High Nb\u2013Ta rhyolites of the Kyihe region, north Daxingan Mountains, China: geochemistry, petrogenesis, and zircon U\u2013Pb geochronology

Abstract

Rhyolites with high Nb–Ta contents were recently discovered in the north Daxingan Mountains, China. We determined the geochemical characteristics and zircon U–Pb ages of these rhyolites to elucidate their tectonic setting of formation and petrogenesis. Zircons from the high Nb–Ta rhyolites are idiomorphic or hypidiomorphic, short prismatic crystals with oscillatory zoning; the zircon trace element has a higher Th/U ratio (> 0.4); zircon rare earth element (REE) content is high (average is 1729 × 10−6) and indicates heavy REE (HREE) enrichment (average is 1561 × 10−6) and shows positive Ce (Ce/Ce* = 2.1–103.4) and negative Eu (Eu/Eu* = 0.18–0.64) anomalies typical of crustal magmatic zircons. Their weighted-mean LA–ICP–MS U–Pb age of 155 ± 1 Ma indicates that they formed in the Upper Jurassic. The rhyolites are characterized by high SiO2 and alkali contents and low Fe, Ca, Mg, and Mn contents, and are weakly peraluminous, indicating that they are high-K calc-alkaline rocks. Trace element compositions are characterized by enrichments in Nb, Ta, Zr, Hf, Ce, and Rb and depletions in Sr, Eu, Ba, P, Ti, Co, and Ni, with significant positive Ce (Ce/Ce* = 2.4–2.7) and negative Eu (Eu/Eu* = 0.06) anomalies. Niobium and Ta are hosted in the zircons. In (Na2O+K2O+FeOT+MgO+TiO2) vs (Na2O+K2O)/(FeOT+MgO+TiO2) and (Al2O3+FeOT+MgO+TiO2) vs Al2O3/(FeOT+MgO+TiO2) mineral characterization diagrams, data for the samples plot in the metamorphic greywacke or basic argillaceous rock fields, indicating that the magma originated from partial melting of crustal material. εSr(t) values cover a wide range (− 18.2 to + 102.9), whereas εNd(t) values have a narrow range (1.9–2.0) with T2DM model ages of 789–785 Ma, indicating that the source was the Neoproterozoic Xinghua Ferry Group crustal basement. This crustal rock suite comprises a volcanic–sedimentary formation of metamorphosed mafic volcanic and terrigenous clastic rocks derived from a mixture of mantle and crustal materials. Residual phases in the source region include Ca-rich plagioclase, amphibole, orthopyroxene, and zircon + garnet. Together with the positive Ce anomalies and low-Sr/high-Yb characteristics of the rhyolites, this indicates that the source rocks melted at relatively shallow depths (< 30 km), low pressures (< 0.8 GPa), and high O2 fugacity. Ga × 104/Al > 2.6, Ta vs Yb, and (Rb/30) vs Hf vs (3Ta) discrimination diagrams, data for the samples plot in the A-type rhyolite and intraplate granite fields, whereas in the Nb vs Y vs Ce diagram, the data plot in the A1-type field. It is concluded that an extensional tectonic setting, resulting from closure of the Mongolia–Okhotsk Ocean at the end-Triassic, or northward subduction of oceanic lithosphere under the Siberian Plate, caused underplating of mantle-derived basaltic magma and partial melting of metamorphic crustal rocks.