Alkali-feldspar Syenite Research Articles

Influence of intensive parameters on the mafic paragenesis of nepheline syenites: the Monte de Trigo alkaline suite, southeastern Brazil

The Monte de Trigo Alkaline Suite is a Late Cretaceous syenite–gabbroid occurrence from the Serra do Mar Alkaline Province, southeastern Brazil. It has a zoned nepheline syenitic stock with nepheline syenite I (NSI), the most SiO 2 -undersaturated, in the centre, surrounded by nepheline syenite II (NSII) and alkali feldspar syenite with nepheline (SN). Synplutonic miaskitic and agpaitic nepheline microsyenite (MNM and ANM, respectively) dykes cut across the stock. Extensive fractionation of alkali feldspar and mafic minerals is attributed to the formation of the syenitic facies and dykes. Mineral chemical data indicate that SN crystallized at relatively high silica activity ( a SiO 2 ; 0.5–0.7), a H 2 O (0.24–0.11) and f O 2 (difference from the fayalite–magnetite–quartz buffer in log units, ΔFMQ + 0.2 to ΔFMQ − 0.2), in contrast to NSII and NSI ( a SiO 2 , ∼0.41; a H 2 O, 0.05–0.02; and f O 2 , ΔFMQ − 1.4 to ΔFMQ − 1.1). These parameters vary with magma evolution towards the phonolite eutectic and appear to control variations in paragenesis, from hastingsite + biotite ± diopside/hedenbergite in SN to hastingsite in NSII and hastingsite + hedenbergite in NSI. The MNM have high a H 2 O (0.31) and hastingsite + biotite + aegirine–augite assemblages. In contrast, the ANM are characterized by an F-rich anhydrous agpaitic assemblage.

Geochemistry of Granites in Panggong Lalat, Gua Musang, Kelantan

Granites are among the most significant rock formations in the Earth’s crust and play a crucial role in various geological processes. This study focuses on examining the geology and granitic rock formations in Panggong Lalat, Gua Musang, Kelantan through field surveys, geological mapping, and geochemical analysis for better understand the geological framework and characteristics of granite in this area. The findings indicate that the lithology of the study area includes granitoids, well-bedded limestone, volcanic rocks, fine-grained sediments, and meta-sedimentary rocks. Petrographic analysis of five samples identified three granitoid types: quartz-rich granitoid, quartz alkali feldspar syenite, and alkali feldspar granite, classified using the IUGS scheme. The X-ray Fluorescence (XRF) data reveals that both samples exhibit typical felsic granite compositions, with high SiO₂ contents in AF 01 (61.61%) and 60.72% in AF02. However, AF 02 displays higher concentrations of TiO₂, FeO, and MnO, suggesting a greater contribution of mafic minerals compared to AF 01. Additionally, CaO content is slightly higher in AF02, indicating a more significant presence of plagioclase feldspar, while AF01 is enriched in K₂O. The Inductively Coupled Plasma - Optical Emission Spectrometry (ICP-OES) data further reinforce the distinction between AF 01 and AF 02. Both samples are enriched in Rare Earth Elements (REE), particularly Ce, Er, and La. A F02 shows significantly higher concentrations of Ce (213.85 ppm) and Er (7554.11 ppm) compared to AF 01 (166.12 ppm for Ce and 3047.54 ppm for Er), indicating a stronger REE enrichment in AF 02.The distribution of Heavy Rare Earth Elements (HREE) is more abundant than that of Light Rare Earth Elements (LREE), with Er exhibiting the highest concentration and Nd the lowest.

Petrogenesis and tectonic implication of the Triassic syenitic complexes in the eastern Qinling orogen, central China

Alkaline igneous rocks usually provide significant information on the mantle-derived components, magmatic processes and crust–mantle interaction due to their deep source. The Songxian syenite, located in the Xiong'er terrane of the Qinling Orogen, is one of the most typical alkaline intrusions in the Early Mesozoic. Geochemical, geochronological and in situ Sr–Nd–Hf isotopic analyses were conducted on the alkali-feldspar syenite and aegirine-augite syenite samples collected from the Songxian syenite, in order to reveal the magma sources, petrogenesis and possible geodynamic implications. All the rock samples are enriched in alkali (Na2O + K2O = 11.74 to 15.14 wt%), light rare earth elements and large ion lithophile elements, and depleted in high field strength elements. Zircon LA–ICP–MS U–Pb dating of medium- and fine-grained alkali-feldspar syenites yield emplacement ages of 235.5 ± 0.9 Ma and 232.7 ± 3.3 Ma, respectively. In addition, the aegirine-augite syenite generally developed as dyke or vein cutting through the two types of alkali-feldspar syenite in the field, indicating multiple pulses of magmatism. In-situ Sr-Nd isotopic analyses on magmatic apatite yielded (87Sr/86Sr)i ratios from 0.707959 to 0.710414 and εNd(t) from −19.3 to −13.9 for alkali-feldspar syenite, while the analyses for aegirine-augite syenite yielded (87Sr/86Sr)i ratios from 0.717948 to 0.721470 and εNd(t) from −25.5 to −24.5. The difference suggests that the Songxian syenite may be generated from multi-stage magmatic episodes. The first magmatic incident mainly originated from the partial melting of the enriched lithospheric mantle, mixed with the lower crust-derived melts. The second mixing magma might involve more proportion of Archean to Paleoproterozoic crust-derived materials. During the Early Mesozoic, the Mian-Lue oceanic slab subducted northward to the asthenosphere mantle and induced asthenosphere upwelling. Under the tectonic setting, the mixture of enriched mantle- and crust-derived melts occurred to form the Songxian syenite.

Gravimetric inversion and three-dimensional geological modelling of the Piquiri Syenite Massif, southern Brazil

The Pelotas Batholith corresponds to the eastern margin of the Dom Feliciano Belt in southernmost Brazil. It comprises multiple intrusions formed by successive tectonic-magmatic processes during the Brasiliano / Pan-African Cycle. One of these intrusive igneous bodies is the Piquiri Syenite Massif (PSM), recently described as a multi-intrusive body formed by three successive pulses, dated by LA-MC-ICP-MS (U-Pb in zircon), with the oldest pulse located at the margins and the youngest in the centre of the intrusion. Pulse 1 (609.3 ± 1.5 Ma) comprises fine- to medium-grained equigranular syenite to quartz syenite and alkali-feldspar quartz syenite, with colour index M′15–30. Pulse 2 (603.4 ± 3.9 Ma) comprises medium- to coarse-grained equigranular alkali-feldspar syenite and alkali-feldspar quartz syenite, with colour index M′ 5–15 and fragments of Pulse 1 varieties. Pulse 3 (588.8 ± 3.1 to 583.2 ± 1.8)comprises medium- to coarse-grained inequigranular quartz syenites, with colour index M′ 2–10, containing fragments of pulses 1 and 2 varieties. As defined by the AMS data, the PSM magnetic fabric is concordant with the magmatic fabric, parallel to the outer edges of the body, dipping towards the centre. However, the construction and emplacement of the body have been the subject of different interpretations. The main objective of this work is to relate the general morphology of the massif and the behaviour of the pulses within the massif with the processes of formation and ascent of the magmas that have built it. For this, terrestrial geophysical surveys were carried out to obtain gravimetric data, with which the Bouguer Anomaly map was generated. Modelling by gravimetric inversion was carried out in nine profiles using tools from the Oasis Montaj software, which was the basis for constructing a three-dimensional geological model using the Leapfrog Geo software. Associating the geological model with the magmatic and magnetic foliations, including the magnetic lineation, it was possible to determine a change in behaviour between the youngest and the two oldest pulses related to the ascension process. In addition, the body’s general shape, with the greatest depths located in the eastern region, allowed us to relate the rise of magma to a structure in the region.

Voluminous Paleoproterozoic plutonic belt of the Bacajá domain in Amazon Craton: Petrology, geotectonic setting and Pb–Pb zircon evaporation ages

To the north of the well-known Archean Carajás Metallogenetic Province, the Bacajá Domain abounds voluminous Rhyacian plutonic bodies predominate largely over remnants of Siderian and Neoarchean crust. The Rhyacian granitoids of the Bacajá Domain present a wide lithological variety and a geological evolution marked by syntectonic emplacement, which printed strongly elongated geometries to the bodies, according to a WNW regional orientation. The Rhyacian granitoids form three distinct units. The Bacajaí intrusive suite comprises 2112 ± 8 Ma metaquartz diorites, tonalites, granodiorites and monzogranites of calc-alkaline signature. Sm–Nd isotopic analyses of these rocks yielded a TDM age of 2310 Ma and an εNd value of +0.56. The Arapari intrusive suite is composed of quartz diorites, tonalites, granodiorites and monzogranites of calc-alkaline signature. A preliminary Pb–Pb zircon dating pointed an age of 2072 Ma to the Arapari intrusive suite. Nd isotopes yielded TDM ages varying from 2100 to 2170 Ma and positive εNd (+2.11 to +3.04). Both Bacajaí and Arapari intrusive suites have similar chemical signatures suggesting continental magmatic arc environment. The origin of Bacajaí and Arapari magmas could be explained by partial melting of mantle and crustal sources. The João Jorge intrusive suite encloses high-K calc-alkaline quartz diorites, monzogranites, syenogranites, alkali feldspar granites, shoshonitic alkali feldspar syenites and monzonites. The alkali feldspar syenites have Pb–Pb zircon age of 2076 Ma, TDM age of 2210 Ma and positive εNd value (+1.51). Spatial relationship between shoshonitic bodies and strike-slip shear zones or geological contacts seems to reflect the influence of crustal anisotropies on mantle-derived magmas ascending. Partial melting of metassomatized mantle after extraction of granite melts could explain the origin of syenites and monzonites. The three granitoid unities show rhythmic layering defined by alternation of quartz-feldspathic and ferromagnesian-enriched levels. Regionally penetrative steep-dipping magmatic foliation was formed in the presence of both crystals and melt, under conditions of increasing deformation and decreasing temperatures. The Bacajaí metagranitoids underwent locally thermal effects of hornblende hornfels facies caused by the later granitoid suites. These hornfelsed granitoids may present granoblastic textures, overgrown muscovite, reddish biotite and clinopyroxene. Late WNW strike-slip ductile zones and brittle faults crosscut some of the plutons, closing the ancient geological evolution of the Bacajá Domain.

Geochemistry and Geochronology (U-Pb and Lu-Hf) of the Soarinho Alkaline Massif (Brazil): Implications on Mantle versus Crustal Signature of Syenitic Magma

The Soarinho Alkaline Intrusion, southeastern Brazil, makes up part of the Serra do Mar Igneous Province, and it is composed of alkali feldspar syenite, alkali feldspar trachyte, quartz syenite, and monzonite. Geochemical and geochronological analyses of the Soarinho were compared with data from its neighbors, Rio Bonito and Tanguá. Zircon U-Pb Laser Ablation ages show that Soarinho is younger than Rio Bonito and Tanguá. Ages obtained from the monzonite (60 ± 2 Ma), alkali feldspar syenite (58 ± 2 Ma), and quartz syenite (58 ± 2 Ma) are indistinguishable within error. One nepheline syenite sample from Tanguá gives an age equal to those of Rio Bonito already published (65 ± 1 Ma). Petrographic and geochemical data show a clear contrast among Soarinho, Rio Bonito, and Tanguá. Lu-Hf isotopic data for the three alkaline intrusions (all negative) point to some similarities in their sources. The data suggest an enriched mantle source for Soarinho; however, an older continental crust contribution cannot be discarded.

Developing an Integrated Petrogenetic Model for Understanding REE Deportment of the Ampasibitika Intrusion and Associated Ion Adsorption Deposits

Abstract Alkaline–peralkaline igneous systems are promising sources of rare earth elements (REEs). Preservation bias has resulted in a gap in the geological record for alkaline–peralkaline magmatic systems, with the hypabyssal plumbing system linking magma chambers to extrusive volcanic rocks poorly represented. Large plutonic varieties of these systems are often proposed to have fed (now eroded) volcanoes, and current peralkaline volcanic systems obscure the plutonic system at depth. The alkaline to peralkaline Ampasibitika Intrusion in Madagascar is a rare example where the magmatic–volcanic interface between a deeper level magma reservoir and its genetically related caldera volcano is exposed. This c. 24 Ma sub-volcanic intrusive system comprises silica-undersaturated to silica-oversaturated units, of peralkaline to metaluminous and peraluminous characters, with varying styles of REE mineralisation, including supergene ion adsorption-style REE occurrences in the overlying weather profiles. There are two main intrusive suites: (1) the concentric Marginal Dyke Swarm (MDS) formed of quartz–microsyenite and peralkaline granite dykes (PGDs), and (2) the Ampasibitika Ring Dyke (ARD) comprising alkali feldspar syenites and subordinate nepheline syenites, trachytes and phonolites. We present new field observations and geochemical data to indicate that the MDS was emplaced as a series of low-viscosity, volatile-rich melt batches, which coalesced in the magma reservoir roof zone and intruded prior to caldera collapse, whereas the ARD was emplaced into the ring fault as a heterogeneous mix of variably evolved syenitic crystal mushes and phonolitic to trachytic-melt batches. As such, we suggest the MDS represents the residual melt fraction of the magma reservoir, whereas the ARD contains portions of the fractionating, silica-neutral to silica-undersaturated syenite, cumulate assemblage. In this revised framework, we assess the major and trace element geochemistry of amphibole- and clinopyroxene-group minerals to gain insight into the magmatic evolution of the Ampasibitika Intrusion and partitioning of REE between early cumulate and residual melt phases. Ultimately, the most REE-enriched units, the PGDs of the MDS, are identified as the product of the most volatile-rich, highly evolved melts from the roof zone of the magma reservoir. However, although REE enriched, the mineralogy does not always enable efficient release of REE for ion adsorption-style mineralisation; instead, lower REE-content protoliths with REE-host phases more amenable to decomposition release a greater proportion of REE.

Two-stage, U-mineralization of A-type granites from the Huangmeijian complex, eastern China

Parts of A-type granites are closely related to some critical mineral resources. The study of origin and geochemical properties of those A-type granites are of great significance in revealing the metallogenic potential. The Early Cretaceous Huangmeijian (HMJ) complex is located in the Lower Yangtze River Belt (LYRB), eastern China, which is proposed to be associated with uranium deposits. Here, we present whole-rock major and trace elements, and zircon U–Pb geochronology to reveal the formation of the two-stage, uranium mineralization of A-type granites. The high Zr + Nb + Ce + Y concentrations (598−1181 ppm) and Ga/Al ratios (2.7−4.2) suggest an A-type geochemical affinity. On the ternary discriminant diagrams, they can be further divided into A1-type. In-situ LA-ICP-MS U–Pb dating on the zircon grains from the HMJ complex yielded weighted mean ages of 129.7 ± 1.6 Ma (quartz alkali feldspar syenite), 131.3 ± 1.0 Ma (quartz alkali feldspar syenite), and 118.7 ± 0.7 Ma (alkali feldspar granite), respectively. The former two age results are consistent within error, and similar to other A-type granites in the LYRB. The latter one is newly identified, and is obviously younger than most A-type granites developed in the LYRB. Geochemical data suggest that these A-type granites are formed under high temperature and low oxygen fugacity conditions in an extensional setting. The presence of huttonite favors for the hydrothermal event and the uranium mineralization. It is important to note that, compared to the ca. 130 Ma quartz alkali feldspar syenites, the ca. 118 Ma alkali feldspar granites have obviously higher U (21.3–79.2 ppm) and Th (81.2–104 ppm) concentrations, which are also higher than those of other A-type granites developed in the LYRB. Moreover, the U concentration is even higher than some uranium mineralized granites of South China. Thus, besides the previously reported ca. 130 Ma A-type granite, the ca. 118 Ma A-type granite is of great probability to be a main source of uranium for the formation of uranium deposits in the LYRB, eastern China.

Evolution of amphiboles of the Serra do Catu intrusive suite: Crystallization conditions of Ediacaran shoshonitic rocks in the sergipano orogenic system, NE Brazil

The Ediacaran shoshonitic Santa Maria, Monte Pedral, Bom Jardim, Niterói stocks, and the Curituba batholith, belonging to the Serra do Catu Intrusive Suite, occur in the Canindé and Poço Redondo domains, in the northern region of the Sergipano Orogenic System, located in the northeast of Brazil. These intrusions occur over a wide region, have different shapes and sizes, and consist of quartz-monzonite, monzogranite, quartz alkali feldspar syenite, alkali feldspar granite, and dioritic enclaves. Calcic amphibole is the dominant mafic mineral in these rocks. Magmatic amphibole corresponds to magnesio-hornblende, edenite, magnesio-hastingsite, hastingsite, and tschermakite, while post-magmatic amphibole consists of actinolite. The amphiboles in these intrusions have similar compositions as those crystals formed in alkaline and subalkaline magmas, a typical characteristic of shoshonitic series magmas. Geothermobarometers were tested and the most convenient, those that consider a greater number of chemical elements, one was selected for the type of magma/rock studied. It was inferred that the primary crystals were formed from mantle magmas and mixed magmas (mantle + crustal). These crystals were formed under high oxygen fugacity. The bodies of the studied suite were emplaced under different temperatures, pressures, and depth conditions: Santa Maria stock (808 °C; 470 MPa; 14.6 km), Niteroi stock (801 °C; 550 MPa; 17 km), Curituba batholith (879 °C; 740 MPa; 22.9 km), Bom Jardim enclaves stock (877 °C; 860 MPa; 26.6 km), and Curituba batholith enclaves (912 °C; 970 MPa; 30 km).

Petrological and Geochemical Characteristics of the Cretaceous Ngaou Boh Anorogenic Complex (Adamawa Plateau, Cameroon Line): Preliminary Constraints

The Cretaceous Ngaou Boh anorogenic complex (NBAC) located in the far North Adamawa Plateau, the centre domain of the Cameroon Line constitutes a plutonic-volcanic ring association. The whole rock K-Ar datation yields a crystallization age of ca. 74 Ma. Plutonic rocks comprise abundant alkali feldspar granites, scarce clinopyroxene-amphibole gabbros and alkali feldspar syenites. Alkali feldspar granites are leucocratic, coarse to fine-grained; quartz and K-feldspars are the major rock-forming mineral, besides minor oligoclase, biotite and accessory phases as sphene, zircon and opaques. Alkali feldspar syenites are mesocratic coarse-grained, mainly composed of K-feldspars with small amounts of quartz and biotite. Volcanic rocks consist of a basanite-trachyte-rhyolite suite. Basanites contain olivine and diopside phenocrysts and a groundmass essentially composed of plagioclase and titanomagnetite. Biotite-clinopyroxene trachytes and clinopyroxene-amphibole rhyolites have an almost homogeneous modal composition, mainly made up of sanidine and anorthoclase microliths, scarce phenocrysts of quartz, and minor crystals of biotite, clinopyroxene (augite) amphibole (pargasite, sandagaite); Fe-Ti oxides (ilmenite, titanomagnetite) and fibreglass are often isolated in the groundmass. Plutonic rocks are alkaline, weakly metaluminous with some alkali feldspar granites displaying agpaitic or peralkaline feature. Incompatible Trace elements (HFSE and LILE) distribution and chondrite-normalized REE patterns evidence a significant petrogenetic link between clinopyroxene-amphibole gabbros, alkali feldspar syenites and alkali feldspar granites. All the analysed samples are enriched in incompatible elements, indicating melts from spinel and garnet-bearing mantle source close to OIB component. Indeed, the (Tb/Yb)N ratios of both basanites (2.3-2.5) and clinopyroxene-amphibole gabbros (1.4-1.9) suggest different parental magma sources. Alkali feldspar granites appear as residue of magma differentiation led by crystal fractionation of liquid derived from the partial melting of spinel peridotite mantle. Clinopyroxene-amphibole rhyolites and biotite-clinopyroxene trachytes (Mg#=0.0-15.4) derive through fractional crystallization from basanites (Mg#=64.3-60.1), the most primitive mafic parental melt. Both plutonic rocks and lavas trends evidence a bimodality highlighted by a pronounced “Daly gap”.

Anisotropy of magnetic susceptibility, gravimetry and geochronology of the Cerne granite, Ribeira Belt, southern Brazil

The Cerne granite is an elongated pluton in the NE-SW direction, parallel with the Ribeira Belt trend. This Neoproterozoic massif crosscuts supracrustal metamorphic rocks and has been considered late to post orogenic in the Ribeira belt evolution (Apiaí domain). The Lancinha and Morro Agudo shear zones are the major tectonic structures that frame the studied area. The southern border of the pluton is deformed by the dextral Cerne brittle-plastic shear zone, which is considered a Lancinha shear zone synthetic component. The intrusion consists mainly of fine-to medium-grained alkali feldspar granites and quartz alkali feldspar syenites. Conversely to its elongated shape, striking N20E, the rocks display isotropic structure. Analyzing field data, and integrating with petrography, gravimetry profiles, anisotropy of magnetic susceptibility (AMS) and geochronology techniques, we sought to contribute to understand the architecture of the Cerne granite and its emplacement mechanisms. The rocks display isotropic structure and very locally an incipient preferred orientation of biotite crystals. Quartz grains do not show preferred orientation but undulose extinction, and bulging-related recrystallization features are more commonly observed on low-temperature mylonites located close to the southern border of the pluton. The Cerne granite gravimetric data show two roots of two inverted teardrop-shaped plutons. ASM fabric is mostly controlled by coarse-grained magnetite, with a mean susceptibility of 7.15 mSI. The magnetic foliation (K3) shows a concentric arrangement shallowly dipping and the magnetic lineation (K1) displays subhorizontal plunging. AMS ellipsoids varies from neutral to oblate (T ≥ 0) in 75% of the sites. Concentric magnetic foliations and lineations seem to follow the plutons boundaries and reflect mostly magma chamber dynamics. The set of data suggests that the Cerne magma emplacement occurred in a place of local distension stresses associated with sinistral strike-slip system. Deep-seated tension gash probably controlled the Cerne granite ascent. SHRIMP U–Pb zircon dating provided an age of 614 ± 8 Ma attributed to the emplacement and crystallization of the granite.

San José de Guaviare Syenite, Colombia: Repeated Ediacaran intrusions in the northwestern Amazonian Craton

The Neoproterozoic igneous rocks found in the municipality of San José del Guaviare include several isolated plutonic bodies that protrude from the Phanerozoic sedimentary cover in belts aligned NW-SE. Limited to the Guaviare department, these intrusions stretch from the La Lindosa mountain range to the corregimiento El Capricho. These plutonic bodies consist of nepheline syeni­tes, nepheline monzosyenites, nepheline-bearing alkali-feldspar syenites, syenites, quartz-syenites, quartz-alkali-feldspar syenites, syenogranites, and quartz-rich granitoids, which have been grouped and termed the San José del Guaviare Syenite unit (SJGS).
 The intrusion of the unit occurred in the Ediacaran (604 ± 7 Ma and 620.5 ± 7.5 Ma) by mantle-derived alkaline magmas formed in anorogenic settings, most likely in rift-like stretching zones. The silica-subsaturated magma may have reacted with host rocks at the crust level, producing some silica-saturated igneous rocks, such as syenogranites and quartz-syenites, which are found in the El Capricho and Cerritos bodies.

Alkaline granitic magmatism of the Western Transbaikalia: Petrogenetic and geodynamic implications from U-Pb isotopic–geochronological data

The Mongolian–Transbaikalian volcanoplutonic belt extends for almost 2000 km through northern Mongolia and Transbaikalia and hosts several hundreds of plutons of alkali feldspar syenites (AFS) and peralkaline granites (PA) that formed at 280–270 and 230–210 Ma. The giant (~220 km2 in area) Sherbakhta pluton in the Vitim plateau (western Transbaikalia) is located and composed of metaluminous (A/CNK ≤1), high ferroan AFS syenites (10.7–12.3 wt% Na2O + K2O; NK/A < 1) and A-type peralkaline granites (8.7–9.5 wt% Na2O + K2O; NK/A 0.95–1.05). The U-Pb (SHRIMP-RG, LA-ICP-MS) age of the pluton is 260 Ma. The isotope and geochemical characteristics of the studied pluton (ɛNd(T) –2.4 to −2.9; TNd(DM-2) 1.25–1.27 Ga; ɛHf(T) 1.1–0.6, THf(DM-2) 1.02–1.05 Ga, δ18О of quartz 7.3–8.22‰) indicate that the parental trachytic–trachyandesitic melt is compositionally similar to high-Sr syenites of the Sherbakhta pluton and derived by the melting of the mafic, moderately potassium-rich lower crustal source. The magmatic evolution of the parental melt toward a residual ferroan peralkaline granitic melt in the intermediate chamber was controlled by fractional crystallization of sodic plagioclase and K-feldspar, followed by amphibole and aegirine under low oxygen fugacity. The A-type granitoids in the Transbaikalia region formed in the intracontinental geodynamic setting, probably as a result of impact of the mantle plume on heated plastic crust of a young orogen, which formed during the Late Hercynian fold thrust deformations.

Nepheline syenite and related rocks at Meruda Takkar hill, northern Kachchh: Neoproterozoic Malani basement or Mesozoic alkaline magmatism?

An extremely small but significant exposure of possibly Precambrian or Pre-Deccan Mesozoic plutonic rocks spanning 200 × 90 m in size with 15 m elevation, remotely located amidst vast salt flats of the Great Rann of Kachchh received attention when it was first reported by Biswas in 1968; however, systematic petrological and petrographical studies remained undone. Therefore, in the present study, we addressed complete geological accounts to establish its possible origin. Our detailed geological study indicates that the complex is made up of nepheline and aegirine bearing alkali feldspar syenite with numerous fine-to-medium grained felsic dykes. It lays two viewpoints: (1) it has affinity with Trans Aravalli Belt anorogenic felsic magmatism, i.e., Precambrian and (2) it is genetically related to the pre-Deccan Mesozoic plutonic intrusives like Nir Wandh in Pachham Island and Mundwara and Sarnu–Dandali complexes in Rajasthan. The nearest felsic magmatic suite is an exposure of gray and pink granites of Nagar Parkar of Neoproterozoic age. Presence of undersaturated minerals and xenoliths of diorites within syenite parent body may suggest partial melting of lower crust probably during Rodinia supercontinent rifting. However, presence of alkali-rich minerals, alkali feldspars, kaersutite, aegirine, apatite and rutile needles and the complex being part of the Kachchh rift basin suggests its genetic resemblance with Pre-Deccan Mesozoic plutonic events. The radiometric age will confirm the age of this plutonic massif. Based on the field geological and petrographical studies of Meruda Takkar syenitic complex, two viewpoints have been raised: Radiometric age determination and geochemical analysis of the Meruda syenite are needed to establish its age and origin.

Characteristics and significance of aegirine and arfvedsonite in Boziguoer Nb-Ta-Zr-Rb-REE deposit related to alkaline granite, Xinjiang

碱性造岩矿物能够记录碱性岩源区特征、岩浆演化以及晚期成矿的重要信息，是开展碱性岩成岩成矿研究的有效手段。波孜果尔碱性花岗岩型铌-钽-锆-铷-稀土矿床位于塔里木北缘-中亚南天山晚古生代造山带，是塔里木地块北缘铌成矿带中典型的碱性岩型矿床。本文通过对含矿岩体中的霓石和钠铁闪石开展矿物学研究，结合全岩成分揭示波孜果尔稀有-稀土金属矿床含矿岩体的岩石类型、演化特征、构造背景及成矿条件。研究发现，含矿碱性岩体由霓石钠铁闪石碱长花岗岩、霓石钠铁闪石石英碱长正长岩、霓石钠铁闪石碱长正长岩组成。不同岩相岩石均表现出相似的稀土和微量元素配分模式，以富集轻稀土元素和高场强元素，亏损大离子亲石元素和重稀土元素为特征，具有显著的负Eu异常，指示三种岩石类型是同源岩浆演化的产物。含矿岩体中的辉石为霓石-霓辉石，角闪石为钠铁闪石。霓石和钠铁闪石稀土和微量元素配分模式相似且含量都较低，富集重稀土、亏损轻稀土，具有显著的负Eu异常，并富集Zr、Hf等高场强元素，亏损Ba、Sr等大离子亲石元素。霓石和钠铁闪石微量元素特征指示三个岩相单元的演化程度由低到高依次为霓石钠铁闪石碱长花岗岩→霓石钠铁闪石石英碱长正长岩→霓石钠铁闪石碱长正长岩，且结晶分异作用控制了岩体的形成。同时，早期霓石钠铁闪石碱长花岗岩的钠铁闪石呈现Ce正异常，晚期岩相中则无异常，指示演化的早期阶段氧逸度较高，随着磁铁矿等氧化物的结晶熔体趋于还原。波孜果尔含矿岩体成矿元素除Rb外分布并不均匀，表现为演化早期的岩相富集Nb，而演化晚期的岩相更富集稀土和Zr。进一步研究发现，塔里木北缘碱性岩带发育的铌、稀土、钽、锆、铷、铀等稀有稀土金属矿化与地幔柱引起的幔源岩浆底侵有关，其构造背景可能为地幔柱对造山带的叠置。;Alkaline minerals can record the important information of source area characteristics, magmatic evolution and late mineralization of alkaline rocks, which is an effective means for the study of diagenesis and mineralization of alkaline rocks. Boziguoer Nb-Ta-Zr-Rb-REE deposit is a typical deposit related to alkaline rocks, which is located in the Late Paleozoic orogenic belt of the southern Tianshan-the northern margin of Tarim block. Based on mineralogical study of aegirine and arfvedsonite in the ore-bearing intrusion, combining with its whole rock compositions, this paper reveals the rock types, evolution characteristics, tectonic setting and metallogenic conditions of the intrusion in the Boziguer rare-earth metal deposit. The rock types of ore-bearing intrusion are aegirine arfvedsonite alkali feldspar granite, aegirine arfvedsonite quartz alkali feldspar syenite and aegirine arfvedsonite alkali feldspar syenite. All of the three types of rocks show similar REE and trace element distributions, with enriched LREE and HFSE, depleted HREE and LILE, and a significant negative Eu anomaly, indicating that they are the products of homologous magmatic evolution. The pyroxene group minerals are aegirine, and the amphibole group minerals are arfvedsonite. Aegirine and arfvedsonite show similar distribution patterns of trace and rare earth elements, which are characterized by enriched HREE and depleted LREE with a significant Eu negative anomaly. They all show enrichment in HFSE (Zr, Hf), depletion in LILE, such as Ba and Sr. Trace element and REE distributions of these minerals indicate that the degree of their evolution from low to high is: aegirine arfvedsonite alkali feldspar granite→aegirine arfvedsonite quartz alkali feldspar syenite→aegirine arfvedsonite alkali feldspar syenite, showing the formation of the intrusion is controlled by crystallization and differentiation. The arfvedsonite of aegirine arfvedsonite alkali feldspar granite shows a positive anomaly of Ce, indicating a high oxygen fugacity environment at the early stages of its evolution. The crystallization of magnetite in the early lithofacies make the evolution environment of the melt tend to be reduced. The Rb in all rock types is equally distributed, and the rocks of early evolution stage is more enriched in Nb, while that of the late-stage more enriched in REE and Zr. The mineralization of rare earth metals such as Nb, REE, Ta, Zr, Rb and U developed in the alkaline rock belt in the northern margin of Tarim basin is related to the mantle magma underplating caused by mantle plume, and its tectonic setting may be the superposition of mantle plume to orogenic belt.

The importance of in situ crystallisation and loss of interstitial melt during formation of the Kærven Syenite Complex, Kangerlussuaq, East Greenland

The Kærven Syenite Complex (KSC) is one of the oldest felsic intrusions in the Tertiary East Greenland province. Here we update our previous description of the KSC and supply a greatly expanded and comprehensive geochemical dataset. New data allow us to present a more detailed petrogenetic model for the evolution of the KSC and to investigate the geochemical characteristics of igneous cumulates subjected to loss and, occasionally, replacement of residual liquid. The KSC comprises eleven mappable units that generally young westwards. Rock types range from quartz syenite to quartz alkali feldspar syenite and alkali feldspar granite. Individual intrusive units are relatively narrow and steep-sided and are collectively suggested to represent a ring dyke complex. Basement gneiss and gabbro host rocks have locally contaminated the oldest quartz syenite KSC unit, but most of the main part of the complex escaped significant influence from host rocks. A late suite of E–W to NE–SW striking peralkaline dykes of trachytic to phonolitic compositions intrude the KSC. Compositions of the KSC rocks span a considerable range in SiO2, 59–73 wt%. Concentrations of several elements vary widely for a given SiO2 (especially at SiO2 &lt; 66 wt%), and variation diagrams do not suggest a single model for the evolution of the units of the complex. A cumulative origin is envisaged for several KSC units. Geochemical modelling suggests that KSC magmas were derived from more than one primary magma, and that the complex evolved through a four-stage process: fractional crystallisation in precursory magma chambers was followed by final emplacement of each unit, establishment of a crystal/melt mush, expulsion of part of the residual melt and, finally, crystallisation of the remaining melt. Trace element disequilibria between alkali feldspar and host rocks in two closely associated quartz alkali feldspar syenite units indicate that highly evolved residual melt was replaced by a less evolved melt phase. Modelling of potential parent melt compositions to the Kærven magmas suggests an origin not in the Iceland plume asthenosphere, but rather in a moderately enriched source, possibly in the continental lithosphere. The course of melt evolution by fractional crystallisation is indicated to have taken place in magma chambers at depth, and repeated rise of magma into the upper crustal magma chambers and crystallisation there formed the KSC. Based on our survey of published geochemical data, the inferred parental magmas seem to have few equivalents in the North Atlantic Igneous Province and may have been generated mainly from melting of enriched dry lithospheric mantle of possibly Archaean age.

The Neoproterozoic alkaline rocks from Fangcheng area, East Qinling (China) and their implications for regional Nb mineralization and tectonic evolution

An integrated study was carried out on samples from the Fangcheng alkaline complex so as to establish the origin and evolution of the Nb-rich alkaline felsic magmas. The Fangcheng complex is composed of nepheline syenites, biotite syenites, alkali-feldspar syenites, quartz syenites and syenogranites. Titanite and zircon U-Pb dating reveals that these rocks were simultaneously emplaced between 867 and 833 Ma. They are strongly enriched in HFSEs (e.g., Nb and Zr) and LREEs, with obvious depletions of Ba, Sr and Eu. Notably, all Fangcheng alkaline rocks are enriched in Nb, and this is particularly true for the nepheline syenites (Nb = 161–461 ppm). The nepheline syenites and biotite syenites yield relatively high εNd(t) values (−0.7 to −1.5) and initial 208Pb/204Pb ratios (36.12–37.61) as compared to the quartz syenites and syenogranites (−4.5 to −4.9 and 23.40–36.72, respectively). Likewise, lower zircon εHf(t) values (−4.2 to −6.9) are recorded by the quartz syenites and syenogranites. Based on our results, we suggest that the Fangcheng alkaline rocks were generated via low degrees of partial melting of an enriched mantle during which the fractionation of an alkali basaltic melt generated a nepheline syenite magma. An ongoing segregation of sodic minerals caused the parent melt to evolve towards a potassium-rich composition, leading to the generation of the biotite syenites and alkali-feldspar syenites. Ultimately, the quartz syenites and syenogranites represent the end products of a highly evolved alkaline magma containing a significant degree of contamination by siliceous-rich crustal materials. The enrichment of Nb in all lithologies was controlled by both magmatic and hydrothermal processes. In an initial F- and HFSE-rich magmatic system, prolonged fractionation of Nb-poor minerals led to the concentration of both F and Nb in the residual melts, as is recorded by the Nb-rich titanite. Hydrothermal metasomatism by Na- and F-rich fluids induced further remobilization and subsequent precipitation of Nb to form secondary titanite, pyrochlore, euxenite and rutile in a post-magmatic stage. We infer that the Neoproterozoic Fangcheng alkaline rocks were emplaced in a post-collisional extensional regime, implying that the assembly of the Rodinia supercontinent might have been completed prior to ca. 833 Ma.

Origin and evolution of ultrapotassic intermediate magma: The Songxian syenite massif, Central China

A series of magmatic syenites containing extremely high K2O contents (up to 15 wt%, mean of 12 wt%) have been identified in the Songxian region of Qinling Orogenic Belt (Central China). An integrated investigation, combining mineral composition, whole-rock elemental and SrNd isotopic geochemistry and zircon U–Pb geochronology, was conducted to track the origin and evolution of the ultrapotassic intermediate magma. The Songxian syenites mainly consist of alkali-feldspar syenite and aegirine-augite syenite, which outcrop as five independent stocks or dykes in the field. Zircon U–Pb dating of the Songxian syenites yields emplacement ages of 233–237 Ma. Alkali feldspar is the dominant mineral for both rock types, while aegirine-augite, aegirine and subordinate riebeckite are characteristic of the aegirine-augite syenite. Both the alkali-feldspar syenite and the aegirine-augite syenite belong to a weakly peraluminous to peralkaline series, and are characterized by extreme enrichment in K and large ion lithophile elements (LILEs; e.g., Rb, Ba, Sr) and prominent negative anomalies of high field strength elements (HFSEs; e.g., Nb, Ta). In comparison to the alkali-feldspar syenites, the aegirine-augite syenites have higher CaO and Na2O contents and Mg# values, but lower Al2O3 and K2O contents. All the syenites show relatively homogeneous and enriched initial SrNd isotopic compositions with initial 87Sr/86Sr = 0.7069–0.7090 and εNd (t) = −14.5 to −22.0, similar to the regional mafic rocks and indicative of an enriched lithospheric mantle source. The composition of clinopyroxene records a prolonged fractional crystallization process of the syenitic magma, which has a key role in promoting K enrichment in the residual melt. The Songxian syenites and the other analogous Early Mesozoic alkaline rocks and carbonatites in the southern margin of North China Craton were generated in a post-collisional extensional regime and represented a regional geodynamic transition from convergence to extension.

Monte Santo suite, an example of Ediacaran-Cambrian deformed alkaline rocks in the Araguaia Belt, Central Brazil. Implications for Western Gondwana evolution

Deformed alkaline rocks and carbonatites (DARCs) can be important tectonic indicators of regions that have experienced two separate events of rifting and collision, but can also represent alkaline magmas that were synkinematically emplaced along major structures and were therefore generated and deformed in the same geologic event. The Monte Santo Suite consists of two foliated and variably deformed massifs (Estrela and Monte Santo) that crop out in the Araguaia Belt and are characterized by igneous and subsolidus microstructures. In this study, we use the chemistry, mineralogy and age of the Monte Santo Suite to understand its genesis and the relations between its emplacement, the Brasiliano-Pan-African orogeny and the final evolution of Western Gondwana. The intrusions are predominantly nepheline syenite and alkali feldspar syenite. Pyroxene varies in composition from omphacite to aegirine-augite to aegirine and has mixed igneous and metamorphic characteristics. Biotite varies from phlogopite to siderophyllite to annite and presents primary, reequilibrated and neoformed trends. Amphibole varies from hastingsite to taramite. Nepheline occurs as both high-temperature and low-temperature varieties. The REE and trace element patterns display parallel trends with significant bulk content variations, positive anomalies for Nb, Ta, Zr and Hf, and negative anomalies for Ba, Sr, P and Ti in multielement diagrams, The Zr/Hf ratios for all the rocks of the massifs are similar and whole-rock εNd values range from −10.1 to −3.0. Trace element and isotopic data suggest that these magmas experienced extensive fractional crystallization, crustal contamination, and evolved from the same source, probably an enriched mantle. The Monte Santo Suite crystallization age is dated to 545 Ma, when these magmas were generated and deformed synkinematically with shearing along major regional faults related to the Transbrasiliano Lineament. Thus, this suite is an example of alkaline rocks generated and deformed in the same geological event throughout Western Gondwana.

Stitch in the ditch: Nutzotin Mountains (Alaska) fluvial strata and a dike record ca. 117–114 Ma accretion of Wrangellia with western North America and initiation of the Totschunda fault

AbstractThe Nutzotin basin of eastern Alaska consists of Upper Jurassic through Lower Cretaceous siliciclastic sedimentary and volcanic rocks that depositionally overlie the inboard margin of Wrangellia, an accreted oceanic plateau. We present igneous geochronologic data from volcanic rocks and detrital geochronologic and paleontological data from nonmarine sedimentary strata that provide constraints on the timing of deposition and sediment provenance. We also report geochronologic data from a dike injected into the Totschunda fault zone, which provides constraints on the timing of intra–suture zone basinal deformation. The Beaver Lake formation is an important sedimentary succession in the northwestern Cordillera because it provides an exceptionally rare stratigraphic record of the transition from marine to nonmarine depositional conditions along the inboard margin of the Insular terranes during mid-Cretaceous time. Conglomerate, volcanic-lithic sandstone, and carbonaceous mudstone/shale accumulated in fluvial channel-bar complexes and vegetated overbank areas, as evidenced by lithofacies data, the terrestrial nature of recovered kerogen and palynomorph assemblages, and terrestrial macrofossil remains of ferns and conifers. Sediment was eroded mainly from proximal sources of upper Jurassic to lower Cretaceous igneous rocks, given the dominance of detrital zircon and amphibole grains of that age, plus conglomerate with chiefly volcanic and plutonic clasts. Deposition was occurring by ca. 117 Ma and ceased by ca. 98 Ma, judging from palynomorphs, the youngest detrital ages, and ages of crosscutting intrusions and underlying lavas of the Chisana Formation. Following deposition, the basin fill was deformed, partly eroded, and displaced laterally by dextral displacement along the Totschunda fault, which bisects the Nutzotin basin. The Totschunda fault initiated by ca. 114 Ma, as constrained by the injection of an alkali feldspar syenite dike into the Totschunda fault zone.These results support previous interpretations that upper Jurassic to lower Cretaceous strata in the Nutzotin basin accumulated along the inboard margin of Wrangellia in a marine basin that was deformed during mid-Cretaceous time. The shift to terrestrial sedimentation overlapped with crustal-scale intrabasinal deformation of Wrangellia, based on previous studies along the Lost Creek fault and our new data from the Totschunda fault. Together, the geologic evidence for shortening and terrestrial deposition is interpreted to reflect accretion/suturing of the Insular terranes against inboard terranes. Our results also constrain the age of previously reported dinosaur footprints to ca. 117 Ma to ca. 98 Ma, which represent the only dinosaur fossils reported from eastern Alaska.