ZIRCONIUM-IN-RUTILE AND SILICA-IN-PHENGITE CONSTRAINTS ON PRESSURE-TEMPERATURE PATH OF DULAN ECLOGITES

Eclogite samples collected from the Dulan area in the North Qaidam ultrahigh-pressure (UHP) terrane in western China, show different peak metamorphic assemblages, and degrees of retrogression. Sample D154 collected in the South Dulan Belt is highly retrogressed and presents a mineral assemblage of grt + ph + zo + omph with accessory qtz + rt. The retrogression in this sample is characterized by the presence of amph and zo, and textural features such as ttn coronas around rt, and symplectitic growth of cpx and plg after omph, which is almost completely replaced in the sample. Sample D167 collected from the North Dulan Belt is less altered, displaying minor symplectite alteration, and has a mineral assemblage of grt + qtz + omph + with accessory ph and qtz pseudomorphs after coesite. X-ray garnet compositional maps shows increasing Ca and Mn content to the core, and increasing Mg content to the rim in both samples, but D154 displays a higher degree of zonation. Pseudosections predict peak conditions for D154 and D167 at 590-800 °C, and 24.5-15.5 kbar and at 634-1140 °C and 28 to greater than 35 kbar, respectively. Eclogite D154 yielded Zr-in-rutile temperatures for rutile in the matrix of 658 ± 11 °C and temperatures for rutile inclusions of 646 ± 8 °C. D167 yielded a temperature of 669±8 °C, with no change between inclusion and matrix rutiles. Ph in eclogite D154 has 3.28 a.f.p.u., whereas eclogite D167 has 3.53 a.f.p.u. The intersection between the Zr-in-rutile thermometer and Si-in-phengite for D154 and D167 is at 627 °C and 24 kbar, and 665 °C and 27.5 kbar, respectively. This intersection represents peak conditions in D167. The intersection for D154 is interpreted to represent the P-T conditions of regressive metamorphism.