Textural and Thermodynamic Model Evidence of Fluid Release from Eclogite, North Qaidam UHP Terrane, Western China

Abstract:

Ultrahigh-pressure (UHP, depths ≥100 km) eclogite samples from the Dulan area, North Qaidam terrane, Western China preserve evidence of decreased whole rock H₂O wt.% along the pressure-temperature (P-T) path. Isochemical phase diagrams (pseudosections), assuming H₂O saturation, predict that more hydrous mineral assemblages change to less hydrous mineral assemblages with increasing P and T along the prograde path, as well as during decompression along the retrograde path. Pseudosections were calculated for two samples from the Dulan area, D5A and D126A. Sample D5A (garnet + omphacite, with minor amphibole + epidote) contains 1.3-2.5 mm garnet porphyroblasts. D5A garnets display strong compositional zoning with increasing Ca from core (Alm₄₉ Prp₂₃ Grs₂₈) to rim (Alm₄₃ Prp₂₄ Grs₃₃). D5A garnet porphyroblasts contain rectangular polycrystalline epidote inclusions, interpreted as lawsonite pseudomorphs that provide evidence for fluid release. Sample D126A (garnet + omphacite + zoisite, with minor quartz + phengite) contains 500-600 µm garnet porphyroblasts, and 2-4 mm zoisite crystals. D126A garnets display strong compositional zoning with decreasing Ca from core (Alm₄₃ Prp₂₅ Grs₃₂) to rim (Alm₄₅ Prp₂₈ Grs₂₇). D126A garnet porphyroblasts contain potential coesite pseudomorphs, seen as a euhedral, tabular quartz inclusion between the core and rim zone.

Thermodynamic models of sample D126A calculate a stable garnet core compositional isopleth intersection in the coesite field at ~660 °C and ~28 kbar. The stable mineral assemblage predicted for these conditions is omphacite + amphibole + phengite + garnet + lawsonite + coesite. The stable rim compositional isopleth is also in the coesite field at ~660 °C and ~31 kbar, within the same mineral assemblage as the core. The predicted peak whole rock H₂O wt.% of 0.9-1.3 wt.% is higher than the 0.4 wt.% H₂O calculated from sample D126A’s present mineral assemblage, stable at >630 °C and <26 kbar. The difference in H₂O wt.% indicates that between 0.5 and 0.9 wt.% H₂O would be released during decompression from peak conditions. Lawsonite breakdown produced the coarse grained zoisite, seen in sample D126A, and polycrystalline epidote, seen in sample D5A.