Origin of the subduction-related Late Carboniferous dacitic tuffs associated with the Bailingshan Fe deposit in Eastern Tianshan, NW China: Geochronological, geochemical, and Sr-Nd-Hf-O isotopic constraints

Abstract

The Bailingshan deposit is an economically important Fe deposit in Eastern Tianshan, located in the southern margin of the Central Asian Orogenic Belt, Xinjiang, northwest China. The orebodies at Bailingshan are mainly hosted in the Carboniferous volcanic rocks. In this study, new secondary ion mass spectrometry (SIMS) zircon U-Pb dating, whole rock geochemistry, zircon Hf-O isotopic analyses and Sr-Nd isotopic analyses were carried out on the Bailingshan dacitic tuff. New SIMS zircon U-Pb dating results indicate that the dacitic tuff occurred in the Late Carboniferous (312.1 ± 3.3 Ma and 312.4 ± 4.4 Ma). The geochemistry of the dacitic tuff is consistent with calc-alkaline arc magmatism, characterized by high SiO2 (70.33–74.70 wt%) and (La/Yb)N values of 1.06 to 3.47. In addition, the Bailingshan dacitic tuff displays variable εHf(t) values (+8.2 to + 14.1), low δ18O values (4.70 to 6.35 ‰), high Mg# values (30–59), together with relatively uniform εNd(t) values from + 2.1 to + 5.8 and (87Sr/86Sr)i ratios ranging from 0.703818 to 0.707168, suggesting a fractional crystallization dominated origination from melting of a juvenile lower crust that mixed with the depleted mantle materials was most likely responsible for genesis of the Bailingshan dacitic tuffs. Combined with regional tectonic evolution and our new data, we infer that the change of subduction angle led to the rupture of magma chamber and the release of the Fe-rich fluids from magma chamber, causing the iron mineralization and relevant alteration of Bailingshan deposit in the Aqishan-Yanmansu arc.