ASSESSING TRACE ELEMENT DISTRIBUTION TO AID IN CONSTRUCTION OF P-T-T PATHS FOR UHP ECLOGITE AND HP GRANULITE SAMPLES FROM THE DULAN AREA, NORTH QAIDAM TERRANE, CHINA

The Dulan area in the North Qaidam terrane, China hosts ultrahigh pressure (UHP) eclogites (>25 kbar/~750˚C) and high pressure (HP) granulites (14-18 kbar/~950˚C) in ortho- and paragneisses. Dulan experienced HP granulite and UHP eclogite facies metamorphism between 449-410 Ma and 462-422 Ma respectively. The UHP eclogite has a mineral assemblage of Grt + Omp + Rt + Qtz + Ep + Phe + Ky. The HP granulite has a mineral assemblage of Grt + Cpx + Pl + Qtz + Ky + Czo + Rt + Ap. Using LA-ICP-MS we collected trace element abundances in Grt, Amp, Omp, Cpx, Pl, and epidote-group minerals. Grt, Amp, and Omp from one Dulan UHP eclogite show very little REE zoning. Grt and Omp are subhedral and relatively fine grained, but Amp is coarse-grained and appears to have formed later. UHP Grt is enriched in heavy REE’s (HREE) with no Eu anomaly and Sm to Yb values 10-100 times chondrite and appears to correlate with Zrn REE, suggesting that the zircon age (432.4 ± 2.4 Ma) records this assemblage. Amp and Omp are enriched in middle REE’s (MREE), particularly Sm between ~10-60 times chondrite, and lack an Eu anomaly. HP granulite Grt is strongly zoned with Mg rich rims and Ti rich cores. HREE’s (Dy, Er, and Yb) are concentrated in the Grt rim, up to 90 times chondrite, and drastically decrease to ~1.0-10 times chondrite towards the core except for Yb. Yb abundance is highest in the rim but varies up to 80 times chondrite in the core and mantle. Light and middle REE’s hover at or below detection limits with most values ~1.0 chondrite and appear to be concentrated in the core-mantle of the Grt. HP granulite Grt cores show a (+) Eu anomaly (2.1 Eu/Eu*), suggesting they could have grown from Pl breakdown. REE patterns of HP granulite Amp is similar to UHP eclogite Amp. HP granulite Czo has consistent concentrations of REE’s from ~1-10 times chondrite, (+) Eu anomalies (~2.0 Eu/Eu*), and one Czo showing LREE concentrations ~10,000 times chondrite. HP granulite Cpx REE patterns are very similar to Amp, but one HP granulite Cpx does show lower concentrations of L-MREE’s (~0.1-1 times chondrite) and a (-) Eu anomaly of 0.53 Eu/Eu*. Zoning in HP granulite Grt rims show similar HREE enrichments as dark CL rims in Zrn. We suggest that these zones in Grt grew contemporaneously and at equilibrium with the Zrn rim suggesting that the mean zircon age of 418 ± 2 Ma records this stage.