مشرف منتدى جيولوجيا البترول
عدد المساهمات : 108
تاريخ التسجيل : 20/03/2010
العمر : 31
|موضوع: sub solidus textures 23rd نوفمبر 2010, 11:26 pm|| |
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).
I. Mineral transformations
Transformation affecting one single minerals: no reactions, but just re-adjustement of the crystalline system.
A. 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.
.. e.g., quartz polymorphs (or feldspars). Little evidence preserved.
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).
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.
II. 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.
III. 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).
A. 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).
• 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.
B. 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.
Hydrothermal, mineral-rich fluids can react with solid rocks to form new (metasomatic) rocks. They also tend to be good ore deposits.
1. 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”).
2. 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).