Ancient and Modern Subduction Zone Contributions to the Mantle Sources of Lavas from the Lassen Region of California Inferred from Lu-Hf Isotopic Systematics

Abstract

Hafnium isotopic compositions have been determined on a suite of INTRODUCTION calc-alkaline and high-alumina-olivine tholeiitic lavas from the The geochemical and isotopic variability of mantle wedge Lassen region of California and are used, in conjunction with peridotites beneath continental volcanic arcs is poorly previously published mineralogical, geochemical, and isotopic data, constrained. This is a result of the fact that most of what to constrain their petrogenesis. Positive correlation between Hf is known about the geochemistry of the mantle wedge is values and geochemical indices of the modern subduction component inferred from the composition of basaltic arc lavas, which, indicates that the isotopic compositions of the calc-alkaline lavas although derived from the wedge, are affected by variable record addition of radiogenic Hf from the subducted slab. However, contributions from the subducting slab and continental the addition of the modern subduction component increases the Hf crust (e.g. Gill, 1981; Tatsumi et al., 1986; Hawkesworth values of most calc-alkaline lavas by <0·5 units over estimates of et al., 1993, 1994). Despite these limitations there is non-subduction enriched peridotites of the mantle wedge. The Lu–Hf significant evidence to suggest that mantle wedge periisotopic systematics of the Lassen lavas suggest that the calc-alkaline dotites are heterogeneous in composition and mineralogy. magmas have equilibrated with garnet at some point in their history, For example, it has been postulated that the compositions whereas the tholeiitic magmas have not. These observations require of mantle peridotites beneath the Cascade arc vary from the two lava types to be derived from different sources. The isotopic compositions that are similar to sources for mid-ocean variability of the Lassen lavas cannot be produced by mixing mantle ridge basalt (MORB), Hawaiian ocean-island basalt sources inferred to be present in the eastern–central Pacific and (OIB), and high-alumina-olivine tholeiites (HAOT) from western USA with a modern subduction component. Instead, the the Basin and Range province (Hughes, 1990; Leeman isotopic variability is consistent with mixing of a depleted mantle et al., 1990; Bacon et al., 1997; Borg et al., 1997; Clynne source, a more fertile mantle source enriched by an ancient subduction & Borg, 1997). The restite mineralogy of the source component, and a modern subduction component. regions for Cascade magmas is poorly constrained as well. For example, experiments conducted by Baker et al. (1994) found that calc-alkaline basalts from the Mt. Shasta region of California did not have garnet on their liquidus, and Reiners et al. (2000) were able to model