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

JOURNAL OF PETROLOGY VOLUME 49 NUMBER 5 PAGES 971^997 2008 doi:10.1093/petrology/egn013 Oxygen Isotope Geochemistry of the Lassen Volcanic Center, California: Resolving Crustal and Mantle Contributions to Continental Arc Magmatism 1 DEPARTMENT OF EARTH SCIENCES, MONTANA STATE UNIVERSITY, BOZEMAN, MT 59717, USA 2 UNITED STATES GEOLOGICAL SURVEY, 345 MIDDLEFIELD ROAD, MAIL STOP 910, MENLO PARK, CA 94025, USA RECEIVED SEPTEMBER 14, 2006; ACCEPTED FEBRUARY 18, 2008 This study reports oxygen isotope ratios determined by laser fluorination of mineral separates (mainly plagioclase) from basaltic andesitic to rhyolitic composition volcanic rocks erupted from the Lassen Volcanic Center (LVC), northern California. Plagioclase separates from nearly all rocks have 18O values (6
1^8
4ø) higher than expected for production of the magmas by partial melting of little evolved basaltic lavas erupted in the arc front and back-arc regions of the southernmost Cascades during the late Cenozoic. Most LVC magmas must therefore contain high 18O crustal material. In this regard, the 18O values of the volcanic rocks show strong spatial patterns, particularly for young rhyodacitic rocks that best represent unmodified partial melts of the continental crust. Rhyodacitic magmas erupted from vents located within 3
5 km of the inferred center of the LVC have consistently lower 18O values (average 6
3ø  0
1ø) at given SiO2 contents relative to rocks erupted from distal vents (47
0 km; average 7
1ø  0.1ø). Further, magmas erupted from vents situated at transitional distances have intermediate values and span a larger range (average 6
8ø  0
2ø). Basaltic andesitic to andesitic composition rocks show similar spatial variations, although as a group the 18O values of these rocks are more variable and extend to higher values than the rhyodacitic rocks. These features are interpreted to reflect assimilation of heterogeneous lower continental crust by mafic magmas, followed by mixing or mingling with silicic magmas formed by partial melting of initially high 18O continental crust (9
0ø) increasingly hybridized by lower 18O (6
0ø) mantle-derived basaltic magmas toward the center of the system. Mixing calculations using estimated endmember source 18O values imply that LVC magmas contain on a molar oxygen basis *Corresponding author. E-mail: approximately 42 to 4% isotopically heavy continental crust, with proportions declining in a broadly regular fashion toward the center of the LVC. Conversely, the 18O values of the rhyodacitic rocks suggest that the continental crust in the melt generation zones beneath the LVC has been substantially modified by intrusion of mantlederived basaltic magmas, with the degree of hybridization ranging on a molar oxygen basis from approximately 60% at distances up to 12 km from the center of the system to 97% directly beneath the focus region. These results demonstrate on a relatively small scale the strong influence that intrusion of mantle-derived mafic magmas can have on modifying the composition of pre-existing continental crust in regions of melt production. Given this result, similar, but larger-scale, regional trends in magma compositions may reflect an analogous but more extensive process wherein the continental crust becomes progressively hybridized beneath frontal arc localities as a result of protracted intrusion of subduction-related basaltic magmas. KEY WORDS: oxygen isotopes; phenocrysts; continental arc magmatism; Cascades; Lassen I N T RO D U C T I O N In the past several decades considerable effort has been devoted to better characterizing the physical and chemical processes affecting magmas in open-system crustal magma chambers (e.g. Bergantz, 1995; Wilson, 1995; Grove, 2000; Spera & Bohrson, 2001; Dufek & Bergantz, 2005; Annen et al., 2006). A large part of this effort has been motivated
The Author 2008. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@ oxfordjournals.org T. C. FEELEY1*, M. A. CLYNNE2, G. S. WINER1 AND W. C. GRICE1 JOURNAL OF PETROLOGY VOLUME 49 MAY 2008 Fig. 1. Tectonic map of the Cascade Range showing areas of late Cenozoic volcanic rocks in stippled patterns after McBirney (1968). Letters next to dots refer to major composite volcanoes and centers: LVC, Lassen Volcanic Center; MS, Mount Shasta; MLV, Medicine Lake volcano; MMc, Mount McLoughlin; CLV, Crater Lake Volcano; NV, Newberry Volcano; TS, Three Sisters; MJ, Mount Jefferson; MH, Mount Hood; SVF, Simcoe Volcanic Field; MSH, Mount Saint Helens; MA, Mount Adams; MR, Mount Rainier; GP, Glacier Peak; MB, Mount Baker, MG, Mount Garibaldi; MC, Mount Cayley; MM, Meager Mountain. Inset shows location of Fig. 2. evaluations of limited datasets from single centers (e.g. Matsuhisa et al., 1973; Blattner & Reid, 1982; Davidson & Harmon, 1989; Singer et al., 1992; Barragan et al., 1998; Pineau et al., 1999). In contrast to whole-rock values, it is now well established that oxygen isotope ratios derived from laser fluorination of separated phenocrysts can avoid problems arising from the susceptibility of glassy volcanic rocks to post-eruptive alteration (Baker et al., 2000; Eiler, 2001; Bindeman et al., 2004). In this paper we build on existing age, petrological, and compositional studies of volcanic rocks erupted from the Lassen Volcanic Center (LVC; Fig. 1), California, by using new oxygen isotope determinations by laser fluorination of mineral separates (mainly plagioclase) to evaluate the proportions of mantle and crustal sources in a continental arc composite volcano as a function of space and time. The LVC is ideally suited for an oxygen isotope investigation because the center has been the subject of detailed studies for well over half a century. As a result, a wealth of data exists on the field relations, compositions, and petrogenesis of the rocks (e.g. Williams, 1932; Eichelberger, 1978; Heiken & Eichelberger, 1980; 972 by the goal of constraining the sources of igneous rocks, as this has important implications for problems such as the mass balance between mantle and crustal contributions to magmas. A key question in this regard is by what means can mantle and crustal contributions to continental arc magmas be resolved? Assessment of magma sources using commonly employed radiogenic isotopic systems (e.g. Sr and Nd) is often difficult in arc settings owing to small compositional contrasts between the magmas and the young arc-related rocks they intrude (e.g. Gill, 1981; Bullen & Clynne, 1990; Davidson et al., 1991; Feeley, 1993; Borg & Clynne, 1998; Hart et al., 2002; Lackey et al., 2005). In contrast, careful examination of oxygen isotope data for rocks at some volcanic centers has allowed identification of shallow and deep crustal differentiation processes and imposed constraints on magma sources and their evolution with development of the centers (e.g. Grunder, 1987; Bacon et al., 198 (...truncated)