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Radiolarian cherts; geochemistry; Andaman Island; Bay of Bengal; India

The radiolarian cherts of Late Cretaceous age are reported in the Andaman-Nicobar ophiolites of Bay of Bengal, India. They are of chocolate-coloured (Type-I) and light pink-coloured (Type-II) varieties, juxtaposed and tectonically associated with phyllites, claystones and basalts and are exposed in eastern part of North Andaman Island. These cherts are composed of radiolarian tests in a fine-grained ferruginous matrix.Type-I radiolarian cherts have low $\rm{SiO}_{2}$, and high $\rm{Al_{2}O_{3}}$, $\rm{Fe_{2}O_{3}^{(T)}}$, $\rm{MgO}$, $\rm{Na_{2}O}$ and $\rm{TiO_{2}}$ and trace elements as compared to Type-II cherts. $\rm{Al–Fe–(\Sigma REE–Ce)}$ diagram of the studied cherts indicates a mixed terrigenous and volcanogenic source. $\rm{La_{N}/Ce_{N}}$ ratios (0.76–0.89 for Type-I and 0.71–0.88 for Type-II) and Ce-anomalies $\rm{(Ce/Ce^{\ast} = 1.15–1.33}$ for Type-I and 1.07–1.38 for Type-II) and other elemental ratios in these cherts suggest that they were deposited in continental margin environments. It has been suggested that the studied samples of cherts were deposited at different places, were scrapped off the subducting plate, became tectonically juxtaposed. They were obducted onto the leading edge of the Eurasian continent during the Late Cretaceous prior to the currently active Andaman–Java subduction, that was probably initiated during the Late Miocene.

S H JAFRI¹ D SRINIVASA SARMA¹ TAVHEED KHAN¹ D K SINGH¹

1. CSIR-National Geophysical Research Institute, Uppal Road, Hyderabad 500 007, India

Published

1 April 2020

Manuscript received

4 July 2019

Manuscript revised

2 January 2020

Accepted

19 January 2020

Early published

Unedited version published online

Final version published online

2 April 2020