MULTIPLE METASOMATISM IN SULU ULTRAHIGH-P GARNET PERIDOTITE CONSTRAINED BY PETROLOGICAL AND GEOCHEMICAL INVESTIGATIONS

Abstract

The pre‐pilot hole (PP1) of the Chinese Continental Scientific Drilling Project (CCSD) recovered drill core samples from a 118 m‐thick section of peridotites located at Zhimafang in the southern Sulu UHP terrane, China. The peridotites consist of phlogopite‐bearing garnet lherzolite, harzburgite, wehrlite and dunite. Some peridotite layers contain magnesite and Ti‐clinohumite, and are characterized by LREE and LILE enrichment and HFSE depletion. Phlogopite (Phl) occurs in the peridotite matrix and is LILE‐enriched with low Zr/Hf ratios (0.19–0.60). Phlogopite shows a mantle signature in H and O isotopes (δ¹⁸O: +5.4‰ to +5.9‰, and δD: −76‰ to −91‰). Ti‐clinohumite (Ti‐Chu) is Nb and Ta‐enriched and has higher Ti and HREE concentrations than phlogopite. Magnesite (Mgs) occurs as megacrysts, as a matrix phase, and as veins (±Phl ± Ti‐Chu), and contains low REEtotal contents (<0.3 ppm) with a flat REE pattern. The δ¹⁸O values (+5.5‰ to +8.0‰) of magnesite are in the range of primary carbonatite, but the δ¹³C values (−2.4‰ to −3.4‰) are slightly more positive than those of the mantle and of primary carbonatite. Petrochemical data indicate that the Zhimafang peridotite was subjected to three episodes of metasomatism, listed in succession from oldest to youngest: (1) crystallization of phlogopite in the mantle caused by infiltration of K‐rich hydrous fluid/melt; (2) formation of Mgs and Mgs ± Phl ± Ti‐Chu veins possibly caused by infiltration of mantle‐derived carbonatitic melt with a hydrous silicate component; and (3) replacement of magnesite, garnet and diopside by dolomite and secondary hydrous phases caused by a crust‐related, CO2‐bearing, aqueous fluid. Stable isotopic compositions of phlogopite and magnesite indicate metasomatic agents for events (1) and (2) are from an enriched mantle. Multiple metasomatism imposed on mantle peridotite of variable composition led to significant compositional heterogeneity at all scales within the Zhimafang peridotite.