TRACE ELEMENT SYSTEMATICS PROVIDE IMPORTANT INSIGHTS INTO THE ROLE OF CRUSTAL ASSIMILATION DURING THE FORMATION OF SOME CALC-ALKALINE DIFFERENTIATION SERIES

Experimental studies have shown that P, H₂O, and oxidation state play key roles in determining major element trajectories of differentiated magma series derived from subalkaline parental basalts; i.e., fractional crystallization of water-rich and oxidized magmas produces calc-alkaline (CA) differentiation trends. Bulk assimilation of granitoids or mixing with partial melts of metavolcanic rocks also raise SiO₂ and alkalies, whilst suppressing an increase in FeO*/MgO. Andesitic to dacitic magmas of the late Cenozoic Taos Plateau volcanic field (Rio Grande Rift, NM) project into the Miyashiro (1974) CA field despite being derived from contemporaneous low-K olivine tholeiite parent magmas. Evolved Taos Plateau magmas contain substantial crustal contributions (Pb-Sr-Nd isotopic variations). Conversely, variably evolved young magmas at the subduction-related volcano Lonquimay (38.45° S, Chilean Andes) define one of the most pronounced tholeiitic (TH) trends on the planet (FeO*/MgO ~10 @ 65% SiO₂), and these can be shown by both trace element and U-series systematics to have evolved by closed-system fractional crystallization. The CA-TH distinction is not entirely pre-ordained by tectonic setting. “Excess” increases in K-Rb-Ba-Th-U relative to Y-HREE-HFSE, as SiO₂ increases in progressively evolved and contaminated magmas, mimic elemental abundances in the crust relative to most mantle sources. Strong correlations between major element CA trends and co-magmatic increases in lithophile trace elements that substantially exceed those which can be generated by closed-system fractionation will be illustrated by a series of non-traditional plots, in the context of along-strike changes in major and trace element differentiation trends at frontal arc volcanoes of the Chilean Andes (SVZ: 33-41° S), wherein both crustal thickness and the tendency for crustal assimilation increase northwards. Open-system contributions to evolving magmas do not contradict the consequences of phase equilibria in wet, oxidized magmas, but a high rate of assimilation will lead to the tendency for magmas to form CA series in cases where TH or ‘transitional’ CA-TH differentiation might otherwise be the outcome.