HR: 0800h
AN: V41G-1545    [Abstracts]
TI: Crustal Assimilation in Parana Flood Basalts Revisited
AU: * Peate, D W
EM: david-peate@uiowa.edu
AF: Dept of Geoscience, University of Iowa, Iowa City, IA 52242 United States
AU: Peate, I U
V41G-1545 AF: Dept of Geoscience, University of Iowa, Iowa City, IA 52242 United States
AU: Kent, A J
V41G-1545 AF: Dept. of Geosciences, Oregon State University, Corvallis, OR 97331 United States
AU: Rogers, N W
V41G-1545 AF: Dept of Earth Sciences, The Open University, Milton Keynes, MK7 6AA United Kingdom
AU: Rowe, M
V41G-1545 AF: Dept. of Geosciences, Oregon State University, Corvallis, OR 97331 United States
AU: Mantovani, M S
V41G-1545 AF: IAG, University of Sao Paulo, Sao Paulo, SP 05508-900 Brazil
AB: Evaluating the potential role of crustal assimilation in modifying the compositions of flood basalt magmas is critical if we are to better understand the ultimate mantle origins and tectonic driving forces behind these immense episodes of melting. This question is particularly important for flood basalts such as the Early Cretaceous Paraná-Etendeka province where the vast bulk of the erupted lavas have compositions far removed from anything expected from melting of the convecting upper mantle or hot-spot sources. Previous studies have demonstrated that the Paraná-Etendeka basalts, particularly the low-Ti magma types of the southern parts of the province, have experienced extensive assimilation of upper crustal material with radiogenic Sr and Pb isotopes (⁸⁷Sr/⁸⁶Sr > 0.725, ²⁰⁶Pb/²⁰⁴Pb > 19.0) and ε-Nd < -8. The issue remains as to what extent even the least contaminated lavas have assimilated some crustal material or whether their distinctive compositional features derive in some way from the sub-continental lithospheric mantle. A combination of new high-precision trace element analyses and a reevaluation of existing compositional data has revealed new complexities in the assimilation history of low-Ti magmas from SE Brazil. For example, we have recognised a suite of flows within the main coastal escarpment profiles that have relatively unradiogenic Sr and Pb isotopes (⁸⁷Sr/⁸⁶Sr < 0.708, ²⁰⁶Pb/²⁰⁴Pb < 18.5) together with low ε-Nd (<-5). Such signatures are more compatible with assimilation of lower crustal material, and demonstrate that magmas were stalling at different crustal levels throughout the eruption of this lava sequence. We will also present preliminary results from major and trace element analyses of homogenised melt inclusions hosted in crystals (mainly plagioclase) from the most primitive and least crustally-contaminated samples available. We are using the melt inclusions as a window into the earlier stages of magmatic differentiation and crustal assimilation, with the aim of seeing whether any of the melt inclusions have compositional features that require smaller extents of assimilation than are seen in the host lavas. This approach should shed fresh light on the lithospheric mantle versus crustal assimilation debate for the Paraná-Etendeka flood basalts.
DE: 1036 Magma chamber processes (3618)
DE: 1037 Magma genesis and partial melting (3619)
DE: 1043 Fluid and melt inclusion geochemistry
DE: 1065 Major and trace element geochemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005