Depositional environments of Cretaceous radiolarian cherts from Andaman-Nicobar Islands, northeastern Indian Ocean

Abstract

Radiolarian cherts of Cretaceous age are tectonically associated with pillow basalts, ultramafic rocks and turbidites in the outer sedimentary arc of the Andaman-Nicobar Islands. These radiolarian cherts are composed of radiolarian tests, quartz, albitic feldspar, basaltic rock fragments, montmorillonite and chlorite, and are classified into three different types: (1) massive tuffaceous radiolarian claystone, (II) bedded tuffaceous radiolarian claystone, and (III) bedded radiolarian argillaceous chert. Radiolarian cherts of Types I and II are similar in composition, characterised by the lower abundance of SiO 2, total REE (ΣREE) and are rich in Fe 2O 3, MgO, TiO 2 and Al 2O 3 and trace elements (e.g., Ni, Co, Cr, V, Rb, Sr, Cu and Zr) as compared to those of Type III radiolarian chert. Based on elemental abundance as well as petrological evidence, it is suggested that both Types I and II cherts have been derived from a mixed continental and basaltic source. In comparison, Type III chert seems to have been derived from a continental source. Low values of Fe 2 O 3 Al 2 O 3 , ΣREE and weakly positive to negative Ce anomalies in Types I and II cherts further suggest that they accumulated close to continental margins as compared to Type III chert, which is suggested to have accumulated in a relatively distal oceanic (hemipelagic) environment. It is inferred that these radiolarian chert sequences, which were originally deposited in different oceanic environments, were scraped off the subducting Indian plate, became tectonically juxtaposed, and now constitute a part of the Andaman-Nicobar ophiolite complex.