Age and Chemical Structure of Lithospheric Mantle

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 to continental crust (as evidenced by their ancient isotopic characteristics).

One fundamental question in understanding 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.

Mantle peridotite xenoliths from the North China Craton were collected from five locations during a field excursion. Participants in this trip are shown here posing in front of columnar joints at the Tuanshanzi volcano (Changle).

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 less refractory peridotites in high Mg diorites. These peridotites may indicate the presence of the cratonic root at this time. By 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.

Tectonic sketch map of the North China Craton (NCC), which is composed of the eastern NCC block, western NCC block, and the intervening Trans-North China Orogen (TNCO), with cross-cutting Paleoproterozoic fold belts: Khondalite Belt, western NCC, and Jiao-Liao-Ji Belt, eastern NCC (modified from Zhao et al., 2005). NSGL: North–South Gravity Lineament; TLFZ: Tan–Lu Fault Zone (from Liu et al., 2015).

Studies of mantle peridotites have shown that the central-eastern North China Craton experienced lithospheric reactivation in the past, which makes the craton an important location to investigate reactivation processes and mechanisms. Our work specifically has shown that: 1) Peridotites from the western-central North China Craton record a north-south composition and age dichotomy. The northern portion of the central region of the craton experienced lithospheric mantle replacement via a ~1.8 Ga collision associated with amalgamation of the craton. The comparatively Late Archean age between crust and lithospheric mantle in the southern portion of the central region suggests that the cratonization in this region occurred at ~2.5 Ga; 2) Lithospheric thinning and replacement beneath the northern edge of the eastern North China Craton occurred prior to ~100 Ma. Phanerozoic lithospheric thinning and replacement in the eastern North China Craton may have evolved from east to west, or from the margins to the interior of the continent with time in the Mesozoic; 3) Highly fractionated highly siderophile element patterns found in a majority of peridotite suites and characterized by Os, Pd and Re depletions relative to Ir were caused by recent sulfide breakdown via interaction with a S-undersaturated oxidizing melt/fluid.

(left) Xenolith from Fanshi locale. Numerous mantle xenoliths were brought to the surface via alkaline basalt eruptions 38-40 million years ago.
(right) “Valley of the Xenoliths”, Fanshi, China. Numerous mantle xenoliths were collected from along this dry stream bed.

(left) A diversity of primitive-upper-mantle-normalized HSE patterns are evident for whole rock peridotites from throughout the North China Craton region: Hannuoba (a; data from Becker et al., 2006 ; Liu et al., 2010), Yangyuan (b; data from Liu et al., 2010; this study), Datong (c), Jining (d), Fansi (e, high Fo group; f, low Fo group with some samples of Pt and/or Pd anomalies), Hebi (g), and Fushan (h). PUM recommended values (Table 2) from Becker et al. (2006).
(right) Histograms of ¹⁸⁷Os/¹⁸⁸Os of whole rock peridotites from the northern region (a), and the southern region (b, with abyssal peridotites also plotted). The cumulative distribution functions (CDF) of ¹⁸⁷Os/¹⁸⁸Os for peridotites from the northern region of the North China Craton and abyssal peridotites worldwide are shown as an inset of the upper panel. The vertical dashed line is at a value of ¹⁸⁷Os/¹⁸⁸Os = 0.1250 for reference. Both figures are from Liu et al. (2011).

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Last Updated June 2024