Laser-ablation Lu-Hf dating reveals Laurentian garnet in subducted rocks from southern Australia

Abstract

Garnet is a fundamental expression of metamorphism and one of the most important minerals used to constrain the thermal conditions of the crust. We used innovative in situ laser-ablation ICP-MS/MS Lu-Hf geochronology to demonstrate that garnet in metapelitic rocks enclosing Cambrian eclogite in southern Australia formed during Laurentian Mesoproterozoic metamorphism. Garnet porphyroblasts in amphibolite-facies metapelitic rocks yielded Lu-Hf ages between 1286 ± 58 Ma and 1241 ± 16 Ma, revealing a record of older metamorphism that was partially obscured by metamorphic overprinting during ca. 510 Ma Cambrian subduction along the East Gondwana margin. Existing detrital zircon age data indicate the protoliths to the southern Australian metapelitic rocks were sourced from western Laurentia. We propose that the metapelitic rocks of southern Australia represent a fragment of western Laurentian crust, which was separated from Laurentia in the Neoproterozoic and incorporated into the East Gondwana subduction system during the Cambrian. The ability to obtain Lu-Hf isotopic data from garnet at acquisition rates comparable to those for U-Pb analysis of detrital zircon means, for the first time, the metamorphic parentage of rocks as expressed by garnet can be efficiently accessed to assist paleogeographic reconstructions.

Paleoarchean metamorphism in the Acasta Gneiss Complex: Constraints from phase equilibrium modelling and in situ garnet Lu–Hf geochronology

“…Garnet from the Högsbo pegmatite (Gothenburg, Sweden) was used as a secondary standard to correct for the effects of matrix‐induced elemental fractionation, producing an age of 1064 ± 6 Ma ( n = 36, MSWD = 2.1) compared to the published U–Pb columbite crystallization age of 1029 ± 1.7 Ma (Romer & Smeds, 1996). This offset towards an older age for laser‐ablation Lu–Hf dating of Högsbo garnet is consistent with previous analysis from other labs (e.g., Brown et al, 2022; Simpson et al, 2021) and has been taken into account for all unknowns by correcting the 176 Lu/ 176 Hf ratios by the discrepancy between the published columbite and measured Lu–Hf garnet age (~3.3%).…”

The metamorphic footprint of western Laurentia preserved in subducted rocks from southern Australia

“…As a consequence of Cambrian-aged subduction and mineralogical recrystallization during high-pressure metamorphism, the primary record of Mesoproterozoicaged metamorphism in samples FMC-1b and FMC-2c is relict coarse-grained garnet (Brown et al, 2022;this study). Therefore, c. 1285-1240 Ma coarse-grained garnet and its inclusion assemblages represent the only repository from which P-T information can be extracted to explore the metamorphic character of Mesoproterozoicaged tectonism.…”

“…Additionally, a quartzite sample was collected for detrital zircon geochronology to constrain the protolith age of the metapelitic rocks in the region (sample FMC-17). The two metapelites were dated by Brown et al (2022) using in situ garnet Lu-Hf geochronology.…”

“…Metapelitic samples FMC-1b and FMC-2c enclose metreto decimetre-scale mafic eclogites which were subducted in the Cambrian (e.g., hypothesis are Cambrian ages for monazite occurring parallel to the fabric in the metapelites (Chmielowski & Berry, 2012) and monazite included within fine-grained, fabric-parallel garnet (Brown et al, 2022). In sample FMC-1b, Neoproterozoic-to Cambrian-aged fine-to medium-grained garnet is located within a weak to moderate fabric and contains inclusions of rutile.…”

In situLu–Hf geochronology with LA-ICP-MS/MS analysis

“…16,17 The in situ Lu–Hf dating technique with LA–ICP–MS/MS analysis was applied to Paleozoic–Precambrian garnet, apatite and xenotime samples for the first time by Simpson et al (2021), 18 and then, this innovative technique has been applied to reveal unidentified multiple-stage garnet growth in subducted rocks from southern Australia. 19…”