Geochemistry of Mesoproterozoic metadolerite dykes and sills of Shillong basin, Meghalaya, NE India

The Shillong Basin is the only Proterozoic (Purana) basin of NE India and is an intracratonic sag basin in nature. The basin is dominantly exposed in Meghalaya and partly in Karbi Anglong district of Assam and confined in the central and eastern parts of the Shillong plateau. This Proterozoic basin was developed during early Mesoproterozoic time. The metasedimentary rock associations of this basin are classified under Lower Metapelitic and Upper Quartzitic Formations closely associated with mafic magmatism in the form of mafic sills and dykes (metadolerite, locally named as Khasi greenstone). They are metamorphosed under greenschist – amphibolite transitional facies. The present study aims at generating geochemical characteristics of the metadolerites of the basin. Mafic dykes and sills of the basement of the Shillong plateau is of Cretaceous age and excluded from the present study. Geochemically, metadolerites are subalkaline and tholeiitic in nature and emplaced in continental setting. Geochemical data, mantle normalized spidergram and chondrite normalized REE patterns suggest that they are genetically related and probably derived from the same parental source magma undergoing shallow level fractionation.Keywords: Shillong Plateau, Shillong basin, Dolerite dykes and sills, Geochemistry

Provenance, prevalence and health perspective of co-occurrences of arsenic, fluoride and uranium in the aquifers of the Brahmaputra River floodplain

The present work focuses on understanding the provenance, prevalence and health perspective of As and F? along with possible co-occurrence of uranium (U) in the aquifers of the Brahmaputra floodplains (BFP), India. Groundwater (n = 164) and sediment samples (n = 5) were obtained from the upper, middle and lower BFP. Energy dispersive spectroscopy (EDX) revealed the presence of As, U and Fe in the sediment matrix. Regression analysis showed a weaker relationship between As and F? co-occurrence. Hierarchical cluster analysis (HCA) and principal component analysis (PCA) suggested reductive dissolution of Fe (hydr)oxides responsible for As release in the BFP, especially in the upper and lower BFP. Bicarbonate appeared to compete with As oxyanions for adsorption on positively charged surfaces leading to As release. Arsenic desorption in presence of PO43?, F? and HCO3? at elevated pH appeared greatest in the upper BFP, suggesting the highest potential for co-occurrence. Co-occurrence, were mainly in isolated aquifers of the upper BFP owing to desorption of adsorbed As and F? from Fe (hydr)oxides at higher pH. Weathering and dissolution of clay minerals in the upper BFP, and competitive desorption in presence of HCO3? and PO43? in the middle and lower BFP, respectively, explain variabilities in F? release. Amorphous Fe (hydr)oxides like ferrihydrite act as sinks of U. Concentrations of As and F? will likely increase in the future as projected from the saturated levels of goethite and ferrihydrite. Hazard indices (HI) revealed that children (3�8 years) were at greater health risk than adults.by Nilotpal Das, Aparna Das, Kali Prasad Sarma and Manish Kuma