Sea water chemistry during Archean-Paleoproterozoic transition: Insight from the Aravalli Supergroup, India.

The Paleoproterozoic Aravalli Supergroup has an unconformable relationship with the Mewar Gneissic Complex (MGC) of Archean age. The MGC in the Umra area, Rajasthan, India is represented by gneisses, granite, and amphibolite, and has development of a paleosol horizon below the unconformity surface separating it from the Aravalli Supergroup. The Aravalli metasediments have undergone metamorphism up to greenschist facies and have a scattered age of about 2.3-1.8 Ga. The lower part of the Paleoproterozoic succession comprises conglomerates, volcanics, carbonates with microbial mats, and carbonaceous phyllites. These metasediments provide geochemical proxies to the seawater conditions during the Great Oxidation Event (GOE) as these happen to lie on the age range where the Archean-Proterozoic transition occurred.  

Conglomerates present in the lower part of the Aravalli Supergroup are polymictic, clast to matrix-supported, and massive. Euhedral magnetite grains scattered in the matrix indicate a suboxic-anoxic condition of deposition. The overlying carbonates of the area show both negative as well as positive δ¹³C_V-PDB   excursions.  The positive δ¹³C excursion can be correlated with the Lomagundi event. This excursion indicates a warm water condition at the time of the GOE and is supported by the presence of microbial mats in the carbonate unit. On the other hand, the negative excursion may be representative of the Paleoproterozoic glaciation event. The PAAS normalized REE data of the carbonates have HREE>LREE, with a flat to positive Ce* and Eu* anomalies and no significant Y-anomaly, indicating a suboxic-anoxic condition of seawater during deposition in a shallow sea. The carbon phyllite present above the carbonate unit in the area hosts uranium mineralization which is associated with chalcopyrite and pyrite. The Δ³⁴S data of the pyrites show a reducing environmental condition of mineralization. The studies related to the metasediments of the Aravalli Supergroup provide a clue to the seawater conditions that prevailed during the Archean-Proterozoic transition and also provide deeper insights into the Great Oxidation Event.