HR: 15:25h
AN: V12F-07    [PDF]
TI: The Origin of Garnet Megacrysts in Sulu UHP Clinopyroxenite, East China
AU: * Liou, J
EM: liou@pangea.stanford.edu
AF: Dept. of Geological & Environmental Sciences, Stanford University, Stanford, CA 94305 United States
AU: Zhang, R
EM: zhang@pangea.stanford.edu
AF: Dept. of Geological & Environmental Sciences, Stanford University, Stanford, CA 94305 United States
AU: Tsujimori, T
EM: tatsukix@rins.ous.ac.jp
AF: Research Inst. Nat. Sciences, Okayama Univ. of Sci., 1-1 Ridai-cho, Okayama, 700-0005 Japan
AU: Li, T
EM: litianfu@ccsd.org.cn
AF: Institute of Geology, Chinese Academy of Geological Sci., #26, Baiwanzhuang Road, Beijing, 100037 China
AU: Chen, S
EM: chenshizhong@ccsd.org.cn
AF: Institute of Geology, Chinese Academy of Geological Sci., #26, Baiwanzhuang Road, Beijing, 100037 China
AB: Abundant garnet megacrysts occur in one garnet clinopyroxenite layer from Rizhao of the Sulu UHP terrane, east China. The clinopyroxenite layers with various contents of garnet (trace to $>$ 40%), ilmenite ($<$ 5 %) and minor olivine form part of a strongly serpentinitzed peridotite body. Megacrystic Grt-bearing clinopyroxenite consists of variable amounts of megacrystic and porphyroblastic Grt, fine-grained matrix of Grt + Di + Ilm $\pm$ Ol (Fo$_{82-85}$), and secondary Amp, Chl and Spl. The megacrystic Grt with ellipsoidal and spherical shapes ranges from 2.5 to 12 cm, are coated with Chl + Amp $\pm$ Cpx $\pm$ Ep $\pm$ Mt/Spl and develops distinct pressure shadow. Megacrystic Grt (Prp$_{44-56}$Grs$_{21-41}$Alm$_{18-26}$) has similar compositions to porphyroblastic Grt (Prp$_{40-43}$Grs$_{37-40}$Alm$_{20-21}$) but differs from matrix Grt (Prp$_{25-32}$Grs$_{46-56}$Alm$_{18-22}$). At some interfaces between Grt and Cpx, Grt contains Ca-rich and Mg-poor compositions (Prp$_{23-25}$Grs$_{58-60}$Alm$_{17-19}$). At least 6 types of inclusions occur: (a) trace spinel $\pm$ magnetite lamellae, (b) trace dolomite, (c) trace amphibole (3.0-3.4 wt% Na$_{2}$O; 0.3-0.4 wt% K$_{2}$O; 0.4-0.5 wt% TiO$_{2}$), (d) ilmenite with magnetite lamellae and rare spinel, (e) abundant coarse-grained Cpx (0.5-1.0 wt% Na$_{2}$O) with characteristic exsolution lamellae of Amp (0.4-1.3 wt% Na$_{2}$O; 0.7-1.0 wt% K$_{2}$O; Mg\# $\sim0.9; 0.1-0.2 $wt% TiO$_{2}$) + Ilm $\pm$ Grt described by Zhang and Liou (2003), and (f) composite phases of Cpx (e) + Ilm + Amp with ilmenite exsolution lamellae. The Grt megacrysts are fractured and veined with amphibolite facies assemblage Amp + Spl + Cpx $\pm$ Pl and do not contain exsolved rutile needles. Available experimental data (4-15 GPa, 1000-1400\deg C) of an Ilm-rich garnet clinopyroxenite composition indicate that Grt coexisting with Cpx contains considerable amount of TiO$_{2}$ ($>$ 1 wt%) only at P $>$ 5 GPa; the results constrain the maximum depth for the formation of Grt megacryst. Parageneses and compositions of inclusion phases in megacrystic Grt and matrix minerals suggest the protolith of the megacrystic Grt-bearing clinopyroxenite probably was a high-Al clinopyroxenite formed at mantle wedge with an initial assemblage Cpx + Ilm + Spl $\pm$ Ol $\pm$ Dol at P $<$ 3 GPa and T $\sim$1200\deg C. Subsequent increase in P and decrease in T as the mantle fragment involved in a continental subduction zone, spinel reacted to form Grt whereas the Al-Ti bearing Cpx exsolved Amp + Grt + Ilm lamellae at subduction depths $< $ 150 km. During exhumation and hydration, Grt coalesced to form megacrysts that engulfed relict lamellae-rich Cpx, Ilm, Spl, Amp and dolomite as inclusions. Porphyroblastic garnet formed subsequently and lamellae-rich Cpx recrystallized to matrix assemblage of Cpx + Grt + Ilm at $\sim$800\deg C, $\sim$3 GPa. Deformation at crustal depths fractured megacrysts and amphibolite facies recrystallization occurred.
DE: 3660 Metamorphic petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting