Garnet peridotite in CCSD-MH in the Sulu UHPM belt:A deep subducted Piaeozoic ultramafic intrusion.

An uitramafic sequence was penetrated between 603.20 and 683.53 m depth in the main hole of the Chinese Continental Scientific Drilling Project(CCSD-MH),located in the Sulu ultrahigh pressure(UHP)metamorphic belt of eastern China.The sequence,which is sandwiched between normal or quartz-rich eclogite at the base and high Ti-Fe eclogite at the top,consists chiefly of garnet wehrlite with a few thin layers of coesite-bearing,high-Mg eclogite.The wehrlite consists mainly of olivine(60-70 modal%), garnet(10-25 modal%),clinopyroxene(5-15 modal%),and orthopyroxene(1-5 modal%),accompanied by minor phlogopite, ilmenite or clinohumite.Inclusions of high-Mg olivine(up to Fo_(89))in garnet and clinopyroxene may represent primary grains,but most of the olivine,which formed by recrystallization during the UHP metamorphism,is less magnesian(mainly Fo_(79_83)).The garnet is characterized by low CaO,and high MgO and Cr_2O_3(up to 3 wt%),with obvious progressive zoning related to the metamorphism. Sodium-rich clinopyroxenes(up to 5 wt% Na_2O)contain both omphacite and normal clinopyroxene,suggesting that these grains formed during UHP metamorphism,an interpretation supported by P-T data derived from equilibrated mineral pairs.There is a 10-wt% difference in MgO between the peridotites and eclogites,which is interpreted as reflecting derivation from different magmatic sources. SHRIMP dating of zircons from the wehrlite indicates a Paleozoic age for the protolith(ca.346-461 Ma),and a Mesozoic age(ca. 220-240 Ma)for the UHP metamorphism.The garnet peridotite in the CCSD-MH is thus interpreted as a Paleozoic intrusion that underwent UHP metamorphism during subduction of the Yangtze plate in the Triassic.