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Geology.Gsapubs.Org on 23 September 2009 Downloaded from geology.gsapubs.org on 23 September 2009 Geology Porphyroblast rotation and strain localization: Debate settled! Scott E. Johnson Geology 2009;37;663-666 doi: 10.1130/G25729A.1 Email alerting services click www.gsapubs.org/cgi/alerts to receive free e-mail alerts when new articles cite this article Subscribe click www.gsapubs.org/subscriptions/ to subscribe to Geology Permission request click http://www.geosociety.org/pubs/copyrt.htm#gsa to contact GSA Copyright not claimed on content prepared wholly by U.S. government employees within scope of their employment. Individual scientists are hereby granted permission, without fees or further requests to GSA, to use a single figure, a single table, and/or a brief paragraph of text in subsequent works and to make unlimited copies of items in GSA's journals for noncommercial use in classrooms to further education and science. This file may not be posted to any Web site, but authors may post the abstracts only of their articles on their own or their organization's Web site providing the posting includes a reference to the article's full citation. GSA provides this and other forums for the presentation of diverse opinions and positions by scientists worldwide, regardless of their race, citizenship, gender, religion, or political viewpoint. Opinions presented in this publication do not reflect official positions of the Society. Notes © 2009 Geological Society of America Downloaded from geology.gsapubs.org on 23 September 2009 Porphyroblast rotation and strain localization: Debate settled! Scott E. Johnson Department of Earth Sciences, University of Maine, Orono, Maine 04469-5790, USA ABSTRACT (reviewed by Johnson, 1999; Carlson, 2002; This contribution shows unequivocally that porphyroblasts rotate relative to one another Johnson et al., 2006). The topic of whether por- during ductile deformation. The porphyroblasts described here have special signifi cance phyroblasts rotate relative to one another and a because they are from the original “millipede” rocks that led to the nonrotation hypothe- fi xed reference frame during ductile deforma- sis. Thus, the debate that has lasted for more than 20 years is settled. Despite this fi nding, tion has attracted particularly energetic debate porphyroblast microstructures continue to provide important evidence for deformation and over the past two decades (reviewed by John- metamorphic histories. Although porphyroblasts clearly rotate relative to one another dur- son, 1999; cf. Fay et al., 2008; Bons et al., ing ductile deformation, there are several factors that contribute to relatively minor rotation 2009). The debate gathered momentum when in many instances, including (1) low strain during and after porphyroblast growth in com- Bell (1985) questioned porphyroblast micro- parison, for example, to mylonitic shear zones; (2) small axial ratios combined with relatively structures that had previously been used as evi- low internal vorticity during growth and post-growth deformation; and (3) strain localization dence for rotation relative to an externally fi xed at the porphyroblast-matrix interface. Thus, given the right circumstances, porphyroblasts kinematic reference frame, suggesting instead may preserve the approximate orientations of deformation fabrics present at the time of their that they may form by growth, without rota- growth, but each case must be individually assessed. tion, during crenulation cleavage development. This hypothesis was based on a geometrical strain fi eld designed to mimic the so-called mil- INTRODUCTION for a wide range of studies in deformed meta- lipede microstructure preserved in and around Porphyroblasts are relatively large metamor- morphic rocks, including those that examine plagioclase porphyroblasts described by Bell phic minerals that commonly show chemical deformation and metamorphic histories, rates and Rubenach (1980). Much of the debate sur- zonation and trap preexisting structural fabrics of diffusion and chemical reaction, deformation rounding porphyroblast rotation can therefore as inclusion trails during their growth (Fig. 1). kinematics, fi nite strain, kinematic vorticity, be traced back to these plagioclase porphyro- For these reasons, they are centrally important pluton emplacement, and folding mechanisms blast microstructures, so they fi gure centrally in the porphyroblast rotation debate. In this paper I show that these same plagioclase porphyro- blasts have rotated relative to one another and to a developing crenulation cleavage, and discuss S some implications for deformation histories and 3 strain localization in rocks. a BACKGROUND b Comprehensive theoretical (Jeffery, 1922) S2 and experimental (Ghosh and Ramberg, 1976) studies provide a fi rm foundation for evaluating the rotational behavior of rigid objects embed- ded in a Newtonian viscous medium, but with A rare exceptions (e.g., Holcombe and Little, 2001) these studies have been diffi cult to suc- cessfully apply to porphyroblasts. Some reasons for this are discussed herein, but in general, the kinematic history of porphyroblasts in region- ally deformed metamorphic rocks is ambiguous compared to, for example, the kinematic history of porphyroclasts in mylonitic shear zones with well-defi ned boundaries (e.g., Passchier et al., 1992; Johnson and Vernon, 1995). The primary evidence used to argue against porphyroblast rotation is the tight clustering of average inclusion-trail orientations over areas ranging in size from sample-scale and outcrop- S2 S2 B scale folds to tens or hundreds of square kilome- Figure 1. Examples of porphyroblast microstructures from rocks discussed in this paper ters (see Bell et al., 1992; references in Johnson and by Bell and Rubenach (1980). A: Thin section that cuts approximately through centers et al., 2006). These data are impressive and give of porphyroblasts marked a and b. B: Magnifi cation of region between porphyroblasts a and good reason to question the direct applicability b illustrating continuity between inclusion trails and matrix S2 foliation. Two individual folia- tions in the matrix that are adjacent, and parallel, to one another diverge and can be traced of the above-mentioned theoretical and experi- into edges of the two different porphyroblasts. See text for discussion. Both images under mental results to deformed rocks. However, cross-polarized light. Width of fi eld in A is 32 mm, in B is 17 mm. close examination of these data reveals a large © 2009 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. GEOLOGY,Geology, July July 2009; 2009 v. 37; no. 7; p. 663–666; doi: 10.1130/G25729A.1; 3 fi gures. 663 Downloaded from geology.gsapubs.org on 23 September 2009 range of inclusion-trail orientations in indi- and consistently oriented regional foliation that OCM geometry in three dimensions. We (John- vidual samples, typically between 40° and 80°. predated pluton emplacement by ~30 Ma. The son and Williams, 1998) developed a new strain This large variance in sample-scale orientations spread of inclusion-trail orientations in the por- tool by comparing the spacing between S2 sur- has generally been interpreted by proponents phyroblasts increases nonlinearly from ~16° to faces inside and outside of the porphyroblasts, of nonrotation as preserving initial variation 75° with increasing strain in the aureole. These calculating an elongation of 172% during the in foliation orientations prior to porphyroblast data provide strong evidence for rotation of the development of S3. growth, possibly refl ecting more than one por- staurolite porphyroblasts relative to one another, phyroblast growth episode (e.g., Bell et al., the amount of relative rotation increasing with PORPHYROBLAST, FOLIATION, AND 1992; Bell and Bruce, 2007). Nevertheless, it increasing strain. The Johnson et al. (2006) INCLUSION-TRAIL ORIENTATION can also be explained by variable rotation of study will not convince some nonrotation sup- RELATIONS porphyroblasts of different shape and orienta- porters, so here I provide a different example in tion (e.g., Passchier et al., 1992; Jiang, 2001; which the pre-deformation orientations of inclu- Orientation Data For All Sections Johnson et al., 2006). sion trails can be confi dently inferred at the thin- I intersected 22 complete porphyroblasts in In addition to measuring inclusion-trail ori- section scale, thus eliminating any uncertainty 42 thin sections from the four serial thin-section entations, a suite of more recent papers shows associated with multiple-sample data sets. blocks. Figures 2A and 2B show the same ori- clustering of relative rotation axes (referred to as entation data from these porphyroblasts plotted foliation intersection-infl ection axes) in porphy- DESCRIPTION OF SAMPLE in two different ways to clarify the relationships. roblasts that contain sigmoidal, spiral-shaped, A hand sample containing millipede micro- All data in these plots were measured in sections and other complex inclusion-trail geometries. structure, better described as oppositely concave that pass approximately through the center of These papers have consistently concluded that microfolds (OCMs), was serially thin sectioned each porphyroblast. the data indicate a lack of porphyroblast rotation at ~1.5 mm intervals and described in Johnson Figure 2A shows porphyroblast axial ratios (see references in Fay et al., 2008). However, and Moore (1996). The sample contained the plotted against both the angle between the as with inclusion-trail
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