Behaviour of rigid objects during deformation and metamorphism: a test using schists from the Bolton syncline, Connecticut, USA

Abstract

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 con...

Prolonged Acadian orogenesis: Revelations from foliation intersection axis (FIA) controlled monazite dating of foliations in porphyroblasts and matrix

“…This is strongly supported by the succession of ages of monazite grains from the core to rim of porphyroblasts in sample V634A (figs. 5 and 6; for example, Adshead- Bell and Hickey, 1999;Hickey and Bell, 1999). Figure 14 and table 2 show that the forty-seven individual monazite grains within the four samples dated range in age from 432 to 309 Ma.…”

Polyphase deformation and garnet growth in pelitic schists of Sausar Group in Ramtek area, Maharashtra, India: A study of porphyroblast-matrix relationship

“…However, there has been strong disagreement and animated debate over the geological significance of strongly curved S i trails in porphyroblasts, especially the so-called 'snow-ball' inclusion patterns. One group of workers (Bell 1985;Bell and Johnson 1989;Bell et al 1992aBell et al , 1992bHickey and Bell 1999) have strongly argued that porphyroblasts do not rotate with respect to an external frame of reference and that even 'snow-ball' type spiral inclusion trails can be reinterpreted as a product of repeated overprinting deformation and transposition of the matrix foliation around a growing and non-rotating porphyroblast. The other group supports the existing and 'traditional' view of rotation of porphyroblasts in non-coaxial flow to explain strongly folded and rotated inclusion trails in porphyroblasts (Schoneveld 1979;Vernon 1988;Busa and Grey 1992;Visser and Mancktelow 1992;Passchier et al 1992;Williams and Jiang 1999;Jiang and Williams 2004).…”

Fan‐shaped staurolite in the Bolton syncline, eastern Connecticut: evidence for porphyroblast rotation during growth

“…On the contrary, Bell et al. (1997) and Hickey & Bell (1999) have cited the same inclusion trails as evidence that during growth, these porphyroblasts were fixed relative to a geographic reference frame and that their sigmoidal form was due to syn‐growth changes in the orientation of the external foliation. In addition to the common prismatic habit, staurolite in the Bolton Notch area of Connecticut (Box Mountain) also occurs as fan‐shaped aggregates of radiating, centimetre‐sized prismatic crystals.…”