The presence of coesite and micro-diamonds in crustal metamorphic rocks from Dabie Shan, China indicates that they were metamorphosed at depths of up to 120 km (Okay et al., 1989). Extreme negative d18O compositions have been recorded in these rocks and are thought to be the result of circulation of meteoric water prior to the ultra-high pressure metamorphism (Baker et al., 1996). To investigate the timing of metamorphism and nature of the alteration we have analysed 143Nd/144Nd, 87Sr/86Sr and d18O ratios for eclogites and their host rocks from a number of localities throughout Dabie Shan and for a suite of whole-rock samples along a well exposed profile perpendicular to foliation at Taihu Reservoir.
Eclogite pods from within the Qiaoling Formation preserve peak ultra-high pressure metamorphic conditions of 30-40 kbars and 650-750°C at their centres. The margins of the eclogites have recrystallised to amphibole, epidote and biotite-bearing assemblages at 10 to 20 kbars at similar temperatures. Garnet-whole rock ages from the eclogite centres across Dabie Shan give 204-230 Ma Sm-Nd ages (cf. Chavagnac and Bor-ming Jahn, 1995). However the samples show more disturbed Rb-Sr systematics and yield an age of 621 ± 35 Ma (0.27 whole-rock d18O) for an eclogite from Taihu, 225 ± 12 Ma (2.92 whole-rock d18O) on an eclogite from Bixiling, ~100 Ma (2.62 whole-rock d18O) for a Mg-rich cumulate at Bixiling and 95 ± 2 Ma (3.45 whole-rock d18O) for a marble hosted eclogite at Shima. Oxygen isotope analyses of eclogites and gneisses give values that span -7 to +11°. Values between +2 and +4 are interpreted as reflecting the primary isotopic composition of the eclogite precursor, whilst values less than 0.5 are interpreted as resulting from interaction with polar meteoric water prior to the ultra-high pressure event. It is interesting to note that the 621 ± 35 Ma age was recorded in an eclogite with a perturbed whole-rock d18O of 0.27, suggesting a correlation between hydrothermal alteration and perturbed isotopic systematics. It is believed that muscovite-rich selvages formed around the eclogite pods during the pre-metamorphic hydrothermal alteration and that these provided the water which promoted recrystallisation to amphibole and biotite assemblages during uplift. This study tests this hypothesis by comparison of the corresponding variation in Sm-Nd and Rb-Sr isotope systematics.
Baker, J., Matthews, A., Mattey, D., Rowley, D.B. & Xue, F., Geochim. Cosmochim. Acta (1996, in press).
Chavagnac, V. & Bor-ming Jahn, Chinese Science Bulletin 40, Suppl. p. 126-127 (1995).
Okay, A.I., Xu, S. & Sengör, A.C.M., Eur. J. Mineral. 1, 595-598 (1989).