Abstract
AbstractDetrital coesite-bearing garnet is the final product of a complex geological cycle including coesite entrapment at ultra-high-pressure conditions, exhumation to Earth’s surface, erosion and sedimentary transport. In contrast to the usual enrichment of high-grade metamorphic garnet in medium- to coarse-sand fractions, coesite-bearing grains are often enriched in the very-fine-sand fraction. To understand this imbalance, we analyse the role of source-rock lithology, inclusion size, inclusion frequency and fluid infiltration on the grain-size heterogeneity of coesite-bearing garnet based on a dataset of 2100 inclusion-bearing grains, of which 93 contain coesite, from the Saxonian Erzgebirge, Germany. By combining inclusion assemblages and garnet chemistry, we show that (1) mafic garnet contains a low number of coesite inclusions per grain and is enriched in the coarse fraction, and (2) felsic garnet contains variable amounts of coesite inclusions per grain, whereby coesite-poor grains are enriched in the coarse fraction and coesite-rich grains extensively disintegrated into smaller fragments resulting in an enrichment in the fine fraction. Raman images reveal that: small coesite inclusions of dimension < 9 µm are primarily monomineralic, whereas larger inclusions partially transformed to quartz; and garnet fracturing, fluid infiltration and the coesite-to-quartz transformation is a late process during exhumation taking place at c. 330°C. A model for the disintegration of coesite-bearing garnet enables the heterogeneous grain-size distribution to be explained by inclusion frequency. High abundances of coesite inclusions cause a high degree of fracturing and fracture connections to smaller inclusions, allowing fluid infiltration and the transformation to quartz, which in turn further promotes garnet disintegration.
Highlights
Since the first application of detrital garnet chemistry to discriminate sediment source regions (Morton, 1985), garnet major-element composition has become a valuable information resource in provenance studies (Mange & Morton, 2007; Krippner et al 2014; Hardman et al 2018; Tolosana-Delgado et al 2018) and first approaches of considering trace elements seem to be promising for future investigations (Čopjaková et al 2005; Hong et al 2020)
We analyse the role of source-rock lithology, inclusion size, inclusion frequency and fluid infiltration on the grain-size heterogeneity of coesite-bearing garnet based on a dataset of 2100 inclusion-bearing grains, of which 93 contain coesite, from the Saxonian Erzgebirge, Germany
By combining inclusion assemblages and garnet chemistry, we show that (1) mafic garnet contains a low number of coesite inclusions per grain and is enriched in the coarse fraction, and (2) felsic garnet contains variable amounts of coesite inclusions per grain, whereby coesite-poor grains are enriched in the coarse fraction and coesite-rich grains extensively disintegrated into smaller fragments resulting in an enrichment in the fine fraction
Summary
Since the first application of detrital garnet chemistry to discriminate sediment source regions (Morton, 1985), garnet major-element composition has become a valuable information resource in provenance studies (Mange & Morton, 2007; Krippner et al 2014; Hardman et al 2018; Tolosana-Delgado et al 2018) and first approaches of considering trace elements seem to be promising for future investigations (Čopjaková et al 2005; Hong et al 2020). Detrital UHP garnet grains often contain inclusions of coesite and diamond, which enables the systematic screening of entire catchments for the presence of UHP rocks as demonstrated in the Western Gneiss Region of Norway (Schönig et al 2018b), the central Saxonian Erzgebirge of Germany (Schönig et al 2019, 2020) and the D’Entrecasteaux Islands of Papua New Guinea (Baldwin et al 2021). In addition to mineral inclusion and chemical data of the coesiteand diamond-bearing detrital garnet of the central Saxonian Erzgebirge presented by Schönig et al (2019, 2020), we here present and comprehensively evaluate the entire dataset of 2100 inclusionbearing garnet grains with the aim of unravelling the distribution systematics of detrital coesite-bearing garnet regarding grain size and source-rock composition. We show that the disintegration of the initially large coesite-bearing garnet crystals during exhumation and processes of the sedimentary cycle is strongly controlled by inclusion size and frequency, leading to a heterogeneous detrital grain-size distribution
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