Geochronology and Hf isotope study of pegmatite in the Xiaoqinling area of NW China: Implication for petrogenesis and regional metamorphism
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Pegmatite
Petrogenesis
Geochronology
The mineralogical studies of rare-element (REL) pegmatites are important for unraveling the ore-forming process and evaluating REL mineralization potential. The Chinese Altai orogenic belt hosting more than 100,000 pegmatite dykes is famous for rare-metal resources worldwide and diverse REL mineralization types. In this paper, we present the results of EMPA and LA-ICP-MS for muscovite from the typical REL pegmatite dykes of the Chinese Altai. The studied pegmatites are Li-Be-Nb-Ta, Li-Nb-Ta, Nb-Ta, Be-Nb-Ta, Be and barren pegmatites. The Li+ accompanied with Fe, Mg and Mn substitute for Al3+ at the octahedral site in muscovite from the REL pegmatites, and the substitution of Rb by Cs at the interlayer space is identified in muscovite from the Be pegmatites. The P and B contents increase with evolution degree and the lenses from the Nb-Ta pegmatite are produced at late fluid-rich stage with high fluxes (P and B). The enrichment of HFSE in muscovite indicates a Nb-Ta-Sn-W rich pegmatite magma for the Be-Nb-Ta pegmatite. From barren pegmatite, beryl-bearing zone, to spodumene-bearing zone, the evolution degrees of pegmatite-forming magmas progressively increase. In the Chinese Altai, the possible indicators of muscovite for REL mineralization types include Rb (ca. 400–600 ppm, barren pegmatite; ca. 1200–4000 ppm, Be pegmatite; >4500 ppm, Li pegmatite), Cs (ca. 5–50 ppm, barren pegmatite; ca. 100–500 ppm, Be pegmatite; >300 ppm, Li pegmatite) and Ge (<3 ppm, barren pegmatite; ca. 4–6 ppm, Be pegmatite; ca. 6–12 ppm, Li pegmatite) coupled with Ta, Be (both <10 ppm, barren pegmatite) and FeO (ca. 3–4 wt%, Be pegmatite; ca. 1–2.5 wt%, Li pegmatite). The plots of Nb/Ta vs. Cs and K/Rb vs. Ge are proposed to discriminate barren, Be- and Nb-Ta-(Li-Be-Rb-Cs) pegmatites. The Li, Be, Rb, Cs and F concentrations of forming liquid are evaluated based on the trace element compositions of muscovite. The high Rb and Cs contents of liquid and lower Be contents than beryl saturation value indicate that both highly evolved pegmatite magma and low temperature at emplacement contribute to beryl formation. The liquids saturated with spodumene have large variations of Li, possibly related to metastable state at Li unsaturation–supersaturation or heterogeneous distribution of lithium in the system.
Pegmatite
Muscovite
EMPA
Spodumene
Tourmaline
Columbite
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Allanite
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Petrogenesis
Pegmatite
Geochronology
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The Precambrian basement of Angola contains several pegmatite fi elds, although most of them have not been studied yet. Some of them include pegmatites devoid of rare elements, as those found close to Caxito, about 100 km ENE Luanda. These pegmatites are exploited for the production of industrial minerals. Moreover, some rare element pegmatite fi elds occur close to Namibe, in the desertic part of the southwest of the country. These pegmatites were prospected for Be and Ta in the 1960’s. The distribution of the pegmatite types, as well as the exceptional quality of the outcrops, due to lack of weathering or soil cover, allow a complete sampling in order to study the evolution of a pegmatite fi eld, from parental granites to barren pegmatites, beryl-columbite-phosphate pegmatites and spodumene pegmatites. Furthermore, the internal evolution of each pegmatite type has been also studied.
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