Abstract
The Demeni-Mocidade domain (DMD) comprises a large area of granitoid rocks, located on the border between the states of Roraima and Amazonas, Brazil, within the inner part of the Ventuari-Tapajos Province of the Amazonian Craton, where large amounts of granitoid rocks, formed between 2.0-1.8 Ga, are predominant. This study examines six granite samples collected from the Mocidade and Demeni mountains, which are related in time to the Uatuma Silicic Large Igneous Province (SLIP), which, in turn, covers several tectonostratigraphic domains whose evolution is associated with an intercontinental setting, allowing a common association with I- and A-type granitoid rocks formed within the same 1.88 to 1.87 Ga time interval. These rocks are monzogranites and correspond petrographically to holocrystalline anisotropic lithotypes with fine to medium grain sizes. Their textural relationships, including some lithotypes containing phenocrysts with oscillatory zoning and resorbed rims, indicatethat crystallization occurred at subvolcanic or hypabyssal depths. Moreover, the association between plutonic and subvolcanic rocks in the same suite shows variations in the crustal development of the magmatic chambers. The granitoids of the DMD share a common geochemical signature with those of the Agua Branca Suite, which occurs within the Uatuma and Trombetas-Erepecuru domains of the Guiana Shield, suggesting that their crystallization occurred in similar magmatic chambers. Six U-Pb zircon ages show that most zircon crystals are concordant and the few ones which are discordant are well aligned along Discordia straight lines descending to zero. The calculated Concordia ages, covering the 1884 to 1877 Ma interval, agree within experimental error, indicating a probably similar crystallizationage. Such apparent age values, close to 1900 Ma for the DMD, made it possible for the domain of the Uatuma SLIP to spread to the West, and the area of the Ventuari-Tapajos Province could extend towards the Amazonas State of Venezuela. Finally, the coexistence between I- and A-type granitoids in the DMD, with ages within the range 1.88-1.87 Ga, stimulates a discussion, and there are three possibilities: (1) The calc-alkaline magmatism can be associated with late (post-collisional) processes related to subduction. (2) The granitoid rocks are formed in an intracontinental setting under more stable (post-orogenic) tectonic conditions. (3) The granitoid rocks are predominantly formed by A-type and alkaline magmatism in intraplate settings.
Highlights
The Ventuari-Tapajós Province is one of the five Proterozoic tectonic provinces of the Amazonian Craton (Cordani and Teixeira 2007; Figure 1A)
Six rock samples from Demêni and Mocidade Mountains were dated by U-Pb geochronology (Figure 1C) at the Geochronology Research Center (CPGeo) of the University of São Paulo (USP) using a SHRIMP-IIe/MC instrument
Nine samples from the Demêni and Mocidade mountains were observed under a conventional optical microscope, and they were classified as monzogranites, with subordinate mylonitic rocks or rocks with garnet
Summary
The Ventuari-Tapajós Province is one of the five Proterozoic tectonic provinces of the Amazonian Craton (Cordani and Teixeira 2007; Figure 1A). Granitoid and volcanic rocks formed between 2.0-1.8 Ga predominate and are present in the Guiana and Central Brazil Shields. Rock samples were collected from the Mocidade Mountain during a field survey conducted by the senior author (initial NR) and we used samples from previous projects (Pinheiro et al 1981) (initials EC and VC). A better knowledge was acquired about the geology of a littleknown Amazon area, from petrographic, geochemical and geochronological studies.
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