Metamorphism occurs where equi P-T is disturbed Steady-state geotherms are disturbed by a variety of processes including plate-tectonic transport of rocks and heat input by intrusion of magma active transport of cold rocks into the hot mantle doubling of the crust by collisional stacking extra source of heat to add to geotherm Stress in the Earth In addition to temperature and pressure, the third main variable that controls the appearance of metamorphic rocks is stress pressure is a uniform force that acts on a rock and compresses it or pulls it apart. When this force field is unequal in different directions, we call it stress a rock responds to this unequal pressure by deforming, or yielding (e.g. flattening), and we call this response to stress strain when this stress is accompanied by movement, we call the response of the rock shear In fact, shear is a general term that describes all types of rock deformation, including that resulting from simple flattening, which is called pure shear Plate tectonic processes introduce stress by forcibly moving masses of rock, but stress even develops in sedimentary basins during burial.
The effects of metamorphism Three main variables that result in metamorphism and produce a metamorphic rock from a protolith: temperature, pressure and stress, but why? a rock will counteract any change it is subjected to: le Chatelier s principle effects of metamorphism: change in mineralogy mineral parageneses reflects the P-T conditions change in texture mineral fabrics give information on intensity and direction of stress and shear change in composition rock composition can point to devolatilisation and melting The effect of P and T on minerals Minerals have a limited stability, and when P and/or T get too high (or too low), they will react to something else that is more thermodynamically stable. 8000 kyanite Gr = Hr - T Sr + P Vr P(bar) 6600 5200 3800 pyrophyllite prl = H2O q ky H = -2593 kj mol -1 V = 4.414 J bar -1 (2) ky = sill sillimanite H = -2586 kj mol -1 V = 4.986 J bar -1 whatever phase has the lowest ΔG, or whatever reaction results in a decrease in ΔGr will be favoured ΔHr ky sill = +7 kj mol -1 ΔHr and sill = +3 kj mol -1 ky = and ΔHr and ky = -4 kj mol -1 2400 1000 1 (1) andalusite and = sill H = -2589 kj mol -1 V = 5.153 J bar -1 673 773 873 973 1073 T(K)
Metamorphic textures develop on a large variety of scales, from km-scale folds to micro-scale mineral alignment in thin-section. Combined, this is called the structure of a metamorphic rock The fabric of a rock is a preferred orientation in the rock, either by alignment of minerals, or by alignment of other features such as fractures, or phase separation Alignment of minerals is again a response in line with le Chatelier s principle: Alignment of minerals is again a response in line with le Chatelier s principle: σ1 10 kg σ3 10 kg σ2 σ2 σ3 σ1 mineral: 1 x 1 x 100 m σ1 > σ2 = σ3 F = m g m = 10 kg; g ~ 10 m s -2 σ1 = F / area σ1 = 100 N / 1 m 2 = 100 Pa σ1 = F / area σ1 = 100 N / 100 m 2 = 1 Pa
Similar response by minerals to simple shear: simple shear results in rotation of minerals into the plane of movement or deformation This gives rise to beautiful textures in thin-section: large biotite grain being pulled apart by shear with infill by a new generation auge (German for eye) of plagioclase with mica foliation wrapping around it
Not all minerals are affected equally by the stress. Shear and strain are most readily observed for platy and elongate minerals, whereas blocky minerals such as quartz and feldspar show only limited effects. two main ways to form a preferred orientation: re-alignment and re-crystallization of pre-existing grains - at low temperature this can take the form of grain fragmentation new growth and replacement by aligned mineral grains - this is a primary preferred orientation and can overgrow (and partially preserve) an older generation alignment of minerals results in two types of fabric (needs to be repetitive): linear fabric, called a lineation - this is generally defined by elongate minerals such as amphiboles, andalusite, tourmaline planar fabric, called a foliation - this is generally defined by platy minerals such as chlorite, biotite and muscovite However, fabrics are not only defined by minerals, but also by other features in a rock: a lineation can result from mineral alignment, intersection of foliation and bedding, intersection of foliation and veining, hinges on folds, alignment of pebbles in a meta-conglomerate And planar features are not necessarily flat, but can be curved, folded, crenulated, and multi-generational
2 nd folding 2 nd folding bedding 1 st folding a planar fabric also develops when minerals segregate into bands in a process called metamorphic differentiation: redistribution of minerals depending on their shape or morphology resulting in an anisotropy in composition and mineral modal abundance on hand specimen scale the resulting fabric is called gneissic banding and the rock a gneiss the driving force for banding is surface tension, with platy and blocky minerals segregating to minimize the surface energy blocky minerals are mainly qtz + plag, whereas the platy and elongate minerals include bt, ms and amp. because qtz + plag are light in colour and amp and bt dark, gneissic banding can generally also be recognized as a colour segregation
Gneissic banding on field and sample scale