minerals Article Chemical Evolution of Nb-Ta Oxides and Cassiterite in Phosphorus-Rich Albite-Spodumene Pegmatites in the Kangxiwa–Dahongliutan Pegmatite Field, Western Kunlun Orogen, China Yonggang Feng 1,2,* , Ting Liang 1,2,*, Xiuqing Yang 1,2, Ze Zhang 1,2 and Yiqian Wang 1,2 1 School of Earth Science and Resources, Chang’an University, Xi’an 710054, China;
[email protected] (X.Y.);
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[email protected] (Y.W.) 2 Laboratory of Mineralization and Dynamics, Chang’an University, Xi’an 710054, China * Correspondence:
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[email protected] (T.L.) Received: 30 January 2019; Accepted: 2 March 2019; Published: 8 March 2019 Abstract: The Kangxiwa–Dahongliutan pegmatite field in the Western Kunlun Orogen, China contains numerous granitic pegmatites around a large granitic pluton (the Dahongliutan Granite with an age of ca. 220 to 217 Ma), mainly including barren garnet-, tourmaline-bearing pegmatites, Be-rich beryl-muscovite pegmatites, and Li-, P-rich albite-spodumene pegmatites. The textures, major element contents, and trace element concentrations of columbite-group minerals (CGM) and cassiterite from three albite-spodumene pegmatites in the region were investigated using a combination of optical microscopy, SEM, EPMA and LA-ICP-MS. The CGM can be broadly classified into four types: (1) inclusions in cassiterite; (2) euhedral to subhedral crystals (commonly exhibiting oscillatory and/or sector zoning and coexisting with magmatic cassiterite); (3) anhedral aggregates; (4) tantalite-(Fe)-ferrowodginite (FeSnTa2O8) intergrowths. The compositional variations of CGM and cassiterite are investigated on the mineral scale, in individual pegmatites and within the pegmatite group.