THE APPLICATION OF U-SERIES DISEQUILIBRIA TO THE ORIGIN OF THOLEIITIC DIFFERENTIATION PRODUCTS IN THE ANDEAN SOUTHERN VOLCANIC ZONE (SVZ): RESULTS FROM LLAIMA AND LONQUIMAY VOLCANOES

Some dominantly mafic volcanoes in a restricted part of the SVZ (37-41° S) have erupted basaltic andesites (52-56 % SiO₂) with high Ti, Fe, and V that are the products of tholeiitic (TH) differentiation related to fractional crystallization and a high proportion of plagioclase in the fractionating assemblage. These volcanoes have not produced voluminous andesite with strong calc-alkaline (CA) affinities during the late Holocene. Abundant CA andesite and dacite at volcanoes from 36.8-33° S are products of variable combinations of relatively high-P (wet) fractionation and crustal assimilation. The most pronounced TH differentiates are young, crystal-poor lavas from Lonquimay (38.45° S: ~2-3 ka to 1989 AD; 53-64 % Si0₂). The most evolved have FeO*/MgO ~10 (comparable to oceanic TH andesites). Major and trace element systematics eliminate open-system processes during differentiation, and U and Pa excesses link these evolved magmas directly to mantle-derived parental basalts. Nine Lonquimay magmas analyzed for U-Th-Pa-Ra activities have (²³¹Pa/²³⁵U) ~1.75-1.85, which is typical of basaltic magmas at Antuco, Llaima, Villarrica, and Osorno (~1.5-2.1). These values extend to the most evolved Lonquimay magmas, verifying an absence of crustal contributions. Minor contamination (up to 5-20%) of some historic eruptive products at Llaima is recorded by major and trace elements and diminished U, Ra, and Pa excesses. Elemental U/Th and (²³⁸U/²³⁰Th) were initially lower at Lonquimay (0.31, ~1.15; 53-64 % SiO₂) than in uncontaminated basalts at nearby Llaima (38.7°; 0.38, ~1.25), in accord with other trace element distinctions between parental magmas. Evolved Lonquimay magmas have Sr-Nd-Pb isotopic compositions typical of mantle-derived, uncontaminated basalts in this part of the SVZ (e.g., ~0.7039-0.7040), which are remarkably similar in terms of Sr and Nd isotopes to local Miocene granitoids. An alternative proposed origin for TH magma series at Zao and Azuma volcanoes in the NE Japan Arc (Tatsumi et al., 2008; Takahashi et al., 2013), as a function of derivation from intrinsically tholeiitic parent magmas produced by partial melting of crustal lithologies under special circumstances, cannot be extrapolated to all TH differentiation trends in continental arcs.