Diamond and coesite inclusions in detrital garnet of the Saxonian Erzgebirge, Germany

Abstract

Local occurrences of coesite- and diamond-bearing rocks in the central Erzgebirge reveal ultrahigh-pressure (UHP) metamorphic conditions during the Variscan Orogeny. Although UHP metamorphism supposedly affected a wider area, implying that rocks which equilibrated under UHP conditions occur dispersed in large volumes of high-pressure country rock gneisses, mineralogical evidence is scarce. Here we applied the new concept of capturing the distribution and characteristics of UHP rocks by analyzing inclusions in detrital garnet. Out of 700 inclusion-bearing garnets from seven modern sand samples draining the UHP area around the Saidenbach reservoir, we detected 26 garnets containing 46 mainly monomineralic coesite inclusions and 22 garnets containing 41 diamond inclusions. Combined with geochemical classification of the host garnets, we show that (1) coesite-bearing rocks are common and comprise eclogites as well as felsic gneisses, (2) small inclusion size is a necessary precondition for the preservation of monomineralic coesite, and (3) for the first time, diamond-bearing crustal rocks can be detected by analyzing the detrital record. Our results highlight the potential of this novel application of sedimentary provenance tools to UHP research, and the necessity to look at the µm-scale to find evidence in the form of preserved UHP minerals.