Provenance information recorded by mineral inclusions in detrital garnet

Abstract

Garnet single-grain analysis is an often used and well established tool in sedimentary provenance studies, especially when metamorphic source rocks are involved. So far, however, solely the geochemical composition of detrital garnets is considered to draw conclusions concerning probable source rocks. The gained information is often limited by (i) geochemical overlap of garnets derived from different lithology and metamorphic grade, (ii) similar probabilities of belonging to more than one source rock type, and (iii) the limitations of discriminating different protolith compositions. Here we present the first attempt of using mineral inclusions in detrital garnet as a provenance tool. We analyzed the inclusions of ~300 fine to medium sand-sized detrital garnets from two proximal modern sand samples taken in the HP/UHP Western Gneiss Region of SW Norway. All mineral inclusions ≥2 μm were identified by Raman spectroscopy, showing that (i) most garnets from HP/UHP metamorphic source rocks contain mineral inclusions ≥2 μm, (ii) Raman spectroscopy is a very powerful tool to characterize the inclusion types, and (iii) less stable mineral phases like kyanite, omphacite, diopside, enstatite, coesite, amphibole group, and epidote group minerals occur as inclusions in garnet. These minerals, which are important for provenance studies, can thus be preserved in the sedimentary record as long as garnet is stable. The combination of inclusion types in garnet and geochemical garnet classification shows that (i) inclusions well reflect the geological characteristics of the sampled catchments, implying that they are useful indicators for HP/UHP provenance, and (ii) inclusions in garnet can be used to support and enhance the provenance information obtained by garnet geochemistry.