HR: 16:45h
AN: V44C-04    [Abstracts]
TI: Continental Crust Formation at Arcs, The "Garnet Pyroxenite" Delamination Cycle, And The Origin of Fertile Melting Anomalies in the Mantle
AU: * Lee, C
EM: ctlee@rice.edu
AF: Rice University, 6100 Main St., Houston, TX 77005 United States
AU: Anderson, D
EM: dla@gps.caltech.edu
AF: Caltech, 1200 E California, Pasadena, CA 91125 United States
AU: Cheng, X
EM: xcheng@rice.edu
AF: Rice University, 6100 Main St., Houston, TX 77005 United States
AB: The total magmatic output in modern arcs, where continental crust is now being formed, is believed to derive from melting of the mantle wedge and is largely basaltic. Globally averaged continental crust, however, has an andesitic bulk composition and is hence too silicic to have been derived directly from the mantle. This imbalance can be reconciled if the parental basalt differentiates into a mafic garnet pyroxenitic residue/cumulate and a complementary silicic melt, the former foundering or delaminating into the mantle due to its high densities and the latter remaining to form the crust. Using the Sierra Nevada batholith in California as a case study, the composition of mature continental arc crust is shown in part to be the product of a cyclic process beginning with the growth of a mafic garnet pyroxenite cumulate layer followed by delamination of this layer and post-delamination basaltic underplating/recharge into what remains of the continental crust. A model is presented wherein continuous arc magmatism and production of garnet pyroxenites in continental arcs is periodically punctuated by a delamination event and an associated magmatic pulse every ~10-30 Ma. The recycling flux of arc pyroxenite is estimated to be no more than ~5-15 % that of oceanic crust recycling by subduction. Delaminated pyroxenites have the necessary trace-element compositions to yield time-integrated isotopic compositions similar to those inferred to exist as reservoirs in the mantle. Because of their low melting temperatures, such pyroxenites may be preferentially melted, possibly forming a component of some hotspot magmas.
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 3640 Igneous petrology
DE: 3904 Defects
DE: 8162 Rheology: mantle (8033)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005