One of the most remarkable discoveries in the field of petrology in the last five years has been the identification of coesite and/or microdiamonds included in garnets in eclogite blocks and boudins in crustal metamorphic rocks from the Dabie Shan in China. These observations imply that crustal material may be subducted to depths of at least 120 km (Okay et al., 1989; Wang et al., 1989). However, the majority of the marbles, calc-gneisses and felsic gneisses that are interlayered with these eclogites record metamorphism at significantly lower pressures within the quartz stability field. Detailed structural mapping indicates that both the eclogites and their country rocks are deformed by structures formed at ultra high pressures (Xue et al., 1996) suggesting that the difference in pressure is the result of preferential recrystallisation of the country rocks during exhumation.
We have carried out a detailed oxygen isotope study to investigate the role of fluids in enhancing the recrystallisation of the country rock. Stable isotope analyses of 80 whole rock samples and garnet separates from eclogites, gneisses and marbles from the ultra-high pressure Dabie Shan terrain show oxygen isotopic compositions that range from -7 to 11, a range that is remarkable in the geological record. This range is the result of the superimposition of a meteoric-hydrothermal system on the original isotopic compositions of Precambrian marbles (9 to 14 d18O), gneisses (?5 to 10 d18O) and mafic intrusives (eclogite precursors, 2 to 4 d18O). The isotopic compositions of these mafic intrusives are perturbed from those of normal mantle-derived basalts, suggesting that these were either contaminated by or derived from hydrothermally-altered source materials. Detailed isotopic sampling along a 5 km traverse reveals that variations of at least 12 in d18O are preserved over length scales less than 1m. This suggests that there has been no significant fluid-mediated exchange between adjacent rock types during ultra-high pressure metamorphism. Thus recrystallisation must have taken place in the presence of very locally (< centimetre-scale) derived fluids.
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Wang X., Liou J.G. & Mao H.K., Geology, 17, 1085-1088 (1989).
Xue F., Rowley D. B. & Baker J., Geology (submitted) (1996).