HR: 16:00h
AN: V32H-01    [PDF]
TI: Recycling Lower Continental Crust: Evidence From High Mg Andesites in the North China Craton
AU: Gao, S
EM: gaoshan@cug.edu.cn
AF: Key Laboratory of Continental Dynamics, Dept. Geology Northwest Univ., Xi'an, 710069 China
AU: Gao, S
EM: gaoshan@cug.edu.cn
AF: Faculty of Earth Sciences, China Univ. Geosciences, Wuhan, 430074 China
AU: * Rudnick, R L
EM: rudnick@geol.umd.edu
AF: Geochemistry Laboratory, Dept. Geology, University of Maryland, College Park, MD 20742 United States
AU: Yuan, H
AF: Key Laboratory of Continental Dynamics, Dept. Geology Northwest Univ., Xi'an, 710069 China
AU: Liu, X
AF: Key Laboratory of Continental Dynamics, Dept. Geology Northwest Univ., Xi'an, 710069 China
AU: Liu, Y
AF: Faculty of Earth Sciences, China Univ. Geosciences, Wuhan, 430074 China
AU: Ling, W
AF: Faculty of Earth Sciences, China Univ. Geosciences, Wuhan, 430074 China
AU: Ayers, J
AF: Dept. Geology, Vanderbilt Univ., P.O. Box 105 Station B, Nashville, TN 37235 United States
AU: Wang, X
AF: Faculty of Earth Sciences, China Univ. Geosciences, Wuhan, 430074 China
AB: Magmatic additions to the crust are dominated by basalts, while the continental crust has an andesitic bulk composition. One way to explain this compositional dichotomy is to recycle mafic-ultramafic lower crust into the deep Earth through density foundering. However, such lower crustal recycling is a difficult hypothesis to test. Geological, geochemical and geophysical lines of evidence suggest that the deep lithosphere of the Archaean North China craton was removed at some point after the Ordovician, but the mechanism and timing of this lithosphere removal event is uncertain. Early Cretaceous high-magnesium adakite, andesite and dacite from western Liaoning, North China craton, have chemical and petrographic features consistent with their origin as partial melts of eclogite that subsequently interacted with mantle peridotite: they have fractionated REE patterns ((La/Yb)$_{n}$ = 12 to 36), high Ni (80-300 ppm), Cr (130-400 ppm) and Mg\# (53-65) and contain chromite and orthopyroxene phenocrysts, the latter of which are chemically zoned with Mg\# rising from 75 in the core, to 84 in the interior and falling again to 60-70 on the rim. Similar features observed in adakites and Archaean tonalite-trondhjemite-granodorite (TTG) have been interpreted in terms of slab melts that interacted with the mantle wedge. However, unlike their arc-related counterparts, the western Liaoning magmas carry inherited Archaean zircons and have neodymium and strontium isotopic compositions overlapping those of Archaean eclogite and garnet clinopyroxenite xenoliths derived from the deep lithosphere of the North China craton. Furthermore, the lack of correlation between isotopic composition and Mg\# argues against the old isotopic compositions arising from AFC-type processes in the deep crust. We suggest that these volcanic rocks represent melts from Archaean mafic lower crust that foundered into the convecting mantle, and were hybridized by interaction with mantle peridotite during their upward migration.
DE: 1010 Chemical evolution
DE: 1020 Composition of the crust
DE: 1040 Isotopic composition/chemistry
DE: 3600 MINERALOGY AND PETROLOGY (replaces
DE: 8120 Dynamics of lithosphere and mantle--general
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting