Abstract :
A garnet–pyroxene rock containing abundant Ti-clinohumite (ca. 40 vol.%) occurs along with eclogites as small blocks in quartzo-feldsparthic gneiss in the southern end of the Chinese Su-Lu ultrahigh-pressure (UHP) metamorphic terrane. It consists of three aggregates: (1) Ti-clinohumite-dominated aggregate with interstitial garnet and pyroxene, (2) garnet+pyroxene aggregate with Ti-clinohumite inclusions, and (3) Ti-clinohumite-free aggregate dominated by garnet. Apatite, phlogopite, zircon, hematite, pentlandite, and an unknown Ni-Fe-volatile-Si (NFVS) mineral, which is replaced by Ni-greenalite, occur as accessories. Serpentine is the major secondary mineral. Garnet (Prp63.9–64.6Alm25.8–26.9Grs1.4–7.9Uva0.5–7.6Sps1.0) in all three aggregates is pyrope-rich with very low grossular component, with that in the aggregate (2) most enriched in Cr (Cr2O3=2.55 wt.%). Orthopyroxene is depleted in Al (Al2O3=0.16 wt.% in the cores) and Ca (CaO=0.06–0.09 wt.% in the cores), with XMg (Mg/(Mg+Fe)) values at ca. 0.900. Clinopyroxene is chromian diopside with Fe3+≥Fe2+. Matrix clinopyroxene has a lower XMg (0.862) than that (0.887) included in Ti-clinohumite. The rock contains modest amount of heavy rare earth elements (HREE) (10 to 12×C1 chondrite), with significant enrichment in Cr, Co, Ni, V, Sr, and light rare earth elements (LREE) (22 to 33×C1 chondrite). The clinopyroxene is very enriched in Cr (Cr2O3 is up to 2.09 wt.% in the cores) and Sr (ca. 350 ppm) and LREE (CeN/YbN=157.7). Ti-clinohumite is enriched in Ni (1981 ppm), Co (123 ppm), and Nb (85 ppm). While it is possible to enrich ultramafites in incompatible elements in a subducted slab, the high Al, Fe, Ti, and low Si, Ca, and Na contents in the Ti-clinohumite rock are difficult to account for by crustal metasomatism of an ultramafite. On the other hand, the similarity in major and trace element compositions and their systematic variations between the Ti-clinohumite-garnet-pyroxene rock of this study and those of Mg-metasomatised Fe–Ti gabbros reported in the literature suggest that crustal metasomatism occurred in a gabbroic protolith, which resulted in addition of Cr, Co, Ni, and Mg and removal of Si, Ca, Na, Al, and Fe. This implies that the rock was in contact with an ultramafite at low pressure. During subsequent subduction, the metagabbro was thrust into the country gneiss, where gneiss-derived hydrous fluids caused enrichment of Sr and LREE in recrystallised clinopyroxene. P–T estimates for the high-pressure assemblage are ca. 4.2 GPa and ca. 760 °C, compatible with those for the eclogites and gneisses in this terrane. It is possible that the Ti-clinohumite-garnet-pyroxene rock and associated eclogites represent remnants of former oceanic crust that was subducted to a great depth.
Keywords :
Garnet–pyroxene rock , Metasomatism , Metagabbro , Ultrahigh-pressure , Su-Lu terrane , Titanian clinohumite
