HR: 0800h
AN: T21A-0507    [Abstracts]
TI: Clinoenstatite Exsolution in Diopside of Dabieshan, China - Evidence of Origin $>$ 250km
AU: Liu, X
EM: xwliu@cug.edu.cn
AF: Graduate school, China Univ. Geosciences, Wuhan, 430074 China
AU: * Jin, Z
EM: zmjin@cug.edu.cn
AF: Fac. Earth Sciences, China Univ. Geosciences, Wuhan, 430074 China
AU: Green, H W
EM: harry.green@ucr.edu
AF: IGPP/Dept. Earth Sciences, Univ. California, Riverside, CA 92521 United States
AU: Qu, J
AF: Graduate school, China Univ. Geosciences, Wuhan, 430074 China
AB: Exsolution lamella of clinoenstatite (Clen) in diopside (Di) were found for the first time by Bozhilov et al. (1999) in the Alpe Arami peridotite, Switzerland. According to its particular micro-characteristics, they deduced that high-pressure clinoenstatite (HPclen) was the originally exsolving phase, which can exist stably only at P-T conditions of more than 250 km in the upper mantle. Here we report similar exsolution lamellae of Clen in Di from garnet peridotite of the Bixiling massif of the Dabie mountains, China. Transmission electron microscopy (TEM) shows oriented exsolution lamellae, 0.3-1$\mu$m long by about 0.1$\mu$m wide. X-ray energy-dispersive spectrometry shows that the host mineral is Di, whereas the composition of the lamella is close to that of enstatite. Selected-area electron diffraction shows that the host mineral is Di with C2/c space group and the lamellae are low clinoenstatite (Lclen) with P2$_{1}$/c space-group. The topotaxy between Lclen and Di is as follows: [100]$_{clen}$$\|$[100]$_{Di}$, [010]$_{clen}$$\|$[010]$_{Di}$ and [001]$_{clen}$$\|$[001]$_{Di}$ approximately, with a deviation of 1-2$\deg$. Lclen exsolution lamellae are oriented parallel to (401) planes of Di. Observation of high-resolution lattice image and diffraction contrast images showed that there exist nanometer-size antiphase domains in the Lclen lamellae, which proves that the Lclen lamellae transformed from a C2/c precursor after exsolution. These micro-characteristics are identical with those reported by Bozhilov et al. (1999). There are two C2/c pyroxenes of approximately enstatite composition, high-temperature clinoenstatite (HTclen) and high-pressure clinoenstatite (HPclen). The Dabie/SuLu UHPM belt has an extremely cold exhumation path, making HPclen the only reasonable choice between these two possible C2/c precursors. We propose extension of the existing exhumation P-T path for the Bixiling massif developed by others to depths in excess of 250 km to reach the stability field of HPclen. In another words, the Bixiling garnet peridotite traveled to the surface from a depth greater than 250 km, placing a new constraint for the peak metamorphic depth of Dabie-Sulu UHP metamorphism belt.
DE: 5112 Microstructure
DE: 3924 High-pressure behavior
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting