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The Andaman Islands are part of the Andaman-Nicobar Ridge; an accretionary complex that forms part of the outer-arc ridge of the Sunda subduction zone. The Tertiary rocks exposed on the Andaman Islands preserve a record of the tectonic evolution of the surrounding region, including the evolution and closure of the Tethys Ocean. Some of the Paleogene sediments on Andaman may represent an offscraped part of the early Bengal Fan. Through field and petrographic observations, and use of a number of isotopic tracers, new age and provenance constraints are placed on the

The Andaman ophiolites form the basement of the Andaman Islands, which is a part of the outer forearc that links the Indo-Burma accretionary complex to the north with the Java-Sumatra trench-arc system to the SE. Upper mantle harzburgite and dunite are overlain by a cumulate peridotite-gabbro complex, highlevel intrusive rocks and a tholeiitic volcanic series. The upper crust in the South Andaman ophiolite shows also a prominent andesite-dacite volcanic suite, suggesting arc volcanism built onto ocean crust. U-Pb zircon dating of a trondhjemitic rock from Chiriya Tapu in South Andaman Island using laser ablation inductively coupled mass spectrometry reveals an age of crustal formation of 95 AE 2 Ma. The trondhjemites have geochemistry comparable with that of plagiogranites associated with ophiolite complexes, and å Nd values around +7 further confirm that they are derived from depleted mantle melts. Basaltic pillow lava and basaltic dykes that cut the trondhjemites have mid-ocean ridge basalt-like trace-element geochemistry. The new data show that the Andaman volcanic arc was built on Cenomanian ophiolite-oceanic crust and that subduction was initiated at this time along Tethys, at least from Cyprus through Oman to the Andaman Islands.

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