K-feldspar-bearing coesite pseudomorphs in an eclogite from Lanshantou (Eastern China)

K-feldspar occurs alongside quartz (after coesite) in microinclusions within omphacite in an eclogite from Lanshantou within the "Su-Lu Coesite-Eclogite Province" of Eastern China. The K-feldspar, investigated by electron and Raman microprobes, is sanidine or orthoclase but not microcline. Modal determination of the inclusions by image analysis revealed a variable proportion of K-feldspar and quartz. Petrofabric analysis demonstrated that some of the inclusions have regular shapes which are not those of negative omphacite but are compatible with those of euhedral coesite. Several hypotheses for the origin of the K-feldspar + quartz microinclusions are discussed: recrystallisation of a syn-eclogite partial melt; replacement of a previous high-pressure mineral; evolution of the KAlSi3O8 component of the eclogitized plagioclase; exsolution of the component KAlSi2O6 from former K-bearing omphacite coupled with reaction with quartz, after or during the coesite to-quartz transformation. This last hypothesis is the preferred one as it is the only one which simultaneously explains the shapes inherited from coesite, the variable proportions of K-feldspar vs, quartz, and the occasional restriction of K-feldspar to a marginal palisade-like zone. The P-T evolution of the microinclusions has been constrained using the quartz-coesite transition, (Al,Si)-ordering in feldspar, and also a computation of the equilibrium: NaAlSi3O8 (in K-feldspar) = NaAlSi2O6 (in omphacite) + SiO2 (quartz/coesite). Rapid retrogression may be inferred from several distinct features. insignificant chemical zoning in garnet and omphacite; limited retrogressive symplectitization; conservation of the pure chemical composition of the K-feldspar; and conservation of moderately disordered K-feldspar. Static retrogression is also indicated by the absence of plastic deformation during retrogression, as shown by the conservation of the euhedral shapes of the replaced coesite. These deductions raise important questions concerning the geodynamical mechanisms responsible for the retrogression of ultrahigh-pressure metamorphic terranes. The inferred rapid and static retrogression could be interpreted as a rapid exhumation from very great depths without plastic deformation.