An Ediacaran back-arc basin preserved in the Transversal Zone of the Borborema Province: Evidence from geochemistry, geochronology and isotope systematics of the Ipueirinha Group, NE Brazil

摘要

The Ipueirinha Group is a ca. 75 km-long by 13 km-wide succession of metavolcano-sedimentary rocks situated in the westernmost Transversal Zone, Borborema Province, NE Brazil. It is mostly composed of marine terrigenous sedimentary rocks represented by immature metarhythmites and quartzites, local metaultramafites, rhyolitic metavolcaniclastic rocks and rare lenses of metamarl. The metavolcaniclastic rocks show highly fractionated chondrite-normalized REE patterns (La-N/Yb-N = 3.68-8.94) with slight to moderately negative Eu anomalies (Eu/Eu* = 0.72-0.94), incompatible trace element signatures similar to those of volcanic arc-related rocks, including a Nb-Ta trough, and negative Nd-epsilon((t)) ranging from -3.4 to -12.2. Metarhythmites show LREE enrichment (La-N/Yb-N = 4.83-15.32), moderate Eu anomalies (Eu/Eu* = 0.68-0.81), and incompatible element signatures consistent with provenance from a continental arc. Nd isotope data indicate important detrital contribution from juvenile sources for these metarhythmites, as reflected by the Nd-epsilon((t)) values between 3.6 and 0.3. Detrital zircon LA-ICP-MS U-Pb data reveal that the main sources of detritus for the metarhythmites are Neoproterozoic rocks (maximum age peak at ca. 845 Ma and younger zircons at ca. 636 Ma). Hf isotope data obtained from detrital zircons show evidence for an important period of juvenile magmatism in the Borborema Province from 895 to 834 Ma (Hf-epsilon((t))) = 2.0-5.7), and suggest that the Neoproterozoic crustal evolution of this province involved mixing between older sources with negative Nd-epsilon((t)) and younger juvenile sources. Metaultramafites associated with the Ipueirinha Group show Nd-epsilon((t)) ranging from -1.3 to -8.5 and TOM = 1.33-1.64 Ga. The whole unit was submitted to four deformation phases and to heterogeneous metamorphic conditions from lower to upper greenschist facies. Additional geochronological data obtained for a granitic sill (578 +/- 13 Ma; zircon U-Pb) along