Zircons from rodingite in the Western Tianshan serpentinite complex: Mineral chemistry and U–Pb ages define nature and timing of rodingitization

The Tianshan rodingite was formed from eclogite tectonically included in ultramafic rocks of the Changawuzi ophiolite in the southwestern Tianshan, China. Rodingites derived from eclogite contain a mineral assemblage of prehnite, clinozoisite, hydrogrossular, diopside, vesuvianite, chlorite and minor zircon. Pervasive rodingitization took place during exhumation of the subducted slab under conditions of 250–350 °C and 2–10 kbar. Cathodoluminescence imaging and geochronological characteristics allow the identification of two types of zircons in the rodingite. Type I zircon consists of sector-zoned grains, commonly with distinct rims and cores. A well-defined weighted mean 206Pb/238U age of 422 ± 10 Ma for the rims records an early metamorphic event that affected the Changawuzi ophiolite during the Silurian, and represents the emplacement of the ophiolite complexes along the South Tianshan Paleo-ocean coast. The type I zircon cores reflect variable degrees of alteration during subsequent serpentinization and rodingitization at ∼ 290 Ma. The U–Pb ages of the cores vary from 416 Ma to 301 Ma, implying patchy and incomplete alteration. The type I zircon shows rather uniform, flat rare earth element (REE) patterns, but with a marked increase in total REE in the cores. Type II zircon has a weighted mean 206Pb/238U age at 291 ± 15 Ma, within error of the youngest ages obtained from the cores, and is therefore considered to define the time of pervasive rodingitization. This age is consistent with known regional geological events recorded from the magmatic arc along the southern margin of the Tarim–Yili suture zone and with low P–high T metamorphism to the north, attributed to dehydration of the subducted oceanic slab when the combined Tarim–North China block collided with the Yili–Central Tianshan micro-plate.