COESITE-BEARING ECLOGITES FROM THE BIXILING COMPLEX, DABIE MOUNTAINS,CHINA - SM-ND AGES, GEOCHEMICAL CHARACTERISTICS AND TECTONIC IMPLICATIONS

The Bixiling mafic-ultramafic complex occurs as a tectonic block withi n biotite gneisses in the eastern part of the Dabie ultrahigh-pressure metamorphic (UHPM) terrane. Geochemical and isotopic (Rb-Sr and Sm-Nd ) analyses were performed on whole-rock and mineral constituents in or der to constrain the age of metamorphism, to discuss the age and natur e of protoliths, and to estimate the rates of initial cooling and upli ft of the Complex as well as much of the Dabie UHPM terrane. The Bixil ing eclogites and ultramafic rocks have narrow ranges of near-chondrit ic epsilon(Nd)(0) values (-2.7 to -0.6) and Sm-147/Nd-144 ratios (simi lar to 0.16-0.19), which is in strong contrast with the majority of ec logites from the Su-Lu and Dabie terranes which have low epsilon(Nd)(0 ) values (-20 to -6) and smaller Sm-147/Nd-144 ratios (similar to 0.1- 0.15). Trace-element and Sr-Nd isotopic characteristics suggest that t he original mantle-derived magma intruded in the lower crust and was c ontaminated by crustal rocks (granulites) during magma differentiation . Sm-Nd isotopic analyses on garnet and omphacite from 6 eclogites and 1 garnet peridotite give tightly grouped isochron ages ranging from 2 10 to 218 Ma, with near-chondritic epsilon(Nd)(T) values (-1.8 to -0.1 ), When all garnet data are treated alone, they form a similar ''isoch ron'' age of 225 +/- 7 Ma with epsilon(Nd)(T) = -25 +/- 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 similar to 210 Ma. These results lead to several impo rtant conclusions: (1) The concordant mineral Sm-Nd ages obtained for both mafic and ultramafic rocks from the same intrusion indicate attai nment of isotopic equilibrium during the ultrahigh-pressure (UHP) meta morphism. (2) The Sm-Nd mineral ages are identical within error limits with the zircon U-Pb ages of similar to 220 Ma from nearby eclogites, Thus, 210-220 Ma is considered the best estimate for the age of the U HP metamorphism and continental collision. (3) The fact that the garne t data points alone also give a similar (or slightly older) age and a slightly lower initial epsilon(Nd)(T) value as the other garnet-omphac ite pairs strongly suggests that the Bixiling Complex intruded shortly before the UHP metamorphism. The same conclusion can be drawn from th e phengite-only Rb-Sr isochron argument. Using the known P-T condition s and probable blocking temperatures for Sm-Nd and Rb-Sr systems in ga rnet and phengite, high rates of initial cooling (similar to 40 degree s C/Ma) and early stage of uplift (similar to 10 mm/yr) were calculate d for the Bixiling Complex. This is in strong contrast with the Weihai eclogites from the Su-Lu terrane in which slow cooling and slow uplif t (similar to 1 mm/yr) rates were estimated. Evidently, differential e xhumation of UHPM rocks prevails in the Dabie and Su-Lu terranes.