Geologia, geoquímica, geocronologia e petrogênese das suítes TTG e dos leucogranitos arqueanos do Terreno Granito-Greenstone de Rio Maria, sudeste do Cráton Amazônico

Abstract

TTG and granite suites are exposed in large domains of the Mesoarchean Rio Maria granitegreenstone terrane (RMGGT), southeastern Amazonian craton. Extensive field work in key areas of the RMGGT, integrated with petrographic, geochemical, and geochronological studies, the latter employing the Pb-Pb evaporation and U-Pb LA-ICP-MS on zircon techniques, indicates that the TTG magmatism record in the RMGGT can be divided into three episodes: (I) A first event at 2.96±0.2 Ga (the older rocks of the Arco Verde tonalite and the Mogno trondhjemite), (II) a second one at 2.93±0.1 Ga (Caracol tonalitic complex, Mariazinha tonalite, and the younger rocks of the Arco Verde tonalite), and (III) a restricted event at 2.86±0.1 Ga (Agua Fria trondhjemite). The new data demonstrate that the Mogno trondhjemite is significantly older than previously admitted, reveal the existence of a new TTG suite (Mariazinha tonalite) and indicate that the volume of TTG suites formed during the 2.87 event was limited. The Arco Verde tonalite yielded significant age variations (2.98 to 2.93 Ga) but domains with different ages could not be individualized so far. The tonalitic-trondhjemitic suites of the RMGGT derived from sources geochemically similar to the metabasalts of the Andorinhas supergroup, which were extracted from the mantle during the Mesoarchean (3.0 to 2.9 Ga) and had a short time of crustal residence. Three groups of TTG granitoids were distinguished in Rio Maria: 1) high-La/Yb group, with high Sr/Y and Nb/Ta ratios, derived from magmas generated at relatively high pressures (≥1.5 GPa) from sources leaving garnet and amphibole as residual phases; 2) medium-La/Yb group which magmas formed at intermediate pressure conditions (~1.0-1.5 GPa), but still in the garnet stability field; and 3) low-La/Yb group, with low Sr/Y and Nb/Ta ratios, crystallized from magmas generated at lower pressures (≤1.0 GPa), from an amphibolitic source that left plagioclase as a residual phase. These three geochemical groups do not have a direct correspondence with the three episodes of TTGs generation and a same TTG unit can be composed of rocks of different groups. The geochronological data indicate that the emplacement of the Archean granites of the RMGGT occurred during Mesoarchean (2.87 and 2.86 Ga) being coeval with the sanukitoid suite (~ 2.87 Ga) and post-dating the main timing of TTG suites formation (2.98 - 2.92 Ga). Three main types of Archean granites were distinguished in the RMGGT on the basis of petrographic and geochemical data: (1) Potassic leucogranites (Xinguara and Mata Surrão granites), that are composed dominantly of biotite-monzogranites with high SiO2, K2O, and Rb contents and fractionated REE patterns with moderate to pronounced negative Eu anomalies. These granites are similar to the low-Ca granites of the Yilgarn craton and to the CA2 Archean granites. Their magmas resulted from the partial melting of sources similar to the older TTG suites of the RMGGT; (2) Amphibole-biotite monzogranites (Rancho de Deus granite) generated by fractional crystallization and differentiation of sanukitoid magmas; (3) leucogranodiorite-granite suites (Guarantã suite and Grotão granodiorite), which are Ba- and Sr-rich rocks with strongly fractionated REE patterns without significant Eu anomalies. These granites have affinity with the high-Ca granites of the Yilgarn craton and the CA1-type Archean granites. On the basis of modeling and geochemical data we suggest that the leucogranodiorite-granite suites were derived from mixing between a granite, similar to the Ba- and Sr-enriched samples of the Guarantã suite, and trondhjemitic liquids. The granite magmas participating in the mixture were originated by fractional cystallization of 35% of a sanukitoid magma of granodioritic composition. The fractionated mineral phases were: plagioclase (46.72%), hornblende (39.05%), clinopyroxene (10.36%), magnetite (3.12%), ilmenite (0.7%) and allanite (0.06%). The large compositional variations observed in the Guarantã suite can be apparently explained by mixing in different proportions between the granite and trondhjemitic liquids. A model involving a subducting slab underneath a thick oceanic plateau was envisaged to explain the tectonic evolution of the RMGGT. In this context, the low-La/Yb group was formed from magmas originated by the melting of the base of a thickened basaltic oceanic crust at comparatively lower pressures (≤ 1.0 GPa), whereas the medium- and high-La/Yb groups were derived from the slab melting at increasing different pressures (1.0-1.5 and > 1.5 GPa, respectively). Part of these TTG magmas react during their ascent with the mantle wedge being totally consumed and leaving a metassomatized mantle. 50 m. y. later, at ca. 2870 Ma, thermal events, possibly related to the slab-break-off, causing asthenosphere mantle upwelling, or to the action of a mantle plume, may have induced the melting of the metassomatized mantle and the generation of sanukitoid magmas. These magmas may have heated the base of the Archean continental crust during their rising to the surface and could have lead to the local melting of the basaltic crust forming the Água Fria trondhjemite magma. This was accompanied by partial melting (at shallower crustal levels) of the Rio Maria tonalitic-thondhjemitic crust and generation of the potassic leucogranite.