Geologists apply the terms allochthonous and autochthonous to various different phenomena which have common characteristics. “Allochthonous” refers to something that has formed elsewhere. This “something” can be a huge mountain block that has been pushed tectonically atop of another block. Or it can refer to mineral grains or rock fragments that were previously parts of another rock which have since decayed and liberated the fragmants. These grains were then picked up by running water, carried to another place, and deposited as an allochthonous (or allogenic) sediment.

Conversely, sediments that formed in place are called autochthonous or authigenic. It does not mean that the material these sediments or rocks are made of can not be from an external source (it usually is) but the resulting rock or its particles did not exist in this form as part of a pre-existing rocks. For example, quartz grains in sandstone were already in existence inside granite before this rock weathered and liberated these grains which were then transported to another place and deposited as sand. These quartz grains are allochthonous. However, sodium ions that were part of the same granite were liberated as well but they combined with chlorine ions to become rock salt. This is entirely different material that was not present in granite. Hence, rock salt is said to be an autochthonous chemical sediment.

http://picasaweb.google.com/107509377372007544953/2015#6190953934974355170
conglomerate is a sedimentary rock which is composed of allochthonous material. Width of sample from Norway is 9 cm.

The bulk of sediments are allochthonous. They are usually referred to as detrital or clastic sediments. Common allochthonous sediments are sand, silt, clay, and gravel. On the other hand, the vast majority of the Earth’s upper crust is autochthonous because the upper part of the crust tends to be relatively young and is usually composed of sedimentary rocks which generally have not moved since they were deposited. Things are different deeper in the crust. Metamorphic rocks beneath the thin veneer of sedimentary rocks are usually formed as a result of regional metamorphism which may include several mountain building episodes which tend to move large blocks of the crust out of its original position. However, these rocks are usually so old, often poorly exposed, and may have suffered multiple episodes of metamorphic overprinting which makes it very difficult to understand the bigger picture. It is a better idea to go and see some younger mountain range up-close if you want to see allochthonous parts of the crust.

Are “autochthonous” and “authigenic” (or “allochthonous” and “allogenic”) entirely synonymous? Actually not. There is a slight difference. Authigenic refers to constituents (sand grains and other sediments) rather than whole formations. Therefore, it is more correct to talk about allochthonous crustal blocks and authigenic sediments¹. However, in real life most people are not so pedantic (or correct) and seem to use especially the term “allochthonous” in both situations.

References

1. Jackson, J. A. (1997). Glossary of Geology, 4th Edition. American Geological Institute.

Xenolith is a fragment of foreign rock within an igneous rock. Xenolith itself may be whatever type of rock but its host rock has to be igneous. Foreign rocks in other rock types are usually known as inclusions. “Xenolith” means literally ‘foreign rock’, but some xenoliths are not entirely foreign to their hosts. They may be genetically related e.g. gabbro xenoliths in basalt. Such xenoliths are called cognate inclusions or autoliths. They are related because they both crystallized from the same magma.

http://picasaweb.google.com/107509377372007544953/Tenerife#5846710708669175778
Xenolith of pyroxenite in trachytic host rock. Width of the xenolith from La Palma is 7 cm.

True unrelated xenoliths are always older than their host rocks because they had to already exist as a solid rock fragment when the magma around them solidified. But this is not necessarily true with cognate inclusions.

Many xenoliths are carried up from the mantle. They are therefore very valuable to scientists because such xenoliths are almost the only way to know for sure what the mantle beneath the crust is made of.

Dunite xenolith in basaltic lava from Hawaii. The sample is 8 cm in width.

Diorite inclusion in granodioritic host rock of Sierra Nevada batholith in California. At least some of these inclusions seem to have been partially plastic and therefore are probably genetically related to the host rock. It is possible that darker, more mafic material that started to crystallize at higher temperature did not fully mix with the rest because their margins were chilled by the lower temperature more felsic material surrounding them. The inclusion is 10 cm in length.
http://picasaweb.google.com/107509377372007544953/Tenerife#5846734621769199858
What about this rock sample from La Palma? Is it full of xenoliths? Actually not, these are pyroxene phenocrysts, they are integral parts of the whole. Rocks that contain lots of phenocrysts are said to be porphyritic.

The principle of inclusions states that inclusions found in other rocks (or formations) must be older than the rock that contain them. This is actually pure logic and it can be applied not only in geology, but it is especially useful for geologists.

http://picasaweb.google.com/107509377372007544953/Rocks#5808533379542828898
Contact between kersantite (rare fine-grained igneous rock that contains phenocrysts of phlogopite with other mafic minerals and also feldspars. It is a variety of lamprophyre¹.) and granite. Width of view 16 cm.

Note that there are one larger and several smaller pieces of granite within kersantite. Which one is older then, granite or kersantite? You probably know it already. It is really that simple. We can safely say that granite has to be older. It was already solid rock when it was intruded by mafic lamprophyric magma that scraped some pieces off of granitic rock and embedded them within the solidifying magma. Geologists call it relative dating — we know which one is older but do not know how old they are.

Inclusions of foreign rocks that are found in igneous rocks are named xenoliths. So we can also say that kersantite contains xenoliths of granite. The same principle is also used in relative dating of sedimentary rocks.

It must be noted, however, that this premise holds water only if the inclusions are really made of matter that is foreign to the rock that contains them. True xenoliths are definitely older than their host rocks but sometimes igneous rocks contain cognate inclusions or restite material. S-type granites for example (granite with a sedimentary protolith) may contain such inclusions which are genetically related to its host rock. If this is the case, we can not say that the inclusion is older than the rock that surrounds it.

References

1. Le Maitre, R. W. (2005). Igneous Rocks: A Classification and Glossary of Terms: Recommendations of the International Union of Geological Sciences Subcommission on the Systematics of Igneous Rocks, 2nd Edition. Cambridge University Press.

The word “hydrothermal” is often mentioned in geological texts. Lots of things can be hydrothermal — hydrothermal vent, alteration, circulation, deposits, veins, metamorphism, mineralization. What is this thing? Are they all related phenomena and how important this topic really is? First of all, it really is important because many mineral deposits that are essential resources to mankind are the direct results of these processes. Think about gold, copper, tin, lead, silver, etc. They are mined from hydrothermally altered rocks or from mineral veins where they formed by the crystallization from hot aqueous solutions.

http://picasaweb.google.com/107509377372007544953/Rocks#5786993618287563826
Epidote vein in a granite (unakite). Epidote is a hydrothermal mineral. There is a crack in the middle which allowed the fluids to flow and alter the rock. Arendal, Norway. Width of sample 11 cm.

What is needed to form these valuable deposits?

1. Water

We need water which is a solvent and a transporter of dissolved chemical compounds. Where is this water coming from? It could be water that was expelled from crystallizing magma. It could be rain water that became ground water. It could be seawater that intruded hot magmatic rocks near the mid-ocean ridges. It could be water that was part of hydrous minerals that were metamorphosed to anhydrous phases which liberated the water into the pore space of rocks.

2. Heat

Why do we need it? Because chemical reactions take place much more easily if the temperature is higher. And we need the water to move. As water heats up its density is lower and it rises. Rising water will be replaced by colder water that sinks (hydrothermal circulation). This way water circulates and chemically attacks (hydrothermal alteration or metamorphism) the rocks through which it moves. What provides the heat? It is usually magma in large intrusions.

3. Source material

Rich hydrothermal deposits contain material that had to come from somewhere. It is often magma again which provides reactive fluid that is rich in many chemical elements that were not incorporated into the crystal structure of most common igneous minerals. Chemical elements may also be liberated from the rocks that were altered by the fluid.

4. Fractures

The Earth’s crust is made of rocks. It is not that easy to just go through a solid rock. Thus, we need fractures, faults, or just permeable rocks which allow the fluids to migrate. Therefore, we should not expect to find hydrothermal deposits everywhere. It takes lots of work to find weaknesses in the crust, which might have been used by these hot fluids.

5. Deposition place

Most hydrothermal minerals are found in rocks through which hot fluids migrated. Epidote and muscovite are common silicate minerals that replaced the original minerals in the rocks. Hydrothermal deposits can be found in shallower cracks where conditions were right (lower temperature and pressure) for the deposition to take place. Such deposits are called hydrothermal veins. They usually contain quartz along with ore minerals because fluids usually contain lots of dissolved silica. Gold, for example, is often found in veins with quartz. But hydrothermal deposits may also form on the ocean bottom. These openings through which hot mineral-rich water enters the sea are called hydrothermal vents (black and white smokers). Hydrothermal deposits may also form subaerally in volcanically active areas.

Hydrothermal mineralization is the precipitation of minerals out of the hot fluid. What type of minerals can we expect to find? Really very many and different type, but sulfides tend to be the most characteristic among those that are valuable to us. Pyrite and marcasite (iron sulfides but not very good iron ores because we have better alternatives), acanthite (silver), cinnabar (mercury), chalcopyrite (copper), molybdenite (molybdenum), sphalerite (zinc), galenite (lead), etc.

Several rock types are the results of hydrothermal alteration. Skarn is an altered carbonate rock that contains lots of calc-silicate minerals (silicates that contains calcium like wollastonite, diopside, and some garnets). Greisen is an altered granitic rock that contains lots of mica and many uncommon minerals. Pegmatite is a coarse-grained rock with unusual mineralogy that represents the latest magma to crystallize which is more hydrous and less viscous than the granitic parent magma. Pegmatites are usually not considered to be hydrothermal sensu stricto, but they are closely associated and form the link between magma intrusion and migrating fluids.

Hydrothermal minerals do not form the bulk of the Earth’s crust. You will not see them everywhere. But they are extremely important because they are the natural concentrates of many less abundant chemical elements. Large number of important but less known minerals crystallize out of hydrothermal fluids. Crystals that grow in cracks often have free room to do so. Therefore, the most beautiful specimens with perfect crystal faces usually come from hydrothermal veins and are highly sought after by rockhounds.

http://picasaweb.google.com/107509377372007544953/Cyprus2#5737567993256792210
Silicified umber in Cyprus. It is a rock type associated with black smokers on the ocean floor. The initially light-weight deposit was quickly hydrothermally altered which explains why it contains so much silica.

Volcanic rock (rhyolite) from the Proterozoic. The original composition has been strongly altered. K-feldspar is mostly replaced by albite and minerals like xenotime and fluorite have been introduced by the hot fluids. The gray spots are quartz phenocrysts in a fine-grained matrix of mostly K-feldspar, albite, and quartz. Width of sample 9 cm.