Coupled Lu–Hf and Sm–Nd geochronology on a single eclogitic garnet from the Huwan shear zone, China

Abstract

We present coupled Lu–Hf and Sm–Nd dates from multiple growth zones within a single large garnet porphyroblast in an eclogite from the Huwan shear zone in the Hong'an orogen, central China. The eclogite sample contains large garnet porphyroblasts up to several centimetres in diameter and a second major population of smaller, millimetre-sized garnet grains. Elemental compositions and mineral inclusions in garnet suggest that the large garnet crystal formed during an early episode of metamorphism, with the abundant small-sized garnets in the matrix growing during a late metamorphic period contemporaneous with the overgrowth of the rim of the large garnet crystal. Ten coupled Lu–Hf and Sm–Nd dates determined from five micro-sawed garnet sections of a ~2cm garnet coherently decrease from core to rim. Lu–Hf dates from each section are consistently older than the corresponding Sm–Nd dates. Petrographic and chemical observations of the eclogite show that three generations of garnet growth occurred, with the latter two generations coincident with growth of the rim of the large garnet, and that of the smaller matrix garnets. Five micro-sawed growth zones were analysed for Lu–Hf and Sm–Nd isotopes. The calculated Lu–Hf dates from the mega garnet record garnet growth initiation at about 397Ma, during prograde to peak metamorphic conditions of the first orogenic episode, lasting about 11Ma. Sm–Nd garnet dates are systematically younger than corresponding Lu–Hf dates, with the Sm–Nd garnet rim date >25Ma younger than the Lu–Hf date. We suggest that the Sm–Nd dates represent a combination of 1) growth and simultaneous diffusion at elevated temperatures during the first metamorphic episode, 2) diffusive age resetting as a result of continued elevated temperatures during residence at mid-crustal depths and/or slow cooling between the two orogenic episodes, and 3) mixing of growth generations. U–Pb dates on three zircon inclusions from the garnet separates range from 386 to 351Ma, possibly suggesting zircon crystallization during exhumation and cooling. This study highlights that coupled microsampling Lu–Hf and Sm–Nd chronology on a single garnet (down to ~cm in diameter) can help decipher the timescales of burial and heating (for Lu–Hf) and cooling from elevated temperatures (for Sm–Nd) from a single orogenic cycle, and that zoned Lu–Hf and Sm–Nd garnet can be utilized as a geo (thermo) -chronometer. Lastly, these Lu–Hf garnet, Sm–Nd garnet and U–Pb zircon dates confirm an early high-pressure metamorphic episode during the Devonian before the final convergence of the South China and North China Blocks in the Triassic.