Peralkaline Granites Research Articles

Petrographic, Radiometric and Paleomagnetic Studies for Some Alkaline Rocks, South Nusab El Balgum Mass Complex, South Western Desert, Egypt

Nusab El Balgum mass complex represents one of the alkaline igneous activities in the south Western Desert of Egypt. Petrographic investigations defined some different rock types in south of the complex represented by alkaline volcanics (pyroclastics [rhyolitic crystal tuffs], spherulitic rhyolites, alkaline rhyolite dykes), sub-volcanic peralkaline granites and structurally controlled mylonitic volcanoclastics. These rocks recorded significant concentrations in terms of the two radioactive elements Th and eU, which displayed considerable spatial variations, especially within the peralkaline granites. The abundance of Th and eU is mainly related to favorable combination of structural and pos-magmatic hydrothermal conditions. Paleomagnetic results give well-defined stable remanent magnetization directions of reliable VGP positions, which are presented and discussed in the context of the African APWP. Rock types, magnetization directions and VGP positions with the corresponding ages are as follows: 1) Rhyolitic crystal tuffs; D/I = 340.0°/—19.4°, α95 = 6.8°; VGP Lat./Long. = 51.4°N/240.5°E, A95 = 5.9° (Late Triassic). 2) Spherulitic rhyolites; D/I = 346.7°/—6.6°, α95 = 3.5°; VGP Lat./Long. = 60.4°N/237.0°E, A95 = 3.0° (Late Triassic/Early Jurassic). 3) Alkaline rhyolite dykes; D/I = 341.3°/16.7°, α95 = 5.4°; VGP Lat./Long. = 67.0°N/262.8°E, A95 = 4.2<span style="fo

Apatite-supergroup minerals in UK Palaeogene granites: composition and relationship to host-rock composition

The Palaeogene granites of Scotland and Northern Ireland range mineralogically from biotite ± amphibole – through fayalite-hedenbergite– to riebeckite-bearing types and compositionally from metaluminous to peralkaline. Five members of the apatite supergroup of minerals are reported from the granites. Apatite is a relatively early-crystallizing phase in all rocks, whilst britholite crystallized during later magmatic stages in about half the samples, independent of granite type. There is a major composition gap between apatite and britholite, ranging from ~10 to 48 % of the britholite component. The dominant substitution mechanism was REE 3+ +Si 4+ = Ca 2+ +P 5+ , although the substitution Na + +REE 3+ = 2Ca 2+ occurred in the more peralkaline granites. Zonation is common in apatite and britholite and is ascribed to kinetic effects.

Late Ediacaran (605–580 Ma) post-collisional alkaline magmatism in the Arabian–Nubian Shield: A case study of Serbal ring-shaped intrusion, southern Sinai, Egypt

The Serbal pluton is a late Neoproterozoic (605–580Ma) post-collisional A-type granites in southern Sinai, Egypt (northernmost Arabian–Nubian Shield, ANS). It is characterized by discontinuous ring-shaped outcrops dislocated by later faulting. The pluton intrudes late Neoproterozoic metamorphic and high-K calc-alkaline rocks. The Serbal pluton mostly comprises an outer zone of alkali feldspar granite surrounding a core of peralkaline granite. Gradational and sharp contacts in the Serbal granites suggest that they were emplaced with a very short time interval, still before complete crystallization of the earlier batch. Serbal granites are highly evolved (75.98–78.52wt.% SiO2) and display the typical geochemical characteristics of post-collisional A-type granites, namely high SiO2, Na2O+K2O, FeO*/MgO, Ga/Al, Zr, Nb, Ga and Y and low CaO, MgO, Ba, and Sr. They are rich in REE and have extreme Eu-negative anomalies (Eu/Eu*=0.01–0.23). The chemical characteristics indicate that the peralkaline granite shares many features of granites with the tetrad REE effect. The Serbal pluton evolved through fractional crystallization of a parental magma derived through partial melting of a juvenile crustal protolith that had been extracted from a source having mantle geochemical and isotopic characteristics. The crystallization temperatures using Fe–Ti oxides of the Serbal granites point to their formation at high temperatures, up to 650–850°C at a shallow depth of emplacement (<10km).

Geochemistry and Petrogenesis of Acid Volcano-plutonic Rocks of the Siner Area, Siwana Ring Complex, Northwestern Peninsular India

Abstract Petrochemical studies on acid plutonic (granite, microgranite) and volcanic (rhyolite, trachyte) rocks occurring in the Siner area of the Siwana Ring Complex, Malani Igneous Suite have been carried out. These rocks are characterized by high concentrations of SiO2, Na2O, K2O, Zr, Nb, Y and REE (except Eu) but low in MgO, Fe2O3(t), CaO, Cr, Ni, Sr; indicating their A-type affinity. Field studies in conjunction with the geochemical characteristic indicate that the magmatism in the Siner area is generally represented by peralkaline suite of rocks which are formed due to rift tectonics. It is also suggested that these acidic rocks could have been derived by low degree partial melting of crustal material. Characteristics of certain pathfinder elements such as Rb, Ba, Sr, K, Zr, Nb, REE and the ratios of K/Rb, Zr/Rb, Ba/Rb along with the multi elemental primitive mantle normalized spidergrams suggest that the Siner peralkaline granites and microgranites have the potential for rare metal and rare earth mineralizations.

Mineral chemistry and genesis of Zr, Th, U, Nb, Pb, P, Ce and F enriched peralkaline granites of El-Sibai shear zone, Central Eastern Desert, Egypt

El-Sibai mineralized shear zone trending NNE-SSW is located at the northern segment of Gabal El-Sibai (500 m in length and 0.5 to 1.5 m in width). Rocks along the shear zone show different types of alterations such as hematization, kaolinitization, fluoritization, and silicification. These alterations are good traps for rare metals of thorite, ferrocolumbite, pyroclore, plumbopyroclore, fluorite, cerite-(Ce), zircon, Th-rich zircon, zirconolite (mixture of zircon & columbite), fluorapatite, titanite, and monazite minerals. The detailed mineralogical study of the El-Sibai shear zone revealed its enrichment in Th, Zr, Nb, Pb, U, F, P, LREE (Ce), especially concerning the hematization processes. The close correlation of ferruginated (hematitized) samples with high radioactivity is related to the high ability of iron oxides for adsorption of radioactive elements from their Solutions. The rare-metal minerals found in altered peralkaline granites (shear zone) are associated with hematitization, albititization, chloritization, fluoritization, and pyritization. Electron probe microanalysis (EPMA) provides an indication of a ränge of solid Solution between thorite and zircon, in which intermediate phases, such as Th-rich zircon were formed. These phases have higher sum of all cations per formula (2.0 to 2.09 atoms per formula unit, for 4 oxygen atoms) than that of ideal thorite and zircon. This is attributed to the presence of substantial amount of interstitial cations such as U, Y, Ca, and AI in these phases. Altered zircon enriched in Th and U (Th-rich zircon) preferentially involves coupled Substitution Ca2+ + (Th,U)4+ ↔ 2Zr4++ 2Si4+, implying that significant amount of U and Th may enter the Zr and Si position in zircon. Thorite and Th-rich zircon are related to hydrothermal fluid. Also the genesis of the studied zircon is related to metasomatic hydrothermal zircon (MHZ). The abundantly detected zircon, Th-rich zircon, Th-bearing minerals and fluorite of demonstrably hydrothermal origin can be attributed to the role of fluorine-rich fluids. Although Zr and Th are generally considered as highly immobile elements, yet the occurrence of zircon indicates that their significant concentrations can be transported under specified F-rich fluids. The hydrothermal origin could be accepted for the thorite, huttonite monazite, zircon, Th-rich zircon, ferrocolumbite, pyroclore, plumbopyrochlore, zirconolite, fluorite, cerite-(Ce), fluorapatite within the El-Sibai altered peralkaline granites (shear zone).

NACARENIOBSITE-(Ce) AND BRITHOLITE-(Ce) IN PERALKALINE GRANITES FROM THE MORRO REDONDO COMPLEX, GRACIOSA PROVINCE, SOUTHERN BRAZIL: OCCURRENCE AND COMPOSITIONAL DATA

Nacareniobsite-(Ce), Na 3 Ca 3 (REE)Nb(Si 2 O 7 ) 2 OF 3 , and britholite-(Ce), (Na,Ca,REE) 10 (Si,P) 6 O 24 (OH,F) 2 , formed during the late stages of crystallization of the most evolved peralkaline granites from the Morro Redondo Complex, Graciosa Province (Southern Brazil). Nacareniobsite-(Ce) compositions are very close to ideal, with Na 2 O and Nb 2 O 5 contents up to 11.1 and 15.5 wt.%, respectively, corresponding to an average formula of Na 3.0 (Ca 2.75 Sr 0.01 ) ∑2.76 (REE,Y) 1.16 (Nb 0.95 Ta 0.02 Ti 0.04 ) ∑1.01 Si 3.99 O 14 (O 1.11 F 2.89 ) ∑4 . Compared to the host rock composition, trace-element patterns for nacareniobsite-(Ce) indicate enrichment in REEs and Sr, and depletion in alkali metals and Zr. The available chemical data are in excellent agreement with a complete solid solution series between rinkite and nacareniobsite-(Ce), controlled by [M +4 M 2+ ] −1 [M 5+ Na + ] and, in part, [M 2+ ] −2 [REE 3+ Na + ] and [M +4 M 2+ ] −1 [REE 3+ ] 2 exchange vectors. Britholite-(Ce) compositions are amongst the most (REE,Y)-enriched reported to date, with REE + Y contents up to 73.6 wt.% oxide. REE and trace element distributions relative to the host rock exhibit some similarities with those observed for nacareniobsite-(Ce); however, Ba, Th, U, and Pb contents are relatively higher. The observed compositions agree very well with [PM 2+ F] −1 [Si(REE,Y)(OH)], [M 2+ ] −2 [Na(REE,Y)], and [M 2+ (Th,U)] −1 [(REE,Y)] 2 exchange vectors. Crystallization of both minerals was favored by the F-, Na-, and HFSE-enriched nature and moderate-to-strong peralkaline signature of the melt, at temperatures below 600 °C in a relatively reduced environment.

Bastnaesite from Kanigiri Granite, Prakasam District, Andhra Pradesh

Abstract For the first time we report bastnaesite and hydroxyl bastnaesite (lanthanum cerium fluoro-carbonate) from the Kanigiri granite. The host granitoids are of A-type and vary in composition from quartz syenites to peralkaline granites. Rare metal and rare earth-bearing minerals identified by X-ray diffraction (XRD) studies in Kanigiri granite are bastnaesite and hydroxyl bastnaesite, besides columbite-tantalite, monazite, fergusonite, thorite and euxenite. Petromineralogical studies have also revealed the presence of bastnaesite. The presence of bastnaesite in Kanigiri granite suggests that the host felsic rocks may also form a potential source for light rare earth mineral, bastnaesite, apart from the already known rare-metal minerals.

Preliminary Study on HFSE Mineralization in the Peralkaline Granites of Nusab El Balgum Area, South Western Desert, Egypt

Nusab El Balgum mass complex represents one of peralkaline volcanic activity phenomena in the south Western Desert of Egypt, which is typical for within-plate event, which formed in Mesozoic period. It consists of acidic volcanic (rhyolite and their pyroclastics) and sub-volcanic granitic rocks (incomplete ring, small stock and dyke of a peralkaline aphanites) as well as dykes (trachyte, bostonite, rhyodacite, rhyolite and porphyritic rhyolite) variable in thickness and the most of run in NNE-SSW trend. The peralkaline granitic rocks, especially those located at the southwestern part of this mass are characterized by extreme enrichments in HFSE (rare metals such as Zr, Nb, U and Th and REEs) which are the highest concentrations (e.g., >1% Zr, 0.5% Nb and 2.6% total REEs, Y up to 1%, eU up to 300 ppm and eTh up to 1100 ppm). The rare metal bearing minerals are thorite, uranothorite, autunite, amorphous secondary uranium, zircon and ferrocolumbite, while the REEs bearing minerals are bastnaesite, monazite and xenotime. The positive relations in all of the binary diagrams of Zr versus Nb, Y, eU and eTh, Nb versus Y, eU and eTh, Y versus eU and eTh in post magmatic intensely hematised peralkaline granites indicated that, this process is responsible for the enrichment in these HFSE. The chondrite-normalized pattern of high-altered peralkaline granites indicates: 1) higher LREEs enriched pattern (La/Gd = 11.34 and 12.25) means the alteration processes taking place under open system and these rocks evolved from magma of lithospheric rifting, 2) ΔCe 2O, and thus very low viscosity, despite its low temperature (ments, as indicated by strong negative Eu anomalies; and c) it had abundances of HFSE cations. Redistribution of elements took place by post magmatic hydrothermal solutions.

Rare Earth Mineralization in Igneous Rocks: Sources and Processes

Deposits of rare earth elements (REEs) in igneous rocks have played an instrumental role in meeting the growing industrial demand for these elements since the 1960s. Among the many different igneous rocks containing appreciable concentrations of REEs, carbonatites and peralkaline silicate rocks are the most important sources of these elements, both historically and for meeting the anticipated growth in REE demand. The contrasting geochemical and mineralogical characteristics of REE mineralization in carbonatites, peralkaline feldspathoid rocks, and peralkaline granites reflect different sources and evolutionary pathways of their parental magmas, as well as differences in the extent of postmagmatic reworking of primary REE minerals by hydrothermal fluids.

Diversity of Rhyacian granitoids in the basement of the Neoproterozoic-Early Cambrian Gurupi Belt, northern Brazil: Geochemistry, U–Pb zircon geochronology, and Nd isotope constraints on the Paleoproterozoic magmatic and crustal evolution

The Gurupi Belt, in northern Brazil, is a Neoproterozoic-Early Cambrian orogen developed at the south-southwestern margin of the São Luís-West African Craton. Several plutonic bodies are exposed as basement units of the Gurupi Belt and represent a variety of granitoid types emplaced at different ages and all show zircon inheritance and chemical and isotopic features that imply participation of reworked Archean to Paleoproterozoic crust in the magma genesis in clear contrast with the juvenile characteristics of the predominant magmatic unit of the neighboring São Luís cratonic fragment, the Tromaí Intrusive Suite of 2167–2147Ma. The weakly peraluminous, high-K, calc-alkaline biotite-bearing Cantão Granodiorite of 2163±4Ma has TDM model ages of 2.21–2.92Ga and ɛNd values of +2.7 to −7.1. The 2142±9Ma-old, weakly peraluminous, biotite- and muscovite-bearing Jonasa Granodiorite underwent metamorphism and deformation at 525Ma and shows TDM model ages of 2.14–2.40Ga, with ɛNd values of +3.9 to −0.2. Both are, probably, related to a continental arc setting. A relatively large event of strongly peraluminous granitic magmatism took place between 2116 and 2079Ma, and comprises the crust-derived, biotite- and muscovite-bearing granites and leucogranites of probable collisional setting belonging to the Japiim, Tamancuoca, Moça and Maria Suprema units. Despite broadly similar, these units show some chemical differences that are interpreted to result from mixtures of variable amounts of sedimentary and igneous sources of Archean to Paleoproterozoic ages. These granites show TDM model ages of 2.19–2.62Ga and the ɛNd values range from +3.9 to −0.2, and probably represent collision rocks. Associated in space and time with part of the strongly peraluminous granites, the 2100±21Ma-old, metaluminous to slightly peralkaline granites, quartz-monzonites and quartz-syenites of the Anelis Intrusive Suite have characteristics of potassic to shoshonitic post-collisional rocks. The suite shows TDM model ages of 2.27–2.48Ga and ɛNd values of +0.1 to −2.6. The last known event is represented by the post-orogenic, 2085±4Ma-old, weakly peraluminous, high-K, calc-alkaline, biotite-bearing Timbozal Monzogranite, with TDM model ages of 2.27–2.58Ga and ɛNd values of +1.1 to −2.8.The strongly peraluminous and different generations of potassic granitoids represent magmatic events with characteristics that may be related to both continental arc subduction and collision processes and indicate that the granitoid bodies represent disrupted fragments of a Rhyacian orogenic belt (continental arc and collision belt) that remains today as basement inliers within the Gurupi Belt in the margin of the São Luís-West African Craton. These sequences, together with the juvenile rocks of the São Luís cratonic fragment, are part of a protracted Rhyacian orogen that evolved from ∼2240Ma to ∼2050Ma, involving accretionary and collision processes. These sequences also correlate in time and several aspects of evolutionary processes with Eburnean-Birimian terranes of the West-African Craton and Trans-Amazonian belts of the Amazonian Craton.

Geochemistry and petrogenesis of Neoproterozoic A-type granites at Nakora in the Malani Igneous Suite, Western Rajasthan, India

The Nakora Ring Complex (NRC) (732 Ma) occurs as a part of Malani Igneous Suite (MIS) in the Western Rajasthan, India. This complex consists of three phases (volcanic, plutonic and dyke). Geochemically, the Nakora granites are peralkaline, metaluminous and slightly peraluminous. They display geochemical characteristics of A-type granites and distinct variation trends with increasing silica content. The peralkaline granites show higher concentrations of SiO2, total alkalies, TiO2, MgO, Ni, Rb, Sr, Y, Zr, Th, U, La, Ce, Nd, Eu and Yb and lower concentrations of Al2O3, total iron, Cu and Zn than metaluminous granites. AI content is ≥1 for peralkaline granites and <1 for peraluminous and metaluminous granites. Nakora peralkaline granites are plotted between 4 to 7 kb in pressure and are emplaced at greater depths (16–28 km and 480–840°C) as compared to metaluminous granites which indicate the high fluorine content in peralkaline granites. The primitive mantle normalized multi-element profiles suggest that Nakora granites (peralkaline, metaluminous and peraluminous) are characterized by low La, Sr and Eu and relatively less minima of Ba, Nb and Ti which suggests the aspects related to crustal origin for Nakora magma. The Nakora granites are characterized as A-type granites (Whalen et al., 1987) and correspond to the field of “Within Plate Granite” (Pearce et al., 1984). Geochemical, field and petrological data suggest that Nakora granites are the product of partial melting of rocks similar to Banded Gneiss from Kolar Schist Belt of India.

Micro-structural and compositional variations of hydrothermal epidote-group minerals from a peralkaline granite, Corupá Pluton, Graciosa Province, South Brazil, and their petrological implications

Epidote-group minerals, together with albite, quartz, fluorite, Al-poor and Fe-rich phyllosilicates, zircon, and minor oxides and sulphides, are typical hydrothermal phases in peralkaline alkali-feldspar granites from the Corupá Pluton, Graciosa Province, South Brazil. The epidote-group minerals occur as single crystals and as aggregates filling in rock interstices and miarolitic cavities. They display complex recurrent zoning patterns with an internal zone of ferriallanite-(Ce), followed by allanite-(Ce), then epidote-ferriepidote, and an external zone with allanite-(Ce), with sharp limits, as shown in BSE and X-ray images. REE patterns show decreasing fractionation degrees of LREE over HREE from ferriallanite to epidote. The most external allanite is enriched in MREE. LA-ICP-MS data indicate that ferriallanite is enriched (&gt;10-fold) in Ti, Sr and Ga, and depleted in Mg, Rb, Th and Zr relative to the host granite. Allanite has lower Ga and Mn and higher Zr, Nb and U contents as compared to ferriallanite, while epidote is enriched in Sr, U and depleted in Pb, Zr, Hf, Ti and Ga. The formation of these minerals is related to the variable concentrations of HFSE, Ca, Al, Fe and F in fluids remaining from magmatic crystallization, in an oxidizing environment, close to the HM buffer. L-MREE were in part released by the alteration of chevkinite, their main primary repository in the host rocks.

The petrogenesis of late Neoproterozoic mafic dyke-like intrusion in south Sinai, Egypt

New field, petrographical and geochemical studies are presented here for the late Neoproterozoic Rimm intrusion (∼15km long) exposed in the southern Sinai Peninsula, Egypt in the northernmost Arabian–Nubian Shield (ANS). Field relations indicate that the Rimm intrusion is younger than the surrounding metamorphic rocks and calc-alkaline syn-tectonic granodiorite and it was not affected by regional metamorphism. The anorogenic peralkaline granite of Gebel Serbal crosscuts the Rimm intrusion. The Rimm intrusion is made up of several consanguineous rock types with gradational contacts. It is composed chiefly of pyroxene–hornblende gabbro, hornblende gabbro and minor quartz diorite. The chemical composition of the mafic minerals indicated that the studied rocks derived from calc-alkaline mafic magma. Geochemically, the studied rocks are characterized by enrichment in LILE relative to HFSE and LREE relative to HREE [(Ce/Yb)N=4.50–6.36]. Quartz diorite display slightly concave HREE pattern and slightly negative Eu-anomaly [(Eu/Eu*)n=0.91] which may be the result of fractionation of amphibole and plagioclase from the source melt, respectively. The Rimm intrusion evolved from mafic mantle magma into different type rocks by fractional crystallization with minor crustal contamination. The initial magma corresponds to pyroxene–hornblende gabbro and the crystallization of hornblende was caused by slight H2O increase in magma after crystallization of near-liquidus clinopyroxene and Ca-rich plagioclase. Amphiboles geobarometer indicate that the gabbroic rocks of the Rimm intrusion crystallized at pressures between 4.8 and 6.4Kb, while quartz diorite crystallized at 1.3–2.1Kb. Crystallization temperatures range between 800 and 926°C for the gabbros and between 667 and 784°C for the quartz diorite. The Rimm intrusion represents a post-orogenic phase formed during the crustal thinning and extension of the Arabian–Nubian Shield.

Geochemistry and zircon U–Pb ages and Hf isotopic composition of Permian alkali granitoids of the Phan Si Pan zone in northwestern Vietnam

The late Permian granitoids exposed in the Phan Si Pan zone of northwestern Vietnam consist mainly of the Ye Yen Sun metaluminous granites and the Nam Xe–Tam Duong peralkaline granites. Laser ablation inductively coupled plasma mass spectrometry U–Pb zircon analysis reveals that both the granite suites were emplaced from 253Ma to 251Ma. They have a distinctive A-type geochemistry of high 10,000×Ga/Al ratios of 3.0–5.7 and are also characterized by elevated contents of high field strength elements, A/CNK values of 0.85–1.58, negative Eu-anomalies. Magmatic zircons from the granitoids exhibit positive initial ɛHf values ranging from 6.4 to 15.9 and yield single-stage depleted mantle Hf model ages of 257–663Ma. This Hf isotopic feature implies significant contribution of juvenile mantle material to the magmas of the spatially and temporally associated Ye Yen Sun metaluminous and Nam Xe–Tam Duong peralkaline granites.

Age constraints on Lower Paleozoic convection system: Magmatic events in the NW Iberian Gondwana margin

The basal units of the allochthonous complexes of NW Iberia are used to examine the Lower Paleozoic geodynamic evolution of the northern Gondwana margin. These units represent the most external continental margin and the sequence of major magmatic events that affected them has been dated. Isotopic dating and field data highlight the existence of two magmatic pulses, dated at 489±4Ma (granodiorites) and 474±3Ma (alkali-granites), and a slightly younger alkaline/peralkaline pulse, dated at ca. 470–475Ma (alkaline and peralkaline granites). Their framing into the regional background has allowed us to explore the major lithosphere-scale processes developed at the Gondwana periphery at that time, as well as to conceive a consistent model for the opening of the Rheic Ocean that reconciles the timing of sea opening and back-arc extension with the timing of intracontinental rifting. The sequence of events is framed in a Cambrian and Ordovician peri-Gondwanan subduction setting where we also explore how subduction may be linked to coeval intraplate magmatism far inboard of the arc–trench. This contribution discusses how such a scenario can be traced in basement areas through a modern analog perspective.

Lower precambrian of the Keivy Terrane, Northeastern Baltic Shield: A stratigraphic succession or a collage of tectonic sheets?

The Keivy Terrane in the northeastern Baltic Shield appreciably differs from the adjacent tectonic blocks. In the northwestern part of this terrane (the Serpovidny Range), an outlier of Paleoproterozoic supracrustal rocks called the Serpovidny structure is surrounded by Archean (?) Keivy high-alumina paraschists. As follows from structural and magnetic data, the Paleoproterozoic rocks are deformed into a tight sheath fold 8 × 2 km in size at the surface and 5 km in length along the sheath axis. Faults parallel to the boundaries of the layers and locally cutting them off at an acute angle are involved in folding as well. The outer boundaries of the Serpovidny structure are tectonic. This structure is complementary to a larger tectonic lens composed of the Keivy mica schists. It is concluded that all of the supracrustal rocks of the Serpovidny Range are in fact tectonic sheets and lenses deformed into sheath folds. The literature data show that kilometer-scale sheath folds occur throughout the Keivy paraschist belt and most likely were formed owing to thrusting of the Murmansk Craton onto the Keivy Terrane in the south-southwestern direction. Foliation and lineation related to thrusting have been established in the Archean silicic metavolcanics and peralkaline granites occupying the most part of the terrane. In contrast, the granitoids and gabbroanorthosites of the Archean basement, which form a block 90 × 20 km in the southwestern Keivy Terrane, were not affected by Paleoproterozoic deformation. In other words, a detached assembly of tectonic sheets composed of the upper and middle crustal rocks that underwent deformation at the initial stage of the Paleoproterozoic Lapland-Kola Orogeny and the Archean basement, which is free of this deformation, are distinguished. The depth of detachment is estimated at 20–25 km. The detachment of the upper and middle crust in the Keivy Terrane and its position in the structure of the Baltic Shield are consistent with a spatiotemporal succession that resulted in the formation of a Paleoproterozoic supercontinent and the Baltic Shield as its fragment. This succession began with the amalgamation and deformation of the Archean terranes in the northeast of the Baltic Shield during the Lapland-Kola Orogeny, the Keivy Terrane showing a record of the earliest reworking (1.97–1.93 Ga). The succession completed in the southern and southwestern parts of the shield (1.80 Ga) after the Svecofennian Orogeny, expressed in the accretion of island-arc terranes composed of Paleoproterozoic juvenile crust to the continent.

The Mineralogical Diversity of Alkaline Igneous Rocks: Critical Factors for the Transition from Miaskitic to Agpaitic Phase Assemblages

Geochemically, the large family of alkaline plutonic rocks (both Qtz-undersaturated and -oversaturated compositions) can be subdivided into metaluminous [(Na 2 O + K 2 O) < Al 2 O 3 ] and peralkaline [(Na 2 O + K 2 O) > Al 2 O 3 ] types. In this paper, we discuss two important aspects of the mineralogical evolution of such rocks. With respect to their Fe–Mg phases, a major mineralogical transition observed is the precipitation of arfvedsonite or aegirine instead of fayalite or magnetite (± ilmenite). The relative stability of these phases is controlled by oxygen fugacity and Na activity in the crystallizing melts. If Na activity in the melt is high enough, arfvedsonite + aegirine form a common assemblage in peralkaline rocks under both reduced and oxidized conditions. Major mineralogical differences within this rock group exist with respect to their high field strength element (HFSE)-rich minerals: most syenitic rocks, known as miaskites, contain zircon, titanite or ilmenite as HFSE-rich minerals, whereas in agpaites complex Na–K–Ca–(Ti, Zr) silicates incorporate the HFSE. Similarly, only a small group of peralkaline granites are found to lack zircon, titanite or ilmenite but instead contain Na–K–Ca–(Ti, Zr) silicates. Here, we present a detailed phase petrological analysis of the chemical parameters (µNa 2 O, µCaO, µK 2 O) that influence the transition from miaskitic to agpaitic rocks. Based on the occurrence of Ti and Zr minerals, several transitional mineral assemblages are identified and two major evolution trends for agpaites are distinguished: a high-Ca trend, which is exemplified by the alkaline rocks of the Kola Province, Russia, and a Ca-depletion trend, which is displayed by the alkaline rocks of the Gardar Province, South Greenland. Both trends show significant Na-enrichment during magmatic evolution. High-Ca agpaites evolve from nephelinitic parental melts that did not crystallize large amounts of plagioclase. In contrast, agpaites showing Ca-depletion originate by extensive fractionation of plagioclase from basaltic parental melts. In some peralkaline granites evolutionary trends are observed that culminate in agpaite-like HFSE-mineral associations in the most evolved rocks.

The Geshere syenite-peralkaline granite pluton: a key to understanding the anorogenic Nigerian Younger Granites and analogues elsewhere

Periodico di Mineralogia (2011), 80, 1 (Special Issue), 199-215 - DOI: 10.2451/2011PM0016 Special Issue in memory of Sergio Lucchesi The Geshere syenite-peralkaline granite pluton: a key to understanding the anorogenic Nigerian Younger Granites and analogues elsewhere Shehu Suleiman Magaji 1 , Robert F. Martin 2,* , Echefu C. Ike 1 and Awajiokan E. Ikpokonte 1 1 Department of Geology, Ahmadu Bello University, Zaria, Kaduna State, Nigeria 2 Department of Earth and Planetary Sciences, McGill University, Montreal, Quebec H1A 1A7, Canada *Corresponding author: robert.martin@mcgill.ca Abstract The Geshere complex, one of the few intrusive centers among the Nigerian Younger Granites that has not yet been characterized, contains an association of syenite and peralkaline granite. The suite shows a classic sequence of rocks related by fractional crystallization of a parental magma, leading to the pseudoternary minimum in the granite system. The syenite is hypersolvus and contains a ferro-richteritic amphibole as the dominant femic mineral, whereas the most evolved granite is subsolvus and contains annite as the dominant femic mineral. Textural evidence points to the crystallization of these minerals from an interstitial femic melt present at the solidus of these rocks, in which magnesium is a trace constituent. Such a late melt appears because of the low thermal stability of these extremely Fe-enriched minerals and the massive removal of alkali feldspar. Calcite inclusions in the primary amphibole and annite, locally defining an emulsion texture, are considered to represent globules of a carbonate melt present at the source. Key words : syenite; peralkaline granite; anorogenic magmatism; Nigerian Younger Granites; ferro-richterite; annite; calcite globules; Geshere; Nigeria.

Note on the stable isotope values of vein calcite in the El-Seboah peralkaline granite (SW Egypt)

Note on the stable isotope values of vein calcite in the El-Seboah peralkaline granite (SW Egypt)Stable isotope data for carbon (δ13C) and oxygen (δ18O) are used to constrain the environments of calcite formation in two veins in the El-Seboah peralkaline granite in south-western Egypt. Vein I with calcite-magnetite-goethite-hematite-quartz, and vein II with calcite-magnetite-goethite-kaolinite-hematite-quartz are texturally distinct. The calcite of each vein has characteristic δ13C- and δ18O-values: + 0.32 and -7.28‰ for vein I and + 1.16 and - 1.21‰ for vein II, respectively. The observed differences between the δ13C values of the two veins indicate that they represent two separate systems of primary dissolved inorganic carbon formed at or near equilibrium with atmospheric CO2. The δ18O values, on the other hand, indicate calcite deposition from meteoric waters which were fresh for vein I and brackish for vein II.

Major- and trace-element composition of REE-rich turkestanite from peralkaline granites of the Morro Redondo Complex, Graciosa Province, south Brazil

AbstractTurkestanite, a rare Th- and REE-bearing cyclosilicate in the ekanite–steacyite group was found in evolved peralkaline granitesfrom the Morro Redondo Complex, south Brazil. It occurswith quartz, alkali feldspar and an unnamed Y-bearing silicate. Electron microprobe analysis indicates relatively homogeneous compositions with maximum ThO2, Na2O and K2O contentsof 22.4%, 2.93% and 3.15 wt.%, respectively, and significant REE2O3 abundances(5.21 to 11.04 wt.%). The REE patterns show enrichment of LREE over HREE, a strong negative Eu anomaly and positive Ce anomaly, the latter in the most transformed crystals. Laser ablation inductively coupled plasma mass spectrometry trace element patterns display considerable depletions in Nb, Zr, Hf, Ti and Li relative to whole-rock sample compositions. Observed compositional variations suggest the influence of coupled substitution mechanisms involving steacyite, a Na-dominant analogue of turkestanite, iraqite, a REE-bearing end-member in the ekanite–steacyite group, ekanite and some theoretical end-members. Turkestanite crystals were interpreted as having precipitated during post-magmatic stages in the presence of residual HFSE-rich fluidscarrying Ca, the circulation of which wasenhanced by deformational events.