The significance of microgranitoid enclave shapes and orientations Scott R. Paterson a, * , Geoffrey S. Pignotta a , Ron H. Vernon a,b a Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089-0740, USA b Department of Earth and Planetary Sciences and GEMOC, Macquarie University, Sydney, NSW 2109, Australia Received 18 January 2003; received in revised form 14 July 2003; accepted 25 August 2003 Available online 17 April 2004 Abstract Enclaves are often incorrectly used to measure magmatic strains in plutons. We emphasize that microgranitoid enclaves are not like other ellipsoidal markers used to determine strain for the following reasons. (1) Adjacent enclaves may form at different times and different places and initially have non-spherical shapes with axial ratios up to 2.7. (2) The final shapes and orientations of enclaves are a complex function of (a) initial shape and temperature of enclaves, (b) subtle changes in composition, melt percents, volatiles, grain sizes, and thus temporally variable viscosity contrasts between the enclave and magma, (c) a competition between strain and interfacial energies, and (d) deformation path, which may include internal strain and rigid rotations caused by magma flow during ascent, convection, expansion, chamber boundary processes, and tectonism. (3) Enclaves spend much of their time in magma as relatively rigid objects, and thus rigidly rotate and potentially break apart, rather than strain at matrix strain rates. (4) In some instances, enclaves do not record or track finite strain. Because of the above, final enclave populations are heterogeneous, and the use of single enclaves or enclave populations as strain markers violates many assumptions needed to complete strain analyses. On the other hand, a comparison of the preserved characteristics of igneous layering, mineral fabrics, and carefully evaluated enclave fabrics, including internal mineral alignment in enclaves, may provide qualitative data on the changing magnitude and kinematics of magmatic strains. q 2003 Elsevier Ltd. All rights reserved. Keywords: Expanding plutons; Rf/f analyses; Granites; Igneous enclaves; Magmatic foliations; Microgranitoid enclaves; Strain analyses 1. Introduction Microgranitoid enclaves or ‘mafic inclusions’, typically formed by mingling of two magmas (Vernon, 1983; Frost and Mahood, 1987), are common in many plutons (Fig. 1). Their wide distribution and common ellipsoidal shapes, as well as the lack of other suitable markers in plutons, make them a natural target for use as strain markers. Williams and Tobisch (1994) explored some of the challenges of using enclaves to determine solid-state strains. However, enclaves are also used to quantify magmatic strains, which are often assumed to occur during emplacement (Holder, 1979; Courrioux, 1987; Hutton, 1988; Ramsay, 1989; John and Blundy, 1993). Even though the use of enclaves as magmatic strain markers has been challenged (Paterson and Vernon, 1995; Paterson et al., 1998), they continue to be used routinely (e.g. Molyneux and Hutton, 2000; Vassallo and Wilson, 2002). Given that the quantitative results of such studies are used to evaluate a wide variety of magma ascent and emplacement mechanisms, and sometimes the nature of syn-emplacement tectonism, we feel that further evaluation of this issue is important. In this paper we emphasize that microgranitoid enclaves are not like other ellipsoidal markers used to determine strain (e.g. conglomerates or volcanic breccia), and thus careful evaluation of enclave data sets is needed prior to using them in any quantitative manner. We suggest that the following need to be addressed if geologists want to obtain meaningful results from enclave shapes and orientations, or their spatial distribution: (1) the initial characteristics of single enclaves and enclave populations and temporal and spatial variation of enclave formation; (2) enclave rheology and enclave/host magma viscosity contrasts; (3) whether enclaves track finite strain, particularly in situations where multiple magmatic fabrics exist; and (4) the final variability in shapes and orientations of enclave populations. We begin by reviewing the assumptions made to successfully complete strain analysis using spherical or ellipsoidal markers in all rock types. We then examine each of the 0191-8141/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsg.2003.08.013 Journal of Structural Geology 26 (2004) 1465–1481 www.elsevier.com/locate/jsg * Corresponding author. Tel.: þ 1-213-740-6103; fax: þ1-213-740-8801. E-mail address: paterson@usc.edu (S.R. Paterson).