Behaviour of rigid objects during deformation and metamorphism: a test using schists from the Bolton syncline, Connecticut, USA
Authors: HICKEY  , BELL  
Published: March 2003

The behaviour of spherical versus highly ellipsoidal rigid objects in folded rocks relative to one another or the Earth’s surface is of particular significance for metamorphic and structural geologists. Two common porphyroblastic minerals, garnet and staurolite, approximate spherical and highly ellipsoidal shapes respectively. The motion of both phases is analysed using the axes of inflexion or intersection of one or more foliations preserved as inclusion trails within them (we call these axes FIAs, for foliation inflexion/intersection axes). For staurolite, this motion can also be compared with the distribution of the long axes of the crystals.

Schists from the regionally shallowly plunging Bolton syncline commonly contain garnet and staurolite porphyroblasts, whose FIAs have been measured in the same sample. Garnet porphyroblasts pre‐date this fold as they have inclusion trails truncated by all matrix foliations that trend parallel to the strike of the axial plane. However, they have remarkably consistent FIA trends from limb to limb. The FIAs trend 175° and lie 25°NNW from the 020° strike of the axial trace of the Bolton syncline. The plunge of these FIAs was determined for six samples and all lie within 30° of the horizontal. Eleven of these samples also contain staurolite porphyroblasts, which grew before, during and after formation of the Bolton syncline as they contain inclusion trails continuous with matrix foliations that strike parallel to the axial trace of this fold. The staurolite FIAs have an average trend of 035°, 15°NE from the 020° strike of the axial plane of this fold. The total amount of inclusion trail curvature in staurolite porphyroblasts, about the axis of relative rotation between staurolite and the matrix (i.e. the FIA), is greater than the angular spread of garnet FIAs.

Although staurolite porphyroblasts have ellipsoidal shapes, their long axes exhibit no tendency to be preferentially aligned with respect to the main matrix foliation or to the trend of their FIA. This indicates that the axis of relative rotation, between porphyroblast and matrix (the FIA), was not parallel to the long axis of the crystals. It also suggests that the porphyroblasts were not preferentially rotated towards a single stretch direction during progressive deformation.

Five overprinting crenulation cleavages are preserved in the matrix of rocks from the Bolton syncline and many of these result from deformation events that post‐date development of this fold. Staurolite porphyroblast growth occurred during the development of all of these deformations, most of which produced foliations. Staurolite has overgrown, and preserved as helicitic inclusions, crenulated and crenulation cleavages; i.e. some inclusion trail curvature pre‐dates porphyroblast growth. The deformations accompanying staurolite growth involved reversals in shear sense and changing kinematic reference frames.

These relationships cannot all be explained by current models of rotation of either, or both, the garnet and staurolite porphyroblasts. In contrast, we suggest that the relationships are consistent with models of deformation paths that involve non‐rotation of porphyroblasts relative to some external reference frame. Further, we suggest there is no difference in the behaviour of spherical or ellipsoidal rigid objects during ductile deformation, and that neither garnet nor staurolite have rotated in schists from the Bolton syncline during the multiple deformation events that include and post‐date the development of this fold.