From mantle metasomatism to crustal anatexis: zircon U-Pb-Hf-O and whole-rock Mg-Fe isotopic constraints on high-Mg^# diorites-granodiorites in the North Qilian orogenic belt

High-Mg^# intermediate rocks in subduction zones serve as pivotal tracers of crust-mantle interaction processes, preserving geochemical evidence of recycled slab components. This study combines zircon U-Pb-Hf-O isotopic systems with whole-rock major and trace element as well as Mg-Fe isotope geochemistry to investigate the petrogenesis of high-Mg^# diorites (Mg^# = 55.7-56.1) and granodiorites from the North Qilian orogenic belt, addressing crustal recycling mechanisms during Early Paleozoic orogenesis.
Precise zircon U-Pb geochronology constrains the crystallization age of these intrusive rocks to a range of 434.8 ±2.0 to 429.5 ±2.4 Ma. The high-Mg^# diorites display characteristic arc-type signatures with high-K calc-alkaline affinity, interpreted as products of partial melting of a metasomatized pyroxenite lithospheric mantle. Their distinctive isotopic composition (ε_Hf(t) = +0.6 to +3.5; δ¹⁸O = +6.34 ±0.03‰; δ²⁶Mg = -0.34 ±0.03‰) unequivocally records the involvement of both hydrous silicate melts and carbonatitic melts derived from subducted oceanic crust. Their δ⁵⁶Fe variations are attributed to fractional crystallization dominated by clinopyroxene and hornblende.
The granodiorites also have arc-like trace element signatures and show calc-alkaline affinities. They have uniform ε_Hf(t) values (+5.21 to +8.25), δ¹⁸O (5.32 ±0.04‰) and δ²⁶Mg values from -0.32 to -0.26‰, suggesting they were formed through anatexis of juvenile lower-crustal materials. Their variable Fe isotope signatures are due to fractional crystallization of hornblende.
Our study, integrated with previous findings, reveals that the Early Paleozoic North Qilian lithospheric mantle experienced complex metasomatic processes involving three distinct components: (1) slab-derived fluids, (2) hydrous silicate melts, and (3) carbonate-rich melts. These geochemical characteristics provide direct evidence for multi-stage mantle enrichment and the recycling of oceanic crustal components into the deep mantle.