Micro‐sampling Lu–Hf geochronology reveals episodic garnet growth and multiple high‐P metamorphic events

Abstract

We present Lu–Hf dates from multiple growth zones within two large garnet porphyroblasts by a micro‐sampling method. A single eclogite sample, taken from the Huwan shear zone in the Hong'an orogen, was investigated with Lu–Hf geochronology by micro‐sampling and traditional bulk separation methods. The sample contains a few large garnet porphyroblasts up to several centimetres in diameter and a second major population of smaller (0.1–2.0 mm) garnet porphyroblasts, comprised of dark cores and pale rims. Elemental compositions and mineral inclusions in the garnet appear consistent with two garnet generations. Lu–Hf dates (c. 400–264 Ma) were determined from twelve micro‐sawed garnet sections from two large garnet porphyroblasts. These Lu–Hf dates overlap with age peaks defined by 115 SIMS zircon U–Pb analyses from the same eclogite sample. Bulk analyses of smaller garnet separates define a minimum date of c. 306 Ma and a maximum date of c. 252 Ma for the dark cores and pale rims respectively. These Lu–Hf dates were interpreted to bracket the period of garnet growth and the broad interval from c. 400 to 264 Ma is best explained by protracted and episodic garnet growth, which may require that these rocks experienced two subduction cycles that were initiated during the Devonian and terminated in the Triassic.

Integrated garnet and zircon–titanite geochronology constrains the evolution of ultra‐high–pressure terranes: An example from the Sulu orogen

“…Mixing different age components of garnet could also be a possible cause of inaccurate age determination (e.g. Cheng, Liu, Vervoort, Wilford, & Cao, ; Herwartz, Nagel, Münker, Scherer, & Froitzheim, ). In both of these studies, two generations of garnet were formed during distinct metamorphic events, which resulted in significant scatter in the garnet Lu–Hf isochron using bulk dissolution methods.…”

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

“…Bulk Lu–Hf dates represent the average age of all growth zones weighted according to the Lu distribution. Possible causes for the variations in garnet dates within a sample include: (a) contamination by inherited, Hf‐bearing inclusions (Scherer, Cameron, & Blichert‐Toft, ), (b) diffusional resetting and resorptive recrystallization (Bloch, Ganguly, Hervig, & Cheng, ; Kelly et al., ), (c) different degrees of mixing between multiple or episodic end‐member age components and various regrowth or preservation of original growth for continuous garnet growth over a protracted time (Cheng, Liu, et al., ; Lapen et al., ; Zirakparvar, Baldwin, & Vervoort, ).…”

“…The microsampling technique within a single garnet applied for the Lu–Hf system has not been attempted until recently (e.g. Anczkiewicz, Chakraborty, Dasgupta, Mukhopadhyay, & Kołtonik, ; Cheng, Liu, Vervoort, Wilford, & Cao, ; Cheng et al., ; Schmidt, Pourteaua, Candan, & Oberhänsli, ), mainly owing to sample size limitations for Hf analysis by MC–ICP‐MS. Nevertheless, all garnet in previous microsampling Lu–Hf geochronology studies have been exceptionally large crystals (several cm diameters).…”

Post-peak metamorphic evolution of the Sumdo eclogite from the Lhasa terrane of southeast Tibet

“…The overpassing of the closure temperature (Dodson, 1973) may reset the Lu-Hf isotope system within garnet, and the date defined by the isochron may reflect cooling rather than growth. The petrological observations and modeling suggest that garnet grew along a thermal relaxation path from ~530 o C to ~620 o C, which appears to be below the closure temperature of the Lu-Hf garnet geochronometer (no less than ~700 °C for garnet grains >1 mm; Scherer et al, 2000;Bloch et al, 2015;Cheng et al, 2016a), although the closure temperature is a function of many factors, such as grain size, thermal history and elemental zoning of garnet. The garnet core has high heavy rare earth element (Lu as a proxy) concentrations that generally decrease toward the rim, a situation that is largely consistent with Rayleigh type behavior (Hollister, 1966).…”

“…Quantitative analyses were performed using wavelength-dispersive spectrometers (WDS) with an acceleration voltage of 15 kV, a beam current of 10 nA, a 5 m beam size and a 10 to 30s counting time (Cheng et al, 2016a). Natural minerals and synthetic oxides were used as standards, and the ZAF procedure was used for data correction.…”