The origin of Mengyin and Fuxian diamondiferous kimberlites from the North China Craton: Implication for Palaeozoic subducted oceanic slab-mantle interaction

摘要

Concentrations of major, trace, and platinum-group element (PGE) compositions were determined for a suite of the Early Palaeozoic (ca. 465 Ma) Mengyin and Fuxian diamondiferous kimberlites from the eastern North China Craton to constrain the petrogenesis of these rocks and thus the mantle processes involved in their origin. Although the Chinese kimberlites are broadly similar to the kimberlites of South Africa (McDonald et al., 1995), there are significant compositional differences between the Mengyin and the Fuxian fields. The Mengyin kimberlites are more refractory (Mg# > 87, Ni = 1207-1786, Cr = 1991-4224) and more strongly enriched in LREE ((La/Yb)_N = 265-517), but have lower Pd/Ir ratios (1.26-2.08) than those of the Fuxian field. The Mengyin kimberlites possess primitive, mantle-like Sr-Nd isoto-pic ratios. These features suggest that the Mengyin kimberlites formed from a mixture of carbonated asthenosphere and lithospheric keel with a minor contribution from a previously subduction-dehydrated oceanic slab. In contrast, the Fuxian kimberlites are less refractory (Mg# = 80-87, Ni = 971-1411, Cr= 1483-2285), less enriched in LREE ((La/Yb)_N = 45-173) and have higher Pd/Ir ratios (1.59-4.24). Their lower ε_(Nd) values (ε_(Nd) ～ -2.3) and large variations in Sr isotopic compositions suggest significant involvement of hydrothermally-altered and previously subduction-dehydrated oceanic lithosphere. Thus, we consider that the formation of the two kimberlites was related to the previously dehydrated subduc-tion of an oceanic slab beneath the North China Craton in the Palaeozoic. The subduction of Palaeo-Teth-yan oceanic lithosphere in the south and/or subduction of the Mongolian Ocean lithosphere in the north could have initiated the destabilization of the eastern North China Craton. Melts released from the previously dehydrated subducted slab migrated and concentrated in regions of thermally perturbed asthenosphere and at the base of lithosphere keel, which in turn triggered the generation of kimberlitic magmas by a small degree of melting of the carbonated asthenosphere and the lithosphere keel.