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

Garnet is a fundamental expression of metamorphism and one of the most important minerals used to constrain the thermal conditions of the crust. Here, we use 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 yield Lu–Hf ages between c. 1285 and 1240 Ma, revealing a record of older metamorphism that has been partially obscured by metamorphic overprinting during c. 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 palaeogeographic reconstructions.