Microstructural analysis of the classical spiral garnet porphyroblasts of south‐east Vermont: evidence for non‐rotation

Abstract

New data strongly suggest that the classical spiral garnet porphyroblasts of south-east Vermont, USA, generally did not rotate, relative to geographical coordinates, throughout several stages of non-coaxial ductile deformation. The continuity of inclusion trails (S,) in these porphyroblasts is commonly disrupted by planar to weakly arcuate discontinuities, consisting of truncations and differentiation zones where quartz-graphite S, bend sharply into more graphitic S,. Discontinuous, tight microfold hinges with relatively straight axial planes are also present. These microstructures form part of a complete morphological gradation between near-orthogonally arranged, discontinuous inclusion segments and smoothly curving, continuous S, spirals. Some 2700 pitch measurements of well-developed inclusion discontinuities and discontinuous microfold axial planes were taken from several hundred vertically orientated thin sections of various strike, from specimens collected at 28 different locations around the Chester and Athens domes. The results indicate that the discontinuities have predominantly subvertical and subhorizontal orientations, irrespective of variations in the external foliation attitude, macrostructural geometry and apparent porphyroblast-matrix rotation angles. Combined with evidence for textural zoning, this supports the recent hypothesis that porphyroblasts grow incrementally during successive cycles of subvertical and subhorizontal crenulation cleavage development. Less common inclined discontinuities are interpreted as resulting from deflection of anastomosing matrix foliations around obliquely orientated crystal faces prior to inclusion. Most of the idioblastic garnet porphyroblasts have a preferred crystallographic orientation. Dimensionally elongate idioblasts also have a preferred shape orientation, with long axes orientated normal to the mica folia, within which epitaxial nucleation occurred.Truncations and differentiation zones result from the formation of differentiated crenulation cleavage seams against porphyroblast margins, in association with progressive and selective strain-induced dissolution of matrix minerals and locally also the porphyroblast margin. Non-rotation of porphyroblasts, relative to geographical coordinates, suggests that deformation at the microscale is heterogeneous and discontinuous in the presence of undeformed, relatively large and rigid heterogeneities, which cause the progressive shearing (rotational) component of deformation to partition around them. The spiral garnet porphyroblasts therefore preserve the most complete record of the complex, polyphase tectonic and metamorphic history experienced in this area, most of which was destroyed in the matrix by progressive foliation rotation and reactivation, together with recrystallization.

Sigmoidal inclusion trails, punctuated fabric development, and interactions between metamorphism and deformation

“…Complex trails involve more than a single antiform/synform pair or have trails that curve through more than 90" within the porphyroblast. These include the spectacular helical or 'snowball' geometries observed in garnet porphyroblasts (Rosenfeld, 1970;Schoneveld, 1977;Hayward, 1992). Although the data presented in this study may have implications for the generation of complex trails, none has been recognized in the two terranes studied here, and the remaining discussion is entirely restricted to simple 'sigmoidal' trails.…”

Porphyroblast inclusion‐trail orientation data: eppure non son girate*!

“…Indeed, strain shadows adjacent to porphyroblasts can be totally destroyed during reactivation of matrix foliations (Bell, 1986), foliation overprinting (fig. 14, Johnson, 1990a;Hayward, 1992), or total destruction of earlier foliations (Bell & Forde, 1992a), and the data indicate that the porphyroblasts have not rotated (Figs 6 & 8). Fig.…”

“…During gravitational collapse, near-horizontal foliations initially develop against rigid objects, but only in that portion of the orogen that is affected by gravitational collapse (figs 25d & 26b in Bell & Johnson, 1989); the transition from predominantly spiral to staircase-shaped trails appears to occur around 4.5-5 kbar within orogen cores, suggesting that horizontal foliations can develop in these regions to at least twice this depth (Bell & Johnson, 1992). Successive periods of porphyroblast growth preserve these predominantly horizontally and vertically orientated truncational foliations as inclusion trails over extremely wide areas that have been deformed subsequently many times (Hayward, 1992;Fig. 6).…”

“…However, matrix foliations show a large range of orientations due to the effect of folding or reactivation of pre-existing foliations (Bell, 1986;figs 7 & 8 in Bell & Hayward, 1991;fig. 1 in Bell & Forde, 1992a;Bell & Johnson, 1992;Hayward, 1992;Johnson, 1992).…”

“…6. Rose diagrams of pitch of truncations or discontinuity surfaces in garnet porphyroblasts from orientated rocks collected around the Chester Dome in Vermont, USA (from Hayward, 1991Hayward, , 1992. For many of these rocks, there are no strain shadows preserved on the margins of porphyroblasts.…”