Dyke Intrusion Research Articles

Neoproterozoic extension and the Central Iapetus Magmatic Province in southern Mexico – New U-Pb ages, Hf-O isotopes and trace element data of zircon from the Chiapas Massif Complex

Final fragmentation of Rodinia occurred during the Ediacaran Period as Amazonia, Baltica and Laurentia drifted apart to form the Iapetus Ocean. Accompanying rift-related mafic dyke swarms of the Central Iapetus Magmatic Province (CIMP) were emplaced between 0.62 and 0.55 Ga, which are preserved in Laurentia and Baltica, whereas no coeval mafic rocks are known from Amazonia. First evidence for the CIMP extending into Oaxaquia (Rodinia-type basement of Mexico) was reported as tholeiitic dykes that intruded the Novillo gneiss, NE Mexico, at 619 ± 9 Ma. In Chiapas, SE Mexico, amphibolite dykes that are chemically similar to the Novillo dykes intruded anorthosite and gneiss. In this paper, a new dating approach to obtain mafic dyke intrusion ages is presented by targeting contact metamorphic zircon with the UPb method, employing Secondary Ion Mass Spectrometry. Zircon that crystallized in anorthosite at intrusive contacts to mafic dykes and at temperatures exceeding 700 °C (Ti-in-zircon thermometry) yields ages between 615 ± 7 Ma and 608 ± 12 Ma, reflecting the time of dyke intrusion. Zircon chemical and isotopic (Hf, O) characteristics suggest a diachronous sequence of metamorphic processes involving Zr release from FeTi oxides, breakdown and recrystallization of other phases, and fluid-mitigated reactions during Ordovician metamorphism. Zircon δ18O values of granulites from Oaxaquia range from +6.2‰ to +9.8‰, whereas Tonian (~0.92 Ga) metamorphic zircon from SE Chiapas yielded low δ18O values from +2.0‰ to +2.8‰ that are explained by the reactivation of major tectonic boundaries during Tonian gravitational collapse. The observations increase the known extent of the CIMP in Mexico, suggesting that a Neoproterozoic superplume was still active during the Early Ediacaran producing a Large Igneous Province that extended over Amazonia, Baltica and Laurentia. The results further suggest that Oaxaquia at the northern edge of Amazonia formed the conjugate margin of Baltica during rifting.

Geochemical characterization of deep-sea sediments on the Azores Plateau – From diagenesis to hydrothermal activity

The Azores Plateau is an active magmatic region in the Central North Atlantic Ocean. In this study, we present a comprehensive data set of major element compositions and 87Sr/86Sr ratios of pore waters from surface sediments (0–9 mbsf) of the Azores Plateau. Based on distinct geochemical signatures we can separate normal marine from hydrothermally affected sediments. Normal marine sediments can further be differentiated by their ash content. Pore waters of ash rich gravity cores (GCs) do not show any deviations from seawater values except of a minor increase in Sr. In contrast, ash poor GCs generally show a trend for decreasing Ca with increasing depth, accompanied by a minor SO4 decrease and a more pronounced Sr increase. We suggest that these deviations are caused by processes such as anaerobic oxidation of methane and carbonate recrystallization. At four additional sample locations we observed a decrease in Mg and SO4 accompanied by a Ca increase in the pore waters, a pattern typical for hydrothermal fluids. The existence of hydrothermal systems in this region are corroborated by multi-channel seismic data, suggesting that sill or dyke intrusions are present in the subsurface close to the core locations. Overall, our observations offer preliminary indications for the existence of submarine hydrothermal systems on the Azores Plateau away from the Mid- Atlantic Ridge.

Stick-slip vs. stable sliding fault behaviour: A case-study using a multidisciplinary approach in the volcanic region of Mt. Etna (Italy)

In active volcanic zones, fault dynamics is considerably fast but it is often difficult to separate the pattern of nearly continuous large-scale volcanic processes (inflation/deflation processes, flank instability) from impulsive episodes such as dyke intrusions or coseismic fault displacements. At Etna, multidisciplinary studies on active faults whose activity does not strictly depend on volcanic processes, are relatively few. Here we present the case-study of the San Leonardello fault, an active structure located in the eastern flank of Mt. Etna characterised by a well-known seismic history. This fault saw renewed activity in May 2009, when pre-seismic creeping along the southern segment preceded an MW 4.0 earthquake in the northern segment, followed by some twenty-five aftershocks. Later, in March–April 2016, creep events reactivated the southern section of the same fault. Both the seismic and aseismic phenomena were recorded by the seismic and GNSS networks of INGV-Osservatorio Etneo, and produced surface faulting that left a footprint in the pattern of ground deformation detected by the InSAR measurements. We demonstrate that the integration of multidisciplinary data collected for volcano surveillance may shed light on different aspects of fault dynamics, and allow understanding how coseismic slip and creep alternate in space and time along the strike. Moreover, we use findings from our independent datasets to propose a conceptual model of the San Leonardello fault, taking into account behaviour and previous constraints from fault-based seismic hazard analyses. Although the faulting mechanisms described here occur at a very small scale compared with those of a purely tectonic setting, this case-study may represent a perfect natural lab for improving knowledge of seismogenic processes, also in other fault zones characterised by stick slip vs. stable-sliding fault behaviour.

Exhumed hydrocarbon traps in East Greenland: A comment on Andrews et al. (2020)

AbstractIn a recent study, Andrews et al. (2020) describe “exhumed hydrocarbon traps” in North‐East Greenland. The basic premise for their interpretation is that dark‐coloured, pyrobitumen‐bearing sandstones represent the remnants of once buried petroleum reservoirs. We do not see the necessary field or analytical evidence to support a model that has strong implications for resource evaluations. Andrews et al. (2020) have not considered previous published information on diagenetic and thermal maturity history of the area. A more probable model would include the intrusion of dykes and sills into a sedimentary succession with immature petroleum source rocks and reservoir‐quality sandstones. The heating caused rapid generation of petroleum components and local hydrothermal circulation systems in adjacent porous sandstones. Any petroleum was rapidly destroyed leaving essentially only black grain‐coatings and minor particles of pyrobitumen—essentially in one short‐lived continuous process. The existence of new plays in the North Atlantic as proposed by Andrews et al. (2020) is in our opinion not substantiated as this requires analytical data from unaltered oils from the less mature parts of the sedimentary succession and considerations of thermal maturity and basin evolution. To draw conclusions that have a serious impact on resource evaluations based on the dark colouration of sandstones without comprehensive analytical data is, in our opinion, ill advised.

Pb–Pb baddeleyite ages of mafic dyke swarms from the Dharwar Craton: Implications for Paleoproterozoic LIPs and diamond potential of mantle keel

The Dharwar Craton in Peninsular India was intruded by a series of mafic dykes during the Paleoproterozoic and these mafic magmatic events have important implications on continental rifting and LIPs. Here we report ten precise Pb–Pb TE-TIMS age determinations on baddeleyite grains separated from seven mafic dykes and three sills, intruding into Archean basement rocks and Proterozoic sedimentary formations of the Eastern Dharwar Craton respectively. The crystallization age of the baddeleyite shows 2366.3 ​± ​1.1 ​Ma, and 2369.2 ​± ​0.8 ​Ma for the NE–SW trending dykes, 2368.1 ​± ​0.6 ​Ma, 2366.4 ​± ​0.8 ​Ma, 2207.2 ​± ​0.7 ​Ma and 1887.3 ​± ​1.0 ​Ma for the ENE–WNW to E–W striking dykes, 1880.6 ​± ​1.0 ​Ma, 1864.3 ​± ​0.6 ​Ma and 1863.6 ​± ​0.9 ​Ma for Cuddapah sills, and 1861.8 ​± ​1.4 ​Ma for the N–S trending dyke. Our results in conjunction with those from previous studies identify eight distinct stages of widespread Paleoproterozoic magmatism in the Dharwar craton. The mantle plume centres of the four radiating dyke swarms with ages of ~2367 ​Ma, ~2210 ​Ma, ~2082 ​Ma, and ~1886 ​Ma were traced to establish their proximity to the EDC kimberlite province. Though the ~2367 ​Ma and ~1886 ​Ma plume centres are inferred to be located to the west and east of the present day Dharwar craton respectively away from the kimberlite province, location of plume heads of the other two swarms with ages of ~2207 ​Ma and ~2082 ​Ma are in close proximity. In spite of the ubiquitous occurrence of dyke intrusions of all the seven generations in the kimberlite province, only few of these kimberlites are diamondiferous. Kimberlite occurrences elsewhere in the vicinity of older Large Igneous Provinces (LIPs) like the Mackenzie, Karoo, Parana-Etendeka and Yakutsk-Vilui are also non-diamondiferous. This has been attributed to the destruction of the lithospheric mantle keel (that hosts the diamonds) by the respective mantle plumes. The diamondiferous nature of the EDC kimberlites therefore suggests that plume activity does not always result in the destruction of the mantle keel.

Extreme rainfall triggered the 2018 rift eruption at Kīlauea Volcano.

The May 2018 rift intrusion and eruption of Kīlauea Volcano, Hawai'i, represented one of its most extraordinary eruptive sequences in at least 200 years, yet the trigger mechanism remains elusive1. The event was preceded by several months of anomalously high precipitation. It has been proposed that rainfall can modulate shallow volcanic activity2,3, but it remains unknown whether it can have impacts at the greater depths associated with magma transport. Here we show that immediately before and during the eruption, infiltration of rainfall into Kīlauea Volcano's subsurface increased pore pressure at depths of 1to3 kilometres by 0.1to1 kilopascals, to its highest pressure in almost 50 years. We propose that weakening and mechanical failure of the edifice was driven by changes in pore pressure within the rift zone, prompting opportunistic dyke intrusion and ultimately facilitating the eruption. A precipitation-induced eruption trigger is consistent with the lack of precursory summit inflation, showing that this intrusion-unlike others-was not caused by the forceful intrusion of new magma into the rift zone. Moreover, statistical analysis of historic eruption occurrence suggests that rainfall patterns contribute substantially to the timing and frequency of Kīlauea's eruptions and intrusions. Thus, volcanic activity can be modulated by extreme rainfall triggering edifice rock failure-a factor that should be considered when assessing volcanic hazards. Notably, the increasingly extreme weather patterns associated with ongoing anthropogenic climate change could increase the potential for rainfall-triggered volcanic phenomena worldwide.

Triggering relationships between magmatic and faulting processes in the May 2018 eruptive sequence at Kīlauea volcano, Hawaii

SUMMARYThe May 2018 activity at Kīlauea Volcano, Hawaii involved magma transport and dyke intrusion along the East Rift Zone (ERZ) and nucleation of the 4 May 2018 M 6.9 earthquake along the basal décollement of Kīlauea's mobile south flank. Combined Global Positioning Systems (GPS) and Interferometric Synthetic Aperture Radar (InSAR) measurements captured the deformation sequence associated with the dyke intrusion, main shock and eruption episode along the ERZ. The earthquake was encouraged by static stress changes from the preceding magma reservoir inflation, ERZ expansion and fault creep on the décollement downdip of the rupture. Slip models derived from the inversion of GPS displacements indicate peak coseismic slip of 2–3 m. Our model analyses, including of the pre-May 2018 deformation, suggest that prior to this event there was no slip on the section of the décollement that ruptured in the earthquake. The observed magma inflation, rapid fault creep on the décollement and coseismic rupture reflect complex cyclic interactions between the magmatic and faulting processes.

Dyke intrusion and stress-induced collapse of volcano flanks: The example of the 2018 event at Mt. Etna (Sicily, Italy)

Magmatic intrusions, eruptions and flank collapses are frequent processes of volcano dynamics, inter-connected at different space and time scales. The December 2018 recrudescent episode at Mt. Etna is an exemplary case where a sudden intrusive event culminated with a short eruption, intense seismicity and a shallow large strike-slip earthquake at the edge of the eastern sliding flank. Here, we show that high resolution velocity models and transient changes of VP and VP/VS resolve the magma intrusion through a dyke and local stress increase at the base of the unstable flank, inducing the collapse. Episodic brittle faulting occurs at the edge of the sliding sector, locally contributed by high fluid pressure. The feedback between magma ascent, stress changes and flank collapse is driving the volcano dynamics, with processes ranging from long term to transient episodes.

Evolution of the uranium mineralisation in the Zoujiashan deposit, Xiangshan ore field: Implications for the genesis of volcanic-related hydrothermal U deposits in South China

Volcanic-related hydrothermal U deposits from the Xiangshan ore field are situated within the Gan-Hang Metallogenic Belt in South China. These deposits are dominantly hosted in late Yanshanian felsic volcanic rocks that were emplaced during the Yanashanian post-orogenic extension. The host volcanic rocks are moderately fractionated high-K calc-alkaline A2-type volcanic rocks and are one of the possible U sources for U ore genesis in the district. Porphyroclastic rhyolite is the main host facies and contains an average of 12 ppm U, which can be released through volcanic glass devitrification and mobilised by oxidised fluids for the formation of hydrothermal mineralisation. However, REE patterns of the U mineralisation suggest multiple U sources with U mobilised from additional felsic basement rocks subjacent to the Xiangshan caldera and/or from magmatic fluids related to a late peralkaline intrusion.At the Zoujiashan deposit in the Xiangshan ore field, structurally controlled albitised zones with brannerite-dominated U mineralisation are constrained by a maximum age of 116 ± 1 Ma on hydrothermal zircon and a minimum crystallisation age of 109 ± 1 Ma on pitchblende. This constitutes the early stage of mineralisation that occurred during the early Cretaceous (ca. 120–105 Ma). The Th-Mo-P-REE-Y-Ti-Zr whole-rock geochemical signature and disseminated Th-brannerite associated with a complex mineral assemblage including hydrothermal rutile, apatite, zircon, uranothorite, sphalerite, pyrite, xenotime and REE-carbonate indicated temperatures >250 °C for the hydrothermal system. The Th-REE-Y-Nb-Zr signature of the U mineralisation concomittent with the intrusion of alkaline mafic dykes suggest that a peralkaline intrusion located at depth, postdating the Xiangshan volcanic-intrusive complex by at least 10 Ma, provided the heat source and U-Th-REE-Y-Nb-Zr-rich fluids to the hydrothermal system for the formation of the early U mineralisation. In contrast, the structurally controlled disseminated- to vein-type pitchblende-dominated U mineralisation that yielded a minimum crystallisation age of 81 ± 4 Ma represents the main stage of mineralisation (ca. 100–80 Ma) and formed during the late Cretaceous crustal extension. The mineralisation occurred at low- to moderate-temperatures (150–250 °C) as Ti-Th-Zr-rich pitchblende associated with extensive illite and fluorite alteration. Here, pitchblende that overprinted the early mineralisation stage most likely inherited the primary chemical signature of this early mineralisation, especially the high Th contents. During this episode of crustal extension, uranium was leached from the host volcanic rocks and subjacent felsic basement rocks by surface-derived oxidised fluids and transported through basement faults as carbonate and/or fluoride complexes in solution. These U-bearing solutions mixed with deep-seated magmatic-derived fluids, and the resulting destabilisation of metal complexes in solution together with pressure decrease in opening fractures promoted U deposition. Therefore, the volcanic-related hydrothermal U mineralisation in the volcanic belt formed during two major stages related to different genetic conditions.

Timing of carbonatite-hosted U-polymetallic mineralization in the supergiant Huayangchuan deposit, Qinling Orogen: Constraints from titanite U–Pb and molybdenite Re–Os dating

The newly-discovered supergiant Huayangchuan uranium (U)-polymetallic (Sr, Se, REEs, Ba, Nb and Pb) deposit is located in the Qinling Orogen, central China. The deposit underwent multistage mineralization, with the main carbonatite ore stage being the most important for the U, Nb, REE, Sr and Ba endowments. According to the mineral assemblages, the main carbonatite ore stage can be divided into three substages, i.e., sulfate (Ba–Sr), alkali-rich U and REE-U mineralization.Main-stage titanite from the Huayangchuan igneous carbonatite are rich in high field strength elements (HFSEs, e.g., Zr, Nb and REEs), and show clear elemental substitutions (e.g., Ti vs. Nb ​+ ​Fe ​+ ​Al and Ca ​+ ​Ti vs. Fe ​+ ​Al ​+ ​REE). High-precision LA-ICP-MS titanite dating yielded a U–Pb age of 209.0 ​± ​2.9 ​Ma, which represents the main-stage mineralization age at Huayangchuan, and is coeval with the local carbonatite dyke intrusion. This mineralization age is further constrained by the Re–Os dating of molybdenite from the Huayangchuan carbonatite, which yielded a weighted mean age of 196.8 ​± ​2.4 ​Ma. Molybdenite Re contents (337.55–392.75 ​ppm) and C-O-Sr-Nd-Pb isotopic evidence of the Huayangchuan carbonatite both suggest a mantle origin for the carbonatite. Our study supports that the Late Triassic carbonatite magmatism was responsible for the world-class U-Mo-REE mineralization in the Qinling Orogen, and that the regional magmatism and ore formation was likely caused by the closure of the Mianlue ocean and the subsequent North China-South China continent-continent collision.

Diverse subduction and exhumation of tectono-metamorphic slices in the Kalatashitage area, western Paleozoic Dunhuang Orogenic Belt, northwestern China

High-pressure mafic granulite and garnet amphibolite are identified as small-scale tectonic slices within pelitic or semi-pelitic gneiss in the Kalatashitage area, which is located in the western Paleozoic Dunhuang Orogenic Belt, northwestern China. These rocks retain three generations of metamorphic mineral assemblages: prograde assemblage (M1) preserved as inclusions within garnet porphyroblasts, metamorphic peak assemblage (M2) consisting of matrix minerals and garnet porphyroblasts, and retrograde assemblage (M3) mainly represented by the symplectic minerals surrounding the embayed garnet and the retrograded hornblende rimming matrix-type clinopyroxene. Metamorphic pressure and temperature (P-T) paths of high-pressure mafic granulite, amphibolite, and metapelite retrieved by thermobarometry are all clockwise, passing from 640 to 720 °C/6.2–12.6 kbar (M1) through 840–920 °C/14.6–16.2 kbar (M2) to 750–815 °C/5.5–7.9 kbar (M3) for high-pressure mafic granulite, from ~650 °C/5.7 kbar (M1) through ~750 °C/9.2 kbar (M2) to ~780 °C/8.1 kbar (M3) for amphibolite, and from ~615 °C/7.9 kbar (M1) through 730–820 °C/8.6–11.7 kbar (M2) to 675–740 °C/5.4–8.7 kbar (M3) for pelitic and semi-pelitic gneiss. Furthermore, pseudosection modeling of high-pressure mafic granulite indicates that the growth zonation of garnet porphyroblast exhibits prograde metamorphism in a P-T range of 510–800 °C/8.5–13 kbar and demonstrates peak metamorphic P-T conditions of ~850 °C/16 kbar, which are consistent with the thermobarometric estimates. The significant pressure differences in peak metamorphism observed in different rocks indicate that the rocks initially subducted to remarkably different depths and were subsequently juxtaposed at shallower crustal levels during exhumation. Sensitive high-resolution ion microprobe (SHRIMP) analysis and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) UPb dating of metamorphic zircon indicates that the metamorphic events occurred at ca. 430–420 Ma (M2) and ca. 400–390 Ma (M3), respectively. Metamorphism was followed by the intrusion of granitic dykes at ca. 244 Ma. Moreover, the metamorphic evolution indicates that the Kalatashitage area was involved in the subduction, collision and subsequent tectonic exhumation in the Paleozoic. Combined with previous literature, it is inferred that the discrepant subduction and exhumation of high-grade metamorphic rocks is a universal phenomenon in the Paleozoic Dunhuang Orogenic Belt, supporting the ubiquitous existence of subduction-collision complexes in this orogenic belt.

Hafnium isotopic record of mantle-crust interaction in an evolving continental magmatic system

Tracing the origin and evolution of magmas on their pathway through the lithosphere is key to understanding the magmatic processes that eventually produce eruptions. For ancient magmatic provinces, isotope-geochemical tracers are powerful tools to probe the source regions and magma-crust interaction during ascent and storage. Here we present hafnium isotopic compositions of ID-TIMS dated zircons to trace the evolution of the Middle Triassic magmatic province in the Southern Alps (northern Italy) at high temporal resolution. Systematic changes in hafnium isotopic composition with time record progressively stronger crustal assimilation over 3.5 million years, followed by a rapid increase in εHf towards more juvenile compositions for the major pulse of mafic intrusions and post-intrusive volcanic ash beds within one million years. We interpret these trends to reflect variations in mantle-crust interaction through time. Initial intrusions of basaltic dykes into the relatively cold lower crust cause only limited crustal melting and assimilation but an ensuing magma injection into progressively hotter crust results in more extensive partial melting and assimilation of crustal material. Subsequent intrusions into the magmatic lower-crustal roots cannibalize previous intrusions with progressively less isotopic contrast due to dilution with mantle-derived magmas. This is potentially accompanied by an increase in magma flux due to delamination of dense lower crustal cumulates into the subcontinental lithospheric mantle. The observed trends in hafnium isotopic composition therefore do not necessarily require tectonic re-organizations or changes in mantle sources. Instead these trends may trace variations in mantle-crust interaction during thermally induced chemical maturation of the lower crustal magmatic roots progressively replacing ancient pelitic to mafic lower crustal lithologies by juvenile cumulates.

Origin and Evolution of the Halo-Volcanic Complex of Dallol: Proto-Volcanism in Northern Afar (Ethiopia)

Contextual early observations on volcano genesis are valuable but scarce. Resembling a shield volcano, the Dallol dome is a complex 40 m-high geological structure on the Danakil depression, a North-South-elongated salt plain lying 120 m below sea level in the North Afar (Ethiopia). Dallol has become a tourist destination famous for its colorful hydrothermal features and raised scientific interest due to its life-challenging polyextreme conditions. Although some general models for its genesis exist, little is known about the origin and temporal evolution of both, the dome and its geothermal activity resulting in hyperacidic and halite-oversaturated brines. In this study, we combine published information with data obtained from our three multidisciplinary field campaigns (January 2016, 2017, and 2019) to refine the geological mapping of the North Danakil and the Dallol dome. The analysis of stratigraphic, geomorphological, geochemical, and hydrogeochemical data as well as satellite, drone and infrared aerial images allows us to shed light in its complex temporal evolution. Our results suggest that the recorded history of the dome began when at least one deep magmatic basalt intrusion occurred later than 6000 years ago, forcing the uplifting of the lacustrine deposits of that age covering the west side of the dome. The interaction of the magma with the buried salt deposit resulted in a halo-volcanic activity with, likely, several melted-salt effusion events. Substrate accommodation after effusion led to the current collapsing crater on the dome top and the geothermal still-ongoing degassing. An important hydrothermal reactivation took place after a dyke intrusion event in October–November 2004. It triggered the appearance of new fractures on the dome top and the northward migration of the hydrothermal activity, as we inferred from the analysis of historical aerial images combined with high-definition visible and infrared images taken from a drone during our field campaigns. Based on our observations, we present an updated hydrogeothermal conceptual model linking deep magmatic activity with halokinetic processes and geothermal fluids to explain the origin and evolution of the Dallol halo-volcanic complex. These geothermal manifestations may potentially inform about rarely documented premises of a volcano’s birth

Flow of partially molten crust controlling construction, growth and collapse of the Variscan orogenic belt: the geologic record of the French Massif Central

We present here a tectonic-geodynamic model for the generation and flow of partially molten rocks and for magmatism during the Variscan orogenic evolution from the Silurian to the late Carboniferous based on a synthesis of geological data from the French Massif Central. Eclogite facies metamorphism of mafic and ultramafic rocks records the subduction of the Gondwana hyperextended margin. Part of these eclogites are forming boudins-enclaves in felsic HP granulite facies migmatites partly retrogressed into amphibolite facies attesting for continental subduction followed by thermal relaxation and decompression. We propose that HP partial melting has triggered mechanical decoupling of the partially molten continental rocks from the subducting slab. This would have allowed buoyancy-driven exhumation and entrainment of pieces of oceanic lithosphere and subcontinental mantle. Geochronological data of the eclogite-bearing HP migmatites points to diachronous emplacement of distinct nappes from middle to late Devonian. These nappes were thrusted onto metapelites and orthogneisses affected by MP/MT greenschist to amphibolite facies metamorphism reaching partial melting attributed to the late Devonian to early Carboniferous thickening of the crust. The emplacement of laccoliths rooted into strike-slip transcurrent shear zones capped by low-angle detachments from c. 345 to c. 310 Ma is concomitant with the southward propagation of the Variscan deformation front marked by deposition of clastic sediments in foreland basins. We attribute these features to horizontal growth of the Variscan belt and formation of an orogenic plateau by gravity-driven lateral flow of the partially molten orogenic root. The diversity of the magmatic rocks points to various crustal sources with modest, but systematic mantle-derived input. In the eastern French Massif Central, the southward decrease in age of the mantle- and crustal-derived plutonic rocks from c. 345 Ma to c. 310 Ma suggests southward retreat of a northward subducting slab toward the Paleotethys free boundary. Late Carboniferous destruction of the Variscan belt is dominantly achieved by gravitational collapse accommodated by the activation of low-angle detachments and the exhumation-crystallization of the partially molten orogenic root forming crustal-scale LP migmatite domes from c. 305 Ma to c. 295 Ma, coeval with orogen-parallel flow in the external zone. Laccoliths emplaced along low-angle detachments and intrusive dykes with sharp contacts correspond to the segregation of the last melt fraction leaving behind a thick accumulation of refractory LP felsic and mafic granulites in the lower crust. This model points to the primordial role of partial melting and magmatism in the tectonic-geodynamic evolution of the Variscan orogenic belt. In particular, partial melting and magma transfer (i) triggers mechanical decoupling of subducted units from the downgoing slab and their syn-orogenic exhumation; (ii) the development of an orogenic plateau by lateral flow of the low-viscosity partially molten crust; and, (iii) the formation of metamorphic core complexes and domes that accommodate post-orogenic exhumation during gravitational collapse. All these processes contribute to differentiation and stabilisation of the orogenic crust.

Inverse Modeling Using Taylor Expansion Approach and Jacobi Matrix on Magnetic Data (Dyke/Magma Intrusion Cases)

The mathematical modelling of geological structures, i.e. magma intrusion or dyke, has been done, based on magnetic data with inversion techniques using MatLab. The magnetic equation is a non-linear equation, and completion is done using a linear approach to non-linear mathematical models of magnetic data using the Taylor expansion approach and Jacobi Matrix. The first step of this research is to make synthetic data forward modelling from the magnetic equation of magma intrusion or dyke cases without errors, and the next stepping then add errors to the data. The next step is to do an inversion to get the parameters sought, i.e. depth and angle of the magma intrusion, by giving initial guesses, and then re-correct iteratively until convergent results are obtained. Finally, parameters of slope dyke or thin magma intrusion and its depth can be determined. The results obtained indicate that this technique can be used to get physical parameters sought from magnetic data for simple geological cases, i.e. dyke and magma intrusion.

A new perspective of the 2009 Al-Ays earthquake episode at western Arabia

The 2009 earthquake swarm that occurred in Al-Ays area, Saudi Arabia was believed to be of magmatic origin and was caused by magma rising underneath the area and installment of a subsurface dyke that reached up to 3.5 km below the ground surface. This study uses the results of an integrated geophysical survey conducted across the major surface rupture, which appeared in the area after the earthquake swarm, along with other evidence, to suggest a different scenario. Contradicting focal mechanisms, temporal and spatial distribution of earthquakes, mixed frequency earthquakes, a previous earthquake swarm, recent similar swarms along the Red Sea coast, and the alignment of earthquakes along a major northwest-southeast trending fault are among the evidence presented to refute the one-decade old subsurface dyke intrusion model. Last but most important, the geophysical data we acquired across the surface rupture in the area show clear evidence of shallow negative flower structure in the ruptured zone, implicating the occurrence of a transtensional tectonic movement, that may have caused the rupture and triggered the associated earthquake swarm. The evidence presented in this article prove that the 2009 earthquakes episode in the Al-Ays area was triggered by regional tectonics readjustment in the area, rather than magma upwelling and subsurface dyke intrusion, that if present, could be a secondary effect of the tectonic movement.

Field model experiments to determine mechanisms of rainstorm-induced shallow landslides in the Feiyunjiang River basin, China

This paper investigates rainfall infiltration processes and the failure mechanisms of slopes in the Feiyunjiang River basin. Field model experiments were conducted on a selected residual soil slope next to the Fuao landslide, which was about 30 m long and 25 m wide, and failed along the interface between soil and bedrock during typhoon Megi. The results indicated that rainfall infiltration over the slope surface had little impact on pore water pressure and deformation of the slope, while the coupled action of slope surface infiltration and preferential infiltration through an artificial ditch at the back of the slope caused rapid saturation near the interface between the residual soil and bedrock, resulting in significant displacement along the interface. Other preferential infiltration channels on slopes in the Feiyunjiang River basin include tension cracks, water impoundment and intrusive dykes and their weathered layers. The conclusions derived from this study are helpful in understanding rainstorm-induced soil landslides in the Feiyunjiang River basin in China, and possibly in other similar slope settings under comparable climate conditions.

Low-temperature thermochronology of the Carlin-type gold deposits in southwestern Guizhou, China: Implications for mineralization age and geological thermal events

The Dian-Qian-Gui area in southwestern China is the second-largest concentration of the Carlin-type gold deposits in the world, and the southwestern Guizhou is the most important gold-producing district of the Dian-Qian-Gui area. Dating of the Carlin-type gold deposits is always a challenge due to the lack of ore-stage minerals which are suitable for conventional radiometric dating. Low-temperature thermochronometers, including fission track and (U-Th)/He, are temperature-sensitive radiometric dating techniques and have been successfully applied to elucidate many aspects of ore deposits, including the timing and duration of mineralization processes.In this study, zircon fission track, zircon (U-Th)/He and apatite fission track dating techniques were applied to the Carlin-type gold deposits in southwestern Guizhou. Samples for zircon and apatite analyses were collected from mineralized or altered sedimentary rocks in Shuiyindong, Taipingdong, Bojitian, Yata and Getang gold deposits. Four groups of thermochronological ages were obtained, i.e., 192–216 Ma, 132–160 Ma, 88 Ma, and 19–40 Ma, which are considered to be the records of four geological thermal events. Of them, most zircon fission track ages and all zircon (U-Th)/He ages concentrate in 132–160 Ma. Thermochronological constraints, combined with field relationships, suggest that the mineralization of the Carlin-type gold deposits in southwestern Guizhou most likely occurred in this period. Partial zircon fission track ages are in 192–216 Ma. They may record the cooling timing of a pre-ore thermal event caused by the Indosinian orogeny, which is supported by the paleogeotemperature restoration. A few zircon grains with high U content yield fission track age of 88 Ma, which may record a post-ore thermal event associated with the intrusion of local mafic dykes. All apatite fission track ages range in 19–40 Ma, which may record a lower temperature but extensive hydrothermal superimposition in the Cenozoic. Additionally, a corresponding relationship between zircon fission track ages and stratigraphic sequences was identified in the Huijiabao anticline, which is considered to reflect the decrease of temperature and duration of ore-forming hydrothermal activity from lower to upper stratigraphic sequences.

Permian felsic magmatism in the Neoproterozoic Nagar Parkar Igneous Complex of the Malani Igneous Suite: Evidence from zircon U–Pb age

AbstractWe report Permian (ca. 272 Ma ±5.4 Ma) felsic dykes that intrude into the Neoproterozoic (ca. 750 Ma) magmatic suite of the Nagar Parkar Igneous Complex (NPIC), the western extension of the Malani Igneous Suite (MIS). The NPIC consists of Neoproterozoic basement amphibolites and granites (riebeckite–aegirine gray granites and the biotite–hornblende pink granites), all of which are intruded by several generations of mafic and felsic dykes. Granitic magmatism occurred in the Late Neoproterozoic (ca. 750 Ma) due to the subduction‐, followed by the rift‐related tectonic regime during the breakup of the Rodinia supercontinent. U–Th–Pb zircon and monazite CHIME age data of 700–800 Ma from the earlier generation porphyritic felsic dykes suggest the dyke intrusion was coeval or soon after the emplacement of the host granites. Our findings of Permian age orthophyric felsic dykes provide new insights for the prevalence of active tectonics in the MIS during late Paleozoic. Textural features and geochemistry also make the orthophyric dykes distinct from the early‐formed porphyritic dykes and the host granites. Our newly obtained age data combined with geochemistry, suggest the existence of magmatism along the western margin of India (peri‐Gondwana margin) during Permian. Like elsewhere in the region, the Permian magmatism in the NPIC could be associated with the rifting of the Cimmerian micro‐continents from the Gondwana.

Parental Melts of Neoarchean Subalkaline Magmatic Rocks of the Keivy Structure (Kola Peninsula)

The Neoarchean subalkaline magmatism of the Keivy structure is manifested by the emplacement of the latite–monzonite–granite (LMG) volcano-plutonic association. The measured 143Nd/144Nd ratio in rocks is 0.511061–0.511365; the values of model ages fall within the interval of 3.14–3.21 Ga with eNd(2670) varying from 2.2 to –1.3. The initial 176Hf/177Hf ratios in zircon are within the range of 0.280950–0.281069, and composition data points make up a field in the region of the evolutionary trend of the chondritic uniform reservoir. As based on isotope and geochemical data, the protoliths of all rocks are assumed to be basic. In terms of the eHf(T) value, the data points of the zircon composition fall within the domain of the crust with 176Lu/177Hf = 0.015 and an age of 3.0–3.3 Ga. This suggests that the parental melts were probably formed predominantly from material that spent a long time in the crust. The parental melts of the LMG association presumably formed as a result of the melting of metasomatically altered basic rocks during the intrusion into the lower crust of basaltic plutons, which were parental for the rocks of the dyke suite and gabbrolabradorite intrusions in the upper crust.