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

• Published in: Journal of structural geology 2016-10, Vol.91, p.88-101
• Main Authors: De Toni, G.B., Bitencourt, M.F., Nardi, L.V.S.
• Format: Article
• Language: English
• Subjects: Dissolution-replacement creep; Granulite facies conditions; Myrmekite; Strain partitioning; Syntectonic magmatism; Syntectonic syenites
• ISSN: 0191-8141; 1873-1201
• DOI: 10.1016/j.jsg.2016.08.002

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 myrmekite 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 syntectonic environments under granulite-facies conditions may extend the field of operation of dissolution-replacement creep, changing significantly the rheology of the lower continental crust. •Pervasive myrmekite formation by coupled deformation and late-magmatic liquid inflow.•Pervasive Ca-rich myrmekite formation reported in syntectonic syenites.•Dissolution-replacement creep may be effective under granulite facies conditions.•Strain partitioning into dry and wet zones in granulite facies environments.