Adakit

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Adakite is an igneous rock that is formed from the partial melting of subducted oceanic crust . Most rocks associated with this term can also be referred to using the more general terms dacite or andesite .

The term adakite was first used in 1990 to describe intermediate igneous rocks on the island of Adak (part of the Aleutian island arc ), which are characterized by relatively high strontium and low yttrium and ytterbium contents. Formation models were later developed to match the characteristics of these rocks, which postulated a partial melting of subducted oceanic crust. Thus they did not belong to the lime-alkali magmatism common on island arcs, which is caused by the supply of fluids in the upper mantle .

controversy

To date, there is no uniformly recognized definition of the term adakit . The features and development models described below are suggestions from individual authors. In China in particular, the term and related petrogenetic models are used for rocks in former subduction zones. According to other authors, some of the special features alone are not sufficient for a particular development of an adakit, and can also be explained differently (see below, section Development ).

features

The characteristic features of an adakit resulting from a partial melting of a subducted plate are as follows:

  • a lot of SiO 2 ≥ 56%
  • a lot of Al 2 O 3 ≥ 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 HFSE (Nb, Ta)
  • low ratio 87Sr / 86Sr <0.704

An even more complete overview with slightly different values ​​can be found in Richards & Kerrich 2007.

Emergence

With these characteristics, the starting rock is not yet drained mafic crust, primarily oceanic crust. The melting of such a rock 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 conditions, relatively young (less than 25 million years old) oceanic crust that is still relatively warm could be subducted. This is currently the case on the west coast of South America, for example . Under these conditions, additional frictional heat could generate magmas.

Magmas that were formed by metasomatic changes in the upper mantle over a subduction zone and that have undergone intensive fractionation during the subsequent ascent can also have similar compositions . Mantle magma, which has been contaminated by the mafic lower crust, is used as a third model for the formation of Adakites. Mafic eclogitized lower crust itself is also believed to be a likely source in some cases.

Some authors therefore completely reject the association of the term adakit with a model of recovery and reduce it to parts of the features mentioned above.

meaning

The discussion about the formation of adakites and the use of the term especially for porphyry deposits , in which copper , molybdenum and gold mineralization occur together, is significant, since magmatites with very similar trace element concentrations occur in these deposits (Hohes Sr / Y and La / Yb ratio).

Adakites could also contribute to the growth of the continental crust and thus take the place of the archaic Trondhjemite-Tonalite-Granodiorites (short: TTG), which could have formed similarly.

Individual evidence

  1. ↑ a b c d Jeremy P. Richards: High Sr / Y arc magmas and porphyry Cu ± Mo ± Au deposits: just add water. Economic Geology, November 2011, v. 106, p. 1075-1081, doi : 10.2113 / econgeo.106.7.1075
  2. ↑ Hecai Niu, Hiroaki Sato, Haixiang Zhang, Jun'ichi Ito, Xueyuan Yu, Takashi Nagao, Kentaro Terada, Qi Zhang: Juxtaposition of adakite, boninite, high-TiO2 and low-TiO2 basalts in the Devonian southern Altay, Xinjiang, NW China. Journal of Asian Earth Sciences, Volume 28, Issues 4-6, 15 December 2006, Pages 439-456, ISSN  1367-9120 , doi : 10.1016 / j.jseaes.2005.11.010 .
  3. ↑ Qiang Wang, Derek A. Wyman, Zhen-Hua Zhao, Ji-Feng Xu, Zheng-Hua Bai, Xiao-Lin Xiong, Tong-Mo Dai, Chao-Feng Li, Zhu-Yin Chu: Petrogenesis of Carboniferous adakites and Nb Enriched arc basalts in the Alataw area, northern Tianshan Range (western China): Implications for Phanerozoic crustal growth in the Central Asia orogenic belt. Chemical Geology, Volume 236, Issues 1-2, 15 January 2007, Pages 42-64, ISSN  0009-2541 , doi : 10.1016 / j.chemgeo.2006.08.013 .
  4. ↑ ZH Zhao, XL Xiong, Q. Wang, DA Wyman, ZW Bao, ZH Bai, YL Qiao: Underplating-related adakites in Xinjiang Tianshan, China. Lithos, Volume 102, Issues 1–2, April 2008, Pages 374–391, ISSN  0024-4937 , doi : 10.1016 / j.lithos.2007.06.008 .
  5. ↑ Paterno R. Castillo: An overview over Adakite petrogenesis. Chinese Science Bulletin Vol. 51, Issue 3, Feb. 2006. ISSN  1001-6538 . http://www.dur.ac.uk/yaoling.niu/MyReprints-pdf/PRCastillo-adakite.pdf
  6. ↑ Adakite-Like Rocks: Their Diverse Origins and Questionable Role in Metallogenesis. Jeremy P. Richards and Robert Kerrich Special Paper, Economic Geology 2007 102: 537-576
  7. ↑ Derek J. Thorkelson, Katrin Breitsprecher: Partial melting of slab window margins: genesis of adakitic and non-adakitic magmas . Lithos, Volume 79, Issues 1–2, January 2005, Pages 25–41, ISSN  0024-4937 , doi : 10.1016 / j.lithos.2004.04.049 .
  8. ^ Sun-Lin Chung, et al: Adakites from continental collision zones: Melting of thickened lower crust beneath southern Tibet Geology, November, 2003, v. 31, p. 1021-1024
  9. ↑ Zhihong Wang, Simon A Wilde, Kaiyi Wang, Liangjun Yu, A MORB-arc basalt – adakite association in the 2.5 Ga Wutai greenstone belt: late Archean magmatism and crustal growth in the North China Craton, Precambrian Research, Volume 131, Issues 3 -4, 15 June 2004, Pages 323-343, ISSN  0301-9268 , doi : 10.1016 / j.precamres.2003.12.014 .

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