Fe isotope fractionation and geochemical evolution of Triassic granitoids in the Eastern Kunlun Orogen, NW China

The East Kunlun Orogenic Belt (EKOB), one of the most important magmatic belts on the Tibetan Plateau, experienced a multicyclic tectonic evolution. In this paper, we present geochemical and geochronological data, especially Fe isotopes for the Triassic granitoids exposed in the EKOB. The granitoids are characterized by relative enrichment of Rb, Pb, U and K and depletion in Nb, Ta and Ti; The granitoid samples show remarkably variable δ⁵⁶FeIRMM014 values, ranging from – 0.055 ± 0.07 to 0.116 ± 0.03‰, which are among the highest values reported from felsic rocks. These values do not correlate with bulk-rock geochemical indicators of fluid exsolution (e.g., Zr/Hf, Nb/Ta, K/Rb and Rb/La ratios), qualitatively indicating that fluid exsolution has an insignificant effect on Fe isotopic fractionation. The most likely mechanism responsible for the heavy Fe isotopic compositions in the granitoids can be narrowed down to fractional crystallization processes. The combination of Fe isotope and mineral composition suggests that the variation in Fe isotopes in our siliciclastic melts during magmatic evolution is primarily attributed to the removal of isotopically light Fe-bearing minerals (i.e., ilmenite, biotite, amphibole, orthopyroxene and clinopyroxene). Additionally, the zircon U–Pb dating of these granitoids reveals their ages to be approximately 250 Ma. In conjunction with the field observations, petrologic and geochronologic features, we assert that the closure of the A’nyemaqen Paleo-Tethys Ocean occurred during the Triassic period (250 Ma) through the formation of syncollisional granitoids in the EKOB, which represents a snapshot of the continental crustal growth.