Abstract

New major, trace and rare earth element (REE) data, and Sr-Nd isotopic compositions of jadeitite and REE-Hf-O isotopic compositions of zircon in jadeitite of the Syum-Keu ultramafic complex of the Polar Urals are used to constrain its origin and source.

The jadeitites have high contents of Na2O (12.80–14.56 wt%), Al2O3 (20.30–23.81 wt%), SiO2 (58.47–59.50 wt%), and are enriched in Sr, Ba, Zr, Hf and depleted in Nb relative to primitive mantle values. Chondrite-normalized REE patterns of the jadeitite display weakly U-shaped distribution patterns, with LaN/YbN ratios of 0.82–2.42, and very weak positive Eu anomalies (δEu = 1.2–1.6). The initial Sr isotopic compositions of the jadeitite range from 0.704000 to 0.703519 (t = 368 Ma), and the initial Nd-isotope ratios (εNd = +0.77 to +5.61) differ from those of ancient ocean water, oceanic sediments and eclogite, metagranite, and metasediments in the nearby Marun-Keu complex.

Zircons from the jadeitite have variable REE contents (37–587 ppm) and are enriched in HREE, with LaCN/YbCN ratios ranging from 0.001 to 0.01, and LuCN/GdCN ratios ranging from 10 to 83. Cerium shows positive anomalies with Ce/Ce* values ranging from 2.8 to 72, and δEu from 0.53 to 1.02. The 176Hf/177Hf ratios of the zircons range from 0.282708 to 0.283017, with initial Hf isotope compositions ranging from 6.5 to 17.4. These characteristics resemble those of zircons from depleted mantle-derived magmas. The δ18O isotope compositions of the zircons range from 5.03‰ to 6.04‰, with an averaged value of 5.45 ± 0.11‰, similar to those of mantle rocks, suggesting that the zircons were acquired from precursor igneous rocks, and then transported and reworked by fluids from the subducting slab. Our new results show that the jadeitite was precipitated from material mainly produced by fluid interaction with mafic-ultramafic rocks in a subduction zone environment.

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