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G314 Advanced Igneous Petrology 2007 Week 5 – Lectures 13--14 Cooling of magmas, textures, sub-solidus evolution See Winter, chap. 3 and 4 1. Final cooling of magmas; textures of plutonic rocks 1.1. Crystal nucleation and growth How crystals form and grow Crystal grow by progressively adding ions to an existing grain. However, the initiation of a grain is a more energy-demanding process (you have to put together a small number of ions, therefore posibely creating unbalanced ionic structures, with high surface/volume ratio (and therefore high surface energy). Small nucleus are therefore very unstable; the amount of crystals in a system is strongly controlled by the amount of crystal nuclei. Crystal forms around… - Spontaneous nuclei (possible but difficult); - Small pre-existing crystals (“seeds”), of the same specie, or of a different mineral; - Pre-existing crystalline faces (“epitaxis”). Competition between growth and nucleation and the textures of rocks Both growth and nucleation rates change as a function of the temperature of the magma (more precisely, of the degree of under-cooling, below the melting point). - For important undercooling (=fast cooled rocks, volcanic), nucleation rate > growth rate; lot of small crystals (microgranular texture). - For moderate undercooling, growth > nucleation. Few, big crystals (plutonic textures). Note: bimodal dstributions (porphyritic lavas): two stages of crystal growth with different degrees of undercooling! Departement of Geology, Geography and Environmental Studies G314 Advanced Igneous Petrology 2007 Water and growth rates Water presence increases both nucleation and growth rate. This results in fairly unpredictable textures, with very coarse and very fine-grained rocks coexisting in close vicinity (aplite-pegmatite association) 1.2. Textures and relations related to crystallization order As we discussed previously, crystals form in a specific sequence that depends on the magma initial composition. Crystal growing in the melt develop their own crystalline shapes (euhedral), whereas crystals developing at a latter stage are likely to be influenced by pre-existing grains, and form intersticial or engulfing grains. On the other hand, fast growing grains can also include slower forming minerals, creating poekilitic grains, more or less euhedral. Being euhedral is therefore not an absolute criteria (engulfing reactions are). Inclusion relations An included grain is older than the surrounding. This allows to propose a sequence of crystallization (can be often interpreted by looking at the relevant phase diagram –see example in lecture W5L1). Just be careful of “pseudo-inclusions” (2D sections of 3D structures!). Simultaneous growth On eutectic or joints, when the crystallization reactions produce several mineral species simultaneously. Intergrowth (or, sometimes, mutually engulfing crystals), e.g. granophyric or graphic textures. 1.3. Textures and relations related to chemical evolution during cooling Normal zoning During cooling, the magma composition evolves. Minerals that form solid solution also have changing composition (see Fo-Fa or Ab-An diagrams from G214). Crystals typically are zoned, with a high temperature core and a lower temperature rim. This reflects only normal cooling and should be expected. 155 7 idus Liqu 150 0 Liquid Plagioclase 140 0 o T C plu s 130 0 Liquid Plagioclase u s lid So 120 0 111 8110 0 A b 2 0 4 6 0 Weight %0 An 8 0 A n Departement of Geology, Geography and Environmental Studies G314 Advanced Igneous Petrology 2007 “Anormal” zonings, resorptions, etc. In some case, the zoning does not obey to this simple evolution (e.g., apparentely low-temperature cores). Or it is more complex, with maybe several cycles, or overgrowth. Or some crystal resorption appears (truncated zoning, etc.). All this indicates that the crystal had a complicated history and probably cooled in a changing (chemical) environement: it was carried to another magma (cf. enclaves and magma mixing), or the magma chamber was refilled by a more primitive melt, etc. Study of crystal zoning (sometimes, in theory) allows to discuss the details of the evolution in the magma chamber. 1.4. Textures related to deformation (syn-tectonic emplacement) Plutonic rocks (granites, especially) are commonly syn-tectonic. They emplace and cool during deformation, and they record the strain they exercised at different stages: - As a liquid with few crystals floating; - As a largely crystallized system with some liquid remaining; - After complete solidification. Modern petrology (1990-onwards) interprets a lot of textures in granites (mostly outcrop or handspecimen scale) as related to deformation in a partially molten “mush”. The n otion of RCMP discussed previously also applies here… Crystals orientation (flow figures) In a liquid dominated system. Alignment of early crystals reflecting either magmatic flow or tectonic stress. Crystal-liquid separation In a largely crystalline system (below RCMP). Evidence for circulation of late, residual magmatic liquids between crystals; “dykes” and “pipes” of magma can sometimes be observed, together with crystal accumulations. Sub-solidus deformation After complete cooling. High-temeprature, solid-state deformation (commonly quartz sub-grain with ondulose extinction, sometimes feldspars fracturation or even orthogneissic textures). Difficult to interpret as syn-plutonic deformation (as opposed to a latter tectonic event), unless you have good context (typically a whole sequence of deformation from magma-dominated to sub-solidus textures). 2. Sub-solidus textures After the final solidification of a plutonic rock, its evolution continues! The solidus is at 700-900 °C, which (for upper crustal intrusions at least) is far from the surrounding thermal conditions. Therefore, mineralogical changes will occur (retrograde metamorphism into greenschist facies, first). 2.1. Mineral transformations Transformation affecting one single minerals: no reactions, but just re-adjustement of the crystalline system. Departement of Geology, Geography and Environmental Studies G314 Advanced Igneous Petrology 2007 Annealing (Ostwald reapening) Minimization of surface energy tends to migrate grains boundary and produce homogeneous grain size distribution of relatively large grains. Affects particularily fine-grained rocks (lavas) or even glasses (devitrification). Low temperatures help to preserve the volcanic textures. Polymorphic transformation .. e.g., quartz polymorphs (or feldspars). Little evidence preserved. Polymorphs Several minerals (e.g., silica) can form different minerals (quartz, cristobalite, etc.) in different parts of the P-T field. Cooling will cause such changes (normaly very discrete, nearly impossible to see). Exsolution Minerals such as feldspars (or pyroxenes) form solid solutions only at high temperatures. At lower temperatures, the minerals become immiscible and form separate phases (solvus). A high-temperature grain will therefore exsolve the minor component into perthites. 2.2. Secondary minerals “Autometamorphic” processes as the rocks cools down, effectively retrograde metamorphic reactions. Reactions involving hydratation of igneous minerals (the most common) are called deuteric reactions. This include, commonly: - successive replacements of pyroxenes -> amphiobles -> biotite; - chlorite replacing any mafic mineral (biotite, very often); - sericite (=fine grained white micas) as alteration products of feldspars or feldspathoids (feldspathoids more than feldspars, and K-feldspars more than plagioclase) - saussurite (an epidote) replacing Ca-plagioclase - iddingsite or serpentine replacing olivine Such replacements are commonly in-situ, resulting in pseudomorphs or in more or less altered crystals (commonly, secondary minerals form along the cleavages). It is also possible to form veins of secondary minerals (commonly calcite + white micas/clays), pointing to (limited) movement of elements. 2.3. Fluid expulsion and movement A typical melt can contain 2-4 wt% of water (even more for low temperature granitic melts). Biotite contains also about 2 wt% of water (and is the most hydrous common mineral in a granite); therefore, crystallizing a granite will yield lots of excess water. The hot fluids are a very potent chemical agent and can dissolve significant amounts of components such as Si, Na or K. In addition to the magmatic water, meteoric fluids infiltrating in the ground will also “feed” the hydrothermal system (and, actually, contribute to a large part of the amount of water present – even more than magmatic fluids in general). Pegmatitic (and aplitic) veins Hydraulic fracturation, resulting in a network of veins reflecting the strain pattern (occasionally it is possible to prove that it’s s the same strain pattern that existed during previous deformation). Departement of Geology, Geography and Environmental Studies G314 Advanced Igneous Petrology 2007 As P and T decrease, precipitation of the dissolved elements in veins => pegmatites and aplites. Changes in pH can also cause precipitation => pegmatites and ore bodies on the contacts between contrasted rock types. Mineralized veins Some very incompatible trace elements tends to be concentrated in the last liquids, and then in the fluids (large cations such as Au, U, Sn…). They will eventually precipitate in pegmatitic veins. Hydrothermalism Hydrothermal, mineral-rich fluids can react with solid rocks to form new (metasomatic) rocks. They also tend to be good ore deposits. In the pluton: Simple leaching of the Si out of the rock (high pH conditions): a quartz-depleted rock called “episyenite”. Complex reactions, forming metasomatic rocks (endoskarns). “Argilitization” of the feldspars into micas/clays (low pH, requires H+). Can lead to kaolinite+quartz rocks (“greisens”). Around the plutons: (exo)skarns. They develop best in carbonates, which supply Ca ±Mg while the metasomatic fluids supply Si, Na, K. This forms calc-silicate rocks, typically rich in (clino)pyroxene and garnet, and a lot of weird minerals (with Mn, Sn, U, Cu, …) All these processes create mineral ore (cf. Economic Geology). Departement of Geology, Geography and Environmental Studies