Fabrics and Origin of Troctolites in the Keketoukeleke Ultramafic–Mafic Complex, South Altyn Tagh, Northwest China

Abstract

Troctolite is relatively rare compared with other ultramafic–mafic rocks, but its origins and rheological deformation are significant for understanding melt–rock interactions and fractional crystallization of mafic magmas. The Keketoukeleke ultramafic–mafic complex in the South Altyn Tagh, northwest China, consists mainly of dunite, coarse- and fine-grained troctolite. Based on petrographic observations, major and trace element variations of the dunites and troctolites, and low olivine Zr/Y and Ti/Y ratios, we propose that the troctolites were formed by fractional crystallization and late localized water-bearing melt injection. In addition, the olivine in the dunites has A- and E-type fabrics, whereas the olivine in the troctolites has a weak fabric and plagioclase has a pronounced fabric with the [010] axes aligned subnormal to the foliation and [100] axes subparallel to the lineation. The results suggest that the troctolite rheological deformation was concentrated mainly in plagioclase, and the olivine only underwent grain rotation or grain boundary slip. Furthermore, the dating of baddeleyite in troctolite suggests they crystallized at 378.6 ± 2.3 Ma, suggesting the Keketoukeleke ultramafic–mafic complex emplaced the continental crust extension after deep subduction–exhumation process of South Altyn.