Adakite
Adakit is an igneous rock that is formed during the partial melting of subducted oceanic crust. Most rocks that are associated with this term can also be referred to with the more general terms dacite or andesite.
The first time the term was used Adakit 1990 in order to describe intermediate igneous rocks on the island of Adak (part of the island arc of the Aleutian Islands ), which are characterized by relatively high strontium and low yttrium and ytterbium contents. Later came to fit the characteristics of these rocks formation models, which postulated a partial melting of subducted oceanic crust. So they did not belong to the usual on island arcs lime -alkali magmatism, resulting from an intake of fluids in the upper mantle.
Controversy
To date, there is no uniformly accepted definition of Adakit. The characteristics and formation models described below are suggestions of individual authors. Especially in China find the term and related petrogenetic models for rocks of former subduction zones use. According to other authors, some of the specific characteristics alone are for a specific emergence of a Adakits not, and can also be explained differently (see below, section emergence ).
Features
The characteristic features of a Adakits resulting from a partial melting of the subducted plate are as follows:
- High level of SiO2 ≥ 56%
- Much Al2O3 ≥ 15 %
- Little MgO <3%
- Much Sr > 300 ppm
- No europium anomaly
- Little Y < 15 ppm
- High ratio Sr / Y> 20
- Little Yb ≤ 1.9 ppm
- High ratio La / Yb > 20
- Little HFSEs (Nb, Ta)
- Low ratio 87Sr/86Sr < 0.704
A more complete overview with slightly different values can be found in Richards & Kerrich 2007.
Formation
As a parent rock comes with these features not yet dehydrated mafic crust in question, primarily oceanic crust. The melting of such rocks is relatively rare due to the high melting temperatures, the low geothermal gradient and the geologically rapid drainage of rocks in subduction zones. Under special plate tectonic relatively young ( younger than 25 Ma) oceanic crust could be subducted, which is still relatively warm. This is currently for example, on the South American west coast of the case. Under these conditions could through surcharges apply frictional heat magmas arise.
Similar compositions may also possess magmas originated by metasomatic changes in the upper mantle above a subduction zone, and have experienced an intense fractionation during the subsequent ascent. As a third model for the origin of mantle magma Adakiten will approach attracted, which was contaminated by mafic lower crust. Also eklogitisierte mafic lower crust itself is kept as a source in some cases as a source of likely.
Therefore, some authors reject the connection of the term Adakit with a genesis model completely off and reduce it to parts of the above characteristics.
Importance
The significance of the discussion about the origin of Adakiten and the use of the term, especially for porphyry deposits in which copper, molybdenum and gold mineralization occur associated, as these deposits igneous rocks with very similar trace element concentrations occur ( High Sr/Y- and La / Yb ratio).
Adakite could also contribute to the growth of the continental crust and thus the place of the archaic Trondhjemit - tonalite - granodiorite (short: TTG ) are taking, which could be similarly developed.