Two parallel belts of late Mesozoic ophiolitic rocks occur along the outer arc ridge and inner volcanic line of the Indonesian arc system. The northernmost occurrence along the outer arc ridge is in the Andaman group of islands where ophiolitic rocks occur as dismembered slices emplaced over Eocene to Oligocene turbidites.

Volcanic members of the ophiolitic suite occur in two thrust slices, one consisting only of pillow basalts while the other is made up of basalts, basaltic andesites and acid differentiates. Certain chemical characters of the basaltic components are comparable to MAR basalts at 45° N. On the basis of comparable REE patterns and the presence of appreciable volumes of acid differentiates, they are further suggested to represent marginal basin crust. Variable (La/Ce)N and (La/Sm)N ratios in these basalts are interpreted in terms of a heterogeneous source.

The basaltic melts have undergone extensive fractional crystallization of clinopyroxene, olivine and plagioclase to produce basaltic andesite and acid volcanic rocks. Between the parent and the differentiates, there is no appreciable change in (Ce/Yb)N ratio or HREE abundance level, suggesting low pressure fractionation in a shallow magma chamber, possibly beneath a slow spreading centre.

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