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

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 epsilon(Hf(t)) values (+8.2 to + 14.1), low delta O-18 values (4.70 to 6.35 parts per thousand), high Mg-# values (30-59), together with relatively uniform epsilon(Nd(t)) values from + 2.1 to + 5.8 and (Sr-87/Sr-86)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.

Automated summary

The Bailingshan deposit in the Eastern Tianshan region of Xinjiang, northwest China is an economically important Fe deposit located in the southern margin of the Central Asian Orogenic Belt. The Late Carboniferous dacitic tuff at Bailingshan is consistent with calc-alkaline arc magmatism and suggests a fractional crystallization dominated origin from melting of a juvenile lower crust mixed with depleted mantle materials. Changes in subduction angles led to the rupture of magma chambers and the release of iron-rich fluids, causing the iron mineralization and relevant alterations in the deposit.