|
José Almeida, Federal University of Para (Brazil)
Marcelo Oliveira, Federal University of Para (Brazil)
Roberto Dall'Agnol, Federal University of Para (Brazil)
Albano Leite, Companhia Vale do Rio Doce (Brazil)
Fernando Althoff, Federal University of Pará (Brazil)
|
|
The Mesoarchean Rio Maria Granite-Greenstone Terrane (RMGGT) is included in the Carajás province the largest exposed Archean domain of the Amazonian craton. Besides greenstone belts, sanukitoid series and potassic leucogranites of calc-alkaline affinity, granitoids of the tonalitic-trondhjemitic series (TTG) are the dominant rocks in the RMGGT. Two groups of TTG granitoids have been be distinguished:
1) Older (2.98 to 2.93 Ga) TTG rocks that are represented by the Arco Verde Tonalite and the Caracol Tonalitic Complex, the oldest granitoids dated so far, in the Carajás Mineral Province;
2) younger (∼2.87 Ga) TTG granitoids, represented by the Mogno and Agua Fria Trondhjemites that are approximately coeval with the sanukitoid series and leucogranites.. These two groups of the TTG rocks display strong petrographic and geochemical similarity and are composed essentially of biotite tonalite and trondhjemite occasionally with small amounts of K-feldspar and hornblende. In terms of chemistry, the TTG of the RMGGT exhibit SiO2 content generally between 65 and 72 wt. % and are metaluminous to peraluminous, showing characteristics of the high-Al trondhjemitic series, K2O/Na2O<0.5, low Rb.and high Sr/Y. Their REE patterns are generally devoid of significant europium anomalies, strongly fractionated and HREE depleted. Geochemical modeling indicated that the Archaean TTG magmas of the RMGGT were probably derived by partial melting (25-30%) of Archean tholeiites transformed in garnet amphibolites. The restite should comprise necessarily large amounts of garnet and hornblende, and possibly subordinate clinopyroxene, plagioclase, orthopyroxene and ilmenite.
The Mesoarchean TTG granitoids of the RMGGT have low #Mg, Ni and Cr, similar to those dominant in Early to Meso Archean TTG (>3.0 Ga). However, Sr contents in the least differentiated TTG approach those values registered in Late Archean TTG (<3.0 Ga). These geochemical data indicate that the parental magmas of TTG granitoids of the RMGGT interacted little with the mantle. In the case of a subduction related tectonic setting, this suggests a relatively small thickness of the overlying mantle wedge, but it could be alternatively explained by partial melting of lower crust in arc zones or in the root zones of oceanic plateaus. In any case, the magma source depth must be great enough to avoid a large retention of plagioclase in the restite, to explain the relatively high Sr in the TTGs of the RMGGT. The occurrence in the RMGGT of sanukitoid series approximately coeval with the younger TTG favors a subduction related tectonic setting for TTG origin, involving slab melting at moderate depths and little interaction with the mantle wedge during ascent. The geochemical compositions of TTGs of the RMGGT suggest that the secular evolution of TTGs can be more complex than admitted.
|
