Incremental pluton emplacement by magmatic crack-seal

Abstract

A growing body of evidence indicates that some, and perhaps most, plutons are highly composite. However, the geometrical forms of increments and the processes by which they are added are poorly known. Magmatic crack-seal probably is an important incremental assembly process, particularly in the upper crust where wall-rock fracture is important. Evidence for magmatic crack-seal is clearest where it is antitaxial, i.e., new fractures form at the contact between wall rock and a growing intrusion. Local deviation of antitaxial cracks into wall rocks isolates wall-rock bodies that therefore mark increment contacts. Wall rock isolated by this process remains in situ and thus is likely to preserve a ghost stratigraphy. Previously described plutons are identified, and interpreted to have grown by antitaxial magmatic crack-seal. In contrast, it remains unclear what observable geological record may remain in plutons formed by syntaxial crack-seal, in which new cracks form in the middle of the growing pluton. Several plutons are identified that preserve possible indirect evidence for growth by syntaxial crack seal, but conclusive identification of a direct record of the process remains elusive. However, plutons with sharp discordant contacts but few xenoliths may have been emplaced incrementally by syntaxial magmatic crack-seal.