Peraluminous Type Research Articles

Petrology and Petrochemistry of Basement Rocks in Ila Orangun Area, Southwestern Nigeria

From field studies, six (6) lithological units were identified to be common around the study area which includes quartzites, granites, granite gneiss, porphyritic granites, amphibolite and pegmatites. The results of petrographical analyses show the mineral assemblages of the various rock types in the area which include quartzites as containing quartz, biotite, muscovite and myrmekite. Granite gneiss contains quartz, biotite, orthoclase, myrmekite, microcline, nepheline, hornblende and plagioclase. Pegmatite contains quartz, myrmekite, plagioclase, hornblende, microcline and muscovites. Porphyritic granites were observed to contain quartz, biotite, microcline, orthoclase and tourmaline. Granite contains quartz, biotite, hornblende, plagioclase and orthoclase while Amphibolite schist contains quartz, biotite, hornblende and muscovites. For the purpose of this study twenty (20) pulverized rock samples were taken to the laboratory for geochemical analysis with their results used in the classification as well as suggest the geochemical attributes of the rocks. Geochemical results obtained and interpreted using various geochemical plots or discrimination plots all classified the rocks in the area as belonging to both the peralkaline metaluminous and peraluminous types. Results for the major oxides ratios produced for Na2O/K2O, Al2O3/Na2O + CaO + K2O and Na2O + CaO + K2O/Al2O3 show the excess of alumina, Al2O3 over the alkaline Na2O +CaO +K2O thus suggesting peraluminous rocks. While the excess of the alkali over the alumina suggests the peralkaline metaluminous rock type. The results of correlation coefficient show a perfect strong positive correlation which shows that they are of same geogenic sources while negative correlation coefficient values indicate a perfect weak negative correlation suggesting that they are of heterogeneous geogenic sources. The result of the factor analysis show the elemental association of the trace elements and heavy metals represented as variables in the statistical analysis. Hence, establishing the elemental association of Component 1 as consisting of Ag-As-Ba-Cd-Co, Component 2 as Cr-Cu-Ni, Component 3 as Pb-Rb, Component 4 as Zn-Fe-V and Component 5 as U-Mn. Component 1 suggest the possibilities of sulphide and gold mineralization. Component 2 suggesting pathfinder elements while Components 3, 4 and 5 suggest weathered or dispersed elements. Hence, the occurrence of heavy metallic minerals in the rocks will serve huge economic significance with focus on the pegmatites in the area.

Early Permian Adakites of the Nora–Sukhotino Terrane in the East of the Central Asian Orogenic Belt: Geochronological (U–Pb, LA-ICP-MS) and Geochemical Data

Abstract ––We present results of geochemical and geochronological (U–Pb, LA-ICP-MS) studies of plagiogranites of the Mt. Zmeinaya massif in the Nora–Sukhotino terrane. The studied rocks are of calc-alkaline series and peraluminous type. They formed in collision settings at the final stage of evolution of the Nora–Sukhotino terrane. The high Sr/Y ratios and low Y and Yb contents of the plagiogranites indicate that these are adakitic rocks. The trace element composition of the rocks suggests their high-pressure (>15 kbar) genesis through the melting of metabasites. The youngest zircon population from the plagiogranite has a weighted average concordant age of 295 ± 4 Ma, which corresponds to the early Permian. With regard to the zircon morphology, the obtained age can be considered the age of the plagiogranites.

Nb/Ta, Zr/Hf and REE fractionation in exotic pegmatite from the Keivy province, NW Russia, with implications for rare-metal mineralization in alkali feldspar granite systems

The White Tundra pegmatites are associated with the Keivy alkali feldspar granite complex in the Kola Peninsula, NW Russia. The host granites are aegirine-arfvedsonite, biotite-arfvedsonite and biotite-ferrohastingsite varieties, ranging compositionally from peralkaline through metaluminous to peraluminous. The pegmatites are remarkable for the diversity of the rare-metal mineralization. The inferred crystallization sequence of the main REE, HFSE and Ti minerals in the pegmatite is: zircon – fergusonite-(Y) – monazite-(Ce) – gadolinite-hingganite series – britholite-(Y) – astrophyllite – titanite – allanite-(Ce) –– chevkinite-(Ce) – ilmenite – kainosite-(Y) – REE carbonates. The pegmatites show unusual parageneses indicating a transition of the mineral associations from agpaitic to miaskitic in the same body (e.g. zirconosilicate → zircon; astrophyllite → titanite). A number of minerals have anomalous minor element contents. Astrophyllite and some titanites have low (even < 1) Nb/Ta and Zr/Hf ratios that contrast with the overall geochemical environment (ratios ≫ 1); titanite with very high Y and HREE contents; elevated Sn, W and V contents in some titanosilicates; allanite-(Ce) with very high Ti contents coupled with low (less than stoichiometric) values of Al, suggesting a Ti → Al substitution. Whole-rock compositions of the host granite show a steady decrease of Nb/Ta and Zr/Hf ratios in peralkaline through metaluminous to peraluminous types. Compositions of Ti minerals suggest that further fractionation of Nb-Ta, Zr-Hf and REE in the White Tundra exotic pegmatite and the crystallization of Ti minerals with anomalous compositions are due to a combination of several factors: assimilation by the primary magma (enriched in Zr, Nb, REE) of upper crustal lithologies enriched in Ta, Hf, LREE, W, Sn; consumption of significant amounts of Nb and Zr by early-crystallized pegmatitic fergusonite and zircon with Nb/Ta and Zr/Hf ≫ 1; transition of HFSE mineral assemblages in pegmatite from agpaitic to miaskitic with consequent lowering of Nb/Ta and Zr/Hf; crystallization of minerals in a low-temperature (hydrothermal) environment with changing F contents, that promoted a sporadic further decrease of Nb/Ta and Zr/Hf (to values < 1) and increase of Y + HREE. The implications of the fractionation mechanisms for ore-forming processes related to rare-metal granites are discussed.

Early Cretaceous A‐type Granites in the Dandong Area, NE China and its Geodynamical Implications

AbstractA‐type granites, as an important petrologic sign to build up environmental recognition, were mainly formed in extensional tectonic settings. A biotite syenogranite from the Fenghuangshan pluton in Dandong, Liaoning Province gave SHRIMP zircon U‐Pb ages of 122.5 ± 1.6 Ma, 124.9 ± 1.7 Ma and 126.9 ± 1.1 Ma. The monzogranites have SHRIMP zircon U‐Pb ages of 118.2 ± 1.6 Ma, 128.1 ± 1.7 Ma and 131.6 ± 1.9 Ma, giving an emplacement age for the pluton in the Early Cretaceous. SiO2 is 65.48–74.49 wt%, whereas that of K2O is 4.16–6.44 wt%, and that of Na2O is 2.99–4.70 wt%. They also contain 13.24–15.76 wt% of Al2O3, with an A/CNK ratio of 0.92–1.10, averaging 1.02. The alkalinity rate (AR) ranges from 2.68 to 5.12, and this range is within the AR of peraluminous type rocks. The granites of Fenghuangshan pluton are characterized by high contents of Na and K and low contents of Ca (thermophile element) and Mg, which are features of A‐type granites. The (Na2O + K2O) – Fe2O3∗ × 5 – (CaO + MgO) × 5 discrimination diagram also shows that Fenghuangshan pluton is an A2‐type granite. The above granite has zircon ∊Hf(t) values ranging from –17.06 to –9.09, with single‐stage Hf model ages (TDM) of 1141 Ma to 1498 Ma and two‐stage Hf crustal model ages (TDMC) of 1762 Ma to 2263 Ma. A comprehensive analysis suggests that the Fenghuangshan pluton might have been formed from the lithospheric‐plate sliding during the late stages of evolution of hot rift structures and might have been closely associated with the tectonic settings of Mesozoic Eurasia and the ancient Pacific Plate.

Isotopic and geochemical record of the active to passive margin transition in NW Iberia during the Cambrian-Ordovician: vestiges of a waning continental arc

Time-resolved geochemical data from the orthogneisses of the basal units of the Allochthonous complexes of NW Iberian Massif allow us to track the changes in composition of the magmas generated in a subduction-related setting that affects a continental margin, which evolves from the building of a volcanic arc to its dismantlement during a rifting episode. The basal units are interpreted as a peripheral section of the North African margin of Gondwana and record Neoproterozoic to Cambrian arc-related magmatism, followed by an episode of Cambro-Ordovician rifting. The intrusive rocks of the basal units, preserved as orthogneisses, can be grouped into two magmatic suites: calc-alkaline (c. 493–475 Ma) and alkaline to per-alkaline (c. 475–470 Ma). The calc-alkaline association has a chemical composition ranging from magnesian calcic metaluminous types to ferroan calc-alkalic and alkali-calcic peraluminous types. The moderately alkaline association and per-alkaline metagranites have ferroan alkalic and per-alkaline to metaluminous composition. The chemical characteristics of the metaigneous rocks from the basal units indicate that the older members of the calc-alkaline suite (tonalites and granodiorites) were generated in a continental arc environment, while the alkaline and per-alkaline rocks were formed in an intraplate extensional setting. The negative values in eNd (− 7.8 to − 1.5) of the calc-alkaline suite suggest a crustal influence in its generation. The eNd values in the alkaline orthogneisses range between − 2.0 and 2.2, and are positive in the per-alkaline group, suggesting a greater participation of a mantle component. The transition from the calc-alkaline suite to the alkaline suite is represented by alkali-granites, which are coeval with the intrusion of basic magmas, and likely derived from a mantle magma underplating that triggered the rifting episode after the cessation of continental arc magmatism during the Ordovician.

МАГМАТИЗМ И ЭПИТЕРМАЛЬНОЕ Ag-Au ОРУДЕНЕНИЕ КРАСНОЯРСКОГО РУДНОГО ПОЛЯ ГОРНОГО АЛТАЯ

Актуальность исследования определяется необходимостью освещения новых данных по петрологии, геохимии субвулканических пород и эпитермальному Ag-Au оруденению Красноярского рудного поля Горного Алтая. С субвулканическими штоками гранитоидов пространственно и парагенетически связаны различные типы оруденения: меди, кобальта, мышьяка, вольфрама и золота. Впервые для Горного Алтая исследован высоко-серный тип Ag-Au оруденения. Цель исследования: изучить петрологические, геохимические особенности пород и оруденения Красноярского месторождения с использованием экспериментальных диаграмм, позволяющих выявлять генетические проблемы и физико-химические особенности петро-рудогенезиса. Методы исследования включали изучение состава горных пород с применением лазерно-абляционного анализа методом индуктивно-связанной плазмы на спектрометре «ОРTIMA&amp;4300» с высокой чувствительностью определения элементов в Лаборатории Института минералогии, геохимии и кристаллохимии редких элементов (г. Москва). Изотопный состав серы и кислорода определён на масс-спектрометре МИ 1201В в той же лаборатории. Результаты. Главными рудогенерирующими объектами в Красноярском рудном поле являются субвулканические штоки гранитоидов среднего девона коргонского комплекса, которые относятся к пералюминиевому типу с варьирующими характеристиками по магнию и железу. В породах проявлен тетрадный эффект фракционирования РЗЭ W-типа. Концентрации Au и Ag в породах увеличиваются с уменьшением величины тетрадного эффекта фракционирования РЗЭ W-типа. Глубинный магматический очаг эволюционировал в сторону повышения его уровня с формированием резко кислотно-серных характеристик и формированием околорудных аргиллизитов. Оруденение Красноярского месторождения представлено серией сближенных кварцевых жил, с образованием своеобразного линейного штокверка. Руды сформированы в три стадии: существенно сульфидную, теллуридную и пострудную. Основные рудные минералы: пирит, галенит, сфалерит, тетраэдрит, теннантит, электрум, тетрадимит, редко – алтаит, гессит, энаргит, люцонит. Основные жильные минералы: кварц, кальцит, хлорит, сидерит, каолинит, монтмориллонит, иллит. Концентрации золота в рудах варьируют от 0,5 до 29,6 г/т, серебра – от 15 до 5000 г/т. Соотношение Ag/Au в рудах варьирует от 135 до 150 и позволяет относить Красноярское месторождение к Ag-Au типу. Температуры гомогенизации газово-жидких включений в кварце разных генераций от стадии к стадии снижались от 265 до 175 °С. Фугитивность серы снижалась от первой стадии (–10–11) ко второй (–15–16), f Te2 повышалась от (–15) до (–10) и f О2 от (–39,5) до (–37). Изотопы серы и кислорода свидетельствуют о магматогенном источнике серы и метеорном – кислорода.

Diversity of Mesozoic tin-bearing granites in the Nanling and adjacent regions, South China: Distinctive mineralogical patterns

The Nanling and adjacent regions of South China host a series of tin deposits related to Mesozoic granites with diverse petrological characteristics. The rocks are amphibole-bearing biotite granites, or (topaz-) albite-lepidolite (zinnwaldite) granites, and geochemically correspond to mealuminous and peraluminous types, respectively. Mineralogical studies demonstrate highly distinctive and critical patterns for each type of granites. In mealuminous tin granites amphibole, biotite and perthite are the typical rock-forming mineral association; titanite and magnetite are typical accessory minerals, indicating high f O2 magmatic conditions; cassiterite, biotite and titanite are the principal Sn-bearing minerals; and pure cassiterite has low trace-element contents. However, in peraluminous tin granites zinnwaldite-lepidolite, K-feldspar and albite are typical rock-forming minerals; topaz is a common accessory phase, indicative of high peraluminity of this type of granites; cassiterite is present as a uniquely important tin mineral, typically rich in Nb and Ta. Mineralogical distinction between the two types of tin granites is largely controlled by redox state, volatile content and differentiation of magmatic melts. In oxidized metaluminous granitic melts, Sn4+ is readily concentrated in Ti-bearing rock-forming and accessory minerals. Such Sn-bearing minerals are typical of oxidized tin granites, and are enriched in granites at the late fractionation stage. In relatively reduced peraluminous granitic melts, Sn2+ is not readily incorporated into rock-forming and accessory minerals, except for cassiterite at fractionation stage of granite magma, which serves as an indicator of tin mineralization associated with this type of granites. The nature of magma and the geochemical behavior of tin in the two types of granites thus result in the formation of different types of tin deposits. Metaluminous granites host disseminated tin mineralization, and are locally related to deposits of the chlorite quartz-vein, greisen, and skarn types. Greisen, skarn, and quartz-vein tin deposits can occur related to peraluminous granites, but disseminated mineralization of cassiterite is more typical.

Generation of felsic rocks of bimodal volcanic suites from thinned and rifted continental margins: Geochemical and Nd, Sr, Pb-isotopic evidence from Haida Gwaii, British Columbia, Canada

The compositionally bimodal volcanic rocks of the Eocene–Miocene Masset Formation from Queen Charlotte basin, Haida Gwaii, British Columbia, Canada, underlie an area greater than 5000km2 where their exposed sections are up to 1.6km thick. The suite of mafic and felsic rocks (dacites and rhyolites) that erupted closely spaced in time, in both submarine and subaerial conditions, was associated with significant crustal extension and thin continental crust (~19–24km thick), with volcanism persisting for ~35Ma (from 46 to 11Ma). Predominant mafic types (mafic:felsic ~2:1) are moderately enriched mid-ocean-ridge-like basalts that were derived by a partial melting of a heterogeneous spinel peridotite source. Felsic rocks are plagioclase-phyric, two pyroxene-bearing, mainly peraluminous types which have Nd, Pb and Sr isotopic compositions overlapping those of basalts including high positive ƐNd(t) values (up to >+6). The chondrite-normalized REE patterns show light REE enrichment but flat heavy REE along with a variable negative Eu anomaly. Mineralogy, major and trace elements, Nd–Sr–Pb isotopic data and model calculations using MELTS are consistent with a derivation of felsic rocks from the basalts by fractional crystallization. The intercalation of basaltic and felsic rocks suggests the existence of separate, simultaneously active plumbing and feeder systems and relatively stable magma chamber(s) to generate large volumes of differentiated felsic magmas by fractional crystallization. The Masset rocks provide an example for the generation of felsic magmas of bimodal volcanic suites during rifting along a thinned continental margin.

U-Pb geochronology and petrogenesis of intrusive rocks: Constraints on the mode of genesis and timing of Cu mineralization in SWSK area, Lut Block

The South-West Sorkh-Kuh (SWSK) area, part of the Tertiary volcanic-plutonic rocks in the Lut Block, eastern Iran, comprises volcanic rocks intruded by intrusive rocks of hornblende diorite and biotite quartz monzonite compositions. Biotite quartz monzonite has been intensively affected by quartz-sericite-pyrite, argillic and silicification alterations and hornblende diorite shows weak propylitic and carbonate alterations. Disseminated, stockwork and vein-type styles of mineralization are associated with intrusive rocks. The vein-type mineralization with NW-SE trend is hosted by hornblende diorite. The ore in vein-type mineralization consist of quartz, chalcopyrite, pyrite, and secondary Cu-Fe minerals. Disseminated and stockwork mineralizations consist of quartz, pyrite, chalcopyrite, and secondary minerals replacing biotite quartz monzonite and hornblende diorite intrusive rocks. The highest grades in terms of Cu (3%), As (1.3%), Mo (150ppm), Pb (362ppm), Zn (743ppm), and Au (278ppb) are associated with vein-type mineralization. Intrusive rocks causing mineralization, are of metaluminous to peraluminous types with calc-alkaline to high-K calc-alkaline affinity formed in a volcanic arc setting. The enrichment of LILE (Ba, K, Cs, Sr), depletion of HFSE (Nb, P, Ti) and enrichment of LREE relative to HREE indicate the formation of source magmas above a subduction zone. The results of zircon U–Pb dating give constraints on the timing of emplacement of biotite quartz monzonite and hornblende diorite bodies at 40.16±0.72 and 20.1±2.1Ma, respectively. The younger date extends the time span of mineralizations in the Lut Block from Middle Eocene to Early Oligocene (42–33Ma.) reported by previous researchers to more recent Early Miocene times. The initial 87Sr/86Sr ratios and εNd values of both biotite quartz monzonite and hornblende diorite reside in the restricted ranges of 0.7055–0.7066 and −0.33 to +1.88, respectively, consistent with an origin above a subduction zone followed by crustal contaminations of the initial magma compositions, especially with regard to biotite quartz monzonite. The zircon U-Pb ages as well as geochemical and radiogenic isotope data for the intrusive bodies associated with mineralization in this study are in accordance with previous studies on intrusive bodies related to porphyry copper deposits in the Lut Block. In contrast to previous studies, this study further extends the span of mineralization events to at least Early Miocene times.

Petrogenesis and tectonic implications of Permian post-collisional granitoids in the Chinese southwestern Tianshan, NW China

Permian porphyritic granite and leucogranite from the Kekesu and Muzhaerte Valleys in the southwestern (SW) Tianshan orogenic belt, NW China have been studied to decipher their petrogenesis and tectonic implications. For porphyritic granite in the Kekesu Valley, in situ LA-ICPMS zircon U–Pb dating yields crystallization ages of 295–291Ma. The granite is a high potassic calc-alkaline, slightly peraluminous type, enriched in large ion lithosphere elements (LILE) and light rare earth elements (LREE), but depleted in high field strength elements (HFSE). Zircon Hf isotopic analysis (zircon εHf(t) of −5.8 to −0.2, two-stage Hf model ages of 1323–1680Ma) and Ti-in-zircon thermometry, which yields crystallization temperatures of 744–749°C, indicate the parent magma was likely formed by partial melting of a Mesoproterozoic crustal source. By contrast, leucogranite in the Kekesu valley yields crystallization ages of 274–267Ma. It contains muscovite and garnet, has high silicon and potassium, and is strongly peraluminous. Multiple inherited zircon cores and low zircon crystallization temperatures (687–701°C), combined with negative zircon εHf(t) values (−7.0 to −4.0), indicate its parent magma was sourced from supracrustal metasedimentary rocks by muscovite–breakdown partial melting. In the Muzhaerte Valley, porphyritic granite has similar major and trace elements characteristics to the Kekesu porphyritic granite. However, its higher zircon εHf(t) values (−0.9 to +3.8) and corresponding lower two-stage Hf model ages (1070–1367Ma) indicate that the parent magma likely included an input from a more juvenile mantle source. Ti-in-zircon thermometry gives lower crystallization temperature of ∼705°C. The intrusive relationships between the Permian granitoids and Paleozoic arc plutons, and the LP-HT and (U)HP metamorphic belts, combined with geochronological studies, suggest that these Permian granitoids were generated in a post-collisional environment. It is suggested that the formation of the porphyritic granites in the Kekesu and Muzhaerte Valleys could be related to slab breakoff, while the leucogranite could be formed by hydrate-breakdown melting in an extensional tectonic regime in response to intra-continental adjustments after collision.

Contrasted crustal sources for peraluminous granites of the segmented Montes de Toledo Batholith (Iberian Variscan Belt)

The Variscan Montes de Toledo Batholith (MTB) is an E–W linear array of peraluminous granite plutons which is chemically segmented. The study is focused on the western segment of the MTB (W-MTB), mainly composed of granites with slightly lower CaO and higher P2O5 contents than associated eastern plutonic units and nearby S-type granites, giving them a more pronounced peraluminous nature. The chemical contrast is also observed in isotopic composition, especially in radiogenic Nd and Pb ratios. The W-MTB granites have higher initial eNd (–5.0 to –5.9) and lower 206Pb/204Pb and 208Pb/204Pb ratios than peraluminous types from the E-MTB segment. A mixed pelitic–greywackeous derivation from regional Neoproterozoic formations is suggested, whereas lower crustal and meta-igneous sources were involved in the origin of the easternmost MTB granites. The presence of igneous muscovite together with coexisting andalusite and sillimanite in some of the studied granites suggests that solidus was reached at 650–700 oC and depth corresponding to the pressure of 2–3 kbar.

Evolved granitic systems as a source of rare-element deposits: The Ponte Segade case (Galicia, NW Spain)

In the Ponte Segade area (Galicia, NW Spain) strongly differentiated granites, often associated with rare-element mineralization (Sn–Ta–Nb–Li–Be–Cs) that could be of economic interest, have recently been discovered. These granites appear in the northern sector of the Ollo de Sapo Antiform (Central Iberian Zone, Variscan Orogen). Three different muscovite-rich synkinematic and peraluminous types of leucogranite (leucogranites s.s., albite-rich leucogranites I and albite-rich leucogranites II) and two pegmatite types associated with the albite-rich leucogranites (zoned and banded) have been identified in the studied area. The geochemistry of whole rock leucogranites indicates an enrichment in incompatible elements (lithium, rubidium, beryllium, cesium and hafnium), Al2O3 and Na2O, and an impoverishment in barium, strontium, zirconium, cerium, yttrium and SiO2. Geochemical studies of zircon, muscovite, K-feldspar and tourmaline in the different types of granites and pegmatites indicate the grade of evolution of the granitic system. With differentiation of the system, the zircon is enriched in hafnium and uranium and is impoverished in zirconium. In muscovite and K-feldspar there is an increase in cesium and rubidium. The opposite behavior is observed with regards to the Mg, Fe and Ti contents. In the case of tourmaline, the increase in Li is the best indicator of the grade of evolution. By contrast, Fe and Mg decrease. The sequence of evolution of the granitic system obtained from the geochemical studies indicates that the leucogranites s.s. are the least differentiated, evolving gradually, in accordance with field relationships, to albite-rich leucogranites I. The albite-rich leucogranites II are the most evolved, but no direct relationship between them and leucogranites s.s. has been found. The banded pegmatites associated with the albite-rich leucogranites II are more differentiated than the zoned pegmatites associated with the albite-rich leucogranites I, but are the poorest in Sn, Ta and Nb.

A Geochemical Classification for Granitic Rocks

This geochemical classification of granitic rocks is based upon three variables. These are FeO/(FeO + MgO) = Fe-number [or FeOtot/(FeOtot + MgO) = Fe*], the modified alkali–lime index (MALI) (Na2O + K2O – CaO) and the aluminum saturation index (ASI) [Al/(Ca – 1·67P + Na + K)]. The Fe-number (or Fe*) distinguishes ferroan granitoids, which manifest strong iron enrichment, from magnesian granitoids, which do not. The ferroan and magnesian granitoids can further be classified into alkalic, alkali–calcic, calc-alkalic, and calcic on the basis of the MALI and subdivided on the basis of the ASI into peraluminous, metaluminous or peralkaline. Because alkalic rocks are not likely to be peraluminous and calcic and calc-alkalic rocks are not likely to be peralkaline, this classification leads to 16 possible groups of granitic rocks. In this classification most Cordilleran granitoids are magnesian and calc-alkalic or calcic; both metaluminous and peraluminous types are present. A-type granitoids are ferroan alkali–calcic, although some are ferroan alkalic. Most are metaluminous although some are peraluminous. Caledonian post-orogenic granites are predominantly magnesian alkali–calcic. Those with <70 wt % SiO2 are dominantly metaluminous, whereas more silica-rich varieties are commonly peraluminous. Peraluminous leucogranites may be either magnesian or ferroan and have a MALI that ranges from calcic to alkalic.

Coexisting monazite and allanite in peraluminous granitoids of the Tribeč Mountains, Western Carpathians

Monazite, a typical light rare-earth element (LREE) mineral of S-type granitoids in the Western Carpathians, was found in the peraluminous biotite granodiorite-tonalite in the Tribec Mountains commonly containing polymineralic inclusions. These inclusions are dominated by anhedral allanite, although allanite also occurs rarely as discrete grains not enclosed by monazite. The monazite studied here is relatively homogeneous and characterized by high Th contents with proportions of huttonite (ThSiO4) and brabantite [CaTh(PO4)2] up to 14.6 and 9.3%, respectively. The discrete allanite grains are highly aluminous with a composition consistent with the peraluminous type of host rock. However, allanite included in monazite is extremely variable in LREE, Al, Fe, and Mg contents. This variation is interpreted to result from entrapment of allanite (+ melt) in monazite before local equilibrium was attained. The change from allanite to monazite as the stable LREE-rich phase is related to an overall decrease in Ca concentration caused by the onset of plagioclase crystallization. The early precipitation of allanite was possible because of the high LREE concentrations in the melt. The crystallization temperature of allanite must have been higher than monazite saturation (>856–845 °C and 798–790 °C for two analyzed samples). The Zr saturation temperature based on zircon solubility and REE thermometry based on monazite solubility reflect an increase in temperature from the edge to the center of the pluton, which coincides with an increase in the huttonite content in monazite. The primary LREE assemblage is accompanied by small grains of late huttonite(?) replacing monazite and brabantite replacing allanite.

Petrogenesis of late Precambrian younger granites from southwest Sinai, Egypt.

The paper represents the field, petrographic and geochemical characteristics of late Precambrian granites of Wadi Baba-Wadi Seih area in southwestern Sinai. They are composed of biotite granites and leucogranites. The biotite granites are fluorite non-bearing, vary from monzo- to syenogranites, which cut by felsic dykes, and probably correspond to the phase II of Egyptian Younger Granites. The leucogranites are fluorite bearing, range from syenogranites to alkali feldspar granites, cut the phase II and felsic dykes and belong to the phase III granites. Petrographic and geochemical data reveal a peraluminous type and potassic-sodic character of the present younger granites. They were originated at 20->30 km depth as shallow- to moderate-level intrusives. They were mostly originated in intraplate environment with extensional regime. In general, the biotite granites have alkaline character, probably exhibit the features of S-type granite, and are derived by partial melting of the lower crustal materials. On the other hands, the leucogranites have strong alkaline and peralkaline charcters, largely similar to the A-type granite with high silica, alkalis and normative acmite, and were formed through partial melting of lower crustal materials followed by crystal fractionation of a fluorine-rich I-type magma.

Permo-Triassic leucorhyolitic ignimbrites at Sierra de Lihue Calel, La Pampa Province, Argentina

The Lihue-Calel ignimbritic plateau in central Argentina represents the easternmost extension of the Permo-Triassic Choiyoi rhyolitic province. The rocks of this pampean plateau are chiefly pyroclastic, with minor intrusives and lavas. Rhyolites are a high-silica, high-potassium, slightly peraluminous type depleted in compatible elements (Sr, Ba, Eu) and variably enriched in incompatible elements (Rb, Cs, U, Th, Y, Nb, Ta). REE patterns are flat (La/Yb = 0–10), showing conspicuous negative Eu anomalies. The geochemistry of the ignimbrites reflects two processes: (1) fractional crystallization leading to a very high degree of differentiation, and (2) the activity of a residual F-enriched fluid phase during the last cooling stages. In this respect, they resemble the topaz rhyolites of the western United States. In both cases, the silicic magmas were erupted in an extensional tectonic setting, subsequent to a previous compressive episode of crustal thickening and uplifting. Thus, the silicic rocks of Lihue-Calel exhibit particular geochemical features that distinguish them from subduction-related calc-alkaline types and from the peralkaline rocks of anorogenic intraplate regions.

“Kerguelen: Continental fragment or oceanic island?”: Petrology and isotopic geochemistry evidence

The Rallier du Baty peninsula in the southwestern part of the Kerguelen archipelago contains five ring complexes composed of rocks of syenitic and granitic affinities. These ring complexes are mutually intrusive and intrude into a series of basaltic flows of about 20–25 m.y. in age. Geologic studies show that these complexes progressively become younger to the north. The main rock types of the ring complexes consist of amphibole syenites and biotite syenites representing two magmatic suites, an peralkaline and an peraluminous type. The isotopic study of the southern syenite-granite ring complex of the Rallier du Baty peninsula is presented. A very precise chronology of the intrusions (A, outermost ring, to H, innermost ring) is given: outer gabbros : K − Ar age = 15 − 13 m . y . ring A : K − Ar r age = 12 m . y . ring B : Whole rock Rb − Sr age = 9.7 ± 0.2 m . y . ( 2 σ ) ring C : Whole rock Rb − Sr age = 8.6 ± 0.1 m . y . ( 2 σ ) ring D , E , F , G , H : Whole rock Rb − Sr age = 7.9 ± 0.2 m . y . ( 2 σ ) The close grouping of initial 87Sr/ 86Sr ratios (0.7057) indicates that different ring complexes have the same source regions. Furthermore, the time interval between production of melt and final crystallization can be estimated at several 10 5 years. As the age differences between ring complexes are about 1 m.y., the different liquids cannot be differentiates of the same magma. The initial ratios of basic plutonic and volcanic rocks from the archipelago range from 0.7041 to 0.7077 with strong grouping around 0.7057. When plotted in a 207Pb/ 204Pb vs. 206Pb/ 204Pb diagram, the lead isotope ratios of all igneous rocks from these islands form a linear array similar to the results from other oceanic islands. Strontium and lead isotope data for all Kerguelen rocks suggest that most of the basic plutonic and volcanic rocks as well as the syenite-granite plutons are probably derived from a similar source located in an undepleted mantle.