Shoshonitic Association Research Articles

Neoproterozoic potassic andesitic and lamprophyric rocks in the tupanci area, southernomost Brazil: Petrology and associated mineralization

Au and sulfide mineralizations occur in the Sul-Rio-Grandense Shield, and important occurrences of these metals are reported and attributed to the Lavras do Sul Shoshonitic Association (LSSA). Several authors point to a significant participation of andesitic and lamprophyric rocks in the formation of these deposits. The Tupanci area has the presence of andesites and lamprophyres, stratigraphically correlated to LSSA volcanic rocks (Hilário Formation), representing the northernmost exposures of this magmatism. The LSSA is one of the most complete Neoproterozoic Shoshonitic associations (ca. 592-590 Ma) in southern Brazil, containing plutonic, volcanic and hypabyssal rocks, related to the post-collisional stage of the Brasiliano/Pan-African orogeny. The andesitic rocks of the Tupanci area occur in the form of the lava flows, overlayed by volcanoclastic deposits, product of the reworking of the volcanic unit. Lamprophyres occur in the form of dykes, with N-NW direction, which are intruded in rocks of the metamorphic basement and in andesitic lava flows, being texturally separated and classified as spessartite and lamprophyric lapilli-tuff. The singular occurrence of the lamprophyric lapilli-tuff stands out, is analogous to Kimberley-type pyroclastic kimberlitic intrusions, depicting explosive manifestations linked to LSSA lamprophyres. The andesites and lamprophyres that ocour in Tupanci area show characteristics of high-K calc-alkaline or shoshonitic rocks, evidenced by LILE and LREE enrichment, negative Ti and Nb anomalies, plus positive Pb anomalies and the absence of Eu anomalies. These attributes reinforce the relationship of andesitic and lamprophyric rocks with related LSSA rocks. The presence of Au and Pt grains in the matrix of lamprophyric lapilli-tuff, highlights the economic importance of these rocks for the mineral occurrences in the region, where future studies can better define the metallogenetic potential for the rocks of the Tupanci area.

Geophysical characterization and modeling of a Neoproterozoic intrusive volcaniclastic lamprophyre in southernmost Brazil

Neoproterozoic spessartitic lamprophyres are linked to the Lavras do Sul Shoshonitic Association in the Sul-Riograndense Shield, southernmost Brazil. Bodies of intrusive spessartitic lamprophyres occur in trachyandesites of this association in the Cerro Tupanci region. There, Neoproterozoic igneous and metamorphic complexes are covered by volcanic-sedimentary Ediacaran sequences of the Camaquã Basin that include trachyandesites, conglomerates, sandstones, and spessartitic lamprophyre dikes of which a spessartitic lapilli tuff N-NE oriented is the main occurrence. Gamma spectrometric and gravimetric data for the lapilli tuff allowed definition of its occurrence as a dike elongated N-S, bearing low values for gamma spectrometry and a strong negative gravimetric anomaly approximately 450 m long and 70 m wide. Two-dimensional modeling of this anomaly suggests that the dike is subvertical to vertical. Magnetometry did not provide significant data regarding the lamprophyre but did show a normal dipolar anomaly to the west, suggesting the presence of a non-outcropping cylindrical intrusion in the area.

New geochronological and geochemical data of the Longsheng mafic-ultramafic suite in northern Guangxi, China, and their implications in Rodinia breakup

The Longsheng mafic-ultramafic suite consists of more than 100 mafic and ultramafic intrusions and are extensively distributed within the Neoproterozoic Danzhou Group along the Jiangnan Orogenic Belt in northern Guangxi, China. The suite has been a hotly debated subject in the context of emplacement age, source of magmas, and tectonic setting for several decades. In this article, we present newly obtained LA-ICP-MS zircon U-Pb age data, whole-rock major and trace elemental data, and Sr-Nd and zircon Lu-Hf isotopic data. These data support a middle Neoproterozoic intrusive emplacement mechanism as a result of asthenospheric underplating and subsequent lithospheric thinning, extension, and rifting. This integrated process might well correspond to the early stage of the Rodinia breakup. For example, five mafic-ultramafic rock samples yielded an emplacement age of 774 ± 2 Ma. The samples contain a large number of inherited zircon xenocrysts dated at 821 ± 3 Ma, 1.4–2.1 Ga, and 2.3–2.8 Ga, indicative of crustal contamination. The mafic-ultramafic rocks are characterized by calc-alkaline to shoshonitic associations; their Zr/Nb, Th/Yb, La/Nb, Ba/La ratios, trace, and REE patterns show an OIB affinity. Their initial 87Sr/86Sr values range from 0.697721 to 0.708579 with an average of 0.702352. They have relatively low initial 143Nd/144Nd values of 0.511530–0.511859 with an average of 0.511661. Sixteen magmatic zircon grains from diabase samples show 176Hf/177Hf values of 0.281326–0.282638, eHf(t) values of − 34.48–11.89, and TDM2 of 925–3819 Ma. Their Nb/U and Th/Ta ratios support crustal contamination as well. It is concluded that the Longsheng mafic-ultramafic suite was originated from partial melting of the enriched upper mantle (EM-I type) due to asthenospheric underplating and lithospheric extension during the incipient Rodinia breakup, and that the magmas were contaminated with crustal material during ascent.

Sr-Nd-Hf isotopic constraints and U-Pb geochronology of the Arroio Pedrado Gneisses, Dom Feliciano Belt, Brazil: A 680 Ma shoshonitic event in the final stages of The Piratini Arc evolution

In this work we present whole-rock and isotopic (Sr-Nd-Hf) chemistry data and zircon U-Pb geochronology from new occurrences of the Arroio Pedrado Gneisses, located near the Brazil/Uruguay border in the southeastern sector of the Dom Feliciano Belt. These monzogranitic orthogneisses are intermediate, metaluminous, with SiO2 concentration ranging from 59.22 to 65.37 wt%, Na2O/K2O ratios < 1, and Ce/Yb and Th/Yb ratios typical of shoshonitic associations. The studied samples are enriched in large ion lithophile elements (LILE) and light rare earth elements (LREE) relative to high field strength elements (HFSE), resulting in a high LILE/HFSE ratios (LaN/LuN ratios between 17 and 33). These features, together with significant negative Ta and Nb anomalies and low Y and Yb concentrations, fit within the characteristics of subduction-related magmatism. The whole-rock isotopic study demonstrates that the samples have intermediate 87Sr/86Sr(i) ratios (0.7095–0.7112), negative εNd(t) values (−3.7 to −4.2) and Nd model ages of 1.48–1.61 Ga. The studied zircons have a wide range of εHf(t) values (−13.44 to −15.16), suggesting a crustal influence in their generation. The geochemistry and isotope characteristics indicate that these shoshonites are derived from the partial melting of a thickened lower crust, which was affected by subduction-related fluids/melts. This magma underwent significant fractional crystallization – K-feldspar, plagioclase, biotite, and amphibole, and accessory minerals such as allanite and Fe-Ti oxides – in oxidative conditions during magma ascent. The LA-ICP/MS U-Pb zircon dating reveals that the Arroio Pedrado Gneisses were emplaced during the Cryogenian, as suggested by a concordia age of 680 ± 2.9 Ma interpreted as the igneous protolith crystallization age. The metamorphic age of 660 ± 2.8 Ma (zircon rims) is interpreted as the amphibolite-facies peak which marks an early collisional event during the convergence of the Río de la Plata Craton and African-related terranes. The Arroio Pedrado Gneisses are here interpreted as representing the latest magmatic event in the construction of the allochtonous Andean-type Piratini Arc, whose evolution is constrained between ca. 780 (initial magmatism) and 650 Ma (continental collision) during the West Gondwana assembly.

Mineral assemblages and temperature associated with Cu enrichment in the Seival area (Neoproterozoic Camaquã Basin of Southern Brazil)

The Neoproterozoic sequence of volcanic rocks in the Camaquã Basin included in the Lavras do Sul Shoshonitic Association hosts disseminated Cu deposits. The volcanic sequence in the Seival Mine area includes andesitic lava flows, lapilli tuff, volcanic agglomerate and andesitic dikes with pervasive alteration. The hydrothermal alteration is interpreted to be a product of late–magmatic fluids or the mixing of magmatic with meteoric fluids or basinal brines. The late–magmatic hydrothermal system started after volatile oversaturation and degassing in a magmatic system, which was partially segregated into vesicles. The similar REE contents in the andesine–labradorite and albite indicate that the fluid temperatures ranged from 600 to 250 °C. The crystallization of titanite and Fe–clinochlore to Mg–chamosite followed the albitization process. The temperature range associated with the chloritization process was estimated using a chlorite geothermometer and varied from 251 ± 56 to 183 ± 39 °C. The precipitation of Mg–saponite and smectite–rich chlorite/smectite mixed–layers in some andesitic lava flows and the precipitation of quartz, calcite, barite and hematite in fractures suggest fluid circulation with temperatures lower than the chloritization process. The alteration minerals are represented by chlorite and albite associated with pyrite–chalcopyrite, while chlorite/smectite mixed–layers and barite or hematite are associated with bornite–chalcocite–covellite. Thermodynamic calculations confirm the potential coprecipitation of pyrite–chalcopyrite with chlorite and albite. The occurrence of bornite–chalcocite–covellite and barite seems to be favored by low–temperature chlorite/smectite with neutral to mildly acidic water influx. Consequently, albitization and the process of chloritization at high temperatures (>ca. 251 ± 56 °C) can be used as an exploration guide for primary pyrite–chalcopyrite enrichment, and chlorite/smectite at low temperatures (ca. 250 to 50 °C) can be related to bornite–chalcocite–covellite or the process of Cu enrichment.

Hydrothermal alteration of volcanic rocks in Seival Mine Cu–mineralization – Camaquã Basin – Brazil (part I): Chloritization process and geochemical dispersion in alteration halos

The Seival Mines are situated in the NE portion of the Lavras do Sul mining district, southernmost Brazil. They are hosted by volcanic and sub-volcanic rocks of Neoproterozoic age, which are part of the post–collisonal Camaquã Basin volcano–sedimentary sequence. The mineralization occurs in shoshonitic volcanic and sub–volcanic rocks of the Lavras do Sul Shoshonitic Association. They are of intermediate composition and exhibit widespread hydrothermal alteration. The mineralization occurs primarily in the form of bornite, chalcocite, covellite, pyrite, and in supergene phases as malachite. Mineral occurrence is always controlled by fractures. Ore is associated with chloritization processes, which produced smectite, chlorite/smectite and corrensite clay minerals and gangue of carbonate, mostly calcite, and barite. In this study field mapping and drill core sampling, petrography with optical microscopy and electron microscopy, X–ray diffraction for clay size fraction characterization and whole–rock geochemistry are used to understand the spatial distribution and relative chronology of hydrothermal alteration products of different lithology in the mineralized zones. The post–magmatic fluid activity and hydrothermal lower–temperature alteration that produced smectite➔chlorite/smectite➔chlorite and corrensite➔carbonate have changed the major, minor, trace and rare earth element (REE) contents. Lavas and sub-volcanic rocks contain Cu–Fe sulfides. In alteration halos, Cu–sulfides with supergene influence is related to circulation of late hydrothermal fluids. The pH variations and sulfide materials are related to dispersion metals around vertical structures.

Geoquímica do magmatismo Ediacarano Serra do Caramuru, NE da Província Borborema, RN, Brasil

Os stocks Serra do Caramuru e Tapuio (magmatismo Serra do Caramuru), localizados no extremo NE do Domínio Rio Piranhas-Seridó (RN), são representantes do plutonismo ediacarano a cambriano da Província Borborema. Análises químicas de rocha total (elementos maiores e traço, incluindo elementos terras raras) em amostras provenientes principalmente do stock Serra do Caramuru permitiram efetuar a caracterização geoquímica deste magmatismo. Foram identificadas as fácies diorítica, granítica porfirítica, granítica equigranular I e granítica equigranular II, além de diques e sheets graníticos. A fácies diorítica é leucocrática a melanocrática, rica em biotita e hornblenda. As fácies graníticas são hololeucocráticas a leucocráticas, com biotita e eventualmente hornblenda. A variação no conteúdo de óxidos e elementos traço sugere o fracionamento de zircão, apatita, titanita, clinopiroxênio (em dioritos), biotita, hornblenda, allanita, plagioclásio, microclina e granada (em diques tardios). O comportamento de Zr, La e Yb indica que dioritos não constituem o magma parental dos granitos. Por outro lado, as fácies graníticas parecem ser cogenéticas, apresentando trends de diferenciação e espectros de elementos terras raras (ETR) análogos [12,3≤(La/Yb)N≤190,8; Eu/Eu*=0,30-0,68]. O padrão de ETR de diques e sheets graníticos [6,96≤(La/Yb)N≤277,84; Eu/Eu*=0,18-0,58] sugere não cogeneticidade ao magmatismo Serra do Caramuru. A fácies diorítica é metaluminosa, shoshonítica, enquanto os granitos são metaluminosos a ligeiramente peraluminosos, cálcio-alcalinos de alto potássio. Diagramas envolvendo óxidos e elementos traço corroboram um ambiente pós-colisional. Os dados aportados permitem correlacionar o magmatismo Serra do Caramuru ao grupo de granitoides pós-colisionais, cálcio-alcalinos de alto potássio com eventuais associações shoshoníticas, do Domínio Rio Piranhas-Seridó.

Os diques latíticos portadores de ouro e sulfetos da Associação Shoshonítica de Lavras do Sul – RS: petrogênese e geoquímica

A Associação Shoshonítica de Lavras do Sul (ASLS), situada no extremo sul do Brasil, tem idade neoproterozóica e hospeda ocorrências importantes de Au-Cu-sulfetos, relacionadas por alguns autores a depósitos do tipo Cu-Au pórfiro. Dados de campo, petrográficos, mineralógicos e geoquímicos permitiram identificar e classificar os diques que ocorrem na ASLS cortando os monzonitos hipabissais, andesitos e lamprófiros, em três tipos principais: diques compostos autoclásticos, diques latíticos e diques latíticos com anfibólio. Sua grande variedade litológica e textural é resultado da diferenciação magmática, segregação mineral e processos de mistura de magmas. Os diques mostram afinidade shoshonítica, com teores de K2O &gt; (Na2O –2), acompanhados de conteúdos elevados de Rb, Ba e Sr e baixos a moderados de Nb, Zr e ETRP, assinatura típica da Associação Shoshonítica de Lavras do Sul. Os dados obtidos permitiram identificar dois trends composicionais: um alto-Ti, enriquecido em Fe2O3t, P2O5, HFS, LILE e ETR, com razões K2O/Na2O &lt; 0,8; outro baixo-Ti, que predomina na ASLS, com menores teores dos elementos anteriormente citados e razão K2O/Na2O &gt; 1,4. Os diques alto-Ti, com menores razões K2O/Na2O indicam uma evolução das fontes e processos no sentido da produção de magmas alcalinos saturados em sílica mais sódicos, como é comum na evolução do magmatismo pós-colisional. A cristalização dos magmas e o progressivo enriquecimento dos voláteis e de complexos de Au-S-Cl geraram uma supersaturação do sistema, causando o segundo ponto de ebulição e a vesiculação de porções dos diques. Admite-se que o magma lamprofírico pode ser a fonte principal do ouro e enxofre do sistema. A mistura dos magmas latíticos ou traquíticos com os lamprofíricos, pode ser responsável por um aumento do potencial de oxidação e dos voláteis no sistema, desencadeando assim a separação da fase volátil, a desestabilização dos complexos e a precipitação dos sulfetos magmáticos portadores de Au, além da própria formação das soluções hidrotermais.

Middle Eocene volcanic shoshonites from western margin of Central-East Iranian Microcontinent (CEIM), a mark of previously subducted CEIM-confining oceanic crust

In western margin of the CEIM (Ashin area), Middle Eocene volcanic shoshonites present very good exposures. This shoshonitic association comprises of all shoshonite group members from basic to acidic. Major minerals of basic members (absarokite and shoshonite) are olivine, clinopyroxene (augite), plagioclase (labradorite), K-feldspar (sanidine and anorthoclase), analcime, calcite, apatite, ilmenite and magnetite. Secondary minerals include chlorite, calcite, epidote and zeolite (mesolite). The Ashin shoshonitic rocks consist of SiO2 undersaturated (absarokite or phonotephrite) to SiO2 oversaturated (toscanite or rhyolite) units. The studied rocks are characterized by wide range of SiO2 (48 to 70 wt %), low content of TiO2 (0.35 to 1.17 wt %), and high values of Alkalis [(Na2O + K2O) = 7.03 to 11.4 wt %]. Other main geochemical characteristics of Ashin shoshonites are potassic to ultra-potassic nature (K2O/Na2O = 1.04 to 5.06), high ratios of LREE/HREE (e.g. La/Yb up to 19.16), and enrichment in LILEs. Primitive mantle normalized spidergram of the studied rocks shows positive anomalies of Cs, U, K, Pb, Hf and negative spikes of Nb, Ta, Sm, Ti and Y. All analyzed samples display markedly negative Nb, Ta, and Ti anomalies, typical features of orogenic magmas. These geochemical signatures point out to the subduction—related nature of Ashin shoshonites and their similarity to potassic volcanic rocks of continental arcs or convergent margins. The parental magma of these shoshonites is an alkalibasalt (absarokite) which produced by low-degree of partial melting of a metasomatized enriched mantle source. Petrographical evidences together with geochemical characteristics (e.g., high values of Pb and U) of the studied rocks conclude crustal contamination of magma during ascending throughout the continental crust. The former subduction of CEIM confining ocean from Triassic to Eocene is too enough for volatile enrichment of the mantle and shoshonitic magmatism in middle Eocene of Ashin area.

Post-collisional polycyclic plutonism from the Zagros hinterland: the Shaivar Dagh plutonic complex, Alborz belt, Iran

AbstractThe petrological and geochronological study of the Cenozoic Shaivar Dagh composite intrusion in the Alborz Mountain belt (NW Iran) reveals important clues to decipher complex relations between magmatic and tectonic processes in the central sectors of the Tethyan (Alpine–Himalayan) orogenic belt. This pluton is formed by intrusion at different times of two main magmatic cycles. The older (Cycle 1) is formed by calc-alkaline silicic rocks, which range in composition from diorites to granodiorites and biotite granites, with abundant mafic microgranular enclaves. The younger cycle (Cycle 2) is formed by K-rich monzodiorite and monzonite of marked shoshonitic affinity. The latter form the larger volumes of the exposed plutonic rocks in the studied complex. Zircon geochronology (laser ablation ICP-MS analyses) gives a concordia age of 30.8 ± 2.1 Ma for the calc-alkaline rocks (Cycle 1) and a range from 23.3 ± 0.5 to 25.1 ± 0.9 Ma for the shoshonitic association (Cycle 2). Major and trace element relations strongly support distinct origins for each magmatic cycle. Rocks of Cycle 1 have all the characteristic features of active continental margins. Shoshonitic rocks (Cycle 2) define two continuous fractionation trends: one departing from a K-rich basaltic composition and the other from an intermediate, K-rich composition. A metasomatized-mantle origin for the two shoshonitic series of Cycle 2 is proposed on the basis of comparisons with experimental data. The origin of the calc-alkaline series is more controversial but it can be attributed to processes in the suprasubduction mantle wedge related to the incorporation of subducted mélanges in the form of silicic cold plumes. A time sequence can be established for the processes responsible of the generation of the two magmatic cycles: first a calc-alkaline cycle typical of active continental margins, and second a K-rich cycle formed by monzonites and monzodiorites. This sequence precludes the younger potassic magmas as precursors of the older calc-alkaline series. By contrast, the older calc-alkaline magmas may represent the metasomatic agents that modified the mantle wedge during the last stages of subduction and cooked a fertile mantle region for late potassic magmatism after continental collision.

A-TYPE GRANITIC ROCKS IN POST-COLLISIONAL SETTINGS IN SOUTHERNMOST BRAZIL: THEIR CLASSIFICATION AND RELATIONSHIP WITH TECTONICS AND MAGMATIC SERIES

A-type granitoids and rhyolites are here considered as those that fulfill at least one of the following four criteria: (i) genetic association with magmatic rocks of sodic or ultrapotassic alkaline affinity. These in general include syenitic rocks, peralkaline rocks, and trachybasalts, basaltic trachyandesites or their plutonic equivalents; (ii) (Na 2 O + K 2 O) and FeO T /(FeO T + MgO) whole-rock values respectively higher than 9 wt% and 0.9; (iii) peralkaline compositions; (iv) (10 4 *Ga)/Al > 2.6, and Ce + Y + Nb + Zr > 340 ppm, plotting in the within-plate field in the (Nb + Y) versus Rb diagram. A-type granitic rocks are widespread in the Neoproterozoic post-collisional settings in southern Brazil. They are positioned either within large shear-zones or in thick volcanosedimentary sequences formed in strike-slip basins. They predominate in the late to final stages of post-collisional magmatism, and follow medium- to high-K subalkaline and shoshonitic associations. The intensity of transcurrent tectonics has an important role in promoting crustal contamination of magmas intruded where shear zones are active. The main effects of crustal contamination observed are increased magmatic oxidation, depletion of HFS elements, and reduction of agpaitic indices. The post-collisional rhyolitic or granitic A-type magmas from southern Brazil are coeval with mafic magmas and, in some cases, with trachytes or syenites of the same chemical affinity, i.e. , most of them belong to the sodic silica-saturated alkaline series and to the ultrapotassic silica-saturated series. Trace-element and isotope data suggest that the plutonic and volcanic A-type rocks from southern Brazil have a variable contribution of crustal components, and in some cases none at all, and have a very strong contribution of OIB–EM1 sources. On the basis of such evidence, we propose that the post-collisional A-type magmas in southern Brazil are derived from basic or intermediate mantle-derived magmas, mostly by fractional crystallization processes, in some cases with contamination by crustal melts, particularly where transcurrent tectonics were active.

THE TRACE-ELEMENT RECORD IN ZIRCON FROM THE LAVRAS DO SUL SHOSHONITIC ASSOCIATION, SOUTHERNMOST BRAZIL

Zircon concentrates from samples of a granodiorite, rhyolite, quartz monzonite, monzonite, and diorite from the Lavras do Sul Shoshonitic Association (LSSA) were analyzed by LA–ICP–MS for trace elements including the REE after petrographic and mineralogical studies. These rocks are representative of a Neoproterozoic comagmatic suite associated with the postcollisional setting and strike-slip basins in southernmost Brazil. Trace-element contents in zircon grains were divided by the whole-rock contents and the calculated zircon/rock values (D) were compared with mineral/melt partition coefficients (Kd). In spite of the very large range for trace-element contents, even in the same grain, the average values are close to those expected from Kd mentioned by several authors. The REE show typical chondrite-normalized patterns, with strong enrichment of HREE and small positive Ce anomalies. The trace-element compositions of zircon from LSSA reflects the magma composition, and hence provide a useful tool to determine provenance of detrital zircon and in petrogenetic studies.

Petrologia e sucessão estratigráfica das rochas monzoníticas da associação shoshonítica de Lavras do Sul (RS)

The monzonitic plutonic and hypabissal rocks from the Lavras do Sul region, southernmost Brazil, are discussed in this paper, mainly with basis on field relationship, and major, trace-element and isotope geochemistry of rocks. The monzonitic rocks of Lavras do Sul Shoshonitic Association (LSSA) occur as epizonal intrusions (Tapera Monzonite and Arroio do Jacques Monzodiorite) and hypabissal bodies, whose distribution and form are related to caldera subsidence. Hypabissal Monzonites and coeval dikes are resurgent intrusions with NW-SE orientation. Their mafic phases are magnesium hornblende, augite-diopside and biotite. Geochemical data suggest that the monzonitic rocks are cogenetic and belong to the LSSA and, were derived from less differentiated magmas through fractional crystallization. The obtained age (587 ± 4 Ma by U-Pb in zircon, SHRIMP) for the Hipabissal Monzonites, which is related to the late stages of shoshonitic magmatism in this area, define a range of at least 16 Ma for this magmatism.

Avaliação de fontes magmáticas de séries shoshoníticas pós-colisionais com base na normalização pela Associação Shoshonítica de Lavras do Sul - aplicação de sliding normalization

The Lavras do Sul Shoshonitic Association is related to the early stages of post-collisional alkaline magmatism in the Brasiliano Orogenic Cycle in southern Brazil. Their magmas were produced through melting of a litospheric mantle metasomatized during a previous subduction. The LSSA is composed of co-genetic rocks derived from magmas evolved by fractional crystallization which show a large silica range. These features are quite adequate for interpretations based on the sliding normalization technique, which allows comparing magmas with different degrees of differentiation in order to evaluate and discuss their potential sources, petrogenetic mechanisms and geotectonic settings. The LSSA composition was used as the pattern for normalizing data available in bibliography for the Late Precambrian post-collisional Tismana pluton, the 1.8 Ga Svecofennian post-collisional shoshonitic magmatism in the Fennoscandian shield, the monzonitic series from the Variscan Tormes Dome, the post-collisional potassic and ultrapotassic magmatism in northern Tibet, and the Baranadag pluton in Central Anatolia, Turkey. This procedure allowed the identification of two distinct geochemical trends: (i) Zr-Nb-Ti-Y-REE enrichment, and (ii) increasing of Rb-K2O with concomitant Nb decrease. The former probably reflect the participation of an astenospheric component, whilst the second trend is related to crustal assimilation. The Nb-Rb-K2O diagram, used for values normalized by this technique, is proposed for discriminating post-collisional shoshonitic associations with different contributions of astenospheric and crustal materials.

Avaliação de fontes magmáticas de séries shoshoníticas pós-colisionais com base na normalização pela Associação Shoshonítica de Lavras do Sul - aplicação de sliding normalization

The Lavras do Sul Shoshonitic Association is related to the early stages of post-collisional alkaline magmatism in the Brasiliano Orogenic Cycle in southern Brazil. Their magmas were produced through melting of a litospheric mantle metasomatized during a previous subduction. The LSSA is composed of co-genetic rocks derived from magmas evolved by fractional crystallization which show a large silica range. These features are quite adequate for interpretations based on the sliding normalization technique, which allows comparing magmas with different degrees of differentiation in order to evaluate and discuss their potential sources, petrogenetic mechanisms and geotectonic settings. The LSSA composition was used as the pattern for normalizing data available in bibliography for the Late Precambrian post-collisional Tismana pluton, the 1.8 Ga Svecofennian post-collisional shoshonitic magmatism in the Fennoscandian shield, the monzonitic series from the Variscan Tormes Dome, the post-collisional potassic and ultrapotassic magmatism in northern Tibet, and the Baranadag pluton in Central Anatolia, Turkey. This procedure allowed the identification of two distinct geochemical trends: (i) Zr-Nb-Ti-Y-REE enrichment, and (ii) increasing of Rb-K2O with concomitant Nb decrease. The former probably reflect the participation of an astenospheric component, whilst the second trend is related to crustal assimilation. The Nb-Rb-K2O diagram, used for values normalized by this technique, is proposed for discriminating post-collisional shoshonitic associations with different contributions of astenospheric and crustal materials.

Geochemistry and zircon geochronology of paleoproterozoic granitoids: Further evidence on the magmatic and crustal evolution of the São Luís cratonic fragment, Brazil

The Tromaí Intrusive Suite is the predominant exposed unit of the São Luís cratonic fragment in northern Brazil. The suite forms batholiths and stocks of granitoids that were emplaced between 2168 ± 4 Ma and 2149 ± 4 Ma and intruded a 2240 ± 5 Ma old metavolcano-sedimentary sequence. The batholiths are composed of a variety of petrographic types that have been grouped in three sub-units, based on the predominant petrographic type, and named Cavala Tonalite, Bom Jesus Granodiorite, and Areal Granite, from the more primitive to the more evolved phases, in addition to subordinate shallow felsic intrusions. The Tromaí Suite is an expanded magmatic association comprising minor mafic rocks to predominantly intermediate and felsic, low- to high-K, and metaluminous to weakly peraluminous granitoids that follow a Na-enriched calc-alkaline trend. Combined rock association, geochronology, Nd isotopes, and geochemical signature indicate that the Tromaí Suite formed from magmas derived from juvenile protoliths modified by fractional crystallization. The juvenile protoliths included ocean plate, mantle wedge, and minor sediments. The data also indicate an intra-oceanic arc setting that possibly transitioned to a continental margin and that the Tromaí Intrusive Suite records the main accretionary stage of the Rhyacian orogen (ca. 2.24–2.15 Ma) that culminated with a collision stage at about 2.1 Ga and gave rise to the present day São Luís cratonic fragment. This time interval is coincident with the main period of crustal growth in the South American Platform and in the Paleoproterozoic terranes of the West African Craton. The beginning of this period is also coincident with the end of a period in which only minor amounts of juvenile crust is found worldwide. The Negra Velha Granite is a distinct unit that forms a few stocks that intruded the granitoids of the Tromaí Suite between 2076 and 2056 Ma ago. Negra Velha is an association of monzogranite and subordinate quartz–monzonite and syenogranite with an alkaline signature that shows high Rb–Sr–Ba enrichments, resembling shoshonitic associations. This granite represents the post-orogenic phase of the Rhyacian orogenesis.

1.8 Ga magmatism in the Fennoscandian Shield; lateral variations in subcontinental mantle enrichment

The ca. 1.8 Ga post-collisional magmatism in the southern part of the 2.1–1.86 Ga Svecofennian domain of the Fennoscandian Shield have been studied with particular reference to the character and sources of the mafic rocks in a traverse from the Archaean craton margin in east, across the juvenile Svecofennian domain to its western margin. For this purpose three key areas were selected in the eastern, central and western parts of the domain: (i) in the western part of the domain, the Tjällmo-Vättern zone (southern Sweden) of the Transscandinavian Igneous Belt (TIB) consists of extensive areas of dominantly alkali-calcic granitoids associated with calc-alkaline to tholeiitic mafic rocks. Initial ɛ Nd for the mafic rocks vary from around 0 to above +3. (ii) In the central part of the domain (SW Finland), the post-collisional rocks are represented by small intrusions consisting of calc-alkaline high-Ba–Sr granites associated with shoshonitic lamprophyres and their plutonic equivalents. Initial ɛ Nd for the rocks in this series vary between 0 and +1. (iii) In the easternmost part of the Svecofennian domain (Russian Karelia and SE Finland) shoshonitic associations occur, comprising lamprophyres and their plutonic equivalents (apatite and magnetite-rich monzodiorites), which are related to syenites and high-Ba–Sr granites by fractional crystallization. All the rock types in this shoshonitic association have strongly elevated contents of P 2O 5, LREE and LILE. Initial ɛ Nd for all rocks in Karelia fall between 0 and −1. Geochemical and isotopic results indicate that the post-collisional rocks in the central and eastern part of the domain stem from lithospheric mantle sources that were enriched during the preceding Svecofennian orogeny. The HFSE depletion, combined with the strong Sr, LILE and LREE enrichment, recall signatures of increasingly carbonate-dominated metasomatism of the mantle eastwards towards the craton margin. In contrast, the mainly LILE enriched mafic rocks from TIB in the west signal sources subjected to H 2O-dominated metasomatism, that could in part be coeval with the magmatism. In all areas the rocks carry a subduction type chemistry with continental arc affinity, however, with strongly increasing enrichment levels eastwards. Rocks in the west are derived by relatively larger degrees of melting at shallower levels, from previously depleted spinel–phlogopite–amphibole lherzolites/harzburgites, while going eastwards successively smaller melt fractions were tapping deeper, more enriched mantle sections in the garnet stability field. The enrichment agents are interpreted to be LILE-bearing H 2O-dominated fluids from dewatering slabs in the west, changing to an increasing role for CO 2-dominated fluids/melts derived mainly from subducted Svecofennian metasediments eastwards. A convergent continental margin setting with transpressional shearing was active during TIB formation in the west. Whether this shearing was instrumental in the formation of the 1.8 Ga magmatism further continentwards, or if the magmatism in the central and eastern areas was the result of extensional collapse or plume activity is presently not known.

Neogene and Quaternary volcanism in Western Anatolia: Magma sources and geodynamic evolution

The Western Anatolia Miocene-to-Present Day magmatism evolved from calc-alkaline and shoshonitic rocks (21–16 Ma) to lamproites (16–14 Ma), and eventually into OIB-type magmas (2–0 Ma) represented by the Kula volcanics. In the calc-alkaline and shoshonitic association, Sr and Nd isotopic ratios and trace element variations suggest that the interaction with the crust was moderate, so that the geochemistry of these rocks is considered to reflect the heterogeneous chemical nature of their mantle source. The ultrapotassic and lamproitic rocks are characterised by a high Sr and low Nd isotopic composition and are strongly enriched in K and Rb with respect to Ba, indicating a phlogopite-bearing lithospheric source. Low Sr and high Nd isotopic compositions, together with low LILE/HFSE ratios, reveal the OIB-type nature of the Kula volcanics. Therefore, the products switch from supra-subduction orogenic suites to volcanics coming from sub-slab astenospheric mantle. The evolution is interpreted as being due to a ‘horizontal’ stretching of the slab (no slab pull break-off) generated by different velocities in the subduction hangingwall lithosphere. This triggered the extensional movement between Greece and Turkey and the stretching into two slabs of the NE-directed African subduction, due to the faster southwestward slab rollback of Africa underneath Greece relative to the slab segment below Cyprus and Anatolia.

ASPECTOS PETROGRÁFICOS E COMPOSICIONAIS DO SISTEMA MULTI-INTRUSIVO DA ASSOCIAÇÃO SHOSHONÍTICA LAVRAS DO SUL (RS) E SEU POTENCIAL PARA MINERALIZAÇÕES DE OURO E SULFETOS

The investigation of hypabissal bodies of the Lavras do Sul Shoshonitic Association in southernmost Brazil, led to the identification of several lithotypes in complex structural pattern, generated in a multi-intrusive system interpreted as the margin of a collapsed caldera. Monzonites related to the evolution of shoshonitic magmas are predominant in the intrusive system. Later magma pulses originated latite dikes and composite rocks formed by mingling of latitic and trachytic liquids. The youngest intrusion is are represented by an originally high-K dacite dike that reached volatile over-saturation, which led to the formation of clastic types enriched in pumices and shards. In the composite and dacite dikes the high volatile activity is evidencee by vesiculation and late-magmatic sulphidation. Petrographic, microprobe and electron microscope studies allowed to the identification of pyrite, chalcopyrite, arsenopyrite. free gold particles, native silver, tetrahedrite, rutile, barite, and sphalerite. Gold and silver are included in a groundmass rich in Si +4 , Al +3 , K + e CI - , which suggest that metals were probably transported as chlorine complexes. Arsenopyrite geothermometry indicates crystallization around 350°C to 420°C. The studied monzonites have shoshonitic affinity, as indicated by K 2 O &gt; (Na 2 O-2), high Rb, Ba, and Sr contents, and low to moderate values of Nb, Zr, and HREE. REE patterns are similar to typical shoshonitic rocks. The dacite dike composition was probably modified by the volatile phase, which promoted the LILE depletion, except for Ba, which was retained by barite precipitation. The studied system is compared to porphyry copper-gold systems related to caldera generated by downsagging, and it is concluded that monzonitic metaluminous hypabissal intrusions, of shoshonitic affinity, in volatile-rich systems of magmatic recharge can represent potential targets for porphyry gold-copper mineralizations.

ASPECTOS PETROGRÁFICOS E COMPOSICIONAIS DO SISTEMA MULTI-INTRUSIVO DA ASSOCIAÇÃO SHOSHONÍTICA LAVRAS DO SUL (RS) E SEU POTENCIAL PARA MINERALIZAÇÕES DE OURO E SULFETOS

The investigation of hypabissal bodies of the Lavras do Sul Shoshonitic Association in southernmost Brazil, led to the identification of several lithotypes in complex structural pattern, generated in a multi-intrusive system interpreted as the margin of a collapsed caldera. Monzonites related to the evolution of shoshonitic magmas are predominant in the intrusive system. Later magma pulses originated latite dikes and composite rocks formed by mingling of latitic and trachytic liquids. The youngest intrusion is are represented by an originally high-K dacite dike that reached volatile over-saturation, which led to the formation of clastic types enriched in pumices and shards . In the composite and dacite dikes the high volatile activity is evidencee by vesiculation and late-magmatic sulphidation. Petrographic, microprobe and electron microscope studies allowed to the identification of pyrite, chalcopyrite, arsenopyrite, free gold particles, native silver, tetrahedrite, rutile, barite, and sphalerite. Gold and silver are included in a groundmass rich in Si +4 , Al +3 , K + e Cl - , which suggest that metals were probably transported as chlorine complexes. Arsenopyrite geothermometry indicates crystallization around 350 0 C to 420 0 C. The studied monzonites have shoshonitic affinity, as indicated by K 2 O > (Na 2 O –2), high Rb, Ba, and Sr contents, and low to moderate values of Nb, Zr, and HREE. REE patterns are similar to typical shoshonitic rocks. The dacite dike composition was probably modified by the volatile phase, which promoted the LILE depletion, except for Ba, which was retained by barite precipitation. The studied system is compared to porphyry copper-gold systems related to caldera generated by downsagging, and it is concluded that monzonitic metaluminous hypabissal intrusions, of shoshonitic affinity, in volatile-rich systems of magmatic recharge can represent potential targets for porphyry gold-copper mineralizations.