Li and B isotopic fingerprint of Archean subduction

摘要

Archean peridotite xenoliths in the similar to 2.52 Ga Zhulagou diorite (Yinshan Block, North China Craton) show chemical and Li isotopic evidence for metasomatism above an ancient subduction zone. The peridotite xenoliths are composed of olivine + orthopyroxene + amphibole + phlogopite + serpentine. The peridotite xenoliths have low whole-rock Mg# (80-81) and low Mg# (81-84) in olivine, indicating that they are cumulates that formed near the crust-mantle boundary. Petrological observations, mineral trace element data and isotopic ages show that the sequence of hydrous minerals is amphibole-serpen tine-phlogopite. SIMS U-Pb dating of zircon from peridotites yielded an upper intercept age at similar to 2.53 Ga, and a U-Pb lower intercept age at similar to 1.8 Ga. The age of similar to 2.53 Ga is interpreted to date the crystallization of zircon from the metasomatized mantle melt that formed the Zhulagou cumulate peridotite. Rb-Sr mineral isochrons date phlogopite formation at similar to 1760 Ma, consistent with the lower intercept age of zircon. Pargasitic amphibole from the Zhulagou peridotites has fractionated REE, pronounced depletions of Nb, Ta, Zr and Ti, and heavy delta(7) Li (similar to+14 parts per thousand) and light delta B-11 (similar to 11 parts per thousand). Combined with slightly depleted mantle whole rock epsilon(Nd) (similar to+1.3) and high zircon delta O-18 (+5.6 to +7.0 parts per thousand), the amphibole composition reflects that the peridotite xenoliths formed from melts that carried the geochemical and isotopic fingerprint typical for a metasomatized mantle wedge above a subduction zone. The Zhulagou peridotite xenoliths have the highest delta Li-7 values (similar to+12 parts per thousand) recorded in Archean peridotites. Isotopically heavy Li and light B in olivine, orthopyroxene, and amphibole from the peridotite xenoliths show that Li and B may decouple during partial melting or fluid release from the subducted slab. The decoupling of Li and B may have a variety of reasons, including different host minerals for Li and B in the source and different protoliths in the subducted slab. The Li and B isotopic record on the recycling of ancient material demonstrates that modern-style subduction operated already in the late Archean. (C) 2019 Elsevier Ltd. All rights reserved.