Zircon U-Pb geochronology and geochemistry of the Hadayang mafic-ultramafic rocks in Inner Mongolia: Constraints on the Late Devonian subduction of Nenjiang-Heihe area, Northeast China

Abstract: The Late Devonian mafic - ultramafic rocks were first discovered in Nenjiang - Heihe area. The rocks outcrop in the Hadayang tectonic mélange in Inner Mongolia. The mafic - ultramafic rocks are allochthonous rock block consisting mainly of amphibole - gabbro and amphibolite. The laser ablation inductively coupled plasma mass spectrometry (LA - ICP - MS) zircon U - Pb geochronology and geochemistry of the rocks were studied. The dating results show that the weighted average 206Pb/238U age of the amphibole - gabbro is （363.3 ± 1.2） Ma (n= 33), and the age of amphibolite is （362.09 ± 0.55 ）Ma (n =26). The ages belong to Late Devonian. The geochemical characteristics of the amphibole - gabbro show high Ti (TiO2 = 1.30%-5.04%), high Na (Na2O = 2.92%-3.14%), relatively high K (K2O = 0.31%-0.45%) and low Mg (MgO =5.94%-8.25%), the fractionation of HREE and LREE is obvious ((La/Yb)N =1.90-3.23), with the depletion of Zr, Nb and enrichment of Th and Sr. The geochemical characteristics of amphibolite show low Ti (TiO2 =0.65%-0.97%), low K (K2O=0.04%-0.12%), high Mg (MgO=20.42%-24.07%), high Fe (FeOT =11.40%-12.74%), obvious LREE and HREE fractionation ((La/Yb) N = 2.35-3.97), enrichment of Th, Hf, Ce and Nb, and depletion of Ba, Sr, K and Zr, Zr/Nb = 10.04-17.12. The characteristics of the Hadayang mafic - ultramafic rocks are similar to the features of the volcanic arc basalts. Combined with existing research results, the authors hold that the rocks probably formed in an island arc and fore arc basin above the subduction zone, and the subduction of the Nenjiang-Heihe tectonic belt already began in Late Devonian. This paper provides new evidence for the study of the tectonic evolution of the eastern part of the Hegenshan - Zhalantun tectonic belt and the Late Paleozoic Paleo - Asian Ocean of the northern Da Hinggan Mountains.