Mineral inclusions in detrital garnet – An excellent petrogenetic tool

Cold and deep subduction is a characteristic feature of modern-style plate tectonics (MSPT) and a prerequisite for the formation of low-temperature/high-pressure (LT/HP) and ultrahigh-pressure (UHP) rocks. Although having strong implications for processes like the movement of materials between surface and mantle, mantle convection, thermal regimes, or the crustal growth rate, the onset of MSPT is poorly understood and controversially debated. One argument for an early onset are local occurrences of Paleoproterozoic retrogressed rocks suggested to have initially formed at LT/HP conditions (e.g., Weller & St-Onge, 2017, Nature Geoscience 10, 305-311). In contrast, a major argument for a late onset is the absence of mineralogical indicators for LT/HP and UHP metamorphism, like glaucophane and coesite, from the pre-Neoproterozoic crystalline rock record (e.g., Stern, 2005, Geology 33, 557-560). A limiting factor for the exploration of LT/HP and UHP rocks through time is the decreasing preservation of crystalline rocks with increasing age. Thus, the sedimentary record represents an important archive that preserves information on ancient continental crust lost due to erosion (e.g., Dhuime et al., 2017, Sedimentary Geology 357, 16-32). Nevertheless, reconstructing metamorphic conditions from a sedimentary perspective is challenging, mainly because we face sand-sized mineral grains that lost their paragenetic context. Here we present a selection of case studies that highlight the applicability of mineral inclusion assemblages in detrital garnet as an excellent petrogenetic indicator. This technique enables to partially reconstruct the paragenetic context and to screen the underexplored sedimentary record for mineralogical evidence supporting the operation of MSPT in deep time.