HR: 1330h
AN: V22B-0588    [PDF]
TI: Majoritic Garnet-bearing Eclogite-Ultramafic Rocks With Rhythmic Layering Structure In The Sulu UHP Terrane, China
AU: * Zhang, R Y
EM: zhang@pangea.stanford.edu
AF: Stanford University, Dept of Geology & Environmental Sciences, Stanford, CA 94305 United States
AU: Liou, J G
EM: liou@pangea.stanford.edu
AF: Stanford University, Dept of Geology & Environmental Sciences, Stanford, CA 94305 United States
AB: The Yangkou ultramafic blocks of tens m size, are enclosed within gneiss at the middle part of the Sulu UHP terrane. The blocks consist of peridotite (Grt+Ol+Di+En+Ilm), clinopyroxenite (Grt+Di+Ilm) and eclogite (Grt+Omp+Rt) with "rhythmic layering" structure in hand specimen and thin section scales. Individual layer ranges from $\sim$1 mm to $>$10 cm thick. Garnet is minor in peridotite, but abundant in clinopyroxenite and eclogite, whereas Cr-bearing pargasite is common in peridotite. Peridotite is nearly equigranular with grain size of 0.3 to 0.5 mm and has a weak to strong foliation. Prismatic enstatite (En$_{92-93}$) is partially or totally replaced by talc, and euhedral orientated amphibole (0.2-0.6 x 0.3-1.75 mm) contains inclusions of olivine and garnet. Pyropic garnet (Prp$_{62-67}$Alm$_{23}$Grs$_{10-15}$Sps$_{1-2}$) is homogenous in composition in individual sample, but varies in Cr$_{2}$O$_{3}$ content ($<$0.5 to 2.25 wt%) from sample to sample. Diopside with low FeO and Al$_{2}$O$_{3}$ (both $<$2 wt%) content has positive correlation with garnet in Cr$_{2}$O$_{3}$ content (0.3 to 1.2 wt%). Most layers of garnet clinopyroxenite and eclogite show porphyroblastic texture. Garnet (Prp$_{51-63}$Alm $_{25-33}$Grs$_{10-19}$Sps$_{1}$) of clinopyroxenite contains much higher pyrope component than eclogitic garnets (Prp$_{34-41}$Alm$_{34-45}$Grs$_{23-36}$Sps$_{1}$). Porphyroblastic garnets of 1-5 mm in size from both rocks contain abundant exsolved rutile needles and minor apatite $\pm$ clinopyroxene rods. A few cores of exsolved rutile needles-bearing eclogitic garnets have high Si of 3.07 $\pm$ 0.01 and low Al of 1.91$\pm$0.02 per formula unit indicating that the garnets contain at least about 8-10 mole% majorite component defined by significant coupled substitution of 2Al$^{3+}$ in the octahedral site by (Mg,Fe)$^{2+}$Si$^{4+}$. The P-T estimates of peridotite using the Grt-Opx barometer and Grt-Cpx thermometer yield about 3.7-4.3 $\pm$ 0.2 GPa at 700-730$\pm$ $50\deg$C. The matrix of pyroxenite and eclogite had lower equilibrium temperature of about 600$deg$C $\pm$ $50\deg$C. The rhythmic layering probably represents a relict cumulate structure, rather than metamorphic differentiation. Preservation of rhythmic layering and majoritic garnet may be attributed to (1) a stable condition for magma differentiation to form layered mafic-ultramafic protoliths, and (2) the original structure has not been significantly modified during UHP metamorphism in a dry environment and rapid subduction and exhumation of the UHP slab. This suggestion is consistent with preservation of gabbroic structures and minerals and occurrence of intergranular coesite in a nearby coesite-bearing eclogite body.
DE: 3660 Metamorphic petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting