Depletion, cryptic metasomatism, and modal metasomatism (refertilization) of Variscan lithospheric mantle: Evidence from major elements, trace elements, and Sr-Nd-Os isotopes in a Saxothuringian garnet peridotite

Abstract

Orogenic garnet peridotites of diverse origins and histories in the Bohemian Massif attest to a variety of mantle processes, including partial melting, cryptic metasomatism, and modal metasomatism (refertilization), all of which are recorded by Saxothuringian garnet peridotite from the T-7 borehole in northern Bohemia. The T-7 peridotite consists of interlayered garnet lherzolite, harzburgite, and phlogopite-garnet pyroxenite lenses that yield peak temperatures and pressures of 1030–1150°C and 36.1–48.0kbar. Olivine crystallographic preferred orientations exhibit [axial](010) slip, corresponding to a pure shear component of deformation under relatively low flow stress conditions. Some lherzolite samples are fertile, resembling primitive mantle in major and trace element composition, but other lherzolites are slightly depleted in incompatible major elements, HREE, and HFSE, and slightly enriched in LREE. Harzburgite is depleted in incompatible major elements, HREE, and HFSE, but enriched in LREE. Harzburgite adjacent to pyroxenite has been refertilized, containing phlogopite, less olivine, more orthopyroxene, and more garnet than distal harzburgite. The T-7 peridotite compositions are the result of variable degrees of partial melting in the spinel stability field, followed by cryptic metasomatism and modal metasomatism by transient basaltic melts in the garnet field. Trace elements, Sr and Nd isotopes, and occurrence of phlogopite reflect a subduction component in the metasomatising melts. Partial melting of the T-7 peridotite was a Proterozoic event, as indicated by Rhenium depletion model ages (TRD); the age of cryptic and modal metasomatism is unconstrained, but is thought to be related to Variscan subduction and amalgamation of the Bohemian Massif.