Lauro Valentim Stoll Nardi

Volcanic cycles and setting in the Neoproterozoic III to Ordovician Camaquã Basin succession in southern Brazil: characteristics of post-collisional magmatism

The Camaqua Basin comprises a volcano-sedimentary succession, located in southernmost Brazil, and represents a molasse basin formed at the post-collisional stage of the Brasiliano/Pan-African orogenic cycle in the Neoproterozoic III to Ordovician period. This basin is one of the most well-preserved ancient volcano-sedimentary sequences undeformed and unmetamorphic in the world, dominantly developed on a continental setting under subaerial conditions. It is composed of five major stratigraphic units, four of them with a distinct volcanic character from the bottom to the top, as: (1) Marica; (2) Bom Jardim; (3) Acampamento Velho; (4) Santa Barbara; and (5) Guaritas Allogroups. A concise sight of geochemical and isotopic rock data is presented, as well as stratigraphic correlation and description of rock structures and textures that lead to the identification of their genetic processes, the aim of this paper, indicating a relation with a coeval plutonism, and volcanism that evolved from high-K calc-alkaline to shoshonitic and ended with a silica-saturated sodic alkaline magmatism, with a crustal component represented by peraluminous granites. Volcanic deposits from bottom to top are made mostly of volcanogenic sedimentary deposits, succeeded by basic to intermediate lava and pyroclastic flows of shoshonitic affinity, followed by intermediate and acid lava flows and ignimbrites of sodic alkaline affinity. The last volcanic event is represented by basalt pahoehoe flows, probably of mildly alkaline sodic affinity.

The evolution of Neoproterozoic magmatism in Southernmost Brazil: shoshonitic, high-K tholeiitic and silica-saturated, sodic alkaline volcanism in post-collisional basins

The Neoproterozoic shoshonitic and mildly alkaline bimodal volcanism of Southernmost Brazil is represented by rock assemblages associated to sedimentary successions, deposited in strike-slip basins formed at the post-collisional stages of the Brasilian/Pan-African orogenic cycle. The best-preserved volcano sedimentary associations occur in the Camaquã and Campo Alegre Basins, respectively in the Sul-riograndense and Catarinense Shields and are outside the main shear belts or overlying the unaffected basement areas. These basins are characterized by alternation of volcanic cycles and siliciclastic sedimentation developed dominantly on a continental setting under subaerial conditions. This volcanism and the coeval plutonism evolved from high-K tholeiitic and calc-alkaline to shoshonitic and ended with a silica-saturated sodic alkaline magmatism, and its evolution were developed during at least 60 Ma. The compositional variation and evolution of post-collisional magmatism in southern Brazil are interpreted as the result mainly of melting of a heterogeneous mantle source, which includes garnet-phlogopite-bearing peridotites, veined-peridotites with abundant hydrated phases, such as amphibole, apatite and phlogopite, and eventually with the addition of an asthenospheric component. The subduction-related metasomatic character of post-collisional magmatism mantle sources in southern Brazil is put in evidence by Nb-negative anomalies and isotope features typical of EM1 sources.

Post-Collisional Alkaline Magmatism on the Taquarembó Plateau: A Well-Preserved Neoproterozoic–Cambrian Plutono-volcanic Association in Southern Brazil

The Taquarembo Plateau plutono-volcanic association (TPPVA), a magmatic association related to a silica-saturated alkaline series, represents a portion of the last episode of post-Brasiliano/Pan-African collisional magmatism in southern Brazil. It was preceded by a postcollisional high-K calc-alkaline and shoshonitic magmatism, which is more pronounced near the collisional belt. The TPPVA is a sequence of volcanic, volcaniclastic, and volcanogenic sedimentary deposits with hypabyssal associated rocks, lying on Paleoproterozoic granulites. Isotope data suggest that this alkaline postcollisional magmatism occurred over the period from 580 to 537 Ma. Two sequences of magmatic liquids, which evolved by mineral fractionation from low-Ti-P and high-Ti-P basaltic magmas, were identified. The former includes mildly alkaline silica-saturated basalts, metaluminous monzodioritic intrusions, and rhyolitic lavas, whereas the second includes hawaiites, mugearites, syenitic intrusions, and peralkaline to intermediate la...

Petrology of dioritic, tonalitic and trondhjemitic gneisses from Encantadas Complex, Santana da Boa Vista, southernmost Brazil: paleoproterozoic continental-arc magmatism

The Encantadas Complex is a unit composed of dioritic, tonalitic and trondhjemitic gneisses with minor hornblendite. This complex is intruded by granites of Neoproterozoic age. Major and trace element data indicate metaluminous to slightly peraluminous composition related to the medium-K calc-alkaline series. Compositional parameters are consistent with a common evolution from less differentiated magmas, probably through fractional crystallization. The orthogneisses show LaN/YbN ratios from 10 to 50, K2O/Na2O varying from 1.1 to 3.0, Y contents from 3 to 39 ppm, Yb from 0.3 to 3.7, and Lu with contents in the range 0.06 and 0.54 ppm. Such geochemical features are similar to those of Archaean tonalitic rocks and are usually described in rocks formed by partial melting of mafic rocks under high-pressure conditions leaving an eclogitic residue. The presence of diorites and hornblendites, associated to tonalitic and trondhjemitic gneisses suggests, on the other hand, that tonalitic magmas could be formed by hornblende-controlled fractionation of hydrous basaltic magmas. Tonalitic gneisses show U-PbSHRIMP zircon age of 2, 263 ± 6 Ma for igneous crystallization and 2, 045 ± 10 Ma for the metamorphism. The geochemical parameters, tectonic and geochronological features of Encantadas Complex are consistent with magmas derived from mantle metasomatized by subduction-related fluids in a continental-arc.

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.

The Cordilheira Intrusive Suite: Late Proterozoic peraluminous granitoids from southern Brazil

The Cordilheira Intrusive Suite is made up of peraluminous two-mica leucogranites related to the Brasiliano Cycle. Located in the east region of the Sul-rio-grandense Shield, in southernmost Brazil, the granitic bodies were emplaced mainly inside mylonitic zones belonging to a transcurrent system with NE orientation. The structural relationship of this intrusion with shear zones indicates a syn- to late-kinematic emplacement. The Cordilheira Intrusive Suite rocks are mainly syenogranites and monzogranites, with inequigranular texture, sometimes modified by mylonitic deformational effects. Mineralogically they are composed of alkali feldspar, plagioclase, quartz, muscovite and biotite, with minor amounts of gamet, tourmaline, apatite, zircon, sphene and fluorite. The analyzed samples show a SiO2 range of 71.55 to 77.68%, and a reduction in the K2O/Na2O ratio with increasing differentiation. Trace-element contents, including REE, are similar to that of granites produced by partial melting of quartz-feldspathic rocks. Field relationships and geochemical data are suggestive of partial crustal melting along transcurrent shear zones developed during the late orogenic stages of the Brasiliano Cycle. The evidence considered in this paper shows the remarkable similarity of the Cordilheira Intrusive Suite to Hercynian granitoids of Portugal and France.

Detrital Minerals of Modern Beach Sediments in Southern Brazil: A Provenance Study Based on the Chemistry of Zircon

Abstract The trace element content of zircon grains from Holocene beach sands from the Rio Grande do Sul state in southern Brazil was investigated in order to discuss their probable primary source. Zircon grains from 10 samples were separated and analysed by laser ablation inductively coupled plasma mass spectrometer (LA ICP-MS) for rare earth elements (REE), niobium (Nb), uranium (U), thorium (Th), yttrium (Y), hafnium (Hf), and tantalum (Ta) and by microprobe and scanning electron microscopy (SEM) for silicon (Si), zirconium (Zr), and phosphorus (P). The results obtained are similar to those suggested by the zircon classification developed by Belousova et al. (2002, Igneous zircon: trace element composition as an indicator on source rock type. Contributions to Mineralogy and Petrology, 143, 602–622), nevertheless, additional information and a more detailed characterisation of zircon sources were yielded. The probable geotectonic setting and geochemical affinity of zircon igneous sources were predicted mainly based on thorium/uranium (Th/U), yttrium/holmium (Y/Ho), and niobium/tantalum (Nb/Ta) ratios; lanthanum (La) and niobium (Nb) contents; and REE patterns of zircons. The statistical groups identified by multivariate analysis were an additional useful tool for identifying compositional groups of zircons. Zircon grains from the studied sediments are probably derived from an association of subalkaline to alkaline granitic rocks with their mafic counterparts and metamorphic rocks of amphibolite to granulite facies. The metamorphic sources were more important in the southern sector of the studied coastal segment, whilst in the northern sector zircons from A-type granites are more abundant. The so-called Pelotas Batholith, situated in the eastern part of the Sul-rio-grandense Shield, which is composed mainly of a postcollisional granitic association of Neoproterozoic age and Paleo to Neoproterozoic high-grade metamorphic sequences, is proposed as the probable source of the studied zircons. The compositional variations of zircons from the southern to the northern sector are coherent with the hypothesis that the studied sediments came from relatively near continental source areas, probably transported by several small streams and rivers during phases of sea-level low stands. The chemical composition of zircons, particularly trace element contents and ratios, can be a powerful tool for determining the primary sources of detrital zircons.

HIDROTERMALISMO NO COMPLEXO GRANÍTICO LAVRAS E VULCÂNICAS ASSOCIADAS, RS

The gold-sulphide mineralization in the Lavras do Sul region (RS) is associated with propilitization, sericitization, and silicification of host rocks. These hydrothermal alterations are evident in the Lavras Granite Complex: and volcanoclastic rocks. Episyenites and beresites have been identified among the hydrothermal products. Majors and traces elements distribution in granitic rocks have been extensively affected just where the hydrothermal alternation is extremely well-developed, reaching its most advanced stages. Loss on ignition, representing the amount of volatile elements, is the best indicator of hydrothermal alteration intensity, particularly in the propilitized and sericitized materials. In the granitic rocks, Au, Ag, and As are the best pathfinders for lithogeochemical exploration. K 2 O and Na 2 O show sharp decrease near the mineralized zones in the Merita area . REE had low mobility during the hydrothermal event identified in the Lavras Granite Complex: LREE depletion has been noticed only in deeply altered products as episyenites. REE have been determined in sericite, chlorite, and epidote concentrates. Even though subsurface data are not available, it is possible to suggest a porph yry copper model for this mineralization based upon the possibility of vein mineralization presently known, representing hydrothermal activity in the suprastructure of the main deposit.

Geochemistry of the Rio Espinharas hybrid complex, northeastern Brazil

The Rio Espinharas pluton, northeastern Brazil, belongs to the shoshonitic series and consists mainly of syenogranite, quartz–monzonite and porphyritic quartz–monzonite, but diorite, quartz–monzodiorite, quartz–syenite and microsyenogranite also occur containing microgranular enclaves, except for the diorite. Most variation diagrams of rocks, amphiboles, biotites and allanites show linear trends, but K, Zr, Sr and Ba of rocks display curved scattered trends. The rocks ranging from diorite to syenogranite define a pseudo-errorchron and have similar REE patterns. Syenogranite and microsyenogranite are derived from two distinct pulses of granite magma with initial 87Sr/86Sr ratio of 0.7083±0.0003 and 0.7104±0.0007, respectively. Modelling of major and trace elements shows that the syenogranite evolved by fractional crystallization of plagioclase, microcline, edenite, biotite and titanite, whereas quartz–monzonite, porphyritic quartz–monzonite, quartz–monzodiorite and quartz–syenite resulted from simple mixing between an upper mantle-derived dioritic magma and the upper crust-derived syenogranite magma. Dioritic enclaves are globules of a mafic magma from the upper mantle.