Ediacaran ophiolite relics in the SE Brazilian coast: Field, geochemical and geochronological evidence from metabasites and paragneisses

Abstract

Metabasites and paragneisses from the high-grade Ediacaran-Cambrian Cabo Frio Tectonic Domain, SE Brazilian coast, are investigated with field, geochronological (U-Pb combined with Lu-Hf), isotopic (Sm-Nd) and geochemical analyses. These units, tectonically interleaved with a Paleoproterozoic basement, record the pre-collisional setting within the eastern Ribeira Orogen. The four analyzed metabasite bodies indicate tholeiitic basalts/gabbros with OIB affinity as protoliths tending to high-Mg terms, also corroborated by their higher LILE, LREE and HFSE contents, in comparison to the E-MORB and N-MORB basalts. The Nb and Ta (Nb/Yb – alkaline proxy) and Ti/Yb (plume-melting proxy) resemble present-day OIBs values. One metabasite provided mainly metamorphic zircon crystals (n = 20) with a U-Pb age of 530 Ma. Only three crystals show oscillatory zoning, revealing ages of 1890 Ma, 676 Ma and 580 Ma. The Paleoproterozoic zircon, interpreted as a xenocryst, and the Epsilon Nd values (+3 and −6) for whole rock indicate crustal contamination. U-Pb data from detrital zircon of two paragneiss layers, structurally below and above the metabasite, constrain maximum depositional ages of 626 Ma and 549 Ma, respectively. The older has a major 750-620 Ma zircon source, similar to the age of Cryogenian magmatic units in the African terranes. The sediments of this layer are here interpreted as deposited in a distal continental passive margin, coeval with the OIB magmatism (metabasites) crystallized near or at the Ocean-Continent Transitional zone. The upper paragneiss layer shows a major juvenile Ediacaran detrital zircon source between 620 and 550 Ma, with mainly positive Epsilon Hf values (0 to +13). It also presents disrupted blocks of the metabasites, forming a sedimentary breccia. This latter evidence, in addition to its medium P-high T metamorphic paragenesis, supports the hypothesis of an accretionary prism system. The metabasites are here interpreted as ophiolite relics from an Ediacaran Ocean that predated the Gondwana amalgamation. The subsequent ca. 550-530 Ma continental collision preserved this tectonic sliver between the Paleoproterozoic continental crust, below, and the Ediacaran forearc sedimentary sequence, above (Palmital Succession). The formed Cambrian suture was later reactivated as one of the Cretaceous rift branches that evolved to the South Atlantic.