The Triassic Saima alkaline complex on the northern margin of the North China Craton (NCC) is composed mainly of nepheline syenite, along with some nepheline syenite pegmatite and mineralized nepheline syenite, and it contains REEs (bastnäsite) + Nb (Nb-rich rutile and pyrochlore) and Zr (zircon) mineralization. However, the effects of magmatic and hydrothermal processes on Nb enrichment and mineralization remain unclear. Thus, we conducted a comprehensive study of the Nb mineralization in the Saima alkaline complex. Biotite in the altered nepheline syenite and pegmatite has inverse isochron 40Ar/39Ar ages of 234.30 ± 6.30 and 230.08 ± 2.75 Ma, respectively. Together with the close spatial relationship and the reported ages of the aegirine nepheline syenite associated with Nb mineralization, we propose that the altered and mineralized nepheline syenite and pegmatite were formed at the same time. In addition, clinopyroxene and biotite compositional trends, an increase in the proportion of hydrous minerals including natrolite, hydromuscovite, and arfvedsonite, and the growth of K-feldspar suggest that the magma evolved from nepheline syenite to mineralized nepheline syenite and nepheline syenite pegmatite. The altered nepheline syenites yield low initial 87Sr/86Sr ratios (0.709625–0.712132) and negative εNd(t) values (−15.32 to −14.15), consistent with previous studies of fresh samples, suggesting that the Saima alkaline rocks were produced by the partial melting of metasomatized lithospheric mantle. The early-crystallized magmatic Nb-rich titanite and rutile in the nepheline syenite and pegmatite indicates an initial Nb-rich parent magma, and fractional crystallization of Nb-poor minerals increased the Nb content of the residual melts. The crystallization of Nb minerals (pyrochlore) and Nb-rich minerals (altered rutile, titanite, and ilmenite) may have been the result of metasomatism by post-magmatic Na- and F-rich fluids, indicating the redistribution and precipitation of Nb. Thus, we conclude that Nb was enriched through magmatic processes, and the Nb mineralization was related mainly to hydrothermal activity.
The Weishan carbonatite-related rare earth element (REE) deposit in China contains both high- and low-grade REE mineralization and is an informative case study for the investigation of magmatic–hydrothermal REE enrichment processes in such deposits. The main REE-bearing mineral is bastnäsite, with lesser parisite and monazite. REE mineralization occurred at a late stage of hydrothermal evolution and was followed by a sulfide stage. Barite, calcite, and strontianite appear homogeneous in back-scattered electron images and have high REE contents of 103–217, 146–13,120, and 194–16,412 ppm in their mineral lattices, respectively. Two enrichment processes were necessary for the formation of the Weishan deposit: Production of mineralized carbonatite and subsequent enrichment by magmatic–hydrothermal processes. The geological setting and petrographic characteristics of the Weishan deposit indicate that two main factors facilitated REE enrichment: (1) fractures that facilitated circulation of ore-forming fluids and provided space for REE precipitation and (2) high ore fluorite and barite contents resulting in high F− and SO42− concentrations in the ore-forming fluids that promoted REE transport and deposition.
A combined study of petrology, mineralogy and stable geochemistry was carried out for nephritites from the Hetian nephrite deposit located in the northern part of the Kunlun Mountains, south of the Tarim Basin, Xinjiang, China. Nephritites from the deposit consist mainly of tremolite, with minor diopside and calcite, and display three main textures: coarse-grained porphyroblast, deformed slender, and fiber crystals of recrystalization. Petrographic observations and backscattered electron imaging revealed two replacements: tremolite replaces dolomite marble (Dol→Tr), or diopside replaces dolomite marble, then tremolite substitutes diopside (Dol→Di; Di→Tr). The nephritites are enriched in light rare earth elements (LREE), but depleted in heavy rare earth elements (HREE) (∑REE, 1.74×10 ~ 6.82×10; ∑REE+Y, 2.03 × 10 ~ 7.71 × 10), having apparent Eu negative anomaly (Eu/ Eu* =0.15-0.97) without apparent Ce anomaly (Ce/Ce*=0.79-1.04). Their Cr and Ni enrichments (1.91 ppm to 307.4 ppm and 2.68 ppm to 352.1 ppm, respectively) are much less than serpentinite-related nephritites. Some high field strength elements and large ion lithophile elements are moderately enriched. Their δO values range from 2.38 to 11.27 , and δD = 55.66 to 102.75 , corresponding to fluid isotope compositions of δO=12.15 to 22.42 , and δD=34.94 to 81.82 , suggesting that the fluids are within, or just adjacent to, the metamorphic water field. Based on these geochemical characteristics and occurrence, it is inferred that igneous rock fluids and sedimentary rocks provide the main ore material for the Hetian nephrite deposite. Re-thinking of the isotope massindependent fractionation (MIF) definition
Abstract Widely distributed in Gyangzê-Chigu area, southern Tibet, NW- and nearly E-W-trending diabase(gabbro)-gabbro diorite dykes are regarded as the product of the large-scale spreading of the late Neo-Tethys Ocean. In order to constrain the emplacement age of these dykes, zircons of two samples from diabases in Nagarzê were dated by using the U-Pb SHRIMP method. Two nearly the same weighted mean 206Pb/238U ages were obtained in this paper, which are 134.9±1.8 Ma (MSWD = 0.65) and 135.5 ± 2.1 Ma (MSWD=1.40), respectively. They not only represent the crystallization age of the diabase, but also documented an important spreading event of the Neo-Tethys Ocean during the late Jurassic and early Cretaceous. This dating result is of great significance to reconstruct the temporal framework of the late Neo-Tethys Ocean in the Qinghai-Tibet Plateau.
Abstract High-pressure (HP) granulites exposed within the eastern Himalayan syntaxis are dominantly felsic rocks. They were displaced southward over the Pei sequence by the NW-dipping Upper Thrust. Twenty-three SHRIMP U/Pb analyses of zircons from one HP sample demonstrate that this rock was derived from Proterozoic sedimentary rocks that underwent high-temperature metamorphism or partial melting to form granitoids or paragneisses at about 500 Ma. During Oligocene collision between India and Asia, these crystalline rocks were buried beneath south Tibet, recrystallizing to form HP felsic granulites at 30-33 Ma, with a metamorphic retrogression at about 23 Ma. Evidently the south Tibetan crust was thickened, and then uplifted at this time.
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