Geochemical Constraints on the Origin of Volcanic Rocks from the Andean Northern Volcanic Zone, Ecuador期刊界 All Journals 搜尽天下杂志传播学术成果专业期刊搜索期刊信息化学术搜索

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Geochemical Constraints on the Origin of Volcanic Rocks from the Andean Northern Volcanic Zone, Ecuador

Authors:	BRYANT J A; YOGODZINSKI G M; HALL M L; LEWICKI J L; BAILEY D G

Institution:	¹ DEPARTMENT OF GEOLOGICAL SCIENCES, UNIVERSITY OF SOUTH CAROLINA, 701 SUMTER STREET, COLUMBIA, SC 29208, USA ² INSTITUTO GEOFISICO, ESCUELA POLITECNICA NACIONAL (IG-EPN), APARTADO 17-01-2759 QUITO, ECUADOR ³ EARTH SCIENCES DIVISION, LAWRENCE–BERKELEY NATIONAL LABORATORY, 1 CYCLOTRON ROAD, MAIL STOP 90-1116, BERKELEY, CA 94720, USA ⁴ DEPARTMENT OF GEOSCIENCES, HAMILTON COLLEGE, CLINTON, NY 13323, USA

Abstract:	Whole-rock geochemical data on basaltic to rhyolitic samplesfrom 12 volcanic centers are used to constrain the role of continentalcrust in the genesis of magmas formed beneath the anomalouslywide subduction-related volcanic arc in Ecuador. Relativelyhomogeneous, mantle-like, isotopic compositions across the arcimply that the parental magmas in Ecuador were produced largelywithin the mantle wedge above the subduction zone and not byextensive melting of crustal rocks similar to those upon whichthe volcanoes were built. Cross-arc changes in ¹⁴³Nd/¹⁴⁴Nd and ${Delta}$ 7/4Pb are interpreted to result from assimilation of geochemicallymature continental crust, especially in the main arc area, 330–360km from the trench. Mixing calculations limit the quantity ofassimilated crust to less than $~$ 10%. Most andesites and dacitesin Ecuador have adakite-like trace element characteristics (e.g.Y <18 ppm, Yb <2 ppm, La/Yb >20, Sr/Y >40). Availablewhole-rock data do not provide a clear basis for distinguishingbetween slab-melting and deep crustal fractionation models forthe genesis of Ecuador adakites; published data highlightinggeochemical evolution within individual volcanoes, and in magmaticrocks produced throughout Ecuador since the Eocene, appear tosupport the deep fractionation model for the genesis of mostevolved Ecuadoran lavas. A subset of andesites, which displaya combination of high Sr (>900 ppm), ${varepsilon}$ Nd >4·1 and ${Delta}$ 7/4Pb <6·0, appear to be the best candidates amongEcuador lavas for slab-melts associated with the subductionof the relatively young, over-thickened, oceanic crust of theCarnegie Ridge. KEY WORDS: andesite; Ecuador; trace elements; isotopes; adakite

Keywords:	: andesite Ecuador trace elements isotopes adakite
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