Detrital Garnet Petrology: Inclusions as a main source of information

Documenting metamorphic conditions through the geologic record is a key for understanding the evolution of plate tectonics on Earth. Minerals characteristic for deep subduction processes (i.e. modern-style plate tectonics) like glaucophane, coesite, and diamond are commonly replaced by their low-pressure polymorphs during exhumation. However, when entrapped as inclusions in resistant host minerals like garnet, these mineral phases are shielded from external metamorphic fluids and may be preserved. Finding evidence for deep subduction processes in host garnets of large volumes of (partially) re-equilibrated crystalline rocks is challenging, time consuming, and often hampered by poor outcrop conditions due to weathering and soil formation. In contrast, by analyzing detrital garnet, natural processes such as erosion and sedimentary transport can sample garnet grains sourced from fresh as well as altered crystalline rocks located in the drainage area, enabling large crustal volumes to be screened using a comparatively low number of samples. Case-studies from Norway (Schönig et al. 2018, Sci. Rep.), Germany (Schönig et al. 2019, Geology; Schönig et al. 2020 Gondwana Res.), Austria, Papua New Guinea (Baldwin et al. 2021, PNAS), and Greenland (Schönig et al. 2023, Eur. J. Mineral.) demonstrate mineral inclusion analysis of detrital garnet integrated with major-element chemistry (Schönig et al. 2021, Contrib. Mineral. Petrol.) to be an efficient tool for screening tectonometamorphic units on the presence or absence of rocks related to modern-style plate tectonic processes (Schönig et al. 2022, Earth-Sci. Rev.). This contribution gives a synopsis of the main findings from the five spatially, chronologically, and tectonically distinct localities.