Oxygen Isotope Geochemistry of the Lassen Volcanic Center, California: Resolving Crustal and Mantle Contributions to Continental Arc Magmatism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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Oxygen Isotope Geochemistry of the Lassen Volcanic Center, California: Resolving Crustal and Mantle Contributions to Continental Arc Magmatism

Authors:	Feeley T C; Clynne M A; Winer G S; Grice W C

Institution:	¹Department of Earth Sciences, Montana State University, Bozeman, MT 59717, USA ²United States Geological Survey, 345 Middlefield Road, Mail Stop 910, Menlo Park, CA 94025, USA

Abstract:	This study reports oxygen isotope ratios determined by laserfluorination of mineral separates (mainly plagioclase) frombasaltic andesitic to rhyolitic composition volcanic rocks eruptedfrom the Lassen Volcanic Center (LVC), northern California.Plagioclase separates from nearly all rocks have ${delta}$ ¹⁸O values(6·1–8·4 ${per thousand}$ ) higher than expected for productionof the magmas by partial melting of little evolved basalticlavas erupted in the arc front and back-arc regions of the southernmostCascades during the late Cenozoic. Most LVC magmas must thereforecontain high ¹⁸O crustal material. In this regard, the ${delta}$ ¹⁸O valuesof the volcanic rocks show strong spatial patterns, particularlyfor young rhyodacitic rocks that best represent unmodified partialmelts of the continental crust. Rhyodacitic magmas erupted fromvents located within 3·5 km of the inferred center ofthe LVC have consistently lower ${delta}$ ¹⁸O values (average 6·3 ${per thousand}$ ± 0·1 ${per thousand}$ ) at given SiO₂ contents relative to rockserupted from distal vents (>7·0 km; average 7·1 ${per thousand}$ ± 0.1 ${per thousand}$ ). Further, magmas erupted from vents situated attransitional distances have intermediate values and span a largerrange (average 6·8 ${per thousand}$ ± 0·2 ${per thousand}$ ). Basaltic andesiticto andesitic composition rocks show similar spatial variations,although as a group the ${delta}$ ¹⁸O values of these rocks are more variableand extend to higher values than the rhyodacitic rocks. Thesefeatures are interpreted to reflect assimilation of heterogeneouslower continental crust by mafic magmas, followed by mixingor mingling with silicic magmas formed by partial melting ofinitially high ¹⁸O continental crust ( $~$ 9·0 ${per thousand}$ ) increasinglyhybridized by lower ${delta}$ ¹⁸O ( $~$ 6·0 ${per thousand}$ ) mantle-derived basalticmagmas toward the center of the system. Mixing calculationsusing estimated endmember source ${delta}$ ¹⁸O values imply that LVC magmascontain on a molar oxygen basis approximately 42 to 4% isotopicallyheavy continental crust, with proportions declining in a broadlyregular fashion toward the center of the LVC. Conversely, the ${delta}$ ¹⁸O values of the rhyodacitic rocks suggest that the continentalcrust in the melt generation zones beneath the LVC has beensubstantially modified by intrusion of mantle-derived basalticmagmas, with the degree of hybridization ranging on a molaroxygen basis from approximately 60% at distances up to 12 kmfrom the center of the system to 97% directly beneath the focusregion. These results demonstrate on a relatively small scalethe strong influence that intrusion of mantle-derived maficmagmas can have on modifying the composition of pre-existingcontinental crust in regions of melt production. Given thisresult, similar, but larger-scale, regional trends in magmacompositions may reflect an analogous but more extensive processwherein the continental crust becomes progressively hybridizedbeneath frontal arc localities as a result of protracted intrusionof subduction-related basaltic magmas. KEY WORDS: oxygen isotopes; phenocrysts; continental arc magmatism; Cascades; Lassen

Keywords:	: oxygen isotopes phenocrysts continental arc magmatism Cascades Lassen
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