STRUCTURES RELATED TO THE EMPLACEMENT OF SHALLOW-LEVEL INTRUSIONS: A TEMPORAL ORGANIZATION

Shallow-level intrusions (SLI) constitute the interface between the hidden kingdom of Pluto below and the fiery realm of Vulcan above. They are characterized first by generally having large aspect ratios with overall tabular forms (i.e. sills, dikes, laccoliths), and second by emplacement into the coolest portion of the crust. These conditions lead to the relatively short “lifetimes” of these bodies between onset of emplacement and complete solidification. Compared to more deep-seated plutons, SLI end up dominated by fabrics and structures directly associated with emplacement since their typical quick crystallization limits the time available to fractionate and develop features associated with in situ crystallization. Thus, well-preserved fabrics in SLI are helpful to better understand the early history of deeper-level plutonic complexes in which many textures are erased by prolonged internal movements and crystallization.

A systematic view of the vast nomenclature used to describe the structures of shallow-level intrusions can be made by organizing structures in four main groups, according to logical breaks in the timing of magma emplacement, independent of the scales of features: (1) Intrusion-related structures, formed as the magma is making space and then develops into its intrusion shape; (2) Magmatic flow-related structures, developed as magma moves with suspended crystals that are free to rotate, recognizing that only the final ‘snapshot’ of this process is preserved; (3) Solid-state, flow-related structures that formed in portions of the intrusions affected by continuing flow of nearby magma, therefore considered to have a syn-magmatic, non-tectonic origin; (4) Thermal and fragmental structures, related to creation of space and impact on host materials. This scheme appears as a rational organization, helpful in describing and interpreting the large variety of structures observed in shallow-level intrusions.