APPLICATIONS OF THE DIFFERENT TYPES OF MINERAL-PAIR GEOTHERMOMETERS TO ECLOGITE AT HUANGZHEN IN THE DABIE OROGEN

Abstract

Geothermometers of garnet-clinopyroxene Fe-Mg partition and quartz-mineral oxygen isotope fractionation are normally used to estimate temperatures of eclogite formation. Lately, natural observations and experimental measurements demonstrate that Zr contents in rutile change linearly with temperature and thus can be used as a geothermometer for high-grade metamorphic rock. This study presents a combined application of the three geothermometer to the same locality of low-T/UHP eclogite and enclosed quartz vein at Huangzhen in the Dabie orogen. Analysis of Zr contents in rutile from the eclogite suggests that temperatures calculated for intragrain rutiles are mainly 528 to 589°C, higher than those of 465 to 528°C for intergrain rutiles. The Zr contents of intragrain rutile in garnet are the highest, but the calculated temperatures of 503 ~ 589°C are still variably lower than the peak metamorphic temperatures of 670°C for the eclogite, The Zr content temperatures for rutile in the quartz vein are mainly between 465 and 528°C. High oxygen isotope temperatures of 650 to 695°C were obtained when pairing quartz with such resistant minerals as garnet, zircon and kyanite, suggesting that the refractory minerals have preserved their oxygen isotope compositions acquired during eclogitization and only suffered much weaker retrograde metamorphism than the other minerals in the eclogite. Thus the calculated oxygen isotope temperatures are concordant with the peak eclogite-facies metamorphic conditions. In contrast, low temperatures of 450 to 510°C were calculated when pairing quartz with such non-resistant minerals as rutile, omphacite, paragonite and zoisite/clinozoisite, indicating the effect of fluid activity during amphibolite-facies retrogression. Temperatures derived from garnet-clinopyroxene Fe-Mg partition can be divided into three groups: (1) 795 to 863°C, (2) 629 to 679°C, and (3) 468 ~ 572°C. The later two groups of temperatures are comparable with the temperatures calculated from the rutile Zr contents and mineral-pair oxygen isotope fractionations, corresponding to equilibration and reequilibration of Fe-Mg and oxygen isotope exchange during eclogite-facies metamorphism and amphibolite-facies retrogression, respectively. The first group temperatures are too high to be compatible with the known conditions for eclogitization, indicating that the omphacite symplectitization results in Fe-Mg partition disequilibria between garnet and some of the omphacite. The fluid activity during the eclogite exhumation may be the basic reason for either reequilibration or disequilibrium of elements and isotope partition by diffusion exchange. The Zr content temperatures in the intragrain rutiles are still lower than the known temperatures for peak eclogitization. This may indicate that these intragrain rutiles were crystallized during prograde metamorphism prior to peak eclogite-facies, but isolated and thus not able to exchange Zr with coexisting zircon to achieve peak eclogite-facies reequilibration. The decrease in intergrain rutile Zr content may be related to recrystallization of rutile, resulting in destruction of Zr exchange equilibrium between rutile and zircon.