17 Emplacement of Plutonic Rocks

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Geology - Emplacement of Plutonic Rocks
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  Emplacement of plutonic rocks  I.    Driving forces for magma ascent Magma is formed at depth (typically 20 km for granites, more for mantle-derived rocks). What allows it to rise to the surface? A.   Buoyancy Magma is typically lighter than the crustal rocks and can “float” upwards. This is  particulary true for felsic magmas, and even more for H2O-rich magmas. On the other hands, basalts will have more difficulties to rise (and will possibely pond at the base of the crust). B.   Tectonic overpressures Deformation and tectonic can cause local overpressures (cf. migmatites) and squeeze the magma out. Whether this can be a significant process or not is discussed… Increasing evidences for magma transfert in shear zones (on a crustal scale) C.   Why do some magmas stay trapped below the surface?    “Neutral buoyancy”: magma rise until they reach a level where they are in equilibrium with the country rock. This can be at different levels depending on the nature of the magma (and the crust): base of the continental crust for  basalts in continental settings, somewhere into the crust for continental granites … or above the surface for oceanic rocks!    Lack of pathways, hindering the upwards movement    Rheological interface acting as a trap (effectively the same idea as above)    Early cooling (cf. shape of granite solidus)  II.    Magma transfert in the crust: dyke vs. diapirs What are the physical ways of magma movement? Three possibilities are considered:  bulk movement (diapirs of magma); infiltration/movement via dykes; and movement helped by deformation (into active shear zones/faults). A.   Diapirism (in the lower crust?) Historically the first model used. Based on    Field observations (sheared margins around plutons)    Experiments (silicone models, Ramberg). 1.   Salt diapirs In fold belts, diapirs of lighter salt in anticline cores are observed to rise through the cover. This shows that the process can be efficient.  2.    Diapiric plutons(?)  Many granitic plutons were thought to be diapiric, on the basis of their field relations. However, the viscosity of the upper crust is simply to high to allow for easy diapirism. It must be restricted to the lower crust, more ductile.  3.    Diapiric migmatite domes Migmatitic complexes ( ≈  metamorphic core complexes) do rise diapirically, or as tectonic-initiated domes srcinating in an extension situation. They can rise quite efficiently, especially if extension is present. But this seems to be more viable in the lower crust… or in very hot upper crust (therefore ductile) (post-orogenic core complexes, cf. Eastern Mediterranean). B.   Upper crustal plutons 1.    Dyke-fed plutons and ballooning Upper crustal plutons appear to be flat-floored, sometimes with a feeder zone. This reflect the strain field during their emplacement. They were probably fed from below by dykes, tapping the magma into the pluton. These dykes are not always seen in the field (probably sealed by tectonics afterwards). The plutons therefore “inflate” in situ, not unlike an inflating balloon… The problem is, the pressure in the magma is probably not sufficient to open the space.  2.   The “room problem” How to make space to accommodate the pluton? -   Assimilation of country rock (will actually not solve the problem…) -   Bulging of the surface (seen in volcanoes, quite likely the ultimate process) -   Deformation/tectonic movement (cf. experiments on fault-related plutons, especially in transtension).  3.    How to open “voids” in the crust? Magma overpressure is probably not enough (actually, it would probably make ring and cone dykes, see below…). Tectonics seems the only viable option. Numerous examples of fault-related  plutons… C.   The role of deformation Exploiting shear zones is an alternate way to bring the magmas to the surface. It’s  basically quite similar to dykes, except that tectonic deformation in the shear zone will help to build the pressure and move the magmas upwards. At high structural levels, the shear zones will turn into faults, that can also be associated with pluton emplacement.  On the field, this will result in close fault-granite association, either as melt-injected (and melt-lubricated!) shear zones in the deep crust, or as fault-related plutons in the upper crust. D.   Ring dykes: transition towards volcanoes In sub-volcanic environements (brittle crust), dyking is the only possible alternative. A common feature is “ring dykes”, associated to caldera collapse and major volcanic eruptions (“cauldron subsidence”).
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