Tholeiitic Magma Research Articles

Permian post-collisional basic magmatism from Corsica to the Southeastern Alps

Post-Variscan Early Permian magmatism is widespread from Corsica to the Eastern Alps. After the formation of the Variscan orogenic belt between Laurussia and Gondwana, felsic and mafic bodies were emplaced during an extensional tectonic phase. This study focuses on a mafic dyke swarm that intruded in the region of Ajaccio (Corsica, France) and on a gabbroic intrusive complex outcropping in the Eastern Alps (Bressanone/Brixen, South Tyrol, Italy) both of which were emplaced during this extensional event. New U-Pb data from zircon show that both of these intrusions were emplaced at ca. 282 Ma. Most Ajaccio dykes display a calc-alkaline affinity and are characterized by enrichment of LILE (Large Ion Lithophile Elements) and LREE over HREE (Light and Heavy Rare Earth Elements) and HFSE (Nb, Ta, Hf, Zr; High Field Strength Elements). Two dykes show tholeiitic affinity and are characterized by marked LREE and Th-U depletion and by minor negative Nb-Ta anomalies on multi-element diagrams, which is typical for within-plate basalts. Bressanone/Brixen gabbros display similar geochemical features to the Ajaccio calc-alkaline samples with LREE enrichment over MREE (Medium Rare Earth Elements) and HREE, and HFSE depletion. Such characteristics are also confirmed by trace element data obtained by LA-ICP-MS on clinopyroxene and plagioclase crystals.The calc-alkaline to tholeiitic dykes are characterized by enriched to depleted Sr-Nd-Pb isotopic compositions, respectively. Some Ajaccio dykes and the Bressanone/Brixen gabbros are clearly affected by crustal contamination. However, the combination of trace element and isotopic variations suggests that mantle source heterogeneities were also involved. Isotopic compositions of the Ajaccio dykes reveal the presence of two distinct mantle source signatures: an enriched mantle (EM) component, which dominates the composition of the calc-alkaline-like suite, while a DMM (Depleted MORB Mantle) component dominates the source of the tholeiitic suite. The EM mantle source likely formed during Variscan subduction, however the composition of the tholeiitic suite requires a source composed dominantly of depleted mantle, with a minor contribution from both the subducting slab and supra-subduction mantle wedge. The mantle source of the tholeiitic magmas possibly originated from greater depths (compared to the calc-alkaline-like suite) during the post-orogenic extensional phase potentially due to break-off and/or roll-back of the Variscan slab or mantle flow from the front or margins of the subducted slab.Notably, coeval Permian mafic intrusive bodies from throughout Corsica (Bocca di Tenda, Porto, Pila Canale) and from the Australpine and Southalpine Alps intruded at a similar time and have geochemical features that are very similar to the Ajaccio dyke swarm and Brixen gabbro. This indicates that a widespread Permian magmatic province developed in a post-orogenic extensional tectonic setting at the margin of the former Variscan belt. During the Middle-Late Triassic magmatism with similar enriched geochemical signatures occurred in the Southern Alps, Dinarides, Australpoine domains and at the end of the Triassic over central Pangea (Central Atlantic Magmatic Province). This would highlight the importance that the Variscan orogeny had in the development of a more fertile mantle.

Comparisons Between Tethyan Anorthosite‐Bearing Ophiolites and Archean Anorthosite‐Bearing Layered Intrusions: Implications for Archean Geodynamic Processes

AbstractElucidating the petrogenesis and geodynamic setting(s) of anorthosites in Archean layered intrusions and Tethyan ophiolites has significant implications for crustal evolution and growth throughout Earth history. Archean anorthosite‐bearing layered intrusions occur on every continent. Tethyan ophiolites occur in Europe, Africa, and Asia. In this contribution, the field, petrographic, petrological, and geochemical characteristics of 100 Tethyan anorthosite‐bearing ophiolites and 155 Archean anorthosite‐bearing layered intrusions are compared. Tethyan anorthosite‐bearing ophiolites range from Devonian to Paleocene in age are variably composite, contain anorthosites with highly calcic (An44–100) plagioclase and magmatic amphibole. These ophiolites formed predominantly at convergent plate margins, with some forming in mid‐ocean ridge, continental rift, and mantle plume settings. The predominantly convergent plate margin tectonic setting of Tethyan anorthosite‐bearing ophiolites is indicated by negative Nb and Ti anomalies and magmatic amphibole. Archean anorthosite‐bearing layered intrusions are Eoarchean to Neoarchean in age, have megacrystic anorthosites with highly calcic (An20–100) plagioclase and magmatic amphibole, and are interlayered with gabbros and leucogabbros and intrude pillow basalts. These Archean layered intrusions are interpreted to have predominantly formed at convergent plate margins, with the remainder forming in mantle plume, continental rift, oceanic plateau, postorogenic, anorogenic, mid‐ocean ridge, and passive continental margin settings. These layered intrusions predominantly crystallized from hydrous Ca‐ and Al‐rich tholeiitic magmas. The field, petrographic, and geochemical similarities between Archean and Tethyan anorthosites indicate that they were produced by similar geodynamic processes mainly in suprasubduction zone settings. We suggest that Archean anorthosite‐bearing layered intrusions and spatially associated greenstone belts represent dismembered subduction‐related Archean ophiolites.

Geochemistry of Basaltic Merbabu Volcanic Rocks, Central Java, Indonesia

DOI: 10.17014/ijog.7.2.161-178The studied area is located along the hiking track of Kajor - Selo, the south flank of Merbabu Volcano, Central Java, Indonesia. Olivine basalt and augite-rich basalt compose the volcanic rocks. A geochemical study recognizes these basalts which tend to originate from the product of tholeiitic magma, in terms of transitional enriched mantle source. It is interpreted to have been formed as primary magma that mixed later with higher degrees of partial melting with a mantle wedge. Both fl uid and melt were derived from the mixing of lower active continental margin and subducting oceanic slab. This study also shows general trends of increasing incompatible elements, i.e. Rb, Ba, Pb2+, and Sr as LIL trace elements and Th, U, Nb, Ce, Zr, Hf, Nb, and Ta as HFS element comparing to basaltic andesites exposed at Thekelan, they show decreasing compatible of MgO, Fe2O3*, Al2O3, CaO, TiO2, Ni, Sr, and Ba in line with increasing SiO2. It was fractional crystallization process, shown by the slightly wide variation of Rb/Zr and La/Sm that indicates random crustal contamination.

Alkalic to tholeiitic magmatism near a mid-ocean ridge: petrogenesis of the KR1 Seamount Trail adjacent to the Australian-Antarctic Ridge

ABSTRACT Coexisting alkalic and tholeiitic basalt lavas has been identified in a seamount chain located near the Australian–Antarctic spreading ridge. The KR1 Seamount Trail (KR1 ST) is a series of volcanic seamounts extending to the southeast in the spreading direction of the Australian–Antarctic Ridge (AAR). We herein report Sr, Nd and Pb isotopic compositions and (U–Th)/He and K–Ar geochronology for dredge samples from the KR1 ST in order to evaluate mantle processes and the role of enriched components for alkalic to tholeiitic magma generation in this region. The KR1 ST is a medium-sized seamount chain that extends for ~60 km, has a maximum height of ~1600 m above the seafloor, and consists of alkaline basalts and tholeiites with formation ages of ~0.4 Ma to ≤1.3 Ma. The isotopic characteristics of the alkaline basalts (206Pb/204Pb = 19.52–19.91; 87Sr/86Sr = 0.7030–0.7033; 143Nd/144Nd = 0.5128–0.5130) from the KR1 ST reflect a dominant ‘PREMA (or FOZO)’ mantle component represented by radiogenic Pb and mildly enriched Sr and Nd isotopic compositions. On the other hand, the weak PREMA (FOZO)-affinity (206Pb/204Pb = 18.89–18.93; 87Sr/86Sr = 0.7028–0.7029; 143Nd/144Nd = ~0.5130; 3He/4He = 7.64 ± 0.13 (R/RA)) coupled with their enriched mid-ocean ridge basalt (E-MORB) characteristics of tholeiites from the KR1 ST largely overlap with the KR1 MORB composition. The potential source materials for the alkaline basalts are considered to be ancient, recycled oceanic crust (i.e. eclogite) as well as sub-KR1 depleted MORB mantle (DMM). Whereas the main source materials for the KR1 ST tholeiites are presumed to be the DMM-dominant lithology with minor recycled material. We interpret the KR1 ST as a submarine hotspot chain that was formed by asthenospheric upwelling and spreading processes that delivered fertile blobs of recycled oceanic crust to the sub-KR1 region. The fundamental reason for sub-KR1 upper mantle enrichment might be attributed to a mantle plume event that possibly occurred prior to the formation of the KR1 ST.

Melt chemistry and redox conditions control titanium isotope fractionation during magmatic differentiation

Titanium offers a burgeoning isotope system that has shown significant promise as a tracer of magmatic processes. Recent studies have shown that Ti isotopes display significant mass-dependent variations linked to the crystallisation of Fe-Ti oxides during magma differentiation. We present a comprehensive set of Ti isotope data for a range of differentiation suites from alkaline (Ascension Island, Afar and Heard Island), calc-alkaline (Santorini) and tholeiitic (Monowai seamount and Alarcon Rise) magma series to further explore the mechanics of Ti isotope fractionation in magmas. Whilst all suites display an increase in δ49/47Ti (deviation in 49Ti/47Ti of a sample relative to the OL-Ti reference material) during magma differentiation relative to indices such as increasing SiO2 and decreasing Mg#, our data reveal that each of the three magma series have contrasting δ49/47Ti fractionation patterns over comparable ranges of SiO2 and Mg#. Alkaline differentiation suites from intraplate settings display the most substantial range of variation (δ49/47Ti = +0.01 to +2.32‰), followed by tholeiites (−0.01 to +1.06‰) and calc-alkaline magmas (+0.06 to +0.64‰). Alkaline magmas possess high initial melt TiO2 contents which enables early saturation of ilmenite + titanomagnetite and a substantial degree of oxide crystallisation, whereas tholeiitic and calc-alkaline suites crystallise fewer oxides and have titanomagnetite as the dominant oxide phase. Positive slopes of FeO*/TiO2 vs. SiO2 during magma differentiation are related to high degrees of crystallisation of Ti-rich oxides (i.e. ilmenite). Bulk solid-melt Ti isotope fractionation factors co-vary with the magnitude of the slope of FeO*/TiO2 vs. SiO2 during magma differentiation.This indicates that the modal abundance and composition of the Fe-Ti oxide phase assemblage, itself is controlled by melt composition, governs Ti isotope fractionation during magma differentiation. In addition to this overall control, hydrous, oxidised calc-alkaline suites display a resolvable increase in δ49/47Ti at higher Mg# relative to drier and more reduced tholeiitic arc suites. These subparallel Ti isotope fractionation patterns are best explained by the earlier onset of oxide segregation in arc magmas with a higher oxidation state and H2O content. This indicates the potential of Ti isotopes to be utilised as proxies for geodynamic settings of magma generation.

Using amphibolite chemistry to assist structural and stratigraphic interpretation at Broken Hill, Australia

Multiple samples taken across the strike of conformable syn-depositional amphibolite (meta-dolerite) bodies in the Willyama Supergroup, Broken Hill, showed that all exhibit chemical variation, mostly consistent with igneous fractionation. The fractionation trends are characterised by very significant reduction in MgO, Ni and Cr from base to top of individual sills, increase in Zr, and less regular variation in Fe and Ti contents. This asymmetric chemical zonation can be used to determine the original ‘way up’ in amphibolite sills as thin as 12 m. Given that the amphibolite sills were emplaced prior to folding of the enclosing sequence, the way up direction of the sills is also the younging direction of the enclosing metasediments, making this a useful tool in stratigraphic and structural studies of folded rocks. Outcropping conformable amphibolite bodies were selected for across-strike sampling in variably well-interpreted stratigraphic positions. Despite uncertainties related to overly wide-spaced sampling and possible internal structural complexity, in most cases the fractionation trend agreed with the prior stratigraphic interpretation, providing an independent stratigraphic ‘way up’ or ‘younging’ direction and demonstrating the potential usefulness of the technique. Subsequent sampling and analysis of drill core through amphibolite sills in the Broken Hill mine area tested alternative interpretations of the enclosing stratigraphy and structure. Fractionation patterns in three intersections of the amphibolite immediately west of the orebody clearly show younging towards the orebody. Intersections of the thicker amphibolite immediately east of the orebody reveal complex and conflicting fractionation patterns indicating internal structural complexity, consistent with this amphibolite being involved in a major thrust zone. The results in the mine area are in agreement with those of a previous study based on graded-bedding observations in the enclosing metasediments. The identification of igneous fractionation patterns in the amphibolites confirms the interpretation that the amphibolite bodies were in fact sills rather than lavas. The extremely high iron contents of some of the amphibolite sills are the result of extreme fractionation of a tholeiitic magma, not metasomatism. KEY POINTS Most Broken Hill amphibolites retain relatively simple igneous fractionation patterns. The fractionation patterns can indicate the stratigraphic younging direction. The stratigraphy west of the Broken Hill orebody youngs eastward towards the orebody. The amphibolite east of the orebody shows complex deformation.

High-pressure metamorphic rocks in the Borborema Province, Northeast Brazil: Reworking of Archean oceanic crust during proterozoic orogenies

We present the first evidence of Archean oceanic crust submitted to Proterozoic high-pressure (HP) metamorphism in the South American Platform. Sm–Nd and Lu–Hf isotopic data combined with U–Pb geochronological data from the Campo Grande area, Rio Grande do Norte domain, in the Northern Borborema Province, reflect a complex Archean (2.9 ​Ga and 2.6 ​Ga) and Paleoproterozoic (2.0 ​Ga) evolution, culminating in the Neoproterozoic Brasiliano/Pan-African orogeny (ca. 600 Ma). The preserved mafic rocks contain massive poikiloblastic garnet and granoblastic amphibole with variable proportions of plagioclase ​+ ​diopside in symplectitic texture, typical of high-pressure rocks. These clinopyroxene-garnet amphibolites and the more common garnet amphibolites from the Campo Grande area are exposed as rare lenses within an Archean migmatite complex. The amphibolite lenses represent 2.65 ​Ga juvenile tholeiitic magmatism derived from depleted mantle sources (positive εHf(t) values of +3.81 to +30.66) later enriched by mantle metasomatism (negative εNd(t) values of –7.97). Chondrite and Primitive Mantle-normalized REE of analyzed samples and discriminant diagrams define two different oceanic affinities, with E-MORB and OIB signature. Negative Eu anomalies (Eu/Eu∗ ​= ​0.75–0.95) indicate depletion of plagioclase in the source. Inherited zircon cores of 3.0–2.9 ​Ga in analyzed samples indicate that the Neoarchean tholeiitic magmatism was emplaced into 2923 ​± ​14 ​Ma old Mesoarchean crust (εNd(t) ​= ​–2.58 and Nd TDM ​= ​3.2 ​Ga) of the Rio Grande do Norte domain. The age of retro-eclogite facies metamorphism is not yet completely understood. We suggest that two high-grade metamorphic events are recognized in the mafic rocks: the first at 2.0 ​Ga, recorded in some samples, and the second, at ca. 600 Ma, stronger and more pervasive and recorded in several of the mafic rock samples. The Neoproterozoic zircon grains are found in symplectite texture as inclusions in the garnet grains and represent the age of HP conditions in the area. These zircon grains show a younger cluster of concordant analyses between 623 ​± ​3 ​Ma and 592 ​± ​5 ​Ma with εHf(t) values of +0.74 to –65.88. Thus, the Campo Grande rock assemblage is composed of Archean units that were amalgamated to West Gondwana during Neoproterozoic Brasiliano orogeny continent-continent collision and crustal reworking.

Record of Early Tonian mafic magmatism in the central Espinhaço (Brazil): New insights for break-up of the Neoproterozoic landmass ancestor of São Francisco-Congo paleocontinent

Petrological characterization, U–Pb geochronology, Lu–Hf analyses and major and trace element data from mafic intrusions in the Central Espinhaço (central portion of the Brazilian shield) are used here to investigate the geological significance of the Early Neoproterozoic magmatism in the context of the São Francisco-Congo paleocontinent. These mafic bodies are represented by medium to coarse-grained metagabbros with plagioclase, amphibole and clinopyroxene. Zircon U–Pb isotopic data from two samples yielded weighted mean 206Pb/238U ages of 895 ​± ​3.4 ​Ma (MSWD ​= ​1.7) and 896 ​± ​2.4 ​Ma (MSWD ​= ​0.64), regarded as the best estimates for the crystallization age of these mafic rocks. Major and trace element data (including REEs) show that the gabbros originated from a subalkaline tholeiitic magma, typical of intraplate magmatism. Such rocks are slightly enriched in LREEs and LILEs and depleted in HFSEs. Our new isotope and geochemical data, along with regional knowledge, indicate that these metagabbros mark the beginning of an important Tonian-age extensional tectonic event of the landmass of which the São Francisco-Congo paleocontinent was part (Rodinia supercontinent or Central African block?). We furthermore suggest that these rocks belong to a prominent suite of Tonian-age mafic rocks that mark a diachronic breakup attempt of this landmass which may have occurred from south to north along the Espinhaço mountain range.

Petrology and geochemistry of mafic and ultramafic cumulate rocks from the eastern part of the Sabzevar ophiolite (NE Iran): Implications for their petrogenesis and tectonic setting

The Late Cretaceous Sabzevar ophiolite represents one of the largest and most complete fragments of Tethyan oceanic lithosphere in the NE Iran. It is mainly composed of serpentinized mantle peridotites slices; nonetheless, minor tectonic slices of all crustal sequence constituents are observed in this ophiolite. The crustal sequence contains a well-developed ultramafic and mafic cumulates section, comprising plagioclase-bearing wehrlite, olivine clinopyroxenite, olivine gabbronorite, gabbronorite, amphibole gabbronorite and quartz gabbronorite with adcumulate, mesocumulate, heteradcumulate and orthocumulate textures. The crystallization order for these rocks is olivine ​± ​chromian spinel → clinopyroxene → plagioclase → orthopyroxene → amphibole. The presence of primary magmatic amphiboles in the cumulate rocks shows that the parent magma evolved under hydrous conditions. Geochemically, the studied rock units are characterized by low TiO2 (0.18–0.57 ​wt.%), P2O5 (<0.05 ​wt.%), K2O (0.01–0.51 ​wt.%) and total alkali contents (0.12–3.04 ​wt.%). They indicate fractionated trends in the chondrite-normalized rare earth element (REE) plots and multi-element diagrams (spider diagrams). The general trend of the spider diagrams exhibit slight enrichment in large ion lithophile elements (LILEs) relative to high field strength elements (HFSEs) and positive anomalies in Sr, Pb and Eu and negative anomalies in Zr and Nb relative to the adjacent elements. The REE plots of these rocks display increasing trend from La to Sm, positive Eu anomaly (Eu/Eu∗ ​= ​1.06–1.54) and an almost flat pattern from medium REE (MREE) to heavy REE (HREE) region [(Gd/Yb)N ​= ​1–1.17]. Moreover, clinopyroxenes from the cumulate rocks have low REE contents and show marked depletion in light REE (LREE) compared to MREE and HREE [(La/Sm)N ​= ​0.10–0.27 and (La/Yb)N ​= ​0.08–0.22]. The composition of calculated melts in equilibrium with the clinopyroxenes from less evolved cumulate samples are closely similar to island arc tholeiitic (IAT) magmas. Modal mineralogy, geochemical features and REE modeling indicate that Sabzevar cumulate rocks were formed by crystal accumulation from a hydrous depleted basaltic melt with IAT affinity. This melt has been produced by moderate to high degree (~15%) of partial melting a depleted mantle source, which partially underwent metasomatic enrichment from subducted slab components in an intra-oceanic arc setting.

Phase petrographic, thermobarometric and petrochemical significance of Cretaceous mafic dykes along Nongchram Fault Zone of Swangkre–Rongmil area of Shillong plateau, NE India: Implications for genetic link to Kerguelen mantle plume

The present study elucidates the phase petrographic and petrochemical signatures of a group of Cretaceous mafic dykes emplaced in the Precambrian gneissic basement complex in the western part of the Shillong plateau, NE India, in order to trace their petrological, geotectonic, geothermobarometric and oxybarometric status. The whole rock geochemistry discriminates the dykes as basaltic andesites and basaltic trachyandesites genetically related to each other and derived from common parent magma. The enrichment in LREE relative to HREE and HFSE, systematic Nb anomalies, moderate MREE to HREE fractionation suggests variable depths of melting of slightly enriched mantle source in the garnet stability field. As per geochemical modelling, the studied dykes are derived by 3–5% non-modal batch melting of garnet peridotite source at melting depth of ~65–80 km. The clinopyroxene thermobarometry reveals a temperature span of 1250–800°C and < 2 kb pressure of crystallization for the dykes. The oxygen fugacity (16.82–18.25) indicates extremely reducing conditions at the time of cooling. The very good correlation of petrochemical and phase chemical data with Kerguelen plume derived Rajmahal Group II basalt, Sylhet volcanics and some ODP (ocean drilling project) sites from Kerguelen basalts implicate a genetic link of the studied dykes with Kerguelen mantle plume. Finally, the present study deciphers subalkaline nature of the studied dyke rocks that have been generated by tholeiitic magmas probably in an anorogenic extensional environment. But we need more geochemical data especially good isotope geochronologic data to get a clear picture of the studied dyke.

Petrogenesis of the Low-TiO2 Batalha Suite in the eastern Parnaíba basin, northeastern Brazil

In the Parnaiba basin, northeastern Brazil, some records of basic tholeiitic magmatism have been recognized as part of large igneous provinces (LIP’s). They are represented by the Mosquito basaltic flows (Triassic–Jurassic; ca. 200 Ma) that occurs in the west portion of the basin as well as the Sardinha diabases (Cretaceous; ca. 124 Ma) eastwards. The Mosquito and Sardinha magmatic rocks of the Parnaiba basin were related to the processes of distension and rupture of Pangea and Gondwana, respectively. This paper presents the petrogenesis of diabase sills that occur in the eastern portion of the basin, probably associated with the Sardinha formation. The studied rocks are mostly equigranular, medium-grained diabases. They are tholeiitic andesite-basalts and stricto sensu basalts, comprising a single Low-TiO2 (TiO2 < 2 wt%) magmatic suite here denominated as Batalha Suite. They show relative enrichment in the broad spectrum of LILE (large ion lithophile elements), HFSE (high field strength elements) and REE (rare earth elements) when compared to MORB, chondrite and primitive mantle, similar to continental flood basalts (CFB) elsewhere. This suite evolved by fractional crystallization as attested by whole-rock geochemistry, mineral chemistry, geochemical modeling and Sr–Nd isotope data. The studied Low-TiO2 basalts derived from an enriched mantle source likely to be represented by the subcontinental lithospheric mantle (SCLM). The diabases are related with 16% of partial melting of either a less enriched residual garnet harzburgitic source or a more enriched residual spinel-harzburgite.

Temporal Evolution of Proto-Izu–Bonin–Mariana Arc Volcanism over 10 Myr: Constraints from Statistical Analysis of Melt Inclusion Compositions

AbstractInternational Ocean Discovery Program (IODP) Expedition 351 ‘Izu–Bonin–Mariana (IBM) Arc Origins’ drilled Site U1438, situated in the northwestern region of the Philippine Sea. Here volcaniclastic sediments and the igneous basement of the proto-IBM volcanic arc were recovered. To gain a better understanding of the magmatic processes and evolution of the proto-IBM arc, we studied melt inclusions hosted in fresh igneous minerals and sampled from 30–40 Myr old deposits, reflecting the maturation of arc volcanism following subduction initiation at 52 Ma. We performed a novel statistical analysis on the major element composition of 237 representative melt inclusions selected from a previously published dataset, covering the full age range between 30 and 40 Ma. In addition, we analysed volatiles (H2O, S, F and Cl) and P2O5 by secondary ion mass spectrometry for a subset of 47 melt inclusions selected from the dataset. Based on statistical analysis of the major element composition of melt inclusions and by considering their trace and volatile element compositions, we distinguished five main clusters of melt inclusions, which can be further separated into a total of eight subclusters. Among the eight subclusters, we identified three major magma types: (1) enriched medium-K magmas, which form a tholeiitic trend (30–38 Ma); (2) enriched medium-K magmas, which form a calc-alkaline trend (30–39 Ma); (3) depleted low-K magmas, which form a calc-alkaline trend (35–40 Ma). We demonstrate the following: (1) the eruption of depleted low-K calc-alkaline magmas occurred prior to 40 Ma and ceased sharply at 35 Ma; (2) the eruption of depleted low-K calc-alkaline magmas, enriched medium-K calc-alkaline magmas and enriched medium-K tholeiitic magmas overlapped between 35 and 38–39 Ma; (3) the eruption of enriched medium-K tholeiitic and enriched medium-K calc-alkaline magmas became predominant thereafter at the proto-IBM arc. Identification of three major magma types is distinct from the previous work, in which enriched medium-K calc-alkaline magmas and depleted low-K calc-alkaline magmas were not identified. This indicates the usefulness of our statistical analysis as a powerful tool to partition a mixture of multivariable geochemical datasets, such as the composition of melt inclusions in this case. Our data suggest that a depleted mantle source had been replaced by an enriched mantle source owing to convection beneath the proto-IBM arc from &amp;gt;40 to 35 Ma. Finally, thermodynamic modelling indicates that the overall geochemical variation of melt inclusions assigned to each cluster can be broadly reproduced either by crystallization differentiation assuming P = 50 MPa (∼2 km deep) and ∼2 wt% H2O (almost saturated H2O content at 50 MPa) or P = 300 MPa (∼15 km deep) and ∼6 wt% H2O (almost saturated H2O content at 300 MPa). Assuming oxygen fugacity (fO2) of log fO2 equal to +1 relative to the nickel–nickel oxide (NNO) buffer best reproduces the overall geochemical variation of melt inclusions, but assuming more oxidizing conditions (log fO2 = +1 to +2 NNO) probably reproduces the geochemical variation of enriched medium-K and calc-alkaline melt inclusions (30–39 Ma).

Rift-related Neoproterozoic tholeiitic layered mafic intrusions at northern Yangtze Block, South China: Mineral chemistry evidence

Neoproterozoic layered mafic intrusions occur along the northern margin of the Yangtze Block in South China. In this paper, we report the chemical compositions of clinopyroxene, plagioclase, and olivine from two major mafic intrusions at Bijigou (BJG) and Wangjiangshan (WJS), to constrain the parental magma characteristics and geodynamic setting. The olivine and plagioclase compositions indicate that the parental magma for BJG and WJS was an evolved tholeiitic magma with high FeOT/MgO ratios. An origin of arc magmatism is not supported by the following mineralogical and petrological observations: (1) the strong rift-affinity indicated by olivine, plagioclase and clinopyroxene geochemistry, (2) the absence of high anorthite in the cumulate rocks, (3) the paucity of abundant cumulus hornblende crystals, and (4) the lack of voluminous intermediate rocks in the rock associations. We propose that the BJG and WJS mafic-ultramafic intrusions formed under a rift extensional setting during 820–780 Ma. The subduction process ceased before 820 Ma and did not directly contribute to the generation of the BJG and WJS mafic-ultramafic intrusions. The arc-like NbTa depleted trace element pattern in the parental magma is inherited from the mantle metasomatized by earlier subduction processes.

A 100 m.y. record of volcanic arc evolution in Nicaragua

AbstractThe processes that result in arc magmas are critical to understanding element recycling in subduction zones, yet little is known about how these systems evolve with time. Nicaragua provides an opportunity to reconstruct the history of a volcanic arc since the Cretaceous. Here we present the stratigraphy of the Cretaceous–Eocene volcanic units in Nicaragua and their relationship to the different tectonic units where the arc developed. We discovered an evolution from an arc‐dominated by calc‐alkaline compositions in the Cretaceous–Eocene, to transitional compositions in the Oligocene–Miocene, to finally tholeiitic magmas common in the modern volcanic front. Our petrographic studies confirm that in the Cretaceous–Eocene the olivine + clinopyroxene cotectic was followed by clinopyroxene + plagioclase ± amphibole. Given the abundance of amphibole and the lack of this mineral in the modern volcanic front, the Cretaceous–Eocene Arc melts were likely more water‐rich than modern Nicaragua, suppressing the crystallization of plagioclase after olivine. We also found temporal changes in element ratios that are sensitive to variations in sediment input. The Cretaceous–Eocene Arc is characterized by a lower Ba/Th compared to the Oligocene–Miocene and modern volcanic front samples, suggesting that the sediment input was lower in Ba, possibly analogous to old deep siliceous sediment subducting in the western Pacific. Both U/Th and U/La are higher in the modern volcanics, reflecting higher U/Th in the subducting sediments following the 'Carbonate Crash'. Finally, we found that the orientation of the arc axis also changed, from northeast‐southwest in the Cretaceous–Eocene to northwest‐southeast after the Oligocene. This change probably records variations in the location of the subduction zone as this region shaped into its current geographic configuration.

Basalt lava flows of the intraplate Newer Volcanic Province in south-east Australia (Melbourne region): 40Ar/39Ar geochronology reveals ~8 Ma of episodic activity

Intraplate basaltic volcanism is one of the most common, yet poorly understood, types of volcanism. Intraplate continental basaltic volcanic provinces (ICBVPs) typically comprise tens to hundreds of usually monogenetic, small-volume eruption centres, often cumulatively producing significant volumes of mainly primitive basaltic magmas. A wide range of mechanisms have been invoked to account for ICBVP melt generation, ranging from deep mantle plumes to local tectonic perturbations of the lithospheric mantle. The key to understanding the cause(s) of intraplate volcanism is understanding the geochemical evolution of magmatism within a regional tectonic framework. Unfortunately, many ICBVPs contain complex arrays of overlying lava flows, with uncertain eruption sources and a lack of precise age constraints.The Newer Volcanic Province (NVP) in south-eastern Australia is a young (~4.6 Ma–5 ka), geochemically and volcanologically diverse ICBVP, comprising ≥416 volcanic centres covering an area of ~23,000 km2. As with other ICBVPs, elucidating the underlying controls on magmatism has proved contentious, with recent studies favouring edge-driven convection, but divided as to the changing nature of volcanism. At the eastern boundary of the NVP, in the Melbourne area, previous work has indicated a complex network of lava flows representing almost the entire age and geochemical range of the NVP, making this an ideal microcosm for studying the evolution of the NVP.In this study, a holistic approach to lava flow mapping is utilised to unravel the complex network, incorporating diagnostic petrography, geochemistry and precise 40Ar/39Ar geochronology. This work reveals seven major lava flows, here named the Tullamarine, Redstone Hill, Aitken Hill, Mount Kororoit, Fenton Hill, Tulloch Hill and Mount Fraser flows, along with eight smaller-volume flows, here named the Pretty Sally, Green Hill, Bald Hill, Mount Cooper, Mount Ridley, Crowe Hill, Springs Hill and Summerhill Road flows. 40Ar/39Ar ages for eleven lava flows and eruption centres span an age range of 7.9–0.8 Ma. These new data reveal that alkalic, small-volume eruptions in the Melbourne area occurred exclusively between ~8 and 3.8 Ma. Post ~3.8 Ma, large-volume, tholeiitic eruptions dominated. A local progression from alkalic to tholeiitic volcanism is contrary to a province-wide progression from tholeiitic to alkalic volcanism inferred by previous studies.The earliest four eruptions (ca. 8 Ma) were aligned with the Cosgrove hotspot track and are probably unrelated to the broader NVP. The ages and geochemical variations of the younger flows are consistent with melt generation due to edge-driven convection. Magma composition might have been controlled by local perturbations within the convective cell, with tholeiitic magmas generated during high-degree melting in regions of more intense upwelling, and alkalic melts reflecting lower-degree melting and less-intense upwelling. The episodic nature of volcanism is likely due to restricted magma transport to the surface, which was only possible during brief periods of favourable tectonic conditions.

The long-lived Chiltepe volcanic complex, Nicaragua: magmatic evolution at an arc offset

The NW-SE striking volcanic front in Nicaragua is dissected into a western and an eastern segment separated by 20 km of N-S offset. The Chiltepe volcanic complex lies at the eastern end of the western segment and at the northern tip of the Nejapa-Miraflores tectonic and volcanic lineament that traces the arc offset. The Chiltepe peninsula attained its present shape and composition during highly explosive and effusive volcanic activity through the Late Pleistocene and Holocene, which formed the Chiltepe Formation (CF) and culminated in the 1.9 ka plinian eruption of the Chiltepe tephra. The previous evolution of this volcanic system is recorded in the volcaniclastic Mateare Formation (MF) exposed west (downwind) of the peninsula and separated from the CF by a large regional erosional unconformity. We divide the MF into the lower MF-1 member (22 volcaniclastic units) and the upper MF-2 member (17 volcaniclastic units), which are separated by a major erosional unconformity. The MF-1 was formed by variably evolved (basalt to dacite) magmas from a mantle source that was moderately metasomatized by fluids derived from subducted sediments. These high-Al moderately hydrous magmas fractionated in a tholeiitic fashion, with early plagioclase but delayed magnetite fractionation (initial Fe-Ti enrichments). Apart from the variable degree of differentiation, magmatic conditions during MF-1 remained fairly constant. While MF-1 contains several erosional unconformities suggesting tectonic activity, MF-2 is conformably stratified and the tholeiitic magmas persisted during this time. However, during MF-2, Al-poor tholeiitic compositions gradually replaced the Al-rich of MF-1 without significant changes in metasomatism or degree of melting at their mantle sources. At the same time, a different mantle source was tapped that was richer in the sediment components, and which produced more hydrous magmas that differentiated in a calc-alkaline fashion with early fractionation of both plagioclase and magnetite. Hence, two mantle source compositions were active during MF-2. The erosional interval between MF and CF, associated with strike-slip motion at the Mateare Fault, correlates with initiation of Nejapa-Miraflores volcanism. We postulate that extension along the Najapa-Miraflores fault system facilitated rapid ascent of mafic magmas from a mantle source laterally away from the arc axis that was less metasomatized than sources directly below the arc. On the Chiltepe peninsula, the Nejapa-Miraflores and Chiltepe magma systems interacted to form tholeiitic, less hydrous types of magmas (andesite to dacite) that erupted intermittently with the dominant calc-alkaline hydrous dacites. While associations of tholeiitic and calc-alkaline magmas at other subduction zones have often been attributed to variable intracrustal processes, we here argue for changes in the mantle source, particularly hydration by slab-derived fluids, as the main control on subsequent differentiation behavior. We further attribute the long-term changes in mantle source conditions through MF and CF, possibly over about 1 My, to result from temporal heterogeneity caused by mantle wedge solid flow and possibly variable fluid flow from the slab.

Clinopyroxene–Liquid Equilibria and Geothermobarometry in Natural and Experimental Tholeiites: the 2014–2015 Holuhraun Eruption, Iceland

Abstract Clinopyroxene–liquid geothermobarometry is a widely used tool for estimating the conditions under which mafic magmas are stored before they erupt. However, redox variability, sector zoning and disequilibrium crystallization present major challenges to the robust estimation of magma storage conditions. Moreover, most recent studies seeking to address these challenges have focused on clinopyroxenes from alkalic systems and are thus of limited use for understanding clinopyroxenes from the tholeiitic systems that dominate global magma budgets. Here we combine observations on natural clinopyroxenes from the 2014–2015 Holuhraun lava in Iceland with observations on experimental clinopyroxenes synthesized during high-pressure, high-temperature experiments on the same lava in order to investigate clinopyroxene–liquid equilibria in tholeiitic systems and optimize of geothermobarometric strategies. Natural clinopyroxenes from the 2014–2015 Holuhraun lava are sector zoned, with {1-11} hourglass sectors being enriched in the enstatite–ferrosillite component at the expense of all other components with respect to {hk0} prism sectors. In contrast with observations on clinopyroxenes from alkalic systems, sector zoning in clinopyroxenes from the 2014–2015 Holuhraun lava is characterized by differences in Ca and Na contents as well as in Ti and Al contents. The products of crystallization experiments performed at 100–600 MPa and 1140–1220 °C on a powdered starting glass at two sets of melt H2O content–oxygen fugacity conditions (∼0·1 wt % H2O and close to the graphite-oxygen redox buffer, and 0·5–1·0 wt % H2O and approximately one and half log units above the quartz–fayalite–magnetite redox buffer) demonstrate that clinopyroxene crystals from nominally equilibrium experiments can preserve strongly disequilibrium compositions. The compositional systematics of experimental clinopyroxenes are consistent with the presence of sector zoning. Furthermore, the magnitude of compositional variability increases with decreasing melt H2O content and increasing deviations of experimental temperatures below clinopyroxene liquidus temperatures (i.e. degrees of undercooling sensu lato), indicating that kinetic processes play a key role in controlling clinopyroxene compositions, even under notionally equilibrium conditions. Few published analyses of experimental clinopyroxene crystals may thus represent truly equilibrium compositions. Stoichiometric calculations on natural and experimental clinopyroxenes show that Fe3+ is a major constituent of clinopyroxenes from tholeiitic magmas under naturally relevant oxygen fugacity conditions. They also show that Fe3+ is most likely incorporated as Ca- and Al- bearing Ca–Fe-Tschermak’s component rather than Na-bearing aegirine component at oxygen fugacities up to one and a half log units above the quartz–fayalite–magnetite buffer. Elevated oxygen fugacities are thus less likely to compromise clinopyroxene–liquid geothermobarometry than previously thought. Guided by our experimental results, we combined published descriptions of clinopyroxene–liquid equilibria with geothermobarometric equations to develop an internally consistent and widely applicable method for performing geothermobarometry on tholeiitic magmas that does not require equilibrium zones to be selected a priori. Applying this method to natural clinopyroxene crystals from the 2014–2015 Holuhraun lava that formed under low but variable degrees of undercooling (perhaps 25 °C or less) returns values in excellent agreement with those from independent methods (232 ± 86 MPa, 1161 ± 11 °C). Robust estimates of magma storage conditions can thus be obtained by performing clinopyroxene–liquid geothermobarometry on tholeiitic magmas when disequilibrium is suitably accounted for.

A back-arc origin for the Neoarchean megacrystic anorthosite-bearing Bird River Sill and the associated greenstone belt, Bird River subprovince, Western Superior Province, Manitoba, Canada

This study presents extensive new field, petrographic, and whole-rock major and trace element and Sm–Nd and Pb isotope data for the Neoarchean megacrystic anorthosite-bearing Bird River Sill, spatially associated Neoarchean supracrustal rocks of the Bird River greenstone belt, and the Mesoarchean Maskwa Lake TTG Batholith I, Manitoba, Canada. Field observations indicate that the 2743 Ma Bird River Sill was emplaced into the Northern Lamprey Falls Formation in an oceanic setting and subsequently intruded by the ca. 2725 Ma Maskwa Lake Batholith II. The Northern Lamprey Falls Formation is in fault contact with the overlying Peterson Creek Formation, which in turn is unconformably overlain by the Bird River Formation. The peridotites of the Bird River Sill were subjected to intense alteration and ductile shearing, resulting in the redistribution of many major and trace elements and resetting of their Sm–Nd and U–Th–Pb isotopic systems. The gabbro, leucogabbro, and anorthosite units of the sill largely retain their primary igneous textures and minerals, REE and HFSE compositions, and Sm–Nd and Pb isotope signatures. These geochemical and petrographic data indicate that these units of the Bird River Sill crystallised from a hydrous, Ca- and Al-rich tholeiitic magma that fractionated from a hydrous primitive arc tholeiitic parental magma. The major and trace element geochemistry and initial eNd values of the Bird River Sill (+ 0.53 to + 4.72) and the Northern Lamprey Falls Formation (+ 1.40 to + 2.66) of the Bird River greenstone belt crystallised from parental magma batches that were derived by partial melting of variably-depleted mantle sources. High-magnesian andesites from the Bird River Formation formed by partial melting of subducting oceanic crust and/or slab sediments and subsequent interaction with sub-arc mantle peridotite, whereas the Peterson Creek Formation dacites and Maskwa Lake TTG Batholith I tonalites formed by partial melting of juvenile lower arc crust. The major and trace element geochemistry of the Bird River Sill and greenstone belt and the Maskwa Lake TTG Batholith I, and geological relationships and lithological associations point to a transition from continental arc to continental back-arc magmatism along the southern active margin of the North Caribou terrane at ca. 2743 Ma, resulting in the formation of the Maskwa Lake microcontinent that occurred prior to subsequent continental arc magmatism along the southern margin of this microcontinent. Combined field observations and geochemical data indicate that the Bird River Sill and Bird River greenstone belt are a dismembered Archean subduction-related ophiolite that marks a suture zone between the Winnipeg River subprovince and the Maskwa Lake microcontinent.

Syncollisional gabbro in the Irtysh shear zone, Eastern Kazakhstan: Compositions, geochronology, and geodynamic implications

New data of petrography, mineralogy, and geochemistry from the Irtysh shear zone in Eastern Kazakhstan reveal three types of gabbro: (1) dolerite and gabbrodolerite, (2) biotite-bearing gabbronorite and gabbrodiorite, and (3) layered peridotite-gabbro intrusions. Rocks of types 1 and 2 occur mainly as small bodies and share similarity in major- and trace-element compositions. Layered gabbro (type 3) forms the largest intrusion in the area and has a different composition: higher CaO and Al2O3 but lower K2O, TiO2, P2O5, Zr, Nb, REE. Correspondingly, the gabbro of two first types may result from differentiation of the same tholeiite magma, whereas the layered gabbro rather derived from a magma intermediate between tholeiite and calc-alkaline series. According to geochemical and isotopic signatures, both types of primary magma originated from the same sublithospheric depleted mantle by low- and high-degree partial melting of spinel lherzolite. The obtained 317–313 Ma U-Pb and Ar-Ar ages of the gabbro fall within the time of collisional events in the area. The suggested model explains the origin of syncollisional gabbro as intrusion of ascending mafic melts from a mantle wedge at the transform boundary of colliding plates along the Irtysh shear zone where strike-slip faulting produced slab windows open to inputs of hot asthenospheric material.

Physical and chemical conditions of basaltic magmatism of archipelago Franz Josef Land

As a result of mineralogical and thermobarogeochemical researches of different-age basaltic complexes of Archipelago Franz Josef Land (FJL) regular changes in time of compositions of plagioclases, clinopyroxenes and melt inclusions are established. Chemical compositions of inclusions directly testify to prevalence in Early Jurassic of plateau basaltic melts similar (according to the content of the basic components, and also trace and rare-earth elements) to typical basalt tholeiitic magma of the Siberian platform. In Early Cretaceous melts already had the enriched subalkaline character. Calculations of conditions of magma generation, spent on the basis of the data on melt inclusions, have shown evolution from Early Jurassic to Early Cretaceous (with allocation of three peaks of magmatic activity: 192.2±2.8, 157.4±3.5 and 131.5±0.8 million years) depths and temperatures (accordingly: 70-110 km and to 120 km, 1430-1580°С; 60-110 km, 1390-1580°С; 50-140 km, 1350-1690°С) of mantle melting with formation of deep sources of the FJL magmas.