Petrology and geochemistry of mafic and ultramafic rocks in the north Tianshan ophiolite: Implications for petrogenesis and tectonic setting

摘要

New petrological and geochemical data for the mafic (pillow basalt, massive basalt, gabbro) and ultramafic rocks (serpentinite, serpentinized harzburgite) in the north Tianshan ophiolite cropping out along the Duku road are combined to constrain the geological evolution of the north Tianshan ophiolite, which marks the boundary between the Yili-central Tianshan and the Junggar terranes. Pillow basalt, characterized by significant enrichments of light rare earth elements (REE) as well as high Nb/Yb and TiO2/Yb ratios, probably represents the seamount fragments accreting in the north Tianshan accretionary prism. Massive basalt and gabbro, displaying flatten REE patterns with low Ti/V ratios, are similar to forearc basalts. Serpentinite and serpentinized harzburgite, belonging to the mantle section of the ophiolite, are depleted in incompatible elements compared to the primitive mantle. The compositions of primary chromian spinel, olivine, and pyroxene in these mantle rocks are comparable to those in forearc peridotites. Such characteristics indicate that the protoliths of serpentinite and serpentinized harzburgite have undergone high degrees of partial melting. They were modified by island arc tholeiitic to boninitic melts during and/or after melt extraction as suggested by the enrichment of Ti in chromian spinel coupled with the occurrence of amphibole inclusions in chromian spinel. These new petrological and geochemical data suggest that the north Tianshan ophiolite was generated in a forearc setting. Retreat and rollback of the north Tianshan subducted slab triggered the upwelling of asthenosphere with minor mass transfer from the slab, leading to the formation of massive basalt and gabbro. Increasing flux of slab-derived fluids/melts largely enhanced partial melting degree of the mantle peridotites, producing island arc tholeiitic to boninitic melts. These melts interacted with refractory mantle residues and formed the mantle rocks (i.e., the protoliths of serpentinite and serpentinized harzburgite) in the north Tianshan ophiolite.