The region of Calçoene (Paleoproterozoic Lourenço Domain), northern part of Amapá, Brazil, belongs to the Maroni-Itacaiunas Province, southeast of the Guiana Shield, which represents an extensive Paleoproterozoic orogenic belt developed during the Transamazonian orogeny (2,26-1,95 Ga). The Lourenço domain consists mainly of metavulcanosedimentary sequences, gneissic complexes, several calc-alkaline granitoid suites related to magmatic arc context (2,26 to 2,09 Ga) and granitic plutons, including charnockites that represent syncollisional to late-orogenic stages (2.1 to 1.99 Ga). The present study is focused on the Cunani
Granite (~2.10 Ga), the main unit of the Calçoene area. Subordinately, the Cricou Suite (2.11-2.09 Ga), adjacent to the Cunani Granite, was studied at the comparison level. In spite of the recent progress in the regional geological knowledge conducted by the CPRM, geochemical and geochronological data are still scarce and make it difficult to establish a reliable chronology and geodynamic context of this magmatism, as well as to the evaluate the processes of juvenile accretion and crustal reworking during the Transamazonian orogeny. In order to better characterize the late-Transamazonian magmatism and to evaluate the influence of Archaean crustal material on the Rhyacian magmatism in the Lourenço Domain, petrographic study, geochemistry, LA-ICP-MS U-Pb zircon geochronology and Nd-Sr isotopic study were performed and provided important contributions about the geological evolution of this portion of the Guiana Shield. The petrographic study allowed to characterize the Cunani Granite as an unit constituted by dominant biotite monzogranites and biotite sienogranites, subordinate hornblende-biotite tonalites and biotite granodiorites, which contains enclaves of granulites (quartz-diorites with orthopyroxene), and hornblende metatonalites. Rocks of enderbitic composition were also found in this unit. The Cricou Suite in the study area is constituted by biotite monzogranites, and subordinate enderbite was also
identified. The geochemical characteristics of these rocks are consistent with both a magmatic arc and syn- to post-collisional context. Most samples displayed enrichment in incompatible elements with high contents of Large Ion Lithophile elements (LILE) like Ba and K, while some High Field Strengh elements (HFSE) such as Th, La, Ce and Nd also show relatively high values. Significant negative anomaly of Nb and P are observed in the rocks of the Cunani Granite and its enclaves. The two samples of the Cricou Suite present Nb positive and negative anomaly, respectively. The Rare Earth Elements diagram showed enrichment of the
light elements in relation to the heavy elements in the two units. Only the Cricou Suite shows accentuated positive anomaly of Eu. Overall, the geochemical features are interpreted as of post-collisional magmatism related to tectonic accretion of magmatic arc to the continent. The LA-ICP-MS U-Pb dating of zircon from lithotypes of the Cunani Granite furnished ages of 2097±17 Ma (upper intercept) for a biotite sienogranite (DAC-08-06), 2017±73 Ma (upper intercept) and 1990±16 Ma (concordant age) for another biotite sienogranite (LKV-06-03) and 2019±53 Ma (upper intercept) and 1995±37 Ma (concordant age) for a sample of biotite
monzogranite (DAC-08-09a). These ages confirm a late-Transamazonian age (Neorhyacian) for this unit and suggest that the Cunani Granite may encompass different magmatic pulses. The granulitic enclaves of the Cunani Granite (DAC-08-07b) furnished an LA-ICP-MS U-Pb zircon age of 2112 ± 10 Ma and may represent deeper crustal-level rocks that correspond in depth to the migmatization event that affected the rocks of arcs around 2.11-2.10 Ga during the collisional stage. The age around 2.0 Ga obtained for the biotite monzogranite (DAC-08-09a) at the same outcrop than the hornblende metatonalite (DAC-08-09b) previously dated at 2151 ± 2 Ma (TIMS Pb evaporation in zircon) allows to reinforce that the latter corresponds
to enclaves of rocks from the Mesorhyacian magmatic Arcs within the Cunani Granite. U-Pb dating by LA-ICP-MS of zircons from a biotite monzogranite (DAC-08-11) of the Cricou Suite provided an age of 1839 ± 62 Ma considered as statistically unreliable. However a resetting of the U-Pb zircon system by younger events cannot be ruled out. The Paleoproterozoic crystallization ages obtained in the analyzed samples and the Archean NdTDM model ages between 3.17 and 2.51 Ga, together with negative values of εNd[2.08 Ga] between -8.67 and -0.72, besides inherited zircons with ages of 3056±63 Ma and 2654±43 Ma identified in a biotite sienogranite, indicate the contribution of Meso-Neoarchean crustal sources during the formation of the Cunani Granite. The Sr-TUR model ages ranged from 2.52
to 2.29 Ga, also indicating Siderian-Neoarchean signature for the source of these granitoids, compatible with tectonic accretion of the magmatic arc with continental landmass.
|
