HR: 0800h
AN: V31C-0641 [Abstracts]
TI: Dating Metasomatism in the Lithosphere Beneath North China Craton
AU: * Chen, L
EM: chenlhnju@yahoo.com
AF: Department of Earth Sciences,Nanjing University, Hankou Road 22, Nanjing, 210093
China
AU: Zhou, X
EM: xhzhou@mail.igcas.ac.cn
AF: Institute of Geology and Geophysics, Chinese Academy of Sciences, Box 9825, Beijing, 100029
China
AB:
Dating of mantle metasomatism had been carried out using zircons in metasomatized mantle xenoliths entrained in kimberlites
(Kinny and Dawson, 1992; Rudnick et al., 1999; Konzett et al., 1998, 2000; Liati et al., 2004), because the U-Pb system in
zircon can remain closed at high temperature (>900‘)(Lee et al., 1997). Here we report a SHRIMP U-Pb dating analysis of
zircons from a unique dunite-orthopyroxenite xenolith entrained in Cretaceous high-Mg diorite of Shandong province, which
provides a timing constraint for the multi-stage metasomatism in the lithosphere beneath North China craton (NCC).
Abundant ultramafic xenoliths had been found in the Tietonggou intrusion, one of the Cretaceous high-Mg diorite-dominated
plutons in North China (Chen and Zhou, 2004). The lithology, mineral chemistry, equilibrium temperature (690-790‘), and
metasomatic characteristics of the ultramafic xenoliths indicate that they might be derived from the shallow lithosphere (the
crust-mantle transitional zone or the uppermost lithospheric mantle) and had suffered multi-stage metasomatism (Chen and
Zhou, 2004, 2005). Xenolith LW0006 is the most extremely metasomatized sample found so far in the xenolith suite of the
Tietonggou pluton. The petrography, mineral chemistry, and major element compositions provide a clear metasomatic record of
the composite xenolith: K (and/or Ca) metasomatism, and Si (Na) metasomatism (Chen and Zhou). We found seven zircons range
from 100-170ÝIm in longest dimension, which is reflected in the unusually high Zr content of the bulk rock (49 ppm) of this
sample.
SHRIMP U-Pb dating reveals that these zircons might be grouped three kinds: Mesozoic (concordia age of 127A3 Ma, 5 zircons),
Paleozoic (430-470 Ma, 1 zircon only) and Mesoproterozoic (1310-1540 Ma, 1 zircon only). Cathodoluminescence (CL) images
reveal that a few Mesozoic zircons and the Paleozoic zircons retain oscillatory zoning. The Mesozoic zircons are
characterized with high Th, U contents and high Th/U ratios, 332-8746 ppm, 141-1205 ppm, and 1.40-7.26, respectively.
Paleozoic and Mesoproterozoic zircons have lower contents of Th (70-425 ppm), U (188-820 ppm), and low Th/U ratios
(0.37-0.52). Zircons from the host diorites have medium Th/U ratios, ranging in 0.54-1.32 with a concordia age of
129.4A1.5Ma. The high Th/U ratios of the Mesozoic zircons and the observed oscillatory zoning suggest that they might
crystallize from a melt with high Th/U ratios. Because the partitioning coefficient of U between garnet/melt is higher than
that of Th (Klemme et al., 2002), the slab melts with eclogite residues can have high Th/U ratios. Therefore, the chemistry
of zircons provide new evidence that the metasomatic liquids of the latest metasomatism might be slab melts.
DE: 1115 Radioisotope geochronology
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 3621 Mantle processes (1038)
DE: 8103 Continental cratons
DE: 9320 Asia
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005