Magmatic Arc Environment Research Articles

Genesis of Luobuzhen Pb–Zn veins: Implications for porphyry Cu systems and exploration targeting at Luobuzhen-Dongshibu in western Gangdese belt, southern Tibet

Porphyry systems are known to form in magmatic arc environment and commonly include porphyry Cu, epithermal Pb–Zn–Au–Ag, skarn polymetallic mineralization, etc. The systems are rarely reported in collisional zones, such as the Gangdese belt in southern Tibet where many postcollisional porphyry copper deposits occurred. In addition, other types of mineral systems are rarely present except porphyry copper mineralization in the Gangdese belt. In this study, we present Pb–Zn-bearing quartz veins at Luobuzhen in the western Gangdese belt. The Luobuzhen Pb–Zn veins cross-cut dacite of the Linzizong Group with zircon U–Pb age of 50.1±0.2Ma and monzogranite with zircon U–Pb age of 17.1±0.1Ma. Ore minerals include sphalerite, galena, chalcopyrite, and pyrite; gangue minerals are quartz with minor chlorite and sericite. Primary fluid inclusions of quartz are liquid-rich, aqueous, and two-phase inclusions. The homogenization temperatures of these primary inclusions are moderate to high (267–400°C), and salinities range from 8.9 to 18.4wt.% NaCl equiv. Quartz has δ18OSMOW values of 6.2–9.3‰, while sulfides have δ34SV-CDT values of −5.1‰ to 0.1‰, 206Pb/204Pb of 18.722–18.849, 207Pb/204Pb of 15.640–15.785, and 208Pb/204Pb of 39.068–39.560. These data suggest that magmatic fluids with contribution from meteoric water, magmatic sulfur, and lead derived from upper crust and metasomatized mantle by Indian continental materials would be critical for the Luobuzhen base metal mineralization.The Dongshibu area, located at ∼2km east of the Luobuzhen, is characterized by high concentrations of Cu (up to 1450ppm) and Mo (up to 130ppm) of stream sediments, which is quite different from high concentrations in Pb, Zn, Ag, and Au shown in the Luobuzhen area. In addition, porphyry copper mineralization-related alteration and veins/veinlets occur in the Miocene monzogranite at Dongshibu. The monzogranite is characterized by high Sr/Y ratios, which are also shown on ore-forming intrusions in the Gangdese postcollisional porphyry copper deposits, and shows similar zircon Hf isotopes to the ore-related high Sr/Y intrusions from the Zhunuo porphyry copper deposit which is located ∼20km northeast of the Luobuzhen-Dongshibu. A comprehensive analysis allows us to infer that the base metal veins at Luobuzhen are components of a porphyry Cu system with porphyry Cu mineralization likely present at Dongshibu and epithermal Au–Ag veins possibly occurring at Luobuzhen, which are indicative of the existence of porphyry copper systems in collisional zones. The potential porphyry Cu mineralization and epithermal Au–Ag veins should be targeted in future exploration at Luobuzhen-Dongshibu.

Petrogenesis of metamorphosed Paleoproterozoic, arc-related tonalites, granodiorites and coeval basic to intermediate rocks from southernmost Brazil, based on elemental and isotope geochemistry

In southern Brazil, three associations of metamorphosed tonalites and granodiorites that are compositionally similar to tonalite–trondhjemite–granodiorite (TTG) or adakitic associations have been identified in the Arroio dos Ratos Complex (ARC) Paleoproterozoic magmatism. The metatonalites of Association 1 (A1; 2148±33Ma) have a well-developed fabric, compatible with strong solid-state deformation. The metatonalites and metagranodiorites of Association 2 (A2; 2150±28Ma) are intrusive in A1 and have a similar composition, but are less deformed, and their primary structures are partly preserved. Both associations display contemporaneity relations with basic to intermediate magmas. Association 3 (A3; 2077±13Ma) is represented by tonalitic to granodioritic gneisses, without any associated basic to intermediate magmatism, and its main characteristic is the banding that resulted from strong solid-state deformation. Partial melting features are locally present in A3. The geochemical compositions of the three associations are similar and indicate sources related to a continental magmatic arc environment. The 87Sr/86Sr(i) ratios (between 0.701 and 0.703), positive ƐNd(t) values (+1.45 to +5.19), and TDM ages close to the crystallization ages indicate juvenile sources for the A1 and A2 associations. The A3 rocks have a 87Sr/86Sr(i) ratio of 0.715, an ƐNd(t) value of +0.47 and a TDM age that is close to the crystallization age, indicating a source composition different from those of the other associations. The Pb isotope ratios of A1 and A2 are similar and compatible with the evolution of mantle and orogen (208Pb/204Pb=37.3–37.6; 207Pb/204Pb=15.62–15.65; 206Pb/204Pb=18.0–18.2). The Pb isotope ratios of A3 differ from A1 and A2, indicating a more Th-poor source (208Pb/204Pb=37.1; 207Pb/204Pb=15.64; 206Pb/204Pb=18.5). The geochemistry of associations A1 and A2 suggests a juvenile source with contamination by crustal material. However, the Sr–Nd–Pb isotope signature of this contaminant is similar to that of the source material that originated these associations. This may be the crust generated in the magmatic arc, which is compatible with the geochronological results. The dataset points to the occurrence of self-cannibalism processes in the generation of the ARC rocks. The similar chemical composition and ƐNd(t) values of A3 relative to A1 and A2 indicate that the A3 source is similar to the one that generated the tonalitic and granodioritic rocks of A1 and A2. However, the slightly lower 208Pb/204Pb, and higher 206Pb/204Pb and 87Sr/86Sr(i) ratios indicate that the A3 association has also the addition of a distinct crustal source. The A3 association high values of the parameter 87Sr/86Sr(i) and its Pb isotope signature indicate a source with high Rb and U, and low Th contents. Such features, and moreover the depletion of HREE combined with TDM values near the igneous age, suggest that the source for A3 may be juvenile arc sediments. The greater degree of crustal contribution, the lack of associated basic to intermediate rocks, and the younger age possibly mark the more mature or late stages of the arc. The major and trace elements, as well as the isotope data obtained in this study suggest that melting of a metasomatized mantle wedge can be the process that generated the ARC basic to intermediate rocks (A1 and A2). The generation of tonalitic and granodioritic rocks with adakitic characteristics (i.e., the depletion of heavy rare earth elements in tonalitic and granodioritic rocks of A1, A2 and A3) may have arisen from the melting of a garnet-rich source or from fractional crystallization of ARC basic to intermediate magmas, which in time increased crustal assimilation under high-pressure conditions. The crustal garnet-rich source could be the basic rocks newly placed at the base of the crust, derived from partial melting of metasomatized mantle. The remobilization of this material by partial melting may have generated tonalitic and granodioritic liquids depleted in heavy REE due to the presence of garnet in the residue. The three associations display microstructures that indicate two episodes of recrystallization, one of a higher temperature and one of a lower temperature. The age of the high-temperature structure remains under discussion and may be attributed to a Paleoproterozoic metamorphic event. The low-temperature event is compatible with the temperature conditions observed in adjacent areas, in the host rocks of the Neoproterozoic post-collisional granitoids that have been emplaced along the Southern Brazilian Shear Belt (SBSB). Zircon crystals with Paleoproterozoic igneous cores exhibit a metamorphic overgrowth at 635±6Ma, compatible with the crystallization ages of the SBSB granitoids.

Synorogenic copper mineralization during the Alpine–Himalayan orogeny in the Zafarghand copper exploration district, Central Iran: petrogrography, geochemistry and alteration thermometry

AbstractThe Zafarghand porphyry copper exploration district is located in the NE of Isfahan (Central Iran), about 15 km SW of the city of Ardestan. This area consists of Eocene extrusive and Miocene intrusive rocks. The Eocene (ca. 56 Ma) units include andesites, dacites and rhyodacites spatially associated with pyroclastic materials (ignimbrites and pyroclastic tuff breccias), which are intruded by a Miocene (26–19 Ma) microgranular I‐type granodiorite. The entire igneous complex is intersected by NW–SE to E–W‐trending andesitic and dacitic dykes and N–S‐trending andesitic basalt dykes. Both the Miocene granodiorite and Eocene extrusives have undergone several hydrothermal alteration stages: (i) potassic, (ii) phyllic, (iii) argillic and (iv) propylitic alteration. The geochemical data of these formations are consistent with (i) the modal mineralogy derived from petrographic analysis of the samples and (ii) the regional tectonic setting of the samples having been emplaced in an active subduction‐related continental volcanic arc setting during the Zagros Orogeny. Copper mineralization in the Zafarghand area shows an average grade of 0.4% Cu, 200 ppm Mo, 0.2 ppm Ag and 0.01 g/t Au and a maximum age of 21 Ma, which implies that copper mineralization was synorogenic, coeval with the Alpine–Himalayan orogeny in this region. Copper mineralization occurs as chalcopyrite and bornite minerals as disseminated and stockwork mineralization associated with remobilized late epithermal vein mineralization. Chlorite geothermometry for the potassic and propylitic alteration zones yields a temperature range of 361–471 and 196–266 °C, respectively, which implies copper mineralization in a cooling synorogenic magmatic arc environment. Chlorite grains in these alteration zones indicate a classification of chlorites mainly as ripidolite, pycnochlorite, brunsvigite and diabantite. Copyright © 2016 John Wiley & Sons, Ltd.

Multiple and prolonged porphyry Cu–Au mineralization and alteration events in the Halasu deposit, Chinese Altai, Xinjiang, northwestern China

The Halasu area is located in the southeastern margin of the Chinese Altai in Xinjiang, China. It is part of the Altaid orogenic collage where a number of porphyry-type Cu–Mo–Au deposits have been discovered in recent years. Geological mapping and drilling indicate the presence of various mineralized porphyritic intrusions in the Halasu Cu–Au deposit, which is currently under exploration. U–Pb dating of zircon crystals from four different mineralized porphyries reveals three significantly different ages of magmatic intrusion, i.e., ca. 372–382 Ma granodioritic porphyry and porphyritic granite, ca. 266 Ma quartz monzonitic porphyry, and ca. 216 Ma quartz dioritic porphyry. Re–Os dating of molybdenite from veinlet-dissemination ores in the granodioritic porphyry yields an age of mineralization of ca. 377 Ma, and Ar–Ar dating of K-feldspar from K-feldspar–quartz–chalcopyrite veins produces ages of ca. 269 and ca. 198 Ma. The mineralization (and alteration) ages correspond broadly to the three episodes of magmatic intrusion, suggesting three overprinting porphyry mineralization events that are significantly separated in time. The first episode of porphyry intrusion and mineralization may be related to the magmatic arc being above a plate subduction zone, and the second was formed in a late-collisional environment during the closing of the Junggar Ocean, whereas the third episode of mineralization took place in the post-collisional stage. This case study suggests that in orogens where major porphyry deposits have been found in magmatic arc environments, the potential of discovering late- to post-collisional porphyry deposits cannot be neglected; conversely, in orogens where most porphyry deposits have late- to post-collisional ages, more attention should be paid to porphyries that were formed earlier in magmatic arc environments.

Petrografia e geoquímica das rochas metamáficas intercaladas em gnaisses do Arco Magmático de Goiás na região de Indiara (GO)

In Indiara region (GO), dozens of metamafic rocks lenses of various dimensions (metric to kilometric) occur, intercalated in gneisses of the Goias Magmatic Arc and oriented according to the direction of the main foliation (Sn). These lenses consist of amphibolite (amphibole schists and amphibolites), have chemical compositions of sub-alkaline basalts to andesitic basalts and sub-alkaline tholeiitic affinity. The distribution of major and trace elements, as well as rare earth elements (REE), suggests that these rocks are derived from a source with similar geochemical signature. The analyzed rocks are enriched in large ion litophile elements (Cs, Rb, Ba, K, Th and U) in relation to elements of high field strength (Nb, Ta, Zr, Hf and Y) and in relation to REE, Sr, P and Ti, showing weak to medium negative anomalies of Nb, Ta and P and suggesting that these rocks were generated in a magmatic arc environment.

Geoquímica e geocronologia U-Pb (SHRIMP) de granitos da região de Peixoto de Azevedo: Província Aurífera Alta Floresta, Mato Grosso

A analise de dados petrograficos, geoquimicos e geocronologicos de granitos do Dominio Peixoto de Azevedo, Mato Grosso, na porcao leste da Provincia Aurifera Alta Floresta, conduziu ao reconhecimento de dois corpos graniticos limitados por grandes falhamentos e zonas de cisalhamento regionais. Na porcao noroeste ocorre biotita granodiorito de granulacao grossa, textura inequigranular a porfiritica, metaluminoso a peraluminoso, calcialcalino de alto potassio e magnesiano. Na porcao sudeste da area ocorre um biotita monzogranito de granulacao grossa, textura equigranular a porfiritica, levemente peraluminoso, calcialcalino de alto potassio e carater dominantemente ferroso. Datacoes U-Pb (SHRIMP) mostraram que o biotita monzogranito apresenta uma idade de 1869 ± 10 Ma, similar a Suite Intrusiva Matupa, enquanto que o biotita granodiorito apresenta idade de 1781 ± 10 Ma, que e a idade esperada para o Granito Peixoto. As duas unidades mostram padroes de elementos terras raras com enriquecimento de leves sobre pesados e anomalia negativa de Eu (LaN/YbN 7,6 a 17,31 e Eu/Eu* entre 0,46 - 0,72 para o biotita monzogranito e LaN/YbN 7,13 a 29,09 com razoes Eu/Eu* entre 0,25 - 0,40 para o biotita granodiorito). O padrao dos elementos traco para ambos apresenta anomalias negativas de Ba, P, Ti e Nb, indicando uma evolucao por fracionamento mineral e associacao com fontes modificadas por subduccao e envolvimento crustal. Neste trabalho, sugere-se que o monzogranito Matupa foi gerado em ambiente de margem continental ativa, num estagio maduro. Para o biotita-granodiorito Peixoto, duas hipoteses sao sugeridas: (a) formacao num ambiente de arco magmatico mais jovem associado ao Magmatismo Colider ou (b) gerado em ambiente extensional, relacionado a quebra do efemero Supercontiente Columbia.

Syn-collisional lower continental crust anatexis in the Neoproterozoic Socorro-Guaxupé Nappe System, southern Brasília Orogen, Brazil: Constraints from zircon U–Pb dating, Sr–Nd–Hf signatures and whole-rock geochemistry

The southern Brasília Orogen occurs along the southernmost border of the São Francisco Craton. This Orogen is structured as a pile of Ediacaran syn-metamorphic thicked-skinned nappes that diachronically migrate toward the cratonic margin. A magmatic arc environment is represented by the Socorro-Guaxupé Nappe (SGN), which is a thick segment of partially molten lower to middle continental crust. The SGN comprises three main units: a lower granulitic, an intermediate diatexitic and an upper metatexitic. High-temperature metamorphic peak was reached at ∼625Ma, when large volumes of high-K diatexitic rocks were formed. Charnockitic and mangeritic magmatic rocks (São José do Rio Pardo suite) and in situ leucosomes of mafic composition are inferred to have formed under hornblende-dehydration melting conditions (i.e. T≥850°C). Granitic diatexites (Pinhal-type) were also formed, and zircon thermometry indicates T≥850°C. Elemental and isotopic geochemistry indicate that these diatexites and in situ leucosomes derived from melting of lower to middle continental crust. TIMS zircon U–Pb ages are presented for the lower and middle units. U–Pb dating coupled to Lu–Hf analysis (LA-MC-ICP-MS) was performed on charno-enderbitic in situ leucosomes from metatexitic rocks that also resulted from hornblende-dehydration melting. Two different zircon typologies were recognized. The first comprises bipyramidal-prismatic grains with oscillatory zoning and high luminescence, sometimes preserving low-luminescence 670Ma cores. The high-luminescence grains show 19 concordant ages at 621±16Ma. The second typology comprises isometric or soccer-ball-type grains. These grains show sector zoning and a concordia age of 608±4Ma. Th/U ratios from soccer-ball grains vary from 1.253 to 2.107, whereas prismatic grains vary from 0.118 to 1.774. Initial ɛHf signatures between −13 and −21 in both typologies provide evidence of a reworked crustal reservoir. The older prismatic grains may be related to the onset of metamorphic peak (∼625Ma), whereas the isometric ones provide evidence of long-lived high-T conditions until ∼610Ma.

Contrasting Ordovician high- and low-pressure metamorphism related to a microcontinent-arc collision in the Eastern Cordillera of Perú (Tarma province)

High-pressure conditions of 11–13kbar/500–540 °C during maximum burial were derived for garnet amphibolite in the Tapo Ultramafic Massif in the Eastern Cordillera of Peru using a PT pseudosection approach. A Sm–Nd mineral-whole rock isochron at 465 ± 24Ma dates fluid influx at peak temperatures of ∼600 °C and the peak of high pressure metamorphism in a rodingite of this ultramafic complex. The Tapo Ultramafic Complex is interpreted as a relic of oceanic crust which was subducted and exhumed in a collision zone along a suture. It was buried under a metamorphic geotherm of 12–13 °C/km during collision of the Paracas microcontinent with an Ordovician arc in the Peruvian Eastern Cordillera. The Ordovician arc is represented by the western Marañon Complex. Here, low PT conditions at 2.4–2.6 kbar, 300–330 °C were estimated for a phyllite–greenschist assemblage representing a contrasting metamorphic geotherm of 32–40 °C/km characteristic for a magmatic arc environment.

A Late Triassic tectonothermal event in the eastern Acatlán Complex, southern Mexico, synchronous with a magmatic arc hiatus: The result of flat-slab subduction?

Basement rocks exposed in the Acatlan Complex of the Mixteca terrane in southern Mexico record two tectonothermal events: (1) a Devonian–Mississippian (ca. 365–318 Ma) event, recording extrusion and exhumation of high-pressure rocks; and (2) an Early to Middle Permian (ca. 289–263 Ma) event, involving N-S dextral shearing, transtensional deformation, and local S-vergent thrusting in a magmatic arc environment. We document an additional, regionally significant, tectonothermal event during the Middle to Late Triassic recorded by 40 Ar/ 39 Ar step-heating laser-probe ages ranging from ca. 239 and 219 Ma (≡ cooling from ca. 525 °C to 300 °C) for amphibole, muscovite, and biotite from: (1) the Carboniferous Amarillo unit, consisting of medium-grade, metasedimentary rocks intruded by mafic dikes; and (2) the Pennsylvanian–Middle Permian, low-grade, clastic-calcareous, arc-related Tecomate Formation. U-Pb laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) data yield an age of 339 ± 6 Ma for the youngest population of detrital zircon grains in the Amarillo unit. Lithogeochemical and Sm-Nd isotopic data for the Amarillo unit dikes are very similar to those of other Carboniferous meta-igneous rocks in the eastern and southwestern part of the Acatlan Complex, displaying affinities transitional between mid-ocean-ridge basalt (MORB) and continental tholeiites, and initial ɛ Nd ( t = 339 Ma) values from -6.6 to +6.4, indicating both depleted and enriched mantle sources, as well as variable contamination by continental crust or by subduction-derived fluids. The 40 Ar/ 39 Ar cooling ages coincide with an apparent hiatus in magmatic activity in southern Mexico, which is inferred to record a change from steep to flat subduction.

The Mejillonia suspect terrane (Northern Chile): Late Triassic fast burial and metamorphism of sediments in a magmatic arc environment extending into the Early Jurassic

The Mejillonia terrane, named after the Mejillones Peninsula (northern Chile), has been traditionally considered an early Paleozoic block of metamorphic and igneous rocks displaced along the northern Andean margin in the Mesozoic. However, U–Pb SHRIMP zircon dating of metasedimentary and igneous rocks shows that the sedimentary protoliths were Triassic, and that metamorphism and magmatism took place in the Late Triassic (Norian). Field evidence combined with zircon dating (detrital and metamorphic) further suggests that the sedimentary protoliths were buried, deformed (foliated and folded) and metamorphosed very rapidly, probably within few million years, at ca. 210Ma. The metasedimentary wedge was then uplifted and intruded by a late arc-related tonalite body (Morro Mejillones) at 208±2Ma, only a short time after the peak of metamorphism. The Mejillones metamorphic and igneous basement represents an accretionary wedge or marginal basin that underwent contractional deformation and metamorphism at the end of a Late Permian to Late Triassic anorogenic episode that is well known in Chile and Argentina. Renewal of subduction along the pre-Andean continental margin in the Late Triassic and the development of new subduction-related magmatism are probably represented by the Early Jurassic Bólfin–Punta Tetas magmatic arc in the southern part of the peninsula, for which an age of 184±1Ma was determined. We suggest retaining the classification of Mejillonia as a tectonostratigraphic terrane, albeit in this new context.

Precious metal-bearing epithermal deposits in western Patagonia (NE Lago Fontana region), Argentina

Precious metal-bearing quartz veins occur at the northeastern sector of the Lago Fontana region in southwestern Argentina, within the context of the Andean continental magmatic arc environment. The deposits and their associated alteration zones are spatially related to a Cretaceous calc-alkaline magmatism represented by silicic dikes and hypabyssal intrusions, and hosted by a Late Jurassic to Cretaceous volcano-sedimentary sequence. The veins and related veinlets crop out discontinuously, in general terms in a NW-SE belt. The primary vein mineral assemblage is composed mostly of pyrite ± galena ± chalcopyrite > hematite ± arsenopyrite in silica gangue minerals. Chemical analyses of grab samples from selected quartz veins show as much as 5.7 ppm Au and 224 ppm Ag, as well as elevated Pb, Cu, and Zn. Hydrothermal fluids caused an innermost silicification and adularia-sericite alteration assemblage, and an external propylitic halo.Sulfur isotope values measured for sulfides (δ34SH2S from −1.90 to +1.56‰), and oxygen and hydrogen isotopes measured on quartz crystals and extracted primary fluid inclusion waters (δ18OH2O = −2.85 to +5.40‰; δDH2O = −106.0 to −103.4‰) indicate that mineralization probably formed from magmatic fluids, which were mixed with meteoric waters. Also, fluid inclusion data from quartz veins point out that these fluids had low salinity (1.7–4.2 wt% NaCl equiv.), and temperatures of homogenization between 180 and 325 °C.Mineralogical, petrographic and geochemical features for mineralized surface exposures indicate a typical adularia-sericite, low sulfidation epithermal system in the Lago Fontana area that represents a promising target for further exploration programs.

Insights on the crustal evolution of the West African Craton from Hf isotopes in detrital zircons from the Anti-Atlas belt

The Lu-Hf isotopic composition of detrital zircons has been used to investigate the crustal evolution of the northern part of the West African Craton (WAC). The zircons were separated from six samples of siliciclastic sedimentary rocks from the main Neoproterozic stratigraphic units of the Anti-Atlas belt, from the Sirwa and Zenaga inliers. The data suggest that the north part of the WAC formed during three cycles of juvenile crust formation with variable amount of reworking of older crust. The younger group of zircons, with a main population clustering around 610Ma, has a predominant juvenile character and evidences of moderate mixing with Paleoproterozoic and Neoarchean crust, which supports that most igneous and metamorphic rocks where zircons originally crystallized were formed in an ensialic magmatic arc environment. The group of zircons in the age range 1.79–2.3Ga corresponds to the major crust forming event in the WAC: the Eburnian orogeny. The isotopic data indicate that the provenance area should represent a crustal domain that was separated from a mantle reservoir at ∼2050–2300Ma, and further evolved with a time-integrated 176Lu/177Hf of ∼0.01, characteristic of continental crust. The evolution of the Eburnian orogeny is, consequently, compatible with new crust formation in an island arc environment, the transition to a continental arc setting and a final continent–continent collision. The Lower Paleoproterozoic and Neoarchean evolution (2.3–2.75Ga) includes a group of detrital zircon ages that has not been identified up to now in the igneous or metamorphic rocks of the north WAC basement. Their Hf isotopic signature points to reworking of mainly juvenile Neoarchean crust with some Meso- to Palaearchean contributions. The significance of these ages is uncertain: they can represent a tectonothermal event not discovered yet in the Reguibat Shield or the zircons can be far traveled from an unknown source.

Evolução petrogenética e tectônica do evento Cariris Velhos na região de Afogados da Ingazeira (PE), terreno Alto Pajeú, Província Borborema

The Tonian Cariris Velhos event was studied in the area around Afogados da Ingazeira town, Pernambuco State, Northeast Brazil, which is situated within the Alto Pajeú terrane, a Tonian-Ediacaran composite domain of the Borborema Province. The exposed rocks belong to the São Caetano Complex, which is formed of biotite-muscovite paragneisses and quartz-feldspathic gneisses, derived from arkosic or felsic volcanic protoliths, with marble and quartzite intercalations. The deformation started with a D1/D2 thrusting episode, when metamorphism ranged from greenschist to amphibolite facies, and was accompanied by emplacement of orthogneiss sheets. The subsequent episode, the D3 phase, had an extensional character and was marked by intrusion of small metadiorite dikes, which were strongly transposed by the D4 phase, a new episode of transcurrent deformation. The main structure of the D4 phase is the Afogados da Ingazeira shear zone (ZCAI), which Ediacaran age was well constrained by the associated Solidão granite, dating from 574 ± 54 Ma. A geochemical study of the metasedimentary rocks shows that the protoliths of the paragneisses located in the southeastern part of the ZCAI are greywackes, whereas those of the northwestern part are greywackes, lithic sandstones and arkoses, although the REE patterns of these samples normalized to NASC (North American Shale Composite) show minor differences between them. These metasedimentary rocks have geochemical signatures and patterns of synorogenic sediments compatible with island and continental arc sources. The geochemical patterns of the orthogneisses are also consistent with a magmatic arc environment, the same conclusion reached by other authors with respect to the metavolcanic rocks of the Alto Pajeú terrane. The occurrence of the pre-transcurrent D3 extensional episode suggests that the D1/D2 thrusting deformation and associated metamorphism may be related to orogenic processes, pointing to the existence of the Cariris Velhos orogeny.

The Crystalline Basement in the Argentinian North-Patagonian Andes: geochronology and tectonic interpretation.

Isotopic data are presented for rocks of the Andean crystalline basement at 40°S, in Neuquén and Río Negro provinces. The methods used were U-Pb in zircon (crystallization ages of plutonic rocks), K-Ar in micas and U-Pb in titanite (metamorphism and cooling ages) and Sm-Nd in whole rock (crustal evolution). No Proterozoic ages were obtained as suggested by previous Rb-Sr and K-Ar data, and two different igneous-metamorphic events were identified. The older one is Devonian and exposed in San Martín de los Andes region, according to 420-380 Ma zircon ages and 375-310 Ma ages in micas of deformed tonalitic and granitic rocks. The younger one, in rocks cropping out to the south of Limay River region, is revealed by Early Permian zircon ages (about 280 Ma) and Late Permian cooling ages (260-250 Ma) of metaigneous rocks. The rocks of both sets were assigned to a magmatic arc environment and correlated respectively with the Chanic orogeny (Devonian; Upper Famatinian Cycle) and Gondwanic Cycle (Upper Paleozoic-Triassic). The Sm-Nd TDM model ages, ca.1907-1165 Ma, eNd for the crystallization age of the rocks ca. -3,0 and -8,4 and eNd(0) ca. -6,9-11,9 suggest reworking of continental proterozoic crust with minor addition of juvenile magmas from the mantle during the Paleozoic.

Modeling multiple melt loss events in the evolution of an active continental margin

The Fosdick migmatite–granite complex in West Antarctica records evidence for crustal melting during two periods of tectonism along the East Gondwana margin. Initial high-temperature metamorphism in the Devonian–Carboniferous (M 1) was broadly contemporaneous with emplacement of calc-alkaline arc magmas during Pacific-style accretionary margin convergence. This event involved metamorphism of arc plutonic rocks soon after their emplacement and partial melting and migmatization of host metasedimentary rocks. Preservation of M 1 garnet-bearing assemblages and mineral equilibria modeling of the metasedimentary rock and calc-alkaline plutonic rock protolith compositions regionally exposed constrain conditions of M 1 metamorphism to 820–870 °C and 7.5–11.5 kbar. A second anatectic event during the Cretaceous (M 2) resulted in metamorphism of plutonic rocks and partial melting of fertile metasedimentary rocks that had remained at a high enough structural level to have been subsolidus during the first anatectic event, and a metamorphic overprint on now residual paragneisses characterized by the growth of M 2 cordierite after garnet, and after biotite + sillimanite. Mineral equilibria modeling of para- and orthogneiss compositions in the Fosdick migmatite–granite complex constrain conditions of M 2 metamorphism to 830–870 °C and 6–7.5 kbar. We use the results of mineral equilibria modeling to assess the constituents of the Fosdick migmatite–granite complex as melt sources and as domains of melt transfer and melt accumulation during the two anatectic events. Modeling the range of metasedimentary rock protolith compositions shows that ~ 4–25 vol.% melt was produced at the conditions of M 1 metamorphism, although most compositions would have been fertile enough to reach the melt connectivity transition (~ 7 vol.%) leading to the development of a melt extraction pathway and subsequent melt loss. The preservation of peak-M 1 assemblages in the paragneiss is consistent with melt loss, and modeling based on a representative protolith composition indicates that a minimum of 70% of the total melt produced must have been extracted from the metasedimentary rock source. The intrusive plutonic rocks produce ~ 2–3 vol.% melt during the M 1 event. Although the plutonic rocks were not a significant melt source at the level exposed, granites derived from these rocks but sourced at a deeper crustal level accumulated within the Fosdick migmatite–granite complex during the M 1 event. The elevated geotherms in a magmatic arc environment allow the possibility that higher crustal levels in the Fosdick migmatite–granite complex remained subsolidus during the M 1 event, and could be fertile sources during the M 2 event. At peak M 2 conditions, these fertile metasedimentary rocks would produce ~ 5–30 vol.% melt, whereas the plutonic rocks were not likely a significant source of melt at this crustal depth. The residual paragneisses that underwent melting and melt loss during the M 1 event are estimated to produce ~ 12 vol.% additional melt during M 2. The mechanical anisotropy created by the residual gneisses likely produced a gradient in melt pressure. This gradient, together with shallow fabrics in the hosting gneisses, could have acted to focus M 2 melts derived from the fertile metasedimentary rocks into a horizontally-sheeted leucogranite complex. The accumulation of these melts would have lead to pronounced weakening of the crust, facilitating the exhumation of the Fosdick migmatite–granite complex during the transition from regional shortening during M 2 to regional extension at ca. 100 Ma.

SHRIMP U–Pb zircon ages of granitoid rocks in the Lüliang Complex: Implications for the accretion and evolution of the Trans-North China Orogen

The Lüliang Complex is situated in the central segment of the Trans-North China Orogen (TNCO), a continent–continent collisional belt along which the discrete Eastern and Western Blocks amalgamated to form the basement of the North China Craton. The complex consists of supracrustal and granitoid rocks, of which the latter can be subdivided into pre-tectonic TTG gneisses, including the Yunzhongshan, Guojiazhuang and Chijianling–Guandishan gneisses; syn-tectonic gneissic granites represented by the Huijiazhuang gneissic granite; and post-tectonic granites, including the Luyashan charnockite, Luchaogou porphyritic granite and Tangershang/Guandishan massive granite. The pre-tectonic TTG gneisses are mostly calc-alkaline and considered to have formed in a magmatic arc environment. SHRIMP U–Pb analyses reveal that the Yunzhongshan gneisses were emplaced at ∼2499 ± 9 Ma, representing the earliest arc-related magmatic event in the Lüliang Complex. This was followed by the intrusion of the Guojiazhuang gneisses at 2375 ± 10 Ma. The most widespread arc-related magmatic event in the region was the emplacement of the Chijianling–Guandishan granitoid gneisses, of which the tonalitic, granodioritic and monzogranitic gneisses were emplaced at 2199 ± 11 Ma, 2180 ± 7 Ma and 2173 ± 7 Ma, respectively. Metamorphic zircon overgrowth rims from a tonalitic gneiss yielded a weighted mean 207Pb/ 206Pb age of 1872 ± 7 Ma, consistent with the metamorphic age range of 1880–1820 Ma defined by metamorphic zircons from various high-grade rocks in the TNCO. Thus, the evolution of the Lüliang Complex involved emplacement of the Yunzhongshan granitoids at ∼2499 Ma, the Guajiazhuang granitoids at ∼2375 Ma and the Chijianling–Guandishan granitoids at 2199–2173 Ma, with the final collision between the Eastern and Western Blocks in this area occurring at 1872 ± 7 Ma, the whole series of magmatic events lasting nearly 650 Ma. This suggests that the Trans-North China Orogen represents a long-lived magmatic arc. The new SHRIMP zircon data, combined with structural and petrological studies, also reveal the existence of both syn- and post-collisional granites in the Lüliang Complex. The former is represented by the Huijiazhuang gneissic granite that was emplaced at 1832 ± 11 Ma, whereas the latter include the 1815 ± 5 Ma Luyashan charnockite, 1807 ± 10 Ma Luchaogou porphyritic granite, 1798 ± 11 Ma Guandishan massive granite and 1790 ± 14 Ma Tangershan massive granite.

Geology and Chemistry of the El Abuelo Calcic Fe-skarn and Related Cu-(Ag)-Bearing Hydrothermal Veins, Chubut Province, Southern Argentina

The El Abuelo Ca-Fe-magnetite skarn and related hydrothermal quartz veins are located at Cerro Pepita Hill, in southwestern Chubut province of southern Argentina, 1700 km southwest of Buenos Aires. These deposits are developed in a continental magmatic arc environment linked to the Andean orogeny. Iron skarn mineralization is mainly hosted by an Upper Jurassic to Early Cretaceous sedimentary sequence interbedded with Upper Jurassic basaltic andesite, and is spatially associated with Early Cretaceous calc-alkaline dikes. Ore grades vary between 40 and 63 wt.% Fe. In addition, anomalous metal contents (>10 000 ppm Cu and up to 81 g/t Ag) are present in the hydrothermal quartz veins. At least three paragenetic mineral assemblages have been identified in the El Abuelo exoskarn: (1) amphibole > epidote ± chlorite ± quartz, formed by isochemical contact metamorphism; (2) Fe-clinopyroxene ± Ca-garnet, formed during prograde metasomatic anhydrous exoskarn formation; and (3) actinolite ± epidote ± chlorite ± quartz ± magnetite > titanite, resulting from hydrous retrograde alteration of exoskarn. Endoskarn alteration involved an early Mg-Fe-clinopyroxene ± Fe-garnet assemblage followed by retrograde actinolite replacement of pyroxene. Lower temperature hydrothermal alteration related to quartz veins was superimposed on the skarn assemblages, showing an innermost sericite ± adularia ± pyrite ± chalcopyrite assemblage and an external propylitic halo. Some metallogenic characteristics of the mineralization and the geological setting resemble those in several iron oxide copper-gold deposits elsewhere in the world.

Geoquímica de Gnaisses do arco magmático de Goiás na Região Sul do Estado de Goiás

Granitic to trondhjemitic gneisses from the Pontalina region in the southern part of Goiás State, Central Brazil, have calcic to calc-alkaline, metaluminous to peraluminous compositions. They have low concentrations of alkaline elements, and are enriched in Ba, Sr, K, Rb in relation to Nb, Y, Zr and REE, and have negative anomalies of Nb and Ti, features which are similar to those of magmas generated in magmatic arc environments. Those rocks were previously interpreted as part of the basement of the Brasília Belt, attributed to the Archean to Paleoproterozoic, but new Sm - Nd isotopic data indicate a neoproterozoic age (T DM = 0,9 a 1,2 Ga), and the preliminary geochemical data reveal compositions similar to the gneisses of the other regions belonging to the Goiás Magmatic Arc.

Geologia, petrografia e petroquímica das rochas relacionadas ao depósito de Au-Cu (Bi) Pombo, Terra Nova do Norte (MT)

The Pombo Au-Cu (Bi) deposit is hosted by the Pombo biotite-hornblende granite, one intrusion of the Terra Nova batolith that contains magnetic, calc-alkaline to alkaline, metaluminous to peraluminous granites. These were formed in a magmatic arc environment, probably during a post-collisional orogeny. Pombo is similar to Australian I type fractionated granites. The batolith rocks underwent a regional tardi-magmatic alteration that chloritized biotite and hornblende, damouritized the feldspar and coat them with albite and disseminated sericite. The hydrothermal phase started with sodic, followed by potassic, propilitic and phyllic alterations. The later, related to faults and fractures, was responsible by ore crystallization and the dissemination of carbonate. The propillitic zone overprinted the potassic. Phyllitization + carbonatization affected all rocks. Overall, metassomatism results in relative gains in Na 2 O, FeO, MgO and P 2 O 5 contents and loss of K 2 O. TiO 2 and Al 2 O 3 grades had small variations and Ba, Sr and Rb contents have risen probably due to hydrothermal feldspatization. Zr, Ta and V were displaced from magmatic magnetite and titanite and crystallized in hydrothermal titanite, apatite, chlorite and biotite. The absence of Eu negative anomaly in granite seems to be a consequence of the metassomatic origin of most feldspar or because there was no magma fractionation and residual feldspar genesis while granite crystallized. The tardi-magmatic alteration zone is enriched in LREE and depleted in HREE and the overprint of the hydrothermal Na and K zones stressed this tendency.

Os terrenos tectônicos da Faixa Ribeira na Serra da Bocaina e na Baía da Ilha Grande, sudeste do Brasil

The central segment of the Neoproterozoic Ribeira belt in the studied area is composed of four tectonic units: the Occidental, Oriental, Paraiba do Sul and Embu terranes. The metasedimentary sequence of the Occidental Terrane represents the reworked margin of the Sao Francisco Craton. The Oriental Terrane comprises the cordilleran magmatic arc that collided with the Sao Francisco Plate at ca. 580 Ma. The Paraiba do Sul Terrane is composed of Paleoproterozoic orthogneisses and metasedimentary rocks, and the Embu Terrane comprises supracrustal succession and undated orthogneisses. Granitoid rocks intrude all terranes. These terranes were thrust-stacked during the main deformational phase related to the collisional stage of the Brasiliano Orogeny, when the Occidental Terrane was covered by the other ones. The nature and the paleogeography of the Paraiba do Sul and Embu terranes are unclear, but a relation to a magmatic arc environment can be suggested.