Tertiary Lithosphere Erosion in Eastern China

Suzanne Y. O'REILLY (GEMOC, School of Earth Sciences, Macquarie University, Sydney, NSW 2109, Australia), Xisheng XU, (Dept. of Geology, Nanjing University, Nanjing, China), Andi ZHANG (Ministry of Geology, Beijing, China) and W.L. GRIFFIN (GEMOC and CSIRO Exploration and Mining, P.O. Box 136, North Ryde, NSW 2113, Australia)

The occurrence of Paleozoic (450-490 Ma; Ordovician) kimberlites in Shandong Province, eastern China, and of Tertiary to Recent xenolith-bearing basalts in the same region, provides an opportunity to quantitatively evaluate the thermal state and lithostratigraphy of the subcontinental mantle in two time slices separated by significant tectonic activity. The results indicate the removal of ca 180 km of Archean cratonic lithosphere, and its replacement by a more fertile Phanerozoic lithosphere.

Lithosphere mapping by means of garnet and chromite concentrates from kimberlites has provided a detailed picture of the Paleozoic mantle section beneath Shandong Province, in the central part of the North China Craton. The Paleozoic paleogeotherm lay near a 40 mW/m2 conductive model up to temperatures near 1200°C, corresponding to a depth of 180-190 km. The degree of melt-related metasomatism increases markedly at depths >180 km, and is associated with pronounced heating; both are interpreted as due to the intrusion of asthenosphere-derived magmas. This level is taken as the lithosphere-asthenosphere boundary, implying a lithosphere thickness on the order of 180-190 km. The lithospheric mantle consisted dominantly of lherzolite; the proportion of harzburgitic rocks reaches 40-50% between 130-170 km, and drops off markedly above and below this zone. The lherzolitic rocks, as defined by their garnet compositions, have low cpx/gnt ratios and are strongly metasomatised with introduction of phlogopite at depths >140 km.

The xenoliths in the Tertiary basalts have been studied in detail at the Middle Pleistocene Nushan volcano, north of Nanjing and ca 300 km south of the Shandong kimberlites; Nushan is thought to lie within the southern boundary of the North China Craton. The volcano contains abundant xenoliths of spinel peridotite, and smaller numbers of garnet- and garnet-spinel lherzolites and pyroxenites. P-T estimates for the garnet-bearing rocks place most of them at depths of 50-70 km, and define a geotherm very close to that derived for Tertiary eastern Australian basalt provinces. The geotherm gives much higher temperatures at shallow depths than standard conductive models, implying advective heat transport by intruding magmas. This geotherm would intersect the mantle adiabat at depths of 100±10 km, defining the top of the asthenosphere and the base of the lithosphere. This is equivalent within error to the depth to the regional seismic low-velocity zone (LVZ) in this region. The depth to the LVZ beneath the Shandong kimberlite province is similar, though poorly defined.

The identification of the LVZ as the base of the lithosphere in Pleistocene time suggests that the lithosphere beneath the Shandong kimberlites today is ²100 km thick, compared to 180-190 km in Ordovician time. The contrast implies a lithosphere thinning of 80-90 km since the Paleozoic. The most likely timescale for this thinning is during regional extension in the early Tertiary.

Spinel lherzolites from Nushan are fertile to mildly depleted, and their Mg#-Mg/Si relations are consistent with an origin as residues from basalt extraction at low P (Boyd and Mertzman, 1989); they probably represent underplated oceanic mantle. They are distinct in these respects from the peridotite xenoliths in kimberlites from Archean areas, which are strongly depleted and have high opx/olivine ratios inconsistent with an origin by basalt extraction. The shallow mantle beneath Nushan therefore cannot be a remnant of thinned Archean lithosphere. If such lithosphere did exist beneath this edge of the North China Craton, then all 180-190 km of it has been displaced by mantle typical of oceanic areas.