NEW AGE CONSTRAINTS ON THE P–T HISTORY OF DEEPLY SUBDUCTED COESITE-BEARING ECLOGITE AT YANGKOU BAY, CENTRAL SULU BELT, CHINA

Constraining burial and exhumation rates of UHP-HP metamorphic rocks requires robust P, T and age information related to both the prograde and retrograde evolution. However, such information may be difficult to obtain from deeply subducted metamorphic terrains for two reasons: 1) fluid deficient conditions limit mineral growth/recrystallization during the late prograde/early retrograde evolution; and, 2) generation of fluid or melt during exhumation facilitates overprinting of the earlier history.

At Yangkou Bay, intergranular coesite-bearing eclogite is preserved within rootless intrafolial F1 fold noses in quartz eclogite and quartz-rich phengite schist, which were exhumed from pressures that may have reached or exceeded 7 GPa. Previous attempts to date thin metamorphic overgrowth rims (2-5μm) or solid recrystallization domains on zircon using SHRIMP, EMP and LA-ICP-MS methods all failed to produce valid metamorphic ages. To address this problem, we used Single Shot-Laser Ablation Split Stream (SS-LASS) ICP-MS analysis, to get multiple dates from the near-surface of unpolished zircon crystals (100nm depth each shot, to 1–2μm total depth). In this case, we were able to obtain robust metamorphic ages from the overgrowth rims.

In polished mounts, zircons from all samples show similar oscillatory-zoned relict cores with similar protolith ages of c. 820–780 Ma. SS-LASS analysis of thin solid-state recrystallized rims of coesite-bearing zircon from a coesite eclogite yields a weighted mean age of 249 ± 5 Ma, thin coesite-bearing metamorphic zircon rims from a quartz-rich phengite schist (~5μm) and quartz eclogite (5-20μm) yield a weighted mean age of 225 ± 9 Ma and 225 ± 8 Ma , respectively.

Structural analysis shows that the intergranular coesite-bearing eclogites were protected from fluid infiltration and retrogression due to strain localization, whereas the other two rock types were retrogressed by an intergranular fluid generated early during exhumation. We interpret the coesite eclogite to record the timing of the later stage of the prograde burial, whereas the other samples record the timing of exhumation from the coesite to the quartz stability field. This indicates that the collision between the Yangtze and North China Cratons had begun prior to c. 249 Ma, earlier than generally thought (240 Ma).