Protracted garnet growth in high-P eclogite: constraints from multiple geochronology and P-T pseudosection

Understanding convergent margin processes requires determination of the onset and the termination of subduction, the duration of subduction-zone metamorphism, and the subduction zone polarity. Garnet growth and intracrystalline zonation can be used to constrain the timing, duration and kinetics of tectonometamorphic processes. An eclogite from the Huwan shear zone in the Hong'an orogen was investigated with combined pseudosection analysis and multiple geochronologies. The pseudosection analysis illustrates that garnet growth is continuous and along an early near-isothermal trajectory followed by a near-isobaric heating path from 1.9GPa/500 degrees C to 2.4GPa/575 degrees C and subsequent near-isothermal decompression. 40Ar/39Ar dating of an amphibole inclusion in garnet from the eclogite yielded an age of 310 +/- 5Ma, which is consistent with a U-Pb age of 305 +/- 3Ma for the metamorphic zircon within uncertainty. Garnet core and rim material produced Lu-Hf ages of 296.9 +/- 3.8 and 256.9 +/- 3.9Ma respectively; the latter is consistent with its Sm-Nd age of 254.3 +/- 4.6Ma for the same aliquots. Similarly, limited zircon U-Pb ages of c. 257Ma were obtained in zircon rims with garnet inclusions. These ages were interpreted to bracket the period of garnet growth and the difference of up to c. 40Ma is best explained by protracted garnet growth. We propose that the rocks represent detachment of part of the downgoing slab and remained free of significant compression/decompression or heating/cooling close to the subduction channel, most likely underplating the mantle wedge, for a long time. These rocks were incorporated into the following subduction channel due to the successive entry of the buoyant materials, and exhumed at some time later than c. 254Ma. The increasing observations of protracted garnetgrowth and long-lived subduction in various orogens worldwide demand more sophisticated geodynamic models.