Dike Formation Research Articles

Observations of avalanche–substrate interactions in the Iymek rock avalanche deposit: A possible causative mechanism

Ubiquitous and complex avalanche–substrate interactions during rock avalanche emplacement have attracted widespread attention in recent years and are regarded as vital processes influencing mobility and damage potential through rapid changes in avalanche mechanical properties. However, the fundamental interaction mechanisms of avalanche mass on substrate and the resultant effect on mobility have yet to be elucidated for natural events. To better understand the mechanics of avalanche–substrate interactions, we present a detailed study on the spectacular synsedimentary deformation structures at the bottom of the gigantic Iymek rock avalanche deposit, including undulose structures, flame structures, mixed textures, clastic dikes, and cracked gravels. The cracked gravels demonstrate that the overriding avalanche mass exited high-energy shearing on the avalanche–substrate interface, where the stress in substrate far exceeded the overburden from the avalanche mass during the avalanche–substrate interaction. Along with high-energy shearing, two different interaction modes with the change of substrate materials are identified, i.e., the Kelvin–Helmholtz instability (KHI) characterized by undulose-to-flame structure transitions, and the Raleigh–Taylor instability (RTI) characterized by the formation of clastic dikes. The KHI is interpreted as a result of the growth of shear instabilities induced by high-energy shearing along the avalanche–substrate interface. The RTI is associated with local liquefaction of the water-bearing sandy substrate and was mainly induced by the high-frequency ground vibrations generated by high-energy shearing. Therefore, we propose that the overriding avalanche mass exited high-energy shearing on the substrate during the avalanche–substrate interaction, which motivated two predominant physical processes of the KHI and RTI along the avalanche–substrate interface. The shear-induced KHI is a potential mechanism of erosion and entrainment in rock avalanches and is responsible for promoting the incorporation of substrate materials into moving avalanche mass. These results not only yield profound insights into the interaction behaviours between rock avalanches and substrates but also provide a fundamental geological prototype to motivate further modelling work to elucidate rock avalanche dynamics.

Syn and post-glacial volcanic activity in the western retroarc area of the andean southern volcanic zone: Overo volcano

Located in a poorly studied zone of the rear arc in the Transitional Southern Volcanic Zone, the Overo volcano presents a complete record of syn-glacial, inter-glacial, and post-glacial activity, marked by significant changes in eruptive mechanisms over time. Following pre-glacial units, an effusive syn-glacial basaltic phase was succeeded by an extensive inter-glacial diking that fed andesitic monogenetic fields. Subsequently, an eruptive pulse, possibly coupled with the destabilization of the main edifice, resulted in a block and ash deposit channeled into a glacial valley towards the southwest, later covered by extensive basaltic lava flows. Then, different types of pyroclastic flows and glacial deposits covered these earlier units. Finally, these younger glacial deposits were overlain by a localized postglacial pyroclastic deposit and widespread lava flows, which ultimately reconstructed the deeply eroded glacial morphology of the Overo volcano. This volcanic record reveals the young and complex activity of this poorly known western rear-arc volcanic center in the Transitional Southern Volcanic Zone of the Andes, whose evolution is more akin to the arc front volcanoes to the west than to their eastern within-plate counterparts. We propose that this magmatic evolution was influenced by the Pleistocene-Holocene Last Glacial Maximum activity (LGM), beginning with an initial phase of volcanic activity encapsulated within glacial conditions. Glacial retreat then triggered isostatic uplift and crustal relaxation, resulting in dike formation, flank instability, and syn-eruptive collapse of the volcanic edifice. Subsequently, a lesser glacial pulse partially eroded the volcano morphology and produced the currently preserved glacial deposits. Finally, a latest and limited post-glacial pyroclastic eruption occurred simultaneously with the formation of an extensive monogenetic basaltic field, shaping the final morphology of the volcano.

Two-Stage Model of Devonian Basic Magmatism in the Vilyui Paleorift (Siberian Platform)

Abstract —We report study results of basic intrusive bodies in the middle Paleozoic Vilyui paleorift (eastern Siberian Platform). Geochemical data for basic sills penetrated by boreholes in the rift’s dike swarms are presented, as well as our data on the time of formation of sills and dikes. We also studied mineral-hosted melt inclusions from a dolerite dike of the Vilyui–Markha dike swarm on the northwestern flank of the Vilyui paleorift. Data on the compositions of homogenous glasses of mineral-hosted melt inclusions yielded the P–T parameters of mantle sources of basic melts responsible for the formation of the Vilyui paleorift gabbro-dolerites. Two depth levels of basic melt generation have been established: 95–65 km at 1480–1400 °C and 55–45 km at 1360–1320 °C. Crystallization of the melts occurred at a shallow depth of 12–4 km with a decrease in temperature from 1185 to 1125 ºС. This occurrence of basic melt sources at two different depths, as well as the heterogeneity of the chemical composition of melt inclusions and their host minerals explains the presence of two pulses of Devonian basic magmatism. Our new numerical thermomechanical model of magma rise during melting of the lithospheric mantle above a mantle plume supports the existence of two chambers at the spinel–garnet peridotite boundary and under the base of the crust, as well as the two-stage nature of Devonian magmatism.

Fracturing and tectonic stress drive ultrarapid magma flow into dikes.

Many examples of exposed giant dike swarms can be found where lateral magma flow has exceeded hundreds of kilometers. We show that massive magma flow into dikes can be established with only modest overpressure in a magma body if a large enough pathway opens at its boundary and gradual buildup of high tensile stress has occurred along the dike pathway prior to the onset of diking. This explains rapid initial magma flow rates, modeled up to about 7400 cubic meters per second into a dike ~15-kilometers long, which propagated under the town of Grindavík, Southwest Iceland, in November 2023. Such high flow rates provide insight into the formation of major dikes and imply a serious hazard potential for high-flow rate intrusions that propagate to the surface and transition into eruptions.

Mineral assemblages of the Shishim mine in the Southern Urals

Mineralogy and petrography of complex rocks in area of the Shishim mine in the South Urals is studied. Various skarn assemblages with dominant pyroxene, garnet, vesuvianite, monticellite, chlorite and epidote are developed after gabbroids and granitoids. The formation of skarns was accompanied by the formation of carbonatite and chloritolite dikes, as well as bodies of carbonatite-pegmatites and chloritolite-pegmatites. The rock-forming minerals are refined, secondary and accessory minerals of rocks are studied including allanite, anhydrite, apatite, baddeleyite, barite, calcirtite, perovskite, valleriite, zircon, and zirconolite. Keywords: skarns, carbonatites, chloritolites, Southern Urals.

Evidence from the Vredefort Granophyre Dikes points to crustal relaxation following basin-size impact cratering

The timescale of the modification stage of basin-sized impact structures is not well understood. Owing to ca. 10 km of erosion since its formation, the Vredefort impact structure, South Africa, is an ideal testing ground for deciphering post-impact modification. Here, we present geophysical and geochemical evidence from the Vredefort Granophyre Dikes, which were derived from the - now eroded - Vredefort impact melt sheet. The dikes have been studied mostly in terms of their composition, while the timing and duration of their emplacement remain controversial. We examined the modern depth extent of five dikes, with three from the inner crystalline core of the central uplift, and two from the boundary between the core and the supracrustal collar of the central uplift, using two-dimensional electrical resistivity tomography. We found that the core dikes terminate near the present erosion surface (i.e., <5 m depth). In contrast, the dikes at the core-collar boundary extend to a depth ≥ 9 m. These observations suggest that the core dikes are exposed near their lowermost terminus. In addition, we obtained bulk geochemical composition of the dikes, finding that the andesitic composition phase is present in the core-collar dikes that is not found in the core dikes. The presence of this phase indicates the episodic emplacement of impact melt into subvertical crater floor fractures.We conclude that the dike formation was protracted and occurred over a time span of at least 104 years. The sequential formation of the Vredefort Granophyre Dikes points to horizontal extension of crust below the impact melt sheet above a kinematic velocity discontinuity, a crustal instability resulting from the dynamic collapse of the transient cavity.

Dense Suspension Flow in a Penny-Shaped Crack, Part I : Theory

We study the dynamics of proppants carried by fluid driven into an evolving penny-shaped fracture. During injection the slurry is modelled using a frictional rheology that takes into account the shear-induced migration and jamming of the proppants. Making pragmatic assumptions of negligible toughness and cross-fracture fluid slip, we find self-similar solutions supporting a range of proppant concentration profiles. In particular, we define an effective viscosity, which equates the fracture evolution of a slurry flow with a given proppant volume fraction, to a Newtonian flow with a particular viscosity. Using this framework, we are able to make predictions about the geometry of the growing fracture and the significance of tip screen-out. We are also able to model the closure of the fracture, after fluid pressure is released, and explore the effect of proppant concentration on the residual geometry of the fracture. The results have important applications to industrial fracking and geological dike formation by hot, intruding magma.

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.

Petrogenesis of Scheelite-Bearing Albitite as an Indicator for the Formation of a World-Class Scheelite Skarn Deposit: A Case Study of the Zhuxi Tungsten Deposit

Abstract Scheelite-bearing albitite is present in the form of rare, highly fractionated felsic dikes in the world-class Zhuxi tungsten deposit. Morphologically, the Zhuxi albitite forms individual dikes with thicknesses from 0.01 to 5.1 m in the orebodies. Additionally, the Zhuxi albitite is characterized by high sodium concentrations (Na2O = 6.08–8.04 wt %), low silicon (SiO2 = 56.81–62.56 wt %) and potassium concentrations (K2O = 1.44–2.62 wt %), and increasing P2O5 (0.1–0.7 wt %), Y (2.72–8.62 ppm), and rare earth element (8.28–28.89 ppm) concentrations from the tops to the bottoms of the dikes, which are controlled by the heterogeneous distribution of apatite grains in the albitite. The trace element geochemical characteristics and Sr-Nd isotope compositions of the albitite and the geochemistry of plagioclase, muscovite, apatite, and scheelite that formed in both the albitite and ore-related (altered) granites strongly suggest a genetic relationship between the two rocks. Given our new data and previous experimental data, as well as natural examples from around the world, we propose that the Zhuxi albitite is the product of a silicate-poor, H2O-rich melt that formed by melt–melt-liquid immiscibility processes in an extremely fractionated residual magma. A deep-seated (&amp;gt;3 kbar) granitic magma reservoir was directly related to the formation of these rare scheelite-bearing albitite dikes. Albitite dikes are the product of extreme fractionation of a granitic magma, and W is highly incompatible during magma evolution regardless of oxygen fugacity; therefore, intense tungsten mineralization development within albitite dikes should serve as an important criterion for judging the tungsten metallogenic potential.

Thermal state of the upper mantle and the origin of the Cambrian-Ordovician ophiolite pulse: Constraints from ultramafic dikes of the Hayachine-Miyamori ophiolite

Abstract Ophiolite pulses, which are periods of enhanced ophiolite generation and emplacement, are thought to have a relevance to highly active superplumes (superplume model). However, the Cambrian-Ordovician pulse has two critical geological features that cannot be explained by such a superplume model: predominance of subduction-related ophiolites and scarcity of plume-related magma activities. We addressed this issue by estimating the mechanism and condition of magma generation, including mantle potential temperature (MPT), from a ~500 Ma subduction-related ophiolite, the Hayachine-Miyamori ophiolite. We developed a novel method to overcome difficulties in global MPT estimation from an arc environment by using porphyritic ultramafic dikes showing flow differentiation, which have records of the chemical composition of the primitive magma, including its water content, because of their high pressure (~0.6 GPa) intrusion and rapid solidification. The solidus conditions for the primary magmas are estimated to be ~1450 °C, ~5.3 GPa. Geochemical data of the dikes show passive upwelling of a depleted mantle source in the garnet stability field without a strong influence of slab-derived fluids. These results, combined with the extensive fluxed melting of the mantle wedge prior to the dike formation, indicate sudden changes of the melting environment, its mechanism, and the mantle source from extensive fluxed melting of the mantle wedge to decompressional melting of the sub-slab mantle, which has been most plausibly triggered by a slab breakoff. The estimated MPT of the sub-slab mantle is ~1350 °C, which is very close to that of the current upper mantle and may reflect the global value of the upper mantle at ~500 Ma if small-scale convection maintained the shallow sub-slab mantle at a steady thermal state. We, therefore, conclude that the Cambrian-Ordovician ophiolite pulse is not attributable to the high temperature of the upper mantle. Frequent occurrence of slab breakoff, which is suggested by our geochemical compilation of Cambrian-Ordovician ophiolites, and subduction termination, which is probably related to the assembly of the Gondwana supercontinent, may be responsible for the ophiolite pulse.

Middle Permian adakitic granite dikes in the Sumdo region, central Lhasa terrane, central Tibet: Implications for the subduction of the Sumdo Paleo-Tethys Ocean

Abstract The Sumdo Paleo-Tethys Ocean was the southernmost part of the Paleo-Tethys Ocean, located on the Qinghai–Tibet Plateau. However, the evolution of the Sumdo Paleo-Tethys Ocean and the origin of eclogites in the Sumdo area of Tibet are still matters of debate. Here we report newly discovered adakitic granite dikes from the Sumdo area of the central Lhasa terrane. The rocks have high SiO2, Al2O3, and Sr contents, high Sr/Y and La/Yb ratios, enrichment in light rare-earth elements (REEs), and depletion in Y, Yb, Th, and heavy REEs. They are characterized by low 87Sr/86Sr (0.703678–0.704427) and high 143Nd/144Nd (0.512825–0.512904) ratios, and positive eNd(t) (+6.41 to +7.38) and eHf(t) (+13.8 to +15.7) values. Zircon U–Pb dating yielded ages of 269.1 ± 4.1 and 266.4 ± 2.0 Ma for the rocks. Our data indicate that the Sumdo adakitic granite dikes were derived from partial melting of subducted oceanic slab, with the magma being influenced by slab-derived fluids and slight interaction with peridotite in the mantle wedge during ascent. By combining our new geochemical data with results of previous studies, we infer a subduction event involving the Sumdo Paleo-Tethys Ocean during the middle Permian, which led to the formation of the Sumdo adakitic granite dikes and the Sumdo, Jilang, and Xindaduo eclogites.

Control of planar fabrics on the development of tensile damage zones under high-speed deformation: An experimental study with granite and gneiss

We have investigated the laboratory-scale, high-strain rate tensile failure processes responsible for Mode-I breccia dike formation in impact structures. Brazilian disc experiments with granite (isotropic) and gneiss (foliated) samples were performed on a Split Hopkinson Pressure Bar, equipped with high-speed photography. For the gneiss samples, the gneissic foliation was oriented (θ) at 0, 45 and 90° to the compression direction. Time-series images show the transient states of tensile rupture localization and propagation, leading to in situ fragmentation of the rocks. Granite samples produced a single incipient tensile rupture, accommodating pulverized clasts, whereas the gneisses underwent failure by way of major fracture and a network of secondary tensile fractures, forming large elongate clasts. For gneisses, θ greatly influenced the secondary crack growth, forcing propagation trajectories to orient preferentially either along or across the foliation. The two types of target rocks produced contrasting clast geometry in the fracture zones. The granite had mostly small clasts (<10 mm), with average aspect ratios around 1:2, whereas the gneisses produced larger clasts (<40 mm) with aspect ratios, 1:5, 1:4 and 1:4 for θ = 0, 45 and 90°, respectively. This study demonstrates that monomict breccia dikes could form in situ, rather than by a tensile dilation followed by infilling.

Cathodoluminescence and trace-element chemistry of quartz from Sudbury offset dikes: Observations, interpretations, and genetic implications

Abstract Offset dikes, radial and concentric fractures infilled with quartz diorite, are important hosts of Ni-Cu-PGE mineralization in the Sudbury area. To better understand their emplacement and evolution, the cathodoluminescence (CL) and trace-element chemistry of quartz were examined in quartz diorite from the Foy, Trill, Whistle, Hess, Parkin (North Range), and Copper Cliff (South Range) offsets. Although the potential causes of the CL response in quartz are considered, the primary focus was the qualitative textures and patterns, as these can provide valuable paragenetic information. Quartz from the North Range displays a strong blue luminescence dominated by homogenous and sharply zoned CL patterns, while that from the Copper Cliff offset displays a weak CL response. Locally recrystallized granoblastic quartz shows diffuse concentric zoning and other heterogeneous CL patterns. Trace-element EPMA-WDS analyses indicate that quartz from the Foy, Trill, and Whistle offsets is enriched in Al (30–600 ppm) and Ti (50–520 ppm) as compared to Fe (&amp;lt;25–490 ppm), while quartz from the Parkin and Hess offsets is enriched in Fe (270–700 ppm) as compared to Ti (44–211 ppm) and Al (95–250 ppm). In contrast to the North Range offsets, quartz from Copper Cliff has low Al concentrations (30–85 ppm) and very low Ti concentrations (&amp;lt;25 ppm). Application of the Ti-in-quartz geothermometer indicates that quartz from the North Range offsets crystallized above 600 °C, while that from the Copper Cliff offset crystallized below 600 °C. The CL responses and trace-element compositions of anhedral quartz from the North Range offsets are consistent with primary crystallization of quartz from magmatic quartz diorite, while those of the granoblastic quartz record dynamic recrystallization and Ostwald ripening. Copper Cliff quartz is anomalous in its CL response, trace-element content, and crystallization temperature, which may reflect overprinting during regional metamorphism of the South Range of the Sudbury Igneous Complex. Quartz CL is demonstrated to be an important tool for discerning and discriminating between paragenetic processes related to the formation of the offset dikes and has clear applications to the study of other quartz-bearing igneous rocks in the Sudbury area.

The epilogue of Paleo-Tethyan tectonics in the South China Block: Insights from the Triassic aluminous A-type granitic and bimodal magmatism

Triassic aluminous A-type granitoids, mostly peraluminous biotite granites, occur in various areas in the South China Block. Here, we focus biotite granites from three plutons and one granitic dike that provide new insights into the Triassic tectono-magmatic evolution of South China Block. The peraluminous biotite granites from Zhulanbu, Xiaotao and Wengshan plutons all emplaced at ca. 225 Ma and contain Fe-rich biotites. Their high FeOT/(FeOT + MgO) and Ga/Al ratios and HFSE contents imply an aluminous A-type affinity. The geochemical features and heterogeneous Sr-Nd-Hf isotopic compositions of these A-type granitoids, together with previous results, indicate their derivation from disequilibrium partial melting of a sedimentary source or fractional crystallization of coeval monzonitic melts. The Xiamao granitic porphyry dike emplaced synchronously with the nearby Xiamao diabase dike at ca. 225 Ma. This granitic dike is highly siliceous and its geochemical characteristics imply derivation from crustal assimilation and fractional crystallization (AFC) of mafic melt. Closure of Paleo-Tethyan oceanic basin and final collision of the South China Block with the Indochina and the North China Blocks were completed by ca. 230 Ma. The onset of A-type granitic magmatism occurred near Triassic dextral shear zones or surrounded by Triassic NE-SW strike-slip shear zones that generated localized extension under an overall compressional regime until 230 Ma. The subsequent formation of abundant A-type granitoids, bimodal dikes and hybrid granitoids hosting mafic microgranular enclaves implies the tectonic regime to a within-plate post-collision extensional setting with more intense mafic magmas underplating, marking the epilogue of Paleo-Tethyan tectonics in the South China Block.

New Constraints on the Timing and History of Breccia Dikes in the Western San Juan Mountains, Southwestern Colorado

In the western San Juan Mountains, clastic (breccia) dikes crop out in Paleozoic to Cenozoic rocks. The dikes are tabular to bifurcating masses up to several meters thick and are exposed on northwest or northeast trends for up to several kilometers. They are matrix- to clast-supported with angular to rounded pebble- to boulder-sized fragments that in most dikes are dominated by Proterozoic igneous and metamorphic rocks. U-Pb age analyses (n = 3) reveal a range of zircon ages in all samples with several containing high proportions of 1820 to 1390 Ma zircons. The majority of Proterozoic zircons are interpreted as direct contributions from basement rocks during breccia dike formation and emplacement. Field relations and U-Pb zircon analyses reveal that breccia dikes formed in intervals from 65 to 30 Ma (Ouray) and 27 to 12 Ma (Stony Mountain); some dikes are closely allied with mineralization. The dikes formed at depths over 500 meters where Proterozoic basement was fragmented, entrained, and transported to higher structural levels along with pieces of Paleozoic to Cenozoic rocks. A close spatial relationship exists between breccia dikes and latest Mesozoic to Cenozoic plutons. This is best exemplified near Ouray where clastic dikes share similar trends with ~65 Ma granodiorite dikes, and there is a clear transition from intrusive rocks to altered-brecciated plutons, and finally to breccia dikes. The preponderance of evidence supports breccia dike formation via degassing and explosive release of CO2-charged volatiles on deep fractures related to emplacement of 70 to 4 Ma plutons or mantle melts. In addition to breccia dikes, several post-80 Ma events in the region involved explosive release of volatile-charged magmas: 29-27 Ma calderas, ~25 Ma diatremes, and ~24 Ma breccia pipes. Causal factors for production of these gas-charged magmas remain poorly understood, but partial melting or assimilation of altered and metasomatized lithospheric mantle could have played a role.

Petrogenesis of the southern Qiangtang mafic dykes, Tibet: Link to a late Paleozoic mantle plume on the northern margin of Gondwana?

AbstractThis study presents 13 new U-Pb zircon ages obtained by laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) together with whole-rock geochemical, Sr-Nd isotopic and zircon Hf isotopic data for a mafic dike swarm in the southern Qiangtang area of Tibet. These data provide the basis for a new model of the late Paleozoic evolution of the Tethys. Combined with the results of previous zircon U-Pb dating, the magmatic zircon grains extracted from mafic dikes yield latest Carboniferous to Early Permian ages (317–279 Ma). The geochemistry of the southern Qiangtang mafic rocks indicates the presence of low-Ti (QLT) and high-Ti (QHT) suites. The magmas that formed the QLT suite underwent higher degrees of partial melting (&gt;5%) and display evidence of crustal contamination, whereas the QHT suite was derived from magmas generated by low-degree (1%–5%) partial melting of a garnet-bearing mantle source, with a greater extent of fractional crystallization than the QLT suite, and no evidence of crustal contamination. We propose that the QHT and QLT suites may have been derived from magmas from different parts of a single mantle plume. The formation of the southern Qiangtang mafic dikes (latest Carboniferous to Early Permian; 317–279 Ma) may have been related to the northward drift of the Cimmerian continent from the northern Gondwana margin, which resulted in the opening of the Meso-Tethys Ocean.

Mesozoic felsic dikes in the Jiaobei Terrane, southeastern North China Craton: Constraints from zircon geochronology and geochemistry, and implications for gold metallogeny

The Jiaobei Terrane in the southeastern North China Craton (NCC) witnessed intense magmatic pulses during Mesozoic in response to lithospheric thinning. These include sporadic felsic-porphyritic dikes which preserve important information on the magmatic processes and of its implications on lithospheric thinning. Here we present a detailed investigation of representative felsic-porphyritic dikes based on whole-rock geochemical analysis, zircon LA-ICPMS U-Pb geochronology, trace elements and hafnium isotopic compositions. Our results identify stages of felsic dikes in the Jiaobei Terrane with ages in the range of 131–127 Ma and 123–116 Ma. The early stage felsic dikes (ESFDs) have negative zircon ɛHf(t) (−22.4 to −11.1), high zircon EuN/EuN* (0.41–0.92) and Ce4+/Ce3+ (62–944), low Ti-in-zircon crystallization temperatures (524–738 °C), and are geochemically comparable to the coexisting Guojialing granodiorite. Abundant inherited zircons and scattered zircon Hf, Yb/Gd, Th/U, and Ce/Sm values suggest that the magmas from which the dikes formed involved crustal contamination. They are interpreted to be the hypabyssal phase of the Guojialing suite sourced from the Paleoproterozoic lower crust of the eastern NCC with input of mantle components, associated with extensive crust-mantle interaction during the peak lithospheric thinning. The late stage felsic dikes (LSFDs) are characterized by variable zircon ɛHf(t), EuN/EuN* and Ce4+/Ce3+ values (−20.1 to −12.2, 0.38–0.88 and 64–956 for the granodiorite- and quartz-porphyritic dikes; −27.6 to −16.2, 0.17–0.75 and 22–503 for the monzogranite-porphyritic dikes), low Ti-in-zircon crystallization temperatures (562 to 774 °C), and display geochemical similarity with the Aishan granitoids. Systematic variations of zircon Hf, Yb/Gd, Th/U and Ce/Sm suggest that fractional crystallization of hornblende played a significant role in the formation of the late dikes. The LSFDs originated from the Neoarchean-Paleoproterozoic lower crust of the NCC with heterogeneous involvement of lesser amounts of mantle materials than the early dikes, consistent with weak crust-mantle interaction during the waning stage of lithospheric thinning. In the LSFDs, the granodiorite and monzogranite porphyries are broadly coeval (123–118 Ma) with large scale gold mineralization in the area as also favored by the high zircon Ce4+/Ce3+ ratios. Our study also provides new insights into the formation of gold deposits.

Possible Role for Tectonics in the Evolving Stability of the Greenland Ice Sheet

AbstractThe history of the Greenland Ice Sheet has been influenced by the geodynamic response to ice sheet fluctuations, and this interaction may help explain past deglaciations under modest climate forcing. We hypothesize that when the Iceland hot spot passed beneath north‐central Greenland, it thinned the lithosphere and left anomalous heat likely with partially melted rock; however, it did not break through the crust to supply voluminous flood basalts. Subsequent Plio‐Pleistocene glacial‐interglacial cycles caused large and rapidly migrating stresses, driving dike formation and other processes that shifted melted rock toward the surface. The resulting increase in surface geothermal flux favored a thinner, faster‐responding ice sheet that was more prone to deglaciation. If this hypothesis of control through changes in geothermal flux is correct, then the long‐term (105 to 106 years) trend now is toward lower geothermal flux, but with higher‐frequency (≤104 to 105 years) oscillations linked to glacial‐interglacial cycles. Whether the geothermal flux is increasing or decreasing now is not known but is of societal relevance due to its possible impact on ice flow. We infer that projections of the future of the ice sheet and its effect on sea level must integrate geologic and geophysical data as well as glaciological, atmospheric, oceanic, and paleoclimatic information.

Late-stage intrusive activity at Olympus Mons, Mars: Summit inflation and giant dike formation

By mapping the distribution of 351 lava flows at the summit area of Olympus Mons volcano on Mars, and correlating these flows with the current topography from the Mars Orbiter Laser Altimeter (MOLA), we have identified numerous flows which appear to have moved uphill. This disparity is most clearly seen to the south of the caldera rim, where the elevation increases by >200 m along the apparent path of the flow. Additional present day topographic anomalies have been identified, including the tilting down towards the north of the floors of Apollo and Hermes Paterae within the caldera, and an elevation difference of >400 m between the northern and southern portions of the floor of Zeus Patera. We conclude that inflation of the southern flank after the eruption of the youngest lava flows is the most plausible explanation, which implies that intrusive activity at Olympus Mons continued towards the present beyond the age of the youngest paterae ∼200–300 Myr (Neukum et al., 2004; Robbins et al., 2011). We propose that intrusion of lateral dikes to radial distances >2000 km is linked to the formation of the individual paterae at Olympus Mons. Two specific dikes to the SE of the volcano are inferred to have volumes of ∼4400 km3 and ∼6100 km3, greater than the volumes of individual calderas and implying triggering of both caldera collapse and lateral dike injection by the arrival of large inputs of magma from the mantle. A comparable disparity between lava flow direction and current topography, together with a tilted part of the caldera floor, has been identified at Ascraeus Mons.

The nature of the heat source of mafic magmatism during the formation of the Vilyui rift based on the ages of dike swarms and results of numerical modeling

Possible mechanisms of rifting and the thermal regime of the lithosphere beneath the rift zone of the Vilyui sedimentary basin are considered based on the available isotopic ages of dike swarms, rates of sedimentation, and results of numerical modeling. Temporal correlations between the intrusion of mafic magma and a sharp increase in the rate of subsidence and sedimentation in the rift basin prove the contribution of both plate-tectonic and magmatic factors to the formation of the Vilyui rift. The results show a relationship between the rapid extension of the lithosphere and the formation of mafic dike swarms in the Yakutsk-Vilyui Large Igneous Province of the Siberian Platform at the Frasnian-Famennian boundary, with a peak at ~374.1 Ma, and at the end of the Late Devonian, with a peak at ~363.4 Ma. There were two pulses of dike formation during rapid subsidence of the basin basement in the period 380-360 Ma, with a sedimentation rate of 100-130 m/Myr, at a background rate of 10-20 m/Myr. Analysis of numerical thermomechanical models revealed that the best-fit model is that combining the mechanisms of intraplate extension (passive rifting) and the ascent of a mantle magmatic diapir (active rifting). A conclusion about the nature of the heat source of trap magmatism has been drawn: The plume-driven regime of the lithosphere can better explain the dynamics of extension during rifting than the decompression melting mechanism.