4. CREATION AND DEATH OF LITHOSPHERIC MANTLE: STUDY OF THE NORTH CHINA CRATON

Archean cratons are underlain by lithospheric mantle that is distinct from off-craton regions: it is generally colder and significantly more refractory. The latter characteristic imparts an intrinsically lower density, and may be a contributing factor to the long-term convective stability of these mantle roots (as evidenced by their ancient isotopic characteristics). A fundamental question in understanding Archean lithospheric evolution and crust-mantle recycling is whether these roots, once formed, are preserved indefinitely, or whether they can be removed. If the latter can be demonstrated, then, depending on the mechanism, their removal may also coincide with loss of the lowermost crust or complete crustal destabilization and thereby be a significant means of crustal recycling.

The North China Craton is perhaps the best example of an Archean craton that was stable for over a billion years, but underwent a significant transformation that resulted in its present state of high heat flow, seismic activity and extensive continental magmatism (which started in the Mesozoic). In the Paleozoic, diamondiferous kimberlites erupted through the craton carrying minerals and xenoliths indicative of the presence of a 200 km thick refractory lithosphere, equilibrated to a cool geotherm. In the Late Cretaceous, refractory peridotitic xenoliths occur side-by-side with with less refractory peridotites in high Mg diorites. These peridotites may indicate the presence of the cratonic root at this time. In contrast, Tertiary intraplate basalts carry fertile mantle xenoliths that record high equilibration temperatures and sample to depths of only 80 km. These observations have been interpreted to reflect the loss of the cratonic root beneath the North China Craton, sometime after the Ordovician. Outstanding questions include: 1) whether all of the cratonic mantle was removed, 2) when it was removed and 3) the mechanism by which it was removed. The current NSF-sponsored project (Rudnick & Walker @ UMd along with Chinese collaborators Gao and Wu) addresses these questions through a detailed petrographic, geochemical and Re-Os isotopic study of mantle xenoliths carried to the surface in the Mesozoic and Cenozoic igneous rocks.

The Re-Os technique is currently the most reliable way to date melt extraction from peridotites, hence lithosphere formation. We are carrying out a systematic characterization of the mantle xenoliths collected in 2007 by electron microprobe, XRF and ICP-MS in order to target samples for Re-Os work. The degree of overprinting via sulfide metasomatism will be quantified by detailed petrography and analyses of sulfides phases, coupled with whole rock PGE analyses. The Os dates for samples from the Late Cretaceous through late Tertiary lithosphere will allow us to determine the extent of Archean lithosphere currently preserved beneath the North China Craton, to define the changes that occurred during the Mesozoic (from xenoliths entrained in Mesozoic magmatic rocks) and to test the diverse hypotheses that are currently invoked to explain the loss of the deep lithosphere.

Mantle peridotite xenoliths from the North China Craton were collected from five locations during a field excursion in May 2007. Participants in this trip are shown here posing behind a large block containing numberous xenoliths at the Yangyuan locale, eastern China.

Xenolith from Fanshi locale (see photo below). Numerous mantle xenoliths were brought to the surface via alkaline basalt eruptions 38-40 million years ago.

“Valley of the Xenoliths”, Fanshi, China. Numerous mantle xenoliths were collected from along this dry stream bed during the 2007 field season.

To learn more about our research concerning the growth and death of lithopsheric mantle, please refer to:

Walker R.J., Carlson R.W., Shirey S.B. and Boyd F.R. (1989) Os, Sr, Nd, and Pb isotope systematics of southern African peridotite xenoliths: Implications for the chemical evolution of subcontinental mantle. Geochim. Cosmochim. Acta 53, 1583-1595.

Wu F., Walker R.J., Ren X-w, Sun D-y and Zhou X-h. (2003) Osmium isotopic constraints on the age of lithospheric mantle beneath northeastern China. Chem. Geol. 196, 107-129.

Lee S. R. and Walker R. J. (2006) Re-Os isotope systematics of mantle xenoliths from South Korea: evidence for complex growth and loss of lithospheric mantle beneath East Asia. Chem. Geol. 231, 90-101.

Wu F-Y., Walker R.J., Yang Y-H., Yuan H-L., and Yand J-H. (2006) The chemical-temporal evolution of lithospheric mantle underlying the North China Craton. Geochim. Cosmochim. Acta 70, 5013-5034.

Yuan H., Gao S., Rudnick R.L., Jin Z., Liu Y., Puchtel I., Walker R.J. and Yu R. (2007) Re-Os evidence for age and origin of peridotites from the Dabie-Sulu ultrahigh pressure metamorphic belt, China. Chem. Geol. 236, 323-338.

Last Revised July 2007