Sm–Nd and Rb–Sr dating of pyroxene–garnetite from North Dabie in east-central China: problem of isotope disequilibrium due to retrograde metamorphism

Abstract

Oxygen isotopes may serve as a guide to evaluate whether equilibria are preserved in the Sm–Nd and/or Rb–Sr systems in eclogites that experience retrograde metamorphism. This study demonstrates that Sm–Nd, Rb–Sr and O isotopes for minerals from two portions of a clinopyroxene–garnetite from North Dabie in east-central China record both equilibrium, peak metamorphic conditions, and disequilibrium conditions resulting from anhydrous, retrograde metamorphism. The minerals are homogeneous in terms of major elements and show no variation across the sample, but clinopyroxene shows variable degrees of alteration. Pure, primary mineral separates that are in O isotope equilibrium yield Triassic Sm–Nd isochron ages of 219±11 and 229±13 Ma that match previously published zircon U–Pb ages for peak metamorphism. Thus, the Sm–Nd system in these minerals equilibrated during peak metamorphism and remained unaffected by retrograde metamorphism. In contrast, altered and impure clinopyroxenes that display oxygen isotope disequilibria fall off the Sm–Nd isochron defined by the pure minerals, implying that, like oxygen, the Sm–Nd systems in these clinopyroxene were disturbed during retrogression. No meaningful isochrons are found from the Rb–Sr dating (even in the pure, primary minerals), implying that these systems were disturbed during retrograde metamorphism and the diffusion rate of Sr is faster than those of Sm–Nd and O in mafic minerals. Our results suggest that the oxygen isotopes in metamorphic minerals can be used to evaluate the validity of Sm–Nd mineral isochrons in dating peak metamorphic events. In addition, variations in the Sm–Nd concentration and isotopic compositions of altered and impure clinopyroxenes from the two portions suggest heterogeneous effects on the minerals from external and internal fluids.