crystals, crystal-rich slurries, solidified blocks, denser magmas) and upward transport of lighter magma are expected. Magma chambers - mafic, felsic, or mixed - may thus solidify from the bottom up and have recognizable stratigraphy.

Processes of material redistribution and stratification have surface analogs - e.g., crystal settling (sedimentation) and compaction and melt expulsion (consolidation). Other processes include mush, solid slope, and dense layer failures (submarine density flows, rockslides, sinkhole formation?) and building of base-of-chamber edifices from dense magma replenishment (volcano construction). Depositional processes are inescapable in the magmatic environment, but the processes and resultant structures are distinct in detail from surface equivalents because of high viscosities, potential for interaction of contrasting magmas, and effects of varying temperature. Rarer surface analogs involving interactions of magma with rock, water, and ice (e.g. jokulhlaups) may be instructive.

Tilted plutons in southern Nevada reveal stratified sections up to 10 km thick with evidence for repeated recharging by both mafic and felsic magmas. Features such as load casts, flame structures, matrix-supported "conglomerates," and asymmetrically quenched, lava-like mafic sheets that built sizable edifices document their analogy to sedimentary-volcanic sequences. Thick sequences of cumulate-textured granite record episodes of accumulation and compaction of mushes. Lateral variability in the depositional sequences can be interpreted in terms of facies models.

Studies of the chronology and depositional features of these plutons, computational models of properties of materials in active chambers, and analog experiments will evaluate depositional processes by which the magma chambers were modified and the plutons formed. These studies will also permit testing of general applicability of magma chamber depositional models.