In-situ U-Pb dating and Nd isotopic analysis of perovskite from a rodingite blackwall associated with UHP serpentinite from southwestern Tianshan, China

Abstract

Metamorphic perovskite associated with diopside + chlorite occurs in blackwalls surrounding rodingitized mafic dikes that are included in ultrahigh-pressure (UHP) serpentinized ultramafic rocks from Changawuzi, Chinese southwestern Tianshan. Perovskite forms discrete crystals in the chlorite-rich matrix or appears at the rim of ilmenite. The metamorphic perovskites from the blackwalls are characterized by high concentrations of REE, Nb, Ta, Th and U. The primitive mantle normalized patterns of perovskite are two to three orders of magnitude higher but parallel to the bulk rock patterns, demonstrating that perovskite is a major host of these incompatible trace elements in the ultramafic rocks. The bulk-rock major and trace element compositions of rodingite dikes and associated blackwalls display variable patterns, consistent with different mafic protoliths of the rodingites.Perovskites from three samples were analysed for U-Pb by in-situ secondary ion mass spectrometry. Two samples contained essentially common Pb whereas one sample had enough radiogenic Pb to define a discordia in the Tera-Wasserburg diagram with an age intercept of 317 ± 11 Ma. If all analyses are pooled together, a 238U-206Pb age of 308 ± 5 Ma is obtained. These ages are in agreement with the age of peak UHP metamorphism in Chinese southwestern Tianshan. In-situ Nd isotopes of perovskite from the three samples were obtained by laser ablation multi-collector inductively coupled plasma mass spectrometry. The εNd(t)310 of two samples are 8.37 ± 0.14 and 7.88 ± 0.25, respectively, which suggests that they have similar protoliths. These samples plot close to a 310 Ma Sm-Nd isochron. In contrast, the third sample has a different εNd(t)310 of 0.90 ± 0.51, indicating a completely different protolith. Thus, in-situ analyses of perovskite can provide important information on the protolith as well as the metamorphic evolution of reaction zones between mafic and ultramafic rocks. Show more