Phase equilibrium modelling and implications forP–Tdeterminations of medium‐temperature UHP eclogites, North Qaidam terrane, China

Abstract

AbstractIn contrast to low‐Teclogites (garnet growth zoning preserved) or high‐Teclogites (garnet diffusionally homogenized at peak conditions), medium‐temperature eclogites pose additional challenges toP–Tdeterminations due to the partial preservation of garnet zoning. The Dulan area, in the southeastern part of North Qaidam ultrahigh‐pressure terrane, exposes minor eclogites hosted by ortho‐ and paragneisses. Four fresh, medium‐temperature eclogites contain the paragenesis Grt+Omp+Rt+Qz/Coe+Ph±Ky±Zo. In all samples, garnetXMgshows little zoning, suggesting diffusional modification, and precludes the use of pyrope+almandine+grossular isopleth intersections to determine aP–Tpath. However, in one sample, sharp zoning in grossular content suggests that grossular growth compositions are preserved. Since garnet pyrope+almandine compositions appear to be modified, we instead use the intersections of grossular and garnet volume isopleths to define a progradeP–Tpath. This approach yields a path from ~17 kbar and ~410°C to ~35 kbar and ~625°C with a gradient of ~5–9°C/km through the lawsonite stability field. PeakP–Tconditions determined from the intersection between Si pfu in phengite and Zr‐in‐rutile isopleths are ~26–33 kbar and ~625–700 °C for the four eclogites. These conditions are close to the limit of the lawsonite stability field, suggesting that fluid released from lawsonite breakdown may have promoted re‐equilibration at these conditions. These peak conditions are also in good agreement (within 3 kbar and 50°C) with garnet–omphacite–phengite (±kyanite) thermobarometry in three of the four samples. We regard the phengite–rutile constraints as more reliable, because they are less sensitive to uncertainties associated with ferric iron compared to conventional thermobarometry. Phase equilibrium modelling predicts that the retrograde assemblage of amphibole+zoisite formed at ~60 km. Infiltration of external fluids was likely required for the growth of these hydrous minerals. Based on the comparison ofP–Testimation methods applied in this study, we propose that the garnet grossular+volume isopleths can recover the progradeP–Tpath of medium‐temperature eclogites, and that the combination of phengite+rutile isopleths represents a more robust approach to constrain peakP–Tconditions.