Comparison of Conventional, Trace Element, and Pseudosection Thermobarometry in UHP Eclogite, North Qaidam Terrane, Western China

In the southeastern North Qaidam terrane, near Dulan, felsic host gneisses enclose minor eclogite lenses. A small (~3°C/km) increase in temperatures to the west based on Zr-in-Rt thermometry is supported by a textural trend of zoned garnet with prograde mineral inclusions in the west to unzoned garnet with only peak-stage inclusions in the east. A western eclogite sample contains minor Ep and trace Phe in addition to Grt-Omp-Qtz-Rt; a strong foliation is defined by banding of Grt and Omp. Garnet Ca falls significantly from core (Alm₄₄Prp₂₃Grs₃₂) to rim (Alm₄₈Prp₂₇Grs₂₃). Phengite contains 3.34-3.36 Si pfu. Conventional Grt-Omp-Phe thermobarometry yields 23-27 kbar, 660-730°C, and Zr-in-Rt thermometry yields 671 ± 9°C (n = 37). An isochemical phase diagram (pseudosection) for the system NCKFMASHO calculated with PerpleX indicates that garnet X_grs decreases with increasing P and T, suggesting that observed garnet zoning reflects prograde growth. Garnet rim compositional isopleths intersect at 23-27 kbar, 580-620°C, depending on choice of effective bulk composition and assumed Fe³⁺/Fe_total. Garnet rims help define the foliation, suggesting that these P-T conditions apply to this deformation. An eastern eclogite sample contains minor Ep and abundant, coarse-grained Phe in addition to Grt-Omp-Qtz-Rt; foliation is weak. Garnet zoning is weak; compositions are Alm_41-43Prp_26-28Grs_29-32. Phengite cores contain 3.48-3.56 Si pfu. Conventional Grt-Omp-Phe thermobarometry yields 32-36 kbar, 700-750°C, and Zr-in-Rt thermometry yields 691 ± 12°C (n = 34). Conventional thermobarometry and pseudosections yield similar peak P-T conditions, but from different mineral compositions: thermobarometry yields peak P from high-Ca Grt, but the pseudosection yields peak P from intermediate-Ca Grt; maximum Ca in Grt is predicted at lower P and T. Pseudosection-based peak Ts are significantly lower than those from Zr-in-Rt and conventional thermometry. Wide isopleth spacing in the observed assemblage field makes isopleth positions very sensitive to the bulk composition; Zr-in-Rt results are therefore regarded as the best estimate of peak conditions, but isopleth patterns provide valuable constraints on possible P-T paths.