Holistic microstructural techniques reveal synchronous to alternating development of andalusite and staurolite during 3 tectonic events resulted from shifting partitioning of growth vs deformation

Bell, T.H., and Fay, C. (2016) Holistic microstructural techniques reveal synchronous to alternating development of andalusite and staurolite during 3 tectonic events resulted from shifting partitioning of growth vs deformation. Lithos, 262. pp. 699-712.

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[img] Video (MP4) ((Porphyroblast movie) Supplemental Material for BELL, SANISLAV and SAPKOTA, 2018. See https://researchonline.jcu.edu.au/47906/)
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View at Publisher Website: http://dx.doi.org/10.1016/j.lithos.2016....
 
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Abstract

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 5 mm. 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.

Item ID: 44686
Item Type: Article (Research - C1)
ISSN: 1872-6143
Keywords: multiple phases of staurolite growth; multiple phases of andalusite growth; synchronous staurolite and andalusite growth; spiral trails from millipeding; porphyroblast nucleation; progressive growth of porphyroblasts
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Figures 2 and 3 can be opened in Adobe Illustrator, Canvas etc. They were created in Adobe illustrator and can be opened or closed in different layers for each foliation, porphyroblast phase, labels etc. The images are very high resolution can readily be examined at 3200% magnification. The inclusion trails are quite fine lines at high magnification and appearing thicker at low magnification because of the way an Adobe pdf handles them at that scale. The trails can be checked against the inclusion trails. Use silicate inclusions (e.g., quartz) rather than opaques such as ilmenite. Ilmenite is a relatively competent crystal and commonly resists internal ductile strain. Therefore, once it has grown, it deforms and rotates differentially due to successive deformations, although it grows aligned with foliations. Examine the way foliations curve towards the next formed axial plane structure with non-platy silicate loss (stage 3), and an increase in the amount of the porphyroblastic phase. The latter results from porphyroblast growth in a later event that replaces the increased phyllosilicate content as other silicates were removed during differentiation. Increased strain eventually leads to the development of a truncational foliation (stage 4). Examine how foliations lie at high angle to porphyroblast boundaries in strain shadow regions (referred to by many incorrectly as pressure shadows) for successive foliations. Examine the full to partial millipede geometries and see how subtly they are preserved.

The uploaded porphyroblast movie is related to the following publication (see the Related URL link): Bell, Timothy H., Sanislav, Ioan V., and Sapkota, Jyotindra (2018) The control of deformation partitioning and strain localization on porphyroblast behaviour in rocks and experiments. Geosciences Journal, 22 (1). pp. 65-77.

Date Deposited: 18 Jul 2016 05:28
FoR Codes: 04 EARTH SCIENCES > 0403 Geology > 040304 Igneous and Metamorphic Petrology @ 50%
04 EARTH SCIENCES > 0403 Geology > 040312 Structural Geology @ 50%
SEO Codes: 97 EXPANDING KNOWLEDGE > 970104 Expanding Knowledge in the Earth Sciences @ 100%
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