Roberto Dall'Agnol

Petrology of the anorogenic, oxidised Jamon and Musa granites, Amazonian Craton: implications for the genesis of Proterozoic A-type granites

The 1.88 Ga Jamon and Musa granites are magnetite-bearing anorogenic, A-type granites of Paleoproterozoic age. They intrude the Archaean rocks of the Rio Maria Granite–Greenstone Terrain in the eastern part of the Amazonian Craton in northern Brazil. A suite of biotite±amphibole monzogranite to syenogranite, with associated dacite porphyry (DP) and granite porphyry (GP) dykes, dominates in these subalkaline granites that vary from metaluminous to peraluminous and show high FeO/(FeO+MgO) and K2O/Na2O. In spite of their broad geochemical similarities, the Jamon and Musa granites show some significant differences in their REE patterns and in the behaviour of Y. The Jamon granites are related by fractional crystallisation of plagioclase, potassium feldspar, quartz, biotite, magnetite±amphibole±apatite±ilmenite. Geochemical modelling and Nd isotopic data indicate that the Archaean granodiorites, trondhjemites and tonalites of the Rio Maria region are not the source of the Jamon Granite and associated dyke magmas. Archaean quartz diorites, differentiated from the mantle at least 1000 m.y. before the emplacement of the granites, have a composition adequate to generate DP and the hornblende–biotite monzogranite magmas by different degrees of partial melting. A larger extent of amphibole fractionation during the evolution of the Musa pluton can explain some of the observed differences between it and the Jamon pluton. The studied granites crystallised at relatively high fO2 and are anorogenic magnetite-series granites. In this aspect, as well as concerning geochemical characteristics, they display many affinities with the Proterozoic A-type granites of south-western United States. The Jamon and Musa granites differ from the anorthosite–mangerite–charnockite–rapakivi granite suites of north-eastern Canada and from the reduced rapakivi granites of the Fennoscandian Shield in several aspects, probably because of different magmatic sources.

Geology, geochemistry, and Pb–Pb zircon geochronology of the Paleoproterozoic magmatism of Vila Riozinho, Tapajós Gold Province, Amazonian craton, Brazil

Abstract Pb–Pb zircon geochronologic data define two different periods of intense igneous activity in the Vila Riozinho region, Tapajos Gold Province (TGP), south-central Amazonian craton. At ∼2.00–1.97 Ga, the Vila Riozinho volcanic sequence (2000±4 Ma, 1998±3 Ma) and the older Sao Jorge granite (1981±2 Ma, 1983±8 Ma) were formed. At ∼1.89–1.87 Ga, the younger Sao Jorge (1891±3 Ma), Jardim do Ouro (1880±3 Ma), and Maloquinha (1880±9 Ma) granites and the Moraes Almeida volcanic sequence (1890±6 Ma, 1881±4 Ma, 1875±4 Ma) were emplaced. Similar age intervals are registered throughout the TGP that is thus a little younger than the Paleoproterozoic terranes of the Maroni-Itacaiunas Province in the Guiana Shield. Geochemical and geochronologic data demonstrate that the Sao Jorge pluton is composed of two different granitoids, the older and younger Sao Jorge granites. These, as well as the Jardim do Ouro granite, are I-type and magnetite series. The Sao Jorge granites are high-K calc-alkaline and show significant geochemical variation, the Jardim do Ouro granite is more iron-rich and less oxidized. The Vila Riozinho Formation is calc-alkaline to shoshonitic and geochemically similar to the Sao Jorge granites. The Maloquinha granite and the Moraes Almeida Formation differ from the other studied rocks in petrographic and geochemical characteristics, have aluminous A-type affinities, and were probably derived by low-temperature crustal melting. The volcanic sequences of the studied area, formerly included into the Iriri Group of the Uatuma Supergroup, are divided into the Vila Riozinho and Moraes Almeida sequences. The presence of these two sequences in the same tectonic domain demonstrates the heterogeneity of the Uatuma Supergroup. The TGP registers accretionary processes related to the formation of the Atlantica supercontinent at ∼2.00 Ga. This was followed (at ∼1.88 Ga) by an intracontinental taphrogenic event that lasted throughout the Mesoproterozoic. The tectonic setting of the TGP was thus transitional between a subduction-related magmatic arc and a stable continental block undergoing extension.

Geology and Pb-Pb Geochronology of Paleoproterozoic Volcanic and Granitic Rocks of Pitinga Province, Amazonian Craton, Northern Brazil

Pitinga Province is one of the main tin provinces of the Amazonian craton. The oldest unit in the studied area is the Iricoumé Group, which consists of rhyolites and rhyodacites with a 207Pb/206Pb zircon age of 1888 ± 3 Ma. This volcanic sequence is intruded by five A-type granite plutons. The studied portion of the Europa pluton is homogeneous, and composed of a peralkaline alkali-amphibole hypersolvus granite that yielded a 207Pb/206Pb zircon age of 1829 ± 1 Ma. The early facies of the Madeira pluton consists of a metaluminous amphibole-biotite syenogranite (rapakivi facies) with a 207Pb/206Pb zircon age of 1824 ± 2 Ma. It is intruded by a 1822 ± 1 Ma, mildly peraluminous biotite syenogranite. The later facies of this pluton consist of a porphyritic, hypersolvus, alkali-feldspar granite and an albite granite. Field relationships and an extensive drilling survey indicate that these two facies are sheet-shaped and were emplaced almost simultaneously. The hypersolvus alkali-feldspar granite has a 207Pb/206Pb zircon age of 1818 ± 2 Ma. Taking in account its field relationships with the albite granite, a similar age is assumed for the latter. The albite granite intrudes the biotite granite and rapakivi granite facies of the Madeira pluton, which was emplaced by shallow-level cauldron subsidence. The albite granite is sheet shaped and consists of a magmatic peralkaline cryolite-bearing core facies partially surrounded by an autometa-somatic peraluminous fluorile-bearing border facies. Both albite granite facies are strongly tin-mineralized and display anomalous contents of Nb, Rb, Zr, and REE. A massive body of cryolite and pegmatitic rocks is associated with the albite granite. The contrast in age between the Iricoume Group and the Europa + Madeira granites demonstrates that the plutons are not subvolcanic intrusions related to the extrusives. The ages of 1824 ± 2 Ma, 1822 ± 2 Ma, and 1818 ± 2 Ma obtained, respectively, for the amphibole + biotite syenogranite, biotite granite, and porphyritic hypersolvus granite of the Madeira pluton are consistent with the emplacement sequence inferred for these facies. These ages indicate that the Madeira pluton was emplaced in a relatively short time. Its facies are a little younger than the peralkaline granite of the Europa pluton, suggesting that the latter is not coeval with the Madeira peralkaline albite granite.

Nd, Pb and Sr isotopes in the Identidade Belt, an Archaean greenstone belt of the Rio Maria region (Carajás Province, Brazil): implications for the Archaean geodynamic evolution of the Amazonian Craton

The Rio Maria granite–greenstone terrain (RMGGT), located in the Amazonian craton at about 250 km south of the Carajas Ridge, is a well preserved Mesoarchaean domain composed of metavolcanic rocks and several suites of granitoids ranging in age from ca. 2.96 to 2.87 Ga. The metavolcanics comprise a basal unit of komatiites and low-K tholeiites overlain by sodic metadacites. A Sm–Nd whole-rock reference isochron for the metabasalts, metagabbros (tholeiites) and metadacites gives an age of 3046±32 Ma (MSWD of 1.6, 1 σ ). Nd model ages ( T DM ) of the metadacites range between 3.04 and 3.11 Ga. The e Nd calculated at 3.0 Ga varies from +1.70 to +2.60 for the tholeiites and +0.30 to +3.22 for the metadacites. A Pb–Pb whole-rock linear array for the metabasalts yielded an anomalous older age of 3.41 Ga. The Sm–Nd age is coincident, within analytical error, with a Pb–Pb whole-rock errorchron age of 2944±88 Ma (1 σ ) obtained for the metadacites. Rb–Sr systematic for the metadacites yielded an isochron age that is ca. 400 Ma youngest. The isotopic, geochemical and geological data suggest that oceanic lithosphere was the probable source of the ultramafic and mafic rocks in such a way that partial melting of the depleted mantle generated komatiites and low-K tholeiites. Partial melting of oceanic crust transformed into garnet amphibolite or eclogite generated the parental magmas of the metadacites. Isotopic and trace element characteristics of both metavolcanic rocks and associated plutonics suggest intra-oceanic island arcs as the most probable tectonic setting of the RMGGT. Two main events of crustal accretion are recognised at about 3.04–2.96 Ga and 2.87 Ga. Geological correlation indicates that the oceanic island arcs were completely amalgamated at around 2.76 Ga and acted as sources for the overlying sediments of the Rio Fresco Group.

Archean crustal sources for Paleoproterozoic tin-mineralized granites in the Carajas Province, SSE Para ´, Brazil: Pb /Pb geochronology and Nd isotope geochemistry

Whole-rock and K-feldspar Pb, single zircon Pb-evaporation, and Nd whole-rock isotopic data are presented for granites of the Velho Guilherme intrusive suite and the volcanic rocks of the UatumaGroup in the Carajas Mineral Province, southern Para ´, Brazil. Pb � /Pb zircon ages of 18679 /4 Ma, 18629 /16 Ma, and 18669 /3 Ma for the Antonio Vicente, Mocambo, and Rio Xingu granite massifs, respectively, show that the granites were emplaced at � /1870 Ma. Pb � /Pb whole-rock reference age of the Uatumavolcanic rocks, 18759 /158 Ma, is comparable within the limits of the error. 1870 Ma thus represents the time of an important magmatic event in the eastern part of the Amazonian craton. Nd model ages of the granites (3.0 � /3.2 Ga) and volcanic rocks (2.9 � /3.1 Ga) and their strongly negative initial oNd values indicate that the magmas were derived from Mesoarchean rocks with a long crustal residence time. A mantle contribution cannot be completely ruled out, however. # 2002 Elsevier Science B.V. All rights reserved.

Tin-Bearing Sodic Episyenites Associated with the Proterozoic, A-Type Água Boa Granite, Pitinga Mine, Amazonian Craton, Brazil

This paper reports the first occurrence of tin-mineralized episyenites in the Amazonian craton. The Agua Boa and Madeira plutons in the Pitinga region are stanniferous Proterozoic A-type granites of the rapakivi series featuring metasomatic episyenitization. The biotite granite facies of the Agua Boa pluton is metasomatically altered to sodic episyenite and minor potassic episyenite and micaceous episyenite. The sodic episyenites formed by: (a) albitization of K-feldspar, (b) vug formation by dissolution of magmatic quartz, (c) vug filling by albite, chlorite, lithian muscovite, cassiterite ± fluorite ± K-feldspar and (d) deposition of late quartz±cassiterite in remaining cavities. In the potassic episyenites, vugs produced by quartz dissolution are filled by secondary K-feldspar, which also replaces magmatic phases. Micaceous episyenites formed by zinnwaldite replacement of secondary albite in the sodic episyenites. In contrast to the sodic episyenites, the potassic and micaceous episyenites contain only traces of cassiterite. Relative to unaltered biotite granite, the sodic episyenites are strongly depleted in SiO2, K2O, LREE and Sr, moderately depleted in HREE, and enriched in Na2O, Al2O3, Rb and Sn. The potassic episyenites resemble the sodic episyenite in SiO2 and Al2O3but exhibit higher values of K2O, Ba, Y and Rb; their Na2O, REE, and Sn concentrations match those of the biotite granite host. Metasomatic episyenitization is attributed to: (1) a high temperature gradient and fluid-rock ratio during subsolidus cooling of the Agua Boa pluton, (2) quartz dissolution in the biotite granite facies by reaction with an high-temperature, silica-undersaturated fluid of probable magmatic origin, (3) hydrothermal cavity filling by a low-temperature, silica- saturated fluid of meteoric, or mixed magmatic/meteoric character. Significant tin values in metasomatic sodic episyenites near the Pitinga mine suggest that exploration of similar rocks in other A-type granites may be worthwhile.

Evolution of Brasiliano-age granitoid types in a shear-zone environment, Umarizal-Caraubas region, Rio Grande do Norte, northeast Brazil

A sequence of Brasiliano-age granitoid types is exposed in a small area near the cities of Umarizal and Caraubas in Rio Grande do Norte State, Northeast Brazil. Porphyritic K-alkali-calcic monzogranite is an important facies of the oldest Caraubas intrusion (RbSr whole rock isochron age of ca. 630 Ma), which suffered solid-state deformation due to movements on a major NE-trending shear zone. The intrusion of the Prado and part of the Quixaba bodies was probably controlled by the shear zone. These two bodies include mafic/intermediate rocks, some of which contain two pyroxenes, and have hybrid, partly alkaline and partly shoshonitic geochemical characteristics. Rock types and ages are similar to those of some Pan-African occurrences in southwestern Nigeria. The Tourao body, intruded at ca. 590 Ma, presents preferred mineral orientations which are probably largely magmatic, since little evidence is found for widespread solid-state deformation. On the other hand, its intrusion may have been facilitated by the presence of the shear-zone faults. The rocks form a monomodal felsic K-alkali-calcic suite. With the exception of the Quixaba body, all these earlier granitoids are magmatic epidote- and magnetite-bearing porphyritic monzogranites with trace element geochemical characteristics of modern syn-collisional granites. The latest intrusion at ca. 545 Ma is mainly represented by potassic quartz syenites and related rocks, some of which contain fayalite or ferrohypersthene. These rocks possess neither well developed mineral orientations of magmatic origin nor signs of solid-state deformation. They are mineralogically similar to, but younger than some of the “bauchites” of central Nigeria. Geochemical signatures are comparable with those of modern within-plate granites. All granitoids present high (87Sr/86Sr)i ratios which range from 0.708 to 0.712, and increase with decreasing age. Such ratios are compatible with important or dominant crustal contributions. On the other hand, the more mafic rocks are likely to have formed from enriched mantle.

The Umarizal Igneous Association, Borborema Province, NE Brazil: Implications for the Genesis of A-Type Granites

Abstract The Umarizal igneous association (NE Brazil) consists of three A-type plutons, intruded over an interval of about 15 Ma, during the post-collisional phase of the Brasiliano orogeny. All the plutons have Sr isotopic compositions which indicate important or dominant crustal contributions. Crystallization of the Umarizal sill, formed of fayalite quartz syenite and syenogranite, and of the Lagoa stocks, formed by mangerite and granite, commenced under water-undersaturated conditions at pressures around 700-800 MPa and temperatures around 900°C, and continued during magma rise with the crystallization of hornblende at about 480-570 MPa. Crystallization of the Acao stock, which contains a rapakivi-like facies, commenced at similar temperatures, and hornblende appeared at slightly less than 800°C and around 500 MPa. Different f O2 conditions controlled the compositions of the ferromagnesian phases and the nature of the oxide mineral assemblage. Simple fractional crystallization models from homogeneous parent magmas are insufficient to explain the chemical variation of the rocks suites.

Petrography, magnetic susceptibility and geochemistry of the Rio Branco Granite, Carajás Province, southeast of Pará, Brazil

The Paleoproterozoic Rio Branco stock intrudes the RESUMO: Petrografia, suscetibilidade magnetica e geoquimica do Archean Cruzadao biotite monzogranite. It occurs west of the Canaa dos Carajas city, close to the Sossego copper mine in the Carajas Prov- ince. It is constituted by undeformed and isotropic, hololeucocratic syenogranites, showing equigranular texture. The granite is formed essentially by perthitic alkali feldspar, quartz, and plagioclase, with additional variable amounts of chloritized biotite. Fluorite, allanite, and zircon are common accessory minerals, whereas pyrite and chal- copyrite are scarces. Albitization and subordinate greisenization are the main alteration processes that affected the granite. The secondary mineralogy is represented by albite, fluorite, topaz, chlorite, musco- vite, siderophyllite, and iron oxides. Magnetic susceptibility values are systematically low and vary between 1.3 x 10 -5 a 6.9 x 10 -4 (SI). The Rio Branco Granite is metaluminous to peraluminous and shows high FeOt/(FeOt + MgO). It has geochemical affinities with ferroan, reduced A2-subtype granites. The rare earth element (REE) patterns are flat with little heavy REE fractionation and show accentuated negative Eu anomalies (Eu/Eu* = 0.08 – 0.13). These characteristics are typical of evolved granites, derived from volatile-enriched liq- uids responsible by the intense subsolidus hydrothermal alteration. The dating of the Rio Branco granite did not give conclusive results, but the obtained data, associated with geological evidence, suggest that it is of Paleoproterozoic age. The comparison between the Rio Bran- co granite and Paleoproterozoic A-type suites of the Carajas Province suggests that the Rio Branco granite has more affinity with the Velho Guilherme suite and, in lesser degree, with the Serra dos Carajas suite. On the other hand, is clearly different from oxidized A-type granites of the Jamon suite. Despite its similarities with tin-specialized gran- ites, the Rio Branco Granite is not associated with tin mineralizations.

Variação morfológica e composicional de zircão e suas implicações metalogenéticas: o exemplo das suítes Jamon, serra dos Carajás e Velho Guilherme, Cráton Amazônico

Zircon from granites of the Jamon suite (JS), Serra dos Carajas suite (SCS) and Velho Guilherme suite (VGS) were studied by scanning electron microscope (SEM) through backscattered electron and cathodoluminescence images and energy dispersive spectroscopy (EDS) analyses. Granites and greisens of the VGS have predominantly anhedral zircons that are altered and intensely corroded, are enriched in Hf and have the lowest Zr/Hf ratios of the studied suites. In the granites, these ratios decrease towards the more evolved varieties. Zircons of the JS are euhedral to subhedral, zoned and slightly altered; they are also comparatively depleted in Hf and display the highest Zr/Hf ratios, indicating limited potential for tin-associated mineralization. Zircons from granites of the SCS are subhedral to anhedral, altered and corroded, and show Hf contents and Zr/Hf ratios intermediate to those of the JS and VGS. The granites of the VGS with Sn-, Ta- and W-associated mineralization contain zircons with Zr/Hf ratios varying from 7 to 22. It is concluded that ratios of similar magnitude can be used as a prospecting guide for specialized granites. Zircons from the greisens associated with the Cigano granite of the SCS have average Zr/Hf ratios of approximately 23, but no cassiterite was found in these rocks, indicating that the zircons preserved their magmatic geochemical signature. This study distinguished the three granitic suites in terms of zircon composition and demonstrated the importance of their geochemical signature, especially in terms of their Zr/Hf ratio, in the identification of specialized granites. EDS-SEM analysis can thus be used in a preliminary assessment of the metallogenic potential of tin granites.