Dating Metasomatism in the Lithosphere Beneath North China Craton

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 (690-790A), 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 100-170 Im 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 127-A3 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 129.4-A1.5Ma. 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.