Interaction between deformation and magma extraction in migmatites: Examples from Kangaroo Island, South Australia

Abstract

Migmatite terranes are structurally complex because of strong rheological contrast between layers with different melt contents and because of magma migration leading to irregular volume changes. Migmatite deformation is intimately linked with magma extraction and the origin of granitoids. We investigate here the relationships between evolving deformation and magma extraction in migmatites formed during the ca. 500 Ma Delamerian orogeny, exposed on Kangaroo Island, South Australia. Here, several phases of deformation occurred in the presence of melt. During an early upright, noncylindrical folding event, magma was channeled toward the hinge zones of antiforms. Funnel-shaped networks of leucosomes formed a root zone that linked up toward a central axial planar channel, forming the main magma extraction paths during folding. Extraction was associated with fold limb collapse, and antiformal hinge disruption by magma accumulation and transfer. During a later deformation phase, melt-rich diatexites were deformed, and schollen, blocks or rafts of source rock, were disaggregated into smaller blocks and schlieren, and these were deformed into asymmetric, sigmoidal shapes indicative of dextral shearing flow. During flow, magma accumulated preferentially along shear planes, indicating a dilatational component during shearing (transtension) and in strain shadows of schollen. At the waning stages of deformation, magma extraction from these diatexites gave rise to N-trending, steeply dipping, funnel-shaped channels not associated with any externally imposed deformational feature. The funnel shape of these structures indicates the direction of magma flow. Structures developed during this phase are comparable with those formed during dewatering of soft sediments. Despite a high degree of complexity, magma migration and extraction features record distinct responses to the evolving deformation that can be used to understand deformation, and the nature and direction of magma extraction. The oldest and youngest magmatic rocks from migmatites were dated by U-Pb SHRIMP (sensitive high-resolution ion microprobe) dating techniques. Both reveal a continuous age spread between ca. 495 and 465 Ma. The age range is interpreted to indicate the duration of anatexis (order of 30 m.y.), with the older ages marking onset of monazite growth as the system approached peak metamorphism and younger ages representing final growth close to the solidus during cooling.