FLUID RELEASE AND POSSIBLE YO-YO SUBDUCTION RECORDED BY ECLOGITE, NORTH QAIDAM UHP TERRANE, WESTERN CHINA

Ultrahigh-pressure (UHP, depths ≥100 km) eclogite samples from the Dulan area, North Qaidam terrane, Western China, preserve evidence of fluid release along a prograde pressure-temperature (P-T) path. An eclogite sample (garnet + omphacite + zoisite, with minor quartz + phengite) contains 500-600 µm garnet porphyroblasts, 2-4 mm zoisite crystals, and zircons with a U-Pb age of 432.6 ± 4.0 Ma. Garnets display strong compositional zoning with Ca decreasing from core to rim (core = Grs₃₄, rim = Grs₂₆), but also a Ca increase within the mantle (Grs_25-28). One garnet porphyroblast contains a euhedral, tabular quartz inclusion between the mantle and rim zone interpreted as a coesite pseudomorph. Isochemical phase diagrams (pseudosections), assuming H₂O saturation, predict a prograde P-T path that begins at garnet core growth (<400 °C, ~16 kbar) and ends at peak P-T conditions (~675 °C, ~28.5 kbar). The intersection of a Zr-in-Rt thermometer isopleth and the max Si in Phe isopleth locate peak P-T conditions within the coesite field, consistent with published UHP conditions and the proposed coesite pseudomorph. The Ca zoning reversal within the garnet mantle produces a P-T path with a P reversal of ~26 kbar to ~25 kbar to ~26 kbar between ~530-610 °C. This P reversal is interpreted as evidence for yo-yo subduction, which may explain the prolonged UHP duration (~20 Myr) in the Dulan area. However, the Ca zoning reversal could also be a result of chemical disequilibrium during garnet growth. The predicted whole rock H₂O wt.% concentration decreases along the prograde P-T path from 5.1 wt.% H₂O (<400 °C, ~16 kbar) to 0.5 wt.% H₂O (~675 °C, ~28.5 kbar). The fluid release rate, based on an assumed constant subduction rate, is initially low, but increases in the middle of the P-T path (~475-610 °C, ~23-26 kbar) as chlorite, amphibole, and lawsonite break down. The increased rate of fluid release occurs during the P reversal when P/depth remains relatively similar, but T and garnet abundance increase (~530 to ~610 °C and ~14 to ~31 vol.% garnet). The fluid release rate more than doubles when amphibole breaks down at ~610 °C, ~26 kbar. Enhanced fluid release is expected to promote zircon growth/recrystallization, suggesting that the 432.6 ± 4.0 Ma zircon age may record the ~475-610 °C, ~23-26 kbar part of the prograde P-T path.