HR: 11:05h
AN: V32A-04 INVITED    [Abstracts]
TI: Recycling of mafic lithologies during continent formation
AU: * Lee, C
EM: ctlee@rice.edu
AF: Rice University, 6100 Main St., MS-126, Houston, TX 77005, United States
AU: Hoink, T
EM: tobias.hoeink@rice.edu
AF: Rice University, 6100 Main St., MS-126, Houston, TX 77005, United States
AU: Luffi, P
EM: pluffi@rice.edu
AF: Rice University, 6100 Main St., MS-126, Houston, TX 77005, United States
AU: Lenardic, A
EM: ajns@rice.edu
AF: Rice University, 6100 Main St., MS-126, Houston, TX 77005, United States
AU: Anderson, D
EM: dla@gps.caltech.edu
AF: Caltech, 1200 E. California Blvd, Pasadena, CA 91125, United States
AB: It is well-known that the continental crust is too silicic to have been derived directly as a melt from the mantle, so in order to balance the crust's composition with respect to basalt a missing mafic reservoir is needed. Thus, a multi-stage scenario is required to generate continental crust. First, basalt is extracted from the mantle. The basalt is then differentiated into felsic and mafic components, the latter of which is disposed of via various processes, loosely termed as delamination. Here, we discuss two of several possible crust formation scenarios. One mechanism is that of island accretion followed by further refinement by continental arc magmatism. Phanerozoic examples include the entire Cordilleran margin, extending from North to South America. The parental magmas are basaltic arc magmas generated in the mantle wedge. These arc basalts differentiate (by both fractional crystallization and re-melting of previously underplated basalt) into felsic and mafic lithologies, the former generating the crust and the latter residing in the deep crust in the form of "arc eclogites". The second mechanism is continent formation by underthrusting and stacking of oceanic lithospheres, a process likely to be confined to the Archean. In this scenario, as the underthrusted lithosphere heats up, serpentinized lithospheric mantle dehydrates, allowing the basaltic oceanic crust to undergo hydrous melting and generate felsic magmas. Such magmas rise up to form the crust, but the mafic residue remains in the underthrusted oceanic crust as eclogite. In both scenarios, the mafic residue or cumulate must be removed because there is little evidence that great quantities of these lithologies exist today within continents. In the arc accretion scenario, the mafic lower crust delaminates only after accretion. In the lithosphere stacking model, we propose that the underthrusted oceanic crust is later evacuated along the thrust faults without disturbing the peridotitic sections of the underthrusted lithospheres. The fate of these lithologies is uncertain. We speculate that underthrusted oceanic crust, once evacuated, may sink into the transition zone or deeper. As for the "arc-eclogites", some may heat up and melt soon after delamination because they are already warm. Such "arc-eclogites" would manifest themselves as fertile melting anomalies in the upper mantle.
DE: 1020 Composition of the continental crust
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 8103 Continental cratons
DE: 8104 Continental margins: convergent
DE: 8413 Subduction zone processes (1031, 3060, 3613, 8170)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting