Clue on ocean redox condition from trace element and rare earth element (REE) composition of iron formation and carbonate rocks from the late Paleoproterozoic Morar Formation, Gwalior Group, central India

摘要

Trace element and rare earth element (REE) composition of iron formation and carbonate rocks from the Morar Formation, Gwalior Group, central India provides valuable information on the redox condition of late Paleoproterozoic Ocean. Facies types of iron formation suggest deposition in various oceanic environments ranging from shoreface-beach to subtidal shelf settings, whereas carbonates belong to shallow and deep subtidal settings. (La/Nd)_(SN) values between 0.53 and 23.60, MREE enrichment and small negative (0.69) to positive (1.46) Ce anomaly in iron formation suggest a stratified character for the Gwalior Sea with development of shallow transitional redoxcline. Whereas deep sea is interpreted as near anoxic and ferruginous, the shallow sea was not very high in dissolved oxygen (DO_2) either. A suboxic to mild oxic shallow sea condition (DO_2 ≥0.2 μM) is interpreted allowing Mn (Ⅱ) oxidation and Ce sequestration. Carbonates, however, do not register any geochemical signature of redoxcline possibly because of the depositional setting either close to or below the redoxcline.