INCREMENTAL PLUTON EMPLACEMENT AND EVOLUTION OF TRANSCRUSTAL SYSTEMS; USING THE PLUTON-HOST ROCK SYSTEM TO RECONSTRUCT PAST INTRUSIVE COMPLEX FOOTPRINTS

Plutons are fossilized sections of incrementally grown transcrustal systems, implying incremental changes in intrusive geometries and host rock removal. We explore how intrusive footprints change during growth by meshing transcrustal models with pluton emplacement models. Footprint is defined as the map pattern of an intrusive complex at a given time during growth and includes the lithologic and structural relationships in the plutonic and host units at that time slice. A well reconstructed footprint history can reflect how the map pattern, internal contacts, and aureoles changed during growth. Plutons are largely a blend of four end-member geometries: horizontally and vertically sheeted bodies, nested elliptical bodies, and migrated elliptical bodies.

We focus on plutons with mostly nested and migrated elliptical pulses. Footprint histories span two scenarios: 1) The current area of a map unit is the original amount emplaced; 2) Units are truncated and originally had larger areas. Pulse location and geometry reveal diversity in growth and preservation histories and indicate the possibility that large units were initiated as isolated smaller bodies that merged later, contrary to some past ideas.

Comparisons of nested and migrated bodies show how footprints can constrain host rock histories removed by forming a framework of cross-cutting relationships that can buttress other data, such as ages. A migrated chain of slightly overlapping pulses will have aureole domains linked to either one or two pulses. Conversely, aureoles of nested bodies are likely compound overprinting structures, suggesting that host rock is usually removed early.

Our data suggest that footprints are mostly established early in growth, implying pervasive magmatic recycling thereafter. The geometries of footprints form a framework for testing growth models by accounting for the coupling of the pluton-host rock system. Footprint-calibrated models help to address issues central to transcrustal systems like magma addition, host removal, crustal thickening rates, as well as the geometries of past magma chambers.