Geochemistry and origin of the Rhyacian tholeiitic metabasalts and meta-andesites from the Vila Nova Greenstone Belt, Guyana Shield, Amapá, Brazil.

Abstract The origin and significance of the metavolcanic rocks of the Vila Nova Greenstone Belt (VNGB) is an important issue in order to understand the evolution of the Guyana Shield in Paleoproterozoic times. This work presents new LA-MC-ICPMS U-Pb zircon data and geochemical analyses carried out on metavolcanic rocks of the VNGB. This information was supplemented by petrography, stratigraphic and structural data acquired through geologic mapping and boreholes descriptions. In the Vila Nova region, the VNGB units rest on the Archean basement composed of orthogneisses of the Tumucumaque Complex. The lower portion of the VNGB consists of metabasalts and andesitic metabasalts, covered by an upper metasedimentary domain with subordinate metavolcanic and exhalative chemical rocks. The metavolcanic rocks include metabasalts, meta-andesites, amphibolites and amphibole schists, deformed and elongated according to the regional NW-SE structure. U-Pb zircon data yielded an age of 2154 ± 6 Ma for a meta-andesite of the lower portion of VNGB. One orogenic metamorphism followed by three deformational events were recognized. The D 1 and D 2 events formed the schistosity (S 1), preserved as intrafolial folds (F 2) and the crenulation cleavage (S 2) associated with the development of the oblique shear zones. Two distinct assemblages: 1) garnet, hornblende and biotite, and 2) hornblende, plagioclase and diopside define two distinct metamorphic peaks: M 1 and M 2 with temperatures of 450 and 650 °C, and lithostatic pressure between 4 and 6 kbar. The metabasites comprise geochemical compositions of Fe- and Mg-tholeiites with komatiitic affinity. The first group is enriched in LREE and HFSE. The second group shows MORB-like REE patterns in association with negative Nb, Ti and P anomalies. The observed features indicate that the metavolcanic rocks of the VNGB represent the evolution of back-arc basins and island arcs associated with the Trans-Amazonian Orogeny. These rocks characterize the Rhyacian evolution of the Maroni-Itacaiunas Province in the eastern portion of the Guyana Shield. Graphical abstract Image 1 Highlights • La-ICP-MS U-Pb zircon geochronology data on meta-andesite established a Rhyacian age (2,154 ± 6 Ma) for the magmatism. • The protoliths are tholeiitic basalts and andesites with geochemistry associated with an island arc-back-arc system. • E-MORB, N-MORB and plume type sources were involved in the formation of the volcanic rocks. • The volcanic rocks were affected by metamorphic events under greenschist-amphibolite and hb-px hornfels facies conditions. [ABSTRACT FROM AUTHOR]