LACK OF
PORPHYROBLAST ROTATION
Deformed metamorphic rocks show evidence for the
partitioning of deformation into narrow shear zones anastomosing around low-strain
lenses at different scales. The crenulation-cleavage planes in the image below
(from Bell & Johnson 1992) are microscopic shear zones enriched in micas
due to preferential dissolution of quartz. The crenulated schistosity was
overgrown by staurolite porphyroblasts probably early during the crenulation
event. Highly consistent orientations of the foliation inside the porphyroblasts
indicate that these crystals did not significantly rotate during deformation due
to a style of deformation partitioning as sketched.
FOLIATION REACTIVATION
The deformation-partitioning model shown above applies readily
to rocks exhibiting a pervasive spaced crenulation cleavage. But what about porphyroblasts
that are surrounded by a single matrix foliation and still show consistently
oriented inclusion trails as shown in the example below (Aerden 1995). In such cases,
careful study commonly reveals evidence for a younger (superposed) deformation
event that reactivated and reoriented the pre-exisrting foliation rather than
forming new crenulation-cleavage planes. This depends on the orientation of the
pre-existing foliation and the bulk vorticity of superposed deformation. Fortunately,
new cleavage planes will still develop locally in low-strain lenses and as
individual shear bands (strain caps) against the margins of rigid porphyroblasts
and porphyriclasts. The latter are key to the recognition and interpretation of
poly-phase deformation histories of rocks with simple fabrics.
INTERPRETATION
OF MORE COMPLEX INCLUSION-TRAIL
PATTERNS
ANIMATION prepared by Dr Ioan Sanislav (James
Cook University, Australia) after the numerical model of Fay et al (2008)
Fay, C., Bell, T. H., and Hobbs, B. E., 2008,
Porphyroblast rotation versus nonrotation: Conflict 296 resolution!: Geology, v. 36, p.
307-310.
Watch this peculiar porphyroblast (from Aerden, 2004)
