Petrologia da Região de São Roque, São Paulo

  • José Moacyr Vianna Coutinho

Resumo

This paper is concerned with São Roque series in the vicinity of the city of São Roque in the State of São Paulo. Pre-Cambrian para-metamorphic rocks form this series. However, many of the more prominent outcrops consist of "Pirituba" granite types formed by large phenocrysts of microcline in a coarse ground mass which are intrusive in the country rocks and, consequently, of later age. These granites are generally supposed to belong to the Pre-Devonian. Tectonic 1) The "granites" (field term which includes: granites, adamellites and granodiorites) show plane-fluidal structures because of upward fluxional movements, during the period in which the physical state of the magma was still plastic or semiplastic. Their structures demonstrate discordant contacts between batholith and country rocks. The flow layers seem to form gentle folds. Their axes are approximately coincident with the horizontal axis of the batholith and, locally, with NE-SW contact: "granite" — limestone. 2) It lacks macroscopic evidence of lineation in the intrusive rocks. 3) The aplitic and pegmatitic formations of the batholith took place only along joints trending 70° NE and dipping steeply northward. Elsewhere in the country rocks aplites and pegmatites are found in the direction of the rock bedding. 4) There are other planes of possible faulted marginal joints in the batholith. The fault observed seems to have taken place much later than the magma consolidation and due, perhaps, to a quite recent regional diastrophism which, by the same way, has also affected the country rocks. 5) The tectonic characteristics of the area seem to show an active intrusion of the magma, which has by this way deformed somehow the country rocks. 6) On the last stages of the chamber-forming process, the magma should have ejected great number of apophyses and dikes which have separated blocks of the country rocks by piecemeal-stoping. Petrography of the igneous rocks. 1) The plutonites found in the area are typical adamellites and granodiorites, according to Johanssen s classification. 2) The sequence of crystallization is the following: magnetite, apatite, epidote, titaniL e, hornblende, biotite, plagioclase, potash feldspar and quartz. 3) Cases of hydrothermal metasomatic alterations were observed in the plutonic rocks. The principal observed modifications were: chloritization and epidotization of the femic minerals; decalcification of the plagioclase and increasing amount of quartz in the rock. 4) In three different observed localities the rocks show appreciable variation in mineral quantities and the plagioclases become more basic. 5) Cataclastic and milonitic textures, as a result of dynamo-metamorphism, take place on the batholithic boundaries along inner faults and in intrusive bodies in the country rocks. 6) Magmatic differentiation of the pegmatite stages includes: pegmatite, pink aplite and purple turmaliniferous aplite. The latter was partially formed before the other two. 7) There are several types of milonitized rocks inside the granitic batholith. The type showing the most intensive cataclastic effects is the so called ultra-milonite, very common in the quarry of the Sorocabana Railway near Sao Roque, which forms pseudo-dikes along the faults. 8) Deposition of hydrothermal minerals through the milonitized faults was observed. Petrography of the xenoliths 1) Two xenoliths found in batholithic boundaries show a calcareous origin. 2) The above mentioned xenoliths were not reabsorbed because of chemical exchange between them and the magma. The xenoliths have provided the magma with a part of the CaO, all of the C02 and possibly of some of the MgO. In the other hand the magma has introduced in the xenolith: SiO.,; Alr,0H; FeO; Fe2Oa and NaoO. The xenolith rock becomes a granular silicatic aggregate where the conspicuous minerals are diopsidc and oligoclase. Petrographic development of the granitic magma. 1) The magmatic orign of the rocks can be seen by their lithological characteristics and their geological relations to the country rocks. 2) Some chemical and mineralogical aspects exhibited by the xenoliths show that the limestone were assimilated by the magma. 3) Daly's hipol hesis of magmatic sloping would explain such a assimilation process. 4) The granitic rock was submitted in some places to a hydrol hermal alteration after the consolidation and after, at least, two cataclastic movements. Metamorphism 1) The limestone, the only rock here studied petrologically, shows the effects of two kinds of metamorphism: dynamo-thermal or regional metamorphism and contactmetamorphism. 2) The regional metamorphism provoked recrystallization, formation of silicates, stress, folding and lifting of the layers. 3) The limestone should be placed at the Tilley's "chlorite zone" because it shows a comparative low degree of metamorphism. Meanwhile, it was observed the formation of diopside, a mineral of more intense metamorphism. It is believed that it was formed during the regional metamorphism. This mineral was not in equilibrium with the other minerals of the rock. A reasonable explanation to this phenomenon is found in the initial chemical composition of the layer, sometimes containing diopside, or by the absence of stress on the same bed. 4) Zones of calcic-silicate hornfels contiguous to the batholith proves the existence of contact-metamorphism. 5) The. contact-metamorphism took place by a process of addition and not by recombinative crystallizations since the hornfels observed in three different outcrops have shown reasonable amount of feldspars. Away from the contact zone there was not observed neither those minerals nor others that could explain their origin. 6) The addition of substances by the magma to the country rock is also suggested by comparing chemical analyses of samples of rocks sucessively more distant from the contact. 7) The addition of substances were made possible by means of magmatic residual solutions of approximately pegmatitic chemical composition that had impregnated and metasomatized the limestone. The effects were more intensive in the contact "hornfels" but it is possible also that the solutions could have affected some beds originally more quartzoses and pervious more distant from the intrusion.

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Publicado
1953-12-17
Como Citar
COUTINHO, J. M. Petrologia da Região de São Roque, São Paulo. Boletim da Faculdade de Filosofia, Ciências e Letras da Universidade de São Paulo. Mineralogia, n. 11, p. 5-144, 17 dez. 1953.
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Artigos