U–Pb geochronology of columbite-group mineral, cassiterite, and zircon and Hf isotopes for Devonian rare-metal pegmatite in the Nanyangshan deposit, North Qinling Orogenic Belt, China

Abstract

The North Qinling Orogenic Belt (NQOB) hosts many rare metal occurrences concentrated in LCT (Li–Cs–Ta) pegmatites. The Nanyangshan pegmatite deposit occurs within this belt and is historically important for mining of these rare metals. To constrain the timing and petrogenesis of the rare-metal-mineralized pegmatite, a systematic study of the U–Pb ages of columbite–group minerals (CGM), cassiterite, and zircon, as well as Hf isotopes and chemical analyses of zircon were undertaken for the Nanyangshan No. 703 rare-metal pegmatite. This study identifies three distinct evolutionary generations of CGM based on textural and compositional features, including low-Ta# (Ta/(Ta + Nb)) core, high-Ta# rim-1, and Ta#-gap rim-2 (chemically expressed by abrupt increase in Ta# values). They have indistinguishable U–Pb isotopic compositions with a U–Pb age of 406.8 ± 3.3 Ma, which is consistent with cassiterite U–Pb age of 410.6 ± 7.9 Ma and zircon U–Pb age of 406.2 ± 5.2 Ma within analytical error from fine-grained quartz–albite zone (zone I) of this pegmatite. These new age data constrain the emplacement of rare-metal pegmatite at ca. 407 Ma, indicating an Early Devonian rare-metal mineralization event associated with Caledonian orogeny of the NQOB. Zircon εHf(t) values range from –8.71 to –7.23 with Hf model ages of 1.94 to 1.85 Ga, implying a dominant source contribution from Paleoproterozoic crust during the magma generation process. This rare-metal mineralized pegmatite (∼407 Ma) is slightly younger than the U-mineralized pegmatites (∼419–405 Ma) in the same pegmatite belt, with formation of rare-metal- and U-mineralized pegmatites being controlled by variations in crustal protolith.