Geodynamics of the North China Craton

In the Palaeozoic, kimberlite magmas intruded the Archean (<3800Ma) and Proterozoic crust of eastern China and entrained garnet-diamond facies mantle, stable at depths of 75-200km. In the Cenozoic similar parts of the crust were intruded by basaltic magmas that entrained spinel-facies mantle (<75 km). This simple observation betrays a change to the nature of the lithosphere in the Phanerozoic and consideration of geological and geochemical data allows us to trace the history of the North China Craton (NCC) over 400 Ma. In the Precambrian the pre-existent Archean lithostratigraphy of the NCC is believed to have been largely intact except for possible movement along major lithospheric shear zones which led to the juxtaposition of different lithospheres. In the Palaeozoic the "cold", refractory, Lithospheric keel extended into the diamond stability field (180-200 km) and a "shield" geotherm predominated (40 mW/m(2)). However Triassic tectonic and minor magmatic activity in the western NCC may mark the onset of a change to that long stable lithosphere, such that by late Jurassic-early Cretaceous, numerous extensional basins had formed across the NCC. Thinning of the Lithosphere is a likely adjunct to such developments. Further evidence of a change to the thermal and tectonic stability of the lithosphere is apparent in the Cretaceous-Tertiary when voluminous tholeiitic magmas were erupted. Extensional alkaline volcanism predominated in the Tertiary-Quaternary providing evidence that the lithosphere may have been hot enough for involvement in magma genesis. Cenozoic volcanic rocks entrained spinel-facies (<75km) xenoliths over several 1000km in eastern China. These xenoliths lie on an "oceanic" geotherm and have Sr-Nd-Pb isotopic geochemistry similar to xenoliths from "oceanic" settings. While these observations are consistent with thinning of the lithosphere since the Palaeozoic, possible accretion of MORE like asthenosphere since the Mesozoic may indicate more recent thickening of the lithosphere. Recent geophysical studies indicate that the present-day lithosphere beneath eastern China is relatively thin (< 80 km) with a shallow low velocity structure and attendant high heat flow (> 100 mW/m(2)).