HR: 15:10h
AN: V32F-07    [PDF]
TI: The Origin of Continental Crust by Intracrustal Differentiation of Basalt in Magmatic Arcs: Trace and Major Element Evidence From Lower Crustal Eclogites Beneath the Sierra Nevada Batholith
AU: * Brill, N E
EM: nebrill@rice.edu
AF: Department of Earth Science, Rice University, MS126, 6100 Main St., Houston, TX 77005 United States
AU: Lee, C A
AF: Department of Earth Science, Rice University, MS126, 6100 Main St., Houston, TX 77005 United States
AB: Continental crust (CC) is defined as that part of the crust that is unsubductable. Based on geologic observations, CC originates from island arcs or from intraplate magmatism (e.g., oceanic plateaus). However, the CC is andesitic in composition whereas island arcs and intraplate magmas are basaltic and represent direct mantle melts. The CC is therefore not mafic enough to be a direct mantle melt. Another fundamental imbalance is the low Nb/Ta ($\sim$12) of CC with respect to primitive mantle (PM) ($\sim$14). Nb and Ta do not fractionate during anhydrous mantle melting, such as beneath mid-ocean ridges. This suggests that during formation of CC, Nb is fractionated from Ta. Key to understanding the origin of CC is finding the "missing" high Nb/Ta and mafic reservoir(s). The two dominant hypotheses solving these chemical imbalances are direct melting of subducted eclogitized oceanic crust to form silicic melts, and intracrustal differentiation of basaltic crust into complementary felsic and mafic components, followed by physical removal of the mafic component. One attraction of the intracrustal differentiation model is that it is viable throughout geologic time, whereas the slab melt model is largely restricted to the Archean. The intracrustal differentiation model, however, is nearly impossible to test if the putative mafic complement to the CC has already been lost as is required by the model. "Eclogite" xenoliths (garnet-clinopyroxenites) erupted in Miocene diatremes in the Sierra Nevada arc, California represent one of extremely few localities where this "missing" mafic complement has been sampled. Here, we show that these "eclogite" xenoliths indeed have the necessary major element and HFSE (Nb, Ta, Zr, Hf) compositions to balance the andesitic composition of CC with respect to a basaltic parent. Ten "eclogites" were analyzed by XRF for major elements and by ICP-MS for trace elements. Preliminary results show that most of these xenoliths have Nb/Ta and Zr/Hf exceeding chondrite and recent PM estimates. On a Nb/Ta vs. Zr/Hf diagram, they lie off the terrestrial array towards anomalously high Nb/Ta. These Nb/Ta ratios are similar to those of Archean oceanic eclogites (AOEs), but differ distinctly in that the AOEs have largely subchondritic Zr/Hf. Mg and Ca concentrations are significantly higher than typical basalt whereas Al and Si concentrations are lower. The Sierran "eclogites" are therefore more mafic than typical basalts in contrast to the currently accepted average composition of the less mafic lower crust. On a MgO versus SiO2 diagram, a simple two-stage model for the origin of CC arises. Crystallization of average Sierran "eclogite" from primitive basalt can drive the composition of the residual melt towards the average composition of bulk CC, which subsequently undergoes further intracrustal differentiation to generate the petrologic differences we see in CC. The lack of Eu anomalies in the Sierran "eclogites" argues against an origin by subsolidus transformation from plagioclase cumulates to garnet-bearing lithologies. Instead, the Sierran "eclogites" represent high pressure cumulates where garnet and clinopyroxene are on the liquidus. We further propose that this "eclogitic" cumulate delaminates or founders back into the convecting mantle, imposing a deficit in mafic component in the CC. Alternatively, the "eclogite" may simply be seismically invisible. Because this simple model can explain most of the geochemical "paradoxes" of the CC and could have operated throughout Earth's history, we suggest that most of the continental crust originates from differentiation of basaltic magmas in arcs.
DE: 1020 Composition of the crust
DE: 1025 Composition of the mantle
DE: 3655 Major element composition
DE: 3670 Minor and trace element composition
DE: 8434 Magma migration
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting