History of Subduction Polarity Reversal During Arc-Continent Collision: Constraints From the Andaman Ophiolite and its Metamorphic Sole

Subduction polarity reversal during arc-continent collision has been proposed as a key mechanism to initiate new subduction zones. Despite often interpreted, well-exposed geological record that document the reversal is sparse. The ophiolitic lithounits of the Andaman and Nicobar Islands have been proposed to have formed during the initiation of a new subduction zone following the collision of the Woyla Arc of Sumatra with Sundaland (Eurasia). We here present new field, petrological and geochronological data to evaluate the timing of the initiation of Andaman subduction. We targeted the previously inferred but unstudied metamorphic sole of the Andaman ophiolites that witnessed juvenile subduction. Thermodynamic modeling reveals that the exposed amphibolites of the sole formed at around 0.9 GPa and 675 °C. We dated two samples of the metamorphic sole using the Ar/Ar method on amphibole, giving cooling ages of 106.4 ± 2.1 and 105.3 ± 1.6 Ma. This is similar to published ages from plagioclase xenocrysts in recent Barren Island volcanics and in zircons from a gabbro sample from the Andaman ophiolite, which we interpret as the age of the original ophiolite formation. The Ar/Ar ages are considerably older than arc magmatic gabbros and plagiogranites of the overlying ophiolite previously dated at 99–93 Ma and thought to reflect the ophiolite age but recently reinterpreted as a volcanic arc built on the ophiolite. Combined with the ages of Woyla-Sundaland collision, we argue that subduction polarity reversal occurred in a transient period of perhaps some 10 Myr, similar to recent settings.