Magma Emplacement Research Articles

Unsupervised Machine Learning on Domes in the Lunar Gardner Region: Implications for Dome Classification and Local Magmatic Activities on the Moon

Lunar volcanic domes are essential windows into the local magmatic activities on the Moon. Classification of domes is a useful way to figure out the relationship between dome appearances and formation processes. Previous studies of dome classification were manually or semi-automatically carried out either qualitatively or quantitively. We applied an unsupervised machine-learning method to domes that are annularly or radially distributed around Gardner, a unique central-vent volcano located in the northern part of the Mare Tranquillitatis. High-resolution lunar imaging and spectral data were used to extract morphometric and spectral properties of domes in both the Gardner volcano and its surrounding region in the Mare Tranquillitatis. An integrated robust Fuzzy C-Means clustering algorithm was performed on 120 combinations of five morphometric (diameter, area, height, surface volume, and slope) and two elemental features (FeO and TiO2 contents) to find the optimum combination. Rheological features of domes and their dike formation parameters were calculated for dome-forming lava explanations. Results show that diameter, area, surface volume, and slope are the selected optimum features for dome clustering. 54 studied domes can be grouped into four dome clusters (DC1 to DC4). DC1 domes are relatively small, steep, and close to the Gardner volcano, with forming lavas of high viscosities and low effusion rates, representing the latest Eratosthenian dome formation stage of the Gardner volcano. Domes of DC2 to DC4 are relatively large, smooth, and widely distributed, with forming lavas of low viscosities and high effusion rates, representing magmatic activities varying from Imbrian to Eratosthenian in the northern Mare Tranquillitatis. The integrated algorithm provides a new and independent way to figure out the representative properties of lunar domes and helps us further clarify the relationship between dome clusters and local magma activities of the Moon.

Laponite gels - visco-elasto-plastic analogues for geological laboratory modelling

Laponite® is a synthetic clay that, depending on concentration, temperature and curing time, forms a clear, transparent thixotropic fluid or brittle visco-elasto-plastic gel when mixed with water. Here we present the results of rheological and mechanical testing of gel-forming Laponite RD (LRD) to evaluate its suitability as a rock analogue in laboratory analogue experiments. Rheological tests of 2–4 wt% concentrations of LRD in deionised water were carried out at temperatures between 20 and 50 °C, and after curing times of 3 to 14 days. Our results show that LRD gels change from a brittle, elastic-dominant, linear viscoelastic material to a plastic material as shear strain increases. The linear viscoelastic region occurs at shear strains, γ < 10% after which the material yields and then undergoes strain hardening before a peak stress occurs at γ = 15–20%. LRD then strain softens up to γ < 26.2%, beyond which it behaves as a plastic material. Empirical equations are provided that predict increases in the Young's and complex shear moduli of LRD with increasing concentration and ageing time. LRD can be used to model elastic deformation when γ < 10% at a shear strain rate of 0.1 s−1 and plastic deformation when γ > 26.2%. LRD is an ideal material for modelling the behaviour of rocks during the emplacement of magma and the propagation of brittle fractures in the upper crust. Its ease of preparation, low surface tension, full transparency, chemical and biological stability and photoelastic properties provide further advantages for analogue laboratory modelling compared to other frequently used visco-elastic gels, such as pig skin gelatine.

A petrochronology window into near-surface fluid/rock interaction within Archaean ultramafic-mafic crust: Insights from the 3.25 Ga Stolzburg Complex, Barberton Greenstone Belt

The Stolzburg Complex is one of several early Archaean ultramafic-mafic layered complexes situated along the northern edge of the Barberton Greenstone Belt on the eastern Kaapvaal Craton. In order to constrain timing and mode of emplacement, as well as post-emplacement processes affecting this igneous complex, we present whole rock trace element compositions as well as isotopic (U-Pb, O, Hf) and REE-Ti concentration data for zircon and titanite from three representative meta-gabbroic samples from the central and western portions of the complex. Zircon (SHRIMP) and titanite (LA-MC-ICPMS) analyses yield precise and identical U-Pb ages of 3247 ± 3 Ma, 3252 ± 16 Ma and 3259 ± 6 Ma (2SE), respectively. Cathodo-luminescence imaging of zircon reveals remnant, faint oscillatory and convolute zoning that is cross-cut by luminescent, irregular fracture fillings. Zircon crystals from one meta-gabbro sample yield oxygen isotopic compositions (δ18O) of <1 to +6‰, which are uncorrelated with the initial Hf isotopic compositions (εHf) of 0 to +3 epsilon units. Zircon REE systematics are highly variable across crystal domains and characterized by enriched light and medium REE. Micro-textural evidence of post-emplacement modification, along with zircon δ18O and trace element systematics, are indicative of pervasive but heterogeneous/irregular overprinting involving high-T hydrothermal fluids (>250 °C) soon after magma emplacement. Some recent metamictization may have also contributed to the low δ18O component identified in the magmatic zircon, but is difficult to disentangle from hydrothermal effects. Based on zircon trace element systematics, the gabbros appear to have formed in an oceanic setting, consistent with the lack of whole rock geochemical evidence for assimilation of more evolved felsic crustal material. Considering analytical uncertainty, zircon εHf values are relatively uniform and point to derivation of the mafic magma from a Depleted Mantle source. Titanium-in-zircon temperatures (assuming αSiO2 = 1.0, αTiO2 = 1.0) range from 840 to 1020 °C. Although such apparent crystallization temperatures do neither constrain magma petrogenesis nor tectonic setting uniquely, they match temperatures recorded by modern mid-oceanic gabbros that experienced extensive hydrothermal alteration. Thus, the meta-gabbros of the ca. 3.25 Ga Stolzburg Complex provide a rare record of fluid-rock interaction at high temperatures within mafic crust on an ancient seafloor.

Comprehensive palaeomagnetic study of San Borja and Jaraguay monogenetic volcanic fields, Baja California (28–30°N): considerations on latitudinal corrections

SUMMARY We report 24 palaeomagnetic directions and 10 high-quality Thellier-derived palaeointensity (PI) values, obtained from 27 sites located in Baja California Peninsula, northwestern Mexico. Sampling was done in four rock units (magnesian andesites, calc-alkaline lavas, ignimbrites, adakites) belonging to San Borja and Jaraguay monogenetic volcanic fields. These units have erupted between ∼15 and 2.6 Ma (previous K-Ar and 40Ar/39Ar data); hence results are presented in two consecutive periods: middle-late Miocene and Pliocene. The identified main magnetic minerals in the sampled sites are titanomagnetite, magnetite and minor hematite, of variable grain size, present as intergrowths or surrounding grains, which reflect varying oxidation/reduction conditions during emplacement of high-temperature magmas. Based on previous geological and geophysical records, the kinematic evolution was carefully considered in the region, allowing for the independent restoration of the palaeoposition of each sampled site. Previous palaeodirections were also evaluated and corrected for tectonic motion in order to combine them with present data. Accordingly, a number of 15 and 36 directional data are used to calculate palaeopole position for Pliocene and middle-late Miocene periods, respectively, selected from a total of 74 data points. Pliocene (Plat = 87.8°, Plong = 147.5°, K = 41.06, A95 = 6.0°) and middle-late Miocene (Plat = 86.0°, Plong = 172.7°, K = 41.08, A95 = 3.8°) palaeopole positions, calculated after tectonic corrections, are not statistically different from expected North American reference pole. Tectonic correction for Middle-late Miocene virtual geomagnetic poles plays an important role in reducing the resultant tilting from 2.7° to –0.8°. PI mean were calculated for Pliocene and middle-late Miocene periods at 29.2 ± 9.1 and 23.2 ± 6.3 μT, respectively. Compiling global filtered PI data, together with our results, indicates that the strength of the geomagnetic field during middle-late Miocene was weak (virtual dipole moment = 5.0 ± 2.2 × 1022 Am2) compared to Pliocene (6.4 ± 2.8 × 1022 Am2), and also relative to the present-day value (7.6 × 1022 Am2). This indicates the global nature of the low dipole moment during the middle-late Miocene period. However, issues related to the spatio-temporal distribution of PI data still present an obstacle to validating these suggestions; therefore, more reliable data are still needed.

Emplacement of the Little Minch Sill Complex, Sea of Hebrides Basin, NW Scotland

The Little Minch Sill Complex consists of a series of stacked, multi-leaved Paleocene dolerite sills, which have primarily intruded into Mesozoic sedimentary rocks and Paleocene tuffs/?hyaloclastites within the Sea of Hebrides Basin. The Sea of Hebrides Basin is situated to the west of the Scottish mainland on the NE Atlantic margin. Two previously proposed models for the emplacement of the sill complex have opposing ideas for the location of magma input and the emplacement mechanisms. Both models have been constructed using data primarily from onshore outcrops on the Isle of Skye, Raasay and the Shiant Isles. However, these onshore outcrops only represent a quarter (1040 km 2 ) of the entire extent of the sill complex, which is largely located offshore. To understand how the sill complex as a whole was emplaced within the basin, both onshore and offshore magma transport needs to be considered. Using high-resolution multibeam bathymetry data (up to 2 m resolution) obtained between 2008 and 2011, along with supporting seismic reflection, sparker and pinger data, a new assessment of the offshore extent and character of the sill complex has been constructed. Mapping of the large-scale relationships between intrusions and the host rock, along with morphological features such as magma lobes, magma fingers, transgressive wings, en echelon feeder dykes and the axis of saucer-/half-saucer-shaped intrusions, has indicated the magma flow directions within the intrusive network. Assessing the flow kinematics of the sills has provided insights into magma transport and emplacement processes offshore. Combining data from previously mapped onshore sills with data from our newly constructed model for magma emplacement offshore has allowed us to construct a new model for the emplacement of the Little Minch Sill Complex. This model demonstrates that major basin-bounding faults may have a lesser role in channelling magma through sedimentary basins than previously thought. Applying the knowledge obtained from this study could further progress our understanding of the effect of sill emplacement on fluid flow within volcanic rift basins worldwide, with direct impacts on the exploitation of petroleum and geothermal systems.

Salt–magma interactions influence intrusion distribution and salt tectonics in the Santos Basin, offshore Brazil

AbstractMany sedimentary basins host thick evaporite (salt) deposits. Some of these basins also host extensive igneous intrusion networks. It thus seems inevitable that, in some locations, magma will interact with salt. Yet how interaction between these materials may influence salt tectonics or magma emplacement, particularly at the basin‐scale, remains poorly understood. We use 3D seismic reflection data from the Santos Basin, offshore Brazil to image 38 igneous intrusions spatially related to thick Aptian salt. Based on identified seismic–stratigraphic relationships, we suggest sill emplacement likely occurred during the late Albian‐to‐Santonian. We show intra‐salt sills are geometrically similar to but laterally offset from supra‐salt sills. We suggest ascending magma was arrested by the salt in some areas, but not others, perhaps due to differences in evaporite lithology. Our mapping also reveals most sills occur within and above the presalt Merluza Graben, an area characterized by Albian‐to‐Neogene, salt‐detached extension. In adjacent areas, where there are few intrusions, salt deformation was driven by post‐Santonian diapir rise. We suggest emplacement of hot magma within evaporites above the Merluza Graben enhanced Albian‐to‐Santonian salt movement, but that crystallization of the intrusion network restricted post‐Santonian diapirism. Our work indicates salt–magma interaction can influence salt tectonics, as well as the distribution of magma plumbing systems, and thus could impact basin evolution.

Combining zircon texture, REE patterns and U-Pb-Hf isotopes to decipher the formation process of orbicular rocks: A case study from Huangling orbicular granodiorite, Yangtze craton, China

The petrogenesis of orbicular rocks is still enigmatic because of the unique texture and variable compositions. In this study, we focus on the textural characteristics and mineral compositions from Huangling Orbicular Granodiorite (HOG), where orbicules with rhythmic growth texture and radial growth texture exist in the same outcrop. Textural and isotopic studies of zircons from different parts of the representative orbicules show three age groups of 1961 ± 19 Ma, 890 ± 6 Ma and 852 ± 6 Ma, corresponding to the ages of protolith crystallization, melt emplacement, and input of a new magma, respectively. Combined with field relationship, Ti-zircon crystallization temperature, trace element modelling and Hf isotopic data, the HOG may be derived from high-temperature contact process that involved the partial melting of granodiorites in contact and interaction with a quartz-bearing dioritic intrusion.Textural features and mineral compositions of the studied orbicules show that different textures are related to different states of crystallization. Orbicules with a rhythmic growth texture have disequilibrium crystallization zone with feldspar An contents of 34–46 and a “W”-shape pattern (forming rhythmic layers), and near-equilibrium crystallization zone with An contents of 42–44 (forming the inner part). Orbicules with a radial growth texture was crystallization in disequilibrium, as An contents show approximate linear variations of 32–44 to form radial growth layer, and with An varies 26–46 to form the inner part.Our observations support the magma quenching hypothesis, proposed as hot orbicule melts underwent rapid cooling with inward crystallization. Moreover, we firstly suggest that the amphibole-plagioclase disequilibrium crystallization could explain different textures of orbicules. Rhythmic growth texture and radial growth texture are formed at undercooling state with fast and slow cooling rates, separately. The numbers of rhythmic layers are controlled by the duration of undercooling state, while the width of each layer is controlled by the cooling rate. If excess amphibole components left in the center of orbicule melt, the “core” will be mafic. Otherwise, the “core” will be felsic.

Structural Setting, Paleomagnetism, and Magnetic Fabric of Miocene Plutons in a Transpressional Sinistral Shear Zone, Tonalá, Chiapas, Mexico: Evidence of Shortening During Magma Emplacement

AbstractWe studied the structure, paleomagnetism, and magnetic fabric of Miocene granitoids intruded along the left‐lateral, transpressional, Tonalá shear zone in Chiapas, southern Mexico. Magmatic fabrics delineate patterns of regional folding striking parallel to the shear zone, suggesting syn‐magmatic shortening. Magnetic susceptibility fabrics are well developed, and mean magnetic susceptibility averages 18.6 × 10−3 SI. The magnetic fabric of the plutons is controlled primarily by MD and finer PSD magnetite, which also carries the magnetic remanence. The orientations of macroscopic mineral fabrics and magnetic susceptibility fabrics are consistently parallel in protomylonites, but their coincidence is good to poor in granitoids that preserve magmatic fabrics. Most site mean directions are NW‐NE directed of moderate positive inclination (or antipodal), but on a local scale site, mean directions are very discordant. The overall mean direction, omitting highly discordant sites, is of Decl. = 359.6° and Incl. = +41.9° (k = 14.4, α95 = 8.1°, N = 24 sites); it is nearly concordant with the North America reference direction, and the slight difference may indicate a gentle, north‐side down regional tilt. High directional dispersion is interpreted to reflect relative rotation about local vertical to sub‐vertical axes of plutons, or parts of plutons, along the shear zone, with some rotations interpreted to be caused by progressive deformation within the shear zone. Paleomagnetic and AMS data from the protomylonite rocks are interpreted to have been affected by the distortional strain, which accommodated shape and volume change in the rock. We suggest that the introduction of granitic magma into middle crustal levels critically enhanced the crustal response to shortening within the shear zone, possibly due to melt compressibility.

Redox series assessment, petrogenetic, and geodynamic appraisal of Neoarchean granites from the Bundelkhand Craton, Central India: Constraints from phase petrology and bulk rock geochemistry

Based on field relationships, microstructure, phase petrology, and whole‐rock elemental geochemistry, we have unravelled the redox conditions and petrogenetic evolution of Neoarchean granites (sensu lato) from the central part of the Bundelkhand Craton, Central India. The studied Neoarchean granites are sanukitoids (sensu lato), high‐K, low‐magnesian granites (HKLMG), and oxidized A‐type granites (OAG). Chemistry of biotite in conjunction with amphibole compositions from sanukitoids indicates their crystallization in calc‐alkaline felsic host magma formed in a subduction setting at elevated fugacity of oxygen (fO2), buffered at +2 fayalite‐magnetite‐quartz (FMQ), corresponding to magnetite series granite. Chemistry of biotite and amphiboles from HKLMG suggests that host melts of these phases buffered above +1 FMQ, that is, moderate fO2 condition corresponding to magnetite series granites. The nature of the HKLMG magma is peraluminous to alkaline which originated at a relatively high‐P condition. The biotites from OAG exhibit pronounced Al‐Fe substitution buffered above +0.5 FMQ to represent oxidized A‐type host granite magma, belonging to magnetite series granite. The sanukitoids bear high ferromagnesian minerals (FeOt + MgO + MnO + TiO2 (FMMT) &gt;4.5; Mg# = 28–45), and enriched incompatible elements without significant Eu anomalies, probably originated from melt‐enriched mantle source in a subduction setting. The HKLMG are characterized by relatively low Mg#, TiO2, high K‐LILEs‐REEs with negative Eu anomalies, and high Th/Nb, Ce/Pb, and La/Nb ratios, which suggest mixed source signatures, derived from the enriched mantle and continental crust. A‐type granites (OAG) bear high NK/A (molar Na2O + K2O/Al2O3 ~ 1.17), FeOt/MgO, K2O/Na2O, Ga/Al, and Ce–Y, which indicate the emplacement of OAG magmas in syn‐ to post‐collisional tectonic environments. Thus, the Neoarchean granites from the Bundelkhand Craton represent a collage of subduction related sanukitoids to high‐K magnesian granites, and syn‐ to post‐collisional oxidized A‐type granites.

India‐Elan Bank‐East Antarctica Breakup, Crustal Architecture, and Margin Evolution: Results From Constrained Potential Field and Process‐Oriented Gravity Modeling of Conjugate Margin Segments

AbstractWe present here the results of a seismically well‐constrained potential field modeling along with process‐oriented gravity modeling across the Eastern Continental Margin of India (ECMI) and its conjugate East Antarctica Continental Margin (EACM). The study revealed both along‐strike variations in crustal architecture and effective elastic thickness (Te), showing a locally asymmetric pattern. Crustal stretching factors (β) estimated at these margins range between 1.5 and 4.0 with slightly lower values varying over the narrow zone in the Cauvery basin‐western Enderby basin and relatively higher values in other margin segments. The models reveal typical magma‐poor margin characteristics in the Krishna‐Godavari (K‐G) and Cauvery basin segments of ECMI. The ocean‐continent transition (OCT) in the K‐G basin and its conjugate (central Enderby basin) indicates asymmetric rifting marked by the development of a detachment fault separating the combined India‐Elan Bank margin and the EACM. The higher β values and magmatic underplating at OCT in the Mahanadi basin segment of ECMI may be related to the post‐breakup magmatic emplacement associated with the proximal activity of the Kerguelen plume after the separation of Elan Bank from India by a ridge jump during M2/M0 (124–120 Ma). The pre‐rift reconstruction model through the palinspastic restoration of margins suggests that the central part of ECMI (16–20°) is the ideal pre‐rift location for the Elan bank.

Petrogenesis of Mesoproterozoic Granites of the Swartoup Hills Region, Kakamas Domain, Namaqua Belt, South Africa

The Swartoup and Polisiehoek plutons in the Swartoup Hills (South Africa) formed during an episode of significant magma emplacement in the Mesoproterozoic Namaqua Sector of the Namaqua Metamorphic Province. They intruded into mid-crustal metasedimentary rocks of the metapelitic Koenap and mafic to carbonate-bearing Bysteek Formations during and shortly after the ∼1,200–1,220 Ma regional metamorphic peak that reached ultrahigh temperatures. Subsequent to pluton emplacement, the crust underwent regional high-temperature deformation during slow near-isobaric cooling. A further episode of pluton emplacement associated with fluid circulation truncated the first-order regional tectonic structures at ∼1,100 Ma. Based on their petrography, the Swartoup pluton is subdivided into leuco-granitoids with biotite as the sole mafic phase, pyroxene granitoids, and garnet-bearing granitoids, which may contain significant biotite. These subgroups display distinctive geochemical variations from one another, and from the Koenap Formation migmatites and the Polisiehoek granites, which are exposed nearby. Incompatible trace element distributions suggest that the Swartoup and Polisiehoek granitoids represent modified A-type granite magma, consistent with derivation from partial melting of quartzo-feldspathic crust. The magmas incorporated significant amounts of juvenile mantle-derived magma (εNd1200 of ∼−5, and LREE-depleted), but do not require older, early to late Paleoproterozoic crust. Particularly close to contacts to the calcic Bysteek Formation, localized contamination of the Swartoup granites by supracrustal carbonates is evident. A relatively pervasive alkali metasomatic effect is manifested strongly in the initial 87Sr/86Sr and LILE profiles of the Polisiehoek granites in particular, as well as in some of the Swartoup pyroxene granitoids, which could be either a symptom of CO2 metasomatism related to the Bysteek Formation carbonates, or to post-magmatic fluid metasomatism, perhaps linked to regional shearing. The comparison of our results with literature data suggests that similar sources, A-type granitic, Meso- to Paleoproterozoic crustal, and enriched mantle, have contributed, in locally differing proportions, to granites in most parts of the Namaqua Sector. Most likely, these plutons were generated during crustal and mantle melting in a long-lived hot continental back-arc environment.

Variant Offset-Type Platinum Group Element Reef Mineralization in Basal Olivine Cumulates of the Kapalagulu Intrusion, Western Tanzania

Abstract The Kapalagulu intrusion in eastern Tanzania hosts a major, 420-m-thick, stratiform/stratabound platinum group element (PGE)-bearing sulfide zone—the Lubalisi reef—within a prominent, chromititiferous, harzburgite unit close to its stratigraphic base. Several features of the vertical base and precious metal distributions (in a composite stratigraphic section based upon two deep exploration drill holes) display similarities to those of offset-type PGE reefs that formed under the overall control of Rayleigh fractionation: (1) composite layering (at several scales) defined by systematic vertical variations of sulfide and precious metal contents and intermetallic ratios, indicating repeated cycles of PGE enrichment and depletion in the order Pd-Pt-Au-Cu, and (2) in the lower part of the reef, stratigraphic offsets of the precious metal peaks below peak sulfide (Cu) content. The form and geochemistry of the reef are consistent with overturns of basal liquid layers within a liquid layering system (i.e., stable density-driven stratification of a magma chamber), plus at least two minor inputs of parental magma during which the resident magma was recharged with sulfur and metals, and the effective depletion of precious metals in the magma midway through reef development. The Lubalisi reef differs from classic offset-type PGE reefs, however, principally because individual Pd, Pt, and Au enrichment peaks are coincident, not offset. The reef is set apart from other offset-type PGE reefs in three additional ways: (1) its association with olivine cumulates that crystallized soon after initial magma emplacement and well below the first appearance of cumulus pyroxene or plagioclase (implying attainment of sulfide saturation and precious metal enrichment without prolonged concentration of sulfur and chalcophile metals by normal magma cooling and differentiation), (2) the probable role of chromite crystallization in not only triggering sulfide segregation during reef formation but also facilitating precious metal enrichment in the early stages of reef development, and (3) its great width. The early stage of fractionation may also help explain the coincident precious metal peaks through its effect on apparent precious metal partition coefficients.

Crustal electrical structure and deep metallogenic mechanism in the Xiongcun and Niangre districts of the Tibetan Plateau

The Gangdese metallogenic belt in the Tibetan Plateau hosts many large ore deposits, of which the Xiongcun and Niangre ore concentration districts are two typical examples. Due to the insufficient geophysical evidence about the deep metallogenic dynamics across these two districts, the electrical structure was obtained by modeling magnetotelluric data to study the relationship of the crustal structure and metallogenic dynamics. Although the mineralization occurred in the past, the continuous magmatic activities after the mineralization could reactivate the previous migration channels to remain some traces, which can be revealed by some geophysical methods. The resistors at depths of less than 20 km may represent the multiple overlapping volcanic rocks related to a serious of magmatic events in the Lhasa-Gangdese terrane. Some conductors that revealed at different depths may be related to the partial melt, aqueous fluids and/or formation of metallic minerals, and were also further indicated as the possible magma chambers. Connecting the conductors at different depths may be further indicated as some ancient migration channels which were probably related to the shear zones and faults. As the ore deposits in both districts belonged to the magmatic-hydrothermal type, the upwelling magma played a crucial role in the metallogenic dynamics. These possible magma chambers at different depths and channels may be also related to the evolution and emplacement of the deep-seated magma, and contributed to the mineralization of both districts. This study can also offer a reference of significance for the deposits in the eastern Gangdese metallogenic belt.

Formation of the Weiya magmatic Fe–Ti oxide deposit and its ore-hosting layered gabbro intrusion, Eastern Tianshan (Xinjiang, NW China)

The Weiya is the largest magmatic Fe–Ti oxide deposit in the Chinese Eastern Tianshan orogen, and is divided into an eastern, central and western section by younger granitoid and diabase intrusions. The Weiya eastern section contain Fe-Ti oxide ores and ore-bearing apatite-poor layered gabbro, which display significant difference in mineral assemblage from the layered gabbro in the central section. We presented new data on the geology, mineralogy, petrology, geochemistry, geochronology, and Sr-Nd isotopes of the gabbroic rocks and the Fe-Ti oxide ores from the eastern section. Igneous zircons from the Weiya anorthosite yielded a SIMS U–Pb concordia age of 237.9 ± 0.71 Ma, coeval with the apatite-rich gabbro in the central section. The gabbroic rocks and Fe-Ti oxide ores in the eastern section and the central section have parallel chondrite-normalized REE patterns and N-MORB-normalized multi-element patterns, together with highly-similar Sr-Nd isotope features, suggesting that the two are comagmatic. Olivine Fo value (62.0–66.7), clinopyroxene Mg# number (68.1–76.2) and plagioclase An contents (39.9–55.4) suggest that the Weiya parental magma was highly evolved after extensive fractional crystallization in the deeper magma chamber, and the residual magma became increasingly oxidized and metals- and volatiles-rich. During the magma emplacement, plagioclase accumulated extensively in the magma chamber roof and further enriched Fe–Ti in the residual melts, leading to Fe–Ti oxide mineralization.

Cretaceous crustal melting records of tectonic transition from subduction to slab rollback of the Paleo-Pacific Plate in SE China

The Mesozoic geology of SE China has been widely considered to relate to subduction of the Paleo-Pacific Plate beneath the Eurasia, accompanied with extensive felsic magmatism. However, the relationship between the origin of the voluminous crust-derived magmatism and the Paleo-Pacific slab subduction remains unclear. Here we present petrology, zircon U-Pb-Hf isotope data, and whole-rock geochemical and isotopic compositions of two episodes of Cretaceous silicic volcanic rocks (~114–113 Ma for Group 1 and ~ 100–93 Ma for Group 2) from southeastern Zhejiang Province in SE China, to provide new constraints on the geodynamic transition of the Paleo-Pacific slab subduction. Both volcanic groups show similarly unradiogenic whole-rock εNd(t) and zircon εHf(t) values (εNd(t) = −8.6 to −6.4 and εHf(t) = −8.5 to −0.7 for Group 1, and εNd(t) = −7.1 to −6.1 and εHf(t) = −7.8 to −2.2 for Group 2). The limited emplacement of coeval mafic magmas and absence of mafic microgranular enclave and intermediate lavas suggest an insignificant role of mantle-derived melt either as a parental magma for differentiation or a mafic component during magma mixing. Instead, the low MgO and high concentrations of SiO2 and incompatible elements and the evolved Nd and Hf isotopic compositions indicate derivation of these two groups of silicic magmas from similar hybrid crustal sources containing the metasedimentary rocks of Cathaysia basement and the newly accreted arc crust. Group 1 rocks contain a higher proportion of hydrous minerals (e.g., biotite) and Sr/Y and La/YbCN ratios but lower zircon saturation temperature than Group 2. Such variations in mineral and geochemical compositions could be mainly attributed to the change of melting conditions from a relatively deeper, colder and water-fluxed crust to a shallower, hotter and less hydrous state. By combining our new results and previous studies of the late Mesozoic volcanic rocks in SE China, we propose that a geodynamic transition from subduction to slab rollback of the Paleo-Pacific Plate was responsible for such P-T-H2O variations of the crustal source.

High-temperature melting of different crustal levels in the inner zone of the Emeishan large igneous province: Constraints from the Permian ferrosyenite and granite from the Panxi region

The Late-Permian Emeishan mantle plume event caused large-scale flood basalts and related mafic-ultramafic layered mafic intrusions in the inner zone of the Emeishan large igneous province (ELIP). However, the related crustal magma response of this mantle plume event in the inner zone of the ELIP remains poorly understood. In this contribution, we report contemporaneous (~259 Ma) ferrosyenite and A-type granite in the inner zone of the ELIP, and their chronological and geochemical data suggest simultaneous crustal melting at different depths. The ferrosyenites display moderate SiO2 (60.14 to 63.58 wt%), high Fe2O3T (4.28 to 8.52 wt%) and alkaline (Na2O + K2O = 10.91 to 12.75 wt%) contents. In combination with their positive zircon εHf(t) values (+1.5 to +12.9), we propose that the ferrosyenites were formed by high temperature melting of Fe-rich refractory juvenile lower crust at H2O-poor condition, which was induced by underplating of the high-temperature mantle plume. Rhyolite–MELTS modeling results show that ~7% melting of ferrodiorite-like source at ~1120 °C and 8 kbar can produce similar compositions with least evolved ferrosyenite under a relatively dry condition. The Baima granites have high SiO2 (70.58 to 74.26 wt%) contents and 10000*Ga/Al ratios (3.00 to 3.28), indicating the affinity to A-type granites. Their evolved zircon εHf(t) values (−8.1 to −0.6) indicate an evolved crustal source region. Rcrust modeling indicates that ~9% partial melting of ancient granodiorite-like crustal material under low pressure condition (4 kbar) can produce the observed granitic compositions. Considering the close spatial and temporal relationship between ferrosyenites and A-type granites, we propose that the emplacement of ferrosyenitic magma resulted in low-degree partial melting at a relatively shallow crustal level, responsible for the formation of the A-type granites. In summary, this study highlights the significant impact on crustal magma response to the high-temperature mantle plume event in the inner zone of ELIP.

Origin of high-Cr stratiform chromitite in the Fangmayu Alaskan-type ultramafic intrusion, North China Craton

The study of chromite from chromitite-bearing layered intrusions can provide significant insights into their petrogenetic origin and tectonic setting. The Fangmayu Alaskan-type ultramafic intrusion in the North China Craton contains layered and massive chromitite associated with a serpentinized ultramafic suite. This study presents zircon and apatite geochronology, chromite mineral chemistry, platinum-group element (PGE) and Re-Os isotopic data for the purpose of constraining the origin, evolution, and composition of the Fangmayu chromitite parental melts. Zircon U-Pb data from the chromitites define various age populations, with oldest age component (>2.49 Ga) interpreted to represent xenocrystic grains incorporated from crystalline basement. A dominant age component (~2.42 Ga) may represent the age of magma emplacement. Ages in the range of 2375–2068 Ma likely represent variable partial radiogenic Pb-loss during subsequent thermal events. Zircon ages in the range of 1.96–1.90 Ga record metamorphic events, potentially related to the collision between the Eastern Block and Western Block in the North China Craton. Ages of 1.85–1.81 Ga likely represent zircon growth during retrograde metamorphic process in a post-collisional setting; near contemporaneous apatite (~1.8 Ga) tracks cooling through ~500 °C. High Fo contents in olivine (Fo90-92) and high Cr# in chromite (72–77) suggest a low Al2O3 ultramafic parental magma formed by a high degree of partial melting. However, total concentrations of PGEs are abnormally low, and the Re-Os system indicates multiple disturbances during later metamorphic events. Experimental data are used to recover the compositions of parental melts to the chromitite, the calculations yield parental melts with 10–11 wt% Al2O3 and 0.7–1.1 wt% TiO2, which define the arc and IAB affinity. The high Cr#s, low Al2O3 (8–10 wt%), and high TiO2 (0.58–0.96 wt%) contents of chromite, combined with calculated parental melt compositions, suggest that the parental magma of Fangmayu stratiform chromitites was a high-Ti island-arc basalt origin and formed within supra-subduction zone setting.

Interplay of tectonics and magmatism during post‐rift inversion on the central West Iberian Margin (Estremadura Spur)

AbstractThe combined effects of post‐rift magma emplacement and tectonic inversion on the hyper‐extended West Iberian Margin are unravelled in detail using multichannel 2D/3D seismic data. The Estremadura Spur, acting as an uplifted crustal block bounded by two first‐order transfer zones, shows evidence of four post‐rift tectonic events each with a distinctive seismic‐stratigraphic response that can be used to demonstrate the tectono‐magmatic interplay, namely: (a) the Campanian onset of magmatism (including the Fontanelas Volcano, the widespread evidence of multiple sill complexes and the detailed description of a &gt;20 km long laccolith, the Estremadura Spur Intrusion; (b) the Campanian‐Maastrichtian NE‐SW event pervasively affecting the area, resulting in regional uplift, reverse faulting and folding; (c) the Paleocene‐mid Eocene inversion that resulted in widespread erosion and; (d) the Oligocene‐mid Miocene evidence of rejuvenated NW‐SE inversion marked by crestal faulting and forced‐fault folding establishing the final geometry of the area. The distinct deformation styles within each tectonic phase document a case of decoupled deformation between Late Cretaceous and Tertiary units, in response to the predominant stress field evolution, revealing that the magnitude of Late Cretaceous inversion is far more significant than the one affecting the latter units. A detailed analysis of the laccolith and its overburden demonstrate the distinct deformation patterns associated both with magma ascent (including extensional faulting, forced‐folding and concentric reverse faulting) and its interference as a rigid intrusive body during subsequent transpressive inversion. This reinforces the role that the combined tectono‐magmatic events played on the margin. Also analysed is the wider impact of post‐rift magmatism and the associate emplacement of sub‐lithospheric magma on the rheology of a thinned continental crust. This takes into account the simultaneous tectonic inversion of the margin, the implied alternative views on characteristic heat flow, and on how these can be incorporated in source rock organic maturity modelling.

Petrology, geochemistry, Ar[sbnd]Ar isotopes of an arc related calk-alkaline pluton from Mamb (Pan-African Yaounde group, Cameroon): A testimony to the subduction of a hot oceanic crust

In Central Africa, the Pan-African Yaoundé group is made up of migmatites and gneisses exhumed and thrusted on micaschists. In a window in the micaschists, the Mamb metagabbro massif occurs as km-size and ca 20 m-high hill crosscut by a 15 m-thick dyke of meta-hornblendite. Minor facies are biotitites, garnetites, and granitic veins. Metagabbro comprises pargasite, biotite, labrador-andesine, pyrope- andradite- grossular-rich almandine, epidote, FeTi oxide, quartz, scapolite, rutile, zircon, and late metamorphic muscovite, whereas meta-hornblendite is made up of abundant edenite-pargasite, and minor amounts of plagioclase, biotite and epidote. Textural features such as epidote needles in plagioclase and rutile exsolutions in amphibole suggests that metamorphism (P-T conditions: 610 °C and 9.1 Kb) post-dated magma emplacement and crystallisation. Based on 40Ar/39Ar thermochronology of amphibole and micas, the proceedings of metamorphic processes extended from 601 to 586 Ma. Chemically, metagabbros and meta-hornblendites display hybrid adakitic and calk-alkaline characteristics. Mantle-normalised element patterns of metagabbros are marked by negative Nb, Ta, Zr, Hf, Ti, Y and positive Sr and Pb anomalies parallel to metadiorites of Yaoundé. Meta-hornblendite patterns show depletion in Th, K, and Pb similarly to alkaline rocks but high values of Y/Nb (> 3) and Zr/Nb (11–52) ratios point to a subalkalic affinity. Characteristics of these metagabbros and meta-hornblendites point to heterogeneous origins by partial melting of a dehydrated subducted hot oceanic crust and the fusion of a metasomatised mantle wedge. Metadiorites from Yaoundé are arc related rocks. Thus it appears that the island arc setting described at Boumnyebel extends eastwards up to Yaoundé, and probably up to Lomié in the south-eastern Cameroon. The northwards extension of this setting is yet to be established taking into account the presence of Archaean-Paleoproterozoic formations north of the continental scale Sanaga shear zone. At the regional scale, the best model to explain the presence of adakites and classic arc magmas at Mamb, Bape, Bangangté and Fomopea is a south-north subduction of the Pan-African Yaoundé hot oceanic crust under the Archaean Adamawa-Yade block.

Windowing petrogenesis of continental flood basalts through mineralogical investigations: a case study from the Eastern Deccan Volcanic Province

Evaluation of the petrogenetic history of continental flood basalt using mineral–chemical data has been sparingly used for several flood basalt provinces of the world. For the Deccan flood basalts, such attempts are still rudimentary. The present contribution highlights several facets of petrogenesis of the Eastern Deccan Volcanic Province using details of constituent phases including glass. The present-study area around Khandwa (21°49′N, 76°21′E) indicates the presence of three distinct lava flows, which were identified on the basis of physical volcanological features based on a three-tier classification. Moreover, the occasional presence of chilled dykes and feeder dykes has been documented. The lava succession rests above (apparent) cumulate gabbro basement. The lava flows, in general, are characterized by dominant pyroxene (mostly augite, locally pigeonite) and plagioclase, opaque (magnetite and ilmenite) and glass, whereas olivine is rare. Both of these principal mineral phases (namely, plagioclase and pyroxene) occur both as phenocrysts and groundmass, while opaque phases occur only as groundmass. Pyroxene and plagioclase are generally zoned due to the changing milieu of crystallization conditions. Pyroxene thermometry data show the equilibration temperature range of 1050–1300 °C. Except for two samples of Flow II and one sample of chilled zone, the pyroxene thermometry data are broadly similar to that obtained from magnetite–ilmenite thermometry. As expected, deduced glass thermometric data reveal a relatively higher temperature. Crystallization history of parental magma was quantitatively modeled using Petrolog 3 software. The results indicate that the parental magma underwent both equilibrium and fractional crystallization (for example initially crystallized olivine reacted with ambient liquid to give rise to pyroxene). Phenocrystal and groundmass plagioclase and pyroxene show distinct patterns of compositional zoning corresponding to fluctuating physical parameters within the magma. Some of the glass [especially from the chilled dyke, feeder dyke, and Upper Colonnade Zone (UCZ) of flow II and flow III] represents ‘higher magnitude pressure ambience’ (fossilized glass) which is distinct from ‘surficial glass’ caused by equilibrium quenching. The glass and pyroxene compositions (using tectonic discrimination diagrams) suggest a non-orogenic continental setting of the parent magma. It is concluded that the studied Deccan volcanism shows a complex interplay of repeated pulses of magma ascent, fluctuation of crystallization condition, effervescence, and local magma mixing.