Abstract
The Kalu’an-Azubai pegmatite field, one of the most important rare-metal metallogenic regions in China, contains a large number of pegmatite dikes belonging to spodumene and lepidolite subtypes. Columbite-group minerals (CGMs) collected from three spodumene subtype pegmatites (No. 802, No. 803, and No. 805 pegmatites) were analyzed for major element contents using EPMA (electron probe micro-analyzer) and dated using LA-ICP-MS (laser ablation-inductively coupled plasma mass spectrometer). The crystallization ages of the CGMs from No. 802, No. 803, and No. 805 pegmatites are 209.5 ± 1.4 Ma (2σ), 198.3 ± 2.0 Ma (2σ), and 224.3 ± 2.9 Ma (2σ), respectively. Oscillatory zoning and/or sector zoning along with the associated mineral assemblages suggest that the dated columbite is of magmatic origin. The crystallization ages of the columbite grains thus represent the emplacement ages of the Li pegmatites. Therefore, our dating results indicate that there were three emplacement events of the Li-rich pegmatite-forming melts in a timeframe of ~30 Ma. In combination with previous studies, we conclude that the Li pegmatites were formed before the Be-Ta-Nb pegmatites (~194–192 Ma), which precludes the genesis of rare-metal pegmatites via fractional crystallization of a granitic magma in the Kalu’an-Azubai region.
Highlights
The Kalu’an-Azubai pegmatite field, located in the Chinese Altay orogen, is one of the most important pegmatite fields for rare element (Li, Be, Nb, and Ta) resources and gemstone exploration in China [1,2,3,4,5]
In combination with previous studies, we conclude that the Li pegmatites were formed before the Be-Ta-Nb pegmatites (~194–192 Ma), which precludes the genesis of rare-metal pegmatites via fractional crystallization of a granitic magma in the Kalu’an-Azubai region
Columbite-group minerals (CGMs) in in all all the the three three investigated pegmatites mainly mainly Petrographic investigated pegmatites occur in the spodumene-albite-quartz zone and show planar contact with spodumene and muscovite occur in the spodumene-albite-quartz zone and show planar contact with spodumene and muscovite (Figure 4)
Summary
The Kalu’an-Azubai pegmatite field, located in the Chinese Altay orogen, is one of the most important pegmatite fields for rare element (Li, Be, Nb, and Ta) resources and gemstone (mainly aquamarine) exploration in China [1,2,3,4,5]. This pegmatite field has attracted increasing attention because of the high global demand for lithium and because of the pegmatite-granite relationship that has long puzzled geologists [2]. Interpretation of the Zircon U-Pb ages reported for the rare-element pegmatites in the region remains ambiguous due to: (1) the presence of xenocrystic zircon, (2) discordancy caused by Pb-loss from high-U zircon, (3) the presence of recrystallized zircons
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