Petrogenesis of the Paleoproterozoic rapakivi A-type granites of the Archean Carajás metallogenic province, Brazil

Three Paleoproterozoic A-type rapakivi granite suites (Jamon, Serra dos Carajás, and Velho Guilherme) are found in the Carajás metallogenic province, eastern Amazonian craton. Liquidus temperatures in the 900–870 °C range characterize the Jamon suite, those for Serra dos Carajás and Velho Guilherme are somewhat lower. Pressures of emplacement decrease from Jamon (3.2±0.7 kbar) through Serra dos Carajás (2.0±1.0 kbar) to Velho Guilherme (1.0±0.5 kbar). Oxidizing conditions (NNO+0.5) characterized the crystallization of the Jamon magma, the Velho Guilherme magmas were reducing (marginally below FMQ), and the Serra dos Carajás magmas were intermediate between the two in this respect. The three granite suites have Archean T_DM model ages and strongly negative _Nd values (−12 to −8 at 1880 Ma), and they were derived from Archean crust. The Jamon granite suite may have been derived from a quartz dioritic source, and the Velho Guilherme granites from K-feldspar-bearing granitoid rocks with some sedimentary input. The Serra dos Carajás granites either had a somewhat more mafic source than Velho Guilherme or were derived by a larger degree of melting. Underplating of mafic magma was probably the heat source for the melting. The petrological and geochemical characteristics of the Carajás granite suites imply considerable compositional variation in the Archean of the eastern Amazonian craton. The oxidized Jamon suite granites are similar to the Mesoproterozoic magnetite-series granites of Laurentia, and they were derived from Archean igneous sources that were more oxidized than the sources of the Fennoscandian rapakivi granites. The Serra dos Carajás and Velho Guilherme granites approach the classic reduced rapakivi series of Fennoscandia and Laurentia. No counterparts of the Mesoproterozoic two-mica granites of Laurentia have been found, however. Following the model of Hoffman [Hoffman, P., 1989. Speculations on Laurentia's first gigayear (2.0 to 1.0 Ga). Geology 17, 135–138], the origin of the 1.88 Ga Carajás granites is related to a mantle superswell beneath the Trans-Amazonian supercontinent. This caused breakup of the continent and was associated with magmatic underplating and resultant crustal melting and generation of A-type granite magmas. The Paleoproterozoic continent that included the Archean and Trans-Amazonian domains of the Amazonian craton was assembled at 2.0 Ga; its disruption was initiated at 1.88 Ga, at least 200 Ma earlier than in Laurentia and Fennoscandia. The Carajás granites were related to the breakup of the supercontinent, not to subduction processes.     

Petrogenesis and tectonic implications of the late Carboniferous calc-alkaline and shoshonitic magmatic rocks in the Awulale mountain,western Tianshan

The widespread late Carboniferous calc-alkaline and shoshonitic magmatic rocks in the Awulale mountain provide crucial constraints on the tectonic evolution of the western Tianshan. Here, we perform detailed petrological investigations as well as zircon U-Pb chronological, whole-rock geochemical and Sr-Nd isotopic analyses on these magmatic rocks from two geological sections along the Duku road. Magmatic rocks in the section I with zircon SHRIMP U-Pb ages of 306.8 Ma and 306.4 Ma are composed of medium-K calc-alkaline to shoshonitic basalt, trachy-andesite and trachyte, while those in the section II consist of shoshonitic trachy-andesite, trachyte with a U-Pb age of 308.1 Ma, and monzonite with a U-Pb age of 309.6 Ma. All these magmatic rocks are characterized by strong enrichments in large iron lithophile elements with depletions of Nb, Ta and Ti, indicating the origination from subduction-modified lithospheric mantle. The ε_Nd(t) values of the rock samples collected from the section I (2.80–5.45) and section II (3.34–5.37) are generally higher than those of the Devonian to early Carboniferous arc-type magmatic rocks in the Yili-central Tianshan, suggesting that depleted asthenosphere might also be involved in their generation. Based on these geochemical data and petrological observations, we suggest that the early-stage (308.1–309.6 Ma) shoshonitic monzonite, trachy-andesite and trachyte in the section II were generated by mixing between mafic magmas and trachytic melts, while the late-stage (306.4–306.8 Ma) medium-K calc-alkaline to shoshonitic basalt, trachy-andesite and trachyte in the section I were produced by partial melting of depleted asthenospheric and metasomatized lithospheric mantle, followed by the processes of fractional crystallization and crustal contamination. Taking into account the available regional geological data, the subduction of south Tianshan ocean was probably ceased at ∼310 Ma, and these calc-alkaline and shoshonitic magmatic rocks in the Awulale mountain formed in a post-collisional setting subsequent to slab break-off.     

PGE and PGM in the Luanga mafic-ultramafic intrusion in Serra dos Carajás (Pará State, Brazil)

The late Archean, Luanga mafic-ultramafic complex intrudes an Archean greenstone belt, that is mainly composed of ultramafic and mafic metavolcanics. The Luanga intrusion consists of dunite, peridotite, gabbro and norite; chromitite seams and layers are present in the ultramafic rocks.A metamorphic overprint transformed the primary paragenesis into a serpentine-talc-chlorite-tremolite and magnetite association. The magnetite is commonly altered to Fe-hydroxides. Unaltered chromite commonly displays atoll-like textures and a chemical composition typical of stratiform chromites (Cr₂O₃ below 45 wt%).Base-metal sulfides, base-metal alloys, platimum-group minerals and platinum group element bearing phases are present in the form of inclusions in the silicate assemblages and in or on the edges of chromite grains. The main minerals detected are pentlandite, pyrrhotite, millerite, chalcopyrite and mackinawite, Fe---Ni alloy, braggite, sperrylite and platinum group elements (PGE) bearing sulfo-arsenides. Braggite is associated with the chromite, whereas sperrylite lies on the edges of or is included in silicates. The PGE content of the massive and disseminated chromities is dominated by Pt (up to 8900 ppb) and the chondrite-normalized PGE profile shows a cuspidal shape with a Pt peak.The main hypothesis for the source of the PGE-rich magma, which fractionated the chromitite-bearing ultramafic magma, consists of a relatively primitive mantle that partially melted in the late Archean.     

Oxidized, magnetite-series, rapakivi-type granites of Carajás, Brazil: Implications for classification and petrogenesis of A-type granites

The varying geochemical and petrogenetic nature of A-type granites is a controversial issue. The oxidized, magnetite-series A-type granites, defined by Anderson and Bender [Anderson, J.L., Bender, E.E., 1989. Nature and origin of Proterozoic A-type granitic magmatism in the southwestern United States of America. Lithos 23, 19–52.], are the most problematic as they do not strictly follow the original definition of A-type granites, and approach calc-alkaline and I-type granites in some aspects. The oxidized Jamon suite A-type granites of the Carajás province of the Amazonian craton are compared with the magnetite-series granites of Laurentia, and other representative A-type granites, including Finnish rapakivi and Lachlan Fold Belt A-type granites, as well as with calc-alkaline, I-type orogenic granites. The geochemistry and petrogenesis of different groups of A-types granites are discussed with an emphasis on oxidized A-type granites in order to define their geochemical signatures and to clarify the processes involved in their petrogenesis. Oxidized A-type granites are clearly distinguished from calc-alkaline Cordilleran granites not only regarding trace element composition, as previously demonstrated, but also in their major element geochemistry. Oxidized A-type granites have high whole-rock FeO_t/(FeO_t + MgO), TiO₂/MgO, and K₂O/Na₂O and low Al₂O₃ and CaO compared to calc-alkaline granites. The contrast of Al₂O₃ contents in these two granite groups is remarkable. The CaO/(FeO_t + MgO + TiO₂) vs. CaO + Al₂O₃ and CaO/(FeO_t + MgO + TiO₂) vs. Al₂O₃ diagrams are proposed to distinguish A-type and calc-alkaline granites. Whole-rock FeO_t/(FeO_t + MgO) and the FeO_t/(FeO_t + MgO) vs. Al₂O₃ and FeO_t/(FeO_t + MgO) vs. Al₂O₃/(K₂O/Na₂O) diagrams are suggested for discrimination of oxidized and reduced A-type granites. Experimental data indicate that, besides pressure, the nature of A-type granites is dependent of ƒO₂ conditions and the water content of magma sources. Oxidized A-type magmas are considered to be derived from melts with appreciable water contents (≥ 4 wt.%), originating from lower crustal quartz-feldspathic igneous sources under oxidizing conditions, and which had clinopyroxene as an important residual phase. Reduced A-type granites may be derived from quartz-feldspathic igneous sources with a metasedimentary component or, alternatively, from differentiated tholeiitic sources. The imprint of the different magma sources is largely responsible for the geochemical and petrological contrasts between distinct A-type granite groups. Assuming conditions near the NNO buffer as a minimum for oxidized granites, magnetite-bearing granites formed near FMQ buffer conditions are not stricto sensu oxidized granites and a correspondence between oxidized and reduced A-type granites and, respectively, magnetite-series and ilmenite-series granites is not always observed.     

Origin and evolution of post-collisional magmatism: Coeval Neoproterozoic calc-alkaline and alkaline suites of the Sinai Peninsula

Two Late Neoproterozoic post-collisional igneous suites, calc-alkaline (CA) and alkaline–peralkaline (Alk), widely occur in the northernmost part of the Arabian–Nubian Shield. In Sinai (Egypt) and southern Israel they occupy up to 80% of the exposed basement. Recently published U–Pb zircon geochronology indicates a prolonged and partially overlapping CA and Alk magmatism at 635–590 Ma and 608–580 Ma, respectively. Nevertheless in each particular locality CA granitoids always preceded Alk plutons. CA and Alk igneous rocks have distinct chemical compositions, but felsic and mafic rocks in general and granitoids from the two suites in particular cannot be distinguished by their Nd, Sr and O isotope ratios. Both suites are characterized by positive ε_Nd(T) values, from + 1.5 to + 6.0 (150 samples, 28 of them are new analyses), but predominance of juvenile crust in the region prevents unambiguous petrogenetic interpretation of the isotope data. Comparison of geochemical traits of felsic and mafic rocks in each suite suggests a significant contribution of mantle-derived components to the silicic magmas. Model calculation shows that the alkaline granite magma could have been produced by partial (~ 20%) melting of rocks corresponding to K-rich basalts. Material balance further suggests that granodiorite and quartz monzonite magmas of the CA suite could form by mixing of the granite and gabbro end-members at proportions of 85/15. In the Alk suite, alkali feldspar and peralkaline granites have evolved mainly by fractional crystallization of feldspars and a small amount of mafic minerals from a parental syenogranite melt. Thus the protracted, 20 m.y. long, contemporaneous CA and Alk magmatism in the northern ANS requires concurrent tapping of two distinct mantle sources. Coeval emplacement of CA and Alk intrusive suites was described in a number of regions throughout the world.     

Metallogenesis of the Carajás Mineral Province, Southern Amazon Craton, Brazil: Varying styles of Archean through Paleoproterozoic to Neoproterozoic base- and precious-metal mineralisation

The Itacaiúnas Belt of the highly mineralised Carajás Mineral Province comprises ca. 2.75 Ga volcanic rocks overlain by sedimentary sequences of ca. 2.68 Ga age, that represent an intracratonic basin rather than a greenstone belt. Rocks are generally at low strain and low metamorphic grade, but are often highly deformed and at amphibolite facies grade adjacent to the Cinzento Strike Slip System. The Province has been long recognised for its giant enriched iron and manganese deposits, but over the past 20 years has been increasingly acknowledged as one of the most important Cu–Au and Au–PGE provinces globally, with deposits extending along an approximately 150 km long WNW-trending zone about 60 km wide centred on the Carajás Fault. The larger deposits (approx. 200–1000 Mt @ 0.95–1.4% Cu and 0.3–0.85 g/t Au) are classic Fe-oxide Cu–Au deposits that include Salobo, Igarapé Bahia–Alemão, Cristalino and Sossego. They are largely hosted in the lower volcanic sequences and basement gneisses as pipe- or ring-like mineralised, generally breccia bodies that are strongly Fe- and LREE-enriched, commonly with anomalous Co and U, and quartz- and sulfur-deficient. Iron oxides and Fe-rich carbonates and/or silicates are invariably present. Rhenium–Os dating of molybdenite at Salobo and SHRIMP Pb–Pb dating of hydrothermal monazite at Igarapé-Bahia indicate ages of ca. 2.57 Ga for mineralisation, indistinguishable from ages of poorly-exposed Archean alkalic and A-type intrusions in the Itacaiúnas Belt, strongly implicating a deep magmatic connection.A group of smaller, commonly supergene-enriched Cu–Au deposits (generally < 50 Mt @ < 2% Cu and < 1 g/t Au in hypogene ore), with enrichment in granitophile elements such as W, Sn and Bi, spatially overlap the Archean Fe-oxide Cu–Au deposits. These include the Breves, Águas Claras, Gameleira and Estrela deposits which are largely hosted by the upper sedimentary sequence as greisen-to ring-like or stockwork bodies. They generally lack abundant Fe-oxides, are quartz-bearing and contain more S-rich Cu–Fe sulfides than the Fe-oxide Cu–Au deposits, although Cento e Dezoito (118) appears to be a transitional type of deposit. Precise Pb–Pb in hydrothermal phosphate dating of the Breves and Cento e Dezoito deposits indicate ages of 1872 ± 7 Ma and 1868 ± 7 Ma, respectively, indistinguishable from Pb–Pb ages of zircons from adjacent A-type granites and associated dykes which range from 1874 ± 2 Ma to 1883 ± 2 Ma, with 1878 ± 8 Ma the age of intrusions at Breves. An unpublished Ar/Ar age for hydrothermal biotite at Estrela is indistinguishable, and a Sm–Nd isochron age for Gameleira is also similar, although somewhat younger. The geochronological data, combined with geological constraints and ore-element associations, strongly implicate a magmatic connection for these deposits.The highly anomalous, hydrothermal Serra Pelada Au–PGE deposit lies at the north-eastern edge of the Province within the same fault corridor as the Archean and Paleoproterozoic Cu–Au deposits, and like the Cu–Au deposits is LREE enriched. It appears to have formed from highly oxidising ore fluids that were neutralised by dolomites and reduced by carbonaceous shales in the upper sedimentary succession within the hinge of a reclined synform. The imprecise Pb–Pb in hydrothermal phosphate age of 1861 ± 45 Ma, combined with an Ar/Ar age of hydrothermal biotite of 1882 ± 3 Ma, are indistinguishable from a Pb–Pb in zircon age of 1883 ± 2 Ma for the adjacent Cigano A-type granite and indistinguishable from the age of the Paleoproterozoic Cu–Au deposits. Again a magmatic connection is indicated, particularly as there is no other credible heat or fluid source at that time.Finally, there is minor Au–(Cu) mineralisation associated with the Formiga Granite whose age is probably ca. 600 Ma, although there is little new zircon growth during crystallisation of the granite. This granite is probably related to the adjacent Neoproterozoic (900–600 Ma) Araguaia Fold Belt, formed as part of the Brasiliano Orogeny.Thus, there are two major and one minor period of Cu–Au mineralisation in the Carajás Mineral Province. The two major events display strong REE enrichment and strongly enhanced LREE. There is a trend from strongly Fe-rich, low-SiO₂ and low-S deposits to quartz-bearing and more S-rich systems with time. There cannot be significant connate or basinal fluid (commonly invoked in the genesis of Fe-oxide Cu–Au deposits) involved as all host rocks were metamorphosed well before mineralisation: some host rocks are at mid- to high-amphibolite facies. The two major periods of mineralisation correspond to two periods of alkalic to A-type magmatism at ca. 2.57 Ga and ca. 1.88 Ga, and a magmatic association is compelling.The giant to world-class late Archean Fe-oxide Cu–Au deposits show the least obvious association with deep-seated alkaline bodies as shown at Palabora, South Africa, and implied at Olympic Dam, South Australia. The smaller Paleoproterozoic Cu–Au–W–Sn–Bi deposits and Au–PGE deposit show a more obvious relationship to more fractionated A-type granites, and the Neoproterozoic Au–(Cu) deposit to crustally-derived magmas. The available data suggest that magmas and ore fluids were derived from long-lived metasomatised lithosphere and lower crust beneath the eastern margin of the Amazon Craton in a tectonic setting similar to that of other large Precambrian Fe-oxide Cu–Au deposits.     

Geophysical structures and tectonic evolution of the southern Guyana shield,Brazil

Aerogeophysical data of an area located on the southern portion of the Guyana shield in Brazil was processed using a fine interpolating mesh, and a corresponding spatial data integration strategy which included the stacking of different high-resolution images, and interpretation following quality control of these. The selected images were correlated to the local known surface geologic units, and to the spatial distribution of the main geochronological provinces of the Amazonian craton. The interpretation of the results also included the available geophysical information for the region, related to Moho depth values, and previously determined SKS shear-wave splitting direction. The observed magnetic regional trends may be strongly influenced by the Proterozoic crustal structure in the area, while radiometric anomalies correlate with the more detailed geologic features. Based on the parallelism among mapped geochronological provinces of the Amazonian craton, and observed geophysical structures on the study area, a geotectonic model is proposed for southern Guyana shield at Proterozoic age.     

Petrogenesis of calc-alkaline,shoshonitic and associated ultrapotassic Oligocene volcanic rocks from the Northwestern Alps,Italy

Along the Western Alps there is geological evidence of late-Alpine (Oligocene) magmatic activity which clearly postdates the Lepontine (Eocene-early Oligocene) metamorphism and related deformation of the Alpine nappe pile. This magmatic activity was notably delayed in relation to the most important convergent processes and may be related to buoyancy of lithosphere, tensional tectonics and thermal updoming subsequent to the collision between the Eurasian and African plates. The geochemical features of the rocks and the geophysical characteristics of the Alpine chain, suggest that: (a) shoshonitic and calcalkaline melts may have been generated by partial melting of metasomatized peridotitic material and subsequent fractional crystallization and crustal contamination; silicic andesites and latites, however, could have been also derived from metasomatized eclogite or deep continental crust material; (b) the ultrapotassic lamprophyres with high K, P, LREE, Th, Zr, U and high ⁸⁷Sr/⁸⁶Sr ratios were generated by partial melting of strongly metasomatized mantle; the varied Sr-isotopic ratios may partially also reflect additional radiogenic component from the continental crust following magma segregation from the source.     

Geology, geochemistry, and U–Pb geochronology of the Archean (2.74 Ga) Serra do Rabo granite stocks, Carajás Metallogenetic Province, northern Brazil

The Carajás region, located in the southeastern part of the Amazon Craton, has been considered one of the most important mineral provinces in the world. The Serra do Rabo Granite (SRG) crops out near the eastern termination of the Carajás fault as two granite stocks, elongated approximately in an E–W direction, concordant with the regional structures. Leucomicrocline granite, hornblende–microcline granite, biotite–hornblende–microcline granite, hornblende syenogranite, and subordinate aplite are identified. The granites are grayish pink and coarse to medium grained and have mainly hypidiomorphic granular texture. Granophyric textures are common. The accessory minerals are ilmenite, apatite, zircon, allanite, and rare pyroxene.The SRG rocks are either massive or foliated, with a slightly anastomosed continuous S₁ foliation (E–W/subvertical) outlined by the preferred orientation of quartz, feldspars, and mafic minerals. Locally, decimeter- to meter-wide mylonite/ultramylonite bands (S_1m) occur along the E–W foliation. The S₁ foliation was developed under higher temperatures than those of the S_1m mylonite foliation. The SRG structural evolution was controlled by progressive deformation under decreasing temperature, indicative of syntectonic emplacement. The SRG also has relatively high SiO₂, K₂O, and Na₂O contents; high FeO*/(FeO*+MgO) ratios; high Zr, Ba, Nb, and Ga; and very high rare-earth element contents. The chemical signature is moderately alkaline and metaluminous, comparable to that of the A-type, A2, and ALK-3 granites. The origin of the SRG magmas may be related to the partial melting of crustal sources, such as previously metamorphosed calc-alkaline granites.The SRG crosscuts supracrustal rocks, promoting low-pressure/high-temperature metamorphism. The interaction between regional compressive stresses and the ballooning effect of the granite stocks promoted slight aureole flattening and rheological changes in the supracrustal rocks. The U–Pb zircon age of 2743±1.6 Ma is interpreted as the age of zircon crystallization, granite stock emplacement, and regional horizontal shortening. Other 2.7 Ga syntectonic alkaline granites (e.g. Estrela, Plaquê, Planalto) have been reported in the region.     

Petrogenesis of the Paleoproterozoic basalt–andesite–rhyolite dyke association in the Carajás region, Amazonian craton

Paleoproterozoic basaltic, andesitic and rhyolitic dykes crosscut the Archaean Carajás basement. Basalts are distinguished into a high and a low TiO₂ group (HTi and LTi), each group consisting of geochemically distinct NE- and NW-trending swarms. The HTi dykes are evolved transitional basalts having essentially EMORB-type geochemistry. The LTi basalts are tholeiites (NE-trending swarm) and high-Al basalts (NW-trending swarm) displaying incompatible trace elements patterns with variably negative Nb anomaly, enrichment in Rb, Ba, K (LILE) and La, Ce and Nd (LREE) and positive Sr anomaly. With respect to orogenic analogues, andesites have lower Al₂O₃, CaO and Ni, higher FeO, LILE, LREE, Nb, Zr and Ti and negative Sr anomaly. Rhyolites have geochemical characteristics comparable with those of A-type granites. At 1.8 Ga, ranges from 0.700 to 0.705 in the HTi basalts and from 0.700 to 0.704 in the LTi group. Andesites define an isochron of 1874±110 Ma (Sr_o=0.7038±0.0010). Rhyolites from Southern and Northern Carajás define two isochrons of 1802±130 Ma (Sr_o=0.7062±0.0046) and 1535±82 Ga (Sr_o=0.7625) respectively, the younger date being interpreted as resetting of the Rb–Sr isotopic system. We propose a petrogenetic model relating LTi basalts with melting of lithospheric mantle metasomatized by acid melts derived from incipient melting of eclogites, representing in turn the subsolidus product of basaltic batches trapped in the mantle. The HTi basalts are explained by melting of the lithospheric mantle containing the complementary residual eclogite. Andesite petrogenesis is consistent with crystal fractionation from a high-Mg andesite parent derived from a mantle source more extensively metasomatized by eclogite-derived melts. Rhyolite composition is consistent with low melting degree of the basement rocks. The basalt–andesite–rhyolite dykes may represent the effects of crustal extension and arching in Carajás, which produced the anorogenic acid to intermediate magmatism (Uatumã group) and affecting a large part of the Amazon craton between 1.85 and 1.7 Ga.     

粤西阳春中生代钾玄质侵入岩及其构造意义：Ⅱ．微量元素和Sr—Nd同位素地球

粤西阳春地区马山二长闪长岩强烈富集K、Sr和LREE，（^87Sr/^86Sr)i ＝0．7046，εNd(t)≈ 1；岗尾－轮水岩体较富集K、Rb、Th和LREE，（^87Sr/^86Sr)i＝ 0．7063，εNd(t)≈－2；石录岩体较富集Sr,K、Rb、Th和LREE相对较低，（^87Sr/^86Sr)i＝0．7084－0．7089，εNd(t)≈－6。马山岩体来源于大离子亲石元素（LILE）和LREE富集的交代地幔；岗尾－轮水岩体来自于放射成因Sr、Nd同位素组成略高或交代时间略早的富集交代地幔，并且经历了明显的结晶分异作用；石录岩体则很可能是前存下地壳底垫基性岩重熔形成的。从早侏罗世到早白垩世，南岭西部的岩浆成分和源区的规律性变化反映了区域软流圈地幔上涌和岩石圈伸展－拉张－减薄的演化过程。     

Petrology, geochemistry and geodynamic setting of Proterozoic igneous suites of the Orós fold belt (Borborema Province, Northeast Brazil)

The Orós belt is a metamorphosed and deformed supracrustal sequence whose deposition started at ca. 1.8 Ga. The volcanic rocks form an essentially bimodal association with a predominance of felsic volcanics. Mafic volcanics show geochemical and Nd isotopic differences which point to separate origins for each type. The mafic rocks are either chemically similar to EMORBs or are transalkaline types, enriched in LILE and LREE and relatively depleted in HFSE but with different isotopic signatures. One mafic type is associated with dominant andesites in a suite which could have evolved by an AFC process involving a Transamazonian-age source rock which later produced some of the overlying felsic volcanics. The rhyolites have variable geochemical signatures, all typical of anatectic products derived from continental crustal rocks. Sedimentary rocks are dominated by pelitic types of different provenances, accompanied by minor arenites and lesser carbonate rocks and calc-silicates. The depositional environment of the supracrustal sequence was continental, and a number of lines of evidence suggest that a rift environment probably developed during relaxation following the Transamazonian orogeny. The most abundant igneous rocks are felsic plutonics, emplaced nearly 100 Ma after the volcanic activity, but whose geochemical signature is similar to that of most of the rhyolites. These are cut by more alkaline anorogenic intrusives. The volcano-sedimentary sequence was intruded at ca. 0.9 Ga by a mafic-ultramafic sill. All deformation and metamorphism occurred during the Brasiliano orogeny, which was accompanied by the intrusion of syn-tectonic granites.The Orós belt, therefore, represents an intracontinental ensialic late Paleoproterozoic volcano-sedimentary basin initially related to strain relaxation of the crust. With subsequent collapse and development of faults, the anorogenic alkaline granites intruded at ca. 1.7 Ga. Based on the data available on this province, the Orós belt forms a part of a series of supracrustal belts of different ages, which were consolidated and welded in the Brasiliano orogeny.Within the Borborema Province, supracrustal sequences with ages of ≈ 2.0-1.9 Ga, ≈ 1.8-1.7 Ga (Orós), ≈ 1.1-1.0 Ga, and ≈ 0.6 Ga are presently known. All were aglutinated or amalgamated to neighboring blocks during the Brasiliano orogenic cycle.     

Crystal clots,amphibole fractionation and the evolution of calc-alkaline magmas

Petrographic and microprobe investigations of calc-alkaline (CA) rocks from the High Cascade Range (i.e., Mt. St. Helens, Mt. Jefferson, Crater Lake and Mt. Shasta) of western North America show that crystal clots represent primary igneous phase assemblages and are not products of amphibole reactions with melt. For each eruptive complex, crystal clots display diverse modal proportions even within a single eruptive unit. Nevertheless, in all cases the crystal-clot minerals are also represented in the rock as phenocrysts or microphenocrysts. Basalts contain clots of ol+plag+mgt, ol+mgt, cpx+ plag+mgt, cpx+mgt and plag+mgt; andesites, clots of cpx+mgt, opx+mgt, cpx+opx+plag+mgt, cpx+plag+mgt, opx+plag+mgt and plag±mgt; and dacites, clots of opx+mgt, cpx+opx+plag+ mgt, opx+plag+mgt, amph+plag+mgt±ilm, amph+mgt±ilm and plag±mgt. The bulk compositions of most of these clot assemblages could not have been derived from amphibole percursors. Although some amphiboles in dacitic rocks display a breakdown reaction of amph=plag+cpx+opx +mag, these mineral clusters, unlike those of clots, typically have a relict amphibole crystal outline and a fine-grained metamorphic texture. Plagioclase grains in the mineral clusters lack oscillatory zoning which is typical of crystal clot plagioclase grains. The euhedral to subhedral shapes of most clot minerals and the oscillatory zoning present in most clot plagioclase grains are not likely to have formed from the breakdown of amphibole. Crystal clots are also observed in Hawaiian and ocean floor basalts, although amphibole fractionation has not been proposed for those lavas. Magnetite fractionation may be the controlling process limiting iron enrichment in CA magmas rather than amphibole fractionation. Textural evidence indicates that magnetite is an early-forming phase in CA magmas. V, which is concentrated in magnetite, shows a strong decrease with increasing silica in many CA rocks, supporting a magnetite fractionation model.Hawaii Institute of Geophysics Contrib. No. 969     

Low- and high-alumina komatiites of Goiás, Central Brazil

Archean komatiites of Goiás, central Brazil, have experienced deformation and low-grade metamorphism, but several outcrops preserve primary volcanic features. Samples from less deformed komatiites of four out of five greenstone belts (Crixás, Guarinos, Pilar de Goiás, and Santa Rita) have been investigated for their geochemical properties. Komatiites from the Crixás greenstone belt have very low Al₂O₃/TiO₂, high CaO/Al₂O₃, and a hump-shaped rare earth element (REE) pattern. Those from the Guarinos and Pilar de Goiás belts have similar REE patterns, characterized by a slight enrichment in LREE coupled with almost flat HREE, but differ in their inter-incompatible element ratios. Compared with those from Pilar de Goiás and Guarinos, samples from the Santa Rita belt have fractionated REE patterns with LREE enrichment, as well as high Al₂O₃ contents, corresponding to Al-undepleted komatiites. Komatiites from Crixás have the lowest (La/Sm)_N, (La/Yb)_N, and Zr/Zr* ratios compared with their equivalents from the other belts, which suggests their source was relatively depleted in LREE and high field strength elements (HFSE), probably due to the retention of garnet in the residue. Komatiites from the Guarinos, Pilar de Goiás, and Santa Rita greenstone belts are enriched in incompatible elements, which can be attributed to either low-degree partial melting at high pressures or a source previously enriched in incompatible elements. Some of the studied komatiites belong to Al- and HREE-depleted and others to the Al- and HREE-undepleted types. The depleted komatiites probably derived by melting at depths greater than 200 km, the undepleted at less than 200 km. Therefore, the komatiites of the four belts may have been derived from either one single mantle plume with different melting depths or sources from distinct plumes.     

北山造山带大地构造相及构造演化

根据1:25万马鬃山幅区调填图资料,从造山带不同构造单元的火山-沉积建造、岩浆岩序列演化、变质变形特征及时空配位关系研究入手,应用大地构造相划分理论,根据北山多旋回复合造山带的特点,识别出15种大地构造相,并探讨了北山古生代构造格局与构造演化模式.     

Cenozoic tectonic events at the border of the Paraná Basin, São Paulo, Brazil

In the last decade, even in areas that had been considered tectonically stable, a great amount of Cenozoic, including the Quaternary period, structural data have been collected throughout Brazil. The main goal of this study is to describe the Cenozoic structures and tectonic evolution of an area that is located at the border of the Paraná Basin in the state of São Paulo.The research methods consisted of the analysis of: (1) brittle structure data, mainly conjugate fractures and fault slip data; (2) lineaments traced on air photos and TM Landsat and radar images; and (3) a second-order base surface map.The study area, during the Cenozoic, has been affected by five strike–slip tectonic events, which generated mainly strike–slip faults, and secondarily normal and reverse ones. The events were named, from the oldest to the youngest, E1-NE, E2-EW, E3-NW, E4-NS, and E5-NNE; and the maximum principal stresses σ1 strike approximately NE–SW, E–W, NW–SE, N–S, and NNE–SSW, respectively. Event E2-EW seems to have been contemporaneous with the deposition of the Rio Claro Formation, the most important Cenozoic deposit of probable Neogenic age, and also to have controlled the distribution of its deposits. Event E3-NW was the strongest one in the area, as is pointed out by structural data, and the maximum principal stress σ1 of event E5-NNE is partially concordant with the orientation of σH-max of well break-out data in the Paraná Basin, suggesting a Neotectonic activity for this event. Finally, discontinuities parallel and correlated to the directions of strike–slip faults of the Cenozoic events seem to have actively controlled the sculpturing of the relief in the study area.     

Contrasting origin of post-collisional high-K calc-alkaline and shoshonitic versus alkaline and peralkaline granitoids. The use of sliding normalization

Abundant high-K calc-alkaline (HKCA) magmatism appears to be post-collisional and often shifts to shoshonitic or alkaline–peralkaline compositions in the final stages of orogeny. The nature and the causes of this transition are studied on the basis of 308 major element and of 86 unpublished trace element (including REE) analyses of the Pan-African granitoids from the Tuareg shield (Adrar des Iforas, Mali and Aïr, Niger). This database covers a wide variety of magmas from subduction-related to intraplate-type including abundant HKCA batholiths. Literature data from geodynamically well-constrained cases are also included. In addition to a conventional geochemical approach of the studied magmatism, the sliding normalization method is proposed. This tool aims at comparing magmatic series: each studied rock is normalized to the interpolated composition of the reference series that has the same SiO₂ content as the sample. This method amplifies differences in sources and in fractionation processes and allows comparison of rocks from basic to acid composition. Two distinct juvenile sources are proposed: a previously enriched phlogopite-K richterite bearing lithospheric mantle or a lower juvenile crustal equivalent for HKCA-shoshonitic magmas, and a lowest lithospheric-upper asthenospheric OIB-type mantle for alkaline-peralkaline magmatism. The first source is melted only shortly after its generation when the lithosphere was still hot, which restricts HKCA magmatism mainly to post-collisional settings. The second asthenospheric/lowest lithosphere source is by definition close to its melting temperature and can generate magma ubiquitously both in space and time. The main melting triggers are lithospheric major structures which are not only operative in a post-collisional setting but also in other environments such as intraplate setting. Geochemistry thus gives indications about the nature of the source and on geotectonic settings. However, the latter is a second rank information, which is partly model-dependant. The post-collisional period differs from other settings by a propensity to generate large amounts of magma of various kinds, among which HKCA magmatism is volumetrically the most prominent.     

Neoproterozoic tectonic evolution of the Hongseong area, southwestern Gyeonggi Massif, South Korea; implication for the tectonic evolution of Northeast Asia

Two types of Neoproterozoic metabasites occur together with regionally intruded arc-related Neoproterozoic granitoids (ca. 850–830 Ma) in the Hongseong area, southwestern Gyeonggi Massif, South Korea, which is the extension of the Dabie–Sulu collision belt in China. The first type of metabasite (the Bibong and Baekdong metabasites) is a MORB-like back-arc basin basalt or gabbro formed at ca. 890–860 Ma. The Bibong and Baekdong metabasites may have formed during back-arc opening by diapiric upwelling of deep asthenospheric mantle which was metasomatized by large ion lithophile element (LILE) enriched melt or fluid derived from the subducted slab and/or subducted sediment beneath the arc axis. The second type of metabasite (the Gwangcheon metabasite) formed in a plume-related intra-continental rift setting at 763.5 ± 18.3 Ma and is geochemically similar to oceanic island basalt (OIB). These data indicate a transition in tectonic setting in the Hongseong area from arc to intra-continental rift between ca. 830 and 760 Ma. This transition is well correlated to the Neoproterozoic transition from arc to intra-continental rift tectonic setting at the margin of the Yangtze Craton and corresponds to the amalgamation and breakup of Rodinia Supercontinent.