Microsampling Lu-Hf geochronology on mm-sized garnet in eclogites constrains early garnet growth and timing of tectonometamorphism in the North Qilian orogenic belt

This study presents Lu-Hf geochronology of zoned garnet in high-P eclogites from the North Qilian orogenic belt. Selected samples have m-sized garnet grains that have been sampled with a micro-drill and analysed for dating. The Lu-Hf dates of bulk garnet separates, micro-drilled garnet cores and the remnant, rim-enriched garnet were determined by two-point isochrons, with cores being consistently older than the bulk- and rim-enriched garnet. The bulk garnet separates of each sample define identical garnet-whole rock isochron date of c. 457Ma. Consistent U-Pb zircon dates of 455 +/- 8Ma were obtained from the eclogite. The Lu-Hf dates of the drilled cores and rim-rich separates suggest a minimum garnet growth interval of 468.9 +/- 2.4 and 452.1 +/- 1.6Ma. Major and Lu element profiles in the majority of garnet grains show well-preserved Rayleigh-style fractionated bell-shaped Mn and Lu zoning profiles, and increasing Mg from core to rim. Pseudosection modelling indicates that garnet grew along a P-T path from 470-525 degrees C and 2.4-2.6GPa. The exceptional high-Mn garnet core in one sample indicates an early growth during epidote-blueschist facies metamorphism at <460 degrees C and <0.8GPa. Therefore, the Lu-Hf dates of drilled cores record the early prograde garnet growth, whereas the Lu-Hf dates of rim-rich fractions provide a maximum age for the end of garnet growth. The microsampling approach applied in this study can be broadly used in garnet-bearing rocks, even those without extremely large garnet crystals, in an attempt to retrieve the early metamorphic timing recorded in older garnet cores. Given a proper selection of the drill bit size and a detailed crystal size distribution analysis, the cores of the mm-sized garnet in most metamorphic rocks can be dated to yield critical constraints on the early timing of metamorphism. This study provides new crucial constraints on the timing of the initial subduction (before c. 469Ma) and the ultimate closure (earlier than c. 452Ma) of the fossil Qilian oceanic basin.