Evolution of nascent mantle wedges during subduction initiation: Li-O isotopic evidence from the Luobusa ophiolite, Tibet. (15th January 2019)

• Record Type: Journal Article
• Authors: Zhang, Peng-Fei; Zhou, Mei-Fu; Robinson, Paul T.; Pearce, Julian A.; Malpas, John; Liu, Qiong-Ying; Xia, Xiao-Ping
• Is Part Of: Geochimica et cosmochimica acta. Volume 245(2019)
• Page Start: 35
• Page End: 58
• Publication Date: 2019-01-15
• Subjects: Luobusa ophiolite -- Subduction initiation -- Slab dehydration -- Asthenospheric upwelling -- High-Cr chromitite -- O-Li isotopes -- Li diffusion; Geochemistry -- Periodicals; Meteorites -- Periodicals; Géochimie -- Périodiques; Météorites -- Périodiques; Geochemie; Astrochemie; Electronic journals; 551.905
• DOI: 10.1016/j.gca.2018.09.037
• Languages: English
• ISSNs: 0016-7037

Abstract

Abstract: The Li-O isotopes of olivine and pyroxene from the mantle sequence of the Luobusa ophiolite, a fragment of Neo-Tethyan forearc lithosphere in Tibet, reveal a series of magmatic processes and geochemical evolution in a nascent mantle wedge during subduction initiation. Olivine grains from the ophiolitic mantle sequence display δ 18 O values similar to those of normal mantle (+5.18 ± 0.2‰), but have large δ 7 Li variations, ∼+4‰ to +13‰ in harzburgites and ∼+1‰ to +11‰ in high-Cr chromitites. Clinopyroxene grains in cpx-bearing harzburgites have finger-like protrusions and LREE-depleted patterns comparable to those of clinopyroxene in abyssal peridotites, and also show high Li concentrations (∼2–8 μg/g) and negative δ 7 Li values (∼−15‰ to −8‰), which are beyond the ranges of normal upper mantle (<1.3 μg/g and +4 ± 2‰, respectively). Our dataset suggests that the nascent Luobusa harzburgitic mantle wedge was penetrated by limited amounts of slab-derived fluids at an early stage, leading to decoupling of high δ 7 Li but normal mantle-like δ 18 O values of the mantle rocks. The hydrated harzburgites were later refertilized by depleted mantle-derived melts. This process generated the cpx-bearing harzburgites with light-δ 7 Li clinopyroxene, and also imposed normal mantle-like Li isotopic signatures on some previously hydrated high-δ 7 Li mantle areas at deeper levels. Subsequent melting in both the high-δ 7 Li and normal mantle-like Li isotopic mantle regimes generated the parental magmas of the high-Cr chromitites with high-Ca boninitic affinities, accounting for the large δ 7 Li variation of olivine in the chromitites. Slab rollback below the nascent mantle wedge induced asthenospheric upwelling and a high geothermal gradient, which played a more direct role than slab dehydration in triggering the magmatic events of refertilization and formation of the high-Cr chromitites. Numerical modeling of inter-mineral Li diffusion in cpx-bearing harzburgites suggests an interval of <1000 years between the two magmatic events and rapid slab rollback before the generation of the high-Cr chromitites. Intra-oceanic emplacement of the Luobusa proto-forearc lithosphere was possibly finished within 10 6 years after the chromitite formation.