Magma Genesis Research Articles

Magma genesis in the supra-subduction zone of the Piranshahr ophiolite complex based on the mineral chemistry of clinopyroxene in the North West basalts of Iran

Magma genesis in the supra-subduction zone of the Piranshahr ophiolite complex based on the mineral chemistry of clinopyroxene in the North West basalts of Iran

Lithogeochemical and Nd-Sr isotope data of the orthogranulites of the Juiz de Fora complex, SE-Brazil: insights from a hidden Rhyacian Orogen within the Ribeira belt

Abstract New petrography, geochemistry and Sm-Nd and Sr data from the orthogranulites of the Juiz de Fora complex in southern Rio de Janeiro State and compiled information provide insights on the petrogenetic and tectonic evolution. The complex comprises several geochemical groups including mafic orthogranulites (three tholeiitic and one alkaline) and felsic orthogranulites (three calc-alkaline and one tholeiitic/low K calc-alkaline). New geochemical and isotope data, combined with available U-Pb data suggest a long evolutionary history from the Paleoproterozoic to the Neoproterozoic. The oldest magmatic episode produced juvenile to slightly contaminated arc-type Rhyacian rocks, as well as granitic rocks related to collision or post-collision episodes. Altogether these rocks integrate part of a dismembered Rhyacian orogen within the Ribeira belt. Few TDM Nd model ages yielded 2.75 to 2.58 Ga, suggesting minor Archean contribution for magma genesis. Some of the basic granulites’ bodies yield Meso- to Neoproterozoic TDM Nd model ages, which may refer to an extensional magmatism. Orthogranulites present granulite facies paragenesis, related to the youngest tectonic episode in the Juiz de Fora Complex (Brasiliano Orogeny). The new data are potentially important for Paleoproterozoic reconstruction models, due to the predominantly juvenile character of the Juiz de Fora complex, as similarly worldwide.

Interactions between volcanism and geodynamics in the southern termination of the Ecuadorian arc

This study focuses on the construction and evolution through time of volcanic edifices located in the southern part of the Ecuadorian arc, with the objective to contribute to a better understanding of the interactions between magmatism, slab geometry and the activity of tectonic faults. Our new groundmass K-Ar ages obtained for a dozen volcanoes from the southern Quaternary arc are rather young, without ages older than 800 ka, and highlight an increasing volcanic activity between 300 and 100 ka. These new temporal constraints suggest that a southward migration of the Ecuadorian arc occurred during the last 600 ka. We interpret this evolution as the result of major and recent geodynamic reorganization in Ecuador related to the activation of lithospheric faults and to the flexure of the slab at depth, following the inland prolongation of the Grijalva fracture zone. Both phenomena could have been induced by the oblique subduction of the Nazca plate towards the convex margin of the South American continent, and the coupling of these two plates along the Wadati-Benioff zone, progressively intensified by the Carnegie ridge subduction. Southward migration of the slab flexure at depth could have changed pressure and temperature conditions, favoring the partial melting of the mantle wedge and magma genesis, thereby inducing a southward migration of the Ecuadorian arc volcanoes. These results contribute therefore to a better knowledge of the current arc dynamics in the Northern Andes, and provide insights into the use of geochronological data applied to volcanic rocks for studies of past tectonic activity in Ecuador.

Magma ascent in planetesimals: Control by grain size

Rocky planetesimals in the early solar system melted internally and evolved chemically due to radiogenic heating from 26Al. Here we quantify the parametric controls on magma genesis and transport using a coupled petrological and fluid mechanical model of reactive two-phase flow. We find the mean grain size of silicate minerals to be a key control on magma ascent. For grain sizes ≳1 mm, melt segregation produces distinct radial structure and chemical stratification. This stratification is most pronounced for bodies formed at around 1 Myr after formation of Ca,Al-rich inclusions. These findings suggest a link between the time and orbital location of planetesimal formation and their subsequent structural and chemical evolution. According to our models, the evolution of partially molten planetesimal interiors falls into two categories. In the magma ocean scenario, the whole interior of a planetesimal experiences nearly complete melting, which would result in turbulent convection and core–mantle differentiation by the rainfall mechanism. In the magma sill scenario, segregating melts gradually deplete the deep interior of the radiogenic heat source. In this case, magma may form melt-rich layers beneath a cool and stable lid, while core formation would proceed by percolation. Our findings suggest that grain sizes prevalent during the internal heating stage governed magma ascent in planetesimals. Regardless of whether evolution progresses toward a magma ocean or magma sill structure, our models predict that temperature inversions due to rapid 26Al redistribution are limited to bodies formed earlier than ≈1 Myr after CAIs. We find that if grain size was ≲1 mm during peak internal melting, only elevated solid–melt density contrasts (such as found for the reducing conditions in enstatite chondrite compositions) would allow substantial melt segregation to occur.

Multi-proxy isotopic tracing of magmatic sources and crustal recycling in the Palaeozoic to Early Jurassic active margin of North-Western Gondwana

We trace source variations of active margin granitoids which crystallised intermittently over ~300 Ma in varying kinematic regimes, by combining zircon Lu-Hf isotopic data from Early Palaeozoic to Early Jurassic igneous and metaigneous rocks in the Mérida Andes, Venezuela and the Santander Massif, Colombia, with new whole rock Rb/Sr and Sm-Nd isotopic data, and quartz O isotopic data. These new data are unique in South America because they were obtained from discrete magmatic and metamorphic zircon populations, providing a high temporal resolution dataset, and compare several isotopic systems on the same samples. Collectively, these data provide valuable insight into the evolution of the isotopic structure of the continental crust in long-lived active margins.Phanerozoic active margin-related granitoids in the Mérida Andes and the Santander Massif yield zircon Lu-Hf model ages ranging between 0.77 Ga and 1.57 Ga which clearly define temporal trends that can be correlated with changes in tectonic regimes. The oldest Lu-Hf model ages of >1.3 Ga are restricted to granitoids which formed during Barrovian metamorphism and crustal thickening between ~499 Ma and ~473 Ma. These granitoids yield high initial 87Sr/86Sr ratios, suggesting that evolved, Rb-rich middle to upper crust was the major source of melt. Granitoids and rhyolites which crystallised during subsequent extension between ~472 Ma and ~452 Ma yield younger Lu-Hf model ages of 0.80 Ga–1.3 Ga and low initial 87Sr/86Sr ratios, suggesting that they were derived from much more juvenile, Rb-poor sources such as mafic lower crust and mantle-derived melts. The rapid change in magmatic sources at ~472 Ma can be attributed either to reduced crustal assimilation during extension, or a short pulse of crustal growth by addition of juvenile material to the continental crust. Between ~472 Ma and ~196 Ma Lu-Hf model ages remain mostly constant between ~1.0 and ~1.2 Ga. The large scatter and the absence of definite trends in initial 87Sr/86Sr ratios suggest that both mafic, Rb-poor, and evolved Rb-rich sources were important precursors of active margin magmas in Colombia and Venezuela throughout the Palaeozoic to the Early Jurassic.Previous studies have shown that the genesis of arc magmas may be stimulated by heat advection to the crust during the underplating of mantle derived melt, but the absence of permanent younging trends in Lu-Hf model ages from ~472 Ma to ~196 Ma suggests that very little new crust was generated during this period in the studied region. An overwhelming majority of the analysed igneous rocks yield zircon Lu-Hf model ages of >1 Ga which may be accounted for by documented local crustal end members of 1 Ga–1.6 Ga, and do not require contributions from the depleted mantle. Therefore, recycling of ~1 Ga and older crust was a dominant process in the north-western corner of Gondwana between ~472 Ma and ~196 Ma.This study shows that whole rock Sm-Nd and zircon Lu-Hf data can be interpreted similarly regarding the age of the source regions, whereas Rb-Sr and O isotope data from the same rocks yield valuable information regarding the geochemical nature of the source.

Geochemical characteristics of basalts from Andaman subduction zone: Implications on magma genesis at intraoceanic back‐arc spreading centres

This study reports new petrological and geochemical data for the ocean floor volcanic rocks dredged from the back‐arc basin of the Andaman Sea and elucidates their geochemical characteristics and petrogenetic aspects to provide insights into the melt generation processes, oceanic crust emplacement, and tectonomagmatic evolution of the Andaman back‐arc basin. The studied samples are porphyritic basalts with olivine, plagioclase, and clinopyroxene representing the phenocrysts and the groundmass composition marked by plagioclase microlites, granular clinopyroxenes, and glass. Geochemical characteristics of these basalts reflect sub‐alkaline, tholeiitic composition of precursor magma that experienced clinopyroxene‐Ti magnetite fractionation. The overall geochemical trend for these basalts reflects LILE–HFSE–REE depleted MORB‐type chemistry which is modified by input from subduction‐derived components. Pronounced enrichment in selected LREE (Nd) and incompatible fluid‐mobile trace elements like Ba with positive primitive mantle‐normalized La and Sr anomalies in the majority of these basalts are attributed to transport and influx of slab‐dehydrated fluids and sediments into the depleted back‐arc mantle asthenosphere from the dehydrating subducted oceanic lithosphere in the volcanic arc front during initial stage of back‐arc rifting in the Andaman Sea. The studied basalts were derived by 10%–30% partial melting of a heterogeneous spinel lherzolitic mantle at a shallow depth. The chemical heterogeneity of the spinel lherozolite source suggests an enrichment‐depletion history of back‐arc mantle correlatable with the transitional tectonic evolution of the back‐arc basin from an initial rifting to matured spreading stage. The depleted MORB‐type mantle (DMM) beneath a juvenile back arc experienced infiltration of fluids and subduction‐derived components (SDC) from proximal subducting oceanic slab that gradually diminished with increasing distance from the arc and progressive back‐arc spreading. The episodes of melt generation and magma genesis invoke a gradual geodynamic transition from incipient back‐arc rifting to matured back‐arc spreading stages in the tectonomagmatic evolution of the Andaman back‐arc basin system. Oceanic crust generated during the incipient rifting stage sampled arc‐type melts enriched and hydrated by subduction inputs and fluid flux, while oceanic crust formed during matured back‐arc extension stage inherited trace element depleted MORB‐type mantle signatures devoid of subduction influence.

Direct analyses of fluorine in aqueous fluids extracted from 1-GPa experiments

Fluorine has been proposed as a useful geochemical tracer for tracking the transport of volatile elements in subduction zones as it is the last volatile element to be exsolved during magma ascent and crystallization. Accurate determination of the concentration of fluorine in geological fluids under high P-T conditions would greatly further the understanding of its geochemical behavior during aqueous transport and magma genesis in arcs. Mass balance is the most widely used method for calculating fluid compositions in hydrothermal experiments. However, the method is inaccurate when phase compositions and/or proportions have large uncertainties. In the current study, we present an alternative method based on the direct measurement of the concentration of F in fluids by extracting the enclosed liquid within experimental samples, and by performing high-pressure liquid chromatography (HPLC) analysis. We then compared the results of the direct analysis with those obtained by mass-balance calculations. Our experimental data demonstrated that direct analysis yielded more accurate concentration values than mass-balance calculation. The direct method is a useful alternative, when i) the phase assemblage is complex, ii) the total F content or the fluid proportion is low, or iii) when there is significant presence of hydrous melts. Caution should be exercised when applying direct analysis to samples that contained insoluble quench phases and/or were low in fluid. Direct HPLC analysis, combined with an improved extraction method that ensures the separation of quench phase, minerals and melts, could be of great utility for studying water-rock reactions.

Petrology, Geochemistry and Tectonomagmatic Evolution of Hezar Igneous Complex (Rayen- South of Kerman- Iran): the First Description of an Arc Remnant of the Neotethyan Subduction Zone

The Hezar Igneous Complex (HIC) in the south-eastern part of Urumieh-Dokhtar magmatic arc, is the most prominent magmatic feature in the Kerman Porphyry Copper Belt, that understanding magmatic evolution of which may shed light on the tectonomagmatic development of this less-studied part of an important magmatic arc in the Neotethys realm. The HIC has been developed in the the intersection of the NS-striking Sabzevaran fault and the NW-SE striking Rafsanjan-Rayen fault. It is indicated that the possible place of the conduit and vent is in Jalas Mountain which has been splitted later by the Sabzevaran fault into Minor and Major Jalas. The current summit had been constructed by ascending magma chamber under the HIC that constitutes the Kamali Mountain at the south of the summit. Some plutonic rocks of the HIC are exposed at Kamali Mountain. The subalkaline rocks of this complex mainly are composed of different pyroclastic and lava flow rocks, acidic to basic in composition, showing the evidences of fractional crystallization and mineral segregation. Sequential explosive and effusive eruptions with Strombolian to Vulcanian types are evident in the successive volcanic layers. The compositional trend shows the melting of spinel lherzolite, not garnet lherzolite. The subduction-related mechanism of the magma genesis has been indicated by IAB nature of the magma formation in geochemical diagrams.

New geochemical constraints on the origins of MARID and PIC rocks: Implications for mantle metasomatism and mantle-derived potassic magmatism

MARID (Mica-Amphibole-Rutile-Ilmenite-Diopside) and PIC (Phlogopite-Ilmenite-Clinopyroxene) rocks are unusual mantle samples entrained by kimberlites and other alkaline volcanic rocks. The formation of MARID rocks remains hotly debated. Although the incompatible element (for example, large ion lithophile element) enrichment in these rocks suggests that they formed by mantle metasomatism, the layered textures of some MARID samples (and MARID veins in composite xenoliths) are more indicative of formation by magmatic processes. MARID lithologies have also been implicated as an important source component in the genesis of intraplate ultramafic potassic magmas (e.g., lamproites, orangeites, ultramafic lamprophyres), due to similarities in their geochemical and isotopic signatures. To determine the origins of MARID and PIC xenoliths and to understand how they relate to alkaline magmatism, this study presents new mineral major and trace element data and bulk-rock reconstructions for 26 MARID and PIC samples from the Kimberley-Barkly West area in South Africa. Similarities between compositions of PIC minerals and corresponding phases in metasomatised mantle peridotites are indicative of PIC formation by pervasive metasomatic alteration of peridotites. MARID genesis remains a complicated issue, with no definitive evidence precluding either the magmatic or metasomatic model. MARID minerals exhibit broad ranges in Mg# (e.g., clinopyroxene Mg# from 82 to 91), which may be indicative of fractionation processes occurring in the MARID-forming fluid/melt. Finally, two quantitative modelling approaches were used to determine the compositions of theoretical melts in equilibrium with MARID rocks. Both models indicate that MARID-derived melts have trace element patterns resembling mantle-derived potassic magma compositions (e.g., lamproites, orangeites, ultramafic lamprophyres), supporting inferences that these magmas may originate from MARID-rich mantle sources.

Geochronological and geochemical constraints on the origin of the Southeast Anatolian ophiolites, Turkey

The Southeast Anatolian ophiolites outcropping in the Southeast Anatolian Orogenic Belt (Southeast of Turkey) mark the closure of the southern branch of the Neotethys Ocean associated with the collision between the Arabian Plate and Anatolian microplate. We present new geochemical, zircon U–Pb age, zircon Lu–Hf, and Sr-Nd isotopic data on the Southeast Anatolian Ophiolites to understand their formation ages, magma genesis, and geotectonic implications. The ophiolites, which are related to island arc igneous rocks, consist of mantle peridotites and crustal rocks (less dunite, gabbros, sheeted dykes, massive, and pillow basalts). The flat rare-earth element (REE) patterns, depletion in Nb and Ta, and enrichment in LILEs (Ba, Rb, Th, Sr, Pb) of gabbros suggest close similarities with very low Ti (boninitic) lavas found in the forearc regions. Using laser ablation inductively coupled plasma–mass spectrometry, zircon separated from leucogabbros, diabase dykes, and plagiogranites yield U-Pb ages of 92 and 83 Ma, which are interpreted to represent the formation ages of the ophiolites. The zircons in the gabbros and plagiogranites are dominated by positive eHf(t) values (between +3.1 and + 17.45) with a few negative eHf(t) values. High eHf(t) features are consistent with derivation from Mid-oceanic Ridge Basalt (MORB)-source mantle. The negative eHf(t) values of the zircons suggest the involvement of subducted sedimentary rocks. The southeast Anatolian ophiolites represent an SSZ-type ophiolite and are part of the Late Cretaceous oceanic lithosphere of the southern branch of the Neotethys Ocean that opened during the Late Triassic and closed during the Late Cretaceous.

Alkaline magmas in zones of continental convergence: The Tezhsar volcano-intrusive ring complex, Armenia

Alkaline igneous rocks are relatively rare in settings of tectonic convergence and little is known about their petrogenesis in these settings. This study aims to contribute to a better understanding of the formation of alkaline igneous rocks by an investigation of the Tezhsar volcano-intrusive alkaline ring complex (TAC) in the Armenian Lesser Caucasus, which is located between the converging Eurasian and Arabian plates. We present new petrological, geochemical and SrNd isotope data for the TAC to constrain magma genesis and magma source characteristics. Moreover, we provide a new 40Ar/39Ar age of 41.0 ± 0.5 Ma on amphibole from a nepheline syenite that is integrated into the regional context of ongoing regional convergence and widespread magmatism.The TAC is spatially concentric and measures ~10 km in diameter representing the relatively shallow plumbing system of a major stratovolcano juxtaposed by ring faulting with its extrusive products. The plutonic units comprise syenites and nepheline syenites, whereas the extrusive units are dominated by trachytic-phonolitic rocks. The characteristic feature of the TAC is the development of pseudomorphs after leucite in all types of the volcanic, subvolcanic and intrusive alkaline rocks.Whole-rock major element data show a metaluminous (Alkalinity Index = 0–0.1), alkalic and silica-undersaturated (Feldspathoid Silica-Saturation Index <0) character of the TAC. The general trace element enrichment and strong fractionation of REEs (LaN/YbN up to 70) indicate a relatively enriched magma source and small degrees of partial melting. All TAC rocks show a negative NbTa anomalies typical of subduction zone settings. The initial 87Sr/86Sr ratios (0.704–0.705) and positive εNd values (+3 to +5) indicate an isotopically depleted upper mantle and lack of significant crustal influence, which in turn suggests the TAC magma has formed via differentiation from lithospheric mantle melts.Regionally, the age of ~41 Ma places the TAC amid a Lesser Caucasian Eocene period of dominantly calc-alkaline magmatism. The TAC's arc-like geochemical signatures are interpreted to result from prior subduction of the Tethyan slab beneath the Eurasian continental margin. The alkaline character, distinct from regional trends, is attributed to Neotethyan slab rollback causing extension and inducing small degrees of decompression melting of metasomatised lithospheric mantle.

Neodymium isotope disequilibria in subducted sediments, and potential consequences for subduction-zone recycling

Neodymium isotope geochemistry is pivotal to understanding magma genesis and crustal recycling processes at convergent plate margins. Here, we present Nd isotope compositions of rare earth element (REE)–rich minerals from blueschist- to eclogite-facies rocks that represent previously subducted continental sediments. We find Nd isotope heterogeneity in all of our samples. Apatite preserves diverse detrital isotopic compositions despite metamorphism up to at least 600 °C and 3.0 GPa, whereas lawsonite appears to be isotopically homogenous. Epidote and monazite are major REE hosts under eclogite-facies conditions; in our samples these phases also retain Nd isotope heterogeneity, even up to ultrahigh-pressure conditions of 700 °C and 4.0 GPa. These findings contest the assumption of Nd isotope equilibration during subduction-zone metamorphism, so caution is needed when using Nd isotopes to calculate subducted sediment contributions to arc magmas or for determining rates of crustal recycling through subduction zones.

Calc-alkaline magmatism associated with salt diapirs in the Shurab and Garmsar back-arc areas (Central Basin, Iran): magma genesis and tectonic implications

Medium- and high-K calc-alkaline magmatism of the Shurab (southeast Qom city) and Garmsar (northwest Garmsar city) areas occurred within the Lower Red Formation in the Central Basin behind the Urumieh-Dokthar Magmatic Arc. In terms of whole-rock geochemical analyses and in agreement with the petrographic features, all representative samples of the Shurab area are classified into three groups: group 1 with mainly intergranular texture comprises basalt/trachybasalt, while groups 2 and 3 with trachytic and porphyritic textures, respectively, have basaltic trachyandesite composition. The overall major constituents are plagioclase with composition in the range from An 49 Ab 23 to An 75 Ab 47 , clinopyroxene with composition in the range Wo 43-45 En 39-45 Fs 9-15 , and olivine with composition in the range from Fo 58 Fa 31 to Fo 67 Fa 40 . Minor minerals consist of opaque minerals and K-feldspar in the range Or 41-65 Ab 33-50 An 0.69-8 . The characteristic accessories are apatite and sphene. In the Garmsar area, rocks are seen as subvolcanic with mafic (basalt) and intermediate (trachybasalt/basaltic trachyandesite) compositions. The Garmsar area rocks represent intergranular, granular, ophitic, and subophitic textures. In these rocks, major mineral phases are plagioclase and clinopyroxene. The minor constituents are olivine, opaque minerals, amphibole, biotite, and quartz. Apatite is the most important accessory mineral. The rocks of both areas display REE patterns characterized by LREE-enriched and HREE-depleted segments typical of arc lavas. Primitive mantle-normalized trace element patterns for samples of both areas exhibit high ratios of strongly incompatible elements with similar bulk partition coefficients (e.g., Th/ Ta and Th/Ce), enrichment in large-ion lithophile elements (LILEs: Cs, Ba, Rb, Th) relative to the high field-strength elements (HFSEs: Ti, Hf, Zr, and REEs), and troughs for Nb, Ta, Ti, and Zr and peaks for Cs, Th, K, and Sr, all of which are indicators for subductionrelated magmatism. Subduction of the Neo-Tethys beneath the Eurasian margin led to upper mantle deformation and metasomatism. Once the Arabian plate collided with the Eurasian margin, subduction ended through a slab breakoff process, and thermal flux of asthenospheric origin uprising through the slab tear induced the thermal erosion of the mantle metasomatized during the previous subduction event and triggered its partial melting. Also, the late Eocene-early Oligocene collision of Eurasian with Arabian plates led to the subsidence and formation of faults and extensions in the Central Basin (i.e. the Shurab and Garmsar areas) such that eruption of medium- and high-K metasomatic magmatism along these faults and extensions caused postcollision volcanism in the Central Basin.

Study of silica-undersaturated magmas through the Kalsilite- Nepheline-Diopside-Silica system at 4.0 GPa and dry conditions

This study experimentally investigates the Kalsilite-Nepheline-Diopside-Silica system at high pressure and temperature, with emphasis on silica-undersaturated volume (leucite-nepheline-diopside — Lct-Nph-Di; and kalsilite-nepheline-diopside — Kls + Nph + Di — planes), at 4.0 GPa (~120 km deep), temperatures up to 1,400ºC and dry conditions, to better understand the influence of K2O, Na2O, and CaO in alkali-rich silica-undersaturated magma genesis. In the Lct-Nph-Di plane, we determined the stability fields for kalsilite (Klsss), nepheline (Nphss) and clinopyroxene (Cpxss) solid solutions, wollastonite (Wo) and sanidine (Sa); and three piercing points: (i) pseudo-eutectic Kls + Nph + Di + liquid (Lct62Nph29Di9) at 1,000ºC; (ii) Kls + Sa + (Di + Wo) + liquid (Lct75Nph22Di2) at 1,200ºC; and (iii) pseudo-eutectic Kls + Di + Wo + liquid (Lct74Nph17Di9) at 1,000ºC. Kalsilite stability field represents a thermal barrier between ultrapotassic/potassic vs. sodic compositions. In the plane Kls-Nph-Di, we determined the stability fields for Klsss, Nphss and Cpxss and two aluminous phases in smaller proportions: spinel (Spl) and corundum (Crn). This plane has a piercing point in Kls + Nph + Di(± Spl) + liquid (Kls47Nph43Di10) at 1,100ºC. Our data showed that pressure extends K dissolution in Nph (up to 39 mol%) and Na in Kls (up to 27 mol%), and that these solid solutions, if present, determinate how much enriched in K and Na an alkaline magma will be in an alkaline-enriched metasomatic mantle. Additionally, we noted positive correlation between K2O and SiO2 concentration in experimental melts, negative correlation between CaO and SiO2, and no evident correlation between Na2O and SiO2.

U–Pb zircon, geochemical, and Sr–Nd–Hf isotopic data for late Mesozoic volcanic rocks along the Tan–Lu fault zone of Shandong Province, eastern China: constraints on magma genesis and lithospheric thinning

ABSTRACTA combined study of whole-rock major and trace elements, Sr–Nd–Hf isotopes, and zircon U–Pb geochronology was conducted on late Mesozoic volcanic rocks along the Tan-Lu fault zone (the Anqiu, Gaoqiao, Juxian, and Junan regions) of Shandong Province, eastern China. The results constrain the petrogenesis of the volcanic rocks and provide important insights into the significance of the Tan-Lu fault zone for lithospheric thinning. The volcanic rocks are composed of basaltic trachyandesite, trachyandesite, trachydacite, andesite, dacite, and rhyolite, which yielded zircon U–Pb ages of 125.4 ± 0.9, 126.2 ± 1, 125.7 ± 1, 127.1 ± 1.2, 120 ± 0.65, and 122.8 ± 1 Ma, respectively, indicating eruption in the Early Cretaceous. A large amount of Palaeoproterozoic and Archaean inherited zircons exist in Anqiu and Gaoqiao volcanic rocks, but Neoproterozoic inherited zircons in Juxian and Junan volcanic rocks, in agreement with the age spectrum of the basement rocks of the North China Craton (NCC) and Neoproterozoic protolith ages of ultrahigh-pressure metaigneous rocks in Sulu orogenic belt, respectively. The volcanic rocks have high Na2O + K2O concentrations (5.94–11.33 wt.%), indicating that they are alkaline. All the rocks are characterized by enriched in light rare earth elements and large ion lithophile elements (e.g. Rb, Th, U, and K) but are depleted in high field strength elements (e.g. Nb, Ta, and Ti). Anqiu and Gaoqiao volcanic rocks have wide initial 87Sr/86Sr ratios (0.707678–0.711259), variable negative εNd(t) values (–15.2 to −11) and εHf(t) values (−19.5 to −2.2). The varied isotopic compositions and the abundance inherited zircons, together with mineral disequilibrium features, suggest that the Anqiu and Gaoqiao volcanic rocks were produced by the mixing of magmas from enriched lithospheric mantle and lower crust. The enriched lithospheric mantle originated from the ancient lithospheric mantle beneath the NCC, which was modified by the subduction of Yangtze crustal material. Juxian and Junan voalcanic rocks have initial 87Sr/86Sr ratios of 0.707502–0.709238, low εNd(t) values of −20.5 to −16.1, and negative zircon εHf(t) values of −24.6 to −16.4, similar to granitoids in Sulu orogenic belt, suggesting that they derived from similar crustal sources. Combining the existence of the Neoproterozoic inherited zircons and the isotope characteristics, we propose that the Juxian and Junan volcanic rocks were dominantly derived from partial melting of the Subducted Yangtze crust but with contributions from the mantle. The volcanic rocks along the Tan-Lu fault zone were produced under an extensional regime linked with lithospheric thinning in the eastern NCC during the Early Cretaceous. According to the characteristic spatial and temporal distributions of late Mesozoic volcanic activity along the Tan-Lu fault zone, combined with the distributions of regional adakitic rocks and the evolutionary history of the fault, we suggested that the Tan-Lu Fault played an important role in the initiation of lithospheric thinning and provide a favourable channel and beneficial conditions for magma activity.

Geochronology and geochemistry of volcanic rocks from the Tanjianshan Group, NW China: Implications for the early Palaeozoic tectonic evolution of the North Qaidam Orogen

AbstractThe widely exposed Tanjianshan volcanic rocks in the North Qaidam Orogen (NW China) encode abundant information about early Palaeozoic magma origin and tectonic evolution. This research utilizes petrological, geochemical, and geochronological methods to investigate the last volcanic cycle (Formation d) of the Tangjianshan Group, aiming to reveal the tectonic evolutionary processes of the North Qaidam Orogen. Formation d can be further divided into two major volcanic sequences, that is, Formation d‐1/d‐4 and Formation d‐3. Formation d‐1/d‐4 is a set of intermediate–basic volcaniclastic rocks intercalated with volcanic lava and contemporary hypabyssal diabase–gabbro intrusive rocks. The rocks are characterized by high average concentrations of Al2O3, K2O, Na2O, and P2O5, pronounced differentiation of light (LREE) and heavy rare earth elements (HREE) and enrichment of LREEs and light‐ion lithophile elements (LILE), showing continental arc affiliation and enriched mid‐ocean‐ridge basalt (E‐MORB) characteristics. They have positive zircon εHf (t) values (7.5 to 16.1) and variable whole‐rock εNd (t) values (−2.7 to 7.0), with crustal Hf model ages (TDMC) and two‐stage Nd model ages (T2DM) ranging from 428 to 970 Ma and from 618 to 1,410 Ma, respectively. The magma may have originated from an enriched mantle source and experienced crustal contamination in a continental back‐arc basin. In contrast, Formation d‐3 is composed of volcaniclastic rocks intercalated with layered basaltic andesite and basalts. It has higher average concentrations of CaO, FeOT, and MgO, lower ΣREE and flatter REE patterns, lower average 87Sr/86Sr ratios (0.70557), higher average 143Nd/144Nd ratios (0.51285), and exclusively positive εNd (t) values (1.3 to 5.0), consistent with normal mid‐ocean‐ridge basalt (N‐MORB) influence. The magma may have been derived from a normal mantle source without significant modification, which probably formed in a rift‐related tectonic setting or even a mid‐ocean ridge that developed within a back‐arc basin. Magmatic zircons from Formation d‐1/d‐4 yield U–Pb ages mostly at 460–440 Ma, whereas zircons modified by continental exhumation‐related orogenic fluids are mainly dated at 430–420 Ma. Magmatic zircons from Formation d‐3 mainly have U–Pb ages of 440–430 Ma. Integrating the magma genesis and rock affiliation interpretations of this study with geological information for the study area and adjacent regions, a four‐stage tectonic evolutionary model is proposed for the early Palaeozoic North Qaidam Orogen: (a) oceanic subduction with arc magmatism (520–460 Ma), (b) continental collision with back‐arc magmatism (Formation d‐1/d‐4) (460–440 Ma), (c) mid‐ocean ridge extension with rift‐related magmatism (Formation d‐3) (440–430 Ma), and (d) continental exhumation with orogenic fluid modification (430–420 Ma).

The nature and evolution of mantle upwelling at Ross Island, Antarctica, with implications for the source of HIMU lavas

The multiple, proximal, young and/or active volcanic centers of Ross Island, Antarctica, provide a unique opportunity to investigate both deep and shallow processes of alkaline magma genesis and the length scales of mantle heterogeneity. Ross Island, Antarctica is an assembly of four silica-undersaturated alkaline volcanic centers, including the active phonolitic Erebus volcano (1.2 to 0 Ma). Mt. Terror, Mt. Bird, and Hut Point Peninsula surround Erebus on the periphery of Ross Island and are mostly older (∼0.3 to 4 Ma) and mainly basanitic in composition. While the geochemical compositions and HIMU isotopic signature of Erebus lavas are well characterized, the geochemistry of the peripheral volcanic centers was, until this study, poorly known. To further investigate the nature of mantle sources and magmatic processes beneath Ross Island, we therefore measured major and trace element and Sr, Nd, Hf, and Pb isotope compositions on fifty-seven samples from the three volcanoes peripheral to Erebus volcano and compared the results to previous geochemical and isotopic studies of the latter. Mt. Terror, Mt. Bird, and Hut Point Peninsula have 87Sr/86Sr ranging from 0.702907 to 0.703147, εNd ranging from +4.3 to +6.3, εHf ranging from +5.6 to +8.6, and 206Pb/204Pb ranging from 19.282 to 20.241. The Sr, Nd, Hf, and Pb isotope compositions of all four volcanoes (Erebus, Terror, Bird, and Hut Point Peninsula) fall on a mixing line between the HIMU and DMM mantle end-members, with a minor contribution from an EM component. Small, but distinct differences in the isotopic compositions of the four volcanoes, most notably Mt. Bird, imply mantle source heterogeneity beneath Ross Island on a length scale comparable to or smaller than inter-volcano distances (i.e., less than 50 km). The major and trace element systematics and isotopic signatures of the Ross Island volcanic samples are distinct from those for samples from Zealandia and older Antarctic samples that typify magmas from the Cenozoic alkaline magmatic province. The chemical and isotopic compositions of the Ross Island lavas and tephras are best explained by small-degree melting of an ancient asthenospheric source containing residual garnet. Additionally, recent global tomographic modeling indicates low shear wave velocity anomalies with depths down to ∼1200 km beneath Ross Island. This suggests a deep-seated upwelling plume impinging on shallower metasomatized mantle which has been gradually eroded away and is no longer a potential source of Ross Island magmatism. Rather, the source of Ross Island lavas is old (Archean to early Proterozoic) material upwelling from the deep mantle as opposed to melting of subcontinental lithospheric mantle metasomatized by Paleozoic subduction. While our findings have local significance in terms of understanding Ross Island volcanism and providing a new perspective on the evolution of volcanism in the surrounding region during the Phanerozoic, they also have important global implications for gauging the nature, age, and evolution of the HIMU mantle source.

Basic Dykes Crosscutting the Crystalline Basement of Valsugana (Italy): New Evidence of Early Triassic Volcanism in the Southern Alps

AbstractBasic dykes crosscutting the crystalline basement in Valsugana (Southern Alps, Italy) have been investigated for the first time in the framework of the known tectonomagmatic cycles. Petrographic observations and bulk rock analyses suggest a serial affinity variable between calc‐alkaline (subordinate) and shoshonitic (prevalent), which are generally ascribed to a convergent plate setting. This is confirmed by Sr‐Nd‐Pb isotopic analyses that display extreme values that are often observed in postcollisional settings. A bulk rock Rb‐Sr pseudoisochron suggests an age of the magmatism around 260 Ma, whereas K‐Ar datings yield ages between 227 and 251 Ma, suggesting that these dykes plausibly represent a transition between the Permian and the Triassic volcanic episodes that are known in neighboring sectors of the Southern Alps. Considering that Permo‐Triassic active subduction beneath the South Alpine is scarcely constrained, we ascribe the metasomatism of the related mantle sources to the Variscan cycle, proposing that magma genesis was delayed with respect to the time of the active subduction(s). According to recent reconstructions, parts of southeastern Europe, including the South Alpine domain, were formed by the breakup of the northern Gondwana margin from the Late Cambrian, in connection with important transtensional movements, leaving rifted continental basins or narrow oceanic seaways. In our view, the subduction processes that induced metasomatism in mantle sources of the South Alpine region occurred in the connection with the subsequent (Carboniferous?) consumption of lithosphere of these basins, a framework that is compatible with pervasive recycling of continental crust components within the mantle wedge. Then, calc‐alkaline/shoshonite magmatism was triggered in the Early Triassic by postcollisional extensional tectonics that followed the Variscan orogenic cycle.

The different sources and petrogenesis of Jurassic intrusive rocks in the southern Lhasa subterrane, Tibet: Evidence from the trace element compositions of zircon, apatite, and titanite

ABSTRACT The Jurassic magmatism distributed in the southern Lhasa subterrane (SLT) has been recognized as a record of the early-stage evolution of the Neo-Tethys ocean; however, the magma compositions and detailed crystallization histories remain an enigma. Zircon, apatite, and titanite in igneous rocks can serve as effective petrogenetic indicators. In this paper, we present the whole-rock geochemical, zircon U Pb data, zircon Hf isotopic analyses, and major and trace elemental compositions of zircon, apatite, and titanite from Baiga granite porphyry (EGP), Shanba granodiorite (EGD), Pengcuolin granodiorite porphyry (MGP), and Dazhuqu monzonite (LM) rocks in the SLT to constrain their source and petrogenesis. The results from this study show that a combination of zircon Ti, Th/U, Hf, and rare earth element (REE) contents, apatite halogen, REE and Sr contents, and titanite REEs can be used to track the detailed magmatic crystallization histories. Higher Sr contents are found in apatite (SrAp) from EGP (ca. 192 Ma) and EGD (ca. 176 Ma) than those in their host rocks (SrWR), indicating that their magma genesis may have involved some mafic magma. The magma genesis of LM (ca. 159 Ma) may have involved more reduced mafic (or least evolved) magma, as evidenced by the higher Th/U but lower Yb/Gd ratios, higher zircon saturation and Ti-in-zircon temperatures in zircon, highest SrAp/SrWR ratios, and large positive Eu anomalies in titanite from LM. The lower Cl contents and higher (La/Sm)N values in apatite from MGP (ca. 172 Ma) are inherited from the low-Cl melts. The MGP rocks have higher zircon Ce4+/Ce3+ and EuN/EuN* ratios, a greater apatite SO3 content, and a higher titanite total REE content, which indicates that MGP has a higher oxidation state than do EGP, EGD, and LM. The MGP also exhibits a higher H2O content (>11 wt%) dissolved in the melt, greater Ba/La and Nb/Ta ratios in whole rock, and higher Nb/Ta ratios in zircon. However, MGP demonstrates lower zircon saturation and Ti-in-zircon temperatures, which suggest that MGP was generated by fluid-fluxed melting below 750 °C. The low Ti-in-zircon temperatures but high La/Yb and Sr/Y ratios of MGP indicate that a local thickened crust likely existed in the Middle Jurassic period in the SLT due to mafic magma underplating during this time.

Sulfur isotope and PGE systematics of metasomatised mantle wedge

At convergent margins fluids liberated from subducting slabs metasomatise the overlying mantle wedge, enriching it in volatiles, incompatible elements and possibly ore-forming metals. Despite the genetic link between this process, the genesis of arc magmas, and formation of porphyry Cu–Au deposits, there is currently little understanding of the behaviour of chalcophile and siderophile elements during subduction-related mantle metasomatism. In this study, we report sulfur isotopic compositions and PGE concentrations of sulfides in a suite of garnet peridotites from the Western Gneiss Region of Norway, sampling mantle wedge from ∼100 to ∼250 km depth. Sulfides hosted in metasomatised samples have δ34S deviated from typical mantle values, ranging between −10.0 and +5.4‰, indicating derivation of sulfur from subducted crust. Sulfides in pervasively metasomatised samples have atypical PGE signatures, with strong enrichment in Os and Ru relative to Ir, whereas channelised fluid flow produced sulfides extremely enriched in Pd, up to 700 times the concentration found in non-metasomatised samples. These signatures are reconcilable with a high oxidation state of the metasomatising agents and demonstrate that subduction can recycle chalcophile and siderophile elements into and within the mantle, along with sulfur. We further show that because the solubility of Os and Ru in fluids is redox sensitive, and Pd is more soluble than the I-PGE, ratios such as Os/Ir, Ru/Ir plotted against Pd/Ir can be used to trace the metasomatic oxidation of mantle samples, mantle-derived magmas and porphyry Cu±Au deposits. This geochemical insight is used to show that Au-rich porphyry Cu deposits are derived from more oxidised mantle wedge than Au-poor porphyry deposits.