MODELING P-T PATHS FOR A GT-KY GNEISS FROM THE N. QAIDAM HP/UHP BELT WESTERN CHINA

A Gt-Ky paragniess from the Dulan region, N. Qaidam HP/UHP, belt contains the assemblage Bt + Pl + Kfs + Gt + Ky + Qtz + Rt + Ap. A pseudosection constructed for the sample’s bulk composition predicts the observed assemblage to form at ~11-15 kbar and ~810-870 °C, which is consistent with previously determined peak conditions of 13-17 kbar and 790-880 °C for mafic granulite pods hosted by the gneiss. The pseudosection predicts ~2-8 vol % melt coexisting with the observed assemblage. Predicted abundances also closely match observed values, with 8-18 vol % Gt and 0-10 vol % Bt in the pseudosection, and 10-15 vol % Gt and 10 vol % Bt in the sample.

Compositional isopleths intersect at lower T than the above values predicted for the observed assemblage field. Pl isopleths intersect at 12.5 kbar , ~700 °C, and Gt rim isopleths converge at ~12-12.3 kbar and ~660-670 °C. This agrees with previous P-T estimates of 13-15 kbar and 730-775 °C for the gneiss using the Gt-Bt thermometer and GASP barometer. Average Gt size is 0.2 mm, and exhibits limited zoning with average core values of Alm₅₈Prp₁₇Grs₂₃ and rim values of Alm₆₂Prp₁₉Grs₁₈. This suggests Gt re-equilibrated ~190 °C down T, representing a segment of the P-T path from the observed assemblage to the isopleth intersections.

To model melt loss potential, a melt-reintegrated bulk composition is estimated by calculating the composition of the initial (low-T) melt, and adding it to the sample’s bulk composition until the melt-in reaction is bounded by H₂O saturated fields. The melt-reintegrated pseudosection predicts a maximum of ~20-35 vol % melt in coexistence with the observed assemblage, ~ 80-85 % of which is lost during peak metamorphism. Melt loss is supported by the lack of retrograde Ms and field evidence for melt escape.

The reintegrated pseudosection displays a Kfs-in reaction, which is consistent with reaction textures in thin section that display Kfs growing around Pl. The prograde P-T path passes through the Kfs-in reaction (~15.5 kbar, 750 °C) to the observed assemblage (~12 kbar, 850 °C), after which melt is lost. This is supported by Gt growth over fields containing Ms, which occurs as inclusions in Gt. This two part P-T path for the host gneiss is consistent with estimates for mafic granulites, supporting a shared P-T history.