The Lavras do Sul Shoshonitic Association: implications for the origin and evolution of Neoproterozoic shoshonitic magmatism in southernmost Brazil

et al. 2006

An. Acad. Bras. Ciênc.

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 volcanosedimentary 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.

Section: The Shoshonitic Magmatismmentioning

confidence: 92%

Section: The Camaquã and Campo Alegre Basinsmentioning

confidence: 99%

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

Sommer

Lima

et al. 2006

An. Acad. Bras. Ciênc.

“…650 to 570 Ma. Plutonic and volcanic acid to basic rocks of shoshonitic affinity are widespread in the 610-590 Ma age interval (Lima and Nardi 1998a), and are followed by (i) voluminous sodic, silica-saturated alkaline magmatism, the Saibro Intrusive Suite (Nardi and Bonin 1991), mostly composed of metaluminous granites, with minor peralkaline components, and (ii) large volcanic plateaus where acid lavas and pyroclastics are dominant, with minor intermediate and basic components (Sommer et al 1999, Wildner et al 1999.…”

Section: Geological Settingmentioning

confidence: 99%

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

Plá-Cid

Bitencourt

et al. 2008

An. Acad. Bras. Ciênc.

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 K 2 O/Na 2 O 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.

“…Granitic rocks are mainly porphyritic to heterogranular, with hypidiomorphic textures, and contain diopside, amphibole and biotite as the main mafic minerals. The Lavras do Sul Shoshonitic Association (Lima & Nardi 1998), Estaleiro Granitic Complex and Monte Bonito granite (Philipp 1998) are typical examples of post-collisional shoshonitic magmatism in Southern Brazil.…”

Section: Figure 2 -Spiderdgrams Of Different Post-collisional Granitimentioning

confidence: 99%

Tectonic Setting and Sources of Magmatism Related to the Southern Brazilian Shear Belt

Bitencourt

2000

RBG

114

The Neoproterozoic-Cambrian Brasiliano/Pan-African orogenic cycle is represented, in southern Brazil, by arc and collisional settings, with very expressive granitic magmatism. The post collisional stage, 650-580 Ma, is recorded by extensive magmatism along major transcurrent shear zones that compose the Southern Brazilian Shear Belt. The early post-collisional magmatism is high-K, calc alkaline, found along gently dipping shear zones; biotite-bearing granites and granodiorites are the most common rock types, often containing mafic microgranular enclaves. This is immediately followed by peraluminous two-mica leucogranites, emplaced either along flat lying or steeply dipping shear zones. The next magmatic manifestation has a shoshonitic affinity and is found either within the shear belt or far from deformation sites, piercing through cratonic areas. Shoshonitic rocks range from acid to basic, either as extrusive or plutonic terms. The latest magmatic event is a silica-saturated alkaline magmatism, occurring within or outside the shear belt. It is controlled by steeply dipping transcurrent structures, or discordantly intrusive in the previous types. Metaluminous biotite granites are widespread, with a coeval basic magmatism of mildly alkaline affinity. The shear belt acted as a conduit for magmas and also promoted decompression of shallower mantle levels, reactivating the sources of previous, calc-alkaline, pre-collisional magmatism. The role of previously modified mantle wedge in the generation of southern Brazilian postcollisional magmatism is here emphasized over the contribution of crustal melting processes.