LAUSR

Abstract The Apiau Alkaline Complex (AAC) represents a rift-related intracontinental magmatism and occurs as intrusions within the Precambrian basement of the central-southern portion of the Guyana Shield, Amazonian Craton. The complex extends for over 200 km and comprises monzonites, syenites, nepheline syenites, nepheline-bearing syenites, trachytes and phonolites. It differs from other alkaline rocks of the craton both in age and tectonic setting. Here we report new petrologic, geochemical, and zircon and baddeleyite U-Pb geochronological information on the nepheline-bearing syenites and phonolites to discuss the petrogenetic and tectonic evolution of the AAC. Our new bulk rock major and trace elements analyses show that the complex is predominantly miaskitic, with potassic and sodic affinity, strong enrichment in light REE and negative Eu, Ba, Sr, P, and Ti anomalies. The phonolites and syenites geochemistry suggests similar fractionation processes from closely related parental melts for their genesis. Zircon and baddeleyite U-Pb crystallization ages of 111 ± 1 Ma and 116 ± 3, respectively, indicate a relationship between the alkaline magmatism and the opening of the Equatorial Atlantic Ocean. The spatial disposition of the alkaline rocks following major faults of the Amazonian Craton denotes a structurally controlled emplacement history by reactivation of ancient weakness zones. Crystallization ages are coincident with the Tacutu basin filling, evidencing a relative tectonic stability period and suggesting that the progressive subsidence contributed to the reactivation of the main structures in the region. The presence of inherited zircon from the orthoderived granitic basement (U-Pb ages of 1931 ± 4 Ma and 1958 ± 7 Ma) in a nepheline-bearing syenite, addition to petrography features and geochemical behaviour indicate that the alkaline magma was contaminated with Precambrian crustal rocks.