Geochemical and geochronological record of the Andaman Ophiolite, SE Asia: From back-arc to forearc during subduction polarity reversal?
Résumé
Ophiolites are widely studied to unravel how new subduction zones form. They may contain crustal and mantle rocks that formed during juvenile stages of intra-oceanic subduction, modifying the pre-existing oceanic lithosphere within which subduction started, and in which a magmatic arc formed upon subduction maturation. Previous geochemical work on the Cretaceous Andaman Ophiolite and its metamorphic sole, located in the forearc of the Sunda subduction zone of south-east Asia, has revealed geochemical signatures that are difficult to reconcile in a single tectonic setting but may rather record different stages in a longer evolution. A recent kinematic reconstruction proposed that the Andaman Ophiolite may have formed during subduction initiation in a former back-arc basin, when the former (Woyla) arc collided with the Sundaland continent. Here, we evaluate whether such a scenario may provide a feasible context to explain the enigmatic geochemical signatures preserved in the Andaman Ophiolite. To this end, we provide new, and review existing geochemical as well as geochronological constraints on the formation of its crustal rocks, as well as the evolution of its mantle portion. We identify mafic magmatic rocks and cogenetic plagiogranites that are consistent with formation in a magmatic arc, whereas other magmatic rocks, as well as metamorphic sole protoliths, have characteristics indicative of a back-arc origin. Three new, and two previous zircon U/Pb ages of arc magmatic rocks give a 99–93 Ma age range, but we also identify an inherited ~105 Ma age. This latter age coincides with Ar/Ar cooling ages of the Andaman metamorphic sole, and with a plagioclase xenocryst age from recent Barren Island volcanics east of Andaman. The geochemical and geochronological constraints of the Andaman ophiolites are straightforwardly explained in the context of the regional kinematic history: (1) The original lithosphere formed in the back-arc basin of the Woyla intra-oceanic arc; (2) Subduction initiation and SSZ ophiolite formation within this basin occurred around or slightly before 105 Ma; (3) This was followed by arc magmatism between 99 and 93 Ma upon subduction maturation.