A cut block of trachyandesite lava from a volcano inAuvergne, France, used as building stone, forming part of the walls ofClermont-Ferrand Cathedral, FranceTrachyandesite is field S3 in the TAS diagram
The termtrachyandesite had begun to fall into disfavor by 1985[1] but was revived to describe extrusive igneous rocks falling into the S3 field of theTAS classification. These are divided intosodium-richbenmoreite andpotassium-richlatite.[3]
Trachyandesite is characterized by a silica content near 58% and a total alkali oxide content near 9%. This places trachyandesite in the S3 field of the TAS diagram.[8][3] When it is possible to identify the minerals present, trachyandesite is characterized by a high content of sodic plagioclase, typicallyandesine, and contains at least 10%alkali feldspar.[1] Commonmaficaccessory minerals areamphibole,biotite orpyroxene. Small amounts ofnepheline may be present andapatite is a common accessory mineral.[2] Trachyandesite is not a recognized rock type in theQAPF classification, which is based on the actual mineral content. However, latite is recognized in this classification, while benmoreite would likely fall into either the latite or the andesite fields.[8]
Trachyandesite magmas can have a relatively highsulfur content, and their eruption can inject great quantities of sulfur into thestratosphere.[9] The sulfur may take the form ofanhydritephenocrysts in the magma.[10] The 1982El Chichón eruption produced trachyandesitepumice rich in anhydrite, and released 2.2 × 107 metric tons of sulfur.[11]
Sodium-rich trachyandesite (with %Na2O > %K2O + 2) is called benmoreite, while the more potassic form is called latite.Feldspathoid-bearing latite is sometimes referred to as tristanite.[12] Basaltic trachyandesite is transitional tobasalt and likewise comes in two varieties,mugearite (sodium-rich) andshoshonite (potassium-rich).[8]
^abcMcBirney, Alexander R. (1984).Igneous petrology. San Francisco, Calif.: Freeman, Cooper. p. 503.ISBN0198578105.
^abNeuendorf, Klaus K.E.; Mehl, James P. Jr.; Jackson, Julia A. (2011).Glossary of geology (Fifth revised ed.). American Geological Institute.ISBN9781680151787.
^Donovan, Amy R; Oppenheimer, Clive (March 2011). "The 2010 Eyjafjallajökull eruption and the reconstruction of geography: Commentary".The Geographical Journal.177 (1):4–11.doi:10.1111/j.1475-4959.2010.00379.x.
^Schmincke, Hans-Ulrich (2004).Volcanism. Berlin, Heidelberg: Springer Berlin Heidelberg. p. 262.ISBN9783642189524.
^Carroll, M. R.; Rutherford, Malcolm. J. (1 October 1987). "The Stability of Igneous Anhydrite: Experimental Results and Implications for Sulfur Behavior in the 1982 El Chichon Trachyandesite and Other Evolved Magmas".Journal of Petrology.28 (5):781–801.doi:10.1093/petrology/28.5.781.
^abLuhr, James F.; Logan, M.Amelia V. (September 2002). "Sulfur isotope systematics of the 1982 El Chichón trachyandesite: an ion microprobe study".Geochimica et Cosmochimica Acta.66 (18):3303–3316.Bibcode:2002GeCoA..66.3303L.doi:10.1016/S0016-7037(02)00931-6.
^Philpotts, Anthony R.; Ague, Jay J. (2009).Principles of igneous and metamorphic petrology (2nd ed.). Cambridge University Press. pp. 140–141.ISBN9780521880060.
^Macdonald, Gordon A. (1983).Volcanoes in the sea : the geology of Hawaii (2nd ed.). Honolulu: University of Hawaii Press. pp. 51–52.ISBN0824808320.
