An experimental study on the brittle–plastic transition during deformation of granite

Abstract

Naturally and experimentally deformed samples of granite show that the deformation mechanisms of plagioclase and K-feldspar are different. To investigate these mechanisms and the brittle–plastic transition that takes place in granitic rocks composed of quartz, plagioclase, and K-feldspar, five leucosome granite samples were deformed with a constant strain rate of 10−5s−1 at different temperatures from 850°C to 1050°C and confining pressure (CP) of 300MPa using a Paterson-type gas deformation apparatus. To consider pressure effects, two more samples were deformed at 950°C but with different CPs, one with CP=100kPa and the other with CP=100MPa. In addition, an eighth sample was deformed under torsion shear at 950°C with CP=400MPa. Microstructures of an undeformed sample and experimentally deformed samples were analyzed using an optical microscope and a scanning electron microscope. The granite is composed of about 36 volume percent quartz, ∼26% plagioclase, ∼34% microcline, ∼3% muscovite, and ∼1% chlorite.The stress–strain curves for all but two samples display weakening. The two exceptions are the sample that deformed with steady-state creep under a CP of 100MPa and the sample that displayed brittle fracture under a CP of 100kPa. For the other samples, peak strengths decreased with increasing temperature or lower CP. Microstructures show that samples underwent a brittle–plastic transition with increasing temperature. Samples fractured by cataclastic flow at 850°C with CP=300MPa and at 950°C with CP=100kPa. Microcline deformed by cataclastic flow at 900–1050°C with CP=100–400MPa accompanied by dislocation glide at temperatures of 1000°C and 1050°C. At 900–1050°C with CP=100–400MPa, plagioclase displayed bulging recrystallization and grain boundary migration recrystallization and quartz deformed by subgrain rotation recrystallization. Diffusion rims were observed between quartz, plagioclase, and microcline grain boundaries at 900–1050°C with CP=100–400MPa. Partial melting appeared around plagioclase, microcline, and quartz grains at 1000°C and 1050°C. Exsolution of albite from microcline took place in a sample deformed at 950°C and CP=100MPa. Electron backscatter diffraction analysis showed that quartz has a lattice preferred orientation in both undeformed and experimentally deformed samples. The basal 〈a〉 slip of the quartz c-axis was found in undeformed samples. The c-axis of quartz closes to the rhomb 〈a〉 slip with a secondary in the basal 〈a〉 slip in samples deformed at 900°C and 950°C with CP’s of 300MPa and 100MPa respectively. The c-axis of quartz changes to prism 〈a〉 slip in samples deformed at 950–1050°C with CP=300MPa and does the same for a sample deformed with torsion shear at 950°C with CP=400MPa.Microstructures show that plagioclase and microcline deform differently. Microcline is a strong phase compared with quartz and plagioclase. Therefore, microcline is important for the brittle–plastic transition in granitic rocks and the depth of that transition in the middle crust will be greater where the granitic rocks in the crust contain large amounts of microcline.