VARIABLE CONTROLS ON OPEN-SYSTEM MODIFICATION OF CONTINENTAL ARC MAGMA; COPAHUE-CAVIAHUE ERUPTIVE COMPLEX, NEUQUEN ARGENTINA

New trace element and isotopic data are presented for pre-caldera to caldera-forming magmas at the Copahue-Caviahue Eruptive Center (CCEC) in the Andean Southern Volcanic Zone (SVZ), Northern Patagonia, Argentina. In detail, pre-caldera lavas record a complex history of both deep and shallow open-system differentiation (to trachyte) of basaltic to basaltic andesite parent magma, although little is known about the nature of compositional variability in SVZ continental crust. At least four episodes of mafic magma-chamber recharge are indicated by sequences of preserved eruptions, volcanic quiescence, and geochemical differentiation indices.

Decreasing Dy/Yb with increasing SiO₂ suggests either anatectic melting with residual amphibole or amphibole fractionation.  Both are consistent with intermediate depths of differentiation and high water contents associated with primitive arc magma. Most CCEC rocks have (La/Yb)_N ~3 to 6 and range in Sm/Hf (~2 to <1.0), but Sm/Hf is always sub-chondritic (< 1.45) when REE patterns are steepest, indicating that Hf may have been incorporated preferentially with respect to REE during ascent and open-system differentiation. Mafic CCEC rocks have nearly constant Zr/Hf while Hf/Yb varies (~1.0 to 2.5), indicating that zircon assimilation has modified HFSE compositions, but not HFSE/HFSE, consistent with an eHf range from ~8.5 to ~7. No zircon was identified petrographically in CCEC rocks, but while typically robust and difficult to re-melt, radiation damage of old grains may increase zircon solubility. The presence of high-concentration brines, particularly if halide-enriched (e.g. Cl^-, F^-), would also increase solubility and help account for high HFSE/REE and low eHf.

Unlike most oceanic arcs, low eHf is coupled with radiogenically depleted eNd indicating that primary partial melts are derived from variably enriched mantle, or assimilation of high-eNd partial melts from refractory cumulates in the lithospheric mantle or lower crust. This is consistent with recent models of lithoshperic underplating and resultant geothermic increase in deep to intermediate crustal levels. Such an increase in temperature effects amphibole sensitivity at intermediate depths and provides a source for solute-rich brines necessary for (at least partial) melting of accessory zircon.