Dike Emplacement Research Articles

Host-rock deformation during the emplacement of the Mourne Mountains granite pluton: Insights from the regional fracture pattern

Abstract The Mourne Mountains magmatic center in Northern Ireland consists of five successively intruded granites emplaced in the upper crust. The Mourne granite pluton has classically been viewed as a type locality of a magma body emplaced by cauldron subsidence. Cauldron subsidence makes space for magma through the emplacement of ring dikes and floor subsidence. However, the Mourne granites were more recently re-interpreted as laccoliths and bysmaliths. Laccolith intrusions form by inflation and dome their host rock. Here we perform a detailed study of the deformation in the host rock to the Mourne granite pluton in order to test its emplacement mechanism. We use the host-rock fracture pattern as a passive marker and microstructures in the contact-metamorphic aureole to constrain large-scale magma emplacement-related deformation. The dip and azimuth of the fractures are very consistent on the roof of the intrusion and can be separated into four steeply inclined sets dominantly striking SE, S, NE, and E, which rules out pluton-wide doming. In contrast, fracture orientations in the northeastern wall to the granites suggest shear parallel to the contact. Additionally, contact-metamorphic segregations along the northeastern contact are brecciated. Based on the host-rock fracture pattern, the contact aureole deformation, and the north-eastward–inclined granite-granite contacts, we propose that mechanisms involving either asymmetric “trap-door” floor subsidence or laccolith and bysmalith intrusion along an inclined or curved floor accommodated the emplacement of the granites and led to deflection of the northeastern wall of the intrusion.

Magmatic intrusion-related processes in the upper lunar crust: The role of country rock porosity/permeability in magmatic percolation and thermal annealing, and implications for gravity signatures

Shallow crustal country rock on the Moon is demonstrably more fractured and porous than deeper crustal bedrock, and Gravity Recovery and Interior Laboratory (GRAIL) mission gravity data have shown that deeper crustal bedrock is more porous than previously thought. This raises the question of how crustal porosity and permeability will influence the nature of magmatic dike intrusions in terms of: 1) the ability of intruding magma to inject into and occupy this pore space (shallow magmatic percolation), 2) the influence of the intruded magma on annealing of this porosity and permeability (thermal annealing) both 1 and 2 densify the country rock), and 3) the effect of crustal porosity on favoring sill formation as a function of depth in the lunar crust. We analyze quantitatively the emplacement of basaltic dikes and sills on the Moon and assess these three factors in the context of the most recent data on micro- and macro-scale porosity of lunar crustal materials. For the range of plausible micro/macro-scale porosity and permeability determined by crack widths (mm to cm) and open crack lateral continuity (mm to tens of cm), we find that 1) rapid conductive cooling of injected magma due to the very large surface area to volume ratio restricts magmatic percolation to very limited zones (extending for at most several tens of cm) adjacent to the ascending dike or intruded sill, even in the upper several hundred meters of the lunar crust; 2) the conductive heat loss from intruded dikes and sills results in a thermal wave decay rate that is predicted to limit the extent of intrusion-adjacent thermal annealing to less than ~6% of the thickness of the intruded body; 3) the extremely rapid rise rate of magma in dikes originating from sources in the lunar mantle disfavors the lateral migration of dikes to form sills in the crust, except in specific shallow crustal locations influenced by impact crater-related environments (e.g., floor-fractured craters). We conclude that, although magmatic percolation and thermal annealing in association with lunar mare basalt magmatic dike and sill emplacement should be taken into consideration in interpreting gravity signatures, the effects are likely to be minor compared with the density contrast of the solidified basaltic magmatic intrusion itself.

Magmato-tectonic links: Ignimbrite calderas, regional dike swarms, and the transition from arc to rift in the Southern Rocky Mountains

AbstractRadial and linear dike swarms in the eroded roots of volcanoes and along rift zones are sensitive structural indicators of conduit and eruption geometry that can record regional paleostress orientations. Compositionally diverse dikes and larger intrusions that radiate westward from the polycyclic Platoro caldera complex in the Southern Rocky Mountain volcanic field (southwestern United States) merge in structural trend, composition, and age with the enormous but little-studied Dulce swarm of trachybasaltic dikes that continue southwest and south for ∼125 km along the eastern margin of the Colorado Plateau from southern Colorado into northern New Mexico. Some Dulce dikes, though only 1–2 m thick, are traceable for 20 km. More than 200 dikes of the Platoro-Dulce swarm are depicted on regional maps, but only a few compositions and ages have been published previously, and relations to Platoro caldera have not been evaluated. Despite complications from deuteric alteration, bulk compositions of Platoro-Dulce dikes (105 new X-ray fluorescence and inductively coupled plasma mass spectrometry analyses) become more mafic and alkalic with distance from the caldera. Fifty-eight (58) new 40Ar/39Ar ages provide insight into the timing of dike emplacement in relation to evolution of Platoro caldera (source of six regional ignimbrites between 30.3 and 28.8 Ma). The majority of Dulce dikes were emplaced during a brief period (26.5–25.0 Ma) of postcaldera magmatism. Some northeast-trending dikes yield ages as old as 27.5 Ma, and the northernmost north-trending dikes have younger ages (20.1–18.6 Ma). In contrast to high-K lamprophyres farther west on the Colorado Plateau, the Dulce dikes are trachybasalts that contain only anhydrous phenocrysts (clinopyroxene, olivine). Dikes radial to Platoro caldera range from pyroxene- and hornblende-bearing andesite to sanidine dacite, mostly more silicic than trachybasalts of the Dulce swarm. Some distal andesite dikes have ages (31.2–30.4 Ma) similar to those of late precaldera lavas; ages of other proximal dikes (29.2–27.5 Ma) are akin to those of caldera-filling lavas and the oldest Dulce dikes. The largest radial dikes are dacites that have yet younger sanidine 40Ar/39Ar ages (26.5–26.4 Ma), similar to those of the main Dulce swarm.The older andesitic dikes and precaldera lavas record the inception of a long-lived upper-crustal magmatic locus at Platoro. This system peaked in magmatic output during ignimbrite eruptions but remained intermittently active for at least an additional 9 m.y. Platoro magmatism began to decline at ca. 26 Ma, concurrent with initial basaltic volcanism and regional extension along the Rio Grande rift, but no basalt is known to have erupted proximal to Platoro caldera prior to ca. 20 Ma, just as silicic activity terminated at this magmatic locus. The large numbers and lengths of the radial andesitic-dacitic dikes, in comparison to the absence of similar features at other calderas of the San Juan volcanic locus, may reflect location of the Platoro system peripheral to the main upper-crustal San Juan batholith recorded by gravity data, as well as its proximity to the axis of early rifting. Spatial, temporal, and genetic links between Platoro radial dikes and the linear Dulce swarm suggest that they represent an interconnected regional-scale magmatic suite related to prolonged assembly and solidification of an arc-related subcaldera batholith concurrently with a transition to regional extension. Emplacement of such widespread dikes during the late evolution of a subcaldera batholith could generate earthquakes and trigger dispersed small eruptions. Such events would constitute little-appreciated magmato-tectonic hazards near dormant calderas such as Valles, Long Valley, or Yellowstone (western USA).

Field and spaceborne imagery data for evaluation of the paleo-stress regime during formation of the Jurassic dike swarms in the Kalateh Alaeddin Mountain area, Shahrood, north Iran

Dike swarms are commonly linked with extensional structures in diverse geodynamic environments. Mafic dyke swarms are typically used to reconstruct the paleo-stress fields of a given region. These dikes are considered paleo-stress indicators and excellent time marker (if related geochronological data are available) of the local and regional stress fields. In the Middle-Late Jurassic, swarms of mafic dikes emplaced into the Neoproterozoic schists and amphibolites in the Kalateh Alaeddin Mountain area in south Shahrood, north Iran. These dikes with different thicknesses show a general east–west strike direction, with mostly a steep dip angle. In this paper, we present structural data of these dike swarms for the sake of assessing the paleo-stress state and the magma pressure ratio at the time of their emplacement. Field and structural data are integrated with ASTER Global Digital Elevation Model (GDEM) and Centre National d’Etudes Spatiales (CNES)/SPOT imagery data, to extract important parameters of the investigated dikes and controlling fault/joint sets. Orientation of the principal paleo-stress axes, quantification of the stress ratio, and the associated magma pressure ratio (driving stress ratio) were calculated using the stereographic projection and Mohr’s circle reconstruction techniques. The results reveal that the maximum paleo-stress component (σ1) was sub-vertical and the intermediate (σ2) and minimum (σ3) paleo-stresses components were sub-horizontal in N264° E and N173° E trends, respectively. Due to the low value of the driving stress ratio (R = 0.05), these dikes developed perpendicular to the minimum principal stress (in E–W direction). The stress ratio value (o = 0.66) indicates a moderately oblate stress ellipsoid. The orientation of the principal paleo-stress axes and the oblate ellipsoid are indicative of the dike emplacement during a N–S-directed tectonic extension, in agreement with the Jurassic subsidence phase and N–S stretching described for the Kalateh Alaeddin Mountain area.

Hydrothermal Alteration, Mineralization, and Structural Geology of the Zijinshan High-Sulfidation Au-Cu Deposit, Fujian Province, Southeast China

Zijinshan is a world-class Au-Cu district located in southwest Fujian Province, southeast China. It contains a diverse array of ore deposits hosted by the Zijinshan granite complex and surrounding volcano-sedimentary rocks. Associated deposits include high-sulfidation epithermal gold-copper, intermediate-sulfidation epithermal polymetallic silver-base metal, and porphyry molybdenum-copper deposits. The Zijinshan high-sulfidation Au-Cu deposit is located in the middle of the Zijinshan district, and ore zones are hosted in, below, and adjacent to the Zijinshan lithocap. The host rocks are part of a Jurassic granite complex emplaced between 165 and 157 Ma. 40Ar/39Ar age spectra of hydrothermal alunite indicate that hydrothermal activity at Zijinshan occurred at 102.86 ± 0.61 to 101.19 ± 0.60 Ma and was associated with dacite porphyry dikes that intruded at 104.8 ± 0.9 Ma. Hydrothermal activity produced pervasive silicic and advanced argillic alteration assemblages that are zoned from a central, massive quartz alteration domain, outward to massive quartz-dickite, disseminated quartz-alunite ± dickite, and disseminated quartz-muscovite-dickite zones. Copper sulfides occur in hydrothermally cemented breccias, veins, and minor disseminations in granite; copper minerals include covellite, digenite, and minor enargite. Oxide Au ores are associated with strongly weathered silicic alteration in the upper parts of the deposit. A set of subvertical NE-striking faults were active prior to mineralization at Zijinshan. During mineralization, a NW-striking fault system controlled the emplacement of veins, breccias, and dacite dikes. Kinematic indicators show that most of the NW-striking faults were active as normal oblique faults during mineralization. Postmineralization ENE-striking dextral strike-slip faults are associated with minor conjugate NNW-striking faults. The premineralization structures at Zijinshan likely formed during NW-directed compression. Synmineralization structures formed under subvertical contraction and subhorizontal NNE-plunging extension. Postmineralization structures developed in a strike-slip regime defined by a NW-oriented axis of compression and NE-trending axis of extension. Changes in the styles of pre- and synmineralization structures are related to fluctuations in the regional stress regime. The transition from compression to extension was fundamental to mineralization. Postmineralization faults are related to transient local stress regime variations at the deposit scale.

Large‐Scale Drainage of a Complex Magmatic System: Observations From the 2018 Eruption of Ambrym Volcano, Vanuatu

AbstractIn late 2018, Ambrym volcano, Vanuatu, erupted for the first time in 3 years. We show that the eruption was the result of a 6.5‐m‐wide, ∼0.7‐km3 dike intrusion which propagated for more than 20 km into the eastern rift zone. The eruptive sequence began with a small shallow dike within the main caldera but ultimately triggered a much larger intrusion, draining both the shallow (1–2 km) and deeper (∼5 km) magmatic systems. Extension of ∼3 m is estimated across the eastern rift zone with more than 2 m of subsidence within the caldera. Modeling suggests that the stress changes induced by the initial dike emplacement helped trigger and guide the propagation of the flank intrusion. Despite the large volume of material removed from beneath the caldera (∼0.7 km3), limited slip is observed along bounding faults supporting the hypothesis that the long‐term formation of the caldera at Ambrym is a result of repeated moderate‐sized events.

Rapid cooling history of a Neotethyan ophiolite: Evidence for contemporaneous subduction initiation and metamorphic sole formation

AbstractThe Beyşehir-Hoyran Nappes, including Mesozoic carbonate platform rocks, deep-sea sediments, and ophiolite-related units, crop out extensively on the western limb of the Isparta Angle in the Central Taurides, Turkey. The ophiolite-related rocks are represented by variably serpentinized harzburgitic mantle tectonites, tectonically underlain by a subophiolitic metamorphic sole and mélange. The harzburgitic mantle tectonites and metamorphic sole are intruded by undeformed isolated dikes. Protoliths of the metamorphic sole are similar to within-plate alkali basalts and associated sediments. The isolated dikes were geochemically derived mainly from tholeiitic magma and, to a lesser extent, from alkaline magma. Five isolated dike samples yielded U-Pb ages ranging from 90.8 ± 1.6 Ma to 87.6 ± 2.1 Ma (zircon) and from 102.3 ± 7.4 Ma to 87.5 ± 7.9 Ma (titanite). Seven amphibolite samples yielded U-Pb age ranges of 91.1 ± 2.1–88.85 ± 1.0 Ma (zircon) and 94.0 ± 4.8–90.0 ± 9.4 Ma (titanite) and a 40Ar-39Ar age range of 93.7 ± 0.3–91.4 ± 0.4 Ma (hornblende). U-Pb and 40Ar-39Ar ages of mineral phases with different closure temperatures (∼900–500 °C) from the isolated dikes and metamorphic sole rocks are almost identical and overlapping within 1σ, suggesting that both the magmatic growth of oceanic crust and formation of metamorphic sole were contemporaneous and cooled very rapidly. Hence, all the data should be interpreted as the crystallization ages of the ophiolite and metamorphic sole pair. Genesis of suprasubduction zone–type oceanic crust, genesis and exhumation of the metamorphic sole, and postmetamorphic dike emplacement within the Inner Tauride Ocean can be best explained by subduction initiation and rollback processes during the Late Cretaceous based on petrological and geochronological data obtained from the ophiolitic rocks of the Beyşehir-Hoyran Nappes.

Bayesian Markov-Chain Monte Carlo Inversion of Low-Temperature Thermochronology Around Two 8 − 10 m Wide Columbia River Flood Basalt Dikes

Flood basalt volcanism involves large volumes of magma emplaced into the crust and surface environment on geologically short timescales. The mechanics of flood basalt emplacement, including dynamics of the crustal magma transport system and the tempo of individual eruptions, are not well constrained. Here we study two exhumed dikes from the Columbia River Flood Basalt province in northeast Oregon, USA, using apatite and zircon (U-Th)/He thermochronology to constrain dike emplacement histories. Sample transects perpendicular to the dike margins document transient heating of granitic host rocks. We model heating as due to dike emplacement, considering a thermal model with distinct melt-fraction temperature relationships for basaltic magma and granitic wallrock, and a parameterization of unsteady flow within the dike. We model partial resetting of thermochronometers by considering He diffusion in spherical grains as a response to dike heating. A Bayesian Markov-Chain Monte Carlo framework is used to jointly invert for six parameters related to dike emplacement and grain-scale He diffusion. We find that the two dikes, despite similar dimensions on an outcrop scale, exhibit different spatial patterns of thermochronometer partial resetting away from the dike. These patterns result in distinct predicted emplacement histories. We extend previous modeling of a presumed feeder dike at Maxwell Lake in the Wallowa Mountains of northeastern Oregon, finding posterior probability distribution functions (PDFs) that predict steady heating from sustained magma flow over $1-6$ years and elevated farfield host rock temperatures. This suggests regional-scale heating in the vicinity of Maxwell Lake, which might arise from nearby intrusions. The other dike, within the Cornucopia subswarm, is predicted to have a $1-4$ year thermally active lifespan with an unsteady heating rate suggestive of magma low flow rate compared to Maxwell Lake, in a cool near-surface thermal environment. In both cases, misfit of near-dike partial resetting of thermochronometers by models suggests either heat transfer via fluid advection in host rocks or pulsed magma flow in the dikes. Our results highlight the diversity of dike emplacement histories within the Columbia River Flood Basalt province and the power of Bayesian inversion methods for quantifying parameter trade-offs and uncertainty in thermal models.

Geophysical reassessment of the Azimuth 125° Lineament: emplacement model and propagation of its dikes

This study analyzes the area along the Azimuth 125° Lineament (Az 125°) supposed to occur from Rondonia to Rio de Janeiro, in Brazil. The lineament is an important structural feature in association with great carbonatite and kimberlite complexes in Brazil (e.g., Goias and Alto Paranaiba Alkaline Provinces), consisting of extensive NW-SE oriented faults. The origin of the Az 125° Lineament and its dikes emplacement are widely discussed. The main intention of this work is to propose a model of emplacement of the Az 125° dikes. Also, some important magnetic characteristics of the lineament are presented. The magnetic data clearly define the features of the Az 125° in the southern portion of the study area, but we found no magnetic geophysical evidence that this lineament occurs from the Mato Grosso state to the Rondonia state. The dikes that compose the great Az 125° feature have distinct magnetic attributes, as their polarities (normal and reversed). It is suggested that the alternating polarity is due to local re-magnetization sometime after their intrusion. It is proposed that the dikes were positioned along a zone of lithospheric weakness and their propagation was facilitated due to the occurrence of mid-crustal axial feeder chambers.

Geological Characterization of the Ina Shield Volcano Summit Pit Crater on the Moon: Evidence for Extrusion of Waning‐Stage Lava Lake Magmatic Foams and Anomalously Young Crater Retention Ages

AbstractIna, a distinctive ~2 × 3 km D‐shaped depression, is composed of unusual bulbous‐shaped mounds surrounded by optically immature hummocky/blocky floor units. The crisp appearance, optical immaturity, and low number of superposed impact craters combine to strongly suggest a geologically recent formation for Ina, but the specific formation mechanism remains controversial. We reconfirm that Ina is a summit pit crater/vent on a small shield volcano ~3.5 billion years old. Following detailed characterization, we interpret the range of Ina characteristics to be consistent with a two‐component model of origin during the waning stages of summit pit eruption activities. The Ina pit crater floor is interpreted to be dominated by the products of late‐stage, low‐rise rate magmatic dike emplacement. Magma in the dike underwent significant shallow degassing and vesicle formation, followed by continued degassing below the solidified and highly microvesicular and macrovesicular lava lake crust, resulting in cracking of the crust and extrusion of gas‐rich magmatic foams onto the lava lake crust to form the mounds. These unique substrate characteristics (highly porous aerogel‐like foam mounds and floor terrains with large vesicles and void space) exert important effects on subsequent impact crater characteristics and populations, influencing (1) optical maturation processes, (2) regolith development, and (3) landscape evolution by modifying the nature and evolution of superposed impact craters and thus producing anomalously young crater retention ages. Accounting for these effects results in a shift of crater size‐frequency distribution model ages from <100 million years to ~3.5 billion years, contemporaneous with the underlying ancient shield volcano.

40Ar/39Ar ВОЗРАСТ ЩЕЛОЧНЫХ ПОРОД ВЕРХНЕАМГИНCКОГО МАССИВА (АЛДАНСКИЙ ЩИТ, ЮЖНАЯ ЯКУТИЯ)

Актуальность исследования обусловлена необходимостью расширения ресурсной базы полезных ископаемых, в том числе и золота. Крупные ареалы щелочного магматизма, такие как Алдано-Становой щит, представляют особый интерес, так как часто со щелочными породами ассоциируют золоторудные месторождения, в том числе крупные и гигантские. Верхнеамгинский массив расположен в Верхнеамгинском рудном районе Алдано-Станового щита, который, в свою очередь, входит в состав Чаро-Алданской металогенетической зоны, протянувшейся более чем на 700 км с запада на восток. Характерной особенностью Верхнеамгинкого рудного района является его приуроченность к Амгинской субмеридиональной структурной зоне тектонического меланжа, отделяющей Центрально-Алданский составной террейн с востока от расположенных к западу и югу от нее соответственно Западно-Алданского и Тындинского составных террейнов. Цели: определить основные типы магматических пород Верхнеамгинского массива, изучить особенности строения и состава и выяснить время внедрения различных фаз массива; сравнить полученные результаты с имеющимися геохронологическими данными по мезозойскому магматизму Алданского щита. Методы: петрографические исследования и 40Ar/39Ar датирование методом ступенчатого прогрева по монофракциям флогопитов. Результаты. В результате петрографических исследований показано, что основные фазы мезозойского щелочного магматизма в пределах Верхнеамгинского массива представлены лейкократовыми сиенитами и дайками мезократовых лампрофиров (минетт). Результаты 40Ar/39Ar датирования показали, что образование массива происходило в несколько этапов: (1) 129,1±2,5 млн лет внедрение сиенитов; (2) 117,7±3,4 млн лет внедрение даек лампрофиров. Мезозойский магматизм, проявленный в Верхнеамгинском районе показывает сходные возрастные рубежи с магматическими процессами, проявленными на Алданском щите в мезозойскую эпоху.

Seafloor Kinematics of the Near-Greenland Region of the Eurasian Basin

The Lincoln Sea floor evolved during stretching of a segment of the continental Greenland –Barents Sea shelf. Prior to the onset of extension, the continental shallow-water Morris Jesup Rise and the Yermak Plateau were a single unit. During rifting, this single continental plateau broke apart, initiating propagation of the Gakkel mid-ocean ridge toward the Atlantic. The breakup continued for ~1.5 Ma, 35.3–33.7 Ma ago. The emplacement of numerous mafic dikes during rifting could have caused the high-amplitude magnetic anomalies on the single plateau. For the first time, the fracture geometry involved in the breakup of the continental crust has been reconstructed, the Euler poles and angle of rotation describing its kinematics have been determined, and the paleobathymetry on the flanks of the fracture have been reconstructed.

Understanding the origin of magmatic necks: insights from Mt. Etna volcano (Italy) and analogue models

Magmatic necks are commonly found in volcanic areas, and they often exhibit a homogeneous structure with a cylindrical shape and a diameter of up to several hundreds of metres. Their massive and uniform structure poses a space problem for their emplacement in the brittle crust. Here, we use field data and analogue models to investigate how necks may emplace at shallow levels. Field analysis focuses on characterising the geometric, structural and magmatic features of two necks outcropping in the eroded portions of Mt. Etna, Italy. These are homogeneous and massive intrusive bodies, related to a single episode of emplacement at 400–600 m below the paleosurface. We further investigated their possible emplacement mechanism through analogue models, injecting vegetable oil within (a) a flat sand pack and (b) a sand cone. Dikes form with both configurations, erupting to the surface through vents. However, dikes injected within the cone are characterised by a larger thickening at shallow levels, in correspondence with the vent, where a neck-like structure forms. This suggests that the gravitational load imposed by a volcanic edifice provides the most suitable conditions for the development of magmatic neck, as the downslope shear stresses enhance the deformation of the cone slope during shallow dike emplacement promoting shallow dilation and thickening of the dike. Therefore, topography should be a further factor enhancing the development of necks, in addition to those mechanisms previously proposed. Our results are consistent with natural examples of feeder dikes thickening towards the surface and dikes transitioning to necks, supporting the reliability of the proposed conceptual model.

The Aksug Porphyry Cu–Mo Deposit (Northeastern Tuva): Chronology of Magmatism and Ore Formation Processes (U–Pb and Re–Os Isotope Data) and Metallogenic Implications

Abstract —The Aksug porphyry Cu–Mo deposit is located in a region of long-lasting magmatic activity. Gabbroids of the Khoito-Oka complex are the earliest intrusive rocks, in which the Aksug granitoid pluton hosting ore-bearing small porphyry intrusions is localized. The intrusive activity was terminated with emplacement of late leucogranite dikes. There are different viewpoints on the age of magmatism and mineralization of the Aksug deposit, with the concept of their Devonian age prevailing. To solve the debatable issue, we performed isotope geochronological studies and analyzed new results of U–Pb (SHRIMP-II) zircon dating and previously published Re–Os molybdenite dates (518 ± 2, 516 ± 2, and 511 ± 2 Ma). The concordant U–Pb zircon ages for igneous rocks are younger than the Re–Os age for mineralization. New U–Pb dating of Khoito-Oka gabbro-diorites has yielded an age of 503 ± 2 Ma. The U–Pb SHRIMP zircon age of tonalites from the Aksug pluton has been estimated at 504 ± 5 Ma. The U–Pb zircon ages for ore-related tonalite porphyry I and tonalite porphyry II are 500 ± 6 and 499 ± 6 Ma, respectively. The obtained SHRIMP age for leucogranite dike is 509 ± 4 Ma. Two groups of U–Pb dates have been obtained for each of the analyzed zircon samples: close to the Re–Os dates (518–511 ± 2 Ma) and younger (507–486 Ma). The weighted average zircon ages calculated for early and late populations from post-ore leucogranites are 515 ± 4 and 500 ± 4 Ma, respectively. We suggest that zircons with an age close to the Re–Os dates found in post-ore leucogranites were assimilated from the underlying substrate and zircons with an age of 500 ± 4 Ma crystallized from melt. The oldest U–Pb dates (509–515 Ma) of individual zircon grains from ore-bearing tonalite porphyry are consistent with the Re–Os molybdenite ages. Zircons from tonalite, tonalite porphyry, and Khoito-Oka gabbroids sometimes show internal textures indicating secondary alteration. The younger U–Pb concordia zircon ages relative to the Re–Os dates might be due to the influence of late thermal processes on the U–Pb isotopic system. The younger dates (486–507 Ma) of individual zircon grains probably reflect the time of the impact of a thermal fluid process. The weighted average of these younger dates (502 ± 2 Ma) falls within the weighted average age of post-ore leucogranites (500 ± 4 Ma). According to the Re–Os dates, the Aksug deposit formed at the end of the early Cambrian. Ore occurrences similar in magmatism and mineralization to the Aksug ore deposit are widespread in Tuva and in the Lake Zone in Mongolia. Therefore, it is necessary to reassess the role of the Cambrian and Devonian magmatism in the development of porphyry Cu–Mo mineralization both in Tuva and in the Altai–Sayan orogenic area.

Petrological and geochemical characteristics of REE mineralization in the A-type French Creek Granite, New Zealand

The French Creek Granite, New Zealand, is an alkaline intrusion with enrichment in rare earth elements (REE). Petrography and whole-rock geochemical signatures demonstrate that the c. 82–84-Ma French Creek Granite is a composite granitoid, dominated by a ferroan, weakly peraluminous biotite alkali feldspar granite to syenogranite, with subordinate quartz-alkali feldspar (QAF) syenite. Maximum total REE + Y contents are higher in the granite and felsic dikes relative to the marginal QAF syenite, cogenetic mafic Hohonu Dike Swarm, and pegmatites. Our findings suggest that REE and high-field-strength element enrichment in the unaltered French Creek Granite is a function of partial melting from an enriched HIMU-like lithospheric mantle source, minor crustal assimilation, and extensive magmatic differentiation. Primary (magmatic) REE enrichment is defined by the occurrence of allanite, zircon, apatite, fergusonite, monazite, perrierite, and loparite, which are commonly associated with interstitial biotite. French Creek Granite samples from a phyllic-sericitic alteration zone in the Eastern Hohonu River are strongly elevated in REE relative to unaltered French Creek Granite, indicating remobilization and secondary REE enrichment by hydrothermal fluids. The REE are hosted in bastnasite group minerals, monazite, xenotime, and zircon. Quartz protuberances, dilatational microfractures, and dike emplacement indicate that this alteration is structurally controlled. The δ13C and δ18O values of secondary carbonate veinlets are consistent with mixed low-temperature (~ 250–260 °C) magmatic hydrothermal fluids containing mantle-derived carbon. These fluids were likely part of a late-stage porphyry-type system operating during the same mantle degassing and extensional episode that was associated with the emplacement of the French Creek Granite and lamprophyric dikes of the Hohonu Dike Swarm, and initial Tasman Sea spreading. Results from this study provide an important insight into the complex processes responsible for REE enrichment in alkaline igneous systems.

The interplay of magmatism and tectonics: An example based on the satellite scoria cones at Llaima volcano, Chile

We present a geochemical and mineralogical study of the satellite scoria cones at Llaima volcano, Chile, providing insights on magmatic processes and how these are affected by the local and regional tectonics. At Llaima, we identify two different and distinctive groups of satellite scoria cones with a common magmatic source but different petrological evolutions. Cones from group 1 (the “glomeroporphyritic group”), predominantly occur in the NE flank of the volcano, are characterized by basaltic to basaltic andesitic compositions (51–55 wt% SiO2) and have glomeroporphyritic textures. We have interpreted these cones as the product of magma replenishment, followed by an unimpeded propagation of their feeder dikes controlled mainly by the regional stress. As a result, dikes are emplaced parallel to the regional σ1, and perpendicular to the axis of local crustal extension. Cones from the group 2 (the “pilotaxitic group”), appear at the NE, NW and SW flanks of the volcano, they have more evolved compositions (55–60 wt% SiO2) and textures lacking glomerocrysts. Their location and morphometric parameters suggest that their feeder dike emplacement is mainly controlled by the stress exerted by the load of the volcanic edifice, where the compressional stress hinders magma ascent, allowing fractionation and volatile overpressure buildup. The characteristic texture of these cones is interpreted as fractionation, followed by a single crystallization event upon eruption. The lack of scoria cones at the SE flank is attributed to the local horizontal compressional stress caused by the local tectonic deformation associated to the Liquiñe-Ofqui Fault Zone.

Cretaceous igneous-related mineralisation in the Reefton Goldfield, New Zealand

ABSTRACTThe Reefton Goldfield is well known for its orogenic style of Au mineralisation although the timing remains largely unconstrained. Here, a first unequivocal occurrence of igneous-related mineralisation is documented and it is suggested to place a lower age constraint on when orogenic Au mineralisation occurred. Emplacement of the 118.5 ± 1.3 Ma (U-Pb zircon) Blackwater granitoid dike into Early Paleozoic Greenland Group caused contact metamorphism up to hornblende hornfels facies temperatures (>500°C) at low pressures (<3.75 kbar). A hydrothermal convective system that developed during cooling led to propylitic and phyllic alteration associated with the precipitation in order of decreasing abundance of: pyrite, chalcopyrite, molybdenite, arsenopyrite-galena-sphalerite-millerite. Quartz veins and stockworks containing combinations of pyrite, chalcopyrite and molybdenite are common within several hundred metres of the dike. This igneous-related mineralisation overprints Greenland Group metasedimentary rocks displaying evidence for a phase of ductile deformation and associated with the occurrence of pyrite and ankerite-cored siderite porphyroclasts, chalcopyrite and cobaltite. Au-in-pyrite trace element data indicate that these pyrite porphyroclasts are more enriched in Au than the pyrite associated with dike emplacement. Quartz lodes containing Au occur within zones of ductile deformation elsewhere in the goldfield, and therefore Early Cretaceous emplacement of the Blackwater dike provides a lower age constraint for the ductile mineralisation event.

Continental Arc and Back‐Arc Migration in Eastern NE China: New Constraints on Cretaceous Paleo‐Pacific Subduction and Rollback

AbstractTectonic evolution models for the Cretaceous Russia Sikhote‐Alin and eastern NE China continental margin and interior remain controversial. To understand the magmatic evolution over time and assess regional geodynamic processes, we sampled a diverse array of igneous rocks and employed zircon U‐Pb dating, hornblende and plagioclase 40Ar‐39Ar dating, whole‐rock major and trace element analysis, and 87Sr/86Sr and 143Nd/144Nd isotopic analysis. The west Sikhote‐Alin Pikeshan Formation volcanics and associated granites occurred at a peak of ~118 Ma and are hosted by the Triassic‐Jurassic accretionary complex. Their whole rock geochemistry shows that SiO2 increased in a linear trend, Eu/Eu* values decreased from 0.91 to 0.38, and εNd(t) values decreased from +0.6 to −2.9, indicating magma mixing of a juvenile mantle wedge source and continental crust, consistent with a continental arc. The arc thickened over time with a felsic dike hosted in the Pikeshan granites showing depletion in heavy rare earth elements. The termination of the arc front is documented by the ~107‐Ma intermediate lamprophyre and felsic dikes with εNd(t) values of +4.5 to +1.1, indicating an increased mantle contribution over time. Lithospheric extension of the Jiamusi Block to the west occurred at ~100 Ma, characterized by bimodal volcanism and composite dike emplacement, suggestive of asthenosphere upwelling. Based on the spatial and temporal distribution of these igneous rocks, the continental arc and intraplate magmatism migrated eastward contemporaneously. We favor a model invoking rollback of the subducting Paleo‐Pacific slab affecting a long‐lived continental arc.

Emplacement and growth of alkaline dikes: Insights from the shonkinite dikes (Elchuru alkaline complex, SE India)

The Mesoproterozoic Elchuru alkaline complex in the Prakasam Alkaline Province (PAP), Eastern Ghats Belt, SE India is intruded by coeval shonkinite dikes. Magmas parental to both the host nepheline syenite and shonkinite dikes formed during ca. 1350 Ma continental-rift related magmatism. The shonkinite dikes show fine-to medium-grained equigranular to foliated textures with clinopyroxene, biotite, amphibole, K-feldspar perthite and nepheline as major mineral phases. The dikes grew by inflation and coalescence of dike segments along intrusive steps, anastomosis and subsequent coalescence, and thermal/mechanical erosion of the host rocks. A few dikes resulted by filling the fractures created by shearing. Inflation of intrusive steps between magma segments by Mode I opening is the major mechanism of shonkinite dike emplacement and growth. However, the shonkinite dikes also demonstrate the application of remote shear stress on the dike walls resulting in anastomosis, entrapment and segmentation of tabular host rock xenoliths, and in extreme cases brecciation of the host rocks under volatile-rich and low-viscous melt conditions. Fractal analysis of xenolith-bearing dike further illustrates that thermal erosion was facilitated and accentuated by mechanical breakdown of host rock xenoliths due to interactions of tensile and shear forces. Cross-cutting relationships between the shonkinite dikes suggest local deviation in the least compressive stress direction during dike emplacement. The shonkinite dikes of Elchuru demonstrate that spatially- and temporally-restricted high-volatile low-viscous alkaline magmas may have distinctly different styles of emplacement, as controlled by interaction of near field (magmatic) tensile stress and far field (tectonic) shear stress, in an evolving continental-rift setting.

Structural processes and dike emplacement mechanism in the Wulong gold field, eastern Liaoning

The Wulong gold deposits are typical representative of deposits in the Liaodong ore concentration area in the eastern North China Craton. The gold deposits occur as the quartz-vein type. However, gold mineralization time of the Wulong gold deposits was uncertain, and structural controlling mechanism for them remains unclear. We present structural and geochronological data to constrain structural evolution and dike emplacement mechanism before and during the gold mineralization. The Wulong monzonitic granite pluton, the host intrusion for the gold deposits, was dated at 173−163 Ma (Middle Jurassic). The pluton suffered WNW-ESE shortening probably at the beginning of Late Jurassic, as shown by penetrative, ductile deformation and local ductile reverse shear belts in the pluton. The Wulong pluton was then subjected to nearly N-S compression that led to formation of several NE-striking sinistral faults and associated conjugate fractures at the beginning of Early Cretaceous. The compression was followed by WNW-ESE extension during the Early Cretaceous (ca. 131−113 Ma). The Early Cretaceous extension caused a series of NE- to NNE-striking normal faults and reactivated previous faults or fractures with varied orientations. Our fault-slip data gave reliable constraints on the stress fields during the compression and extension. The extension was also associated with emplacement of the Sangouliu pluton (131−120 Ma) in the south, the Wulongbei pluton (127−126 Ma) in the south and a series of diorite, granite, granitic pegmatite, lamprophyre and diabase dikes in the gold mine field. Previous work demonstrates that the diorite and granite dikes often coexist with the auriferous quartz veins. Our zircon U-Pb dating gave 127−113 Ma intrusion ages for the dated diorite dikes, 126−113 Ma ages for the granite dikes and a 121 Ma age for the granitic pegmatite dike. These demonstrate that the diorite and granite dikes in the Wulong gold field are synchronous and their emplacement of 127−113 Ma happened during the extensional stage. These Early Cretaceous dikes were emplaced along both the newly- formed, NE- to NNE-striking normal faults and the previous faults or fractures reactivated during the extension. The dike emplacement along the new and previous structures resulted in varied orientations in the study area. We propose that the auriferous quartz veins share the similar emplacement mechanism with the synchronous dikes in the Wulong gold field. This inference is supported by the common coexistence of the both. It is suggested therefore that the gold mineralization in the Wulong gold field took place during the period of 127−113 Ma when the North China Craton experienced the peak destruction. However, direct dating results from ore minerals are needed to constrain the accurate gold mineralization time in the Wulong gold field.