Multi-stage granite and granitic pegmatites in the eastern North Qinling orogen: Petrogenesis, geodynamic setting, and metallogenic potential

Abstract

Numerous granitic pegmatite dikes intrude the Wuduoshan batholith in the eastern North Qinling orogen (NQ). However, the petrogenesis, interrelationships, and geodynamic setting of these granitoids and associated dikes remain poorly understood. This study investigates the Erlangchuan and Sikeshu plutons and their related dikes in the Qinling orogen, central China, through integrated petrographic, whole-rock geochemical, zircon U-Pb geochronological, and Lu-Hf isotopic analyses. Zircon U-Pb dating yielded crystallization ages of 430.8 ± 2.0 Ma for the Erlangchuan monzogranites, 415.2 ± 1.7 Ma for associated biotite granitic pegmatites, 416.9 ± 2.2 Ma for associated aplite dikes, and 403.6 ± 4.7 Ma for Sikeshu muscovite granitic pegmatites. All samples display geochemical characteristics consistent with a crustal origin, showing enrichment of large-ion lithophile elements (LILE; e.g., Rb, Th, Ba, and Cs) and depletion of high field strength elements (HFSE; e.g., Nb, Ta, and Ti). The granitic dikes exhibit higher SiO₂ contents compared to their respective host plutons: the K-rich Erlangchuan monzogranites and the Na-rich, two-mica Sikeshu granites. Zircon Lu-Hf isotopic data reveal higher ε_Hf(t) values for the Erlangchuan monzogranites (average = +2.6) compared to the associated biotite pegmatites (+2.0), aplites (+1.6), and the Sikeshu muscovite pegmatites (+0.9). The ε_Hf(t) values of both plutons and their dikes fall between those of the Qinling Group and juvenile mafic crust, suggesting derivation from a mixed source. Variations in ε_Hf(t) values likely reflect differing proportions of these two endmembers. The Erlangchuan monzogranites formed via partial melting of the lower crust, triggered by mafic magma underplating during Shangdan Ocean subduction. The Sikeshu two-mica granites, associated aplites, and biotite pegmatites likely formed during subsequent collision and slab break-off. Finally, the Sikeshu muscovite pegmatites may represent a later magmatic pulse related to post-collisional lithospheric delamination. Regionally, granitic pegmatites in the eastern NQ define three distinct age peaks at 437 Ma, 413 Ma, and 367 Ma. These pegmatites can be broadly classified into two series: (1) a rare metal-bearing series, predominantly located in the northern eastern NQ, enriched in Nb, Ta, Be, Rb, Li, and Cs, and characterized by negative ε_Hf(t) values indicative of a Qinling Group source; and (2) a U-bearing series, primarily found in the southern eastern NQ, positive or slightly negative ε_Hf(t) values, suggesting derivation from a mixed Qinling Group and juvenile crustal source.