Petrology, geochronology and geophysics of the mesozoic alkaline rocks from northern Amazonian Craton

The Roraima Alkaline Province (RAP) is rift-related magmatism emplaced into main faults and shear zones central-southern part of Guyana Shield, northern Amazonian Craton. It corresponds to the first alkaline province proposed to Guyana Shield, which timing of emplacement coincides with early rifting stage in the Equatorial Atlantic. The RAP encompasses mainly felsic rocks, mostly foid syenites and phonolites as large and small plutons or dykes, while basic rocks are confined to scarce lamprophyre, tephrite and tephriphonolite dykes. In this paper, we combine petrographic, geochemical and isotopic evidence, as well as U-Pb ages, to evaluate whether indeed these rocks are cogenetic and propose a model for the origin of the RAP. Besides, we analysed airborne radiometric and magnetometric data from seven foid and foid-bearing syenites from RAP, as well as the aeromagnetic signature of the Precambrian basement to understand its relationships with the alkaline magmatism. The studied rocks are nepheline-normative, from metaluminous to peralkaline affinity and with mineralogy that varies from miaskitic to agpaitic. The geochemical patterns indicate that the rock-types seem to be controlled mainly by fractional crystallisation and the increase in peralkalinity conditions is attributed to the extensive fractionation of feldspar. The possible relationship between basic and felsic magma was evaluated by MELTS thermodynamic modelling which demonstrated that some phonolites and syenites may be the result of 51–68% crystallization of a tephritic magma. Geochemical, isotopic, geochronological and petrographic features show that crustal assimilation process played a role in the genesis of the studied rocks. The U-Pb ages obtained for RAP range from ~100 to 116 Ma, indicating that alkaline magmatic activity in central Guyana Shield was a short-lived event. The Sr-Nd isotopic signatures suggest a mixture of the depleted and enriched mantle as responsible for generating those rocks. Furthermore, depleted-mantle model ages present relatively short intervals, between 548 to 657 Ma, which coincides with model ages for tholeiites of Central Atlantic Magmatic Province in Guyana Shield, suggesting the same metasomatic event for Mesozoic alkaline and tholeiite magmatism. Therefore, the lithospheric mantle metasomatism took place shortly before magmatism, triggered by an extensional tectonic environment regarding the Tacutu rift and the Atlantic opening. Gamma-ray spectrometry data allowed us to delineate the area of high favourability to other alkaline bodies. The rocks are characterized by high concentrations of eU, eTh and K relative to the host basement and exhibit mostly low to intermediate amplitude in the ASA image. Therefore, interpretations of gamma-ray spectrometry and magnetic data constrained by field observations proved to be efficient in prospecting alkaline rocks and deciphering the regional structures in remote regions and areas of the limited outcrop. The major deformational events in the central Guyana Shield, such as Paleoproterozoic Trans-Amazonian, Mesoproterozoic K'Mudku and Mesozoic reactivation, provides a framework from which we can derive the magnetic framework of the area and insights over its structures, as well as understand their relationships with the alkaline magmatism. Alkaline magmatism occurs associated with reactivations of Precambrian NE and NW-trending weakness zones and, subsequently, E-W strike-slip faults during early rifting stage in the Equatorial Atlantic Ocean. (author)