New Sr-Nd Isotope Data Record Juvenile and Ancient Crust-Mantle Melt Interactions in the Vijayan Complex, Sri Lanka

Abstract

The Vijayan Complex (VC) of Sri Lanka is a Neoproterozoic arc terrain having no convincingly proven counterpart elsewhere in the Gondwana supercontinent. Therefore, little is known about the nature of its pre-Gondwanan ancestry. We present here new elemental and Sr-Nd isotope systematics of orthogneisses to envisage the origin and evolution of the VC. Samples with high and variable Sr-initials and negative εNd > +4 are compatible with their derivation by melting of older rocks with a long crustal residence history. Most of the studied rocks show depleted-mantle-model ages (1.4–3.3 Ga) considerably older than the ∼1 Ga magmatic crystallization age of the VC. The rest of the samples show model ages of ∼0.9–1.1 Ga, coeval with the crystallization age of the VC, indicating separation from the mantle as juvenile inputs. Therefore, protolith sources of the majority of these rocks are mixtures of distinctive components formed much earlier than the time of predominant magmatism (∼1 Ga) in the VC. Major and trace elements and rare earth element ratios divide these samples into two categories. The “Depleted Group” represents protoliths similar to Proterozoic oceanic island basalt–like mantle materials that intensively interacted/mixed with different proportions of melts derived from juvenile and ancient lower crust. The “Enriched Group” represents mid-ocean ridge basalt–like mantle materials subsequently interacted/mixed with variable amounts of melts derived from Archean felsic and mafic crust. Geochemical-mixing modeling with 87Sr/86Sr and 143Nd/144Nd confirms the involvement of above multiple source components. Thus, our findings illustrate evidence for hybridization of mantle-derived melts from various juvenile and ancient crustal components. Therefore, this first finding of Paleoproterozoic to early Archean–aged components within the VC provides new insights to elucidate its pre-Gondwanan geodynamic setting, revising the previous understanding of it as an “exotic terrain” to Gondwana with only largely juvenile Neoproterozoic materials.