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In contrast to low-T eclogites (garnet growth zoning preserved) or high-T eclogites (garnet diffusionally homogenized at peak conditions), medium-temperature eclogites pose additional challenges to P-T determinations 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, garnet X-Mg shows little zoning, suggesting diffusional modification, and precludes the use of pyrope+almandine+grossular isopleth intersections to determine a P-T path. 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 prograde P-T path. This approach yields a path from similar to 17 kbar and similar to 410 degrees C to similar to similar to 35 kbar and similar to 625 degrees C with a gradient of similar to 5-9 degrees C/km through the lawsonite stability field. Peak P-T conditions determined from the intersection between Si pfu in phengite and Zr-in-rutile isopleths are similar to 26-33 kbar and similar to 625-700 degrees 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 degrees 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 similar to 60 km. Infiltration of external fluids was likely required for the growth of these hydrous minerals. Based on the comparison of P-T estimation methods applied in this study, we propose that the garnet grossular+volume isopleths can recover the prograde P-T path of medium-temperature eclogites, and that the combination of phengite+rutile isopleths represents a more robust approach to constrain peak P-T conditions.
Hernández‐Uribe, D, Mattinson, CG, Zhang, J. Phase equilibrium modelling and implications for P–T determinations of medium‐temperature UHP eclogites, North Qaidam terrane, China. J Metamorph Geol. 2018; 36: 1237– 1261. https://doi.org/10.1111/jmg.12444
Journal of Metamorphic Geology
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