Testing in-situ U Pb garnet geochronology on a polymetamorphic schist from the Grenvillian basement of the Western Sierras Pampeanas, Argentina

Abstract

Garnet, a common mineral in metamorphic rocks, frequently records complex textural and chemical zoning resulting from polymetamorphism. Dating metamorphic events using conventional methods (e.g., U–Pb zircon or Lu–Hf garnet) is challenging and often fails to yield an accurate reconstruction of the P-T-t history in complex metamorphic regions. Recently, a technique has been developed for in situ U–Pb LA-ICP-MS dating in garnets with very low concentrations of U and Pb. In this work we tested the method on polymetamorphic garnet from a sample of garnet-biotite schist from the Sierra de Maz (Western Sierras Pampeanas, Argentina). This rock belongs to a Grenvillian-age tectonic unit (nappe) that was reworked by the Famatinian orogeny (Ordovician-Silurian). The garnet porphyroblasts show a complex internal arrangement with three textural and compositional zones: Grt1 (cores), Grt2 (replacements) and Grt3 (overgrowths). Dating of a single porphyroblast yielded ages of 1045 ± 31 Ma for Grt1 and 476 ± 18 Ma for Grt2, confirming polymetamorphism resulting from the two main tectonometamorphic events recorded in the Western Sierras Pampeanas. The garnet core (Grt1) age is within error of a 1037 ± 7 Ma zircon-rim U–Pb age from the same rock. However, garnet (Grt2) is slightly older than ages recorded by different methods from the same tectonic unit (nappe), that are mostly in the range ca. 440–410 Ma including a CHIME monazite age of 426 ± 15 Ma from the same schist. This discrepancy remains open to question, as Ordovician ages of metamorphism of ca. 470 Ma are common elsewhere in the Sierras Pampeanas.