Maria de Fátima Bitencourt

Geochemistry and petrogenesis of post-collisional ultrapotassic syenites and granites from southernmost Brazil: the Piquiri Syenite Massif

The Piquiri Syenite Massif, southernmost Brazil, is part of the post-collisional magmatism related to the Neoproterozoic Brasiliano-Pan-African Orogenic Cycle. The massif is about 12 km in diameter and is composed of syenites, granites, monzonitic rocks and lamprophyres. Diopside-phlogopite, diopside-biotite-augite-calcic-amphibole, are the main ferro-magnesian paragenesis in the syenitic rocks. Syenitic and granitic rocks are co-magmatic and related to an ultrapotassic, silica-saturated magmatism. Their trace element patterns indicate a probable mantle source modified by previous, subduction-related metasomatism. The ultrapotassic granites of this massif were produced by fractional crystallization of syenitic magmas, and may be considered as a particular group of hypersolvus and subsolvus A-type granites. Based upon textural, structural and geochemical data most of the syenitic rocks, particularly the fine-grained types, are considered as crystallized liquids, in spite of the abundance of cumulatic layers, schlieren, and compositional banding. Most of the studied samples are metaluminous, with K2O/Na2O ratios higher than 2. The ultrapotassic syenitic and lamprophyric rocks in the Piquiri massif are interpreted to have been produced from enriched mantle sources, OIB-type, like most of the post-collisional shoshonitic, sodic alkaline and high-K tholeiitic magmatism in southernmost Brazil. The source of the ultrapotassic and lamprophyric magmas is probably the same veined mantle, with abundant phlogopite + apatite + amphibole that reflects a previous subduction-related metasomatism.

Cumulatic Diorites Related to Post-Collisional, Brasiliano/Pan-African Mafic Magmatism in the Vila Nova Belt, Southern Brazil

Abstract The Capivaras Diorite, in the Vila Nova region, NW of the Sul-rio-grandense Shield, is composed of six NE- to NNE-oriented rock bodies of late-tectonic emplacement relative to the D3 deformation phase which forms subvertical high-strain zones in basement gneiss sequences. Within these intrusive bodies, a shape foliation is present, generally parallel to contacts and displaying a local solid-state deformational component. The internal structure of the Capivaras Diorite main intrusion is marked by a zone of intense flow and mingling, characterized by strong shape foliation and layers of variable texture and composition, which result from cumulative processes, heterogeneous flow and interaction of coeval, compositionally contrasting magmas. The central part of this intrusion is texturally homogeneous and slightly foliated, even though cumulative processes have remained important during its formation. Along the contact with basement gneisses, fine-grained diorites are found, which are considered to be compositionally close to the parental magma of the Capivaras Diorite. This magma has a mildly alkaline affinity and shows moderate to high contents of Zr, Ti and P. Highly-fractionated REE patterns, low Nb contents, as well as high contents of K, Sr, Ba, and Rb, are suggestive of its provenance from mantle sources which have been previously affected by subduction processes, such as those of mature magmatic arcs or post-collisional settings. Magmatic evolution was controlled by cumulative processes and gave origin to pyroxene orthocumulates, plagioclase-pyroxene orthocumulates, pyroxene adcumulates, and more rarely plagioclase adcumulates. The cumulative origin of these rocks is indicated by field, textural and geochemical features, which are distinct from those of crystallized liquids. The compositional diversity of cumulates has led to the generation of compositionally different melts. The early-formed pyroxene cumulates have caused Ca, Al, Na, Ba, and Sr enrichment in the magmatic liquid, leading to plagioclase crystallization and accumulation. Coarse-grained mafelsic cumulates were formed during the late stages of magmatic crystallization, due to volatile enrichment of the intercumulus liquid. Considering geological relations, as well as tectonic and compositional features of the Capivaras Diorite, it is interpreted as part of Neoproterozoic magmatism related to the post-collisional stage of Brasiliano/Pan-African Orogenic Cycle in southern Brazil.

Caracterização geoquímica e petrogenética dos Granitoides Arroio Divisa, região de Quitéria, Rio Grande do Sul

The Arroio Divisa Granitoids (ADG), situated in the Quiteria region, eastern Sul-rio-grandense Shield, conform an elongate, NE-SW oriented body about 30 km long and 1 to 6 km wide. They are intrusive in Paleoproterozoic metatonalites, metagranodiorites, and tonalitic to dioritic gneisses at the northern border, whilst in the south they are intruded by Neoproterozoic granites and rhyolites. The ADG rocks are predominantly foliated granodiorites to granites, with medium- to coarse-grained equigranular textures, containing amphibole and biotite. Titanite, zircon, and apatite are accessory minerals. Dioritic to tonalitic rocks occur as mafic microgranular enclaves, bodies with metric dimensions, and synplutonic dikes, with interpenetrated and sinuous contacts, as usual for magma mingling products. Near the diorites, the mafic contents of the granitoids is increased. Centimeter- to meter-sized xenoliths of gneisses and metatonalites from the Arroio dos Ratos Complex, and of hornblende-biotite granodiorites correlated to the Cruzeiro do Sul Granodiorite are frequently observed. Where mingling and large xenoliths are abundant, the ADG granodiorites change their texture to heterogranular and porphyritic, in meter-wide zones. Magmatic foliation is marked by the shape orientation of plagioclase and biotite. Parallel to the magmatic foliation, a mylonitic one is developed with variable intensity and sinistral transcurrent movement. The steeply-dipping, ENE-striking structures are rotated towards NE strike at the eastern part of the body, where a regional cataclastic zone has controlled the emplacement of later intrusions. Quartz-mylonites and phylonites are found within the ADG along high-strain, low-temperature zones, sometimes hundred-meters wide. The ADG and associated mafic rocks show geochemical features that indicate their genetic relationship with medium to high-K magmas of tholeiitic affinity contaminated by crustal melts produced from garnet-bearing gneissic protoliths. The integrated interpretation of stratigraphic, tectonic and geochemical evidences indicates that the ADG and associated mafic rocks have formed during the early period of Neoproterozoic post-collisional magmatism in southernmost Brazil.

Paleoproterozoic late-orogenic and anorogenic alkaline granitic magmatism from northeast Brazil

Abstract During the Transamazonian cycle (2.0±0.2 Ga), two silica-saturated alkaline granite suites were intruded along the border of the Sao Francisco craton, northeastern Brazil. The Couro de Onca intrusive suite (COIS) is late tectonic relative to major deformational events, and the Serra do Meio suite (SMS) postdates this event, being interpreted as anorogenic type, deformed during the Brasiliano cycle (0.65–0.52 Ga). The COIS encompasses alkali-feldspar granites, syenogranites, monzogranites, with dykes of similar composition and age (2.157±5 Ma). They are metaluminous rocks of alkaline affinity, whose evolution was probably controlled by mineral fractionation processes involving mainly plagioclase, amphibole, and Fe–Ti oxide. The SMS, whose probable age is 2.01 Ga, is composed of metaluminous and peralkaline granites, with associated quartz syenites, with Ti-aegirine transformed during the Brasiliano deformation to riebeckite+titanite+aenigmatite. The temperatures of two stages of metamorphic transformation, overprinted during the Brasiliano event, are estimated at about 550 and 400–450°C. Both magmatic suites are derived from enriched mantle sources. The metaluminous and peralkaline trends observed in the SMS, characterized by high concentrations of HFS and RE elements controlled mainly by F-activity, reflect the evolution of different primary magmas produced by successive melting of the same mantle sources, in an anorogenic setting.

Caracterização cinemática de uma zona de cisalhamento transcorrente de direção NNE marcada na evolução de seu magmatismo granítico sintectônico na região de Camboriú, SC

The Corre-mar Granite (GCM) is located in the region of Itapema-Camboriu, Santa Catarina, Brazil, in the northeastern part of Dom Feliciano Belt. A subvertical NNE-trending shear zone controls its emplacement, and a subvertical S-C fabric is generated with sinistral shear sense. The S-planes (magmatic) strike NE-SW and are marked mainly by the orientation of feldspar crystals and xenoliths, while the C-planes (shearing), strike NNE-SSW, marked by mineral stretching, tails of asymmetric recrystallization in feldspars, and inflection of the structures previously oriented along S. Low-temperature deformation zones parallel to the GCM local C-foliation, best developed near the contacts with the host rocks, do not extend more than a few meters into the country rocks. The GCM bodies have a systematic variation in thickness, where wider zones show the least developed foliations. Microstructures indicative of progressive deformation under decreasing temperature are represented by chessboard-pattern subgrains in quartz, and granoblastic K-feldspar grains. These are overprinted by lower temperature microstructures, such as fine-grained recrystallized quartz, frequently by bulging, sealing of fractures in larger K-feldspar grains by the finely recrystallized matrix, and neoformation of fine grains around the feldspar crystals. The integration of structural and petrological data indicate that the GCM was positioned when the NNE-trending shear zone was active, and its geometry indicates an opening component along the NE direction, typical of transtensional regimes. The transtensional structure is compatible with its positioning in a low-strain zone of the Southern Brazilian Shear Belt, situated between the higher strain zones represented by the Major Gercino and Itajai shear zones, both active in the post-collisional stages of Brasiliano Cycle in southern Brazil.