RECONCILING INCREMENTAL EMPLACEMENT AND WIDESPREAD DEFORMATION WITH THE APPARENT HOMOGENEITY AND ISOTROPY OF PLUTONIC ROCKS

A persistent criticism of the incremental-emplacement hypothesis for plutons is the apparent lack of internal contacts between the required intrusive increments even though an abundant body of geochronologic data indicates that many plutons were emplaced over time spans much longer than the time required to solidify even huge magma bodies. Such data require the plutons to have grown at least in part in increments rather than to reflect the ascent of giant magma blobs, but the lithologic homogeneity and isotropy of many plutons appears to contradict this. If plutons grow in myriad small increments, where are the internal contacts?

Here we argue that the paucity of visible internal contacts is largely a product of post-emplacement textural modification, and that such modification can erase evidence for widespread brittle and ductile deformation. Crack-seal appears to be an important intrusive process over multiple length scales. However, the textural record of cyclic crack opening and magma injection can be preserved at the 10 cm scale in composite dikes, but lost at the km scale in plutons owing to pervasive recrystallization. Evidence for textural modification includes: (1) absence of sharp internal contacts in plutons in which sheeting and in-situ enclosure of wall-rock screens indicate growth by amalgamation of dikes; (2) mineral compositions that have reequilibrated at low, nominally subsolidus temperatures; (3) growth of large, low-temperature K-feldspar crystals across brittle fractures; (4) clear evidence for high-strain brittle and ductile shear zones that cut markers within plutons but are unrecognizable in the host pluton; and (5) folding of lithologic markers in otherwise isotropic plutons. Mapping of these latter structures indicate significant bulk deformation of classic “undeformed” plutons such as the Half Dome Granodiorite of Yosemite National Park.

We propose that the map view of a pluton does not provide reliable information about the size, number, dimensions, or shapes of the magma bodies from which the pluton grew. A major question in plutonic geology thus is how to see through textural patterns that have guided so much thought but mainly reflect post-emplacement processes. Only when this is accomplished will it be possible to discern the actual processes by which a pluton was constructed.