Trace Elements and U-Pb Ages of Zircons from Myanmar Jadeite-Jade by LA-ICP-MS : Constraints for Its Genesis

Jadeite-jade originates in subduciton-related HP/LT metamorphic zone, thus study of it is of great importantance for shedding lights in subduction research. Zircon is often used to determine the age of rocks, and it's usually applied to the investigation of genesis and formation time of jadeite-jade. Myanmar is the most primary origin of high quality jadeite-jade, of which zircon is a common accessory mineral. To explore the genesis and formation time of Myanmar jadeite-jade, raw jadeite-jade rock with enriched zircon inclusions were studied. In addition, Cathodoluminescence (CL) images were obtained to characterize distribution of colors and diversity of oscillating zones from well-crystallized zircons. Moreover, trace elements composition and U-Pb ages of these zircons were further determined using Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) according to characteristic CL images. The results showed that zircons with different colors and oscillating zones shapes have different trace elements distributions. The overall performance was that: chondrite-normalized REE pattern of zircons displays depleted LREE, enriched HREE, with edivdent Ce(Ce/Ce* =19.2-74. 2) positive anomalies and slight Eu(Eu/Eu* = 0. 49-0. 72) negative anomalie. High total REE (Sigma REE =810 similar to 3 984 mu g . g(-1)), (Yb/Sm)(N) values ranged from 23. 1 to 195. 1 (most of them less than 100), and Th/U ratios 0. 28 to 1. 18. These zircons can be classified into magmatic zircons, originated from mafic basalt magma in the upper mantle wedge of subduction zone. This type of magma was formed by partial melting of subcontinental peridotite which reacted with fluids released from the subduction process. Moreover, Ti thermometer reveals that the forming temperatures of those zircons ranged from 762 to 923 degrees C. The mean (206) Pb/U-238 age is (157. 3 (+/-)1. 3) Ma, which represents the formation time of these zircons. Distribution of colors and diversity of oscillating zones in CL images are irrelevant to forming ages of studied zircons. This distribution might be caused by trace element imparity in zircons, which reflects the dynamic evolution of magmatic melt compositions during zircon formation.