HR: 17:30h
AN: V54A-07    [Abstracts]
TI: Assimilation of Consanguineous Mafic Intrutions: Layered Crustal Sill Complexes as Reactive Filters for Continental Basalts
AU: * Shervais, J W
EM: shervais@cc.usu.edu
AF: Utah State University, Dept Geology, Logan, Uta 84322, United States
AU: Hanan, B B
EM: bhanan@mail.sdsu.edu
AF: San Diego State University, Dept Geological Sciences, San Diego, CA 92182, United States
AU: Vetter, S K
EM: svetter@centenary.edu
AF: Centenary College, Dept Geology, Shreveport, LA 71134, United States
AB: Continental basalts commonly display variations in their chemical compositions that are inferred to reflect fractionational crystallization (FC), recharge-FC (RFC), assimilation-FC (AFC), or recharge-AFC (RAFC). The dominance of AFC-related processes reflects the intrinsic linkage between crystallization (which releases latent heat) and assimilation (which consumes latent heat). One of the central questions in any assimilation process, however, is what exactly is being assimilated. It is commonly assumed in most AFC models for the intrusion of basalt into continental crust that the contaminant is pre-existing continental crust – that is, felsic gneiss of roughly granodioritic to tonalitic composition, which is enriched in K₂O and other large ion lithophiles relative to mantle-derived basalts. These continental gneisses are commonly Precambrian in age and are enriched in the lithophilic isotope ratios ⁸⁷Sr/⁸⁶Sr, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb, and depleted in ¹⁴³Nd/¹⁴⁴Nd. As a result, AFC-related processes involving this ancient continental crust component typically result in basaltic lavas that are enriched in LILE (e.g., K) relative to high-field strength elements (e.g., Ti, P) and enriched in the heavy isotopes of Sr, Pb, and Nd compared to the primary or parental magma. Contrary to these expectations, basalts of the Snake River volcanic province that display chemical variations diagnostic of AFC (e.g., increasing La/Lu with decreasing mg#) are commonly characterized by essentially constant isotopic ratios of Sr, Pb and Nd, and by LILE/HFSE ratios (e.g., K/P) that decrease with decreasing mg#. We propose that these basalts assimilated a ferrogabbro derived from a parent magma that was the same or similar to the magmas being intruded to recharge the system. Melts derived from this ferrogabbro would be low in K and enriched in Fe, Ti, P, and La/Lu relative to the primitive recharge magma; the isotopic composition would be the same as the primitive recharge magma. We infer that this exchange took place within a 10 km thick mafic sill complex that has been imaged seismically at depths of 12-22 km the middle crust. We propose that this process may apply to a wide range of continental basalts.
DE: 1040 Radiogenic isotope geochemistry
DE: 1065 Major and trace element geochemistry
DE: 3640 Igneous petrology
DE: 3643 Layered magma chambers
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting