FLUID-MECHANICAL INSIGHTS ON GRAVITY-DRIVEN INTRUSION OF MAFIC MAGMA INTO A FELSIC HOST: DEVELOPMENT OF MAFIC-FELSIC LAYERING PUNCTUATED BY FELSIC “PIPES”

Alternating mafic and felsic layers are relatively common features in plutons (e.g. Wiebe, J. Geol., 102, 423-437, J. Petrol., 34, 461-89; Patrick and Miller, Proc. 30th IGC, 30, 121-135). Typically the mafic layers are decimeters to meters thick; felsic layers are generally thinner but are highly variable (millimeters to decimeters, occasionally several meters). This stratigraphy likely forms by repeated intrusions of mafic magma into a mostly felsic host chamber, wherein the mafic layers spread laterally as gravity-driven flows that override and “trap” felsic layers (granitic melt plus crystal mush) beneath them. Felsic “pipes” (Wiebe, J. Petrol., 34, 461-89) with diameters on the order of a decimeter locally protrude upward though the mafic layers, roughly perpendicular to them. Following the work of Snyder and Tait (J. Fluid Mech., 369, 1-21), our scaled laboratory experiments of this phenomenon, together with simple fluid mechanical theory, reveal how a mafic intrusion overrides a felsic sublayer due to viscous planing. During intrusion sublayer fluid is not so much “squeezed” out by the overlying denser fluid as it is “dragged” out by the motion of this overlying fluid. But a mismatch in the timescales of emplacement of mafic fluid versus extraction of felsic fluid leaves a thin felsic sublayer. At rest, the dense mafic layer over the less dense felsic layer is mechanically unstable. Inasmuch as the mafic-felsic fluid interface remains mechanically weak (mushy rather than rigid), possibly ensured by thawing of this interface associated with advection of heat during intrusion of the mafic layer, the natural onset of a Rayleigh-Taylor instability leads to the growth of interface waveforms that develop into diapiric pipes which, in turn, intrude upward through the mafic layer. The fastest-growing wavelengths predicted by linear stability theory match the wavelengths of fluid-interface waveforms in our experiments (~ cm scale), and are consistent with the spacing of felsic pipes observed in the field (~ m scale). Our experiments moreover suggest that these pipes transport sublayer (felsic) fluid upward through the mafic layer, eventually draining the melt-rich part of the sublayer, consistent with field observations. These linked processes may be important in incremental growth of magma chambers, hybridization of magmas, and triggering of eruptions.