Strain partitioning into dry and wet zones and the formation of Ca- rich myrmekite in syntectonic syenites: A case for melt-assisted dissolution-replacement creep under granulite facies conditions G.B. De Toni a, * , M.F. Bitencourt a, b , L.V.S. Nardi a, b a Programa de P os-graduaç~ ao em Geoci^ encias, Instituto de Geoci^ encias, Universidade Federal do Rio Grande do Sul, Av. Bento Gonçalves, 9500, Porto Alegre, 91500-000, RS, Brazil b Departamento de Geologia, Instituto de Geoci^ encias, Universidade Federal do Rio Grande do Sul, Av. Bento Gonçalves, 9500, Porto Alegre, 91500-000, RS, Brazil article info Article history: Received 11 January 2016 Received in revised form 26 July 2016 Accepted 7 August 2016 Available online 11 August 2016 Keywords: Syntectonic magmatism Syntectonic syenites Myrmekite Dissolution-replacement creep Strain partitioning Granulite facies conditions abstract The formation of Ca-rich myrmekites is described in syntectonic syenites crystallized and progressively deformed under granulite facies conditions. The syenites are found in high- and low-strain zones where microstructure and mineral composition are compared. Heterogeneously distributed water-rich, late- magmatic liquids were responsible for strain partitioning into dry and wet high-strain zones at outcrop scale, where contrasting deformation mechanisms are reported. In dry high-strain zones K-feldspar and clinopyroxene are recrystallized under high-T conditions. In wet high-strain zones, the de-stabilization of clinopyroxene and pervasive replacement of relatively undeformed K-feldspar porphyroclasts by myr- mekite and subordinate micrographic intergrowths indicate dissolution-replacement creep as the main deformation mechanism. The reworking of these intergrowths is observed and is considered to contribute significantly to the development of the mylonitic foliation and banding. A model is proposed for strain partitioning relating a positive feedback between myrmekite-forming reaction, continuous inflow of late-magmatic liquids and dissolution-replacement creep in the wet zone at the expenses of original mineralogy preserved in the dry zones. Melt-assisted dissolution-replacement creep in syn- tectonic environments under granulite-facies conditions may extend the field of operation of dissolution- replacement creep, changing significantly the rheology of the lower continental crust. © 2016 Elsevier Ltd. All rights reserved. 1. Introduction Myrmekite is one of the most commonly described and dis- cussed microscopic feature in geology. This is partly due to its common ocurrence in a diversity of rocks and environments, and also because of the controversy generated since the early debate between the two proposed models for its genesis by replacement (Becke, 1908) and exsolution (Schwantke, 1909). With the advances in both science and technology, mainly with the advent of the scanning-electron microscope and microprobe, detailed observa- tions of composition and morphology of minerals and micro- structures have lead to great advances in the understanding of replacement reactions. Years of research on the subject have produced extensive literature (see Phillips, 1974 for a review of early concepts), and a strong tendency is observed toward a balance of arguments from both schools of thought (Ashworth, 1972; Phillips, 1980). In a compromising paper, Phillips (1980) suggests that the exsolution model is best applied to undeformed granitoids, whilst the replacement model generally fits better to deformed and metamorphosed rocks. He also argued that both models could operate together (as agreed upon by Ashworth,1972; Tsurumi et al., 2003). Myrmekite is strictly defined as a symplectitic intergrowth of sodic plagioclase (generally oligoclase) and vermicular quartz in contact or pseudomorphically replacing K-feldspar (Phillips, 1974; Smith and Brown, 1988; Vernon, 2004). Calcic myrmekites are re- ported (e.g. Dymek and Schiffries, 1987; Efimov et al., 2010; Mukai et al., 2014) as resulting from the replacement of intermediate-An content plagioclase by more calcic ones in mafic rocks. However, some authors (as Smith and Brown, 1988) suggest that such * Corresponding author. E-mail address: gdetoni@ufrgs.br (G.B. De Toni). Contents lists available at ScienceDirect Journal of Structural Geology journal homepage: www.elsevier.com/locate/jsg http://dx.doi.org/10.1016/j.jsg.2016.08.002 0191-8141/© 2016 Elsevier Ltd. All rights reserved. Journal of Structural Geology 91 (2016) 88e101