grain-tail complexes (inclusions; porphyroclasts; porphyroblasts)

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1 grain-tail complexes (inclusions; porphyroclasts; porphyroblasts)
grains in matrix may have tails that form during deformation …tails are distinguishable from matrix …grains may be …inclusions …porphyroclasts (relics from protolith) …porphyroblasts (grow during deformation) grain tail …tails may be.. ..attenuated, preexisting minerals ..dynamic recrystallization at grain rim ..synkinematic metamorphic reactions grains are rigid bodies that rotate during deformation …tails give sense of displacement… to use grain-tail complexes to indicate shear-sense, need reference frame…relative to shear zone foliation.. …two “winged” types of tails: s-type and -type

2 two asymmetric types: s-type and -type
wedge-shaped tails that do not cross reference plane when tracing tail away from grain; looks like s s right-lateral tails wrap around grain so they cross-cut reference plane when tracing tail away from grain; looks like   right-lateral from van der Pluijm and Marshak, 1997 reference plane is shear zone foliation right-lateral (dextral) shear: clockwise rotation left-lateral (sinistral) shear: counter-clockwise rotation

3 Sense of Shear Indicators

4 relationship between rate of crystallization and rotation of grain
two-types related: relationship between rate of crystallization and rotation of grain …formation fast relative to rotation: s-type …rotation fast relative to formation: d-type (tail dragged and wrapped around grain) presence of both types indicates different: • rates of tail growth • initial grain shape • times of tail formation development of s from d from van der Pluijm and Marshak, 1997

5 Shear Sense Indicators

6 other minerals, such as phyllosilicates, display useful geometry
phyllosilicate grains (micas) connected by mylonitic foliation …basal planes oriented at oblique angle to foliation… …point in direction of instantaneous elongation grains have stair-step geometry in direction of shear when large enough to see in hand specimen, …look like scales on a fish (“mica fish”) from van der Pluijm and Marshak, 1997 from: Simpson, Microstructures CD-ROM

7 can determine asymmetry of mica fish by
observing reflections in sunlight… …fish flash… • mark north arrow on sample • put back to sun and sample in front of you • view parallel to lineation • tilt sample • note if flashy or dull from: Davis and Reynolds, 1996

8 …veins commonly associated with shear zones…
…form perpendicular to instantaneous extension… …initially form at 45° to shear zone… …subsequently rotate to steeper angle while new part of vein forms at 45° from: from:

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10 Evidence for Rotation during non-coaxial deformation
Garnet in Qtz-mica schist

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