科研之友
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摘要
The Planalto Suite is located in the Canad dos Carajasd subdomain of the Carajas Province in the southeastern part of the Amazonian Craton. The suite is of Neoarchean age (similar to 2.73 Ga), ferroan character, and A type affinity. Magnetic petrology studies allowed for the distinction of two groups: (1) ilmenite granites showing low magnetic susceptibility (MS) values between 0.6247x10(-3) and 0.0102 x 10(-3) SI and (2) magnetite-ilmenite-bearing granites with comparatively higher but still moderate MS values between 15.700x10(-3) and 0.8036 x 10(-3) SI. Textural evidence indicates that amphibole, ilmenite, titanite, and, in the rocks of Group 2, magnetite also formed during magmatic crystallization. However, compositional zoning suggests that titanite was partially re-equilibrated by subsolidus processes. The amphibole varies from potassian-hastingsite to chloro-potassian-hastingsite and shows Fe/(Fe + Mg) > 0.8. Biotite also shows high Fe/(Fe + Mg) ratios and is classified as annite. Plagioclase porphyroclasts are oligoclase (An(25-10)) , and the grains of the recrystallized matrix show a similar composition or are albitic (An(9-2)). The dominant Group 1 granites of the Planalto Suite were formed under reduced conditions below the FMQ buffer. The Group 2 granites crystallized under more oxidizing conditions on or slightly above the FMQ buffer. Pressures of 900-700 MPa for the origin and of 500-300 MPa for the emplacement were estimated for the Planalto magmas. Geothermometers suggest initial crystallization temperatures between 900 degrees C and 830 degrees C, and the water content in the magma is estimated to be higher than 4 wt%. The Neoarchean Planalto Suite and the Estrela Granite of the Carajas Province reveal strong mineralogical analogies, and their amphibole and biotite compositions have high total Al contents. The latter characteristic is also observed in the same minerals of the Neoarchean Matok Pluton of the Limpopo Belt but not in those of the Proterozoic rapakivi A-type granites. On the other hand, in terms of the degree of magma oxidation, the Planalto and Estrela granites approach the reduced Mesoproterozoic rapakivi granites and the reduced to moderately oxidized Paleoproterozoic granites of the Velho Guilherme and Serra dos Carajas Suites, respectively, and differ from the oxidized granites (Damon Suite) of the Carajas Province as well as those of Matok pluton. The high total Al content of amphibole and mica could be caused by crystallization at high pressures that, in turn, can be a reflex of the association of the studied granites and Matok with charnockitic rocks.
- 出版日期2016-4
