Ca 2.85 Ga Nb‐enriched mafic dikes from the southern Huangling dome of the Yangtze Block, southern China: Implications for Mesoarchean subduction zone processes

Abstract

In this study, we report the first occurrence of Archean Niobium‐enriched mafic dikes in the southern Huangling dome within the Yangtze Block, southern China. Zircon U–Pb dating of three mafic dike samples yielded weighted mean 207Pb/206Pb ages of 2,858 ± 24 Ma (MSWD = 1.14), 2,889 ± 11 Ma (MSWD = 0.98), and 2,840 ± 11 Ma (MSWD = 0.17), respectively, which are interpreted as the intrusion ages of the dikes. Zircons from the host granodioritic gneiss show different trace element patterns and variable Eu anomalies, and yield an upper intercept age of 2,891 ± 32 Ma. On the basis of previous studies, the 2,891 Ma age is interpreted as the metamorphic recrystallization time caused by late tectono‐thermal events. The mafic dikes possess high Nb contents ranging from 4.7 to 18.4 ppm, which are similar to those reported from Archean, Proterozoic, and Phanerozoic Nb‐enriched basaltic rocks occurring worldwide. The Nb‐enriched mafic dikes show HFSE‐depleted patterns with significant negative Zr, Hf, and Ti anomalies, and higher Nb/U (8.29–48.42), Nb/Thpm (0.59–1.71), and Nb/Lapm (0.76–1.34) ratios than “normal” island arc volcanic rocks that are consistent with a subduction zone geochemical affinity. Positive εNd(t) values (+5.0 to +11.8) of the Nb‐enriched mafic dikes point to an extended depleted mantle characterized by elevated Sm/Nd ratio, whereas εNd(t) values (+0.3 to −2.3) and tectonic environment of the host granodioritic rocks, as well as the most widely accepted petrogenetic models of the TTG series indicate enriched mafic crustal sources. Therefore, we propose that the Nb‐enriched mafic dikes were generated from a depleted mantle with an extended history of elevated Sm/Nd ratio that was metasomatized by slab‐derived melts at ca. 2.90–2.85 Ga.