UNUSUAL UPPER-MANTLE BENEATH GUANIAMO, GUYANA SHIELD, VENEZUELA - EVIDENCE FROM DIAMOND INCLUSIONS

The geochemistry of mineral inclusions in diamond is an important sour ce of information about the composition of the continental lithospheri c mantle at depths exceeding 120-150 km. At these depths, two main typ es of geochemical environment support diamond formation; they are ultr amafic (or peridotitic) (U-type) and eclogitic (E-type) environments a s shown by minerals that occur as inclusions in diamonds. In primary d iamond-bearing kimberlite or lamproite rock the ratio of diamonds from these two geochemical environments varies widely between localities. The U-type environment dominates for the majority of diamond occurrenc es worldwide, whereas the E-type environment dominates for a very limi ted number of localities in South Africa, North America, and Australia . The present study shows that an uncommonly high percentage (99.4%) o f E-type diamonds with extremely variable inclusion assemblages is fou nd in kimberlites in the northwestern part of the Guyana shield at Gua niamo, Venezuela. These variations range from previously unknown silic a-undersaturated corundum eclogite to abundant silica-rich coesite ecl ogite assemblages representing 22% of all E-type diamonds, some of whi ch contain syngenetic ihmenite and magnetite, The compositional variat ions of garnet and omphacite inclusions are extremely broad. The wide variability of the eclogitic source in which diamonds formed beneath t he Guyana shield indicates a broadly basaltic chemistry in this enviro nment, which may represent ancient subducted oceanic crust. Such speci fic features have not been reported for any other diamond occurrence a nd reflect an unusual composition of the deep lithospheric mantle in t his area.