Deep subduction of felsic rocks hosting UHP lenses in the central Saxonian Erzgebirge: Implications for UHP terrane exhumation

Abstract

Contrasting metamorphic conditions determined by chemical geothermobarometric investigations of ultrahigh-pressure (UHP) lenses surrounded by high-pressure (HP) and medium-pressure (MP) 2 felsic country rocks are an enigmatic feature of UHP terranes. One of the major questions arising is whether the UHP lenses and the country rocks are a product of different peak metamorphic conditions corresponding to different maximum depth or whether country rocks also experienced UHP conditions but equilibrated and/or re-equilibrated at a different metamorphic stage. Here we address this question to the central Saxonian Erzgebirge in the northwestern Bohemian massif, Germany. In order to screen the variety of garnet from lithologies occurring in the study area, we analyzed the detrital garnet record from seven modern stream sands. In addition to 700 inclusionbearing garnet grains previously studied from the 125-250 µm grain-size fraction, we analyzed the 63-125 and 250-500 µm fractions and extended the dataset to overall 2100 inclusion-bearing grains. The new findings of coesite and diamond inclusions in several garnet grains, which are in compositional contrast to garnet of the known UHP lenses but match with those of the felsic country rocks, show that considerable parts of the country rocks underwent UHP metamorphism.Melt inclusions containing cristobalite, kokchetavite, and kumdykolite in garnet derived from the country rocks point to partial melting and re-equilibration during exhumation at HP/HT conditions. Although an amalgamation of rocks which reached different maximum depth may be responsible for some of the contrasting peak metamorphic conditions, the mineralogical evidence for UHP conditions in the felsic country rocks surrounding the UHP lenses proves a largely coherent slab subducted to UHP conditions. Furthermore, the presence of coesite in the subducting voluminous felsic crust and its transformation to quartz during exhumation have great implications for buoyancy development during the metamorphic cycle, which may explain the high exhumation rates of UHP terranes.

Life-cycle analysis of coesite-bearing garnet

“…by the presence of coesite or pseudomorphs after coesite (Schmädicke et al 1992;Massonne, 2001;O'Brien & Ziemann, 2008;Gose & Schmädicke, 2018). However, previous investigations of mineral inclusions in detrital garnet of the sampled catchments revealed that coesite-bearing UHP rocks occur in all studied catchments; they are not solely confined to eclogite and paragneiss lenses, but also include country-rock gneiss which re-equilibrated during high-temperature (HT) exhumation at HP conditions (Schönig et al 2019(Schönig et al , 2020. All lithologies occurring in the six catchments therefore represent a potential source for coesitebearing garnet grains and can be roughly subdivided into mafic sources, that is, eclogite and felsic sources (i.e.…”

“…Measurement conditions include an accelerating voltage of 15 kV and a beam current of 20 nA. Counting times were 15 s for Si, Mg, Ca, Fe and Al, and 30 s for Compared with the map of Schönig et al (2019Schönig et al ( , 2020, an eclogite lens was added in the catchment of sample JS-Erz-8s according to Kossmat & Reinisch (1931).…”

“…In contrast, in a beach placer of the D'Entrecasteaux Islands of Papua New Guinea, coesite-bearing garnet has solely been found in the > 200 μm fraction (Baldwin et al 2021). In modern sands from the central Saxonian Erzgebirge of Germany, coesite-bearing garnet grains show an even more complex distribution (Schönig et al 2019(Schönig et al , 2020. In some samples they are enriched in the 250-500 μm fraction and in others in the 63-125 μm fraction, while some do not show a clear trend.…”

“…This becomes particularly important for sediments being derived from high-pressure (HP) to ultra-high-pressure (UHP) source rocks, as state-of-theart discrimination schemes based on garnet composition do not enable their distinction. In contrast, 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(Schönig et al , 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 (2019Schönig et al ( , 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. In addition, the substantial quantity of 93 coesite-bearing grains, which contain a total of 193 coesite inclusions, enables the evaluation of the role of inclusion size, inclusion frequency and fluid availability for coesite preservation.…”