Syenite Plutons Research Articles

Petrography and geochemical characterization of the Baïbokoum syenitic pluton (Southern Chad): Implication for the magma genesis

Petrography and geochemical characterization of the Baïbokoum syenitic pluton (Southern Chad): Implication for the magma genesis

Classification, Tectonic Setting, and Mineralization Potential of Ayetoro and Sasaro Syenitic Plutons in Igarra Schist Belt, Southwestern Nigeria

Classification, Tectonic Setting, and Mineralization Potential of Ayetoro and Sasaro Syenitic Plutons in Igarra Schist Belt, Southwestern Nigeria

Structural analysis of the Banyo syenitic pluton: Implication for its synkinematic emplacement and correlations to Godé Gormaya and Rocher du Loup shear zones – Related Pan-African N–S syenites

The Banyo Syenitic Pluton (BSP) is located on the south western extension of the Mayo Nolti Shear Zone (MNSZ). It is a NNE-SSW elliptical pluton. On the petrographic view, the BSP displays two rock types namely: hornblende-pyroxene-quartz syenite (HPQS) and hornblende-biotite granite (HBG), intruded in a plutono-metamorphic basement rock consists of biotite granite (BG) and hornblende-biotite gneiss (HBGn). Structural investigations indicate that the study area recorded three deformation phases: D1, D2 and D3. D1 is a flattening phase characterized by WNW-ESE to NNW-SSE (N110°E to N160°E) trending metamorphic foliation (S1) with moderate (50°–60°) dips toward NNE to ENE. D2 trends N–S and is characterized by crenulation cleavages (S2), N–S sinistral shear and coeval S2 foliation in hornblende-biotite gneiss. The emplacement of the HPQS and (HBG) took place during this phase considering the N–S global trend of the entire pluton and the NNE-SSW shape of the HPQS in the one hand and sinistral shear deformation microstructures display by plagioclase and K-feldspar crystals in HPQS. D3 displays NE–SW (N30E to N45E) trend in HBGn and HPQS. Magnetic data indicate an inward-dipping NNE-SSW concentric pattern around station N32. The NNE-SSW elliptic shape and concentric magnetic foliation trajectories displayed by the BSP indicate its synkinematic emplacement during the D2 N–S sinistral activation of the MNSZ with the feeder zone (station N32) located on the north eastern border. The sinistral activation is related to the Saharan Metacraton convergence over the Cameroon northern margin. This emplacement was disturbed by an overprinting E–W to NNW-SSE dextral syn-D3 shear phase probably due to the dominant convergence (during its late stage) of the West African Craton over the Cameroon western border. Structural field data and magnetic fabrics infer that the Banyo syenitic pluton was emplaced in a N–S to NNE-SSW oriented fracture initiated during the transcurrent strike-slip MNSZ. The BSP is intruded in HBGn basement rock whose deformation ages are bracketed between 600 Ma (for the early syn-D1 deformation structures) and 550 (for the late syn-D3 deformation structures). The location of the BSP on the N–S to NNE-SSW sinistral MNSZ, between the GGSZ to the north and the RLSZ to the south, displaying similar structural features and N–S syn-to late kinematic syenitic plutons respectively dated at 593 Ma and 590 Ma help in dating the BSP pluton at 593 - 590 Ma. This age range dates several synkinematic emplacements of Pan-African younger syenite and granitoids along N–S and NE–SW shear zones during the western Gondwana post-collisional history.

U–Pb and 40Ar/39Ar geochronology and tectonic setting of the peralkaline magmatism in the Borborema Province (NE Brazil)

Aegirine-augite granite and syenite plutons and dikes form a distinct magmatic pulse aligned in the NE-SW direction in the central and northern Borborema Province. We use SHRIMP zircon U–Pb and laser step-heating 40Ar/39Ar determinations to constrain the crystallization and cooling ages of syenites and alkali-feldspar granites and gabbros deformed by shear zones. Concordant zircon ages of the Triunfo and Cacimbas syenites vary between 596 ± 4 and 594 ± 4 Ma while sodic amphiboles from a granitic dike of Manaíra swarm yield 40Ar/39Ar plateau ages of 606 ± 5, 605 ± 6 and 599 ± 4 Ma. Muscovite from host mylonitic gneisses, in turn, yields plateau ages of 600 ± 3 Ma and 586 ± 2 Ma. Close to the Patos shear zone which limits the Central and Northern Borborema domains, foliated felsic dike of the Serra do Algodão suite yields a concordia age of 592 ± 2 Ma whereas the Catingueira granitic pluton records numerous inherited zircons. The Taperoá gabbro stock, situated next to the connection between E− and NE-trending shear zones, yields concordant ages of 592 ± 2 Ma. These results constrain the alkaline magmatism of the Central and Northern domains between 600 and 590 Ma, i.e., shortly after the continental collisions that amalgamated the province in the Ediacaran. Cooling was faster in the Triunfo-Manaíra suite and apparently slower close to the Patos shear zone. Host rock structures indicate that the Triunfo-Manaíra suite emplaced in the brittle upper crust, whereas the alkaline rocks of the Serra do Algodão suite emplaced in HT/LP ductile transtensional setting. A possible tectonic scenario includes slab break-off in a continental collision setting, partial melting of the sub-continental mantle and lower crust and magma transfer to the middle-upper crust. The distribution of these rocks next to the margins of volcanosedimentary belts suggests that their ascent and emplacement explored deep fault segments reactivated by crustal extension.

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

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

Origin of Early to Middle Triassic ultrapotassic syenites in South China: A record of block amalgamation in East Asia

Alkaline intrusions such as ultrapotassic syenites are typically formed in extensional tectonic settings including continental rift, post-orogenic and post-collisional regimes. Here we present two early Mesozoic ultrapotassic syenite plutons (Tieshan and Yangfang) in South China that were formed in a continent collisional setting. LA-ICP-MS zircon UPb chronological, major and trace elemental, and SrNd isotopic data of these syenites are used to explore their origin and to further constrain the geodynamics of early Mesozoic magmatism in South China. Our new data indicate that the Tieshan and Yangfang plutons were formed in Early Triassic (249 Ma) and Middle Triassic (240 Ma), respectively. The Tieshan and Yangfang syenites have SiO2 contents of 50.84–56.95 wt% and 59.13–68.34 wt%, respectively, both are enriched in alkalis with very high K2O (5.60–9.68 wt%) and K2O/Na2O (1.8–15.7) and in LILE (Ba = 2464–8813 ppm; Sr = 652–3499 ppm), REE (498–1355 ppm) and HFSE (Zr = 264–766 ppm). They show enriched SrNd isotopic compositions with 87Sr/86Sr (t) and εNd (t) of 0.7096–0.7102 and − 5.2 to −6.5, respectively for Tieshan and of 0.7112–0.7119 and − 8.8 to −9.7, respectively for Yangfang. Geochemical data suggest that the studied ultrapotassic syenites were derived by partial melting of ancient subduction-modified lithospheric mantle including slab-derived melt hybridization and slab-derived carbonate fluid metasomatism, with the source depth of 60–90 km for Tieshan and at least 90 km for Yangfang. We further suggest that their origins are related to the transtension of the regional NE–trending strike-slip faults caused by continental collisions during the early Mesozoic amalgamation in East Asia.

Petrology of the late Pan-African Guider syenite pluton (Cameroon): Implications for magma genesis and emplacement

The Guider pluton, located in the eastern border of the North-West Cameroon domain close to the Mayo-Kebbi domain, is one of the rare high potassic magmatic bodies in the northern portion of Central African Orogenic Belt. Previously studied for its structural and geochronological aspects, its petrogenesis remained uncertain. Petrography, mineralogy, and whole rock geochemistry data are presented. The pluton is made of pink and grey quartz syenites and contains numerous enclaves of quartz diorites. These enclaves are highly concentrated at the center of pluton, previously identify as the pluton's feeder zone. The transition between syenitic types is gradational while the contact between grey quartz syenite and quartz diorite is sharp. Geochemically, they display a high-K signature. Our results including field observations, petrography, mineral chemistry and geochemistry suggest that these rocks derive from a single parental magma differentiated by fractional crystallization and sedimentation in a magmatic chamber. These results also suggest that the parental magma was generated by partial melting of metasomatized lithospheric mantle. Combining previous results with former structural data, we envisage that the Guider pluton derive from two successive steps: (i) a static step of fractional crystallization and sedimentation in a deep magmatic chamber; and (ii) a dynamic step during which the stratified magma ascent toward the upper crust, the evolved syenitic magma being tapped first and the diorite later. The magmatic structures are consistent with this magma emplacement scenario.

Textures and Chemical Compositions of Nb-Bearing Minerals and Nb Mineralization in the Shuangshan Nepheline Syenite Pluton, East Qinling, China

The Shuangshan alkaline complex located in the Henan province of China is a newly discovered, potentially giant niobium (Nb) deposit. A variety of Nb-bearing minerals including pyrochlore, zircon, and titanite have been identified in this deposit. Distinct textural and chemical differences of pyrochlore and zircon indicate that both have different origins. The magmatic pyrochlore and zircon both have euhedral grains with small sizes. On the other hand, hydrothermal pyrochlore is mainly intergrown on the edge or inside of hydrothermal zircon in the form of an aggregate. Compared with magmatic pyrochlore, the contents of F, Ca, and Na in hydrothermal pyrochlore are obviously high. The texture and composition of hydrothermal pyrochlore and zircon indicate that Ca-bearing hydrothermal alteration resulted in the migration of Nb from Nb-bearing zircon and the reprecipitation of Nb to form aggregate pyrochlore. However, the quantitative calculation shows that the amount of Nb migrated from zircon is very small. Therefore, this study suggests that hydrothermal alteration plays a certain role in the redistribution of Nb, but the enrichment of Nb is limited.

Neoproterozoic tectonic transition from subduction-related convergence to continental extension of the Tarim Block, NW China

The Tarim Block, one of the three largest cratons in China, plays an important role in the reconstruction of the Rodinia supercontinent. Knowledge of the Neoproterozoic tectonic evolution of the Tarim Block, especially the tectonic transition from convergence to rifting, remains unclear. The Aksu terrane, an integral part of the Tarim Block, is marked by the occurrence of Neoproterozoic blueschists, conglomerates, and mafic rocks that may provide important constraints on its tectonic history. We present an integrated study involving detailed field observations, whole-rock geochemistry, and zircon U-Pb geochronology for rhyolitic clasts in a Neoproterozoic conglomerate, a syenite pluton and mafic rocks in the Aksu region. Zircon U-Pb dating of the rhyolite yielded a crystallization age of 840 ± 4 Ma. Geochemically, the rhyolite is characterized by enrichment of light rare earth elements and depletion of high field strength elements, indicating a subduction-related arc setting. Syenites interspersed with mafic intrusions were crystallized at ~755 Ma, representing fractional crystallization products from intra-plate mantle-derived basaltic magma. Based on a compilation of previous studies, we find that the Late Neoproterozoic–Early Cambrian mafic rocks in the Aksu region can be divided into three phases. Phase I mafic intrusions (~760–745 Ma) occurring as dikes intruded into the blueschist-bearing Aksu Group. Phase II mafic intrusions (~755 Ma) intruded into the Neoproterozoic Qiaoenbulake Formation. Phase III mafic rocks (~520 Ma) are sills or basalts hosted within the Sugaitebulake Formation. Detailed studies suggest that subduction of oceanic lithosphere in the Aksu area persisted until at least ~840 Ma. The transition from ocean-continent subduction to continental extension took place around ca. 760 Ma. At 755 Ma, the northwestern Tarim region experienced significant extension.

The age and geochemistry of the Bardkish syenite, northwest Iran: Syenite formation during Neo‐Tethyan subduction

AbstractSHRIMP U–Pb dating of zircon from two representative samples of the Bardkish syenitic pluton, Urumieh plutonic complex, northwestern Iran, yielded a Late Cretaceous weighted mean 206Pb/238U age of 91.1 ±0.8 Ma (Turonian). The pluton is characterized by high K2O content, Ti–Nb–Ta depletion, enrichment in light rare earth elements and large ion lithophile elements compared to high field strength element, Mg‐rich biotite and calcic amphibole, suggesting a shoshonitic affinity. Geochemical features of the Bardkish syenite are consistent with derivation by fractional crystallization of initial mafic parent magmas. It is most likely that primary magma of the syenite originated from a metasomatized subcontinental lithospheric mantle source with minor crustal component, and underwent a fractional crystallization process during its emplacement. The estimates of trapping pressures and temperatures suggest that crystallization began at the middle crustal level (less than 11 km) near 700–730 °C. The generation of the Bardkish syenite could be related to the subduction of Neo‐Tethyan oceanic crust beneath the Central Iranian microcontinent and it possibly marks a major magmatic episode prior to the final closure of the Neo‐Tethyan ocean in northwest Iran.

Petrography and Mineralogy of the Quartz and Quartz-Feldspar Sulphide Veins in the Pan-African Syenitic Massif of Guider (North Cameroon)

In the syenitic pluton of Guider (593 ± 4 Ma) in the North-West Cameroon domain of Central African Fold Belt, mineralized N-S to NE-SW vertical or sub-vertical quartz and quartz feldspar veins has been recently identified. In this contribution, we present petrography and mineralogy of these veins, in order to constrain their genesis and emplacement mechanisms based on detailed field work, petrographic studies and chemical characterization of minerals by using an electron probe microanalyser (EPMA). Field observations and vein microstructures show that the emplacement of the veins has been controlled by the dextral N-S trending strike-slip shear zones related to the regional D2 deformation phase. The results of mineralogical analysis reveal the co-presence of silicates and metallic minerals that include magnetite, ilmenite, pyrite, bismuthite, galena (very rare) and sulphide complexes (BiPbS, BiAgPbS, FeBiPbCuS, BiFe(TeS), FeBiPbS, BiPbCuS). The gangue is represented by quartz (quartz 1, quartz 2 and quartz 3), feldspars, sericite, chlorite, yellowish brown clay minerals, and hematite. The textural relationships between sulphides, quartz and alteration products show that the mineralization is essentially syn- to late-D2 and suggest that syenitic country rock and dextral shear zones have played an important role in the metallogenesis of these veins. This mineralization shows characteristics for copper-bearing calc-alkaline deposits, but differs from these by its more extensive alteration and its abundance in hematite. Substitutions of Al(IV) by Si(IV) in sericite associated with the sulphide mineralization and cataclastic deformation suggest that the temperature of trapping of the fluids is between 230°C and 275°C.

Magnetic fabrics of the neoproterozoic piquiri syenite massif (Southernmost Brazil): Implications for 3D geometry and emplacement

The study of magnetic fabrics and rock magnetic properties, together with geological and structural mapping, was carried out in a syenite pluton to investigate its shape and emplacement history. The Piquiri Syenite Massif (PSM) is an alkaline pluton which exhibits S >> L magmatic fabric and is interpreted to be part of the last Neoproterozoic post-collisional magmatic episodes in southernmost Brazil. Thermomagnetic curves, hysteresis data and coercivity spectra obtained from representative samples of different facies in the massif reveal that magnetic susceptibility is dominated by ferromagnetic minerals, especially magnetite. Magnetic fabric data were determined by using Anisotropy of Magnetic Susceptibility (AMS) and Anisotropy of Anhysteretic Remanence (AARM). Both fabrics are coaxial, and the parallelism of AMS and AARM tensors in more than 84 % of the sampled sites rules out the possibility of significant effects of Single Domain (SD) crystals. The magnetic foliation is concordant with the magmatic foliation field measurements, both parallel to pluton contacts, with high, inward dip angles. The magnetic lineation shows distinct but related behaviour from one facies to another. It is dominantly subvertical in the marginal facies rocks and plunges at moderate to shallow angles in the main facies. It is sub-horizontal in the quartz-syenites and plunges at shallow angles in the granitic rocks. Oxidizing conditions determined from the study of magnetic mineralogy leads to challenge former interpretation of in situ differentiation and crystallization and points to the multi-intrusive character of the pluton. Field relations such as fragments of marginal facies rocks found within the main facies rocks, which are in turn intruded by quartz-syenites, together with the general absence of contact metamorphism except near the marginal facies, lead to interpret that a sequence of magmatic pulses have built up the pluton. Thus, a first magmatic pulse may have heated the host rocks and resulted in the marginal facies which was followed by the next pulses to form the main facies and the quartz-rich varieties, therefore constructing the pluton from outside inwards.

Insights on the origin of the Graciosa A-type granites and syenites (Southern Brazil) from zircon U-Pb geochronology, chemistry, and Hf and O isotope compositions

The Neoproterozoic, post-collisional, Graciosa Province (southern Brazil) comprises several A-type intrusions and related volcanics of alkaline and subalkaline (or aluminous) petrographic associations. U-Pb age determinations, trace element, and Hf and O isotopic compositions were obtained for zircons from six distinct A-type granitic and syenitic plutons from both alkaline and subalkaline associations from the province. Geochronological results indicate that their emplacement, petrogenetic evolution and crystallization spanned over a ~9 million years interval, with peak magmatism at ~580 Ma. Plutons were emplaced in a post-collisional extensional regime related to the geodynamic evolution of the south-southeastern part of the Gondwana supercontinent. Hf isotopic compositions are highly variable, with εHf(t) values between +1.4 to −31, whereas δ18O signatures are mainly between +4.7 and +6.0‰. These features, along with trace element signatures, especially for Hf, U, Nb, and rare-earth elements, suggest that generation of the Graciosa rocks occurred via partial melting of a lithospheric mantle previously metasomatized by slab-derived fluids, followed by variable degrees of crustal contamination at depth.

Genesis of porphyry Mo deposits linked to gradually dehydrating subcontinental lithospheric mantle metasomatised by previous subduction in northeastern China

Many Mesozoic Mo-bearing granites, present along the northern margin of the Precambrian North China Block (NCB) and Phanerozoic Xing'an-Mongolian Belt (XMB) within the Central Asian Orogen (CAO) have hydrated sources. These granites are temporally and spatially related to biotite-rich mafic, alkali syenitic plutons, are highly evolved, have an arc-like trace element geochemical pattern, and were emplaced between ca. 245–129 Ma bracketed by A2- and A1-type granites. This conflicts temporally with the current model involving the westward subduction of the Pacific oceanic plate. We use Sm/Nd and Th/Yb ratios of Mesozoic nepheline-bearing pyroxene-biotite syenites (biotite-syenites), mantle-peridotite xenoliths from Palaeozoic diamondiferous kimberlites, and Mesozoic to Cenozoic alkali basalts to define a hydrous melt-extraction trend. The results indicate a Palaeozoic mantle-peridotite source that is more metasomatised than the lower continental crust and the source of the Mesozoic (<129 Ma) to Cenozoic mantle-peridotite xenoliths (Th/Yb < 1). This indicates that the sub-continental lithospheric mantle (SCLM) beneath northeast China has been gradually dehydrating since the Palaeozoic, and the lower continental crust and Cenozoic SCLM in northern NCB are too depleted to be the magmatic source for the Mo-bearing granites. The results of the study also indicate that the Palaeozoic SCLM beneath the northern margin of the NCB was more metasomatised than beneath its central zone. The development of Mo-bearing granites in both the Precambrian northern margin of NCB and Phanerozoic XMB indicate that the presence of an ancient SCLM is not necessary for the genesis of Mo mineralisation. From these observations, it is proposed that the genesis of Mo mineralisation in the region is attributed to a hydrated SCLM metasomatised by the subduction of the Paleo-Asian oceanic plate rather than the younger Pacific oceanic plate.

Tracing magma mixing and crystal–melt segregation in the genesis of syenite with mafic enclaves: Evidence from in situ zircon Hf–O and apatite Sr–Nd isotopes

Alkaline rocks contain essential information on post-collisional/intraplate extensional magmatic processes within the continental lithosphere. The Erhulai syenitic pluton in the Liaodong Peninsula, North China Craton, consists of quartz syenite, mafic enclaves, and porphyritic K-feldspar granite. Field observations and zircon U–Pb dating reveal that the quartz syenite and the hosted mafic enclaves were coeval (with identical emplacement ages of 129–126 Ma), and slightly older than the porphyritic K-feldspar granite (ca. 123 Ma). The mafic enclaves have relatively low SiO2 and high MgO contents (up to 3.0%), and lack Ba or Eu anomalies. Whole-rock analyses of the mafic enclaves and accessory minerals in the enclaves show relatively low initial 87Sr/86Sr ratios (0.7056 to 0.7078) and variable εNd(t) (−11.2 to −2.0) and εHf(t) (−15.8 to −5.5) values. Large proportion of zircons from the mafic enclaves possess mantle-like δ18O values. These geochemical features suggest an enriched mantle source, with the addition of crustal material. The quartz syenites have relatively high SiO2 and low CaO and MgO contents, as well as significantly positive K, Ba, and Eu anomalies. All the quartz syenites show variable (87Sr/86Sr)i ratios (0.7073 to 0.7081) and εNd(t) values (−10.4 to −8.6) for the whole-rocks and apatites, εHf(t) values of −12.4 to −3.6 for the zircons, and zircon δ18O values of +4.4‰ to +7.0‰ (mainly greater than +5.3‰). All these features can be isotopically produced by the partial melting of ancient crustal material, with the involvement of mantle-derived magma. Crystal fractionation process, such as K-feldspar crystallization and accumulation, played an important role in the formation of the quartz syenite. The porphyritic K-feldspar granites possess relatively evolved geochemical compositions with high silica contents (SiO2 > 69 wt%) and remarkable negative Ba and Eu anomalies, which are chemically complementary to the quartz syenite. A crystal mush model was employed to establish the genetic link between the porphyritic K-feldspar granite and the quartz syenite, with the granite representing the interstitial melt extracted from the crystal mush and the quartz syenite representing the residual accumulation of crystals. The variable isotopic compositions of the quartz syenite and the porphyritic granite were mainly the result of extensive interaction of isotopes between mantle- and crust-derived magmas during magma mixing.In situ analyses of the Sr, Nd, Hf, and O isotopes of accessory minerals, coupled with field observations and whole-rock geochemical data for the Erhulai syenitic pluton, indicate a complex multi-stage magmatic process involving magma mixing, crystal–melt segregation, and wall-rock assimilation. Our study also shows that in situ isotopic analyses of accessory minerals provide unique information on, and are powerful geochemical tracers of, mantle–crust interactions and magmatic processes.

The Overmaraat-Gol Alkaline Pluton in Northern Mongolia: U–Pb Age and Preliminary Implications for Magma Sources and Tectonic Setting

A new Wenlockian zircon U–Pb age (~426 Ma) of the Overmaraat-Gol nepheline syenite (foyaite, juvite) pluton in the SW Lake Hovsgol area (Northern Mongolia) prompts a long history of alkaline magmatism in the western Central Asian Orogenic Belt, exceeding the duration of the Devonian and Permian–Triassic events. The LILE and HFSE patterns of pluton samples analyzed by X-ray fluorescence (XRF) and inductively coupled plasma (ICP-MS) methods indicate intrusion in a complex tectonic setting during interaction of a mantle plume with accretionary-collisional complexes that previously formed on the active continental margin. As a result, the parent magma had a heterogeneous source with mixed mantle (PREMA and EM) and crustal components. This source composition is consistent with Nd–Sr isotope ratios of the Overmaraat-Gol alkaline rocks, from −0.1 to −1.2 εNd(t) and from ~0.706 to 0.707 87Sr/86Sr(t).

Petrogenesis of the Middle Devonian syenites in the Miyun–Huairou Region of Beijing and its tectonic implications

Three Devonian magmatic rocks have been identified from the mid‐western part of the Yanshan Thrust‐and‐Fold Belt (YTFB). Here, we present new geochronological, Sr–Nd–Hf isotopic, and whole‐rock chemical data. Our new LA‐ICP‐MS U–Pb dating recognized syenite plutons of ca. 385 Ma, which include Laogongying, Shicheng, and Qiaomaiyu, respectively, around the Yunmeng Mountain in the Miyun–Huairou area of Beijing. Rocks from three plutons are characterized by low SiO2 and high alkali contents, especially high K2O, and they show syenite and foid syenite compositions. They display slight–middle light REE enrichment, no negative or positive Eu anomalies, strong depletion in Rb, Th, U, Nb, Ta, P, Zr, Hf, and Ti, enrichment in Ba, K, and Sr, low contents of Y and Yb, and high Sr/Y ratios. They have a relatively narrow range of isotopic compositions with initial 87Sr/86Sr ratios from 0.708203 to 0.710477, εNd(t) values from −12.26 to −13.28, and εHf(t) values from −19.92 to −17.96. This study shows the Middle Devonian intrusions were produced by fractional crystallization of a parental magma, which was derived by interaction between mantle‐derived alkaline magmas of a type I‐enriched mantle and pre‐existing continental crust. Tectonic background was likely emplaced in a back‐arc extension environment related to southward subduction of the Palaeo‐Asian oceanic plate or during the cessation of the subduction. Feldspar geothermometer and aluminium‐in‐hornblende barometry studies of the Middle Devonian plutons yielded emplacement temperature of ~700°C and pressure of ~6.5 kbar, indicating the emplacement depths of about 24 km. Combined with previous geobarometry results on the Middle Devonian plutons within the Inner Mongolia Palaeo‐uplift (IMPU) on the northern margin of the NCC, the strong uplift and exhumation the IMPU were identified during Middle Devonian to Late Triassic times. The exhumation was more than 4–5 km than those of the IMPU, resulting in widely absent of Palaeozoic strata within the YTFB.

Kinematics, strain pattern and geochronology of the Salem-Attur shear zone: Tectonic implications for the multiple sheared Salem-Namakkal blocks of the Southern Granulite terrane, India

Structural mapping, strain analysis, and a variety of geochronological studies were carried out to determine the tectonothermal evolution of the Salem-Attur shear zone in the Southern Granulite terrane of South India. The Salem-Namakkal blocks containing the shear zone consisted of quartzofeldspathic gneiss, charnockite and mafic granulite, and had undergone multiple phases of magmatism spanning over a period of 3.2–0.5 Ga. The rocks were deformed by four phases of deformation D1–D4. The D1 deformation was characterized by isoclinal and recumbent NE-SW trending F1 fold with a pervasive subhorizontal axial planar granulitic fabric, S1, and associated quartzofeldspathic leucosomes. Granulite metamorphism was dated at ca. 2.5–2.3 Ga. The F1 fold and S1 fabric were coaxially refolded by tight to isoclinal, upright to steeply inclined NE-SW trending F2 folds during D2 deformation. The D2 deformation was associated with F2 axial planar shear zones, crenulations and leucosomes, S2 fabric. Large-scale D2 shear zones characterized by high-temperature ductile shear fabric with a vertical flow host syntectonic syenite pluton which was dated at ca. 2.5–2.4 Ga. A P-T condition of 7 kb/600 °C was inferred for the D2 deformation. The D3 deformation was characterized by NW-SE to E-W trending F3 folds and the Salem-Attur shear zone. The shear zone was a greenschist to amphibolite facies shear zone being characterized by mylonitic foliation and dominantly down-dip stretching lineation defined by quartz, biotite and hornblende minerals and dated at ca. 2.0 Ga. It indicated N-NNE vergence of thrusting with the mean kinematic vorticity number, Wm, as 0.7 suggesting general simple shear strain with 50% pure shear component. The D4 deformation was manifested as NNE-SSW striking strike-slip faults and NW-SE striking extensional normal faults. Pseudotachylite veins having an age of 1.9 Ga injected during strike-slip faulting and granite-pegmatite veins showing age of 0.8–0.5 Ga intruded during normal faulting. The Salem-Namakkal blocks thus recorded a long-lived shearing history. We suggest that the Salem-Attur shear zone and other shear zones such as Palghat-Cauvery, Moyar, Bhavani, Karur-Kambam-Painavu-Trichur and Achankovil shear zones, were Paleoproterozoic intraterrane shear zones which were overprinted by Meso-Neoproterozoic-Cambrian ductile and brittle deformations.

U-Pb geochronology, bulk-rock geochemistry and petrology of Late Cretaceous syenitic plutons in the Gölköy (Ordu) area (NE Turkey): Implications for magma generation in a continental arc extension triggered by slab roll-back

Late Cretaceous felsic plutons in the Eastern Pontides orogenic belt of Turkey are found in varying ages, sizes and compositions. The Late Cretaceous syenitic bodies, namely Direkli and Yeniköy Tepe plutons in the Ordu area, intruded into the Late Cretaceous volcanics and volcanoclastics, and compositionally consist of syenite and minor quartz-syenite with mainly granular and rarely porphyric, poikilitic and perthitic textures. The studied plutons contain K-feldspar, plagioclase, quartz, clinopyroxene, biotite and minor hornblende, and Fe-Ti oxide, apatite, and zircon. Mineral thermobarometer estimations imply that the syenitic plutons were crystallized in P-T conditions representing mid to shallow crustal levels. Petrochemically, these syenitic plutons show I-type, metaluminous-peraluminous transitional (A/CNK = 0.85–1.04) and shoshonitic features. LA-ICP-MS U-Pb zircon dating on the syenitic plutons yielded weighted mean ages of 78.5–72 Ma. Primitive mantle-normalized trace element patterns of the studied plutons are characterized by negative anomalies of Ba, Sr, Nb, Ta and Ti, and enrichment in Rb, Th, K, Ce and La. The chondrite-normalized rare earth element (REE) plots of the studied plutons show moderately enriched concave-shaped patterns (LaN/LuN = 10.3–10.7 for the Direkli pluton; LaN/LuN = 6.8–11.7 for the Yeniköy Tepe pluton) with negative Eu anomalies. The plutons have 87Sr/86Sr ratios (0.705368–0.706954), 143Nd/144Nd ratios (0.512600–0.512672), 206Pb/204Pb ratios (19.29–19.52) and 207Pb/204Pb ratios (15.65–15.67). The geochemical data, as well as regional geology, suggest that the Late Cretaceous syenitic plutons have formed in an extensional continental arc setting triggered by slab roll-back. The parental magma of syenitic plutons was derived from melts of lower crustal materials (metabasalt/metaandesite) and lithospheric mantle component.

Triassic alkaline magmatism and mineralization in the Xiong'ershan area, East Qinling, China

The Qinling orogenic belt is a complex subduction–accretion–collision orogen that welded the North China Craton and the Yangtze Craton during the final continental collision in the Triassic. The Xiong'ershan area, located in east Qinling, exposes a typical Triassic syenite pluton and several contemporaneous Mo, Au, and Cu deposits. The aegirine–augite syenites and syenites from the Mogou pluton are characterized by alkalic to peralkalic (total alkali Na2O + K2O = 13.95–14.63 wt.%, CaO = 0.06–2.87 wt.%), and shoshonitic features (K2O = 11.86–14.34 wt.%). Zircon LA–ICP–MS U–Pb dating of the aegirine–augite syenite and syenite yield emplacement ages of 232.5 ± 0.6 and 221.8 ± 0.7 Ma, indicating multiple pulses of magmatism. Evidence from zircon Hf isotopes; occurrence of mafic microgranular enclaves; heterogeneous peralkaline composition; and wide ranges of MgO, Ni, and other trace elements suggest that the parental magma was mainly sourced from partial melting of Archean to Paleoproterozoic crustal sources, mixed with juvenile mantle‐derived mafic magmas. The Mogou pluton was probably emplaced in the tectonic transition from syn‐collision to post‐collision settings and accompanying slab break‐off process, from the commencement of collision at approximately 245 Ma and post‐collisional extension at approximately 210 Ma. Gold, molybdenum, and copper deposits formed during the interval of 255–208 Ma, and the close temporal and spatial relationship between these Triassic polymetallic deposits and the Mogou alkaline pluton invokes a genetic linkage. The heat source for magmatism and related metallogeny is correlated to a hot upwelling asthenospheric mantle that caused partial melting of the Archean to Paleoproterozoic crustal basement, resulting in magma mixing between the two end‐members.