THE ONSET OF SUBDUCTION IN INTERIOR OCEANS: SIGNIFICANCE OF A HIGHLY DEPLETED MANTLE SOURCE FOR MAFIC COMPLEXES

Supercontinents appear to form by two end-member processes: extroversion, in which the oceanic lithosphere surrounding the supercontinent (exterior ocean) is preferentially subducted (e.g. Pannotia), and introversion, in which the oceanic lithosphere formed between dispersing fragments of the previous supercontinent (interior ocean) is preferentially subducted (e.g. Pangea). Extroversion can be explained by “top-down” geodynamics, in which a supercontinent breaks up over a geoid high and amalgamates above a geoid low. Introversion may begin when subduction zones are initiated along boundaries between the interior and exterior oceans and become trapped within the interior ocean. The Paleozoic evolution of mafic complexes within the Rheic Ocean may provide insights into subduction initiation that led to the amalgamation of Pangea. These complexes fall in the age range ca. 400-330 Ma and preserve many of the features of ophiolites. But they are characterized by anomalously high εNd, well above widely accepted model depleted mantle curve values. These data indicate derivation from a highly depleted mantle source displaying time-integrated depletion in Nd relative to Sm, The extent of mantle depletion indicates the melting event occurred up to 500 million years before the Rheic Ocean formed. If so, the mantle lithosphere that gave rise to the Rheic Ocean mafic complexes must have been captured from an adjacent, older oceanic tract. Transfer of this captured lithosphere to the upper plate enabled it to become preferentially preserved. Possible Mesozoic-Cenozoic analogues include the capture of the Caribbean plate from the Pacific to the Atlantic oceanic realm. These data also imply that virtually all oceanic lithosphere generated during the opening phase of the Rheic Ocean was consumed by subduction during Laurentia-Gondwana convergence.