METAMORPHIC EVOLUTION OF A KYANITE ECLOGITE, NORTH QAIDAM TERRANE, WESTERN CHINA

The North Qaidam Mountains of western China near Dulan house areas of UHP terranes, these terranes contain lenses of kyanite-bearing eclogite.

A kyanite eclogite sample exhibits a mineral assemblage of garnet, omphacite, kyanite, amphibole, zoisite and quartz. Garnet occurs as inclusion rich porphyroblasts within a foliated matrix defined by kyanite, omphacite and zoisite. Idiomorphic garnets are zoned with increasing Ca and Mg content from core to rim, as well as a decreasing content of Fe and Mn from core to rim. Omphacite and quartz are present as inclusions in garnet. Omphacite (X_jd =0.31) is partially replaced by fine grained symplectite making rims. Xenomorphic kyanite is also partially replaced by symplectite of fine grained margarite and zoisite.

Rutile grains of ~60 µm contain Zr concentrations of 84-140 ppm, leading to an average of 117 ±13 ppm. These concentrations yielded temperatures of 622-660 ºC with an average of 647 ºC ± 8 ºC calculated at 30 kbar.

Phase equilibria modeling predicts that the stability field that best fits the observed mineral assemblage (garnet, omphacite, kyanite and quartz) is within a temperature range of 620-800 ºC and a pressure range of 19-30 kbar. However, amphibole and zoisite are present at 500-800 ºC and less than 23 kbar.

Preliminary phase equilibrium diagrams as well as Zr-in-rutile thermometry indicate that this sample reached peak P-T conditions within the lawsonite stability field. Also these models suggest that the amphibole and zoisite grew during the early retrograde path.