No abstract

The Saidikulam nephrite deposit, which is famous for its high-quality white nephrite in history and nowadays, is located in the eastern part of the Western Kunlun Orogen, in the northwestern margin of the Tibetan Plateau, Yutian County, Xinjiang, China. Field investigations and petrographic observations revealed the presence of five mineral assemblage zones from granodiorite to dolomitic marble in the Saidikulam nephrite deposit: slightly epidote-altered granodiorite → epidote-altered diopsidite → nephrite → calcite tremolitite → calcite-bearing dolomitic marble. However, little is known about the relationships among these five zones or the formation age of the deposit. The slightly epidote-altered granodiorite consists of plagioclase, orthoclase, quartz, biotite, and epidote, with accessory apatite, zircon, and allanite. They exhibit slightly negative Eu anomalies (0.69–0.72), with declined LREE and flat HREE patterns. They are also strongly enriched in Rb, Zr, U, and Sm, and depleted in Hf, Ti, P, and Nb. The epidote-altered diopsidite close to the granodiorite intrusions is a relatively high-temperature metasomatic rock composed of Fe-rich diopside and actinolite, while the calcite tremolitite adjacent to the dolomitic marble is a relatively low-temperature rock composed of prismatic tremolite and calcite. The main mineral component of the nephrite is tremolite, with minor epidote, chlorite, apatite, calcite, graphite, and allanite. The nephrites have low ∑REE contents (4.38–18.53 ppm), with declined LREE and flat HREE patterns. All of the analyzed nephrite exhibits pronounced negative Eu anomalies (δEu = 0.20–0.82), and they are strongly enriched in Rb, U, and Ti and relatively depleted in Ba and HFSEs. SHRIMP U-Pb dating of zircons from the granodiorite yielded a Concordia age of 453.8 ± 3.7 Ma, which is interpreted as the upper limit of the Saidikulam nephrite formation age. The zircons in the epidotized syenite in the deposit yielded a SHRIMP Concordia U-Pb age of 489.2 ± 4.2 Ma. These two ages indicate that the dolomitic marble experienced at least two thermal events. The approximate ages of the nephrite deposits along the giant Kunlun-Altyn Tagh belt indicate that the closure of the Proto-Tethys may have been the predominant driving force for the formation of the whole nephrite deposits. The multistage metasomatism and the Fe absorption of epidote-altered diopsidite had a positive impact on the formation of high-quality nephrite.

The Mogok metamorphic belt (MMB), Myanmar, is one of the most well-known gemological belts on Earth. Previously, 40Ar/39Ar, K-Ar, and U-Pb dating have yielded Jurassic-Miocene magmatic and metamorphic ages of the MMB and adjacent areas; however, no reported age data are closely related to the sapphire and moonstone deposits. Secondary ion mass spectrometry (SIMS) U-Pb dating of acicular rutile inclusions in sapphire and furnace step-heating 40Ar/39Ar dating of moonstone (antiperthite) in syenites from the MMB yield ages of 13.43 ± 0.92 and 13.55 ± 0.08 Ma, respectively, indicating both Myanmar sapphire and moonstone formed at the same time, and the ages are the youngest published in the region. The ages provide insight into the complex histories and processes of magmatism and metamorphism of the MMB, the formation of gemstone species in this belt, and the collision between India and Asia. In addition, our high field strength element data for the oriented rutile inclusions suggest an origin by co-precipitation rather than exsolution. In situ age determination of this nature is particularly significant since rutile inclusions in other gemstones, such as rubies, can be used to help constrain the geological history of their host rocks elsewhere.