Spiral-shaped Inclusion Trails Research Articles

Tectonic evolution of the Karakoram metamorphic complex (NW Himalayas) reflected in the 3D structures of spiral garnets: Insights from X-ray computed micro-tomography

Spiral garnet porphyroblasts are known to record lengthy periods of deformation and metamorphism by preserving single or multiple FIAs (Foliation Intersection Axis) formed normal to tectonic shortening directions. Thanks to technological advances in X-ray computed micro-tomography (XCMT), FIAs can now be readily determined in relatively large samples in contrast to previous methods that require the preparation of a set of radial vertical and horizontal thin sections of samples. XCMT scanning not only alleviates tedious thin section based procedures but also illuminates the complete internal architecture of a rock sample allowing three-dimensional (3D) quantitative shape analysis of an individual porphyroblast as well as precise measurement of FIAs. We applied the technique to a sample from the Hunza Valley in the Karakoram metamorphic complex (KMC), NW Himalayas, containing numerous garnet porphyroblasts with spiral-shaped inclusion trails. The XCMT imaging reveals an E–W trending FIA within the sample, which is consistent with orthogonal N–S collision of the India-Kohistan Island Arc with Asia. Garnet long axes (XGT) have variable plunges that define a broad sub-vertical maximum and a small sub-horizontal maximum. The XGT principle maxima lie at N-090 and N-120. Smaller maxima lie at N-020 and N-340. Geometric relationships between XGT axes and FIA orientation in the sample suggest that porphyroblast shapes are controlled by the geometry of the lens-shaped microlithons in which they tend to nucleate and grow. The orientation of inclusion trails and matrix foliations in the sample are correlated with three discrete tectono-metamorphic events that respectively produced andalusite, sillimanite and kyanite in the KMC. Late staurolite growth in the sample reveals how the rocks extruded to the surface via a significant role of roll-on tectonics, which can be correlated with the Central Himalayas.

Holistic microstructural techniques reveal synchronous and alternating andalusite and staurolite growth during three tectonic events resulted from shifting partitioning of growth vs deformation

Excellent inclusion trails in a sample containing both staurolite and andalusite porphyroblasts are used to demonstrate techniques that allow the intimate relationships between deformation and porphyroblast growth to be recognized, described in detail and understood. This approach reveals three main phases of growth of both mineral phases, some of which was demonstrably synchronous, during three tectonic events. Each main period of growth occurred during the early stages of three deformations that were successively near orthogonal. However, extra periods are distinguishable in andalusite in some of these events because this phase occurs as clusters of large crystals that vary in orientation by 2° to >10°. All foliations defined by all inclusion trails within every porphyroblast inflect/intersect about an axis trending at 025° (called a FIA). This indicates that the direction of the horizontal component of bulk shortening was identical for the first and third of the three deformations recorded by porphyroblast growth. Portions of sigmoidal to slightly spiral-shaped inclusion trails in most porphyroblast clusters locally diverge in opposite directions due to overprinting orthogonal bulk shortening typical of that which forms millipede geometries. These microstructures confirm the role of coaxial bulk shortening in initiating porphyroblast growth in an environment that locally becomes strongly non-coaxial as the deformation intensifies in the same event.In this sample, increasing non-coaxiality as the deformation intensified resulted in the same asymmetry for each of the three events and thus an overall spiral-like shape. Differing stages in the development of these bulk-shortening geometries preserved in adjacent or touching phases negate any role for porphyroblast rotation during ductile deformation. Andalusite and staurolite grew without any inter-reaction in locations where they lie in contact. This multiply repeated growth behaviour initiated within zones of spatially partitioned crenulation deformation of pre-existing foliations. This varied slightly in timing and in the subsequent development of successive surrounding foliations from location to location over distances around 5mm. Growth ceased adjacent to any zone where metamorphic differentiation associated with the development of new foliations initiated. This also varied in timing locally as a result of migrating and/or shifting partitioning of the deformation at similar to smaller scales. A pseudosection for the bulk composition of this sample shows that andalusite and staurolite grow simultaneously only over a very tightly constrained range of P–T conditions. Yet these occurred for at least three tectonic events, two of which produced strong schistosities. This required bulk coaxial orogen-scale deformation with slight crustal thickening during two periods of bulk horizontal shortening balanced by gravitational collapse in the intervening event.

Early formed regional antiforms and synforms that fold younger matrix schistosities: their effect on sites of mineral growth

In the metamorphic cores of many orogenic belts, large macroscopic folds in compositional layering also appear to fold one or more pervasive matrix foliations. The latter geometry suggests the folds formed relatively late in the tectonic history, after foliation development. However, microstructural analysis of four examples of such folds suggests this is not the case. The folds formed relatively early in the orogenic history and are the end product of multiple, near orthogonal, overprinting bulk shortening events. Once large macroscopic folds initiate, they may tighten further during successive periods of sub-parallel shortening, folding or reactivation of foliations that develop during intervening periods of near orthogonal shortening. Reactivation of the compositional layering defining the fold limbs causes foliation to be rotated into parallelism with the limbs. Multiple periods of porphyroblast growth accompanied the multiple phases of deformation that postdated the initial development of these folds. Some of these phases of deformation were attended by the development of large numbers of same asymmetry spiral-shaped inclusion trails in porphyroblasts on one limb of the fold and not the other, or larger numbers of opposite asymmetry spirals on the other limb, or similar numbers of the same asymmetry spirals on both limbs. Significantly, the largest disparity in numbers from limb to limb occurred for the first of these cases. For all four regional folds examined, the structural relationships that accompanied these large disparities were identical. In each case the shear sense operating on steeply dipping foliations was opposite to that required to originally develop the fold. Reactivation of the folded compositional layering was not possible for this shear sense. This favoured the development of sites of approximately coaxial shortening early during the deformation history, enhancing microfracture and promoting the growth of porphyroblasts on this limb in comparision to the other. These distributions of inclusion trail geometries from limb to limb cannot be explained by porphyroblast rotation, or folding of pre-existing rotated porphyroblasts within a shear zone, but can be explained by development of the inclusion trails synchronous with successive sub-vertical and sub-horizontal foliations.

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

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.

Three-dimensional reconstruction and modelling of complexly folded surfaces using Mathematica

In this paper we provide the following three examples of how the software system Mathematica can be used to reconstruct or model the three-dimensional shapes of folded surfaces. (1) First, we revisit the reconstruction of the central inclusion surface within a garnet porphyroblast that contains spiral-shaped inclusion trails. (2) Next, we revisit the reconstruction of five foliation surfaces that define oppositely concave folds within and surrounding a plagioclase porphyroblast. (3) For the main part of this paper we model superposed folds, and the many interference patterns that can be found in two-dimensional sections through these folds. Because this special issue is accompanied by a compact disk, we have included a series of reconstructions, models and animations to illustrate these three examples. Our reconstructions and models have, in some instances, provided important constraints on the interpretations of complex or controversial microstructures, and in all instances have provided useful teaching aids.

Testing models for the development of spiral-shaped inclusion trails in garnet porphyroblasts: to rotate or not to rotate, that is the question

Abstract The formation of spiral-shaped inclusion trails (SSITs) is problematical, and the two viable models for their formation involve opposite shear senses along the foliation in which the porphyroblasts are growing. One model argues for porphyroblast rotation, with respect to a geographically fixed reference frame, whereas the other argues for no such porphyroblast rotation, but instead rotation of the matrix foliation around the porphyroblast. Thus, porphyroblasts with SSITs cannot be used as shear-sense indicators until it is conclusively determined which model best explains them. Any successful model must explain features associated with SSITs, including: (1) foliation truncation zones, (2) smoothly curving SSITs, (3) millipede microstructure, (4) total inclusion-trail curvature in median sections, (5) porphyroblasts with SSITs that have grown together, (6) evidence for relative porphyroblast displacements, (7) shear-sense indicators inside and outside porphyroblasts; (8) crenulations associated with porphyroblasts and (9) geometries in sections subparallel to spiral axes (axes of rotation). A detailed study of these features suggests that most, if not all, can be explained by both the rotational and non-rotational models, in spite of these models involving diametrically opposed movement senses. Therefore, geometrical analysis of individual porphyroblast microstructures may not determine which model best explains SSITs until the kinematics required to form these microstructures are better understood, in particular the sense of shear along a developing crenulation cleavage. Specific tests for determining the shear sense along crenulation cleavages are proposed, and results of such tests may conclusively resolve the debate over how SSITs form.

Unravelling the spirals: a serial thin‐section study and three‐dimensional computer‐aided reconstruction of spiral‐shaped inclusion trails in garnet porphyroblasts

Abstract Seventy‐seven spatially orientated, serial thin sections cut from a single rock reveal changes in the geometry of spiral‐shaped inclusion trails (SSITs) in garnet porphyroblasts. The observed SSITs are doubly curved, non‐cylindrical surfaces, with total inclusion‐trail curvature decreasing systematically from the cores to the rims of porphyroblasts. The three‐dimensional geometry of the SSITs, reconstructed with the aid of computer graphics, shows that the orientations of spiral axes defined by the SSITs are not related in any expected nor predictable way to the main foliation in the matrix. This suggests continued deformation after or during the latest stages of porphyroblast growth, which has important implications for the use of SSITs as shear‐sense indicators. Whether the formation of SSITs involves significant porphyroblast rotation with respect to a geographically fixed reference frame cannot be determined from the available data.

Surface reconstruction from parallel serial sections using the program mathematica: Example and source code

A “package”—module of commented source code—for the computer program Mathematica has been designed to reconstruct surfaces from curves, collected in parallel serial sections through these surfaces. The package contains routines to interpolate digitized curves using continuous functions, to interpolate transversally between the resulting continuous curves, and to render the results of this later operation as a parametrized surface in three dimensions. This paper provides a listing of the package and an example of surface reconstruction using curves which were collected from serial thin sections through a garnet porphyroblast with spiral-shaped inclusion trails.

Do smoothly curving, spiral-shaped inclusion trails signify porphyroblast rotation?: Comment and Reply

Research Article| May 01, 1993 Do smoothly curving, spiral-shaped inclusion trails signify porphyroblast rotation?: Comment and Reply Gordon S. Lister; Gordon S. Lister 1Victorian Institute of Earth and Planetary Sciences, Monash University, Melbourne, Victoria 3001, Australia Search for other works by this author on: GSW Google Scholar T. H. Bell; T. H. Bell 2Department of Geology, James Cook University, Townsville, Queensland 4811, Australia Search for other works by this author on: GSW Google Scholar A. Forde; A. Forde 2Department of Geology, James Cook University, Townsville, Queensland 4811, Australia Search for other works by this author on: GSW Google Scholar N. Hayward N. Hayward 2Department of Geology, James Cook University, Townsville, Queensland 4811, Australia Search for other works by this author on: GSW Google Scholar Author and Article Information Gordon S. Lister 1Victorian Institute of Earth and Planetary Sciences, Monash University, Melbourne, Victoria 3001, Australia T. H. Bell 2Department of Geology, James Cook University, Townsville, Queensland 4811, Australia A. Forde 2Department of Geology, James Cook University, Townsville, Queensland 4811, Australia N. Hayward 2Department of Geology, James Cook University, Townsville, Queensland 4811, Australia Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1993) 21 (5): 479–480. https://doi.org/10.1130/0091-7613(1993)021<0479:DSCSSI>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Gordon S. Lister, T. H. Bell, A. Forde, N. Hayward; Do smoothly curving, spiral-shaped inclusion trails signify porphyroblast rotation?: Comment and Reply. Geology 1993;; 21 (5): 479–480. doi: https://doi.org/10.1130/0091-7613(1993)021<0479:DSCSSI>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract No abstract available This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Do smoothly curving, spiral-shaped inclusion trails signify porphyroblast rotation?

Smoothly curving, spiral-shaped inclusion trails may form in porphyroblasts during the overprinting of near-orthogonal foliations in schists that are relatively quartz rich and, in general, graphite poor. The smooth curvature results from the combined effects of (1) the protection from progressive shearing provided by projecting crystal faces and (2) greater competency and generally wider zones of progressive shearing in these rock types than in ones that are less quartz rich and more graphtic. Therefore, lack of truncations or abrupt changes in compositional zoning profiles and the presence of smoothly curving, spiral-shaped inclusion trails do not necessarily signify that the porphyroblast containing them rotated as it grew. This has considerable significance for shear-sense determinations in deformed and metamorphosed rocks.

Comment and Reply on "Do smoothly curved, spiral-shaped inclusion trails signify porphyroblast rotation?"

Comment and Reply on "Do smoothly curved, spiral-shaped inclusion trails signify porphyroblast rotation?"