Coesite-bearing eclogites from the Bixiling Complex, Dabie Mountains, China: SmNd ages, geochemical characteristics and tectonic implications

Abstract

The Bixiling mafic-ultramafic complex occurs as a tectonic block within 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 order to constrain the age of metamorphism, to discuss the age and nature of protoliths, and to estimate the rates of initial cooling and uplift of the Complex as well as much of the Dabie UHPM terrane. The Bixiling eclogites and ultramafic rocks have narrow ranges of near-chondritic ϵ Nd(0) values (−2.7 to −0.6) and 147 Sm 144 Nd ratios (∼ 0.16-0.19), which is in strong contrast with the majority of eclogites from the SuLu and Dabie terranes which have low ϵ Nd(0) values (−20 to −6) and smaller 147 Sm 144 Nd ratios (∼0.1-0.15). Trace-element and SrNd isotopic characteristics suggest that the original mantle-derived magma intruded in the lower crust and was contaminated 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 210 to 218 Ma, with near-chondritic ϵ Nd( T) values (−1.8 to −0.1). When all garnet data are treated alone, they form a similar “isochron” age of 225 ± 7 Ma with ϵ Nd( 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 ∼ 210 Ma. These results lead to several important conclusions: 1. (1) The concordant mineral SmNd ages obtained for both mafic and ultramafic rocks from the same intrusion indicate attainment of isotopic equilibrium during the ultrahigh-pressure (UHP) metamorphism. 2. (2) The SmNd mineral ages are identical within error limits with the zircon UPb ages of ∼ 220 Ma from nearby eclogites. Thus, 210–220 Ma is considered the best estimate for the age of the UHP metamorphism and continental collision. 3. (3) The fact that the garnet data points alone also give a similar (or slightly older) age and a slightly lower initial ϵNd( T) value as the other garnet-omphacite pairs strongly suggests that the Bixiling Complex intruded shortly before the UHP metamorphism. The same conclusion can be drawn from the phengite-only RbSr isochron argument. Using the known P-T conditions and probable blocking temperatures for SmNd and RbSr systems in garnet and phengite, high rates of initial cooling (∼ 40°C/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. Evidently, differential exhumation of UHPM rocks prevails in the Dabie and Su-Lu terranes.