Traditional geochronological methods targeting accessory phases such as zircon and monazite to date high-grade metamorphic events commonly fail to directly link metamorphic ages to the respective pressure–temperature (P–T) conditions of formation. Recent developments in laser-ablation ICP–MS/MS have made it possible to obtain in situ Lu–Hf ages in garnet (Simpson et al., 2021), a mineral that is common in most high-grade metamorphic rocks and widely utilised in thermobarometric studies. The combination of high spatial resolution and preservation of textural relationships makes the method a valuable tool in unravelling complex metamorphic histories of polymetamorphic terranes and constraining P–T–time paths. Here, we apply campaign-style in situ laser-ablation ICP–MS/MS Lu–Hf garnet geochronology to a variety of high-grade metamorphic rocks from across the Western and Central Dharwar Craton, as well as parts of the Mesoarchean Coorg block and Mercara shear zone, to constrain the timing of Archean metamorphism. Our results show a wide distribution of Neoarchean to early Paleoproterozoic ages (2.6–2.4 Ga) along the eastern and southern margin of the Western Dharwar Craton, reflecting its accretion with the Central Dharwar Craton and other crustal blocks. Records of a potentially older metamorphic event (3.2–3.0 Ga) that did not experience widespread Neoarchean overprinting are mainly preserved in the Coorg block and the cratonic cores of the Western and Central Dharwar blocks, and may aid in unravelling their crustal evolution prior to Neoarchean accretion. Finally, this study outlines the advantage of in situ Lu-Hf garnet dating over conventional (time and resource intensive) isotope dilution dating, along with its pitfalls and limitations.

Reference(s)

Simpson, A, Gilbert, S, Tamblyn, R, Hand, M, Spandler, C, Gillespie, J, Nixon, A and Glorie, S 2021, In-situ Lu–Hf geochronology of garnet, apatite and xenotime by LA ICP MS/MS: Chemical Geology, v.577, p. 120299.