Syenite Bodies Research Articles

Britholite-group minerals as sensitive indicators of changing fluid composition during pegmatite formation: evidence from the Keivy alkaline province, Kola peninsula, NW Russia

The Keivy alkaline province, Kola Peninsula, NW Russia, consists of vast alkali granite massifs and several dike-like nepheline syenite bodies. It contains numerous rare-metal occurrences, formed by a complex sequence of magmatic, late-magmatic and post-magmatic (including pegmatitic) processes. The Sakharjok nepheline syenite pegmatite contains a remarkably diverse number of britholite group minerals, pointing to different physico-chemical conditions in the fluid. REE and actinides distribution in the host rock indicates that the late-magmatic (and pegmatitic) fluids were alkaline, with significant amounts of F and CO2. From REE and F variations of the britholite group minerals possible fluid compositions at different stages are suggested. The earliest fluorbritholite-(Ce) formed locally from a late magmatic, high temperature F-rich fluid. Fluorbritholite-(Y) presumably crystallized from a F-bearing and CO2-rich fluid; marked F saturation resulted in precipitation of abundant fluorite due to a temperature drop. Variations in REE and F contents in the most abundant fluorcalciobritholite indicate a successive decrease of F in the fluid during its evolution. The relationship between intergrown fluorapatite and fluorcalciobritholite and the presence of zones with a REE-rich fluorapatite between them indicate a continuous to sudden crystallization in this mineral sequence. The сrystallization of the latest “calciobritholite” is related to the input into the fluid of CO2 and/or H2O.

Spatial Confirmation of Major Lineament and Groundwater Exploration using Ground Magnetic Method near Mecheri Village, Salem District of Tamil Nadu, India

Geophysical methods are widely used in various applications, especially to know about the subsurface features of the Earth. In the present study, the ground magnetic method has been done to map the NE-SW trending major fault traversing Mecheri block, Salem district for the spatial correlation study and also to locate possible groundwater potential zones with in the study area. The instrument used is Proton Precession Magnetometer-600 which produces a weak magnetic field that is picked up by an inductor, amplified electronically, and fed to a digital frequency counter whose output is typically scaled and displayed directly as field strength. The survey is done across the major fault marked by the GSI geologist with 100 m spacing in profile direction and 30 m sample spacing along the profile and for each profile line, 14 to 15 samples have been collected along with coordinates, time and magnetic value with all the necessary precautions. Then the data is processed and diurnal corrections were made for the interpretation using geophysical software. After the necessary corrections, profiles, contours and maps were generated for quantitative and qualitative analysis which includes magnetic contour map, total magnetic intensity, reduction to pole, analytical signal, upward and downward continuation, horizontal and vertical derivative, and radially average power spectrum. Based on the visual interpretation and interpreter’s knowledge, it was identified that the major NE-SW trending magnetic break is present at the southeastern corner of the map which is spatially correlated with the major fault marked by the GSI geologist. Apart from that there are two magnetic highs were notice in the southwestern part of the map which is mainly due to presence of isolated granite and syenite bodies. A small another magnetic break in the E-W direction has also been noticed. Intersection point of the NE-SW and NW-SE fault zones are favorable zone for groundwater potential zone. Other than this, the magnetic anomaly depth has been inferred from the radially average spectrum method shows the anomaly at 11 m, 21 m and 51 m depth.

Petrology of Intrusive Bodies of Azizabad-Tinamo Area in the South of Songour, Iran

In terms of structural divisions of the Iran’s earth crust, the studied area is considered as a part of the Sanandaj-Sirjan structural zone and Zagros Thrust. In this area, there are two gabbroic-dioritic and syenitic igneous intrusive bodies. The host rocks of these intrusive bodies are upper Cretaceous limestones and formed by contactmetamorphism at the contact zone between hornfels rocks and skarn. Field and geochemical datasets represent that all various rocks within these intrusive bodies have been originated from the same magma. The process of fractional crystallization and probably a small amount of contamination and assimilation has also played a role in the evolution of parent magma. Generally, these magmas are sub-alkaline and lie in themagmatic range of tholeiitic and calc-alkaline series. Samples investigated are metaluminous and peralkaline.

A trachyte–syenite core within a basaltic nest: filtering of primitive injections by a multi-stage magma plumbing system (Oki-Dōzen, south-west Japan)

Oki-Dōzen (Japan) is a Late Miocene (7–5 Ma) intraplate alkalic volcano composed of a central trachytic pyroclastic complex surrounded by a ring-shaped succession of basaltic to trachybasaltic lavas and pyroclastic rocks and dispersed trachytic bodies. The central trachytic complex is in contact with a syenite that was intruded into the basement early Miocene volcano-sedimentary succession. In the centre of the system there are no alkalic basaltic rocks that are correlative of the outer flank ring. We present whole-rock major and trace element chemistry, Sr–Nd–Pb isotopic compositions and petrological data from the central trachytic volcanic complex and the intrusive syenite body, as well as from the outer ring basaltic succession. We also present and discuss a new set of zircon U/Pb ages collected from the central trachyte and syenite bodies. All the eruptive products of Oki-Dōzen, as well as the syenite, plot on a single liquid line of descent initiated from a mantle-derived alkalic basaltic parent. A younger (2.8 Ma) basaltic eruption (Uzuka basalt) has isotopic compositions that distinguish it from the rest of the system. Geochemical modelling indicates that magmatic differentiation through crystal fractionation and minor crustal assimilation occurred in crustal and shallow sub-volcanic magma reservoirs. In the central part of the system, a number of vertically spaced reservoirs acted as a filter, capturing basaltic dykes and hindering their ascent. In the outer region, dykes either reached the surface unhindered and erupted to form the basaltic/trachybasaltic succession or stalled at crustal levels and differentiated to trachyte before forming dispersed domes/flows. The central plumbing system “filter” resulted in a nest-shaped volcano, with a trachytic core surrounded by basaltic products, and stopped direct injection of basaltic magmas into the shallow syenitic magma reservoir, likely preventing its destabilization and explosive eruption.

Wadeite (K2ZrSi3O9), an alkali-zirconosilicate from the Saima agpaitic rocks in northeastern China: Its origin and response to multi-stage activities of alkaline fluids

The Triassic Saima alkaline complex in the Liaodong Peninsula of northeastern China covers an area of about 20km2 and is dominated by nepheline syenite, with phonolite at its center, and a concealed body of eudialyte-bearing nepheline syenite in the northwest of the complex. The phonolite has similar features to miaskite, while the nepheline syenites are classified in the agpaitic group according to their mineral assemblage, and the alkalinity and aluminum saturation indexes. Zircon is the dominant Zr-bearing mineral in the phonolite, whereas wadeite occurs as the only primary Zr-bearing mineral in the nepheline syenites. The transitional crystallization from zircon to wadeite reveals an increase in alkalis and a high K/Na ratio as the magmas evolved from the volcanic to the intrusive stage. The primary wadeite grains underwent varying degrees of hydrothermal alteration. Overall, the areas of weak, medium, and strong alteration are characterized by the following respective associations: (1) wadeite+secondary catapleiite/gaidonnayite, (2) wadeite+secondary catapleiite/gaidonnayite+zircon, and (3) pseudomorphs after wadeite. The pseudomorphs are widespread and mainly consist of residual wadeite, secondary zircon, catapleiite/gaidonnayite, K-feldspar, calcite, and some Zr-bearing titanite and vesuvianite. All of the secondary zircon grains in the three associations are typically enriched in Ca and Al compared with the primary Ca-free zircons of the phonolite. The progressive alteration of wadeite suggests that the Saima complex underwent multiple episodes of fluid activity during a hydrothermal stage, including an initial Na-metasomatism via alkaline fluids, then stages most likely involving progressively CO2-rich fluids, and an intensive episode involving a mixture of these fluids with externally derived Ca-rich fluids.

Zircon U–Pb dating, geochemistry and Sr–Nd–Pb–Hf isotopes of the Wajilitag alkali mafic dikes, and associated diorite and syenitic rocks: Implications for magmatic evolution of the Tarim large igneous province

Abstract The Early Permian Tarim large igneous province (Tarim LIP) consists mainly of basaltic lavas, mafic–ultramafic intrusions including dikes and, syenite bodies in the Tarim Basin, NW China. A major unit of the Tarim LIP, the Wajilitag intrusive complex, consists of olivine pyroxenite, clinopyroxenite and gabbro units (from bottom to top), diorite and syenite rocks occurred in the upper part of the complex and alkali mafic dikes intrude the clinopyroxenite phase. Here we report the zircon U–Pb age and Hf isotopes, geochemical characteristics and Sr–Nd–Pb isotopic data of the alkali mafic dikes, and diorite, aegirine–nepheline syenite and syenite porphyry units in the Wajilitag intrusive complex. Zircons from the diorite and alkali mafic rocks yield concordant crystallization ages of 275.2 ± 1.2 Ma and 281.4 ± 1.7 Ma, respectively. The diorite and syenitic rocks in Wajilitag area have a narrow range of SiO 2 contents (51.9–57.3 wt.%), and are enriched in total alkalis (Na 2 O + K 2 O = 8.3–14.3 wt.%), among which the aegirine–nepheline syenite and syenite porphyry have the geochemical affinity of A-type granites. The alkali mafic rocks and syenitic rocks have high Al 2 O 3 (19.4–21.1 wt.%), Zr, Hf, Ba contents, total rare earth element abundances and LREE/HREE ratios and low Mg# value, K, P and Ti contents. Diorites have lower Al 2 O 3 contents, total REE abundances and LREE/HREE ratios and higher Mg# values than the alkali mafic rocks and syenitic rocks. The diorites and syenitic rocks have low initial 87 Sr/ 86 Sr ratios (0.7034–0.7046), and high eNd( t ) values (0.1–4.1) and zircon eHf( t ) values (− 0.9–4.4). All the diorites and syenitic rocks show the 206 Pb/ 204 Pb ratios ranging of 18.0–19.5, 207 Pb/ 204 Pb of 15.4–15.6 and 208 Pb/ 204 Pb of 38.0–39.9. Sr–Nd isotopic ratios indicate a FOZO-like mantle source for the diorite and syenitic rocks, similar to that of the mafic–ultramafic rocks in the Wajilitag complex. In contrast, zircon Hf isotopes of basalt and syenite elsewhere in the Tarim LIP indicate a FOZO-like component may also contribute to Tarim LIP magmatism. Geochemical and Sr–Nd–Pb–Hf isotopic features reflect that diorites and syenitic rocks are probably derived from a FOZO-like mantle source, consistent with a plume mantle origin and then underwent crystal fractionation process.

The Pan-African quartz-syenite of Guider (north-Cameroon): Magnetic fabric and U–Pb dating of a late-orogenic emplacement

The small Guider pluton (70km2) in-between the cities of Maroua and Garoua (north Cameroon) is one of the quartz-syenite bodies that intrude Pan-African orthogneisses, and that are aligned along the western side of the NNE-directed Poli-Léré volcano-sedimentary corridor. The Guider syenite is here studied for its internal structures, using field and anisotropy of magnetic susceptibility measurements, and for its emplacement age of 593±4 Ma (U–Pb zircon), ∼20Ma younger than the beginning of gneissification of the country-rocks, mainly made of deformed granodiorites and diorites. In these orthogneisses, constantly NNE-trending vertical foliations, variously plunging stretching lineations and fold axes, as well as kinematic markers attest to a transpressive and possibly dextral regime. The Guider syenite, rich in K-feldspar and magnetite, displays typical magmatic microstructures with incipent solid-state features such as chess-board subgrains in quartz. Its magnetic fabric displays a well-defined feeding-zone in its centre and western side, attesting to its unrooted nature. However, N- to NE-trending lineations at its northern and southern ends, parallel to the overall linear structure of the country-rocks, point to its late-orogenic emplacement. Our results reveal that the NE-trending and dextral transpressive shear zone that acted in the Guider area ended after the emplacement of the syenite body; they specify the age of collage of the NW-Cameroon domain with the Mayo-Kébbi domain. They also show that the tectonic evolution of Guider area is closer to the E-Nigerian domain, to the west, than to its eastern neighbouring Mayo-Kébbi and Adamawa-Yadé domains. The connection of our sector with E-Brazil is briefly discussed.

Alkali feldspar syenites with shoshonitic affinities from Chhotaudepur area: Implication for mantle metasomatism in the Deccan large igneous province

Two petrologically distinct alkali feldspar syenite bodies (AFS-1 and AFS-2) from Chhotaudepur area, Deccan Large Igneous Province are reported in the present work. AFS-1 is characterized by hypidiomorphic texture and consists of feldspar (Or55Ab43 to Or25Ab71), ferro-pargasite/ferro-pargasite hornblende, hastingsite, pyroxene (Wo47, En5, Fs46), magnetite and biotite. AFS-2 exhibits panidiomorphic texture with euhedral pyroxene (Wo47-50, En22-39, Fs12–31) set in a groundmass matrix of alkali feldspar (Or99Ab0.77 to Or1.33Ab98), titanite and magnetite. In comparison to AFS-1, higher elemental concentrations of Ba, Sr and ∑REE are observed in AFS-2. The average peralkaline index of the alkali feldspar syenites is ∼1 indicating their alkaline nature. Variation discrimination diagrams involving major and trace elements and their ratios demonstrate that these alkali feldspar syenites have a shoshonite affinity but emplaced in a within-plate and rifting environment. No evidence of crustal contamination is perceptible in the multi-element primitive mantle normalized diagram as well as in terms of trace elemental ratios. The enrichment of incompatible elements in the alkali feldspar syenites suggests the involvement of mantle metasomatism in their genesis.

Ore geochemistry, zircon mineralogy, and genesis of the Sakharjok Y-Zr deposit, Kola Peninsula, Russia

The Sakharjok Y-Zr deposit in Kola Peninsula is related to the fissure alkaline intrusion of the same name. The intrusion ∼7 km in extent and 4–5 km2 in area of its exposed part is composed of Neoarchean (2.68–2.61 Ma) alkali and nepheline syenites, which cut through the Archean alkali granite and gneissic granodiorite. Mineralization is localized in the nepheline syenite body as linear zones 200–1350 m in extent and 3–30 m in thickness, which strike conformably to primary magmatic banding and trachytoid texture of nepheline syenite. The ore is similar to the host rocks in petrography and chemistry and only differs from them in enrichment in zircon, britholite-(Y), and pyrochlore. Judging from geochemical attributes (high HSFE and some incompatible element contents (1000–5000 ppm Zr, 200–600 ppm Nb, 100–500 ppm Y, 0.1–0.3 wt % REE, 400–900 ppm Rb), REE pattern, Th/U, Y/Nb, and Yb/Ta ratios), nepheline syenite was derived from an enriched mantle source similar to that of contemporary OIB and was formed as an evolved product of long-term fractional crystallization of primary alkali basaltic melt. The ore concentrations are caused by unique composition of nepheline syenite magma (high Zr, Y, REE, Nb contents), which underwent subsequent intrachamber fractionation. Mineralogical features of zircon-the main ore mineral—demonstrate its long multistage crystallization. The inner zones of prismatic crystals with high ZrO2/HfO2 ratio (90, on average) grew during early magmatic stage at a temperature of 900–850°C. The inner zones of dipyramidal crystals with average ZrO2/HfO2 = 63 formed during late magmatic stage at a temperature of ∼500°C. The zircon pertaining to the postmagmatic hydrothermal stage is distinguished by the lowest ZrO2/HfO2 ratio (29, on average), porous fabric, abundant inclusions, and crystallization temperature below 500°C. The progressive decrease in ZrO2/HfO2 ratio was caused by evolution of melt and postmagmatic solution. The metamorphic zircon rims relics of earlier crystals and occurs as individual rhythmically zoned grains with an averaged ZrO2/HfO2 ratio (45, on average) similar to that of the bulk ore composition. The metamorphic zircon is depleted in uranium in comparison with magmatic zircon, owing to selective removal of U by aqueous metamorphic solutions. Zircon from the Sakharjok deposit is characterized by low concentrations of detrimental impurities, in particular, contains only 10–90 ppm U and 10–80 ppm Th, and thus can be used in various fields of application.

Geology of Pulikonda and Dancherla alkaline complexes, Andhra Pradesh

Geological studies on saturated to oversaturated and subsolvus aegirine-riebeckite syenite bodies of the Pulikonda alkaline complex and Dancherla alkaline complex were carried out. The REE distribution of the Dancherla syenite shows a high fractionation between LREE and HREE. The absence of Eu anomaly suggests source from garnet peridotite. The Pulikonda syenite shows moderate fractionation between LREE and HREE as reflected by enrichment of HREE and moderate enrichment of LREE. The negative Eu anomaly indicates role of plagioclase fractionation.Three distinct co-eval primary magmas i.e. mafic syenite-, felsic syenite- and alkali basalt magmas — all derived from low-degrees of partial melting of mantle differentiates and enriched metasomatised lower crust played a major role in the genesis and emplacement of the syenites into overlying crust along deep seated regional scale trans-lithospheric strike-slip faults and shear zones following immediately after late-Archaean calc-alkaline arc magmatism at different time-space episodes i.e. initially at craton margin and later on into the thickened interior of the Eastern Dharwar craton. The ductile sheared and folded Pulikonda alkaline complex was evolved dominantly from the magmas derived from partial melting of lower crust and minor juvenile magmas from mantle. Differentiation and fractionation by liquid immiscibility of mafic magma and commingling-mixing of intermediate and felsic magmas followed by fractionational crystallisation under extensional tectonics during waning stages of calc-alkaline arc magmatism nearer to the craton margin were attributed as the main processes for the genesis of Pulikonda syenite complex. Commingling and limited mixing of independent mantle derived mafic and felsic syenitic magmas and accompanying fractionation resulting into soda rich and potash rich syenite variants was tentatively deduced mechanism for the origin of Dancherla, Danduvaripalle, Reddypalle syenites and other bodies belonging to Dancherla alkaline complex at the craton interior. The Peddavaduguru syenite was formed by differentiation of alkali mafic magma (gabbro to diorite) and it’s simultaneous mingling with fractionated felsic syenitic magma under incipient rift. Vannedoddi and Yeguvapalli syenites were derived due to desilicification and accompanying alkali feldspar mestasomatism of younger potash rich granites along Guntakal-Gooty fault and along Singanamala shear zone respectively. Dancherla syenite body has yielded a six point Rb-Sr whole rock isochron age of 2211±110 Ma with low initial 87Sr/86Sr ratio of 0.7004±0.00046; MSWD = 5.8.The low initial Sr(i) ratio suggests mantle derived juvenile additions to the crust. The Pulikonda syenite body yielded a six point whole rock isochron age of 1500 ±100 Ma with initial 87Sr/86Sr ratio of 0.7085±0.0047 and MSWD = 2%. The higher initial Sr(i) value for this body suggests metasomatised source. Resetting of Rb-Sr systematics has resulted younger age for Pulikonda syenite though it is supposed to be older as per the geological setting and this anomaly is interpreted interms of repeated deformation and reactivation of shear zone, emplacement of alkali granite into Pulikonda syenite and the role of Rudravaram shear zone (basement tectonics) in the formation of the Nallamali fold belt of the Cuddapah Basin. Alkaline magmatism was initially triggered along microplate margin tectonic setting and later confined to the intraplate tectonic setting.

Geochronology of the Arkansas Alkaline Province, Southeastern United States

Abstract The Arkansas alkaline province (AAP) consists of intrusive bodies ranging in lithology from carbonatite and lamproite through nepheline syenite. Apatite and titanite fission‐track ages fall into two groups—101–94 Ma and ∼88 Ma. New 40Ar/39Ar ages and those reported in the literature define a third age group of ∼106 Ma. Apatite and titanite fission‐track ages are concordant, indicating rapid cooling due to the emplacement of these intrusions at high levels during a time of regional uplift. There is a relationship between age and petrology: (1) lamproites are emplaced at ∼106 Ma, (2) carbonatites and associated silica‐undersaturated rocks are emplaced between 101 and 94 Ma, and (3) large nepheline syenite bodies are emplaced at ∼88 Ma. Chemical and isotopic data support the inference that the lamproites are derived from subcontinental lithosphere, while the other sequences are derived from the asthenosphere. The ages for the AAP and other conflicting information do not support the hypothesis that t...

Geology and geochronology of neoarchean anorogenic magmatism of the Keivy structure, Kola Peninsula

Anorogenic magmatic complexes were formed during protoplatformal evolution of the Keivy structure. This evolution ended with development of aluminous schists, which were derived by deep disintegration and redeposition of the rocks from the lower parts of the sequence and surrounding of the structure. The anorogenic rocks of the region are represented by the following magmatic complexes: gabbro-labradorite-latite-monzonite-granites; ophitic gabbro and gabbrodiabases; quartz syenite-alkaline granites; alkaline and nepheline syenites. The magmatic activity of this period, starting from the emplacement of gabbrolabradorite massifs and ending with alkaline and nepheline syenite bodies, was caused by ascent of mantle asthenolith, which destructed the Earth’s crust basement in this area. The anorogenic magmatism of the Keivy structure lasted for no more than few or few tens of million years. The granitoid subcomplex of the gabbro-labradorite-latite-monzonite-granite complex is dated at 2674 ± 6 Ma, which is comparable with an age of alkaline granites of the Ponoy and Beliye Tundry massifs (2673 ± 6 Ma). The considered complexes are separated in time by intrusion of amphibole-biotite plagiomicrocline granites with an age of 2667 ± 8 Ma. Gabbrolabradorites of the Shchuch’e Ozero and Tsaga massifs have close ages (2663 ± 7 and 2668 ± 10 Ma, respectively, Bayanova, 2004), but were formed earlier than granitoids (Bayanova, 2004). Formation of alkaline syenites of the Sakharijok I Massif, which finalized the Neoarchean anorogenic magmatism of the region, falls in the same interval. During Paleoproterozoic transformations, the rocks of the Keivy structure were sheared and uranium was introduced in the contact zones of the alkaline granite massifs, which caused formation of palingenetic melts and subsequent formation of pegmatites in the outer contact zones of the granite bodies.

Petrological and geochemical characteristics of Marunthurkota syenites from the Kerala khondalite belt, southern India

This note reports new occurrences of syenite bodies around Marunthurkota area from the Kerala khondalite belt (KKB). Petrological and geochemical studies suggest that the syenites have a pronounced A-type affinity, metaluminous characteristics with high concentrations of alkalies, Rb, Sr, Zr, and high K 2 O/Na 2 O ratio. Miaskitic nature (agpaitic index<1) of syenite suggest involvement of CO 2 related phase in their genesis. The petrological characteristics signify crystallization of the rock at shallow levels within the crust. Geochemistry favours mantle origin of the magma and enrichment of Ba and Sr are indicative of involvement of carbonatite melt in the source region. The study envisages the presence of a juvenile CO 2 enriched upper mantle below the southern Indian continental crust during the Pan-African time.

Gravity Data Support for Large-Scale Silicate Liquid Immiscibility in the Triunfo Batholith, Northeastern Brazil

The Triunfo batholith, located in the Transverse Zone of Borborema Province in northeastern Brazil, consists of ultrapotassic alkali-feldspar syenite containing comagmatic alkali pyroxenite inclusions, and cut by synplutonic alkali pyroxenite dikes. Field, chemical, mineralogical and isotopic studies on rocks of this batholith have led to the hypothesis that coexisting syenite and pyroxenite melts were produced by large-scale liquid immiscibility from a parent mafic syenitic magma. A model for the spatial distribution of the unmixed syenitic and pyroxenitic melts proposed that volumetrically subordinate but denser pyroxenite underlies the main syenite body. In this study, three gravity traverses along the major and minor axes of the batholith were carried out. The spatial distribution hypothesis was tested using direct and inverse gravity modeling, which suggested that a 200 m thick pyroxenite layer with a total width of 8.5 km forms the base of the batholith whose geometric form is intermediate between that of a laccolith and a lopolith.

The Mordor Alkaline Igneous Complex, Central Australia: PGE-enriched disseminated sulfide layers in cumulates from a lamprophyric magma

The Mordor Alkaline Igneous Complex (MAIC) is a composite intrusion comprising a body of syenite and a funnel-shaped layered mafic–ultramafic intrusion of lamprophyric parentage, the Mordor Mafic–Ultramafic Intrusion or MMUI. The MMUI is highly unusual among intrusions of lamprophyric or potassic parentage in containing primary magmatic platinum-group element (PGE)-enriched sulfides. The MMUI sequence consists largely of phlogopite-rich pyroxenitic cumulates, with an inward dipping conformable layer of olivine-bearing cumulates divisible into a number of cyclic units. Stratiform-disseminated sulfide accumulations are of two types: disseminated layers at the base of cyclic units, with relatively high PGE tenors; and patchy PGE-poor disseminations within magnetite-bearing upper parts of cyclic units. Sulfide-enriched layers at cycle bases contain anomalous platinum group element contents with grades up to 1.5 g/t Pt+Pd+Au over 1-m intervals, returning to background values of low parts per billion (ppb) on a meter scale. They correspond to reversals in normal fractionation trends and are interpreted as the result of new magma influxes into a continuously replenished magma chamber. Basal layers have decoupled Cu and PGE peaks reflecting increasing PGE tenors up-section, due to increasing R factors during the replenishment episode, or progressive mixing of between resident PGE-poor magma and more PGE-enriched replenishing magma. The presence of PGE enriched sulfides in cumulates from a lamprophyric magma implies that low-degree partial melts do not necessarily leave sulfides and PGEs in the mantle restite during partial melting.

Petrogenesis of the Nanling Mountains granites from South China: Constraints from systematic apatite geochemistry and whole-rock geochemical and Sr–Nd isotope compositions

The widespread Mesozoic granitoids in South China (∼135,300 km 2) were emplaced in three main periods: Triassic (16% of the total surface area of Mesozoic granitoids), Jurassic (47%), and Cretaceous (37%). Though much study has been conducted on the most abundant Jurassic Nanling Mountains (NLM) granites, their rock affinities relative to the Triassic Darongshan (DRS) and Cretaceous Fuzhou–Zhangzhou Complex (FZC) granites which are typical S- and I-type, respectively, and the issue of their petrogenetic evolution is still the subject of much debate. In this study, we discuss the petrogenesis of NLM granites using apatite geochemistry combined with whole-rock geochemical and Sr–Nd isotope compositions. Sixteen apatite samples from six granite batholiths, one gabbro, and three syenite bodies in the NLM area were analyzed for their major and trace element abundances and compared with those collected from DRS ( n = 7) and FZC ( n = 6) granites. The apatite geochemistry reveals that Na, Si, S, Mn, Sr, U, Th concentrations and REE distribution patterns for apatites from DRS and FZC granites basically are similar to the S and I granite types of the Lachlan Fold Belt (Australia), whereas those from NLM granites have intermediate properties and cannot be correlated directly with these granite types. According to some indications set by the apatite geochemistry (e.g., lower U and higher Eu abundances), NLM apatites appear to have formed under oxidizing conditions. In addition, we further found that their REE distribution patterns are closely related to aluminum saturation index (ASI) and Nd isotope composition, rather than SiO 2 content or degree of differentiation, of the host rock. The majority of apatites from NLM granites (ASI = 0.97–1.08 and εNd( T) = −8.8 to −11.6) display slightly right-inclined apatite REE patterns distinguishable from the typical S- and I-type. However, those from few granites with ASI > 1.1 and εNd( T) < −11.6 have REE distribution patterns (near-flat) similar to DRS apatites whereas those from granites with ASI < 1.0 and εNd( T) > −6.6 and gabbro and syenite are similar to FZC apatites (strongly right-inclined). In light of Sr and Nd isotope compositions, magmas of NLM intrusives, except gabbro and syenite, and few granites with εNd( T) > −8, generally do not involve a mantle component. Instead, they fit with a melt derived largely from in situ melting or anatexis of the pre-Mesozoic (mainly Caledonian) granitic crust with subordinate pre-Yanshanian (mainly Indosinian) granitic crust. We suggest that an application, using combined whole-rock ASI and εNd( T) values, is as useful as the apatite geochemistry for recognizing possible sources for the NLM granites.

Permian bimodal dyke of Tarim Basin, NW China: Geochemical characteristics and tectonic implications

This study reports for the first time the occurrence of bimodal dyke in the Shuigongtuan area of Bachu County, Tarim Basin, NW China. Here, quartz syenite porphyry and diabase dykes occur in direct contact showing bimodal feature. The quartz syenitic porphyry is metaluminous, enriched in K 2O + Na 2O (10–11 wt.%) and total rare earth elements (REE), with low Mg/(Mg + Fe) ratios, high LREE/HREE, and negative Eu anomalies. The chemical characteristics and tectonic discriminative diagrams show that the rocks have geochemical affinity with A-type granites. The diabase dyke shows 45–52 wt.% SiO 2 and Mg/(Mg + Fe) ratio in the range of, with high total REE, high LREE/HREE ratios and lack of Eu anomalies, broadly corresponding to tholeiitic composition. Based on low Y/Nb (as low as 0.4, and less than 1.2), enrichment in LILE and HFSE, and uniform Nb-enrichment patterns in spider diagram for the quartz syenitic porphyry, together with the geochemical patterns of the diabases, this biomodal association is interpreted to be derived from a mantle source and formed under typical within-plate environment. The quartz syenitic porphyry and diabase have Daly gap of 46 wt.%–67 wt.% in SiO 2, which is explained through formation under bimodal rifting. The quartz syenitic dyke probably formed during Early Permian (277 Ma) and has geochemical affinity with the Xiaohaizi syenitic body. We propose that magmas sourced from the mantle intruded into middle–upper crust and were emplaced as dykes, which indicate large-scale extension during the Permian in Tarim Basin. The bimodal dyke has genetic affinity with the huge volume of Permian basalts and igneous rocks (248–292 Ma) that occur in the Tarim Basin. The magmatism manifests the culmination of the major thermal event in the Tarim Basin.

Influence of oxygen fugacity on mineral compositions in peralkaline melts: The Katzenbuckel volcano, Southwest Germany

The igneous rocks of the Katzenbuckel, Southwest Germany, represent a unique and unusual alkaline to peralkaline association within the European Volcanic Province. The magmatic activity can be subdivided into two main phases. Phase I comprises the main rock bodies of phonolite and nepheline syenite, which were later intruded by different peralkaline dyke rocks (tinguaites and alkali feldspar syenite dykes) of phase II. The dyke assemblage was accompanied by magnetite and apatite veins and was followed by a late-stage pneumatolytic activity causing autometasomatic alterations. As is typical for alkaline to peralkaline igneous rocks, early mafic minerals of phase I rocks comprise olivine, augite and Fe–Ti oxides, which are substituted in the course of fractionation by Na-amphibole and Na-pyroxene. For the early magmatic stage, calculated temperatures range between 880 and 780 °C with low silica activities (0.4 to 0.6) but high relative oxygen fugacities between 0.5 and 1.9 log units above the FMQ buffer. Even higher oxygen fugacities (above the HM buffer) are indicated for the autometasomatic alteration, which occurred at temperatures between 585 and 780 °C and resulted in the formation of pseudobrookite and hematite. The unusually high oxygen fugacities (even during the early magmatic stage) are recorded by the major element compositions of the mafic minerals (forsterite content in olivine between 68 and 78 mol%, up to 6.2 wt.% ZrO 2 and 8.5 wt.% TiO 2 in clinopyroxene), the unusual mineral assemblages (pseudobrookite, freudenbergite) and by the enrichment of Fe 3+ in the felsic minerals (up to 2.8 wt.% Fe 2O 3 in alkali feldspar and up to 2.6 wt.% Fe 2O 3 in nepheline). These observations point to a metasomatically enriched and highly oxidized lithospheric mantle as a major source for the Katzenbuckel melts.

Cooling history of the Puttetti alkali syenite pluton, southern India

The Puttetti alkali syenite pluton in southern India belongs to the suite of felsic magmatic intrusives emplaced during the Late Neoprotrozoic-Cambrian time during the final phase of amalgamation of the Gondwana supercontinent. In this study, we evaluate the cooling history of this pluton based on various isotopic systems. We present whole-rock Pb-Pb data on the syenite which yields an isochron age of 508±25Ma. Three phlogopite separates from the syenite pluton give K-Ar ages of 454.0±9.0, 448.5±8.9 and 445.6±8.8 Ma indicating cooling age at temperatures of ∼415°C. U-Pb analyses of zircons from this syenite yielded an age of 572±2 Ma in a previous study. With U-Pb closure temperatures >800 o C, this age probably indicates the timing of emplacement of the Puttetti pluton. Collectively, we estimate from the isotopic age data and respective closure temperatures that the syenite body cooled at about 3.2 o C/Ma from about 800 o C to about 415 o C. The markedly low cooling rate of the syenite pluton, absence of chilled margin effects, and common occurrence of pyroxene, feldspar, phlogopite and zircons megacrysts in the rock indicate that the host granulites were at high temperatures during the emplacement of the syenite magma. The cooling history of Puttetti syenite estimated in this study is closely comparable with the 3–4 o C/Ma cooling rate estimated for a granite pluton in a previous study from Madagascar. Our study suggests protracted cooling rates for the late Pan-African intrusives emplaced within the Gondwana crust, with a long residence history in a hot crust bore they were exhumed to shallower levels.

Geochemical and Sr-Nd isotopic study of alkaline syenites in Liangtun-Kuangdonggou, Liaoning Province, China: Evidence for enriched mantle before 1.86 Ga and implications

The 1.86 Ga Liangtun-Kuangdonggou complex (LKC) is one of the oldest alkaline syenite bodies so far discovered in China. This syenite suite has elevated contents of total alkali (K2O+Na2O), with an average of 10.50%, and a mean Rittmann Index (σ) of 6.48. The intrusions have slightly higher concentrations of K2O than those of Na2O on a weight percent basis, indicating the rocks belong to potassium-rich alkaline syenite series. Total rare-earth element concentrations (ΣREE) of the rocks are relatively high, ranging from 324×10−6 to 1314×10−6, with a mean value of 666×10−6. The REE patterns are subparallel and rightward steep with (La/Yb)N>33, showing mild negative to positive Eu anomalies (δEu: 0.63–1.15). All samples exhibit strong LILE and LREE enrichments and TNT (Nb, Ta, Ti) and P depletions in multi-element spidergrams. On theɛSr(t)-ɛNd(t) correlation diagram, most analytical data points plot within the enriched mantle field with low (87Sr/86Sr)i ratios (0.7045–0.7051) and negativeɛNd(t) values (−3.72–−3.97), falling among those kimberlites from Fuxian County, Liaoning Provinve, from Mengyin County, Shandong Province and the II-type kimberlites from South Africa. These characteristics imply that the LKC-rocks may have the same source as the above-mentioned kimberlites, i.e., they have close connections to the materials derived from enriched mantle reservoirs, further revealing that the upper mantle beneath the northeastern part of the North China Plate had been highly enriched before 1.86 Ga. Geodynamically, the LKC-rocks were formed in a within-plate environment with close genetic connections to rift-related alkaline magma activities possibly controlled by ancient mantle plumes.