Abstract
The basic and ultrabasic metavolcanics, occurring within the about 1.0 Ga Kuanping Group in the Qinling orogenic belt, China, have been investigated for bulk-rock major and trace-element data. Studied rocks fall into three geochemical categories: (1) komatiites, resembling Barberton-type Al-depleted komatiites; (2) OIB-like basalts, spatially associated with and geochemically similar to komatiites; and (3) MORB-like tholeiites, resembling mid-ocean ridge and back-arc basin basalts. Combining our observations and recent high-pressure experimental data, we accordingly considered an unusual pressure condition at the depth of mantle transition zone and a low-degree critical melting (5-6%) event of a majorite mantle source for the generation of komatiitic initial melt. The OIB-like basaltic melt may have been produced by another low degree melting event of the komatiite-derived source or by high-pressure segregation of initial komatiitic magma at the depth greater than 300 km, whereas the MORB-like tholeiite by 15-30% partial melting of a depleted shallow spinel-peridotite source. An unusually hot rising plume would have controlled the generation of komatiitic and basaltic magma and the magmatic evolution. This plume magmatism may have occurred either in a marginal ocean basin where the mantle plume would also controlled the development of the marginal basin, or in a major ocean basin which had developed between the North-China and the South China platform at about 1.2 to 1.0 Ga.
