The Extremely Depleted Peridotites in Sulu UHP Belt: Geochemistry of PP3 Ultramafic Rocks

PP3 ultramafic massif is located in the east of Sulu UHP belt, as a third pre-pilot hole of Chinese Continental Scientific Drilling (CCSD) Program; PP3 had drilled 705.45 m depth. Ultramafic rocks about 480 metres sandwiching eclogites and gneisses are revealed. Coesite occurred as an inclusion in omphacite, which means the eclogite ever metamorphosed in the UHP condition. Ultramafic rocks include garnet peridotite, dunite and serpentinite with few hargzburgite; contain olivine, chromium spinel, clinopyroxene, or/and garnet, and few orthopyroxene. About 55 ultramafic rock samples are analyzed systematically, all the samples in PP3 peridotites concentrate in hargzburgite area in the modal mineralogy compositions. SiO₂ content of PP3 peridotites varies from 37% to 43.16%, Al2O3 from 0.24% to 1.03%, MgO from 38.7% to 45.4%, Cr₂O₃ from 0.27% to 0.41%£»NiO from 0.26% to 0.39%. Most of CaO and Na₂O values from under detection to 0.29% and 0.32% respectively. These samples characterize relatively lower contents of CaO, Al₂O₃, TiO₂, Na₂O, K₂O and FeO and higher contents of MgO, compared to the depletion trend. Mg# (Molar Mg/(Mg+Fe) A100) is generally high (92 to 93) and Cr# (Molar Cr/(Cr+Fe) A100) is also high (30 to 45), and variable in a small scale. Garnet-bearing peridotites have the same Mg# and Mg/Si ( ∼ 1.6) as garnet-free peridotites, indicating that they are residues of high degrees of partial melting (between 35-50%), which demonstrating depleted mantle. PP3 peridotites were extremely depleted, not only major and trace elements, but minerals as well. Mg# of olivine is about 92.8, Cr# of chromium spinels is from 60 to 90.Compared to previously published compilations of the average composition of either spinel lherzolites, garnet lherzolites or spinel harzburgites, The compositions of PP3 peridotites is significantly lower CaO and Al₂O₃, and higher Mg#, suggesting that they are the extremely depleted ultramafic rocks in east part of China. PP3 peridotites show huge variations in rare earth element (REE) abundances (from ∼ 0.07 to ∼ 0.53 times chondritic values£¬the average value is 0.18, and exhibits depleted, but smooth chondrite-normalized REE patterns, which suggests that REE compositions of PP3 garnet peridotites and dunites are severly depleted. (La/Yb)_N ratio is from 6.9 to 51.2£¬the average is 16.1, which shows slightly rich in LREE. PGE in PP3 peridotites is higher than mantle nodule and orogenic peridotites, but less than one-two order of magnitute comparing with chondrite. (Pd/Ir)_N varies from 0.11 to 0.92, the average is 0.52, (Rh/Ir)_N from 0.32 to 0.91£¬the average is 0.61 and (Pt/Ru)_N varies from 0.02 to 0.54, the average is 0.09. Their PGE patterns are similar to chromite PGE patterns in the world. The relatively incompatible elements Pd and Pt are more depleted than that of pyrolite, represents the most depleted mantle and is apparently different from other ultramafic rocks in the east part of China.