HR: 0830h
AN: V21D-0560 [PDF]
TI: Sr and Nd isotopic constraints on the protoliths of the Chinese Tianshan UHP metamorphic complex, West
China
AU: * Zeng, L
EM: lzeng@gps.caltech.edu
AF: Division of Geological and Planetary Sciences, MC 100-23
California Institute of Technology, Pasadena, CA 91125 United States
AU: Saleeby, J B
EM: jason@gps.caltech.edu
AF: Division of Geological and Planetary Sciences, MC 100-23
California Institute of Technology, Pasadena, CA 91125 United States
AU: Gao, J
AF: Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, 100029
China
AB:
The Chinese Tianshan high- to ultrahigh-pressure metamorphic belts resulted from multiple Paleozoic collisional events.
Recent discovery of coesite pseudomorphs within garnets in eclogites extends the metamorphic conditions into the
ultrahigh-pressure field up to pressures of ~26 kb and temperatures in the range of $\sim$490 to 590$\deg$C. Unlike most of
the UHP metamorphic belts in the world which are thought to result from continent-continent collisions, the Chinese Tianshan
UHP metamorphic belts developed during the subduction of. oceanic lithosphere. We present new Sr and Nd isotopic composition
data on a suite of metabasites (including eclogite and blueschist, and their retrograde counterparts) and metagraywackes to
better constraint the protoliths of the metamorphic assemblage. The metabasites have initial $^{87}$Sr/$^{86}$Sr and
$\epsilon$$_{Nd}$ ranging from 0.70248 to 0.70890, and -3.03 to +8.51, respectively. The metagraywackes have initial
$^{87}$Sr/$^{86}$Sr and $\epsilon$$_{Nd}$ ranging from 0.70658 to 0.71091, and -5.53 to -9.41, respectively. A t = 350 Ma,
the age of peak metamorphism, is assigned to calculate the initial Sr and Nd isotopic compositions, and thus it is a minimum
age correction. Sr-Nd isotopic systematics as well as published major and trace element data suggest that: (1) the
metabasites were derived from protoliths similar to the ocean island basalts which extend the depleted mantle array into the
relatively enriched E-MORB domain; (2) they bear an Sr isotopic signature suggesting various degrees of seawater alteration;
and (3) the metagraywackes may represent recycled sediments from nearby continental basements. It has been shown that the
pillow basalts in nearby ophiolite zones have been experienced high degrees of seawater alteration by their high
$^{87}$Sr/$^{86}$Sr ($\sim$0.7080) and $\delta$O$^{18}$ ($\sim$13.0$\permil$) values. A similar conclusion is reached based
on the high $\delta$O$^{18}$ values ranging from 9.0 to 10.0$\permil$ of eclogite samples(Gao et al., 1999). Covariation
patterns in the Sr and Nd isotopic compositions of the metabasites suggest that they might have experienced a long period of
intensive seawater alteration before they were subducted and metamorphosed under high- to ultrahigh-pressure conditions. A
similar pattern has been resolved in the Sr-Nd isotopic systematics of the pillow basalts within the ophiolite zone.
DE: 1020 Composition of the crust
DE: 1030 Geochemical cycles (0330)
DE: 1040 Isotopic composition/chemistry
DE: 1749 Volcanology, geochemistry, and petrology
DE: 3660 Metamorphic petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting