Voluminous Paleoproterozoic plutonic belt of the Bacajá domain in Amazon Craton: Petrology, geotectonic setting and Pb–Pb zircon evaporation ages

Abstract

To the north of the well-known Archean Carajás Metallogenetic Province, the Bacajá Domain abounds voluminous Rhyacian plutonic bodies predominate largely over remnants of Siderian and Neoarchean crust. The Rhyacian granitoids of the Bacajá Domain present a wide lithological variety and a geological evolution marked by syntectonic emplacement, which printed strongly elongated geometries to the bodies, according to a WNW regional orientation. The Rhyacian granitoids form three distinct units. The Bacajaí intrusive suite comprises 2112 ± 8 Ma metaquartz diorites, tonalites, granodiorites and monzogranites of calc-alkaline signature. Sm–Nd isotopic analyses of these rocks yielded a TDM age of 2310 Ma and an εNd value of +0.56. The Arapari intrusive suite is composed of quartz diorites, tonalites, granodiorites and monzogranites of calc-alkaline signature. A preliminary Pb–Pb zircon dating pointed an age of 2072 Ma to the Arapari intrusive suite. Nd isotopes yielded TDM ages varying from 2100 to 2170 Ma and positive εNd (+2.11 to +3.04). Both Bacajaí and Arapari intrusive suites have similar chemical signatures suggesting continental magmatic arc environment. The origin of Bacajaí and Arapari magmas could be explained by partial melting of mantle and crustal sources. The João Jorge intrusive suite encloses high-K calc-alkaline quartz diorites, monzogranites, syenogranites, alkali feldspar granites, shoshonitic alkali feldspar syenites and monzonites. The alkali feldspar syenites have Pb–Pb zircon age of 2076 Ma, TDM age of 2210 Ma and positive εNd value (+1.51). Spatial relationship between shoshonitic bodies and strike-slip shear zones or geological contacts seems to reflect the influence of crustal anisotropies on mantle-derived magmas ascending. Partial melting of metassomatized mantle after extraction of granite melts could explain the origin of syenites and monzonites. The three granitoid unities show rhythmic layering defined by alternation of quartz-feldspathic and ferromagnesian-enriched levels. Regionally penetrative steep-dipping magmatic foliation was formed in the presence of both crystals and melt, under conditions of increasing deformation and decreasing temperatures. The Bacajaí metagranitoids underwent locally thermal effects of hornblende hornfels facies caused by the later granitoid suites. These hornfelsed granitoids may present granoblastic textures, overgrown muscovite, reddish biotite and clinopyroxene. Late WNW strike-slip ductile zones and brittle faults crosscut some of the plutons, closing the ancient geological evolution of the Bacajá Domain.