Late Paleozoic subduction-related magmatism in NE China and its implication: Insights from intrusions in the Handagai Fe[sbnd]Cu deposit

Abstract

North East (NE) China forms the easternmost part of the Central Asian Orogenic Belt. However, its Paleozoic geological evolution process remains controversial, especially the complex subduction-accretion-orogeny processes of the Paleo-Asian Ocean plate, as well as the linking magmatism-metallogenic with tectonic evolution during that time. This paper reports the results of geochemical, geochronological, and isotopic analyses of igneous rocks at the Handagai skarn Fe–Cu deposit to elucidate the late Paleozoic tectonic framework and evolution of NE China and the setting of ore formation. New zircon LA–ICP–MS U–Pb data for monzogranite, quartz diorite, and quartz monzodiorite record Carboniferous ages of 350 ± 2 Ma, 321 ± 3 Ma, and 319 ± 2 Ma, respectively. The monzogranites have high SiO2 (69.57–71.97 wt%) and total-alkali (6.68–7.56 wt%) contents and relatively low MgO contents (0.60–1.03 wt%) and Mg numbers (33–41). They have arc-type trace-element signatures, with enrichment in large-ion lithophile elements and light rare earth elements, together with depletion in Nb, Ta, P, and Ti. Their whole-rock Sr–Nd–Pb and zircon in-situ LuHf isotopic compositions ((87Sr/86Sr)i = 0.7008–0.7062; εNd(t) = −1.9 to +1.9; (206Pb/204Pb)t = 17.900–19.006; εHf(t) = 5.04–15.28) and young two–stage Nd–Hf model ages (TDM2 = 1105–749 Ma and 899–574 Ma, respectively) indicate that their precursor magma was derived from partial melting of Mesoproterozoic to Neoproterozoic juvenile lower crust that had been metasomatized by fluids/melts from an enriched mantle wedge. The quartz diorites and quartz monzodiorites are characterized by intermediate SiO2 (58.21–61.47 wt%), high MgO (3.62–5.42 wt%), and low CaO (3.15–4.96 wt%) contents, and high Cr (151–222 ppm) and Ni (97–131 ppm) contents, as well as elevated Sr and Ba. They have strong Nb and Ta depletion and highly fractionated rare earth elements, indicating a geochemical affinity to typical high-Mg andesites. These rocks exhibit mid-ocean ridge basalt (MORB)-like whole-rock Nd–Sr–Pb–Hf isotopic characteristics, with high εNd (t) and εHf (t) values of +1.60 to +3.20 and + 10.21 to +13.61, respectively, and relatively low (87Sr/86Sr)i and (206Pb/204Pb)i ratios of 0.7028–0.7045 and 16.768–17.856, respectively. They also have low Sr/Y (30–59), Ba/Th (75–213), and Ba/La (26.05–38.24), but elevated Th/Yb (4.51–7.36), Th/Nb (0.78–1.17), and Rb/Y (2.79–6.29) ratios. We conclude that they were generated by partial melting of depleted lithospheric mantle previously metasomatized by subduction-related fluids and sediment melts. These results, integrated with regional geological, geochronological, and geochemical data, constrain the tectonic evolution of NE China during the late Paleozoic. This involved normal subduction at 400–335 Ma, ridge subduction at 333–319 Ma, and slab break–off at 315–272 Ma. The high-Mg andesitic/dioritic magmas had high oxygen fugacities and high water and volatile (e.g., Cl, S) contents, and sufficient ore resources (e.g., Cu, Au, Fe) from the mantle wedge to be favorable for Cu (–Au–Fe) mineralization.