Basaltic andesite

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Basaltic andesite from Parícutin volcano in Mexico

Basaltic andesite is a volcanic rock that is intermediate in composition between basalt and andesite. It is composed predominantly of augite and plagioclase. Basaltic andesite can be found in volcanoes around the world, including in Central America[1] and the Andes of South America.[2]

Description

TAS diagram for classifying volcanic rock, with the basaltic andesite field (O1) highlighted

Basaltic andesite is a fine-grained (

plagioclase feldspar, and feldspathoids, but would fall in the basalt-andesite field.[3] This corresponds to rock in which feldspathoid makes up less than 10% and quartz less than 20% of the total QAPF fraction, and in which at least 65% of the feldspar is plagioclase.[4] Basaltic andesite would be further distinguished from basalt and andesite by a silica content between 52% and 57%.[5]

Although classification by mineral content is preferred by the

IUGS, this is impractical for glassy or very fine-grained volcanic rock, and then the chemical TAS classification is used. Basaltic andesite is then defined as volcanic rock with a silica content from 52% to 57% and a total alkali content (K2O plus Na2O) of less than about 6%, corresponding to the O1 field in the TAS diagram.[4]

A basaltic andesite (or andesite) enriched in

TiO2, is termed a boninite.[6]

Occurrence

Basaltic andesite in the Cascade Range

Basaltic andesite is found in volcanic arcs, such as those of Central America,[1] the Andes of South America,[2] and the Cascades of North America.[7]

Basaltic andesite is also common where the Earth's crust is undergoing extension. For example, mid-Cenozoic volcanic sequences in western Mexico, southwestern New Mexico, and southeastern Arizona are capped by basaltic andesite of the Southern Cordilleran Basaltic Andesite (SCORBA) suite, which may be the most extensive Cenozoic mafic suite in North America.[8] Mount Mazama, a large composite volcano in south-central Oregon, consists of several overlapping basaltic andesite shields.

Basaltic andesite is a component of back-arc basin basalt suites.[9] The Columbia River Basalt Group is composed of 80% basaltic andesites on the basis of silicon content.

Thick sequences of basaltic andesites were erupted in a shallow-marine environment during the

Kaapval craton lithosphere.[10]

The Mars Global Surveyor has found evidence for basaltic andesite on Mars.[11]

References

  1. ^ a b Jordan, B.R.; Sigurdsson, H.; Carey, S.; Lundin, S.; Rogers, R.D.; Singer, B.; Barquero-Molina, M. (2007). "Petrogenesis of Central American Tertiary ignimbrites and associated Caribbean Sea tephra". In Mann, Paul (ed.). Geologic and tectonic development of the Caribbean plate boundary in northern Central America. Geological Society of America. p. 161.
  2. ^ a b Kay, Suzanne Mahlburg; Copeland, Peter (2006). "Early to middle Miocene backarc magmas of the Neuquén Basin: Geochemical consequences of slab shallowing and the westward drift of South America". In Kay, Suzanne Mahlburg; Ramos, Victor A. (eds.). Evolution of an Andean margin: a tectonic and magmatic view from the Andes to the Neuquén Basin (35 degrees-39 degrees S lat). Geological Society of America. p. 209.
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  9. ^ Gaetani, Glenn A.; Grove, Timothy L.; Bryan, Wilfred B. (1994). "Experimental phase relations of basaltic andesite from hole 839B under hydrous and anhydrous conditions". Proceedings of the Ocean Drilling Program, Scientific Results. 135.
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