Basaltic Trachyandesite Research Articles

Geochemical and isotopic features of the Early Cretaceous volcanism of the Torey Volcanic Field (Eastern Transbaikalia, Russia) as a record of the transition from pyroxenite to eclogite mantle source

The Mesozoic magmatic activation in Central and Northeast Asia resulted in the formation of a large volume of volcanic rocks with diverse compositions. The most dramatic compositional change occurred at the end of the Early Cretaceous, when mainly alkaline basaltic lavas began to erupt after subalkaline differentiated lavas. The nature of crustal and mantle processes that led to this change in volcanism remains unclear. The Torey Volcanic Field (TVF) of Eastern Transbaikalia (Russia) demonstrates a similar compositional change. Therefore, the TVF is crucial to studying the cause of the difference in geochemical and isotopic signatures of the Mesozoic volcanism in Central and Northeast Asia.TVF belongs to the northeastern end of the Eastern Mongolia Volcanic Area (EMVA). Like other volcanic fields of the EMVA, TVF formed in two stages: early (∼121–129 Ma) and late (∼101–119 Ma). The TVF is composed of subalkaline and alkaline basaltic trachyandesites – trachyandesites. All TVF rocks are characterized by negative Ti and Sr anomalies and positive Ba and Pb anomalies. Compared to the late TVF rocks, the early TVF rocks have distinct negative Ta and Nb anomalies and are highly enriched in light rare earth elements relative to heavy rare earth elements. TVF rocks have the following isotopic characteristics: 87Sr/86Sr(t) = 0.70477–0.70540, εNd(t) = − 0.9 – +2.4, 206Pb/204Pb(t) = 17.9–18.4, and 207Pb/204Pb(t) = 15.5–15.6. However, older rocks mainly have higher εNd(t) values and 206Pb/204Pb(t) and 207Pb/204Pb(t) ratios.Geochemical and isotopic data of samples suggest that the TVF formed by melting in the continental metasomatized lithospheric mantle (CMLM). Phlogopite-amphibole-rutile-bearing pyroxenite veins played a major role in the formation of older rocks. Eclogite, represented by the recycled oceanic crust or the buried lower continental crust, was dominant in the source of younger rocks. The common source for both groups of the TVF rocks was metasomatized hydrous peridotite.Lithospheric extension and subsequent asthenospheric upwelling led to melting in the CMLM and the formation of the TVF. At the early stage of the volcanism, melting occurred at relatively low temperatures where amphibole, phlogopite, and rutile were stable. Due to ongoing lithosphere extension, the melting of eclogite occurred at a higher temperature and/or lower pressure at the late stage of the volcanism.

The magma plumbing system of the potentially hazardous Laoguipo volcano in the Tengchong Volcanic Field, SW China

The Late Pleistocene-Holocene Laoguipo volcano in the Tengchong Volcanic Field (TVF), southwestern China, displays significant geochemical and geophysical anomalies characteristics. Here we present petrographic observations, mineral chemistry, bulk rock geochemistry, thermobarometry, and thermodynamic simulation to evaluate the crystallization conditions and pre-eruptive magmatic processes occurring within the magma plumbing system. This study reveals the existence of two magma reservoirs beneath the Laoguipo volcano. The deep magma reservoir is composed of basaltic trachyandesite (SiO2 = 54–57 wt%), which is located at 15–19 km depths with 1087–1160 °C, 1.5–2 wt% H2O content, oxygen fugacity of ΔNNO+1 (Ni-NiO buffer), melt viscosity of 101.7–102.6 Pa·s, and density of 2.5–2.6 g/cm3. The formation of the deep magma reservoir is attributed to the 31% mass fractional crystallization of primitive basalt in the TVF. The shallow magma reservoir is composed of trachyte (SiO2 = 63–64 wt%), which is located at 6–11 km depths with 780–825 °C, 5.9–6.5 wt% H2O content, oxygen fugacity of ΔNNO+1 (Ni–NiO buffer), melt viscosity of 103.9–104.8 Pa·s, and density of 2.2–2.3 g/cm3. The shallow magma reservoir formed after the basaltic trachyandesite had assimilated 19% mass of the upper crustal material and fractionated 41% mass of the crystals. This study suggests that the shallow trachyte magma reservoir is being heated by the ascending deep basaltic trachyandesite magma, resulting in crystal dissolution, remobilization of crystal mush, and magma convection, which may be the main factors responsible for the geochemical and geophysical anomalies characteristics. The Laoguipo volcano is forming a mature magma plumbing system, which is of significance for forecasting future volcanic eruptions.

Mineral chemistry and P-T conditions of the Karakaya volcanites at Kırka-Afyon-Isparta volcanic province, Afyon, Turkey

The Kırka-Afyon-Isparta Volcanic Province (KAIVP) is one of the best known regions in Turkey for the origin and petrological evolution of the high potassium volcanic activity. The temporal and spatial variability of volcanic rocks in the region exerts significant control over their geochemical diversity. Alkaline and ultrapotassic volcanic rocks of the Afyon volcanism are the first products of asthenospheric origin after the orogenesis in western Anatolia. We have determined the mineralogical and petrographic properties of the Karakaya volcanites surrounding Afyon with the help of microprobe analyses. Estimated thermobarometers are calculated. The Karakaya volcanites have been grouped into four different units according to their their mineralogical, petrographic and geochemical characteristics: Seydiler ignimbirite, basaltic trachyandesite, trachyandesite, trachyte and lamproite. Most samples display hypocrystalline porphyritic texture, whereas samples of lamproite unit have a holocrystalline texture. Generally, volcanic units also exhibit some textural evidence of disequilibrium crystallisation, such as sieve texture and corrosion in plagioclase phenocrysts, zoning and inclusions in clinopyroxene phenocrysts. Mineral thermobarometric estimations in all suites were tested on clinopyroxene and feldspar compositions, considering different authors’ approaches. Values of temperatures and pressure range from 1105 to 1273ºC and 5.6 to 12.2kbar, respectively. The temperature and pressure values calculated from the mineral-melt associations in the volcanics suggest that the Afyon Volcanites were affected by magma mixing processes and crystallised at different depths during the transport of magma.

Geochemistry of hydrothermal illitizations in Eocene Kösedağ magmatic rocks, Zara-Suşehri area, NE Sivas, East-Central Anatolia: Origin and age of alteration

The study area located at the periphery of the collision zone between the Eurasian plate (i.e. Pontides) and Tauride-Anatolide platform, NE of Sivas in the east-central Turkey, which is part of the Tethyan Metallogenic Belt. Mixed-layer illite-smectite (IS) and illite minerals are derived within the hydrothermal alteration zones with a few km2 surface areas (up to 30 km2) in Eocene volcanic and plutonic rocks. The representative IS and illite samples taken from plutonic- and volcanic-hosted alteration zones are investigated by optical and scanning electron microscopy (SEM), X-ray diffraction (XRD), major and trace elements and OH isotope geochemistry and KAr dating methods. Different types of hydrothermal alterations, such as propylitic and phyllic alteration in the plutonic rocks and argillic alteration in the volcanic rocks were developed as a result of intrusion of Kösedağ Pluton (syenite) into Karataş Volcanics (basaltic trachy-andesite and trachyte) with relations of hot-hot contact. The main phyllosilicate/clay minerals are characterized by kaolinite and IS in volcanic-hosted argillic alteration zones, whereas IS and illite in plutonic-hosted phyllic zones. The ordering types (Reichweite) of IS and illites are represented by R1 IS (I = 65–80 % in IS) + R3 IS (I = 90 % in IS) in the volcanic-hosted rocks, and R3 IS (I = 90 % in IS) and illite (S = 3–5 %). Dioctahedral (d060 ≤ 1.500 Å) R3 IS and illites have 1 Md + 1 M and 1 Md + 1 M + 2 M1 polytypes, respectively. The major and trace elements such as TiO2, Fe2O3, MgO, Na2O, P2O5, Sc, V, Cu, Ge, Sr, Hf, Zr and Y increase in the volcanic-hosted IS, whereas SiO2, Al2O3, K2O, Pb, W, Mo, As, Sb, Rb and U in the plutonic-hosted IS and illites. The chondrite-normalized distributions of IS and illites present a great similarity to those of host rocks. The chondrite-normalized rare earth element (REE) concentrations are more enriched in the volcanic-hosted IS in comparison with the plutonic-hosted IS and illites having distinctive Eu negative anomaly, which indicate deriving from volcanic matrix and K-feldspar, respectively. Oxygen and hydrogen isotope data of illitic clays indicate that the hydrothermal fluids are originated from magmatic water. According to stable isotopes and fluid inclusion data, IS and illites were formed at the temperature conditions ~150 °C in volcanic-hosted argillic zone, whereas ~250 °C in plutonic-hosted phyllic zones. KAr dating of alunite, IS and illite minerals indicate that the hydrothermal alteration was started at 40.45 ± 1.28 Ma, almost 2 Ma after the Q-syenite intrusion, within the plutonic body as phyllic alteration stage, and continued up to 35.27 ± 2.81 Ma, with a duration of ~5 Ma, and finalized before the exhumation of the Kösedağ Pluton (28–30 Ma). The geochemical characteristics of IS and illites were controlled by host-rock, condition, origin, and ages of alterations and they can be used as an important tool for magmatic-hydrothermal systems.

Geochemical modeling of basic pegmatites from Foz do Iguaçu in the Itaipu region, western Paraná, Brazil

In the westernmost portion of the state of Paraná, in the city of Foz do Iguaçu, basic pegmatites occur within inflated pahoehoe basaltic flows of the Paraná Igneous Province. Both rock groups are composed of similar mineral assemblages which include labradorite, augite, ilmenite and titano-magnetite, but pegmatites are medium to coarse grained, whereas basalts are fine grained. Host rocks are geochemically classified as basalts (s.s.), while pegmatitic segregations are basaltic andesite and basaltic trachy-andesite, according to the TAS [(Na2O + K2O) x SiO2] diagram. Based on the AFM [(Na2O + K2O) – (FeOtot) – (MgO)] diagram, all rocks are included in the tholeiitic series, classified as high-Fe tholeiite basalts in the cation plot [Al x (Fetot + Ti) x Mg]. Variation diagrams indicate enrichment of SiO2, TiO2, FeO, Na2O, K2O e P2O5 coupled with depletion of Al2O3, MgO e CaO from basalts to pegmatites. La/LuN ratios range between 5.4 to 6.5 for basalts and 6.3 to 7.9 for pegmatites reflecting a greater degree of fractionation in the pegmatitic segregations and, therefore, their more evolved character. Plagioclase crystals in pegmatites have higher sodic contents when compared to those found in host basalts, while CaO content in basaltic pyroxenes is higher than those from pegmatitic segregations. Trace element geochemical modeling results show that it is possible to generate basic pegmatites from basalts comprised of 55 wt.% plagioclase, 35 wt.% clinopyroxene and 10 wt.% titanium and iron oxides. The process occurs as fractional crystallization with 50% residual melt.

Miocene Volcanism in the Slovenský Raj Mountains: Magmatic, Space, and Time Relationships in the Western Carpathians

The Miocene volcanic-intrusive complex in the Slovenský Raj Mountains, middle Slovakia, comprises a swarm of subalkaline basalts and basaltic andesites with alkaline basalts, trachybasalts and basaltic trachyandesites. Basaltic to doleritic feeder dykes and sporadic hyaloclastite lavas are exposed in contact with the Triassic Bódvaszilas Formation of the Silica Nappe. The primary clinopyroxene, plagioclase, and Fe-Ti oxide assemblage also contains calcite spheroids inferred to represent carbonatitic melt. These spheroids are associated with subsolidus chlorite, actinolite, magnetite, titanite, calcite, and epidote. Micropoikilitic clinopyroxene, albite, and Ti-magnetite formed due to rapid quenching. There was an incorporation of host rock carbonate during the eruption. The erupted products are the result of magmatic differentiation of the parental basaltic tholeiitic magma with a redox of ∆QFM = +1 to +3, affected by varying degrees of 0%–50% fractionation and the assimilation of carbonate material in a shallow magmatic reservoir. REE geochemistry shows N-MORB-like type patterns with both LaN/YbN and LaN/SmN < 1 at near constant Eu/Eu* (~0.9). This is supported by εNd(t=13 Ma) values of +8.0 to +7.4 determined from the basaltic rocks. The REE values can be modeled by 1% fractional melting of garnet peridotite mixed with 7% melting of spinel peridotite of PM composition (1:9 proportions). SIMS and LA-ICP-MS U/Pb analysis of zircons yields a concordant age of 12.69 ± 0.24 Ma and a 13.3 ± 0.16 Ma intercept (Serravallian) age. The Middle Miocene volcanic activity was related to subduction-collision processes along the boundary of the Cenozoic ALCAPA (Alps–Carpathians–Pannonia) microplate and the southern margin of the European plate.

Two-stage evolution of a bimodal reservoir: The case of Holocene lavas of the Lanín composite volcano, Southern Volcanic Zone, Chile

Lanín is a composite volcano (∼39.38°S; 71.30°W) located in the Central Southern Volcanic Zone of the Andes. It is the southernmost member of the NW-SE volcanic alignment with the Villarrica and Quetrupillán composite volcanoes. During the Holocene, Lanín volcano erupted many lava flows, which we classify into three compositional groups: BAT (basalt, andesite, and basaltic trachyandesite), intermediate, and trachyte samples. The main recognised phenocrysts in the Lanín lavas are plagioclase, clinopyroxene, orthopyroxene, Fe–Ti oxides, and apatite. These mineral phases allow us to constrain the pre-eruptive (from the magma reservoir) conditions: temperature (985–1089 °C), pressure (1.8–4.1 kbar; equivalent to 6.5–15 km depth), and oxygen fugacity patch (∼QFM+0.4). In addition, textural features combined with pre-eruptive conditions suggest the existence of a crystal mush within the reservoir, which developed in two stages. The first stage considers the emplacement, crystal mush development, and eruption related to a melt of BAT composition. The residual melt evolved from BAT to trachytic primarily through fractional crystallisation. The second stage involves an evolved trachytic melt in the reservoir and the intrusion of a new, more mafic magma (presumably of BAT composition), which generates heating of the trachytic residual melt and crystal mush disaggregation. The fractional crystallisation also gives rise to scarce products of intermediate composition. This observation is consistent with the chemical constituents plots (TAS and Harker) and thermodynamic modelling.

Experimental Equilibrium and Fractional Crystallization of a H2O, CO2, Cl and S-Bearing Potassic Mafic Magma at 1.0 GPa, With Implications for the Origin of Porphyry Cu (Au, Mo)-Forming Potassic Magmas

ABSTRACT The origin of intermediate to felsic potassic magmas is debated, and not much is known about the volatile content of potassic magmas associated with porphyry Cu (Au, Mo) deposits. To better understand the liquid line of decent of mafic potassic magmas and the behavior of volatiles during magma differentiation, we performed 19 experiments at 1.0 GPa and 1150 °C to 850 °C using piston cylinder presses. We developed a new experimental technique that involves a capsule liner made of single-crystal zircon to prevent the loss of Fe and S in the starting material to the noble metal capsule. The starting material is a high-Mg, basaltic trachyandesite (52 wt% SiO2, 12 wt% MgO, 1.9 wt% Na2O and 5.3 wt% K2O) from the Sanjiang region in southwestern China, doped with geologically realistic amounts of volatiles (i.e. 4.0 wt% H2O, 0.34 wt% CO2, 0.27 wt% Cl and 0.25 wt% S). The addition of 0.25 wt% S in the form of anhydrite internally buffered the experiments at an oxygen fugacity of 2.0 ± 0.5 log units above the fayalite–magnetite–quartz buffer, which is similar to the redox state of the Sanjiang variously evolved potassic magmas. The experimentally produced silicate melts match well with the Sanjiang intermediate to felsic magmas in terms of major, minor and trace element compositions, and also with regard to the S and Cl contents. The sequence of crystallizing minerals (olivine + clinopyroxene –> biotite ± orthopyroxene –> apatite –> K-feldspar) also fits with the one observed in the Sanjiang mafic to intermediate magmas. These results suggest that the Sanjiang intermediate to felsic magmas, including the porphyry Cu (Au, Mo)-forming magmas, can form solely by differentiation of potassic mafic magmas without any involvement of old crustal material. During experimental differentiation at 1.0 GPa, the S content of the evolving silicate melt first increased until ~57 wt% melt SiO2, and then decreased in response to precipitation of sulfides, sulfate melt and/or anhydrite, whereas the H2O and Cl contents of the evolving silicate melt increased exponentially until saturation in a CO2-rich fluid was reached at 60 to 65 wt% melt SiO2 and ~ 8 wt% melt H2O. During further magma differentiation the H2O and Cl contents of the evolving silicate melt remained constant until ~70 wt% melt SiO2, after which point the Cl content of the silicate melt decreased due to increased partitioning of Cl into the fluid phase ± increased fluid/melt ratio. Based on these experimental results and petrographic and geochemical evidence from natural samples, the Sanjiang porphyry Cu (Au, Mo)-forming magmas (65–70 wt% SiO2) are interpreted to have formed through differentiation of primitive, mantle-derived, potassic magmas in the lower crust (≥1.0 GPa), and to have ascended ±directly from the lower crust to shallow crustal levels. They likely contained 8 to 13 wt% H2O, 0.37 to 0.90 wt% Cl and 0.07–0.29 wt% S. This case study on the magma evolution in the Sanjiang region may have implications for the origin and nature of intermediate to felsic potassic magmas in various tectonic settings.

Geochemistry, geochronology, and tectonic setting of the Cretaceous volcanic rocks in east Mongolia

Late Mesozoic volcanic rocks in East Mongolia exhibit high-K calc-alkaline and shoshonitic series and display features of bimodal-like volcanic with a mafic and felsic members. The mafic member is composed mainly of trachybasalt and basaltic trachyandesite, with a minor amount of trachyandesite. The mafic rocks have elevated incompatible trace element concentrations and significantly negative Nb-Ta and Ti anomalies, features of typical subduction-related magmas, distinguished from OIBs. The mafic magmas have undergone fractional crystallization dominated by pyroxene and/or olivine and crustal contamination. The mafic magma was derived from low-degree partial melting of an enriched lithospheric mantle source that might have been metasomatized by subduction-derived fluids. The felsic member consists of trachydacite and rhyolite. The geochemical data indicate they produced from partial melting of a crustal source that is dominated by juvenile mafic rocks. Our new K-Ar dating and previous age data demonstrated that the late Mesozoic volcanism in East Mongolia took place during Late Jurassic-Early Cretaceous at between 156-99 Ma. The model of the arc‒back-arc extension possibly induced by slab roll-back of the westward (paleo) Pacific-subduction can explain the geodynamic setting and the eastward young trend of the late Mesozoic volcanism in East Mongolia and its adjacent NE China.

Cretaceous intraplate volcanism of Govorov Guyot and formation models of the Magellan seamounts, Pacific Ocean

ABSTRACT The OIB-type volcanic rocks of Govorov Guyot in Magellan Seamounts (Pacific Ocean) include basalts, trachybasalts, basaltic trachyandesites, and trachyandesites dated by the 40Ar-39Ar plateau from 121 ± 2.8 to 98.5 ± 1.4 Ma. Some rocks bear amphibole-spinel wehrlite xenoliths and clinopyroxene-amphibole (Cpx-Amp) mantle vein fragments in xenoliths and xenocrysts, which are remnants of metasomatized oceanic lithosphere. The pressure and temperature crystallization for pargasitic amphibole were the highest (1170–1130°C and 2.5–1.6 GPa, 73–53 km depth) in wehrlite peridotite and the lowest (1.6–1.1 GPa, shallower than 52 km, and 1070–980°C) in Cpx-Amp metasomatic mantle veins. Ti-bearing Amp phenocrysts and microlites crystallized from the parent melts of basaltic rocks at 1060–910°C and 1.2–0.4 GPa (40–15 km). The ascending basaltic melts lost H2O from 8.6 ± 1.2 to 2.6 ± 0.4 wt% upon decompression. Amphibole mantle xenocrysts became replaced by Ti-magnetite- and rhönite-bearing mineral assemblages in intermediate magma chambers within oceanic crust (3–7 km). Pargasitic Amp in wehrlite and metasomatic mantle veins may have crystallized at different depths from hydrous (6–4.5 ± 0.7 wt% H2O) mafic melts that percolated through peridotite. The sources of heat and metasomatic agents (silicate melts and/or fluids enriched in volatiles) most likely had no relation to the Pacific Plate subduction but may be associated with the Southern Pacific Superplume in the South Pacific Thermal and Isotopic Anomaly region, or may be located at the Lithosphere-Asthenosphere Boundary (LAB) level in the plume absence. Repeated events of Cretaceous intraplate volcanism in the Magellan guyots can be explained in terms of plume (hotspot) activity or decompression partial melting of enriched or metasomatized asthenosphere and/or metasomatized oceanic lithosphere caused by Pacific Plate deformation and fracturing from the LAB level.

Petrogenesis of the Paleogene potassium-rich volcanic rocks in the western Yangtze Craton, southeastern Tibetan Plateau

Paleogene potassium-rich volcanic rocks (PVRs) are widespread in the southeastern Tibetan Plateau, providing an opportunity to illustrate the architecture of the lithosphere and further constrain the development of the SE Plateau. Their petrogenesis, together with the geodynamic mechanism, remains in dispute. Mg isotopic compositions of the PVRs in western Yangtze Craton, along with new elemental and Sr-Nd-Pb isotopic data, are utilized to elucidate their petrogenesis and reveal the plausible metasomatic and genetic mechanisms. The PVRs give a compositional spectrum from basaltic trachyandesite to trachyte and are characterized by high large ion lithophile elements concentrations, negative anomalies in high field strength elements, and enriched Sr-Nd-Pb isotopes, suggesting an origin of enriched lithospheric mantle. The correlations among varying major oxides and fractional crystallization simulation results jointly reveal fractional crystallization of olivine and clinopyroxene in the magmatic process. The less-evolved PVRs present similar major oxide contents (e.g., Ti, Na, K) to those melts originated from carbonated peridotite + silicate sediment. Apart from enriched Sr-Nd-Pb isotopes, they also show high La/Yb ratios (11.73–58.95), low Ti/Ti* (0.08–0.71), Hf/Sm (mostly <0.70), and varying δ 26 Mg values (−0.13 to −0.36‰). Combined with the Sr-Nd-Mg isotope numerical simulations, it points out that the mantle source of the PVRs was replenished by a hybrid carbonate-silicic sediment, which was plausibly caused by the Neoproterozoic seafloor subduction. These volcanic rocks also present relatively high K 2 O (3.29–7.50 wt%) and K 2 O/Na 2 O ratios (1.00–2.85). Besides, varying elevated Rb/Sr and low Ba/Rb ratios, together with the La/Yb-Yb non-modal batch melting result, suggest that the enriched mantle source recorded by these PVRs is predominantly constituted by a hybrid phlogopite-bearing garnet-spinel lherzolite. Constrained by petrology studies and seismic tomography, the generation of the PVRs is proposed to be closely associated with the delamination of the thickened lithosphere and thereafter asthenosphere upwelling. • Paleogene K-rich volcanic rocks in the western Yangtze Craton are variably evolved. • These volcanic rocks are sourced from phlogopite-bearing garnet-spinel lherzolite. • Recorded mantle source was replenished by a hybrid carbonate-silicic sediment.

Experimental investigation of trace element partitioning between amphibole and alkali basaltic melt: Toward a more general partitioning model with implications for amphibole fractionation at deep crustal levels

Abstract Time-series experiments were carried out in a piston-cylinder apparatus at 0.8 GPa and 1030–1080 °C using a hydrous K-basalt melt as the starting material to determine the element partition coefficients between amphibole and silicate glass. Major, minor, and trace element compositions of amphibole and glass were determined with a combination of electron microprobe and laser ablation inductively coupled plasma mass spectrometry. Results show that the main mineral phase is calcic amphibole, and the coexisting glass compositions range from basaltic trachyandesite to andesite. We estimated the ideal radius, the maximum partition coefficient and the apparent Young’s modulus of the A, M1-M2-M3, and M4-M4′ sites of amphibole. The influence of melt and amphibole composition, temperature, and pressure on the partition coefficients between amphiboles and glasses has also been investigated by comparing our data with a literature data set spanning a wide range of pressures (0.6–2.5 GPa), temperatures (780–1100 °C), and compositions (from basanite to rhyolite). Finally, we modeled a deep fractional crystallization process using the amphibole-melt partition coefficients determined in this study, observing that significant amounts of amphibole crystallization (&amp;gt;30 wt%) well reproduce the composition of an andesitic melt similar to that of the calc-alkaline volcanic products found in Parete and Castelvolturno boreholes (NW of Campi Flegrei, Italy).

Chemistry and Crystallization Conditions of Minerals in Metasomatized Oceanic Lithosphere and Basaltic Rocks of Govorov Guyot, Magellan Seamounts, Pacific Ocean

Studies have provided first detailed data on the chemistry of rock-forming, minor, and accessory minerals of Govorov Guyot volcanic rocks (basalts, trachybasalts, basaltic trachyandesites, and trachyandesites). Some basalt samples bear pargasitic amphibole and clinopyroxene xenocrysts, mantle vein fragments in xenoliths, as well as wehrlite xenoliths, which are remnants of metasomatized oceanic lithosphere. Amphiboles make up a continuous series from pargasite –Mg-hastingsite in wehrlite xenoliths and xenocrysts to Mg-hastingsite–kaersutite end-members in phenocrysts and microlites of basaltic rocks. The discussed issues include the trace element chemistry of Ti-amphibole and clinopyroxene phenocrysts; fractionation of OIB melts; and P-T equilibration of minerals during the formation of mantle and basaltic rocks. Pargasitic amphibole may have crystallized at P-T conditions (2.5–0.6 GPa, 1170–980 °C) corresponding to the spinel facies of peridotite at different depths (73–21 km) in hydrous (6.0–4.5 wt% H2O) silicate mafic melts that percolated through peridotites of the oceanic lithosphere. Ti-amphibole in basaltic rocks crystallized at 1.2–0.4 GPa (40–15 km), and 1060–910 °C from melts containing 8.6–2.6 wt% H2O. As the high-temperature (~1100 °C) basaltic magmas reached chambers at the oceanic crust level (7 to 3 km), the Ti-bearing amphiboles of xenocrysts and phenocrysts became replaced by Ti-magnetite- and/or rhönite-bearing mineral assemblages.

FRACTIONATION AND ASSIMILATION DURING THE FORMATION OF THE DEVONIAN ANTIDROMIC IGNEOUS SERIES OF THE SISIM AREA IN THE MINUSINSK TROUGH: GEOCHEMICAL AND Sr-Nd ISOTOPIC EVIDENCE

In the Minusinsk trough, being the large fragment of the Devonian Altai-Sayan rift system, the antidromous series along with the homodromous volcanic series are widespread, sialic rocks with SiO2 content over 65 wt. % being predominant. Abundance of high-silica rocks in these series implies an extensive involvement of crustal igneous sources. However, in contrast to the homodromic series, for which the fractionation of basaltic magmas to trachyte compositions and assimilation processes during the formation of rhyolites are reconstructed, the rocks of the antidromic series are poorly studied in terms of their geochemical and isotope characteristics, which makes it difficult to assess the mechanisms of their formation. This article reports geological, petrographic, geochemical and Sr-Nd isotope characteristics of the igneous series of the Sisim area of the Minusinsk trough, its Early Devonian age is reliably identified by previous researchers, e.g. U-Pb dating by zircon. The X-ray fluorescence and mass spectrometric analyses were used to estimate the material composition. This series is composed of moderately alkaline basalts, dolerites, basaltic trachyandesites, trachyandesites, trachytes, trachydacites, rhyodacites and rhyolites. The intermediate and sialic rocks occur in the lower part of the section, prevailing over mafic rocks. As SiO2 increases in the rocks of the Sisim antidromous series, similarly to the rocks of the homodromous series, the contents of incoherent elements are dispersed, and there is a trend of decreasing εNd and increasing εSr values. This evidence points to a disruption in fractionation resulting from the appearance of anatectic melts, which are close in composition to the upper continental crust. Thus, regardless of the sequence of formation of mafic, intermediate, and sialic rocks and their volume ratios in the homodromous and antidromous series, a combination of fractional crystallization of basaltic magmas and their assimilation of silicate crustal substrates is reconstructed for them.

40Ar/39Ar ages and geochemistry of the Intersalar Range of the Bolivian Altiplano: A volcanological transect spanning the arc and reararc of the Central Andean Plateau

The volcanic Intersalar Range of western Bolivia provides a unique opportunity to examine geochemical variations spanning the arc and reararc regions of the Central Andean Plateau. In this study we report 23 new 40Ar/39Ar ages, 15 whole-rock Sr-Nd-Pb isotope analyses, and 50 whole-rock major and trace element analyses from samples collected across ∼115 km of the Intersalar Range. Most samples are classified as trachyandesites and trachydacites, with the most mafic lavas (slightly alkaline, basaltic trachyandesites) erupting from the Pliocene Coracora volcano in the central Altiplano. We identify two distinct pulses of reararc magmatism: a Miocene phase between 20 Ma and 14 Ma that corresponds with local compressional shortening, and a Plio-Pleistocene phase between 5 and 1 Ma that postdates observed structural deformation in the region. 87Sr/86Sr values (0.70512–0.70600) and 143Nd/144Nd values (0.51226–0.51255) are generally higher, and lower, respectively, in the younger phase, whereas Pb isotopes (206Pb/204Pb = 17.7315–18.5095; 207Pb/204Pb = 15.5714–15.6279; 208Pb/204Pb = 37.7862–38.6156) show little variation with age. Isotope values are only loosely correlated with distance from the modern Central Volcanic Zone. Higher Sr/Y, Dy/Yb, and [La/Yb]N values in the Plio-Pleistocene samples are consistent with homogenization at the base of a thicker continental crust compared to the Miocene samples. Nb concentrations show the strongest correlation with distance into the reararc compared to all other trace elements (arc Nb = 6–16 ppm; reararc Nb = 12–26 ppm). Nb/Nb* values (a measurement of the depth of the negative Nb anomaly) correspondingly increase into the reararc (indicating smaller anomalies), reaching a maximum at Coracora volcano before decreasing in the far rear arc region. Compiled data across the Central Andean Plateau reveal a strong correlation between Nb/Nb* and the presence of intact mantle lithosphere beneath the central Altiplano. We interpret this distinct Nb signal to reflect melting triggered by the breakdown of Nb-rich hydrous minerals within foundering (delaminating) mantle lithosphere. In conjunction with spatiotemporal data, Nb systematics provide the clearest indication of mantle lithosphere in regions where mafic samples are not present.

Reworking of Yangtze crust into the mantle lithosphere of the North China Craton along the Dabie–East Qinling Orogen: Evidence from the Early Cretaceous volcanic rocks

Abstract Mineral chemistry, zircon U-Pb geochronology, and elemental and Sr-Nd-Pb-Hf-O isotopic data for the Early Cretaceous volcanic rocks from the Dabie and East Qinling orogens of China constrain the reworking history of the Yangtze crustal materials in the North China lithosphere. These data provide new insights into the recycling of the deepsubducted crustal materials into the mantle and the transformation process from continental subduction to collision. Our data show that the Early Cretaceous volcanic rocks are characterized by shoshonitic and high-K calc-alkaline basaltic trachyandesite, trachyandesite, and trachyte. They synchronously erupted at ca. 135–120 Ma, and have zircon in situ εHf(t) values ranging from −29.0 to −17.2, and δ18O values from 4.89‰ to 6.84‰. These samples share similar “crust-like” geochemical signatures, whole-rock enrichment in the large-ion lithophile elements, depletion in high field strength elements (Nb/La = 0.12–0.38), highly enriched Sr-Nd isotopic compositions, as well as (206Pb/204Pb)i = 15.97–17.59, (207Pb/204Pb)i = 15.33–15.54, and (208Pb/204Pb)i = 36.87–38.59. The εNd(t) values range from −24.4 to −15.6, −21.8 to −13.1, and −16.8 to −10.9 for the eastern and western North Huaiyang belts of the Dabie Orogen and the East Qinling Orogen, respectively. Such geochemical similarities suggest that the northward deep-subduction of Yangtze crustal materials have been reworked into the North China Craton enriched lithospheric mantle. The spatial pattern of the Early Cretaceous volcanic rocks suggests that the Yangtze subduction extended northward beyond the southern margin of the North China Craton and ran into its interior. The surface and deep lithospheric boundaries are decoupled between the Yangtze Block and North China Craton. The Early Cretaceous igneous rocks in the Dabie–East Qinling Orogen were induced in response to the post-collisional unrooting of the Triassic over-thickened lithosphere coupling.

Volcanism at the end of continental rifting: The Cretaceous syn-rift to post-rift transition in the Songliao Basin (NE China)

The evolution of volcanism at the latest stages of continental rifting and its transition to the post-rift stage are poorly known. A Cretaceous rifting episode characterized the Songliao Basin (NE China) between 150 and 105–100 Ma. The transition to the post-rifting stage was marked by an increase in the subsidence rate due to thermal contraction. Here we present new major, trace element and Sr-Nd isotopic data on the volcanic rocks sampled in wells located in the Songliao Basin. These rocks are representative of two major eruptive cycles: K1yc1 (115–106 Ma, syn-rift stage) and K1yc3 (105–102 Ma, transition to the post-rift stage). The K1yc1 and K1yc3 rocks were erupted during the late phases of the Songliao rifting and have been found in the two members of the Yingcheng Formation. The K1yc1 rocks consist of trachybasalts, basaltic trachyandesites, and rhyolites. The K1yc3 rocks show a homogeneous rhyolitic composition. Both the K1yc1 and K1yc3 rocks display an alkaline, Within-Plate affinity. The trace element patterns and the isotopic composition of the K1yc1 mafic rocks indicate that they are the product of the fractional crystallization from a parent magma with an OIB signature. The felsic rocks from K1yc1 and K1yc3 have similar major and trace element patterns and originate from fractional crystallization of the K1yc1 trachybasalt and assimilation of upper crust material. The bimodal K1yc1 volcanism is related to fractionation processes producing low amounts of magmas with intermediate compositions. The lack of mafic eruption in K1yc3 is due to the reduction of porosity of the crustal rocks and a transition from ‘hot’ to ‘cold’ crust related to the thermal subsidence from syn-rift to post-rift stage. The late stage of the Songliao rifting volcanism coincides with a shift from a roll-backing phase of the Paleo-Pacific slab to its passive sinking in the asthenosphere.

Geochemical, fluid inclusion and sulfur isotope studies of the Daghkesemen Au-bearing polymetallic deposit (Northwestern Azerbaijan) in Lesser Caucasus: Implications for ore genesis

The Daghkesemen Au-bearing polymetallic vein-type deposit, which is the most important ore deposit of the Kazakh graben located in the northwestern part of the Lesser Caucasus, is hosted by calc-alkaline volcanic rocks (andesite, dacite, basaltic trachyandesite, trachyandesite, rhyodacite and their pyroclastics), subvolcanic rocks (andesite porphyry, dolerite, albitophyre/rhyodacite porphyry), and volcanic breccias. The mineralization is commonly observed as gold-bearing quartz-sphalerite-galena-chalcopyrite veins. The Daghkesemen Au-bearing polymetallic deposit, mostly observed in the propylitic alteration, is commonly composed of primarily sphalerite, galena, chalcopyrite, pyrite, native gold, bornite and magnetite, and secondarily digenite, cerussite, covellite, chalcocite, malachite, azurite, hematite, jarosite, and anglesite. The Daghkesemen Au-bearing polymetallic deposit mostly comprises Zn, Pb, Cu, Au, Ag, Fe and Cd. Homogenization temperatures (Th) of the fluid inclusions in sphalerite and quartz range from 200.1 °C to 281.2 °C and 266.4–266.7 °C, respectively. The low salinities for the fluid inclusions in sphalerite and quartz show a range of 0.7–6.6 wt % NaCl equivalent and 2.7 to 3.1 wt % NaCl equivalent, respectively. Eutectic temperatures measured in the fluid inclusions vary between −27.4 and −18.2 °C, indicating H2O–NaCl–KCl system. The δ34SH2S values (−0.8 to −4.4‰) of sulfides (chalcopyrite, sphalerite, galena and pyrite) suggest a magmatic-hydrothermal origin for sulfur. The calculated sulfur isotopic geothermometer is value of 190 °C for sphalerite-chalcopyrite pair. The geochemical, mineralogical, sulfur isotopic and fluid inclusion data of the Daghkesemen Au-bearing polymetallic deposit suggest an intermediate sulfidation epithermal deposit associated with volcanic rocks.

The Ouarzazate Supergroup and its plutonic keel: the relicts of an Ediacaran silicic large igneous province in North Africa

Ouarzazate igneous rocks from Sidi Ifni, SW Morocco, define a &gt;3 km thick pile of mostly pyroclastic falls and flows of rhyolitic and dacitic ignimbrite and tuff, underlain by co-magmatic granitoid plutons. These rocks formed during two closely spaced periods of magmatism dated to c. 595–585 and 575–560 Ma, with the majority of rocks comprised of ferroan alkali feldspar rhyolite, syenorhyolite or monzorhyolite. They can be divided into high and low heavy rare earth element types that discriminate as both volcanic arc and within-plate magma types. Coeval, but less common, mafic rocks consist of olivine-normative trachybasalt and basaltic trachyandesite. Temporal and compositional equivalents of the Ouarzazate volcanic and intrusive rocks crop out widely across North Africa and extend into the previously contiguous regions of NE North America. Collectively, these erupted/shallowly emplaced Ouarzazate-like rocks represent asthenospheric partial melts that are variably hybridized by additions from the lithospheric mantle and crust. The root cause of asthenospheric upwelling remains unknown, although the foundering of pre-collison subducted oceanic lithosphere may provide a plausible explanation. The vast geographical extent over which Ouarzazate-like igneous rocks are found, together with their composition and intra-plate setting, support the conclusion that these rocks represent the remnants of a late Ediacaran silicic large igneous province. Supplementary material: A tabulation of the geochronologic and geochemical datasets from Sidi Ifni is available at https://doi.org/10.6084/m9.figshare.c.5945756

Geodynamic framework of the magmatic activity of Cretaceous Léré basin (Southwestern Chad, Central Africa)

Numerous 5 to 20 m-wide dolerite dykes crosscut vertically to sub-vertically basement granitoids around and within the Cretaceous Léré basin, SW Chad (Central Africa). They extend along strike on several hundred meters length. Léré dolerites exhibit various textures, from classical doleritic texture to sub-ophitic via ophitic and intersertal textures. They are composed of euhedral and subhedral crystals of andesine and labradorite, anorthoclase, augite, olivine, ilmenite and Ti-magnetite. Calcite and amphibole are present. Estimated crystallization temperatures are between 1050 and 1105 ± 50 °C. Occurrence of dyke swarms in Léré basin is controlled by Pan-African strike-slip faults created during an extensional tectonic phase during Paleozoic to Early Mesozoic. These lavas, analyzed with ICP-MS and ICP-AES, are basalt, trachybasalt and basaltic trachyandesite with some continental tholeiite affinity. Dolerites of Léré basin have evolved by limited fractional crystallization differentiation, accompanying by minor assimilation process. Three groups of rocks are identified (according to Y/Nb ratios), linked to different mantle sources which have intermediate compositions between E-MORB and OIB.