PLUTON-HOST ROCK COUPLED SYSTEMS: A DOG'S BREAKFAST OF PROCESSES AND THE REBIRTH OF THE SPACE PROBLEM.... THANKS, CAL! (Invited Presentation)

Cal Barnes’ contribution to the study of magmatism has impacted our understanding of melt formation/melt segregation in migmatites, magma transfer mechanisms in the middle and upper crust, AFC mechanisms at a variety of depths and scales, and recently the significant role that crystal accumulation plays in the evolution of plutonic systems. A natural outgrowth of Cal’s work, and one that has impacted my research, is the nature of the pluton-host rock coupled system and a return to the “space problem”. A number of pluton-host rock systems Cal and I and others have studied demonstrate a sequential spatial and temporal variation in host rock rheology during magma emplacement and internal magmatic evolution. Thermal and structural contact aureoles are narrow, ~0.2 body radii of a given pluton. ‘Walls’ of these plutons are characterized by ductily-deformed host rocks that show gradients of increasing strain approaching pluton margins. However, in all cases, these host rock structures are locally found to be truncated at a sharp intrusive contact. In contrast, the roofs of these plutons (when preserved) display no evidence of dynamothermal metamorphism and are often discordant (shallow laccoliths, aside). This would imply that the mappable shape of the plutons was established by brittle deformation at the roof, early ductile deformation along the walls with continued intrusion and discordant contact formation resulting in the above cross-cutting relationship. Within this complexly forming cauldron, magmatic processes yield the compositional diversity of the mapped pluton. The age-old question remains, where did the host rock material go? An interesting implication is that periodic fluctuations in host rock strain rate (ductile to brittle) may occur throughout the evolution of the growing pluton. Such rapid strain rates have been suggested to be evidence for construction of dike-like pluton shapes that are volumetrically small. However, it is unlikely that a simple dike model would produce the plastic strain gradients observed around these plutons. The inability to restore maps or cross sections of plutons remains problematic. Thus, it seems that the plutonic "space problem" remains an enigmatic question to ponder.