2014 GSA Annual Meeting in Vancouver, British Columbia (19–22 October 2014)

Paper No. 75-10
Presentation Time: 3:35 PM

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


DUNGAN, Michael, Department of Earth Sciences, University of Oregon, Eugene, OR 97402, DAVIDSON, Jon, Department of Earth Sciences, Durham University, Durham, DH1 3LE, United Kingdom, FREY, Fred, Department of Earth, Atmospheric, and Planetary Sciences, MIT, Cambridge, MA 02139, HILDRETH, Wes, Volcano Science Center, U. S. Geological Survey, 345 Middlefield Road, MS-910, Menlo Park, CA 94025, LANGMUIR, Charles H., Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, MA 02138, RHODES, J. Michael, Department of Geosciences, University of Massachusetts, 611 North Pleasant Street, 233 Morrill Science Center, Amherst, MA 01003, COOPER, L.B., Geochemistry and Petrology, ETH-Zurich, Zurich, 8092, Switzerland and REUBI, O., Department of Earth Sciences, University of Lausanne, Lausanne, 1015, Switzerland

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.