The geodynamic setting of Dulan eclogite-type rutile deposits in the North Qaidam orogen, western China

Abstract

Abundant eclogite-type rutile deposits have recently been discovered along the North Qaidam ultrahigh-pressure (UHP) metamorphic belt in China. The identified titanium reserves exceed 3 million tons and range in grade from 1.0% to 4.5%. The rutile deposits of economic interest are found within two types of eclogite. However, the origin and original geodynamic setting of the two types of rutile-bearing eclogites remain controversial. This study therefore investigates ore and mineralization bodies belonging to the two types by way of detailed field work and geochronological, geochemical and Sr–Nd–Hf isotopic compositional analysis, taking the Dulan eclogite-type rutile deposit as its case study. Type-1 rutile-bearing eclogites enclosed in granitic gneisses are found to have Neoproterozoic protolith ages of 845 ± 4 Ma and an eclogite-facies metamorphic age of ca. 433 Ma. Their high TiO2, TFe2O3, and P2O5 contents cannot have been produced by rutile-formation during UHP metamorphism but could have arisen through the accumulation of ilmenite and apatite in a high-Ti protolith. They have ocean-island basalt (OIB)-like trace element patterns, with enrichment in light rare-earth elements (LREEs) and high field strength elements (HFSEs) (Nb, Ta, Zr, and Hf). The Hf isotopic compositions of the residual magmatic zircon cores in Type-1 rutile-bearing eclogites (εHf (845) = +1.4 to +6.0, TDM = 1.2–1.5 Ga) indicate that their parental magmas were derived from the Proterozoic mantle. However, they show initial 87Sr/86Sr values of 0.71059–0.71326 and εNd (845) values of −1.5 to −2.5, suggesting significant crustal contamination. Thus, the Type-1 rutile-bearing eclogites may have originated through partial melting of an OIB-type mantle plume source during the Neoproterozoic rifting of the Rodinia supercontinent. In contrast, the Type-2 rutile-bearing eclogites, which are hosted in a mafic–ultramafic ophiolitic complex, have protolith ages of 449 ± 3 Ma and eclogite-facies metamorphic ages of ca. 435–433 Ma. Correlations between the whole-rock MgO and the major and trace element contents of Type-2 rutile-bearing eclogites suggest that they are derived from cumulates with alternating olivine and pyroxene-rich and ilmenite-rich layers. They have mid-ocean-ridge basalt (MORB) and OIB-like trace element patterns, indicating extensive fractional crystallization and the possibility that there were multiple injections of magma during the formation of the protolith. Meanwhile, the +0.6 to +4.3 εNd (449) values and −1.8 to +11.8 εHf (449) values of the Type-2 rutile-bearing eclogites indicate that the protoliths may have been generated by partial melting of a mantle wedge source metasomatized by slab-derived components. The short interval (ca. 14 my) between magma generation and eclogite-facies metamorphism, in combination with the type of rock assemblage within which they are found, indicates that the Type-2 rutile-bearing eclogites originated in a supra-subduction zone (SSZ) ophiolite. Type-1 and Type-2 rutile-bearing eclogites have similar eclogite-facies metamorphic ages of ca. 435–433 Ma, indicating that the Type-1 and SSZ-related Type-2 protoliths were incorporated into a subduction mélange that was dragged down the subduction channel and underwent eclogite-facies metamorphism via subduction of the Qaidam Block, eventually forming eclogite-type rutile deposits.