The development of spiral‐shaped inclusion trails during multiple metamorphism and folding

Abstract

Three periods of mineral growth and three generations of spiral‐shaped inclusion trails have been distinguished within folded rocks of the Qinling‐Dabie Orogen, China, using the development of three successive and differently trending sets of foliation intersection axes preserved in porphyroblasts (FIAs). This progression is revealed by the consistent relative sequence of changes in FIA trends from the core to rim of garnet porphyroblasts in samples with multiple FIAs. The first and second formed sets of FIAs trend oblique to the axial planes of macroscopic folds that dominate the outcrop pattern in this region. The porphyroblasts containing these FIAs grew prior to the development of the macroscopic folds, yet the FIAs do not change orientation across the fold hinges. The youngest formed FIAs (set 3) lie subparallel to the axial planes of these folds and the porphyroblasts containing these FIAs formed in part as the folds developed. The deformation associated with all three generations of spiral‐shaped inclusion trails in garnet porphyroblasts involved the formation of subhorizontal and subvertical foliations against porphyroblast rims accompanied by periods of garnet growth; pervasive structures have not necessarily formed in the matrix away from the porphyroblasts. The macroscopic folds are heterogeneously strained from limb to limb, doubly plunging and have moderately dipping axial planes. The consistent orientation of Set 1 FIAs indicates that the development of spiral‐shaped inclusion trails in porphyroblasts with FIAs belonging to Set 2 did not involve rotation of the previously formed porphyroblasts. The consistent orientation of Sets 1 and 2 FIAs indicate that the development of spiral‐shaped inclusion trails in porphyroblasts with FIAs belonging to Set 3 did not involve rotation of the previously formed porphyroblasts during folding. This requires a fold mechanism of progressive bulk inhomogeneous shortening and demonstrates that spiral‐shaped inclusion trails can form outside of shear zones.

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

“…In the case of non-coaxial deformation producing strong monoclinic shape symmetry in the rock, sections normal to the vorticity vector (rotation axis of porphyroblasts, if any) will give the maximum amount of information. Some workers, however, prefer serial sectioning at different angles to identify the foliation intersection axis (FIA) for the study of complex spiral inclusions in porphyroblasts (Bell and Chen 2002). In the present study we prepared thin sections perpendicular to the S 2 foliation and parallel to the mineral lineation (L 2 ) as those give us the best option of observing a section perpendicular to the possible rotation axis.…”

Extensional v. contractional origin for the southern Menderes shear zone, SW Turkey: tectonic and metamorphic implications

“…SSITs appear so obviously a product of rotation of the porphyroblast within a foliation as progressive shearing occurred that no one considered the possibility that this might not be the case until the 1980s. However, a technique for quantitatively measuring the axes of SSITs was developed (Hayward, 1990;Bell, Forde & Wang, 1995) and a large amount of data has now been published on the orientation of spiral axes around folds (Bell & Hickey, 1997;Hickey & Bell, 2001;Bell & Chen, 2002;Timms, 2004), around oroclines (Bell & Mares, 1999;Yeh & Bell, 2004) and along and across orogens (Bell & Bruce, 2006;Bell, Hickey & Upton, 1998;Bell, Ham & Hickey, 2003, Bell, Ham & Kim, 2004Bell et al 2005;Cihan, 2004;Cihan & Parsons, 2005). He also generated a non-coaxial strain field diagram that mimicked asymmetric crenulation cleavages and straight to slightly sigmoidal inclusion trails that are common in so many porphyroblasts.…”

Ninety million years of orogenesis, 250 million years of quiescence and further orogenesis with no change in PT: Significance for the role of deformation in porphyroblast growth

“…More than 10 years of research and data have already been published using this technique from tectonically complex regions around the world (e.g., Bell et al 1998Bell et al , 2003Bell et al , 2005Bell and Chen 2002;Cihan 2004;Kim and Bell 2005;Sayab 2005Sayab , 2006Bell and Bruce 2007;Sanislav 2010;Sanislav and Shah 2010;Ali 2010;Sanislav and Bell 2011).…”