Cryptic metasomatic agent measured in situ in Variscan mantle rocks: Melt inclusions in garnet of eclogite, Granulitgebirge, Germany

Abstract

Garnet of eclogite (formerly termed garnet clinopyroxenite) hosted in lenses of orogenic garnet peridotite from the Granulitgebirge, NW Bohemian Massif, contains unique inclusions of granitic melt, now either glassy or crystallized. Analysed glasses and re‐homogenized inclusions are hydrous, peraluminous, and enriched in highly incompatible elements characteristic of the continental crust such as Cs, Li, B, Pb, Rb, Th, and U. The original melt thus represents a pristine, chemically evolved metasomatic agent, which infiltrated the mantle via deep continental subduction during the Variscan orogeny. The bulk chemical composition of the studied eclogites is similar to that of Fe‐rich basalt and the enrichment in LILE and U suggest a subduction‐related component. All these geochemical features confirm metasomatism. In comparison with many other garnet+clinopyroxene‐bearing lenses in peridotites of the Bohemian Massif, the studied samples from Rubinberg and Klatschmühle are more akin to eclogite than pyroxenites, as reflected in high jadeite content in clinopyroxene, relatively low Mg, Cr, and Ni but relatively high Ti. However, trace elements of both bulk rock and individual mineral phases show also important differences making these samples rather unique. Metasomatism involving a melt requiring a trace element pattern very similar to the composition reported here has been suggested for the source region of rocks of the so‐called durbachite suite, that is, ultrapotassic melanosyenites, which are found throughout the high‐grade Variscan basement. Moreover, the Th, U, Pb, Nb, Ta, and Ti patterns of these newly studied melt inclusions (MI) strongly resemble those observed for peridotite and its enclosed pyroxenite from the T‐7 borehole (Staré, České Středhoři Mountains) in N Bohemia. This suggests that a similar kind of crustal‐derived melt also occurred here. This study of granitic MI in eclogites from peridotites has provided the first direct characterization of a preserved metasomatic melt, possibly responsible for the metasomatism of several parts of the mantle in the Variscides.

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

“…Such inclusions are also called "nanogranitoids" and likely represent melt droplets entrapped during garnet growth at HP/HT conditions (e.g., Cesare et al, 2009;Ferrero et al, 2016;Ferrero et al, 2019). Similar melt inclusions containing kumdykolite and/or kokchetavite have been found in the "Gneiss-Eclogite Unit" drilled in the Eger Crystalline Complex farther south (Kotková et al, 2014) and in the Granulitgebirge farther north (Borghini et al, 2020). The reason for crystal structures other than usual at HP/HT conditions, i.e., quartz, alkali feldspar, and plagioclase, is not yet fully understood.…”

Embryos of TTGs in Gore Mountain garnet megacrysts from water-fluxed melting of the lower crust

“…4b). At high T and lower crustal conditions, especially in presence of Na and Si (Mysen, 2015 and references therein), HFSE, U and Th can be transported in H2O-rich fluids and subsequently transferred into the melt (Borghini et al, 2020). The melt shows a high concentration of Zr (1400 ppm average), twice the expected amount (~690 ppm) based on zircon solubility (Boehnke et al, 2013) at the corresponding temperature and inferred major-element melt composition.…”

Highly evolved miaskitic syenites deciphering the origin and nature of enriched mantle source of ultrapotassic magmatism in the Variscan orogenic root (Bohemian Massif, Moldanubian Zone)

“…Such widespread mantle refertilization is probably recorded by the presence of K-rich melt inclusions trapped in Saxonian eclogites from Granulitgebirge (Borghini et al, 2020), exhibiting chemical features typical of common durbachitic rocks (Janoušek et al, 2020). Accordingly, occurrences of…”