Eclogitic metapelites in the western segment of the north Qaidam Mountains: Evidence on "in situ" relationship between eclogite and its country rock

In Yuka-Luofengpo area of the north Qaidam Mountains, eclogitic metapelites are recognized. The metapelites enclosed lenses of eclogites, and locally intercalated with eclogites.Their typical mineral assemblages are garnet + kyanite + chloritoid + phengite + quartz + rutile.Strong growth zoning is preserved in garnets of metapelites, and phengite contains up to 3.4 Si per formula units. The petrographic observations and textural relations testify to the following sequence of mineral assemblages connected to three metamorphic stage: (1) Grt+ Chll + Cldl+ Phel ± Stl + Qtz; (2) Grt+Ky +Phell±Cldll +Qtz; and (3) Grt + Cldlll + Chlll + Phelll +Stll + ky + Qtz. Applying THERMOCALC Program, Grt-Phe thermometer and Grt-Ky-Phe-Qtz barometry,P-T conditions for three metamorphic stages were obtained: P=1.07±0.31GPa, T=564±22℃ (prograde stage); P=2.3-3.1GPa, T=615-700℃(peak stage); and P =1.22 ± 0.26GPa, T=581± 20℃ (retrograde stage). A hairpin shape P-T path similar to that of adjacent eclogite is inferred.In combination with eclogitic mineral relics in marbles and orthogneisses enclosing eclogites, we thought that the relationship between eclogites and country rocks is "in situ" rather than "tectonic emplacement".