Constraints on the age and geodynamic setting of the iron formations and anhydrite Fe-(Ba) deposits in the Bulunkuole Group of the Taxkorgan area, NW China

Abstract

• Bulunkuole Group (BG) and associated mineral deposits is both early Cambrian in age. • Volcanic-sedimentary rocks in the BG reveal a back-arc basin depositional setting. • Iron formations deposited in newborn back-arc basin from redox-stratified water column. • Anhydrite Fe-(Ba) deposits formed in restricted sulfate-rich water during back-arc basin collapse. Here we present detailed field observations, zircon U-Pb-Hf isotopes, whole-rock geochemical, and Sm-Nd isotopic data for the volcanic-sedimentary rocks within the Bulunkuole Group (BG) in the Taxkorgan area, northwestern China in order to reveal significant information on the formation age and tectonic setting of the iron formations (IFs) and anhydrite Fe-(Ba) deposits hosted by this group. Exposed rocks in the BG are broadly grouped into three subgroups: the lowermost Subgroup 1 is a bimodal volcanic suit (basalts and dacites) interbedded with IF layers; the overlying Subgroup 2 is a clastic sedimentary sequence hosting abundant anhydrite Fe-(Ba) mineralization; and the uppermost Subgroup 3 is dominated by carbonate rocks. Zircon U-Pb ages provide constraints on the formation age of the BG. Specifically, the bimodal volcanism occurred at ~ 540–521 Ma and the sedimentary rocks deposited after 540 Ma. Basalts from the BG bimodal suit are characterized by slightly enriched LREE patterns and pronounced to insignificant negative Nb anomalies when normalized to primitive mantle compositions. These features are comparable with those of modern N-MORB and back-arc basin basalts. Combined with highly heterogeneous Nd isotopes ( ε Nd (t) = 0.2–4.4), these suggest that their magmatic precursors were derived from partial melting of a depleted mantle modified by slab-derived fluids, and interacted with minor crustal components during ascending. By comparison, the BG dacites display gently sloped to flat LREE patterns with a FII-type felsic affinity and a range of negative to positive ε Nd (t) values (-5.8~+0.7). They are inferred to be generated by partial melting of the old continental crust at relatively shallow levels with variable addition of juvenile material. The BG clastic sediments were dominated by greywacke and arkose. Their diagnostic geochemical features, detrital zircon age spectra, and in situ zircon Hf isotopic data indicate that they were mainly derived from weathering of the early Cambrian felsic rocks. Together, these characteristics above, integrated with previous studies, suggest that the BG and related IFs and anhydrite Fe-(Ba) deposits were formed on an active continental margin (possibly back-arc basin) during the southward subduction of the Proto-Tethys Ocean beneath the Taxkorgan-Tianshuihai Terrane.