Evolução geológica da sequência metavulcanossedimentar Ipueirinha, Província Borborema, Piauí: petrografia, geoquímica e geocronologia

Abstract

The Ipueirinha Belt is a ca. 75 km-long and 13 km-wide belt of metavolcano-sedimentary rocks in westernmost Transversal Zone, Borborema Province, NE Brazil. Such belt is mostly constituted by marine terrigenous sedimentary rocks represented by immature metarhythmites (Caridade do Piauí Formation) and quartzites (Cacimbas Formation). Metarhythmites yield negative Eu anomalies (Eu/Eu* = 0.68-0.81), LREE enrichment (LaN/YbN = 4.83-15.32), and incompatible elements coherent with provenance from a continental arc. Overall Nd isotopic data (Nd(t) between -3.6 and 0.3) indicate detrital contribution from juvenile sources for deposition of these metarhythmites. Associated to this clastic sedimentary sequence, there are metarhyolitic tuffs (Barrocão Formation), rare lenses of metamarl, and kilometric-long bodies of metaultramafites (grouped in an informal unit named Serpentinito Carnaubal). The metarhyolitic tuffs exhibit highly fractionated chondrite-normalized REE patterns (LaN/YbN = 3.68-8.94), negative Eu anomalies (Eu/Eu* = 0.72-0.94), and incompatible trace elements patterns similar to those of volcanic arc-related rocks (including a Nb-Ta trough), besides negative Nd(t) ranging from -3.4 to -12.2. Zircon U-Pb analyses performed on crystals recovered from metarhythmites and from a granitic sill suggest that sedimentation and volcanism in the Ipueirinha Belt took place at 626-591 Ma. Such detrital zircon data reveal that the main sources of detritus for the Ipueirinha Basin are Neoproterozoic rocks (maximum age peak at ca. 845 Ma and youngest zircons at ca. 620 Ma). Isotopic data attained from detrital zircons provide evidence for an important period of juvenile magmatism in the Borborema Province from 895 to 834 Ma (Hf(t) = 2.0-5.7), as well as for a crustal renewal of this province during the Neoproterozoic, due to juvenile mantellic inputs. Additionally, U-Pb and Hf data attained for the metarhythmites resemble the geological evolution of the Tamboril-Santa Quitéria Complex (Northern Borborema Province). Hence, the Ipueirinha Belt might have constituted a Cryogenian/Ediacaran back-arc basin to the Tamboril-Santa Quitéria magmatic arc, which is one of the few records of initial convergence tectonics in the West Gondwana margin of the Borborema Province. Metaultramafites associated with the Ipueirinha Belt yield Nd(t) ranging from -1.3 to -8.5 and TDM = 1.33-1.64 Ga. In the context of a back-arc basin, these rocks could represent slices of oceanic crust that once floored the Ipueirinha basin. The whole belt was submitted to four deformational phases and to heterogeneous metamorphic conditions from lower to upper greenschist facies. Additional geochronological data attained for a chloritite blackwall (monazite U-Pb) along with the maximum crystallization age of the granitic sill constrains deformation, metamorphism and syn-orogenic magmatism in the Ipueirinha Belt at 591-565 Ma.