Bühn & Trumbull,2003

Comparison of petrogenetic signatures between mantle-derived alkali silicate intrusives with and without associated carbonatite, Namibia  

B. Bühn1,2
R.B. Trumbull3

Lithos, 2003, 66: 201-221

Keywords: Liquid immiscibility; Carbonatite; Ijolite; Nepheline foidite; Nepheline syenite; Namibia

1 Institut für Geowissenschaften und Lithosphärenforschung der Universität, Senckenbergstrasse 3, D-35390 Giessen, Germany 
2 Instituto de Geociências, Universidade de Brasília, 70910-900 Brasília DF, Brazil, e-mail: berbu@unb.br 
3 GeoForschungsZentrum Potsdam, Telegrafenberg, D-14473 Potsdam, Germany 

ABSTRACT

We compare the petrogenetic and chemical signatures of two alkali silicate suites from the Cretaceous Damaraland igneous province (Namibia), one with and one without associated carbonatite, in order to explore their differences in terms of magma source and evolution. The Etaneno complex occurs in close spatial proximity to the Kalkfeld bimodal carbonatite-alkali silicate complex, and is dominated by nepheline (ne)-monzosyenites and ne-bearing alkali feldspar syenites. The Etaneno samples have isotopic compositions of 87Sr/86Sr(i)=0.70462-0.70508 and Nd=-0.5 to -1.5, with the highest 87Sr/86Sr(i) and lowest Nd values observed in evolved samples. The magma differentiated via olivine, feldspar, clinopyroxene, and nepheline (ne) fractionation in a F-rich system, which fractionated Zr from Hf, and Y from Ho. Partly glassy, recrystallized inclusions in some samples are less evolved than their host rocks and contain a cumulate component (nepheline, plagioclase). The Kalkfeld ne-foidites (ijolites) and ne-syenites have 87Sr/86Sr(i)=0.70285-0.70592 and Nd=0.5 to 1.1. The isotope ratios show no consistent variation with rock composition, and they are in the same range as the associated carbonatites. The Kalkfeld silicate magma fractionated nepheline and alkali-feldspar in a CO2-dominated, F- and Ca-poor system. As a result, the rocks display some major and trace element trends distinctly different from those of the Etaneno samples.
We suggest that the Etaneno and the Kalkfeld magmas represent different melt fractions of a heterogeneous mantle source, resulting in different compositions especially with respect to CO2 contents of the primitive, parental magmas. In this scenario, the carbonated alkali silicate Kalkfeld parental melt contained a critical CO2 concentration and underwent liquid separation of carbonate and silicate melt fractions at crustal depths. The resulting silicate melt fraction experienced a very different mode of differentiation than the carbonate-poor Etaneno parental magma. Thus, the Kalkfeld rocks are depleted in Ca and other divalent cations, as well as F, rare-earth elements (REE), Ba, and P relative to the Etaneno syenites. We interpret these differences to reflect the partitioning of these elements into the carbonate melt fraction during immiscible separation. 

Bühn et al., 2003

Fluid/rock interactions and element fractionation during progressive migration of carbonatitic fluids, derived from small-scale trace element and Sr,Pb isotopic distribution in fluorite

B. Bühn
J. Schneider
P. Dulski
A.H. Rankin

Geochimica et Cosmochimica Acta(2003),67:4577-4595.

Fortes et.al.,2003

Sm–Nd studies at Mina III gold deposit, Crixás greenstone belt, central Brazil: implications for the depositional age of the upper metasedimentary rocks and associated Au mineralization

Abstract

The Crixás greenstone belt, central Brazil, is a volcanosedimentary sequence comprised of ultramafic and mafic metavolcanic rocks at the base and detrital and chemical metasedimentary rocks at the top. The sequence is part of the Archean sialic block exposed in the central part of the Neoproterozoic Brasília belt and therefore is strongly affected by the Brasiliano orogeny. Important gold deposits, such as Mina III, are mainly associated with carbonaceous and Fe- and Al-rich metasedimentary rocks of the upper unit.

Sm–Nd isochron data for the basal metakomatiites and metabasalts indicate an age of 3.00±0.07 Ga. However, Nd isotopes for the upper metasedimentary rocks show T_DM model ages of mostly 2.33–2.49 Ga, which indicates that neither they nor the associated Au mineralization are Archean. Also, garnet whole-rock dating for a Au-bearing chlorite–garnet schist yields an age of 505±7 Ma, suggesting that the mineralization is Neoproterozoic. The Nd model ages suggest that the metasedimentary sequence of Crixás represents a thrust sheet of Proterozoic rocks emplaced over the Archean terrains during the Neoproterozoic Brasiliano orogeny.

Resumo

O greenstone belt de Crixás, Brasil Central, é uma seqüência vulcano-sedimentar constituída, na base, por rochas metavulcânicas ultramáficas e máficas e, no topo, por rochas metassedimentares detríticas e químicas. A seqüência é parte do bloco siálico arqueano exposto na região central da Faixa Brasília, de idade Neoproterozóica, e foi, portanto, fortemente retrabalhada pela orogenia Brasiliana. Importantes depósitos auríferos, como a Mina III, estão associados a rochas metassedimentares carbonosas e ricas em Fe e Al da unidade de topo.

Dados Sm–Nd de metakomatiitos e metabasaltos da unidade basal indicaram idade de 3.00±0.07 Ga. Entretanto, as idades modelo T_DM de rochas metassedimentares do topo situam-se predominantemente entre 2.33 e 2.49 Ga, demonstrando que as mesmas, assim como a mineralização, não são arqueanas. Adicionalmente, uma isócrona Sm–Nd construída para granada e rocha total de clorita-granada xisto, hospedeiro da mineralização, forneceu idade de 505±7 Ma, sugerindo idade neoproterozóica para a mineralização. As idades modelo das rochas metassedimentares de topo sugerem que as mesmas sejam parte de uma ou mais lâminas de cavalgamento formadas por rochas supracrustais Proterozóicas posicionadas tectonicamente sobre os terrenos arqueanos, provavelmente durante a orogenia Brasiliana.

Palavras chave;Brasil central; Crixás; Idades modelo; Isótopos de Nd; Ouro

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Hollanda et.al.,2003

Paleoproterozoic subduction-related metasomatic signatures in the lithospheric mantle beneath NE Brazil: inferences from trace element and Sr–Nd–Pb isotopic compositions of Neoproterozoic high-K igneous rocks

Maria Helena Bezerra Maia de Hollanda
Márcio Martins Pimentel
Emanuel Ferraz Jardim de Sá

 Journal of South American Earth Sciences (2003), 15(8): 885-900.
(Ver texto integral)

Keywords: Mafic-intermediate rocks; Subduction-modified lithospheric mantle source; Transamazonian tectonics

ABSTRACT

Late Neoproterozoic (ca. 580 Ma), high-K, mafic-intermediate rocks represent voluminous bimodal magmatism in the Borborema Province, northeast Brazil. These rocks show the following chemical signatures that reflect derivation from a subduction-modified lithospheric mantle source: (1) enrichment in large ion lithophile elements (Rb, Ba, K, Th) and light rare-earth elements (REE) (La/Yb_CN=11–70), (2) pronounced negative Nb anomalies, and (3) radiogenic Sr (0.71202–0.7059) and unradiogenic Nd (_Nd from -9.3–-20.1) isotopic compositions. T_DM model ages suggest that modification of the lithospheric mantle source (metasomatised garnet lherzolite) may have occurred in the Paleoproterozoic during the Transamazonian/Eburnean tectonics that affected the region. Interaction with asthenospheric fluids is believed to have partially melted this enriched source in the Neoproterozoic, probably as a result of asthenosphere-derived fluid percolation in the Brasiliano/Pan-African shear zones that controlled the emplacement of these mafic-intermediate magmas. The involvement of this asthenospheric component is supported by the nonradiogenic Pb isotopic ratios (²⁰⁶Pb/²⁰⁴Pb=16–17.3, ²⁰⁷Pb/²⁰⁴Pb=15.1–15.6, ²⁰⁸Pb/²⁰⁴Pb=36–37.5), which contrast with the enriched Sr and Nd compositions and thereby suggest the decoupling of Rb–Sr, Sm–Nd, and U–Pb systems at the time of intrusion of the mafic-intermediate magmas in the crust.

Marques et.al.,2003

Re–Os and Sm–Nd Isotope and Trace Element Constraints on the Origin of the Chromite Deposit of the Ipueira–Medrado Sill, Bahia, Brazil

J. C. MARQUES
C. F. FERREIRA FILHO
R. W. CARLSON
M. M. PIMENTEL

Journal of Petrology ( 2003), 44(4): 659-678

Key words: mafic–ultramafic sill; isotopes and trace elements; chromite deposit

ABSTRACT

The chromite deposit of the Paleoproterozoic Ipueira–Medrado sill is hosted in a single, thick (5–8 m), massive layer, which sets severe constraints for the origin of chromitites. It is divided from bottom to top into: (1) a Marginal Zone (5–20 m); (2) an Ultramafic Zone (<250 m) consisting of dunite and harzburgite that host the chromitite layer, in which intercumulus amphibole is important and more abundant toward the top; (3) a Mafic Zone (<40 m). The parental magma was large ion lithophile element and light rare earth element enriched and high field strength element depleted. Sm–Nd isotopic compositions are consistent with a 2 Ga age, but suggest a variable initial Nd isotopic composition that correlates with the abundance of amphibole. The more negative Nd (mean –6·5) of the amphibole-rich intervals argues for crustal contamination, although the Nd (mean –4·4) of the amphibole-free samples suggests an old, enriched, subcontinental lithospheric mantle source. Chromite separates have initial Os values that range from –4·6 to +3. The negative Os values are typical of old, Re-depleted, lithospheric peridotitic mantle and give Re-depletion model ages of up to 2·75 Ga. An integrated assessment suggests that the very high-Mg parental magma probably originated from Archean, subcontinental, metasomatized, peridotitic lithospheric mantle and was subsequently contaminated with up to 30% of crust, which triggered the chromitite crystallization.

Geochemistry and Sm–Nd isotopic characteristics of bimodal volcanic rocks of Juscelândia, Goiás, Brazil: Mesoproterozoic transition from continental rift to ocean basin

Renato Moraes^a
Reinhardt A. Fuck^b
Márcio Martins Pimentel^b
Simone Maria Costa Lima Gioia^b
Ana Maria Graciano Figueiredo^c

Precambrian Research (August 2003), 125(3-4):317-336
Full Text + Links

Keywords: Bimodal volcanism; Juscelândia Sequence; Brazil; Continental rift-ocean basin transition; Mesoproterozoic; Sm–Nd isotope geochemistry

^a Departamento de Geologia, Instituto de Geociências, Universidade Federal do Rio de Janeiro, Ilha do Fundão, Rio de Janeiro, RJ 21949-900, Brazil
^b Instituto de Geociências, Universidade de Brasília, Brasília, DF 70910-900, Brazil
^c IPEN-CNEN/SP, Supervisão de Radioquímica, Caixa Postal 11049, Pinheiros 05422-970, São Paulo, SP, Brazil

Abstract

The ca. 1.28 Ga Juscelândia Sequence, in Goiás, Brazil, comprises a unit of bimodal metavolcanic rocks, amphibolite and felsic gneiss, intercalated with metapelite and metachert. The bimodal nature of the magmatism is evident as the metavolcanic rocks have basalt and rhyolite compositions. Bimodality is also shown by the trace elements and Nd isotope data. Amphibolites yield positive _Nd(T) values that can be separated into two groups. In the first group, amphibolite samples from lower and middle portion of the sequence are characterized by _Nd between +2.8 and +5.4. These fairly positive values and enrichment in LREE and LILE suggest they were derived from a depleted mantle source with subsequent assimilation of continental crustal material. The second group is formed by amphibolite of the upper portion of the sequence that have _Nd around +6.0 and their low LILE and LREE contents resemble normal mid-ocean ridge basalts (N-MORB). Initial ¹⁴³Nd/¹⁴⁴Nd ratios represent the composition of the depleted mantle at 1.28 Ga. Felsic rocks yield negative _Nd(T) values, between ca. −2.8 and −4.5, and Paleoproterozoic model ages (T_DM values in the interval between 1.8 and 2.1 Ga). The original felsic magmas represent the product of melting of older (Paleoproterozoic) continental crust with trace element chemistry indicating that the felsic magmas were the product of continental crustal melts with small mantle contribution. Previous models proposing an oceanic setting for the origin of magmatism of the Juscelândia rocks, within a mid-ocean ridge or back-arc basin are not supported by the data presented here. Transition from a continental rift to an ocean basin represents the most likely tectonic setting for the origin of the Juscelândia rocks.

Oliveira & Santos,2003

Isotopic domains controlled by transtensional and transpressional sectors in the auriferous Diadema shear belt, northern Brazil

Claudinei Gouveia de Oliveira
Roberto Ventura Santos

Keywords: Carbon isotopes; Gold mineralization; Oxygen isotopes; Shear zone; Structural domains

Abstract

This paper presents carbon and oxygen isotope data from the auriferous Diadema shear belt, Carajás mineral province, northern Brazil. This area was affected by large-scale deformation accompanied by metamorphism and carbonate alteration of the metavolcanic rocks. Carbonate carbon-isotope data (¹³C_PDB=−2.2 to −3.6‰, n=11) reveal that the hydrothermally altered zone within the shear zone has a different isotopic composition than that of the country rock (+0.4 to −1.7‰, n=12). Similarly, initial strontium isotope compositions reveal different isotopic compositions between the country rock of the shear zone (⁸⁷Sr/⁸⁶Sr=0.70454–0.70642) and the transpressure and transtensional zones (⁸⁷Sr/⁸⁶Sr=0.71286–0.71841), which indicates that the alteration zone resulted from large-scale fluid flow derived from an external reservoir. In addition to these differences, there are small variations in the ¹³C_PDB values (from −2.8 to −2.0‰) in the shear zone, which may be related to preferential carbonate decomposition (and CO₂ devolatilization) in the transpressional domains. Evidence of carbonate decomposition is supported by calc-silicate rocks, which derived from the reaction between calcite and silicate minerals.

The hydrothermal alteration and gold mineralization at the Diadema shear belt comprised both fluid infiltration from an external source and contemporaneous fluid generation in the shear zone. Compared with other gold deposits related to shear zones, the Diadema gold deposit developed at hypozonal crustal levels in the transition between greenschist and amphibolite metamorphic facies mineralogical assemblages.

SHRIMP AND CONVENTIONAL U-PB AGE, SM-ND ISOTOPIC CHARACTERISTICS AND TECTONIC SIGNIFICANCE OF THE K-RICH ITAPURANGA SUITE IN GOIÁS, CENTRAL BRAZIL

The Gameleira deposit is one of several important Cu–Au deposits associated with the late Archean (ca. 2.7 Ga) volcanic rocks of the Itacaiúnas supergroup in the Carajás mineral province, southeastern Pará. It comprises mainly biotite- and sulphide-rich veins and quartz–grunerite–biotite–gold hydrothermal veins that cut andesitic rocks. It is interpreted as representative of the Fe oxide Cu–Au class of deposit.
Sm–Nd isotopic data indicate an age of 2719±80 Ma (MSWD=3.0) and _Nd(T) of -1.4 for the host meta-andesites. Metavolcanic rocks and cogenetic gabbros give an age of 2757±81 Ma (1) with _Nd(T) of -0.8. This is considered the best estimate for the crystallization age of the Gameleira volcanic and subvolcanic rocks. Negative _Nd(T) and Archean T_DM model ages (mostly between 2.8 and 3.1 Ga) suggest some contamination with older crustal material. The andesitic/gabbroic rocks are cut by two generations of granite dykes. The older has striking petrographic and geochemical similarities to the ca. 1.87 Ga alkali-rich Pojuca granite, which is exposed a few kilometers to the northwest of the deposit. The younger is a leucogranite with a U–Pb SHRIMP age of 1583+9/-7 Ma. Neodymium isotopic analyses of the two generations of granites indicate a strong crustal affinity and possible derivation from reworking of the Archean crust.
The quartz–grunerite–gold hydrothermal vein yields a Sm–Nd isochron (MSWD=.83) age of 1839±15 Ma (1) with _Nd(T) of -9.2. Pervasive potassic alteration, represented by the widespread formation of biotite in the country rocks, is dated by Ar–Ar at 1734±8 Ma, and a similar age of 1700±31 Ma (1) is indicated by the Sm–Nd isochron for the biotite–sulphide veins. Similar to that for the quartz–grunerite vein, the _Nd(T) value for the sulphide-rich veins is strongly negative (-8.2), thereby suggesting that the original fluids percolated through, leached, or were derived from igneous rocks with an Archaean Nd isotopic signature. The geochronological data suggest that the Gameleira Cu–Au mineralization is related to a Paleoproterozoic (ca. 1.83 Ga) episode of hydrothermal activity and is not Archaean. The younger ages of ca. 1.70–1.73 might be interpreted as products of the lower blocking temperatures of biotite in relation to the Ar–Ar and Sm–Nd isotopic systems. Combined with previous geochemistry and stable isotope data, the Nd isotopic data suggest that the mineralizing fluids were derived from, or strongly interacted with, a Paleoproterozoic crustal granite, possibly similar in age and composition to the Pojuca granite.

RESUMO

Palavras chave: Carajás; Cu-Au; Gameleira; Geocronologia

O depósito Gameleira é um dos importantes depósitos de Cu–Au associados com rochas vulcânicas de ca. 2.7 Ga do Supergrupo Itacaiúnas, na Província Mineral de Carajás, sudeste do Pará. É constituído dominantemente de veios de biotita e sulfetos e veios hidrotermais de quartzo–grunerita–biotita–Au que cortam rochas andesíticas do Grupo Salobo–Pojuca. O depósito é interpretado como representativo da classe de depósitos de óxido de Fe–Cu–Au. Dados isotópicos Sm–Nd indicam a idade de 2719±80 Ma (MSWD=3.0) e _Nd(T) of -1.4 para os meta-andesitos encaixantes. Quando plotados juntos, as rochas metavulcânicas e gabbros associados indicam a idade de 2757±81 Ma (1) e _Nd(T) of -0.8. Essa idade é interpretada como representativa da época de cristalização das rochas ígneas encaixantes do depósito. Valores negativos de _Nd(T) e idades T_DM arqueanas (predominantemente entre 2.8 e 3.1 Ga) sugerem contaminação com material crustal mais antigo. As rochas andesíticas/gabróicas são cortadas por duas gerações de diques graníticos: o mais antigo apresenta marcante similaridade petrográfica e geoquímica com o granito Pojuca com idade de ca. 1.87 Ga, exposto a noroeste do depósito, e o mais jovem é um leucogranito com idade U–Pb SHRIMP de 1583+9/-7 Ma. Composições isotópicas de Nd revelam o caráter crustal dos granitos e sugerem que sejam derivados da refusão de crosta continental arqueana.
Amostras do veio hidrotermal de quartzo–grunerita–Au produziram uma isócrona Sm–Nd bem definida (MSWD=0.83) com a idade de 1839±15 Ma (1) with _Nd(T) of -9.2. A alteração hidrotermal potássica, representada pela formação de biotita nas rochas encaixantes, foi datada pelo método Ar–Ar em 1734±8 Ma, e uma idade similar, de 1700±31 Ma (1)(_Nd(T) of -8.2), é indicada pela isócrona Sm–Nd para amostras dos veios de biotita e sulfeto. Os valores fortemente negativos de _Nd(T) para os veios hidrotermais sugerem que os fluidos originais percolaram/lixiviaram ou, alternativamente, foram inteiramente derivados de rochas ígneas com assinatura isotópica arqueana.
Os dados geocronológicos sugerem que a mineralização de Gameleira está relacionada com evento(s) hidrotermal(is) há aproximadamente 1.83 Ga, e não com alteração hidrotermal arquena. As idades mais jovens, de aproximadamente 1.70–1.73 Ga podem ser atribuídas às mais baixas temperaturas de bloqueio da biotita em relação aos sistemas isotópicos Ar–Ar e Sm–Nd. Combinados com dados prévios de geoquímica e isótopos estáveis, os dados isotópicos de Nd são compatíveis com derivação (ou forte interação) dos fluidos mineralizantes com granitos crustais paleoproterozóicos do tipo Pojuca.

Neoproterozoic granulite facies metamorphism and coeval granitic magmatism in the Brasilia Belt, Central Brazil: regional implications of new SHRIMP U–Pb and Sm–Nd data

Abstract

New SHRIMP U–Pb zircon ages combined with Sm–Nd isotopic characteristics of granulites and associated granitic rocks of the Anápolis–Itauçu Complex in the central-southern part of the Brasília Belt are presented and discussed in this study. Igneous crystallization ages obtained in zircon grains of orthogranulite and granites vary between 760 and 650 Ma. Growth of new zircon at ca. 650–640 Ma dates the high-grade metamorphism. Zircon cores from paragranulites and granites give U–Pb ages between 2.0 and 0.8 Ga.
T_DM model ages of the granulitic rocks fall into two age intervals: 2.3–1.9 and 1.7–1.4 Ga. _Nd(T) values are negative, varying between −9.29 and −1.42. SHRIMP U–Pb ages of zircon cores indicate that the granulite sedimentary protoliths were deposited after 800 Ma ago, contradicting previous models which assigned Paleoproterozoic or Archean ages for these rocks. Their Nd isotopic signature indicate that Paleoproterozoic sources (within the São Francisco Craton?), as well as younger sources, such as the Goiás Magmatic Arc, contributed to the sediment infilling of the former basin.
The intrusive granites display Nd model age pattern similar to that of the granulitic rocks, with most T_DM model ages ranging between 1.45 and 1.2 Ga. _Nd(T) values are between −2.61 and −7.96, indicative of assimilation of older material by the original magma. SHRIMP U–Pb data for the granite intrusions indicate that granitoids metamorphosed under amphibolite facies show strong inheritance (between 2.1 and 0.8 Ga) and magmatic(?)/metamorphic age at ca. 660–650 Ma. The syn-granulite facies granitoid ANA 1 gave magmatic and metamorphic ages of ca. 650 Ma, without any discernible inheritance. The growth of new zircon occurred at the same time of the high-grade metamorphism, probably related with the collision event between the Goiás Magmatic Arc to the west–southwest and the São Francisco Craton, to the east.
The data suggest that the Anápolis–Itauçu Complex, instead of representing the ancient sialic basement of the supracrustal rock units of the Brasília Belt, constitutes a Neoproterozoic high-grade metamorphic complex related with the evolution of the Brasiliano orogen.

Piuzana et.al.,2003

SHRIMP U–Pb and Sm–Nd data for the Araxá Group and associated magmatic rocks: constraints for the age of sedimentation and geodynamic context of the southern Brasília Belt, central Brazil

Abstract

SHRIMP U–Pb and Sm–Nd isotopic data for Araxá Group micaschist, associated amphibolite and intrusive tonalite were used to estimate the age and tectonic setting of deposition of the original detrital sediments, in the southern part of the Neoproterozoic Brasília Fold Belt. One amphibolite sample from a layer interleaved tectonically within the metasedimentary rocks has the U–Pb zircon age of 838±20 Ma. Its tholeiitic composition and depleted mantle isotopic signature (_Nd=+5.1) suggests that it represents a tectonic slice of the former oceanic lithosphere separating the São Francisco and Amazon continents. U–Pb ages of detrital zircon grains and T_DM model ages of the Araxá Group micaschists show a bimodal pattern, interpreted as the result of provenance from two contrasting sources: (i) young juvenile sources, probably represented by igneous rocks of the Goiás Magmatic Arc in the west and (ii) Paleoproterozoic sialic sources, most probably belonging to the São Francisco Craton, in the east. The detrital population is largely dominated by ca. 666–682 Ma old zircon grains derived from both mafic and felsic igneous source rocks. Detrital metamorphic grains and rims dated at ca. 715 and 643 Ma indicate that the original basin received sediments from Neoproterozoic sources with a previous metamorphic overprint, implying that young metamorphic rocks have been uplifted, exposed and eroded, providing sediment for at least part of what is presently mapped as the Araxá Group. The youngest detrital zircon age of ca. 643 Ma represents the upper limit for deposition of the original detrital sediments. Their minimum age is constrained by the crystallisation of an intrusive tonalite at 638±11 Ma. The data suggest, therefore, that the Araxá Group metasediments were deposited and metamorphosed within a very short time interval, most probably during the tectonic events related with the accretion of the Goiás Magmatic Arc to the eastern part of the Brasília Belt and final closure of the Goiás Ocean.

Santana et.al.,2003

 Estudio de inclusiones fluidas en el stockwork del sector La Union. Provincia de Camagüey, Cuba.

Abstract:

Study of fluid inclusions in La Unión stockwork. Province of Camagüey. Cuba. La Unión is located in the center-oriental region of Cuba, municipality of Najasa, in Camagüey province. Alteration rocks are represented by zoned quartz-sericite with quartz veins, related to intrusive bodies and locally La Unión stockwork. The porphyry copper (Cu-Au) stockwork mineralization was defined by studies of ground geochemistry, occurring a close correlation between gold and copper contents. Petrography studies based on phases number, nature and proportions at room temperature and preliminary microthermometric studies of the mineralized stockwork's quartz veins fluid inclusions (IF) allowed us to identify three types of fluid inclusions, type 1 (V), type 2 (V+L), type 3 (V+L+Ss). The association of vapor rich IF (type 2) and high-salinity inclusion(type 3 THL) suggests the relationship with an immiscible magmatic fluid (L+V+H), hosted in a subvolcanic environment, related to granitic rocks (quartz diorite porphyries), where the primary fluid boiled in two phases, one low density aqueous and another of high salinity (30 to 40 % wt eq. NaCl) at a temperature around 300 - 400 C, typical condition of fluids associated to porphyry type mineralization. .

Trumbull et.al.,2003

The petrology of basanite-tephrite intrusions in the Erongo Complex and implications for a plume origin of Crateaceous alkaline complexes in Namibia 

R.B. Trumbull1
B. Bühn2,*
R.L. Romer1
F. Volker2 

1 GeoForschungsZentrum Potsdam, Telegrafenberg, D-14473 Potsdam, Germany 
2 Institut für Geowissenschaften und Lithosphärenforschung der Universität, Senckenbergstrasse 3, D-35390 Giessen, Germany 
* Present address: Bernhard Bühn, Instituto de Geociências, Universidade de Brasília, 70910-900 Brasília DF, Brazil, e-mail: berbu@unb.br 

Journal of Petrology, 2003, 44: 93-112
Keywords: Sr–Nd–Pb isotopes; Namibia; plume; basanite; petrogenesis

ABSTRACT

Basanite intrusions from the Early Cretaceous Erongo complex, Namibia, have compositions consistent with near-primary mantle melts derived from a depth of at least 100 km. These rocks provide a key reference for the mantle component(s) involved in breakup-related magmatism in this region. Initial Sr–Nd–Pb isotope ratios of the Erongo basanites and associated tephrites and phonotephrites (87Sr/86Sr = 0•70425–0•70465; Nd = +1•8 to +2•7; 206Pb/204Pb = 18•63–18•91) are independent of the degree of differentiation and correspond closely to an estimated range for the Tristan plume at 130 Ma. Incompatible trace element ratios also overlap with ratios of ocean island basalt (OIB) from the South Atlantic islands of Tristan da Cunha, Gough and Inaccessible associated with the modern Tristan hotspot. The Tristan plume signature of Erongo basanite–tephrite intrusions is shared by at least six other Early Cretaceous mafic alkaline complexes in Namibia, whereas the associated flood basalts in general lack a plume signature. We attribute the contrast in mantle sources for the flood basalts and alkaline complexes to their relative timing with respect to lithospheric thinning. Thick lithosphere during the main flood basalt event prevented direct melting of the Tristan plume and magmas were generated mostly from the lithosphere. The alkaline complexes intruded later, when the lithosphere was sufficiently thinned to allow decompression melting of the underlying plume mantle.