Coesite-bearing Eclogites from the Bixiling Complex, Dabie Mountains, China: Sm-Nd Ages, Geochemical Characteristics and Tectonic Implications

Valerie Chavagnac Géosciences Rennes, UPR 4661-CNRS, Université de Rennes 1, 35042 Rennes Cedex, France and Mineralogisch-petrographisches Institut, Gruppe Isotopengeologie,

Universität Bern, Erlachstrasse 9a, 3012 Bern, Switzerland

Bor-ming Jahn Géosciences Rennes, UPR 4661-CNRS, Université de Rennes 1, 35042 Rennes Cedex, France


The Bixiling mafic-ultramafic complex occurs as a relatively large (× 1.5 km2) tectonic block within biotite gneisses in eastern part of the Dabie ultrahigh-pressure metamorphic (UHPM) terrane. The complex consists mainly of banded eclogites with many lenses of ultramafic rocks. The protoliths most probably represent a layered intrusion undergone UHP metamorphism as a result of deep subduction of continental mass. Geochemical and isotopic (Rb-Sr and Sm-Nd) analyses on whole-rock and mineral constituents yield many interesting results concerning the age of metamorphism, the age and nature of protoliths, and the high rates of initial cooling and uplift of the Complex, and probably much of the Dabie UHPM terrane.

Results and Discussion

The Bixiling eclogites and ultramafic rocks have narrow ranges of near-chondritic Nd isotopic compositions (eNd(0) = -0.6 to -2.7) and 147Sm/144Nd ratios (× 0.16 to 0.19). This feature is in strong contrast with the majority of eclogites from the Su-Lu and Dabie terranes which have low eNd(0) values (-6 to -20) and smaller 147Sm/144Nd ratios (× 0.1 to 0.15). Trace element and Sr-Nd isotopic characteristics suggest that the original magma was derived from the upper mantle and intruded in the lower crust where the magma was probably contaminated by variable amounts of lower crustal rocks (granulites) during the differentiation of this layered complex.

Sm-Nd isotopic analyses on garnet and omphacite from 6 eclogites and 1 garnet peridotite give very coherent isochron ages ranging from 210 to 218 Ma with only one exception of 231 ± 4 Ma. All isochrons have near-chondritic eNd(T) values (-0.1 to -1.8). In addition, all garnet data points are well aligned to form a similar "isochron" age of 225 ± 7 Ma with eNd(T) = -2.5 ± 1.1. Similarly, Rb-Sr isotopic analyses on phengite and other minerals yield almost the same age results; and the phengite-only data also define a similar isochron age of about 210 Ma.


The above results lead to several important conclusions: (1) The coherent mineral Sm-Nd ages obtained for both mafic and ultramafic rocks from the same intrusion indicate attainment of isotopic equilibrium during the UHP metamorphism. (2) The Sm-Nd mineral ages are identical within error limits with the zircon U-Pb ages of × 220 Ma (Ames et al., 1993, 1996) from nearby eclogites. Thus, the age of 210-220 Ma is probably the best estimate for the time of the UHP metamorphism and the continental collision, a conclusion reached earlier by Li et al. (1993). (3) The fact that the garnet data points alone also give a similar (or slightly older) age and a slightly lower initial eNd(T) value as the other garnet-omphacite pairs strongly suggests that the Bixiling complex was intruded shortly before the UHP metamorphism. The same conclusion can be drawn from the phengite-only Rb-Sr isochron argument. The available whole-rock Sm-Nd data are too scattered to define an isochron, but their disposition is consistent with a young (late Paleozoic to early Mesozoic) intrusive age. The recently reported Archean age of × 2.8 Ga (Cao and Zhu, 1995) and an early Proterozoic age of × 2.2 Ga (Liu et al., 1995) could be interpreted as the ages of zircons from the lower crust contaminant. The interpretation of the Bixiling complex as the oldest (late Archean) UHP metamorphic terrane is considered fallacious.

Using the known P-T conditions and probable blocking temperatures for Sm-Nd and Rb-Sr sytems in garnet, phengite and biotite, very high rates of initial cooling (× 50°/Ma) and early stage of uplift (× 10 mm/yr) were calculated for the Bixiling Complex. This is in strong contrast with the Weihai eclogites from the Su-Lu terrane in which slow cooling and slow uplift (× 1 mm/yr) rates were estimated (Jahn et al., 1995). Evidently, the exhumation of UHPM rocks was not of the same style everywhere in the Dabie and Su-Lu terranes.


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