Subalkaline Granitoids Research Articles

Первые данные по петрохимии и минералогии месторождения плитчатого камня Талатшинское-2 (Южный Урал)

The article provides a novel detailed petrographic, petrochemical and mineralogical description of the Talatshinskoye-2 slab stone deposit. Fergusonite was found in the rocks of the Uraltau anticlinorium for the first time. The correlation of the studied object with other deposits of slab stone was carried out. An increased content of alkalis was revealed, which is an atypical characteristic for shales of the eastern flank of the Uraltau anticlinorium. The contents of niobium and yttrium directly indicate that the protolith belongs to acidic alkaline formations. The first manifestations of rare-metal-rare-earth Y-Nb-Ta-mineralization were found in shales of the Uraltau anticlinorium. Based on a set of features, including petrochemical, geochemical, and mineralogical, it was concluded that subalkaline granitoids served as the protolith for the rocks of the deposit.

A complex history of extension, subduction and collision in west Gondwana: Clues from the Riacho do Pontal orogen, Borborema Province (NE Brazil)

The geodynamic understanding of the Riacho do Pontal Orogen, in the Borborema Province, NE-Brazil, has progressed substantially in the last decade, mainly from geochemical and isotopic studies. However, the role of the Neoproterozoic magmatism has not totally been unraveled and has left some crucial questions unanswered. In order to contribute to this debate, we present petrographic, geochemical and isotopic (U–Pb, Sm–Nd and Sr–Sr) data obtained for a set of granitoids and metavolcanic rocks from the Internal and Central domains of the Riacho do Pontal Orogen. The Neoproterozoic granitoids comprise a major alkaline pluton of the Serra da Aldeia suite; two syenitic plutons – Nova Olinda and Caboclo –, and high-K, sub-alkaline granitoids from the Betânia granite. The alkaline pluton from the Serra da Aldeia suite yielded a zircon U–Pb age of 578 ± 5 Ma, while the Nova Olinda and Caboclo syenites yielded ages of 613 ± 2 Ma and 620 ± 3 Ma, respectively. The Betânia granite, displaying a volcanic-arc geochemical signature, yielded the age of 627 ± 4 Ma. Tonian ages were also obtained between 956 ± 8 Ma and 965 ± 3 Ma for three unclassified orthogneisses from the Internal Domain of the Orogen. A sample of metarhyolite from the Paleoproterozoic basement yielded an age of 1631 ± 21 Ma. The isotopic data for the Nova Olinda and Caboclo syenites revealed εNd(620 Ma) values between −7.72 and −8.45 and TDM values from 1.76 to 1.97.87Sr/86Sr ratios of 0.71594 ± 2 and 0.74432 ± 1 suggest mixing of crustal and mantle materials during their evolution. In contrast, the Betânia granite presented a ratio of 0.70669 ± 1, reinforcing the correlation with mantle sources. The integrated analysis of the data indicate that the Betânia granite is part of an Ediacaran magmatic-arc system, with the Nova Olinda and Caboclo syenites characterized as products of the late-orogenic magmatism, while the alkaline rocks of the Serra da Aldeia suite represent the final of post-orogenic stages of one of the multiple collisions of the West Gondwana.

Late Mesozoic Adakite Granites in the Northern Framing of the Eastern Flank of the Mongol–Okhotsk Orogenic Belt

The examination of original and available petrochemical, geochemical, and isotope (Sm-Nd, Rb-Sr) data showed that subalkaline granitoids in the northern framing of the eastern flank of the Mongol–Okhotsk orogenic belt belong to the adakite series. The granitoids are characterized by high Sr/Y, Sr, Al2O3, and LREE, extremely low HREE concentrations, and the absence of negative Eu anomalies. The results obtained and the analysis of the geodynamic setting of their formation made it possible to assume that the granitoids were formed at a depth of more than 45 km through melting of garnet-bearing (20–50% garnet) rocks, which are likely represented by lower crustal Precambrian complexes widespread in the southern framing of the Siberian craton. The source of parental melts involved both mantle and crustal matter. This likely occurred in a subduction setting through melting of frontal or lateral parts of oceanic slab in subduction “windows”. The Late Jurassic–Early Cretaceous Chubachin adakite complex (149–138 Ma) was distinguished. Its formation preceded the initiation of suprasubduction differentiated calc-alkaline magmatism (140–122 Ma) in the northern framing of the eastern flank of the Mongol–Okhotsk orogenic belt.

Petrographic and isotopic studies in Kamoo area, northeast isfahan, Iran

The study area is located 5.5 km north-east of Kamo County and 30 km north-east of Meymeh, Isfahan. Kamo Index comprises an anticline with a northwest-southeast trend, the outcrop of which consists of siltstone-sandstone and Jurassic shale rocks that lie along a cretaceous unit. Based on field studies as well as petrographic and geochemical investigations, mineral assemblages and minerals in the area include chalcopyrite, pyrite, marcasite, and magnetite as primary ores and malachite, bornite, covellite, chalcocite, hematite, goethite, and hydrous iron oxides as secondary ores. The secondary ores have been formed by oxidation and biogenic processes. The primary textures are granular, disseminated, vein-shaped, and layered and the secondary textures are boxed and martitized, shear zones, replacements, and veins. Tectonic, metamorphic, and oxidation factors are involved in the formation of these textures. Due to tectonic activity in this area and the presence of faults, replacement, vein, and shear textures, which have developed after mineralization, are observed. Among alterations, hematite, propylitic, sericitic, limonitic, and kaolinitic are highly prevalent and they are important in the formation of iron deposits. The sub-alkaline granitoids of the region are related to the continental subduction-zone orogenic series. Studies on the decomposed iron samples from Kamo show that most of the samples are in the sub-alkaline series and the chemical and mineralogical composition of the magma derived from this series is calc-alkaline with a large amount of iron. According to mineralogical, lithological, and geochemical evidence and based on isotopic results, Kamo deposit is a skarn-type deposit.

Geochemistry of Nb and Та in magmatic rocks of the Ukrainian Shield

It was made an attempt to generalize geochemical data of Nb and Ta concentration in the most common igneous rocks of the Ukrainian Shield (USh). In the majority of widely distributed rocks of the USh (normal and subalkaline granitoids) the Nb and Ta concentration are similar to upper crust but lower the accepted Clark value for acidic rocks. In the more differentiated rapakivi granites concentrations of these elements reach or exceed the Clark’s concentrations (up to 35 ppm). Only highly differentiated alkaline rocks and alkaline feldspar granites (Perga, Kamiani mogyly, Ruska Poliana massifs) have high Nb concentrations (up to 800, 120 and 370 ppm, respectively). Medium rocks of the normal range are the least geochemically studied and typically are characterized low Nb and Ta concentrations. Almost all gabbroids as well as their metamorphosed analogues in the greenstone structures, are characterized by very low Nb (and Ta) concentration (two or more orders of magnitude) compared with Clark’s values for basic rocks (20 ppm Nb and 0.48 ppm Ta) according to A.P. Vinogradov. Against this background, increased of Nb and Ta concentration is observed in the main rocks of anorthosite-rapakivi-granite plutons. Howover even in these rocks concentration of these elements rarely reach or does not achieve the values which are typical for subalkaline and alkaline basalts of continental rifts. The regional heterogeneity in Nb and Ta distribution is observed in alkaline rocks of different composition: in the Azov Sea region these rocks are characterized by high concentrations, while in the western part of the USh the content of these elements is extremely low. The increased concentration of these elements is also revealed in kimberlites from the Azov Sea region and the Kyrovohrad megablock of the USh. The authors give some considerations and assumptions about the dependence on geochemical features of the Precambrian igneous rocks of the USh from the geodynamic conditions of their formation.

Geochemical evolution of the early Paleozoic collisional magmatism from autochthonous migmatites and granitoids to multiphase granite intrusions (Sharanur and Aya complexes, Baikal Region)

Overall petrologic and geochemical data indicate that the early Paleozoic magmatism in the Olkhon area of the Baikal Region exhibits diverse types of granitoids, whose time of formation is estimated at a narrow age interval of 500-465 Ma. This magmatism was responsible for the formation of both autochthonous gneiss-migmatite-granitoid suites (Sharanur complex) and multiphase intrusions (Aya complex) emplaced into the upper horizons of the continental crust. In major-element chemistry, K2O/Na2O values, and rare-element composition the migmatite-plagiogranites and calc-alkaline and subalkaline granitoids of the Sharanur complex are similar to the host gneisses and schists, as they were likely derived from melting of the ancient metamorphic substratum of the Olkhon series. In new isotope-geochemical characteristics (ICP MS method) the Sharanur granitoids are close to the first-phase biotite granites of the Aya massif, whose further geochemical evolution was governed mainly by intrachamber magmatic differentiation leading to the production of second-phase leucogranites enriched in HREE and HFSE (in particular, Ta and Nb) and depleted in Sr, Ba, Eu, Li, and LREE. The origin of the autochthonous and intrusive granitoids is related to early Paleozoic collision events within the Olkhon metamorphic terrane, while the formation of syncollisional granitoids is best explained by both melting of the crust protolith (Sharanur complex) and magmatic differentiation (multiphase Aya intrusion). All mineralogical and geochemical characteristics indicate that these granitoids are distinguished from rare-metal pegmatoid granites and Li-F and Rb-Be-Nb pegmatites, whose vein bodies crosscut the granitoids, and are regarded as middle Paleozoic rocks, which mark the transition to within-plate magmatism in the Baikal Region.

Late Paleozoic anorogenic magmatism in Southern Mongolia: Evolutionary stages and structural control

Geochronological studies of anorogenic magmatic complexes in the South Mongolian Hercinide have been carried out. Series of the massifs composed of alkaline and subalkaline granitoids with some monzonite formed during the interval of 318–316 Ma have been found. Taking into account previously obtained data, two groups of Late Paleozoic A-type granitoids different in age have been identified in the region: Late Carboniferous and Early Permian. These two occupy different structural positions. The location the early magmatic massifs is controlled by the extended Trans–Altai fault zone of northwest strike: the massifs cluster in areas of the fault intersection with boundaries of structural blocks formed by the pull-apart mechanism. The later Early Permian igneous complexes are associated with the Gobi–Tienshan rift zone of sublatitudinal strike.

Synchronous alkaline and subalkaline magmatism during the late Neoproterozoic–early Paleozoic Ross orogeny, Antarctica: Insights into magmatic sources and processes within a continental arc

Extensive exposure of intrusive igneous rocks along the Ross orogen of Antarctica—an ancient accretionary orogen on the margin of East Gondwana—provides an exceptional opportunity to study continental arc magmatism. There is significant petrologic and geochemical variability in igneous rocks within a ~500-km-long segment of the arc in southern Victoria Land. The conspicuous occurrence of carbonatite and alkaline silicate rocks (nepheline syenite, A-type granite, and alkaline mafic rocks) adjacent to large complexes of subalkaline granitoids is not adequately explained by traditional models for continental arc magmatism. Extensive geochemical analysis (>100 samples) and zircon U–Pb geochronology (n=70) confirms that alkaline and carbonatitic magmatism was partially contemporaneous with the emplacement of large subduction-related igneous complexes in adjacent areas. Major pulses of subalkaline magmatism were compositionally distinct and occurred at different times along the arc. Large bodies of subalkaline orthogneiss and granite (sensu lato) were emplaced over similar time intervals (ca. 25Myr) to the north (ca. 515–492Ma) and south (ca. 550–525Ma) of the alkaline magmatic province, although the initiation of these major pulses of magmatism was offset by ca. 35Myr. Alkaline and carbonatitic magmatism spanned at least ca. 550–509Ma, overlapping with voluminous subalkaline magmatism in adjacent areas. The most primitive rocks from each area have similarly enriched trace element compositions, indicating some common characteristics of the magma sources along the arc. The samples from the older subalkaline complex have invariably low Sr/Y ratios (<40), consistent with relatively shallow magma generation and differentiation. The younger subalkaline complex and subalkaline rocks within the area of the alkaline province extend to higher Sr/Y ratios (up to ~300), indicative of generation and differentiation at deeper levels. The significant spatial and temporal diversity in magmatism can be explained by a tectono-magmatic model that invokes alternating phases of extension and contraction in the overriding plate. The low Sr/Y magmatism of the earlier subalkaline complex and the largely contemporaneous alkaline and carbonatitic magmatism occurred during a phase of extension in the overriding plate ca. 550–525Ma. The differences in composition of this early phase of magmatism can be explained by variable lithospheric thickness along the margin, with the alkaline magmas derived from partial melts of thick metasomatized lithosphere. The younger subalkaline complex was emplaced during a phase of contraction in the overlying plate ca. 515–505Ma. The latest phase of (post-orogenic) magmatism occurred during extension in the overriding plate ca. 505–492Ma.

Geochemistry of Early Paleozoic granitoids of the Baikal region and their geodynamic setting exemplified by the Khamar-Daban Ridge and Olkhon Island

Comparative study of geological and isotope-geochemical features of the Early Paleozoic granitoids of the Khamar-Daban Ridge and Olkhon Island located in the Baikal region has revealed their close age and composition. Besides, they were referred to as syncollisional S-type formations derived from gneiss-schistose substratum of metamorphic sequences.Granitoids of the Solzan massif in the Khamar-Daban Ridge, as well as the Sharanur complex on Olkhon Island, occur in the autochthonous and allochthonous facies. They primarily consist of migmatites, plagiogranites, gneiss granites, and K–Na-granites. The magmatic rocks of the Sharanur complex include subalkaline granosyenites and quartz syenites spatially proximal to K–Na-granites. In the north of the island we investigated alkaline syenites which might be related to the Budun massif of basic rocks. On Olkhon Island in the Tashkiney valley, the surveyors recognized the geochemical type of pegmatoid rare-metal granites bearing beryllium mineralization. As was found, they are distinguished from Be-muscovite and spodumene pegmatites of the Khamar-Daban by high Rb, Cs, Sn, Nb, Ta, and W but low Li concentrations, which is probably due to Li-enrichment in the protolith of the Kornilova Formation relative to the Olkhon sequence. This points to the inheritance of the protolith composition at all stages of syncollisional granite formation.The geochemical study has shown similarity of calc-alkaline and subalkaline granitoids of the Khamar-Daban Ridge and Olkhon Island and their affinity in age and average composition of the regional continental crust. In addition, it has revealed the evidence for the existence of the Olkhon–Khamar-Daban block occurring as a single terrane in the Baikal region.

Early precambrian A-granitoids in the Aldan Shield and adjacent mobile belts: Sources and geodynamic environments

Systematized geological, geochronologic, geochemical, and Sm-Nd isotopic geochemical data obtained over the past decade on A-granitoids in the Aldan Shield and in adjacent mobile belts surrounding it in the south make it possible to identify the sources from which the rocks were derived and the geodynamic environments in which they were generated. The territory in question provides evidence of five episodes of Early Precambrian within-plate magmatism, including the derivation of A-granites: at 2.62, 2.40–2.52, 2.07, 1.87–1.88, and 1.70–1.74 Ga. Although all of the granitoids were derived within plates, the environments of their derivation were different: (i) postcollisional lithospheric extension at 2.64 and 1.87–1.88 Ga in an anorogenic environment and (ii) in relation to the activity of mantle plumes at 2.40–2.52, 2.07, 1.74–1.70 Ga. The postcollisional magmatism generated only potassic granitoids of the subalkaline type, whereas the anorogenic magmatic rocks comprise both subalkaline granitoids (of K series) and alkaline granites (of Na series), which are intensely fractionated and strongly enriched in incompatible elements. A-granitoids in the Aldan Shield and its surrounding folded structures were derived from mixed mantle-crustal sources. The sources of the subalkaline granitoids were dominated by the material of the continental lower crust, while the alkaline granitoids were derived from mantle sources. Thereby the mantle source material of the anorogenic granitoids consisted of an OIB-type component, and the postcollisional granitoids were derived from MORB and OIB sources.

Geochronological limits of subalkaline magmatism in the Ket-Kap–Yuna igneous province, Aldan Shield

Comparative U-Pb (zircon- and sphene-based, for the first time) and K-Ar (biotite-, amphibole, and whole rock-based) dating of monzonitoids and subalkaline granitoids of the Ket-Kap and Uchur volcanic-plutonic complexes (Ket-Kap-Yuna Igneous Province is one of the areas of tectonic-magmatic activation of the Aldan Shield), respectively, has been made. The new data, which are chiefly acquired by us for the first time, are close to each other and fall within the relatively narrow time interval of the mid-Early Cretaceous. The U-Pb sphene-based isochron datings indicate the formation time of the subalkali complexes at the Barremian-Aptian boundary. The values of SHRIMP age for zircons from the Uchur Complex subalkali diorites vary from 124.4 to 120.1 Ma; variations in U-Pb dates for monzonitoids of the Ket-Kap Complex fall in the narrower interval of 125.8–123.6 Ma. The extraordinary cases of highly polymodal distribution of SHRIMP-derived zircon-based dates are explained by the presence of detrital zircon crystals probably belonging to the protolith; this is interpreted by evidence for participation of the ancient continental crustal material in the formation of the source for the KKYuIP subalkali magmas. The range of the new K-Ar datings based on the whole rock composition of subalkali rocks and on the results of monomineral determinations is wider; however, it corresponds to the Early Cretaceous as well. This is significantly in contrast with the data given in the literature and geological survey reports. It was found for the first time that the duration of the subalkali magmatism manifestation in the KKYuIP is only several Ma and, according to the precise U-Pb dating, falls within the Aptian age of the Early Cretaceous epoch; this is sharply in contrast to the previously derived dates that referred to Early Jurassic-Cretaceous age of the magmatism. This time interval probably marks the main stage (relative quiet phase under strong compression) in the evolution of the setting of mutual sliding between the Siberian Craton and Amurian megablock, according to geodynamical reconstructions by A.I. Khanchuk and his colleagues.

Compositional variation in the chevkinite group: new data from igneous and metamorphic rocks

AbstractElectron microprobe analyses are presented of chevkinite-group minerals from Canada, USA, Guatemala, Norway, Scotland, Italy and India. The host rocks are metacarbonates, alkaline and subalkaline granitoids, quartz-bearing pegmatites, carbonatite and an inferred K-rich tuff. The analyses extend slightly the range of compositions in the chevkinite group, e.g. the most MgO-rich phases yet recorded, and we report two further examples where La is the dominant cation in theAsite. Patchily- zoned crystals from Virginia and Guatemala contain both perrierite and chevkinite compositions. The new and published analyses are used to review compositional variation in minerals of the perrierite subgroup, which can form in a wide range of host rock compositions and over a substantial pressure- temperature range. The dominant substitutions in the various cation sites and a generalized substitution scheme are described.

Distribuição e origem dos minerais detríticos pesados das areias praiais holocênicas do litoral norte do Rio Grande do Sul

This study discusses the primary sources of detrital minerals of the Holocenic beach sediments from southern Brazil, based mainly on their chemical composition. Five samples of beach sand were collected at each 40km along the northern littoral of southernmost Brazil. Trace elements were determined by LA-ICP-MS and major elements by microprobe and electron microscopy. Pyroxene compositions indicate the contribution of granulites and post-collisional basic rocks. High-grade metapelitic rocks and amphibolites are important sources suggested by staroulite, rutile, aluminium silicates, almandine, grossular, zoisite, Ca-rich tourmaline, and commingtonite, identified in all samples. A-type granitoids in the source area are indicated by zircons with high Nb and ETR contents, Nb/Ta ratios > 20, and amphiboles with Fe/(Fe+Mg) > 0.9. Peraluminous granitoids are indicated by Mn-rich garnet and Fe-rich tourmaline, whilst the occcurrence of shoshonitic and medium to high-K subalkaline granitoids is pointed out by magmatic epidote, titanite, calcic amphiboles with moderate Fe/(Fe+Mg) ratios and zircons with Th/U = 0.3–0.5 and Nb/Ta = 5 - 15. Magnesian amphiboles indicate ultramafic sources. Hydrothermal products are also indicated in the primary sources by Li-rich tourmaline, actinolite, and hydrothermal epidotes. The homogeneity of abundance and typology of detrital minerals in the northern littoral indicate a common source for the studied sediments. The northeastern part of the Pelotas Batholith, is the probable source region. This study confirms that the chemistry of detrital minerals, including trace elements in zircons, can yield detailed and accurate information about their primary source, and so be of high relevance for provenance studies.

Sources of Archean sanukitoids (High-Mg subalkaline granitoids) in the Karelian craton: Sm-Nd and Rb-Sr isotopic-geochemical evidence

The Sm-Nd systematics of sanukitoids with an age of 2715–2740 Ma in the Western, Eastern, and Central domains of the Karelian craton with various crustal evolutionary histories indicates that the mafic and acid rocks of the sanukitoid series were derived from two contrasting sources: enriched lithospheric mantle and lower crust. The basic sanukitoids of the Western domain were derived from the mantle enriched long before its melting [ɛNd(2715) = −0.48 ± 0.22]. The source of the acid magmas was the young juvenile crust of TTG composition [ɛNd(2715) increases to +1.2]. The mantle source of mafic sanukitoids in the Eastern domain was enriched shortly before melting [ɛNd(2740) = +1.58 ± 0.01], whereas the acid melts came from an ancient crustal source [ɛNd(2740) decreases to −3.0]. For sanukitoids in the Central domain, the time span between the enrichment of the mantle source and its melting was the shortest [ɛNd(2725) = +2.05 ± 0.15], and the contribution of the juvenile TTG crust was insignificant [ɛNd(2725) deceases to +1.7]. The variations in the isotope characteristics of the acid members of the sanukitoid series are consistent with the known age heterogeneity of the crust of the domains. The lateral isotopic-geochemical heterogeneity of the lithospheric mantle source of the sanukitoids is thought to have been related to its two-stage reworking (at 3.2 and 2.8–2.9 Ga) under the effect of TTG granitoids, which are regarded as the melting products of the subducted oceanic crust. The sanukitoids provide information on the geochemical structure of the Archean lithosphere, which is reflected in Archean crust-building processes. The Rb-Sr isotope system of the Neoarchean sanukitoids underwent transformations on the mineralogical scale and within small massifs in the course of at least two Paleoproterozoic tectono-thermal events. A trace of the event at ∼2.1 Ga is left in the Rb-Sr system of monomineralic fractions from a weakly deformed syenite of the sanukitoid series in the Central Domain. Later event (∼1.7 Ga) was recorded in the minerals of the Teloveis sanukitoid massif, which hosts a gold mesothermal deposit in the Western domain. Monomineralic fractions of muscovite and biotite from the wall-rock metasomatites and of plagioclase, microcline, and biotite from metasomatites away from the orebodies yield isochron ages of 1719 ± 60 and 1717 ± 27 Ma. This age of the metasomatic alterations of the Neoarchean sanukitoids is able to explain the broad and unsystematic variations in the Rb-Sr isotope-geochemical characteristics of these rocks. Our data on the Paleoproterozoic age of the mesothermal gold ore mineralization at the Teloveis deposit provide additional lines of evidence for the complex tectonic and metallogenic evolution of the Karelian GGT in the Early Precambrian.

Magma pulses in the Central Variscan Belt: episodic melt generation and emplacement during lithospheric thinning

ABSTRACTMagmatism in the central part of the Variscan orogenic belt in Europe has occurred in several distinct episodes. Each episode spanned a few million years and was characterized by a distinct geochemical signature and geodynamic setting: (i) between 340 and 330 Ma (Visean), high‐K diorites and granitoids within a zone extending parallel the orogen axis, possibly tracing a palaeosuture; (ii) at 310–307 Ma (Westphalian), calc‐alkaline granitoids in the Intra‐Alpine domain; (iii) at 304–295 Ma (Stephanian), subalkaline granitoids in Intra‐Alpine and Pennine domains; and finally (iv) at 280–270 Ma (Permian) granitoids in Pennine, Austroalpine and South‐Alpine domains. The generation and emplacement of granitoid melts in the Variscan crust are suggested to be short‐term pulses that are controlled by extensional processes in the lithosphere such as thermal erosion and relaxation of the lithospheric mantle, and decompressional melting.

Calc-alkaline arc granitoid activity in the Sierra Norte-Ambargasta Ranges, central Argentina

A petrological and geochemical study of an area of approximately 750 km 2 has been carried out on representative lithological units of the extensive Sierra Norte-Ambargasta batholith (SNAB), in the Sierra Norte-Ambargasta range of the easternmost Sierras Pampeanas system of central Argentina. This batholith has intruded a dominantly metasedimentary basement of Precambrian age, which is mainly composed of quartzofeldsphatic-biotite or sericite-chlorite schists, and minor quartz-arenites of local distribution. The most primitive rocks of the magmatic sequence are diorite to tonalite microgranular enclaves (ME), which are included in the granodiorite-monzogranite (GM) basement. A large differentiated dacite-rhyolite (DR) stock intruded the earlier granitoids. All of these units were later intruded by highly evolved subalkaline granitoids (HESG) composed of miarolitic monzogranites, granite porphyries and large aplite dykes, which emplacement was controlled by old regional structural corridors. Geochemical parameters show that these granitoids series were derived from crystal fractionation of calc-alkaline magmas of metaluminous to weakly peraluminous composition. Discriminatory diagrams for tectonomagmatic settings, and the petrological and geochemical signature of the studied lithologies indicate that most granitoids of the intrusive sequence are the result of I-Type calc-alkaline magmatism. These granitoids are typical representatives of a precollisional volcanic arc environment, spatially related to an active continental margin. A significant chemical gap along the fractional crystallization trend separates the most evolved silica-rich granitoids from the less evolved units. This gap is attributed to the absence of representative magmatism of the syncollisional stage within the orogenic cycle. Silica and K-feldspar rich-rocks of the HESG-group are the result of intrusions that occured after the main magmatic activity, in a late-to post-orogenic tectonic setting. The occurrence of molybdenum, copper and other base-metal sulfides, associated with alteration assemblages that are common in porphyry systems, are in line with I-Type dacitic-rhyolitic magmatism in a volcanic arc geotectonic regime. Barren quartz-seriate greisen alteration is characteristic of the late-stage evolution of miarolitic monzogranites of the HESG-group. Radiometric ages for the SNAB granitoids suggest Upper Precambrian to Lower Paleozoic magmatic activity. Calc-alkaline arc magmatic activity of this age in the Sierra Norte of Córdoba, the easternmost block of the Oriental Sierras Pampeanas, raises the important question whether to assign the main magmatic activity of this region to the Pampean or to the Famatinian orogenic cycles. Our data suggest that the main igneous basement of the Sierra Norte-Ambargasta batholith consist of precollisional granitoids related to an orogenic cycle that most likely overlaps the G1 magmatic cycle of the Pampean orogen.

Reedmergnerite and associated minerals from peralkaline pegmatite, Dara-i-Pioz, southern Tien Shan, Tajikistan

Reedmergnerite occurs in pegmatites associated with a Jurassic complex dominated by subalkaline granitoids. It forms aggregates up to several cm across in coarse quartz and microcline, and is extensively embayed by microveinlets of fine-granular quartz containing minor microcline and accessory albite, aegirine, eudialyte, pyrochlore, leucosphenite, leucophanite and zektzerite. The coarse microcline is cut by microveinlets of finer albite and microcline containing accessory hyalotekite and aegirine. Microcline is Or97-99 with 0.02 wt % B 2 O 3 ; albite is An0-1 with 0.005 wt % B 2 O 3 . Aegirine contains 83-98 mole % NaFeSi 2 O 6 , ≤ 5 % NaAlSi 2 O 6 , ≤ 10 % Ca(Mg,Mn)Si 2 O 6 and ≤ 14 % Na(Fe 2+ ,Mn) 0.5 Ti 0.5 Si 2 O 6 . Polylithionite composition is K 2 Li 3.9 Al 1.6 Ti 0.24 Fe 0.1 Si 4 O 10 F 4 . Reedmergnerite contains no impurities in amounts exceeding 0.05 wt % oxide. Average electron microprobe analyses of reedmergnerite from Utah (type) and Tajikistan give nearly stoichiometric NaBSi 3 O 8 , whereas individual analyses follow a trend whereby Na + B varies inversely with Si for Si = 2.91-3.07 per formula unit. Reedmergnerite had nearly ideal B-Si order at the 450-500 °C temperatures and P ≤ 2 kbar pressures estimated for the pegmatitic assemblage orthoclase + reedmergnerite + quartz. We suggest that this assemblage crystallized from a highly alkaline, sodic residual melt that evolved under conditions whereby B saturation was attained prior to vapor saturation

The Brasiliano São Francisco craton revisited (central Brazil)

The Brasiliano (∼600 Ma) belt of Paramirim divides the São Francisco craton (as defined by Almeida, 1967, 1969) into two parts: to the west is the São Francisco craton proper; to the east is the Salvador craton, forming an extension of the Congo craton. The basement of the Paramirim belt, of Archean and Early Proterozoic age, is intruded by subalkaline granitoids dated around 1700 Ma. The cover, known as the Espinhaço Supergroup, consists of felsic magmatic rocks, overlain by clastic sediments depoited in a continental rift during the time interval ∼1700–1100 Ma. The Espinhaço Supergroup was deformed during the Brasiliano orogeny, at about 600 Ma. It is characterized by shallow-dipping, submeridional, large-scale ductile shear zones with western vergence and mineral and/or extension lineation indicating tectonic transport from east to west. These ductile shear zones extend south into the outer units of the Brasiliano Araçuaí mobile belt and the Lower Proterozoic of the Quadrilatero Ferrifero. There are no structural data supporting the hypothesis of a Middle Proterozoic deformation of the Paramirim belt that should have been obliterated during the Brasiliano orogeny. However, a possible pre-Brasiliano deformation does not affect the question of the division of the Brasiliano-São Francisco craton.

Granitoid chemistry and associated mineralization in the French Variscan

The orogenic granitoids of the French Variscan are classified into three principal associations: aluminopotassic, subalkaline, and calc-alkaline. Each of these associations is characterized by the petrographical and geochemical composition of the members and the nature of the related mineralization.Aluminopotassic granitoids contain most of the observed Sn, W, and U mineralization and all the Be, Nb-Ta, and Li mineralization. They are associated with two-mica leucogranites, with a separate occurrence of Sn, W, Li, Be, Nb-Ta mineralization related to evolved sodic leucogranites and U mineralization related to potassic leucogranites.The Cu and Mo potential of the subalkaline granitoids is assessed by the occurrence of genetically related mineralized pegmatites and K feldspar veins and of postmagmatic veins and stockworks. Complex Sn-Cu and W-Mo mineralization occurs in areas and bodies characterized by the emplacement of both subalkaline and aluminopotassic granitoids. Evolved subalkaline granites and leucogranites contain U mineralization (occurrences, deposits) which can be enhanced by late aluminopotassic two-mica leucogranite intrusions.The calc-alkaline granitoids are conspicuously barren, except for some W skarn occurrences, a type of mineralization which is also observed around subalkaline and aluminopotassic intrusions. No Cu-Mo porphyry mineralization is observed in either calc-alkaline or subalkaline granitoids.The regional pattern of distribution of the granitoids and their related mineralization leads to the definition of three principal regions which differ by the volumic development of the various granitoid associations and the related mineralization. No clear magmatic or metalliferous polarity can be observed across the French Variscan. The observed pattern of distribution results from the ensialic evolution of the French Variscan orogeny and from the mainly crustal origin of the granitoids emplaced between 280 and 360 Ma in response to the postcollisional thrusting and shearing tectonics.Similar relationships between the mineralization and granitoid varieties are observed in the adjacent European segments of the French Variscan. Other classification schemes suffer some limitations, including being less discriminating, so that comparisons are not always useful.