Calc-alkaline Affinity Research Articles

The Açdif Gold-Bearing Shear Zone (Zenaga Inlier, Central Anti-Atlas, Morocco): New Petro-Structural and Geochemical Data

The Açdif gold deposit is located in the Zenaga Inlier (central Anti-Atlas), approximately 120 km southwest of the city of Ouarzazate. It hosts gold mineralization associated with a shear zone affecting the metamorphic and magmatic formations of the Eburnean basement. It mainly consists of alternating micaschists, augen gneisses, and orthogneiss. These crystalline rock assemblages are intruded by a variety of granitoids. These are the Azguemerzi granitoids, which are locally affected by Eburnean deformation. Subsequently, these facies are intersected by a swarm of mafic dykes, which show a variety of rocks ranging from microgabbro-dolerite to quartz diorite. Detailed mapping, petro-mineralogical investigations, and geochemistry of the major and trace elements of these magmatic intrusions suggests the following: (i) for the granodiorite rocks (deposited before the basic dykes), a calc-alkaline affinity, with a chemical signature similar to a syn-collisional context; (ii) for mafic dykes, a contemporaneous emplacement of these mafic intrusions with an evolutionary process controlled by fractional crystallization of the same magma of continental tholeiites, whose chemical composition is comparable to that of enriched MORBs (EMORBs). These continental tholeiites are related to a distensive tectonic context that would have affected the Zenaga inlier prior to the Pan-African orogeny.

Geochemistry and geochronology of the Imbicuí Complex, Western Dom Feliciano Belt, Brazil: Tonian adakite-like subduction-related rocks

Dioritic-tonalitic-trondhjemitic metamorphic rocks represent the Imbicuí Complex in Western Dom Feliciano Belt, São Gabriel Terrane, Lavras do Sul region, Brazil. This complex presents rocks with an adakite-like signature showing high Sr contents (573.5–1337 ppm), Sr/Y ratio higher than 64.08, low Y content (< 33.7 ppm), (La/Yb)N > 10, and YbN < 16 being classified as Type 1, high silica and high-Al2O3 adakites. The adakites present a low-K calc-alkaline affinity, with a U–Pb LA-ICP-MS in zircon crystallization age of 828 ± 8 Ma, the product of an eclogitic melting source. Subordinate, metagabbroic rocks occur with tholeiitic affinity, presenting a U–Pb LA-ICP-MS in zircon crystallization age of 845 ± 5 Ma. The adakitic rocks showed εNd(828) (+ 8.2 to + 7.5), model ages Nd-TDM of 0.8 and 0.9 Ga, similar to the age of crystallization, suggesting that these magmas represent juvenile rocks generated in an orogenic environment mixed with reworked rocks (ratios 206Pb/204Pb and 207Pb/204Pb, 17.70 to 18.40 and 15.49 and 15.55) during an oceanic slab melting associated to the closure of Charrua Ocean throughout Tonian in the Western Dom Feliciano Belt.

Titanium and iron isotopic records of granitoid crust production in diverse Archean cratons

Archean granitoid provides critical clues on how Earth's felsic crust was established and its geodynamics evolved. In this study, we present Fe-Ti isotope and trace-element data for granitoids (mostly TTGs) from the ∼3.8-3.6 Ga Itsaq Gneiss Complex and ∼3.3 Ga East Pilbara Terrane, Pilbara Craton. TTGs from those localities and several other cratons follow the same Ti isotopic fractionation trend as modern calc-alkaline rocks. This similarity hides important petrogenetic differences as partial melting could have played a much more important role in establishing the felsic character of TTGs compared to modern granites of calc-alkaline affinity. Our thermodynamics-based modeling of isotopic fractionation shows that partial melting of hydrated metabasite can explain the Fe and Ti isotopic compositions of many TTGs but reworking of tonalite crust is needed to explain the most extreme compositions. The same Ti isotopic fractionation trends have been found now in several cratons (West Greenland, Slave, Pilbara, and Kaapvaal cratons) in granitoids formed in different geothermal gradients, meaning that it was likely a global signature of the Archean crust. Sediment provenance studies that use those TTG compositions and point to a primarily felsic crust in the Archean are therefore valid.

Genesis of the Kuh-Panj porphyry copper deposit, Kerman, Iran: Constraints from mineralization, geochemistry, fluid inclusion, and zircon U[sbnd]Pb isotope systematics

The Miocene Kuh-Panj porphyry copper deposit, with reserve estimated at about 100 Mt. at 0.21 % Cu, is located 120 km SW of Kerman in the southern part of the Urumieh-Dokhtar magmatic arc. The Eocene host rocks consist of tuff, lithic tuff, breccia tuff, and lava flows, ranging in composition from rhyodacite to andesite. A late Miocene granodiorite porphyry stock (8.57 ± 0.49 Ma, U/Pb zircon) intruded into the Eocene sequence. Subvolcanic dykes cut the Eocene rocks and porphyry stock. The Kuh-Panj porphyry system has an extensive and typical hydrothermal alteration zoning pattern, which is comprised of potassic, phyllic, argillic, and propylitic alteration types from inner proximal to outer distal zones. Copper mineralization occurs in potassic and to some extent in phyllic zones. Mineralization is mostly observed as disseminated, stockwork, and vein-veinlets forms in the porphyry stock. Vein paragenesis includes barren quartz, M-type (magnetite+biotite), A1-type (quartz+magnetite+chalcopyrite±chlorite±biotite), A2-type (quartz+chalcopyrite±pyrite), D-type (quartz+pyrite±chalcopyrite), and L-type (galena+sphalerite±chalcopyrite±pyrite) veins. The I-type granitoids with calc-alkaline affinity have enrichments of LILE (Pb, K, U, Sr, and Cs), depletion of HFSE (P, Nb, Ta, Zr, and Ti), and enrichment in LREE displaying geochemical similarity to arc-related magmatic rocks. The high SiO2 (ave. 68.9 wt%), high Al2O3 (ave. 14.5 wt%), low MgO (ave. 1.3 wt%), low P2O5 (< 0.2 wt%), high Sr (ave. 365 ppm), high Sr/Y (ave. 45), relatively high LaN/YbN (ave. 28), relatively high Th (ave. 8 ppm), low Y (ave. 8 ppm), and low Yb (0.5 ppm) contents are similar to those of fertile porphyries and lower crust-derived adakites. The most dominant type of fluid inclusions in quartz from mineralized veins are liquid-vapor-halite-solid (LVHS), liquid-vapor-halite (LVH), liquid-vapor (LV), and vapor-liquid (VL) fluids. Homogenization temperatures of LVHS, LVH, LV, and VL fluids range from 306 to 574 °C, 141–566 °C, 215–422°, and 349–571 °C, respectively. Salinities of LVHS, LVH, LV, and VL fluid inclusions range from 30 to 69, 29.0–65.5, 0.35–22.0, and 0.7–13.0 wt% NaCl eq., respectively. The extreme variations in salinities of fluid inclusions represent the combined effects of varying salinity of the magmatic fluid, aqueous fluid-phase immiscibility (boiling), and mixing with lower salinity meteoric water later in the cooling history of the porphyry system. Normal cooling, water-rock interaction, surface fluid dilution, and boiling are dominant factors that control both the distribution and abundance of mineralization. Zircon from the Kuh-Panj system has high Th/U (0.53 to 0.83), low Dy/Yb (<0.3) ratios, positive Eu/Eu* (1.28–1.85), strong positive Ce/Ce* (1.36 to 197) reflecting high Ce4+/Ce3+ ratios, and enrichment in HREEs relative to LREEs, which indicate the magmatic origin of zircon, oxidized, and hydrous conditions of the primary magma. This magma with adakitic affinities is favorable for producing fertile porphyry systems. Based on these multidisciplinary studies, the Kuh-Panj deposit is classified as a continental arc-related porphyry copper system hosted predominantly at the transition zone between potassic and phyllic alteration zones due to decreasing temperature, boiling, and fluid-rock reaction that all enhance CuFe sulfide saturation.

Sr-Nd-Ca isotopic variations of Cenozoic calc-alkaline and alkaline volcanic rocks above a slab tear in Western Anatolia, Turkey

The tectonic mechanisms that may trigger a transition from calc-alkaline to alkaline volcanic activity above a subduction zone are enigmatic. We report major/trace elemental and Sr-Nd-Ca isotopic compositions of a suite of magmatic samples from the Miocene Gördes calc-alkaline dacites and the Quaternary Kula alkaline basalts from Western Anatolia, Turkey. Zircon sensitive high-resolution ion microprobe dating shows that the Gördes dacites formed at ca. 18 Ma. They are characterized by high-K calc-alkaline affinities, enrichment of light rare earth elements and large-ion lithophile elements (Rb, Th, U), positive Pb anomalies and negative Eu and high field strength element (HFSE) (Nb, Ta, and Ti) anomalies with high 87Sr/86Sri (0.70977−0.71010) and low εNd(t) (−7.3 to −7.0). They have small δ44/40Ca (0.59‰−0.69‰) values lower than that of mid-ocean ridge basalt (0.83% ± 0.11%). The Kula basalts exhibit alkaline affinities and are more enriched in incompatible elements (e.g., Rb, Ba, Th, Nb, Ta, Pb, and Sr) than those of typical oceanic island basalts. Their low 87Sr/86Sri (0.70307−0.70343) and high εNd(t) (+4.2 to +6.5) are consistent with an asthenospheric mantle source. Their δ44/40Ca values, ranging from 0.67‰ to 0.81‰, are higher than those of the dacites and lower than that of the bulk silicate earth (0.94‰ ± 0.10‰). Geochemical modeling suggests that variable degrees of partial melting of an asthenospheric mantle involving recycled oceanic crust may have resulted in low δ44/40Ca values of the Kula basalts. Combined with reported mantle tomographic images, we interpret that sub-slab asthenospheric mantle upwelling through a slab tear during subduction roll-back may have played a key role in the Sr-Nd-Ca isotopic variability from Miocene calc-alkaline dacites to Quaternary alkaline basalts in Western Anatolia.

Volcanic rocks from Amensif-Tnirt district in the Western High Atlas (Morocco): Geochemistry, magma features and new age dating

The High Atlas Mountains include mafic, intermediate and felsic volcanic-subvolcanic intrusive rocks, representing a result of magmatism related to “Cambrian rift” during the Lower Cambrian.The investigated magmatic rocks in the Amensif-Tnirt area are divided into three main groups as the mafic (SiO2 48.31 to 53.06 wt%), intermediate (SiO2 59.16 to 67.38 wt%), and felsic (SiO2 76.39 to 79.65 wt%) rocks. The mafic rocks correspond to microgabbros with high TiO2 contents (1.98–2.12 wt%) and FeOt/MgO ratios (1.65–1.90), La/Yb (9.09–11.60) and Nb/Zr (0.03–0.04) ratios, providing good evidence of extensive intracontinental tholeiite. The Nb/Yb and Zr/Y ratios ranging between 0.36 to 0.52 and 12.76 to 13.69, respectively suggest their derivation from an enriched mantle source during the opening of the Rheic ocean along the NW edge of Gondwana. The mafic rocks may be interpreted as the mixtures of melts derived from two contrasting mantle sources: (1) a garnet-bearing clinopyroxenitic ultramafic source and (2) an ultramafic source composed of spinel and peridotite.The intermediate and felsic rocks correspond to andesite-dacite and rhyolite with TiO2 contents and FeOt/MgO ratios ranging from 0.09 to 1.36 wt% and 1.32 to 4.69, respectively, suggesting their calc-alkaline affinities setting up during a compressive context. The youngest 206Pb/238U ages for intermediate rocks are 290.29 ± 4.68 and 291.27 ± 4.31 Ma, while the youngest felsic rocks yield a concordia age at 272.83 ± 0.73 Ma (MSWD = 0.25). Petrographic and geochemical data indicate that mafic magmatism in the Amensif-Tnirt region may be the equivalent of the first Cambrian volcanic episode in the High Atlas. Intermediate rocks are chronologically correlated to the Permian volcanic activities recorded in Central Morocco (Tiddas, Khenifra, Chougrane …) whereas felsic activity is correlated to the Permian Azegour granite from the High Atlas.

Magmatic evolution in an open and polybaric system feeding the Miocene calcalkaline lava domes of Cixerri (SW Sardinia, Italy)

The lava domes of the Cixerri half-graben (SW Sardinia) are part of the subduction-related igneous activity that developed in Sardinia during the Upper Eocene-Middle Miocene (38–12 Ma), with calcalkaline and high-K calcalkaline affinity. The investigated rocks are porphyritic basaltic andesites and andesites, with amphibole, plagioclase and minor clinopyroxene phenocrysts included in a groundmass composed of the same phases together with feldspar, quartz and opaque oxides. 40Ar/39Ar analyses of plagioclase and amphibole separates from an andesite indicate a crystallization age of 21.31 ± 0.05 Ma. The high abundance of amphibole in the Cixerri rocks, unlike andesites from the neighboring Sardinian districts, could be explained with the high calculated oxygen fugacity (NNO+1–NNO+2) and H2O content (up to 9 wt%) in the Cixerri magmas. Bulk-rock major and trace element variations, and mass balance calculations are consistent with a magma evolution mainly driven by fractional crystallization of amphibole and plagioclase, which occurred in a polybaric plumbing system, as highlighted by the calculated pressure of amphibole crystallization (2–4 kbar and 6–9 kbar). Bulk rock isotope variations (87Sr/86Sri = 0.70701–0.70786 and 143Nd/144Ndi = 0.512328–0.512436) indicate that the magma evolution took place in open-system conditions with a low degree of crustal assimilation. The HREE flat patterns point to a magma source in the spinel stability field located in a mantle wedge depleted in incompatible elements and metasomatized by slab and sediment derived fluids and melts.

The Sr, Nd, Pb and Hf isotopes and crystallization conditions of the middle Eocene Dağdibi Pluton in the eastern Sakarya Zone, Turkey

AbstractMagmatic activity in the Sakarya Zone, an important segment of the Alpine orogenic belt, continues intermittently from the middle Carboniferous to Miocene. In this study, we provide geochronological and geochemical data from the Dağdibi Pluton in the eastern Sakarya Zone to present some inferences on the source region and petrogenesis of the middle Eocene magmatism. U–Pb zircon geochronology from two granodiorite samples gives middle Eocene ages of 44.75 ± 0.92 and 45.01 ± 0.59 Ma. The pluton is mainly composed of K-feldspar, plagioclase, quartz, Mg-hornblende/actinolite, Fe–Ti oxides and small amounts of biotite, and secondary chlorite and epidote. Parental magma of the intrusive rocks has a high-K calc-alkaline affinity with metaluminous character. The oxygen fugacity values vary between −18 and −17. The rocks show slightly radiogenic 87Sr/86Sr(i) (0.704845–0.705726) ratios and ϵNdi values between −0.96 and +0.52. Pb–Pb isotope ratios are typical for those of the lower continental crust. ϵHf(i) values of the zircons range from 0.14 to 10.26. The geochemical and isotopic features of the pluton point to a parental magma derived from a depleted mantle that was metasomatized by fluids during previous subduction events. The volumetric abundances of the rock types are decreased as the silica content increase, implying that the fractional crystallization is the most important process during the formation of the present compositional range of the pluton. Amphibole, plagioclase and Fe–Ti oxides are the fractionated phases while K-feldspar is largely accumulated. In the light of the data presented above, slab breakoff is regarded as the geodynamic process responsible for the formation of the Dağdibi Pluton in the middle Eocene.

Geological and geochemical characteristics of Late Ordovician volcanic levels of the Gözdağ Formation in the İstanbul-Zonguldak Tectonic Unit (NW Turkey): Implications for global events during the Ordovician

The İstanbul-Zonguldak Tectonic Unit is regarded as the easternmost fragment of Avalonia-Carolina and stands for Far East Avalonia. Its stratigraphy also is characterized by discontinuous sedimentation from Late Ediacaran to Late Carboniferous. In the western part of the block, known as the İstanbul Terrane, the Gözdağ Formation is represented by lagoonal sedimentary rocks consisting of shale-sandstone with limestone of Middle Ordovician-Lower Silurian age. Here, I report on stratigraphic positions and petrographical and geochemical data of fine- and coarse-grained tuffs and lavas in the Late Ordovician strata of the Gözdağ Formation. The fine- and coarse-grained tuffs have pyroclastic and the lavas have porphyritic, vitrophyric and aphanitic textures. The fine-and coarse-grained tuffs are Sandbian and Katian ages, and the lavas are Hirnantian ages, according to the stratigraphic positions of the Late Ordovician volcanic rocks. Geochemically, the-fine grained tuffs have High-K calc-alkaline character and the coarse-grained tuffs and lavas have calc-alkaline affinity. They are devoid of noticeable with-in plate components, as deduced by the presence of obvious negative Nb anomalies, and they have subduction signatures. In conjunction with the data from literature, the Sandbian fine-grained tuffs were deposited in a lagoonal depocenter in the İstanbul-Zonguldak Tectonic Unit during the Taconic orogeny formed in the earliest late Ordovician between the Piedmont Terrane and Laurentia. The Katian coarse-grained tuffs were deposited in a shallow marine in it during the last phase of the consumption of the Teisseyre-Tornquist Ocean from volcanoes occurred in volcanic arc-related settings. Besides, I the Hirnantian lavas flowed in a lagoonal depocenter in it during the soft-docking of Avalonia and Baltica, known as the pre-Caledonian orogeny.

Zircon U–Pb Dating and Geochemical Characteristics of Chahannuo Gabbros in the Northern Margin of Qaidam Basin, Northern Tibetan Plateau

Late Paleozoic–early Mesozoic intrusive rocks are distributed widely along the northern margin of the Qaidam Basin in the northern Tibetan Plateau. To constrain the tectonic evolution, we carried out petrological, chronological, and geochemical studies of the Chahannuo gabbros. LA-ICP-MS Zircon U–Pb dating yields an age of 255.0 ± 0.9 Ma for the gabbros, which confirms the existence of Indosinian tectono-magmatic activity on the northern margin of the Qaidam Basin. The Chahannuo gabbros have low whole-rock SiO2, Fe2O3 contents, and high Al2O3 contents, which suggests a calc-alkaline affinity. In addition, the gabbros have high MgO, Cr, and Ni contents and Mg#, similar to those predicted of the regional basaltic melts, and indicating that they were affected mainly by fluid from the subducted slab. The Chahannuo gabbros are characterized by arc-like trace element patterns, with enrichment in LREE and LILE, and depletion in HREE and HFSE. No obvious negative Eu anomalies also indicate that no significant magmatic differentiation has occurred. The low Nb/La ratio and Ti content in gabbros samples suggests that the Chahannuo gabbros were partially contaminated by the crust during their formation. The Chahannuo gabbros have high incompatible element ratios (Rb/Sr, Th/Nd, and Th/La), which are closer to the category of enriched mantle. Combing our data with previous data from contemporaneous magmatism in the region, we suggest that the Chahannuo gabbros formed in a continental arc environment related to the northward subduction of the Paleo-Tethyan oceanic plate.

Relación entre vulcanismo dacítico monogenético y poligenético. Caso de estudio del domo Tapias (&lt;95 ka) y volcán Cerro Machín (&lt;50 ka), Colombia

The Tapias lava dome is located ~5 km SE of the explosive, Cerro Machín polygenetic volcano, both located on the Colombian Central Cordillera. The edifices have been interpreted as generated by the same magmatic system; however, the genetic and temporal relations between both edifices have not been established. In this work, the Tapias dome is studied using petrographic characterizations, mineral-, whole-rock- and isotopic chemistry, geothermobarometric calculations, and a K/Ar radiogenic age, and subsequently it is explored the relationship with the Cerro Machín. The rocks from Tapias dome are porphyritic with phenocrysts of plagioclase (An28-48), amphibole (tschermakite and magnesiohastingsite), biotite, quartz, and Fe-Ti oxides, embedded in a glassy groundmass. Petrographically, two types of amphibole crystals were recognized: type I, which is homogeneous, and type II, which displays different rims. The composition of Tapias is dacitic (SiO2: 66.59 wt.%) with calc-alkaline affinity. Our geothermobarometric calculations indicate that type I amphibole crystallized at 871-874°C, between 10.2 and 10.4 km depth. The core of the type II amphibole crystallized at 852-920°C, between 8.5 and 12.3 km depth, while its rim did it at higher temperatures (880-973°C), but at similar depths (9.1-11.8 km). The K/Ar radiogenic age obtained for the groundmass shows an emplacement age not older than 95 ± 0.05 ka. When comparing the results to the Cerro Machín, it is noted that the mineral and compositional characteristics are nearly identical. These results suggest that indeed both volcanoes share the same magmatic system and are fed by magma hosted between ~9 and ~12 km depth. Moreover, it is proposed that the vent location of both edifices, as well as the time gap between the eruption at Tapias and the beginning of the activity at the Cerro Machín (~50 ka), are the result of crustal deformation caused by the interaction between the Otú-Pericos and Ibagué faults.

Petrological, geochemical and geochronological evolution of massif type charnockite from the Eastern Ghats Province, India: Implications on the regional tectonics of the Rayner-Eastern Ghats orogeny

Massif type charnockite was emplaced in the lower crust of the Eastern Ghats Province and is now exposed over a large geographic area. The rock shows clear evidence of magmatic emplacement in lower crustal metasedimentary rocks, which occur as enclaves within the charnockite. Textural data suggest the magma underwent subsolidus cooling and subsequently metamorphosed to granulite facies (up to ∼ 910 °C, 9 kbar). Geochemical data show that the charnockite magma had variable chemistry which was acquired by differentiation and possible crustal contamination. The rock shows both high- and low-SiO2 types, with weakly peraluminous to metaluminous characters. Trace and REE fractionation trends suggest the magma had calcic to calc-alkaline affinities and was emplaced in a continental arc type collisional setting. Theoretical modelling suggests that such a magma could be generated by melting of a hydrated basaltic slab under CO2-rich fluid. U-Pb analysis on oscillatory zoned zircon domains from eight samples yields crystallization ages for the magma. While the majority of the samples show crystallization ages within ca. 980–940 Ma (978 ± 16 Ma, 968 ± 22 Ma, 951 ± 9 Ma, 954 ± 8 Ma, 951 ± 13 Ma and 939 ± 27 Ma), two samples yield crystallization ages of 1002 ± 13 Ma and 1020 ± 16 Ma. This implies two-phase emplacement of the charnockite magma which can be correlated with the tectonometamorphic evolution of the province. While the earlier pulse of charnockite magmatism is broadly synchronous with the first cycle (M1) of metamorphism, the later pulse followed when the lower crust was still hot. The two pulses of charnockite magmatism are broadly synchronous with those of the Mawson charnockite of the Rayner Province, East Antarctica. It is argued that the charnockite magmatism in the combined Rayner-Eastern Ghats Province was extensive and resulted from arc-continent accretion and collision between the India and East Antarctica during ca. 1030–900 Ma.

Petrology and amphibole-plagioclase chemistry of amphibolites in the Ilesa area, Southwestern Nigeria

Amphibolites of Itagunmodi and Igun, within the Ilesa Area were studied in order to determine their petrochemical affinities, tectonic setting and conditions of formation; ultimately for an improved understanding of the evolution of the Ilesa Schist Belt. The amphibolites are massive in texture and occur as small to large lensoid boulders associated with quartz-biotite schist, quartzite and talc-chlorite schist. They comprise magnesio-hornblende, actinolite, labrodorite, albite and accessory titanite. Average elemental composition and ratios in the amphibolites were: Ba (872 ppm), Sr (609 ppm), Ni (89 ppm), Rb (72 ppm), Y (27 ppm), Zr (168 ppm), Nb (83 ppm), Cr (200 ppm), La (34 ppm), Cr/Ni (2.30), Y/Nb (0.63), Rb/Sr (0.10), Nb/La (2.55), and Zr/Nb (4.62). The major and trace element characteristics revealed that they are ortho-amphibolites with tholeiitic and calc-alkaline affinities, plotting in Within Plate Basalt field of the Zr vs Zr/Y diagram. The amphibolites exhibit enhanced mid-ocean ridge basalt (E-MORB) elemental characteristics with LREE > HREE enrichment and slight negative Eu anomaly. The calc-alkalic affinity of some of the amphibolites is a result of crustal contamination, and not an arc-related signature as earlier proposed. Elemental abundances and ratios revealed that the amphibolites were formed in an extensional tectonic setting where mantle plume activity and lithospheric contamination occurred. The mineralogical assemblage reflects amphibolite facies metamorphism, at temperature of formation of 450–687 °C and pressure of 0.5–4.3 kbar constrained by plagioclase-hornblende thermobarometry.

Petrogenesis and Metallogenic Evolution of Leucogranites from the Paleoproterozoic Malanjkhand Granitoids, Central India

Abstract The Birsa and Devgaon leucogranites represent contemporaneous episodes of highly fractionated felsic magmatism associated with the Paleoproterozoic Malanjkhand granitoids in the Bastar craton, Central India. Major element oxides geochemistry reveals that two leucogranites are strictly granite in composition and both show calc-alkaline affinity. Geochemical discrimination diagrams inferred that these leucogranites shows affinity towards I-type magma with metaluminous characters. Geochemical classification inferred that the Birsa leucogranite preferred to be magnesian whereas Devgaon leucogranite designated to be ferroan type. Geothermobarometry estimation suggested that 1.2-4.5 kbar pressure and 750°-830°C temperature conditions at the shallow depth of the continental crust is responsible for the emplacement of leucogranites. Crystallization evolution of feldspar in the leucogranite provided evidences of late stage felsic magmatism in the Malanjkhand pluton. Molybdenum enrichment in the leucogranite possibly due to crystal fractionation under high oxygen fugacity conditions inherited from the magma chamber in the continental crust. Anomalous high concentration of Mo in the Devgaon leucogranite is contributed from episodic hydrothermal system with poor supply of sulphur. Occurrences of molybdenite in the Devgaon leucogranite provides high exploration potential to investigates granite molybdenite system in the Bastar craton.

Syn-orogenic tectonomagmatic evolution of the Qilian Orogen: Insights from the Lumanshan gabbro–granite association in the Qilian Block, Northwest China

Gabbro–granite associations provide critical geological clues for unraveling crust–mantle interactions, regional tectonics and geodynamic processes. Many Late Ordovician–Early Devonian granitoids occur in the Qilian Block, display high-K calc-alkaline affinities but have less well-constrained geodynamic mechanisms. Here, we report the newly discovered Late Ordovician-Early Devonian Lumanshan (LMS) gabbro–granite association in the Qilian Block that is composed mainly of syenogranites and alkali-feldspar granites, with minor gabbro. Zircon U Pb dating yields crystallization ages of ca. 440 Ma for the gabbros and ca. 450 Ma and 414 Ma for the syenogranite and alkali-feldspar granite, respectively. The gabbros are calc-alkaline series and display enriched mid-ocean ridge basalt ( E -MORB)-like rare earth element (REE) patterns; U, Cs, and Rb enrichments; Nb, Ta, Zr, and Hf depletions; and depleted Sr–Nd–Hf isotope compositions. The results indicate that the gabbros were possibly derived from partial melting of an asthenospheric mantle wedge metasomatized by aqueous fluids. Two granite types within the LMS pluton are peraluminous and have a transitional geochemical affinity between S- and I-type, low Mg# values, and similarly evolved Sr Nd isotope compositions, which overlap with those of Neoproterozoic basement rocks exposed in the study area. The syenogranites show adakitic geochemical characteristics with high La/Yb and Nb/Ta ratios, whereas the alkali-feldspar granites are non-adakitic granites with flat heavy REE patterns and significant Ba, Sr, and Eu depletions, suggesting that they formed at lower and middle crustal levels, respectively. The LMS ca. 450–440 Ma gabbro and syenogranite were emplaced during a back-arc extensional regime associated with roll-back of the northward-subducting South Qilian Ocean slab. Synthesized data from this and previous studies suggest that the final closure of the South Qilian Ocean and continental collision between the Qilian and Qaidam blocks likely occurred at ca. 435 Ma, and the ca. 414 Ma LMS alkali-feldspar granites generated in syn -collisional setting related with exhumation of the subducted continental crust. The LMS alkali-feldspar granites also show decoupled Nd Hf isotopic systems, which should result from significant disequilibrium melting of metasedimentary materials with the apparent abundance of inherited zircons in their source due to a less intense thermal regime. • A gabbro–granite association in the Qilian Block formed in early–middle Paleozoic. • The ca. 472–435 Ma magmatism formed in an extensional setting related to slab roll-back. • The ca. 435–400 Ma magmatism occurred in syn-collisional setting.

New insights on the fossil arc of the Tyrrhenian Back-Arc Basin (Mediterranean Sea)

Geology, geophysics and geodynamics of the Tyrrhenian Back-Arc Basin (BAB; central Mediterranean Sea) have been studied extensively during the last 50 years. However, some topics are still open: for example, the possible migration of the volcanic arc during the Ionian subduction of the past few Ma. We improved our knowledge of the geodynamics of the Tyrrhenian BAB in the area South of the Vavilov Volcano by analyzing multibeam bathymetry and unpublished single-channel reflection seismic and magnetic data. Furthermore, we studied the petrology of igneous rocks as well as facies and microfaunas of carbonates dredged from the Aurelia and Augusto seamounts. The Aurelia basement is made of basalts with calc-alkaline affinity. Carbonates from the Aurelia and Augusto seamounts consist of cemented Mg-calcite biomicrite crusts rich in planktonic foraminifera not older than Early Pleistocene. Based on our results, we interpret the Augusto and Aurelia seamounts as part of the active volcanic arc seaward of the Tyrrhenian BAB in Late Pliocene–Early Pleistocene.

Paleozoic tectonic setting and evolution of the Central Tianshan Block: Insights from Devonian arc-related granitoids

• Distinct Devonian arc-related granitoids record three phases of magmatism . • Devonian granitoids were derived from remelting of Mesoproterozoic rocks with addition of mantle and juvenile crustal components. • The Central Tianshan Block was a continental arc during the Paleozoic bipolar subduction of the Kangguer oceanic slabs. The Central Tianshan Block of the Central Asian Orogenic Belt (CAOB) contains Precambrian metamorphic basement and voluminous Paleozoic granitoids, but its Paleozoic tectonic setting remains debated. This study presents new geochronological and geochemical data for the granitoids in the Shuangqing district in combination with published data from the Central Tianshan Block to constrain the tectonic setting and evolution of the Central Tianshan Block. Zircon U–Pb dating of the different granitoid plutons identified three phases of magmatism, a foliated granite (408.9 ± 2.6 Ma), a K-feldspar granite (397.7 ± 2.5 Ma), and a granodiorite (porphyry) (ca. 390–388 Ma). These Devonian granitoids show medium- to high-K calc-alkaline affinities, large ion lithophile elements (LILE; e.g., Rb and Ba) enrichment, and high field strength elements (HFSE; e.g., Nb, Ta, and Ti) and Eu depletion, consistent with subduction-related I-type granitoids. They have negative ε Nd (t) values (−5.2 to −3.4), old zircon T DM C ages (1623–1036 Ma), and variable zircon ε Hf (t) values (−3.54 to +5.70), indicating they were mainly sourced from remelting of Mesoproterozoic rocks with variable incorporation of mantle and juvenile crustal components in their magma sources. Combining with published data and this study, we suggest the Central Tianshan Block was a continental arc in the Ordovician to Carboniferous during the bipolar subduction of the Kangguer oceanic slabs. During the Devonian, the subduction of the Kangguer oceanic slabs formed the Central Tianshan continental arc to the south, accompanied by emplacement of multiphase arc-related granitoids in the Shuangqing district.

Evolution of arc magmatic cycles from the Carboniferous to the Early Cretaceous in the western paleomargin of Gondwana, north of the Andes

This work presents a considerable volume of new and compiled data indicating that arc magmatism in the western paleomargin of Gondwana began in the Carboniferous and continued during the Permian and Early Triassic. Subsequently, the magmatism reactivated during the Early and Middle Jurassic due to the subduction of the Farallon Plate under the continental paleomargin. The arc pluton belts are distributed from the edge of the paleomargin toward the interior of the continent in the same orientation as the slab (west–east direction). During the Carboniferous, between ca. 333 Ma and ca. 300 Ma, magmatism formed small calcic metaluminous gabbro and leucotonalite plutons of tholeiitic to calc-alkaline affinity on the western margin of Gondwana. Later, the second belt of arc plutons formed during the Permian/Triassic (between ca. 300 Ma and ca. 234 Ma) and are represented by metaluminous to peraluminous calc-alkaline to high-K calc-alkaline batholiths and stocks of heterogeneous composition (granites, granodiorites, diorites, quartz monzonites, and monzonites), which were intruded by dikes and minor granite bodies during the Middle Triassic. Between ca. 214 Ma and ca. 186 Ma, peraluminous plutons of batholithic dimensions of monzogranitic to syenogranitic composition developed in the back-arc. Between ca. 197 Ma and ca. 186 Ma, back-arc magmatism occurred, while a new magmatic cycle began along the arc axis. At the end of the Jurassic, the magmatic arc cycle ended in the northwestern paleomargin of Gondwana (ca. 164 Ma). The intrusion ca. 159 Ma of porphyritic bodies of alkaline andesitic basalts toward the edge of the continental margin suggests the strangulation and collapse of the subduction zone in the mantle. To the west, off the continental margin, a new arc magmatic cycle began over a different continental terrane ca. 171 Ma and extended to ca. 138 Ma, giving rise to a belt of calcic to calcic-alkaline plutons emplaced in the Ordovician metamorphic (Anacona Terrane), Triassic (Tahamí Terrane), and Upper Jurassic (Tierradentro Orogen) rocks. The assemblage amalgamated to the western margin of Gondwana in this period.

40Ar–39 Ar dating, whole-rock and Sr-Nd isotope geochemistry of the Middle Eocene calc-alkaline volcanic rocks in the Bayburt area, Eastern Pontides (NE Turkey): Implications for magma evolution in an extension-related setting

Discussions continue about whether Middle Eocene magmatism in the Eastern Pontides is associated with collision or subduction. This paper presents new whole-rock geochemistry, Sr-Nd isotopic and 40Ar-39Ar age data for Middle Eocene volcanic rocks from the Bayburt area of the Eastern Pontides (NE Turkey) to investigate their sources and evolutionary history. The new 40Ar–39Ar ages reveal that these volcanic rocks erupted between 44.6 ± 0.1 Ma and 43.5 ± 0.1 Ma, within the Lutetian (Middle Eocene). The studied volcanic rocks are composed of basalt, andesite, basaltic andesite and minor dacite lava and pyroclastic rocks. These rocks consist of plagioclase, amphibole, pyroxene, olivine, biotite, sanidine and minor quartz phenocrysts with Fe-Ti oxides. They have microlithic, hyalo-microlithic, porphyritic and rarely glomeroporphyritic textures. The volcanic rocks have low to high-K calc-alkaline affinities. They display enrichment in large-ion lithophile elements and depletion in high-field strength elements with high Th/Yb ratios, which indicate that the magmas forming the volcanic rocks were derived from lithospheric mantle sources enriched by mostly slab-derived fluids in the spinel stability field. 87Sr/86Sr(i) values vary between 0.70485 and 0.70551 and 143Nd/144Nd(i) values vary between 0.51255 and 0.51267. These data correspond to the mantle array on the isotope ratio diagram. The main solidification processes consist of fractional crystallization with minor assimilation. In light of the data obtained in this study together with data from previous studies, petrogenetic character of the Middle Eocene magmas from the southern parts of the Eastern Pontides may be explained by melting of an enriched lithospheric mantle source initially metasomatized by subduction fluids in a post-collisional extensional-related tectonic setting.

Geochemical and zircon U-Pb geochronological constraints on late mesozoic Paleo-Pacific subduction-related volcanism in southern Vietnam

Late Mesozoic volcanic rocks comprising mainly basalt and basaltic-andesite to dacite occur in south-central Vietnam (Dalat zone) and to a lesser extent in southwestern Vietnam (Bay Nui area). Mineral and whole-rock chemistry indicate a calc-alkaline affinity for samples in the Dalat zone and a high-K calc-alkaline to shoshonitic affinity for rocks in the Bay Nui area. Mineral characteristics and variation diagrams of selected elements suggest that fractional crystallization dominated during magma differentiation. The Bay Nui volcanic rocks generally are more enriched in potassium and LILEs (large-ion lithophile elements) than volcanic rocks from the Dalat zone, which may indicate a more evolved nature or crustal assimilation. The similar chemical characteristics and eruption/emplacement age range of volcanic and plutonic rocks (ca. 90–110 Ma) of equal silica concentration indicate that the magma feeding the volcanic eruptions had the same source as that of the plutonic rocks. The observed mineral and whole-rock compositions with enrichment in LILEs, depletion in HFSEs (high field strength elements), and noticeably negative Nb, Ta, and Ti anomalies are characteristic for arc signatures. Zircon U-Pb geochronological data for the volcanic rocks indicate an age range of 95–105 Ma for the eruption. These geochemical and geochronological data link this Late Mesozoic volcanism with continental arc magmatism driven by the subduction of the Paleo-Pacific beneath eastern Indochina. Zircon xenocrysts with a likely magmatic origin cluster around 350 Ma and 250 Ma, indicating two earlier magmatic events most likely related to the subduction of the Paleo-Tethys beneath western Indochina and the subsequent Indosinian orogeny.