SOLIDIFICATION, ZONING, AND HOMOGENIZATION IN SIERRAN PLUTONS

Geophysical images of melt-poor mid-ocean ridge magma bodies, combined with recent age measurements documenting protracted (>10⁶yr) assembly of the Tuolumne Intrusive Suite (Coleman, Glazner, Bartley), challenge the interpretation that plutons solidify from melt-rich magma reservoirs of dimensions similar to the resulting intrusion. Gradual assembly from multiple smaller additions of magma, and existence of much of the intrusion as near-solidus crystal-rich mush, or as hot, but sub-solidus rock are possibilities. And yet, how to explain the textural, mineralogical, and compositional uniformity, systematic internal zoning, and inconspicuous internal contacts that characterize so many plutons? Either deep melting and differentiation are capable of long supplying magmas of limited compositional variety, that solidify with characteristic textures, and that organize in spatially reproducible ways, or the intrusions are capable of widespread mixing and homogenization at the level of emplacement through much of their assembly. Homogenization requires mobility, and thus either maintenance or creation of melt fractions sufficient for magma flow. Shallow underplating by mafic intrusive complexes (Onion Valley, Aberdeen, Rockslides, Kaweah Gap) could supply heat and maintain or create melt fractions, but underplating events can be expected to leave chemical fingerprints absent or unrecognized in many plutons. An alternative or complementary process may be the periodic-to-continuous percolation of high-temperature magmatic gas emanating from the roots of the plutonic system. Recent calculations (Bachmann & Bergantz) show that gas percolation events are capable of remobilizing largely solidified mush. If so, then steady gas percolation at lower rates can be expected to slow the cooling of intrusions and may maintain melt fractions over time periods sufficient for internal homogenization - without leaving obvious chemical fingerprints. Plutons could be mushy, bubbly, and capable of mixing over periods much longer than conductive cooling calculations would indicate, while their common mafic-to-felsic zoning probably results from temporally waning temperatures, hence increased degrees of evolution, of the average magmas supplied to the intrusion.