(post two of three. The first gives background on metamorphic reactions.)
Starting point: at some period of time in the distant past, a collection of clays, small quartz grains (less than sand sized), maybe some carbonates and oxides collected in a relatively quiet sedimentary basin. If we were to sample the sediment, it would feel like mud in our hands (though it might be slightly gritty if we tasted the seds).
Let’s take a moment and looks at the composition of our components:
- quartz (maybe also some chert) – SiO2
- clays: usually kaolinite – Al2Si2O5(OH)4, montmorillonite - (Na,Ca)0.33(Al,Mg)2(Si4O10)(OH)2·nH2O, and/or illite - (K,H3O)(Al,Mg,Fe)2(Si,Al)4O10[(OH)2,(H2O)]
- carbonates: normally calcite / aragonite – CaCO3, though you may have some ankerite – FeCO3 or dolomite – CaMg(CO3)2
- oxides: hematite – Fe2O3, limonite – FeO(OH)·nH2O and/or goethite - FeO(OH)
- other random things that may or may not be present: albite – NaAlSi3O8, organic material – C + H2O
What this boils down to is that our quiet depositional environment seds are Si + Al + water-rich with some K, Ca, Na, Fe, Mg, CO2 mixed in. Whatever minerals are going to be stable in the future of this rock, they’ll probably be Si + Al-rich.
The first event to occur is more sediment being deposited on top of “our” seds. This new weight forces compacts the seds, driving off some water +/- carbon dioxide, reducing the porosity of the rock, and increasing the density. If you look at the image above, the clays on the left are “looser” and on the right “denser” due to their placements (towards top & towards bottom) within the core.
(Here’s a link to a great animation of a filling basin over time to give you an idea of what occurs over time within a depositional environment.)
As more & more sediment is deposited in the basin, our seds are compacted more & the temperature starts to gradually go up simply due to burial. More than driving off water, though, some of our clays (and some of the other random minerals such as goethite, aragonite, etc.) become unstable. At this point, usually the clays are replaced by other clay minerals that are slightly more dense and contain a bit less water. If aragonite was present, calcite may form. Goethite would be replaced by an anhydrous oxide such as hematite.
The further compaction, driving off of water / carbon dioxide, and recrystallization of some of the phases takes our loose seds and turns them into a sedimentary rock–in this case, a shale. Shales are still very, very fine grained so that individual minerals are not visible to the naked eye. They are also “fissible” or form in thin layers along which they are easy to break in relatively smooth planes.
If our basin simply continues to fill, our shale may lose more water / carbon dioxide, but it won’t become “interesting” (at least to a metamorphic petrologist). In order to take that next step, we need to either heat the rock by intruding a pluton (igneous body) next to the shale or involve the shale within an orogenic event that will increase both the P & T.
Let’s first deal with contact metamorphism:
The rocks within the contact aureole will be heated up with higher temperatures near the igneous body & lower temperatures further away. Frequently, there is a minimal P change associated with the intrusion of an igneous pluton, but it may cause a differential pressure field.
Normally the changes from unmetamorphosed to low-T contact metamorphism to moderate-T to high-T are gradual and may be difficult to pin down exactly to as easily identifiable line in the field. Detailed work with samples made into thin sections is usually need to pin down exactly where a new mineral becomes stable (in-isograd). The rock may either be unfoliated (no alignment of the grains due to a differential stress) or foliated.
If unfoliated, we’ll see a sequence of “hornfelses” in which our fine grained shale will become more & more coarse grained. The other main characteristic as the temperature increases is that the rock will lose more & more water / carbon dioxide. The clays / carbonates / oxides may no longer be stable and instead plagioclase, chlorite, muscovite &/or biotite will form. Quartz is still stable. (Just as a quick check – micas + quartz + plag are Si + Al rich with some Fe + Mg + Ca + Na, so compositionally we’re on track.)
As the temperature continues to go up, the rock become more & more anhydrous and instead of micas (chl, bt, mu) either anhydrous minerals (e.g. plagioclase, andalusite, garnet, sillimanite, K-feldspar) or minerals with only very low amounts of water (e.g. cordierite, staurolite) become stable. Note the increase in grain size below.
How hot the contact aureole will get (and therefore how high grade of metamorphism will be present) depends on a few things:
- what is the temperature of the igneous body itself? granites will be colder than diorites or gabbros
- how large is the igneous body? small bodies will lose their heat quickly & therefore won’t be able to heat as large a region or to as high a temperature
- is there free fluid in the system? fluid-flow convection around a pluton transfers heat much more efficiently than simple conduction–wider contact aureole that may reach higher temperatures simply because the heat reaches the country rocks before the pluton can cool off much
- how deep within the Earth is the system? shallow intrusions cool off much quicker because the surrounding country rocks are colder and absorb the heat almost instantly (in geological timescales), deeper intrusions have a less drastic temperature differential between the pluton & the country rocks and allow for a slower cooling of the igneous material and a protracted timing of metamorphism — also if the rocks are warmer to start with, it doesn’t take as much heat to bump them up to a higher grade of metamorphism
If the conditions are right, the rocks directly in contact with the pluton may start to melt and form migmatites. I’m going to talk about migmatites at the end of the regional metamorphism post, so you’ll have to wait a sec…