HR: 1340h
AN: V13B-0528    [Abstracts]
TI: Nevado de Longavi Dacites: Adakite Genesis by Intra-Crustal Fractional Crystallization in a Cold Subduction Zone
AU: * Rodriguez, C
EM: Carolina.Rodriguez@terre.unige.ch
AF: University of Geneva, Dept. Mineralogy, 13 Rue des Maraichers, Geneva, 1205 Switzerland
AU: Selles, D
EM:
AF: University of Geneva, Dept. Mineralogy, 13 Rue des Maraichers, Geneva, 1205 Switzerland
AU: Dungan, M
EM:
AF: University of Geneva, Dept. Mineralogy, 13 Rue des Maraichers, Geneva, 1205 Switzerland
AU: Langmuir, C
EM:
AF: Dept. of Earth and Planetary Sciences, Harvard University, 20 Oxford St., Cambridge, MA 02138 United States
AU: Leeman, W
EM:
AF: Dept. of Earth Science, Rice University, 6100 Main St., Houston, TX 77005 United States
AB: Holocene dacites (5000-7000 y.B.P.; 63-65 wt% SiO₂) of Nevado de Longav'{i} volcano (NLV, 36.2°S, Chile) are the only occurrence of adakitic magmas in the central Chilean Andes, showing high Na₂O (4-4.6 wt%) and low K₂O/Na₂O (0.25-0.33), coupled with high Sr/Y (62-81), La/Yb (10-15), and exceptionally low Y and HREE (Y= 7-9 ppm; Yb=0.6-0.9 ppm). NLV dacites have low abundances of incompatibles elements (K, Rb), inconsistent with crustal assimilation, whereas Sr and Nd isotopic ratios suggest a mantle origin for these melts. The mineral assemblage of these dacites involves plagioclase, amphibole, and minor orthopyroxene and Fe-Ti oxides (cpx absent). Similar assemblages, including high volumes of amphibole (30 vol%) are found in quenched mafic enclaves (53-58 wt% SiO₂) in dacites, suggesting a broadly co-genetic relationship. Adakitic dacites can be derived from hydrous enclave magmas by 50% fractionation in the deep crust (0.46 amph+0.38 plag+0.07 oliv+0.02 cpx+0.07 opx+0.04 mt+0.08 ap+0.015 garnet). This model is consistent with amphibole-rich cumulate xenoliths in NLV lavas, and garnet fractionation at deep crustal pressures (35-40 km) is supported by experimental results on water-saturated arc-basalt melts [1, 2, and 3]. High B abundances in dacites (37-49 ppm) and mafic enclaves (17-33 ppm) relative to other southern Andean magmas support a scenario in which NLV adakitic dacites are related to transient high inputs of slab-derived fluids from the oceanic Mocha Fracture Zone, perhaps by serpentinite dehydration [4]. Fractional crystallization is a viable mechanism for adakite generation in cold-subduction zones where large fluxes of slab-derived fluids occur. [1] Grove, T.; Elkins-Tanton, L.; Parman, S.; Chatterjee, N.; Muentener, O.; Gaetani, G. 2003. Contrib. Mineral. Petrol., 145, p.515-533; [2] Muentener, O.; Kelemen, P.; Grove, T. 2001. Contrib. Mineral. Petrol., 141, p.643-658; [3] Ulmer, P.; Muentener, O.; Alonso-Perez, R. 2003. Geophys.l Res. Abstract, Vol.5, 08308. [4] Ulmer, P.; Tromsdorff, V. 1995. Science, 268, p.858.
DE: 8410 Geochemical modeling (1009, 3610)
DE: 8413 Subduction zone processes (1031, 3060, 3613, 8170)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005