Oxygen isotope geochemistry of the Lassen volcanic center, California;resolving crustal and mantle contributions to continental arcmagmatism

Abstract

This study reports oxygen isotope ratios determined by laserfluorination of mineral separates (mainly plagioclase) from basalticandesitic to rhyolitic composition volcanic rocks erupted from theLassen Volcanic Center (LVC), northern California. Plagioclaseseparates from nearly all rocks have delta (super 18) O values(6.1-8.4ppm) higher than expected for production of the magmas bypartial melting of little evolved basaltic lavas erupted in the arcfront and back-arc regions of the southernmost Cascades during thelate Cenozoic. Most LVC magmas must therefore contain high (super 18)O crustal material. In this regard, the delta (super 18) O values ofthe volcanic rocks show strong spatial patterns, particularly foryoung rhyodacitic rocks that best represent unmodified partial meltsof the continental crust. Rhyodacitic magmas erupted from ventslocated within 3.5 km of the inferred center of the LVC haveconsistently lower delta (super 18) O values (average 6.3ppm+ or-0.1ppm) at given SiO (sub 2) contents relative to rocks erupted fromdistal vents (>7.0 km; average 7.1ppm+ or -0.1ppm). Further, magmaserupted from vents situated at transitional distances haveintermediate values and span a larger range (average 6.8ppm+ or-0.2ppm). Basaltic andesitic to andesitic composition rocks showsimilar spatial variations, although as a group the delta (super 18) Ovalues of these rocks are more variable and extend to higher valuesthan the rhyodacitic rocks. These features are interpreted to reflectassimilation of heterogeneous lower continental crust by mafic magmas,followed by mixing or mingling with silicic magmas formed by partialmelting of initially high (super 18) O continental crust (approximately 9.0ppm) increasingly hybridized by lower delta (super18) O ( approximately 6.0ppm) mantle-derived basaltic magmas towardthe center of the system. Mixing calculations using estimatedendmember source delta (super 18) O 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(super 18) O values of the rhyodacitic rocks suggest that thecontinental crust in the melt generation zones beneath the LVC hasbeen substantially modified by intrusion of mantle-derived basalticmagmas, with the degree of hybridization ranging on a molar oxygenbasis from approximately 60% at distances up to 12 km from the centerof the system to 97% directly beneath the focus region. These resultsdemonstrate on a relatively small scale the strong influence thatintrusion of mantle-derived mafic magmas can have on modifying thecomposition of pre-existing continental crust in regions of meltproduction. Given this result, similar, but larger-scale, regionaltrends in magma compositions may reflect an analogous but moreextensive process wherein the continental crust becomes progressivelyhybridized beneath frontal arc localities as a result of protractedintrusion of subduction-related basaltic magmas.

112 samples from this reference