Rhyacian-Orosirian Khondalite Belt in the Borborema Province (NE Brazil): An active margin setting based on U–Pb zircon and monazite constraints

Khondalite belts are of special importance is delimiting old suture zones and in palaeocontinental reconstruction of Precambrian terrains. In the South-American Platform, khondalitic rocks surround cratonic areas and Archean blocks reworked in the Neoproterozoic, an example of which being the Ceará Khondalite Belt (CKB). Located in the northern portion of the Borborema Province in NE Brazil, CKB is here described by means of its petrographic, structural, and LA-SF-ICP-MS zircon and monazite U–Pb geochronological characteristics. CKB constitutes an E-W to NE–SW striking belt comprising migmatitic spinel (hercynite)- and graphite-bearing cordierite-garnet-sillimanite-biotite gneisses, small granulitic boudins, garnet-biotite quartzite, lenses of garnet amphibolite, calc-silicate rock, gondite, and mylonitized leucogneiss. Relicts of Palaeoproterozoic island and continental arcs surrounding CKB are represented by tonalitic and dioritic gneiss, respectively. The contact between CKB and the Neoproterozoic Independência Unit is marked by a transpressional shear zone to the north, whereas the contact between CKB and Archean and Palaeoproterozoic rocks to the south is via a N-verging thrust fault. The metamorphic peak is given by the parageneses: spinel (hercynite)-quartz, spinel (hercynite)-cordierite, and cordierite-sillimanite-garnet, possibly stable under UHT conditions. Crystallization ages of 2,086 ± 8 Ma and 2,063 ± 9 Ma were obtained by dating CKB migmatitic dioritic orthogneisses using the zircon U–Pb method. The CKB para-derived rocks yielded detrital zircon U–Pb age peaks at 2,190, 2,130, and 2,060 Ma. Three metamorphic stages are identified in the Palaeoproterozoic via U–Pb dating of zircon rims and monazite at 2,094 ± 4 Ma, 2,055 ± 5 Ma, and 1,958 ± 3 Ma. A fourth metamorphic event with associated migmatization in shear zones took place in the Neoproterozoic between 595 ± 6 Ma and 573 ± 15 Ma, as indicated by dating zircon rims and monazite, respectively. Our study shows that the metamorphic events occurring in CKB in the Palaeoproterozoic share similarities with those that affected the khondalitic rocks bordering the São Francisco and the North China cratons, which suggests the existence of a continuous orogen during the Rhyacian-Orosirian related to the Columbia Supercontinent amalgamation.

Ticiano José Saraiva dos Santos