Porphyroclast systems as kinematic indicators
Porphyroclasts of feldspar and other relatively rigid minerals in mylonites commonly have mantles of dynamically recrystallized material that extend as tails into the matrix. The internal shape symmetry of such porphyroclasts is usually orthorhombic or monoclinic; the orientation of the porphyroclast with respect to the foliation (external symmetry) can also be described by these symmetry classes. An identical monoclinic external symmetry of most porphyroclasts in a given sample indicates non-coaxial flow in the matrix during at least the last stages of deformation. Two classes of porphyroclast systems with monoclinic symmetry have been identified on geometrical grounds. sigma-type porphyroclasts are characterized by wedge-shaped tails of recrystallized material. Median lines of the tails lie on opposite sides of, and do not cross, a marker line drawn parallel to the mean foliation. sigma-type porphyroclasts may lie isolated in a homogeneous matrix (sigma/sub a./-type) or may be in clusters associated with shear bands or S-C mylonites (sigma/sub b/-type). delta-type porphyroclasts commonly occur in ultramylonites and have highly attenuated recrystallized tails. Median lines of the tails cross the marker line adjacent to the porphyroclast which results in an embayment of matrix material adjacent to the host grain. More complex porphyroclast systems include ellipsoidal overturned delta-types, complex sigma - delta types and folded porphyroclast aggregates. In all cases, the symmetry of porphyroclast aggregates with respect to the foliation can be used to accurately determine the sense of vorticity in the mylonites.
- Research Organization:
- Virginia Polytechnic Institute and State Univ., Blacksburg (USA)
- OSTI ID:
- 6070284
- Report Number(s):
- CONF-8510489-
- Journal Information:
- Geol. Soc. Am., Abstr. Programs; (United States), Journal Name: Geol. Soc. Am., Abstr. Programs; (United States) Vol. 17; ISSN GAAPB
- Country of Publication:
- United States
- Language:
- English
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