A Systems Approach to Understanding a Basaltic Upper-Crustal Magmatic Complex

The 201-Ma Central Atlantic Magmatic Province is part of the basaltic Large Igneous Province formed during rifting of Pangaea. Upper-crustal magma systems such as the Morgantown-Jacksonwald complexes (MJC, western Newark basin, NJ-PA, USA) are investigated using a systems framework (Morin, 2008, On Complexity) that considers interactions of phase equilibria - kinetics and magmatism - seismicity within the active rift-to-drift tectonic setting. First we establish the interconnected intrusive architecture and geochemistry of the MJC. Fractionation trends are predicted using alpha-MELTS models. Whole-rock and mineral compositions of basalt and 19 chill margins quantify the variability of initial magmas and crystal cargos. Observed textures and mineral compositions result from dialogic interplay of thermodynamic processes that strive towards equilibrium by crystallization and release of latent heat, and irreversible heat loss that drives changes in phase equilibria but opposes the establishment of equilibrium. Thus, in rapidly-cooled samples minerals have heterogeneous non-equilibrium compositions. With somewhat slower cooling, rates of diffusion within crystals and melt and crystal growth reach a balance such that the predicted evolution of the liquid is tracked by mineral compositional zoning. With even slower cooling, crystallization should follow phase equilibria; however, mineral assemblages and zoning patterns in MJC intrusion interiors deviate significantly. Slower cooling allows the operation of other processes. A dimension stone quarry in a deep MJC sill reveals details of pyroxene-plagioclase modal layering and melt migration structures that are predicted by models of vibro-agitation of crystal-rich mushes by seismic activity (Davis, 2007, JVGR 167). Magmatism and seismicity are recursive: magma movements both result from and produce seismicity such as harmonic tremor and brittle-failure events. Seismic shocks lead to instantaneous underpressure at high strain rates that can enhance antecryst layering and promote melt segregation into dilatant structures. In turn, melt migration changes bulk composition and phase equilibria, and reactive transport of displaced interstitial liquid can produce localized assemblages different from predictions for trapped liquid.