Deciphering subduction polarity reversal from the Cretaceous Andaman Ophiolite, SE Asia
Switching of subduction zone polarity (subduction polarity reversal) is believed to be a key mechanism to initiate new subduction zone system, however inferring it from rock record is challenging. Ophiolites are widely studied to unravel the evolutionary history of intra-oceanic subduction, from the juvenile stages to subduction maturation. Here we present an integrated geochemical and geochronological synthesis of various rock types of Andaman Ophiolite to evaluate the feasibility of a previously speculated process of subduction initiation via polarity reversal when the former (Woyla) arc collided with the Sundaland continent. The Andaman Ophiolite Suite is exposed on the Andaman and Nicobar Islands in the outer arc ridge of the Sunda trench of the India-Eurasia subduction system. The Ophiolite Suite is made up of incoherent Penrose sequence including contrasting crustal (especially volcanic) and mantle rocks along with underlying metamorphic sole and mélange, and an overlying Palaeogene and younger forearc sedimentary sequence. Detailed field observation indicated that the existing geochemical and geochronological results on the Andaman Ophiolite and its metamorphic sole are difficult to reconcile in a single tectonic setting but may rather record different stages in a longer evolution. 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. 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 that collided in the mid-Cretaceous with Sumatra; (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. This study thus directly contradicts the previously inferred ~95 Ma age of the Andaman ophiolite and the concurrent subduction initiation as well as provides new insights into the possible petrological manifestation of subduction polarity reversal scenario.
1. Bandyopadhyay, D., Ghosh, B., Guilmette, C., Plunder, A., Corfu, F., Advokaat, E.L., Bandopadhyay, P.C., van Hinsbergen, D.J.J. (2021) Geochemical and geochronological record of the Andaman Ophiolite, SE Asia: From back-arc to forearc during subduction polarity reversal?, Lithos, vol-380-381, 105853. [DOI: 10.1016/j.lithos.2020.105853] 2. Plunder, A., Bandyopadhyay, D., Ganerød, M., Advokaat, E.L., Ghosh, B., Bandopadhyay, P.C., van Hinsbergen, D.J.J. (2020) History of subduction polarity reversal during arc-continent collision: constraints from the Andaman Ophiolite and its metamorphic sole, Tectonics, vol-39, e2019TC005762, [DOI: 10.1029/2019tc005762] 3. Bandyopadhyay, D., van Hinsbergen, D. J. J., Plunder, A., Bandopadhyay, P. C., Advokaat, E., Chattopadhaya, S., Morishita, T., and Ghosh, B. (2020) Andaman Ophiolite: An Overview. In: Ray, J. S.and Radhakrishna, M. (eds.), The Andaman Islands and Adjoining Offshore: Geology, Tectonics and Palaeoclimate, Springer International Publishing, Cham, pp.1-17. [DOI: 10.1007/978-3-030-39843-9_1]