Paleomagnetic constraints on the primary iron minerals of the late Paleoproterozoic Xuanlong-type ironstone in the North China Craton

Abstract

The Xuanlong-type ironstone is a late Paleoproterozoic (∼1.64 Ga) hematite-dominated granular iron formation (GIF) in the North China Craton (NCC). However, recent geochemical studies indicated this GIF was precipitated in a weakly oxygenated environment. The conflicts between the mineral composition of the Xuanlong-type ironstone and its geochemical interpretations can be attributed to the potential occurrence of secondary hematite. Here we used paleomagnetic, rock magnetic measurements and petrographic analysis to determine the mineralization age of the hematite. Our objectives were to determine the primary iron mineralogy and to further explore the paleoenvironmental information preserved in the Xuanlong-type ironstone. Rock magnetic measurements and petrographic analysis identified Al-substituted hematite and siderite as the magnetic minerals. The high-temperature characteristic remanent magnetizations, which passed a C-classification reversal test and a fold test, yielded a paleomagnetic pole position at 51°N, 197°E. This pole position is in close proximity to the coeval paleomagnetic pole reconstructed from the ∼1.64 Ga Cuizhuang Formation in the southern NCC. Therefore, the high-temperature component of the natural remanent magnetization is interpreted as primary remanent magnetization acquired through crystal growth of Al-substituted hematite during early diagenesis. The degree of Al-substitution in hematite seems to be influenced by the alternating dense and porous laminae within the hematitic ooids of Xuanlong-type ironstones. It is hypothesized that these laminae are a result of periodic sea level fluctuations, considering that the ironstones were likely formed in a shallow water setting.