Coupled trace element mobilisation and strain softening in quartz during retrograde fluid infiltration in dry granulite protoliths

This study focuses on the retrograde rheological and chemical evolution of quartz and the behaviour of quartzites during retrograde metamorphism following dry high grade metamorphism at 750°C, 7 kbar. SEM-CL and LA-HR-ICP-MS are applied to document quartz texture and chemistry, respectively. Four generations of quartz were distinguished by SEM-CL; Qz1, Qz2, Qz3 and Qz4. Qz1, brecciated and partly dissolved old grains, is enriched in B, Al and Ti when compared with the other types. Qz2, formed during brecciation and partial dissolution of Qz1, has low Al contents (<50 ppm) but, due to rutile inclusions, variable Ti contents when occurring in amphibolite (210–10 ppm) but more consistent values when occurring in quartzites (peak value 32 ppm). Qz3, dark grey luminescent quartz forming fluid migration channels (fluid pathways), has Ti < 5 ppm and Al contents below 10 ppm and B < 1 ppm. Qz4, comprises are group of quartz later than Qz3 filling micron thick cracks and pods with very low luminescent quartz, i.e. darker than Qz3. The textural and chemical evolution of quartz in our study is explained by two major influxes of aqueous fluids during regional uplift and retrogression. They facilitated rehydration and recrystallisation in the otherwise dry high grade quartzites. The first introduction of aqueous fluids was associated with brecciation of the high grade quartz (Qz1) and dissolution/precipitation of quartz (Qz2). Ti in quartz geothermometry (Wark and Watson, Contrib Mineral Petrol 152(6):637–652) gives 626°C in agreement with the retrograde PT-path deduced from phase diagrams. Later fluid influx associated with scapolitisation of amphibolite caused localised recrystallisation (Qz3) and alteration of biotite to muscovite along mm-wide fluid migration channels. During subsequent deformation, Qz3 deformed plastically and recovered by subgrain rotation recrystallisation (SGR), resulting in a reduction of grain size, whereas Qz1 quartz formed micro faults. Qz2 was plastic but did not experience SGR to the same degree as Qz3 quartz. Increased plasticity and recovery rates most likely relate to an increased H₂O fugacity and the depletion in trace elements of the quartz lattice by promoting strain softening processes dislocation climb and recovery. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.