TESES DE DOUTORADO EM GEOCIÊNCIAS SOBRE REGIÕES BRASILEIRAS
(DEFENDIDAS EM UNIVERSIDADES ESTRANGEIRAS)

GEOLOGIA E METALOGENIA DA REGIÃO DE SANTANA DA BOA VISTA, SUL DO BRASIL

Palavras-chave: geologia regional, estratigrafia, geologia estrutural, metalogenia, Precambriano tardio, Brasil sul

THE UNIVERSITY OF GEORGIA, ATHENS - GEORGIA, USA

Tese de Doutoramento defendida em março/1981
Orientador: GILLES O. ALLARD

RESUMO

Key words: regional geology, stratigraphy, structural geology, metallogeny, Late Precambrian, Southern Brazil

THE UNIVERSITY OF GEORGIA, ATHENS - GEORGIA, USA

DATE OF ORAL PRESENTATION: march 1981
ADVISOR:  GILLES O. ALLARD

ABSTRACT

The geology and mineral deposits of the Santana da Boa Vista region, State of Rio Grande do Sul, Southern Brazil are described and discussed.
Metamorphic units are distributed into two major structures: the Santana Dome and the Cerro da Árvore nappe Complex. The suprastructure of the dome and the nappe are grouped under the Porongos Metamorphic Suite.
The Santana Dome has an ensialic core, or basement of gneisses in the upper amphibolite to granulite (?) facies, which are here termed the Encantadas Gneisses (2,036 M.y.). The gneisses are overlain through a transposed unconformity by a 4,000 to 5,000 m thick sequence of northeast to northwest-trending low-grade (greenschist facies) metasediments of the Cerro dos Madeiras Group (Braziliano Orogeny, 650 to 450 M.y.). The Group is subdivided into the lower Arroio dos Neves Formation (metarkoses and quartzites), the middle Arroio Olaria Formation (rhythmic quartzites, schists, and metarkoses), and the upper Irapuazinho Formation (schists and marbles). The Cerro dos Madeiras Group is interpreted as a sedimentary wedge deposited on a Late Precambrian Atlantic-type continental margin. A Barrovian type metamorphism affected the unit during orogenesis.
The Cerro da Árvore Complex is a fragment of a westerly transported, overturned nappe bounded on the lower side by extensive mylonite zones. It consists of a complex sequence of high Al203, high K20 metandesites, metadacites and fine-grained metatufs. Volcanic and pyroclastic rocks are interlayered with pelitic schists, graphite schists, and rare quartzites and marbles. The Cerro da Árvore Complex comprises five informal stratigraphic units. It is interpreted to have been formed in an island arc environment, which developed during the closing of the Late Precambrian proto-South Atlantic Ocean. Metamorphic grade in the complex ranges from the chlorite zone (greenschist facies) to the staurolite zone (lower amphibolite facies). The assemblage andalusite-staurolite-muscovite-quartz indicates a Buchans type metamorphism. Structural evidence suggests that the westerly transport of the nappe took place when the Santana Dome already existed as a structural height.
There are two categories of granitic rocks. Coarse-grained granites, alaskites, and quartz-syenites comprise the Encruzilhada Complex. A fine-grained porphyritic quartz-monzonite (Campinas Stock) intrudes and alters the Encruzilhada Complex granitic rocks and the schists of the Cerro da Árvore Complex. Intrusion of the stock was controlled by transcurrent faults, which form pronounced topographic lineaments. The absolute age of the granitic rocks has not been determined. However, the granites of the Encruzilhada Complex may have been emplaced during the synorogenic phase of the Brasiliano Orogeny (650 to 450 M.y.), whereas the Campinas Stock is younger.
Younger units that cover the area consist of the following (oldest to youngest): (1) a tilted, allochtonous set of slices (4,000 m thick) of gray, petromictic clastics of the Arroio dos Nobres Formation (Late Precambrian conglomerates, sandstones, graywackes, and siltstones); (2) a 300 m thick sequence of red fluviatile conglomerates, sandstones, and siltstones of the Early Paleozoic Guaritas Formation, (3) a 100 m thick sequence of red fluviatile sandstones, siltstones, shales, and fossiliferous mudstones, locally capped with basalt flows, of the Caneleiras Formation (Early to Middle Mesozoic); and, (4) quaternary alluvium.
Three folding events occur in the Encantadas Gneisses. The first set of folds are suggested to be related to peak metamorphism whereas the second and third generations of folds are postulated to have been formed during the uplift of the Santana Dome. The later are not accompanied by metamorphic reactions. The Cerro dos Madeiras Group underwent two major deformations. The first corresponds to peak metamorphism and development of a metamorphic foliation, which has lately been folded to conform the Santana Dome. Metamorphic reactions are lacking in the second deformation event of the Cerro dos Madeiras Group. Four deformation events took place in the Cerro da Árvore Complex, more or less accompanied by metamorphic reactions. Based on microstructural and mesoscopic evidence it can be shown that the acme of metamorphism (second folding event) predates the formation of the nappe, which is related to a third folding event. The last major folding of the Cerro da Árvore Complex resulted from drag folding due to transcurrent displacement along the contact between the Encruzilhada and the Cerro da Árvore Complexes. An additional deformation of the Encantadas Gneisses and the Cerro dos Madeiras Group in the Santana Dome area is time-related to the displacement of the nappe onto the dome.
In decreasing age, major faults comprise: (1) north-south to northeast-trending and easterly dipping major thrusts and reverse faults; (2) northeast- to north-trending steep transcurrent lineaments; (3) northwest strike-slip faults; and (4) north-east trending, vertical, newly opened or reactivated older fault systems. The intensity, width, and length of cataclastic deformation and metamorphism decreases with the age of the fault Systems. Joint systems are in general oriented with respect to the major compression axes of the area.
Three mineral deposits occur in the area. Sedimentary iron and iron-manganese deposits form small massive pods in the Cerro dos Madeiras Group. Volcanogenic and volcano-sedimentary Cu-Fe-Pb-As sulfide occurrences have been recently discovered in the Cerro da Árvore nappe Complex. Low-grade Sn-W mineralization in greisens comprise the Encruzilhada tin mining district.
The author concludes that the area evolved in five major stages as follows: (1) the Pre-Orogenic Stage is postulated to correspond to the opening of the proto-South Atlantic Ocean during the Late Precambrian with the development of a passive continental margin, and deposition of the Cerro dos Madeiras Group on an ensialic basement (Encantadas Gneisses); (2) the Orogenic Stage is represented by the formation of an Island Arc against the continental wedge, due to consuption, deformation of the passive margin accompanied by metamorphism; pilling up, deformation and metamorphism, and westerly gravity gliding of volcanic and volcano-sedimentary sequences from the rears of the island arc; and, beginning of molasse deposition; (3) the Late Orogenic Stage took place under general uplift, large-scale transcurrent faulting, and peralkaline magmatism, and, (4) the Post Orogenic Stage is represented by faulting, peraluminous intrusions, and later molasse sedimentation; and, (5) the Epicratonic Stage is represented by the transgression of Triassic fluviatile sediments, normal faulting, flood basalts, and intrusion of basic and alkaline dikes, followed by Late Mesozoic to Cenozoic erosion.