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Paper No. 15
Presentation Time: 1:30 PM-5:30 PM
A HETEROGENEOUS AND METASOMATIC MANTLE RECORDED BY TRACE ELEMENTS OF MINERALS FROM DONGHAI GARNET PERIDOTITE IN SULU UHP TERRANE, CHINA
Trace elements in the minerals of Donghai garnet peridotites from the Sulu ultrahigh-pressure terrane, China were analyzed by laser ablation ICPMS. The studied samples include one Zhimafang and four Xugou garnet peridotites, which show porphyroclastic and near-equigranular texture, respectively. The Zhimafang peridotite consists of olivine (Fo91.2-91.8), Opx (En92.0), Grt (Prp66.5-70.7) and Cpx (90.8 Mg#) and minor phlogopite. The Xugou peridotite contains same assemblage but is phlogopite-free and shows higher and more variable Mg# (Fo90.6-92.2, En92.0-92.7, Prp63.2-87.1, Cpx93.7-95.7). Garnets from both peridotites have similar, normal REE patterns (low LREE and flat HREE) but Zhimafang garnet contains higher HREE ((GrtHREE)N=10) and lower LREE than Xugou garnets ((GrtHREE)N=5). Zhimafang diopside shows a sinusoidal REE pattern with flat HREE. Xugou diopside displays a LREE-enriched pattern with large variation in HREE. Diopsides from both peridotites have low abundances of Yb (0.03-0.06 ppm), Y (0.04-0.61 ppm) and Ti (22.5-131 ppm), and low Ti/Eu (517-1158), but have high and variable ratios of Sr/Nd (4800-22000) and (La/Yb)n (40-100). Modeling indicates that the concentrations of moderately incompatible trace elements (such as Y, Ti, Yb and Dy) in diopside fit a fractional melting trend with about 30% partial melting of a primitive mantle source. However, Zr and Gd contents of the Zhimafang peridotite, and highly incompatible trace elements (such as Nb and especially LREE, Sr, Th and U) of both peridotites cannot be described by this model, and are attributed to carbonatitic metasomatism in the upper mantle. The presence of hydrous silicates such as phlogopite and amphibole suggests silica metasomatism. The major and trace element data suggest that the Zhimafang peridotite is less depleted than the Xugou peridotite and both were derived from a heterogeneous mantle prior to Triassic UHP metamorphism.