Abstract
The Chaqiabeishan area is characterized by small Li-rich granitic pegmatites in the Quanji Massif (QM), northwest China. In this study, the columbite-tantalite group minerals (CGMs) from a typical Li-rich pegmatite dike were analyzed for major element contents using an EMPA (electron microprobe analyzer), for trace element contents using LA-ICP-MS (laser ablation-inductively coupled plasma mass spectrometry), and for ages using LA-ICP-MS U-Pb dating, respectively. The CGMs from the sample can be divided into two types, i.e., magmatic Type 1 and metasomatic Type 2. Although these two types of CGMs do not exhibit distinct major and trace element variations from core to rim within an individual grain, the Ta# values, Mn# values, and some trace element contents (such as Zr, Hf, W, and Sr) of Type 1 CGMs are distinct from those of Type 2 CGMs. The overall compositional changes from Type 1 CGMs to Type 2 CGMs are consistent with the typical evolutionary trend described for many lithium-cesium-tantalum (LCT) pegmatites and the complex spodumene trend described by Černý and Ercit (Bull. Mineral., 1989, 108, 499–532). The Type 2 CGMs have formed later and must be a metasomatic product of Type 1 CGMs. Eighteen Type 1 CGMs yielded a weighted mean 206Pb/238U age of 240.6 ± 1.5 Ma. The slight oscillatory zoning and/or sector zoning suggest that the dated Type 1 columbites have a magmatic origin. Thus, the crystallization ages of Type 1 columbites represent the emplacement ages of Li-rich pegmatites. One of the Type 2 CGMs yielded a 206Pb/238U age of 211.0 ± 4.7 Ma, which is hardly interpreted to be an age representing the later hydrothermal metasomatism, because one dataset has no apparent statistical significance. Therefore, our dating results can only indicate that the Li-rich pegmatite-forming melts were emplaced at approximately 240.6 Ma. Based on these results and previous studies of the 240–254 Ma granitoids in the QM, we conclude that the 240.6 Ma Li-rich granitic pegmatites, as well as 240–254 Ma granitoids in the QM, were both emplaced during the southward subduction of the Zongwulong Ocean Plate in the Late Permian to Middle Triassic.
Highlights
The Chaqiabeishan area in Tianjun County in Qinghai Province is tectonically located in the Quanji Massif (QM) and is one of the most important granitic pegmatite fields for rare element (Li and minor amounts of Be, Nb, and Ta) resources in Western China
We believe that columbite-tantalite group minerals (CGMs) are paragenetic with the other rare-element-rich minerals in the Li-rich pegmatites because recent studies have shown that CGMs mainly crystallize from pegmatite-forming melts or during metasomatism (Van Lichtervelde et al, 2007)
The Type 2 CGMs have irregular or complicated internal textures, some of which have both bright patches and dark patches in the Back-scattered electron (BSE) images, indicating that they are perhaps related to latestage metasomatic replacement caused by highly reactive fluids exsolved from the residual melts (Cerný and Ercit, 1985; Tindle and Breaks, 2000; Badanina et al, 2015)
Summary
The Chaqiabeishan area in Tianjun County in Qinghai Province is tectonically located in the Quanji Massif (QM) and is one of the most important granitic pegmatite fields for rare element (Li and minor amounts of Be, Nb, and Ta) resources in Western China. These Li-rich pegmatites have attracted increasing amounts of attention because of the global demand for lithium and the Li prospecting potential in this area. Recently reported 235.9 and 217.0 Ma U-Pb ages of zircons within pegmatites by Wang et al (2020) indicate amphibolous ages as well as an amphibolous genetic link between the rare-element pegmatites and the nearby Triassic granitic pluton. The emplacement ages of the Li-rich pegmatites in the Chaqiabeishan region need to be robustly determined to reconstruct the timeframe of the pegmatite emplacement and to explore the genesis of the Li-rich pegmatites
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