Abstract

Detailed fabric development histories have been deduced for Neoproterozoic foliated quartz diorite complexes of the Channel Islands, UK. The plutons were emplaced during regional (D2) deformation. Magmatic fabrics, formed at low crystal contents (pre-rheologically critical melt percent) are only locally preserved due to overprinting by solid-state deformation which occurred at moderate temperatures (400–550°C). This is demonstrated by brittle microcracking of plagioclase and amphibole with only local ductile bending and dynamic recrystallization; marginal recrystallization and myrmekite development on alkali-feldspar; ductile deformation of quartz to form types 2–3 ribbons; quartz recrystallization by sub-grain rotation associated with a-slip but not c-slip: biotite recrystallization within micro-shears; and variable amphibole recrystallization. The deformation histories display a distinct division between magmatic state and moderate temperature solid-state deformation features with little evidence for intervening high temperature solid-state fabrics. This contrasts with criteria widely considered diagnostic of syn-tectonic plutons which envisage development of a continuum of magmatic through high temperature solid-state deformation fabrics. However, we consider the plutons syn-tectonic and attribute the hiatus to contrasting strain and pluton cooling rates, length and nature of deformation events and host rock ambient temperatures. Intermediate magmas, emplaced at moderate to shallow levels in the crust may be more prone to develop such apparently punctuated deformation histories than SiO2,-richer granitoids.

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