Petrogenesis of eclogites enclosed in mantle-derived peridotites from the Sulu UHP terrane: constraints from trace elements in minerals and Hf isotopes in zircon

• Published in: Lithos 2009-05, Vol.109 (3), p.176-192
• Main Authors: Zhang, R.Y., Liou, J.G., Zheng, J.P., Griffin, W.L., Yang, Y.-H., Jahn, B.-M.
• Format: Article
• Language: English
• Subjects: Eclogite; Garnet peridotite; Hf isotope; Sulu UHP terrane; Trace elements; Zircon
• ISSN: 0024-4937; 1872-6143
• DOI: 10.1016/j.lithos.2008.08.002

Garnet clinopyroxenite and eclogite lenses are widespread in mantle-derived garnet peridotites from the Sulu UHP terrane, eastern China. In order to constrain their origin, major and trace element compositions of minerals from 6 eclogites and one garnet clinopyroxenite, and Hf isotopes of zircons from one eclogite, were analyzed by electron probe, LA-ICP-MS and LA- (MC) ICP-MS, respectively. Eclogitic garnets show a large variation in composition (Alm 20–48Prp 24–50Grs 15–38Sps 1), weakly correlated with garnets in their host peridotites. Garnets show LREE depletion and flat HREE patterns, with La n of 0.02–0.52 and Lu n of 5–75. Some garnets show zoning in HREE and Y with a U-shaped, concave-downward profile, but locally exhibit positive sharp variations in HREE and Y contents; such feature reflects disturbance in chemical–physical conditions during garnet growth. Clinopyroxene exhibits humped (mostly) or sinusoidal REE patterns, with La n of 0.3–6.0, Nd n of 2–14 and (La/Lu) n of 1.8–113 with a few exceptions. Partition coefficients of REE ( D) between coexisting clinopyroxene and garnet in eclogite show a large variation: D LREE and D HREE ranging from 10 to100 and from 0.1 to 0.01, respectively, similar to D REE values of the natural systems at 800–1100 °C. The D values (0.1–1) of HREE between Cpx and Grt from Grt clinopyroxenite and one eclogite differ distinctly from the other eclogites, which are consistent with high- T experimental data on garnet lherzolites. Zircons from the Xugou eclogite have no inherent core, which have low U/Th (most < 0.20) and 176Lu/ 177Hf (0.000008–0.0000637), and relative high 176Hf/ 177Hf (0.28241–0.28262) ratios. These features indicate a metamorphic signature, and document that metasomatism affected the eclogite protolith. The model ages ( T DM1) of zircons mostly cluster around 1.0–1.1 Ga, which probably reflects the metasomatic source. Bulk compositions of eclogite and Grt clinopyroxenite differ from those of Sulu eclogites in gneiss and MORB. The protoliths of these rocks may reflect the intrusion of asthenosphere-derived melts in Proterozoic. Eclogites with convex REE pattern and garnet clinopyroxenite were formed by high-pressure crystal accumulation whereas the LREE-enriched eclogites may represent crystallized melt; both eclogite and clinopyroxenite together with their host peridotite were metamorphosed at 700–965 °C and ≥ 4 GPa during the Triassic northward subduction of the Yangtze craton.