U-Pb zircon age of the andaman ophiolite: Implications for the beginning of subduction beneath the andaman-sumatra arc

摘要

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 ± 2 Ma. The trondhjemites have geochemistry comparable with that of plagiogranites associated with ophiolite complexes, and 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.