Basic Sills Research Articles

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.

Proterozoic - basal Cambrian stratigraphy across Nares Strait: correlation between Inglefield Land and Bache Peninsula

In the Kane Basin region of Nares Strait (78° to 79°N) unmetamorphosed Proterozoic and basal Cambrian platform strata overlie the Precambrian Shield both in Inglefield Land in Greenland, and on Bache Peninsula in Ellesmere Island. The platform succession is composed of two shallow-water elastic sequences separated by an erosional disconformity. The lower formation, of multicoloured sandstone and siltstone with minor shale and stromatolitic dolomite, is assigned to the redefined Rensselaer Bay Formation; the upper, composed of red and overlying white sandstone with dolomitic sandstone and siltstone at the top, is assigned to the Dallas Bugt Formation. Basic sills from the Rensselaer Bay Formation have yielded isotopic ages as old as 1200 m.y., while the trace fossils Rusophycus and Skolithos suggest a basal Cambrian age for the Dallas Bugt Formation.
 The platform sequences of Inglefield Land and Bache Peninsula show striking similarity in lithology and thickness and the detail correlation is supported by the evidence from isotopic ages of basic sills and from trace fossils. In both areas the bipartite succession is overlain with apparent conformity by dolomites that pass upwards into limestones containing Early Cambrian macrofauna.
 The Rensselaer Bay Formation thickens southwards and passes into the much thicker sedimentary sequence of the Thule Basin, while the Dallas Bugt Formation and the overlying dolomites thicken northwards, indicating that the two formations were laid down in separate sedimentary basins. The close correlation of the platform successions and the corresponding configuration of the depositional slopes on both sides of the Kane Basin clearly indicate that the Proterozoic - Early Cambrian sedimentary pattern has not been radically disturbed by tectonism or by any appreciable transcurrent movement along Nares Strait.

The genesis and age of the Grunehogna Granite and Rb–Sr and Sm–Nd chemistry of the Annandagstoppane Granite, Ahlmanryggen, Dronning Maud Land, Antarctica

Interpretations of Rb–Sr and Sm–Nd data from the Grunehogna Granite demonstrate that anatexis of and mixing with Høgfonna Formation sedimentary rocks, caused by hot basic sills of ~1100 Ma Borgmassivet Suite, are consistent, with the granite εNdεSr100Ma data, partially overlapping Borgmassivet Suite data and are marginally displaced toward published data from the Høgfonna Formation. Upper intercept LA-ICPMS U–Pb data from the Grunehogna Granite and Borgmassivet Suite zircons are within error at 1088 ± 17Ma and 1088 ± 68Ma, and are consistent with with unpublished data for the gabbro of 1107Ma. Interpretations of syn-depositional anatexis of semi-consolidated sediments under, high pH2O, are consistent with deposition at ~1130-1107Ma for the Høgfonna Formation. Sm–Nd data define an age of 3058 ± 116Ma for the Annandagstoppane Granite, consistent with a reported U–Pb zircon age of 3067± 8Ma. Rb–Sr data for the Annandagstoppane Granite define an errorchron of 2252 ± 82Ma. Differences between Rb–Sr and Sm–Nd data suggest that Rb–Sr systematics in many samples from all units have experienced open system behaviour in contrast to Sm–Nd systematics from which no disturbance is apparent. Localised tonalitic partial melts of gabbro yield an upper intercept LA-ICPMS U–Pb age from zircon of 1114 ± 47Ma.

Appraisal of Veldurti–Kalva–Gani (VKG) fault, Cuddapah Basin, India: Gravity and magnetic approach

A ~60 km long Veldurti–Kalva–Gani (VKG) fault is one of the identified strike-slip faults extending from Eastern Dharwar Craton (EDC) to Cuddapah basin in South Indian Shield. The recorded recent seismic activity during year 2012–2016 show occurrences of three microseismic events (< Mw 2.0) in the vicinity of this fault. Historically, no major seismic events are recorded near this fault except magnitude of 5.0–5.9 (1843) earthquake at about ~80 km west of this fault near Bellary. In the present study, analysis of available gravity, aeromagnetic and newly acquired ground gravity and magnetic data in the vicinity of the fault has been carried out to understand subsurface characteristics of this VKG fault and nearby structural features related to recent seismic activity. Analysis of aeromagnetic and gravity data shows shallow origin of the fault and earthquakes are associated with the zone of intersection like cross faults/lineaments which are parallel and perpendicular to the VKG fault. The calculated log normalized radially averaged power spectrum of the available gravity and aeromagnetic data shows four average depths h0 (12.7 km), h1 (6 km), h2 (2.0 km) and h3 (0.5 km). These estimated depths are possibly, bottom of the upper crust, thickness of the Cuddapah basin sediments, horizon of the basic sills, flows and the ferruginous quartzites and cumulative stratigraphic thickness of the Tadpatri shales and the Kurnools in the areas, respectively. The jointly inverted 2-D model from the ground gravity and aeromagnetic data along 2.7 km profile across VKG fault shows, faulting between Banganapalli Quartzite and Tadpatri Shales. The estimated average focal depth from the observed microseismic events is around 13 km. It is concluded from the present study that the observed microseismic events in the vicinity of VKG fault are associated with the intersection zones of cross faults/lineaments near the VKG fault and originated at an average depth of 13 km might be bottom of the upper crust. The estimated depths from the present analysis are well corroborated with previous geophysical studies.

The Orestes Melt Zone, McMurdo Dry Valleys, Antarctica: Spatially Distributed Melting Regimes in a Contact Melt Zone, with Implications for the Formation of Rapakivi and Albite Granites

Abstract The Orestes Melt Zone (OMZ) is a massive contact melt zone (∼20 m thick by several kilometers long), located in the McMurdo Dry Valleys of Antarctica. The OMZ formed at shallow crustal depths by melting of the A-type Orestes Granite owing to intrusion of the underlying, doleritic Basement Sill. The OMZ can be divided broadly into two melting facies. The upper melting facies is distal from the contact and formed by melting at low temperature and water-saturated, or near water-saturated, conditions. The lower melting facies is proximal to the contact and formed by melting at high temperature and water-undersaturated conditions. Separate melting reactions occurred in both of the melting facies, resulting in distinct textures and melt compositions. Melting in the distal facies generated melts with compositions that plot near a predicted eutectic composition. Melting in the proximal facies was accomplished in part by replacement reactions in restitic feldspars. These reactions resulted in the development of plagioclase mantles on both restitic plagioclase and K-feldspar, and melt compositions that diverged from predicted minimum melt along an unexpected path, towards enrichment in orthoclase component. Thermal modeling indicates that this melt zone was active for a minimum of ∼150 years, with a contact temperature of ∼900 °C. Upon cooling, recrystallization generated ocellar textures around restitic quartz, as well as faceted albite as a late-stage product. Observations of the OMZ, combined with thermal modeling, provide new insights into the origin of rapakivi and albite granites. This study has implications for the origin of these two associated granite types in other geological settings.

Fossil subduction zone origin for magmas in the Ferrar Large Igneous Province, Antarctica: Evidence from PGE and Os isotope systematics in the Basement Sill of the McMurdo Dry Valleys

Mantle plumes provide an attractive mechanism for generating short-duration, voluminous magmas in large igneous provinces (LIPs) while at the same time providing an explanation for the frequently associated break-up of supercontinents. This model has also been invoked for the Ferrar large igneous province (FLIP) in Antarctica, which zircon and baddeleyite U–Pb dating shows was emplaced over a short duration at 182.7 ± 0.5 Ma, contemporaneously with fragmentation of the supercontinent Gondwanaland. Here, we present platinum-group-element (PGE) and Os-isotopic data for the Basement Sill in the McMurdo Dry Valleys – a part of the FLIP – that challenge the plume interpretation. The Basement Sill samples studied are cumulate-textured gabbro to norite, and pyroxenite with minor ferro- or leuco-lithofacies with MgO ranging from 2 to 19 wt%. The 187Os/188Os values range from 0.1609 ± 0.003 (2σ) to 8.100 ± 1.600 (2σ); the minimum value overlaps with a previously published estimated initial 187Os/188Os ratio for Ferrar magmas of 0.145 ± 0.049 (2σ). The PGE abundance patterns for the Basement Sill define positive, convex-shaped slopes between the IPGE (Os, Ir and Ru) and PPGE (Pt, Pd and Rh). The most significant feature of the entire data set is the extreme sub-chondritic Os/Ir ratios (<0.33), values which are atypical of plume-derived magmas. These low Os/Ir ratios are more consistent with the alternative view that FLIP resulted from the decompression melting of mantle with a fossil subduction zone signature along the proto-Pacific margin of Gondwanaland, disaggregated by rifting related to plate rearrangements during supercontinent break-up. We propose that hydrated fossil subduction zones elsewhere on Earth might account for other short-lived voluminous magmatic events that form LIPs. The remarkably short duration of these events may be due to rapid decompression of hydrated mantle allowing instantaneous large-volume melting which then peters out quickly (<1 Myr) as H2O is expelled from the source rocks and into the melt.

Reativações pós-silurianas do Lineamento Transbrasiliano na porção sul da Bacia do Parnaíba

O Lineamento Transbrasiliano (LTB) apresenta direção NE-SW. Cerca de 900 km desse sistema interceptam o substrato da Bacia do Parnaíba (BPar), inferido, a partir de dados geológicos e geofísicos, como uma zona de cisalhamento plástica transcorrente dextral. Este trabalho objetivou abordar a assinatura estrutural e os marcadores estratigráficos associados às reativações do LTB na região sul da bacia, ao leste de Palmas (TO). No embasamento cristalino, um estágio tardio do LTB se expressa como uma zona dúctil-frágil também de cinemática dextral de idade ediacarana-cambriana, no NW do Ceará. Nas unidades litoestratigráficas da BPar são distinguidos eventos de reativação em regime frágil ou hidroplástico. Na região de estudo, um evento mais antigo registra cinemática transcorrente sinistral expressa principalmente como bandas de deformação e falhas com direção NE, combinadas a estruturas dilatacionais (incluindo juntas e falhas normais) ou conjugadas/antitéticas de rejeito direcional ou oblíquo, com orientações que variam de NNE a NNW, todas observadas nos litotipos das Formações Sambaíba e Pedra de Fogo, bem como nas mais antigas. Ao sudeste de Alto Parnaíba (MA) são observadas evidências de atividade tectônica sindeposicional durante o Permiano superior. Um segundo conjunto de estruturas composto por falhas normais associadas a uma distensão N/NNW, impressas nos corpos básicos de idade eojurássica da Suíte Mosquito, que, por sua vez, é capeada por depósitos com seixos das vulcânicas, correlacionados à Formação Corda. Um terceiro conjunto de estruturas, caracterizado por falhas normais ou oblíquas com direção NE, registra distensão NW, sendo associada ao evento de rifteamento da Margem Leste brasileira, durante o Cretáceo Inferior. Finalmente, um quarto evento, de ocorrência restrita, envolve distensão NE e está registrado em arenitos correlacionados ao Grupo Urucuia, implicando uma idade máxima neocretácea.

Gravimetric and magnetic fabric study of the Sintra Igneous complex: laccolith-plug emplacement in the Western Iberian passive margin

The geometry and emplacement of the ~ 96 km2, Late Cretaceous Sintra Igneous complex (SIC, ca. 80 Ma) into the West Iberian passive margin is presented, based on structural data, gravimetric modeling, and magnetic fabrics. A granite laccolith (~ 76 km2, < 1 km thick, according to gravimetric modeling) surrounds a suite of gabbro–diorite–syenite plugs (~ 20 km2, ~ 4 km deep) and is encircled by cone sheets and radial dykes. Anisotropy of Magnetic Susceptibility was interpreted from 54 sites showing fabrics of para- and ferro-magnetic origin. Most fabrics can be interpreted to have a magmatic origin, according to the scarcity of solid-state deformation in most part of the massif. Magnetic foliations are shallowly dipping in the granite laccolith and contain a sub-horizontal ENE–WSW lineation. The gabbro–syenite body displays concentric magnetic foliations having variable dips and steeply-plunging lineations. The SIC can be interpreted to be intruded along an NNW–SSE, 200 km-long fault, perpendicular to the magnetic lineation within the laccolith, and was preceded by the intrusion of basic sills and plugs. The SIC intruded the Mesozoic series of the Lusitanian Basin during the post-rift, passive margin stage, and its geometry was only slightly modified during the Paleogene inversion that resulted in thrusting of the northern border of the intrusion over the country rocks.

IEBOOK-based Experimental Operation Platform of Computer Basic Course

With the development of information technology, educators combine the teaching contents of basic computer knowledge with the abundant materials available online, having raised the interestingness and attraction of the teaching practice of computer basic course. This paper aims to design an IEBOOK-based experimental operation platform of computer basic course which can overcome the defects of traditional experimental operation platform of inconvenience and less abundance of contents by combining IEBOOK software and an interactive display technology (including input module, picture processing module, touch screen, image addition module, and output module) of electronic courseware. The results show that the IEBOOK-based experimental operation platform effectively combine classroom teaching and after-class review, Q&amp;A and resources exchange, and largely raise students’ interest in computer basic course learning, self-management sills, and communicative competence via mobile devices such as mobile phone and tablet PC.

Multiple reinjections and crystal mush compaction in the solidification evolution of the Karaj Dam basement Sill, Northern Iran

The Karaj Dam basement Sill is a 460m–thick saucer shaped sill, situated in the Alborz Magmatic Belt, Northern Iran. The results of geochemical, textural and field relations reveal characteristics of a sill with a well-developed S-shaped compositional profile which could be subdivided into distinct parts and suggest that the sill was repeatedly split and reinjected with fresh magma in the upper half of the previous emplacements (over- to partly intra-accretion). Whole rock and mineral compositional profiles have recorded five to six discrete injections of magma, each of which individually show an upward increase in terms of primitivity which represents partial crystallization in feeder conduits. The first three small successive pulses of magma, emplaced in the basal 150m of the floor sequence, were followed by voluminous fourth and fifth pulses in the upper portion of the sill. During final two pulses the system acts as a closed system for each independently, and evolves through compositional convection or compaction aided in-situ crystallization. Theoretical models for convection and compaction shows the significance of both processes. Considering the final porosity of 0.1<φ<0.45, calculated convective velocities (0.2<Vc<227m/year) are higher than the calculated crystal accumulation rate of the basal 150m and upper portion of the sill (Va=1 and 0.1m/year), when the crystal mush varies between 1 to 0.01m with the liquid viscosity ranging from 85 to 15Pa·s. Our calculations further indicate that compaction driven velocity of liquid expulsion (ω−w) hardly exceeds the Va in the basal 150m of the floor sequence. The highest velocity is reached (ω−w=~1m/year) only if the crystal mush thickness is no less than 240m with the porosity of 0.6 and the liquid viscosity of no more than 15Pa·s. On the other hand, compaction is highly effective in the upper portion of the sill. Transfer of residual liquid from the compacting lower solidification front to the dilating upper solidification front resulted in characteristic chemical and mineralogical effects, such as the depletion of the lower half of the sill and the enrichment of the sandwich zone in incompatible elements and modal granophyre. Crystallization of the fourth pulse of magma produced a peak in incompatible clement concentrations at a sandwich horizon located at the 410m level, where the floor and roof of the sill appeared to converge at this stage. Subsequent to the time of this enriched zone crystallizing, the fifth pulse of magma was emplaced near this level and inflated the chamber vertically for ~200m. Cumulus material containing interstitial melt and subsequent buoyancy driven upward transport of interstitial melt in this pulse concentrated incompatible elements at ~47m below the main peak, at the stratigraphic height of 360–370m. The boundaries between the successive pulses were cryptic and represent a gradational contact in terms of grain size, chemical composition and crystallization sequence. This implies a short time interval between the emplacements of the magma. This sill is comparable with Antarctica Beacon Sill and Hudson River Palisades Sill, and the recognition of evidence for reinjection and compaction in these macroscopically uniform sills, as well as the Karaj Dam basement Sill, suggests that these processes may be common in the construction of sills with a thickness of more than 100m.

Image-based modelling of lateral magma flow: the Basement Sill, Antarctica.

The McMurdo Dry Valleys magmatic system, Antarctica, provides a world-class example of pervasive lateral magma flow on a continental scale. The lowermost intrusion (Basement Sill) offers detailed sections through the now frozen particle microstructure of a congested magma slurry. We simulated the flow regime in two and three dimensions using numerical models built on a finite-element mesh derived from field data. The model captures the flow behaviour of the Basement Sill magma over a viscosity range of 1–104 Pa s where the higher end (greater than or equal to 102 Pa s) corresponds to a magmatic slurry with crystal fractions varying between 30 and 70%. A novel feature of the model is the discovery of transient, low viscosity (less than or equal to 50 Pa s) high Reynolds number eddies formed along undulating contacts at the floor and roof of the intrusion. Numerical tracing of particle orbits implies crystals trapped in eddies segregate according to their mass density. Recovered shear strain rates (10−3–10−5 s−1) at viscosities equating to high particle concentrations (around more than 40%) in the Sill interior point to shear-thinning as an explanation for some types of magmatic layering there. Model transport rates for the Sill magmas imply a maximum emplacement time of ca 105 years, consistent with geochemical evidence for long-range lateral flow. It is a theoretically possibility that fast-flowing magma on a continental scale will be susceptible to planetary-scale rotational forces.

In-situ Crystallization and Thermal Migration in Karaj Dam Basement Sill (Northern Iran)

Abstract: The Karaj Dam basement sill (KDBS) shows layering between upper and lower chilled margins. Geochemically, the KDBS belongs to high-K calc-alkaline series and displays the geochemical characteristics related to an active continental margin environment. The rocks of the chilled margins are gabbroic in composition and porphyritic, becoming coarse-grained toward the center of the sill. The layers are dominated by gabbro, monzogabbro, and monzodiorite, formed from the lower to the upper chilled margin, with gradational transitions between rock types. The KDBS shows an S-shaped profile of MgO concentration from the base to the top of the sill. In situ crystallization is indicated by an approximately constant modal abundance of pyroxene and plagioclase, a narrow range of major element contents in the main lithologies and variable trace element concentrations. Calculated densities of successive cumulates and estimated Rayleigh number also confirm extraction of the residual liquid by compositional convection at the crystallization front and existence of turbulent flow in the main magma chamber. Disequilibrium crystallization, caused by saturation gradient thermal diffusion and to some extent Soret effect (Thermal migration) in the marginal series, is also indicated by an unexpected overall enrichment trend in MgO concentration and average Mg# of pyroxenes from the chilled margin (olivine-bearing gabbro) to marginal gabbro.

Petrology and geochemistry of the Karaj Dam basement sill: Implications for geodynamic evolution of the Alborz magmatic belt

The northeastward subduction of the Neo-Tethyan oceanic lithosphere beneath the Iranian block produced vast volcanic and plutonic rocks that now outcrop in central (Urumieh–Dokhtar magmatic assemblage) and north–northeastern Iran (Alborz Magmatic Belt), with peak magmatism occurring during the Eocene. The Karaj Dam basement sill (KDBS), situated in the Alborz Magmatic Belt, comprises gabbro, monzogabbro, monzodiorite, and monzonite with a shoshonitic affinity. These plutonic rocks are intruded into the Karaj Formation, which comprise pyroclastic rocks dating to the lower–upper Eocene. The geochemical and isotopic signatures of the KDBS rocks indicate that they are cogenetic and evolved through fractional crystallization. They are characterized by an enrichment in LREEs relative to HREEs, with negative Nb–Ta anomalies. Geochemical modeling using Sm/Yb versus La/Yb and La/Sm ratios suggests a low-degree of partial melting of a phlogopite–spinel peridotite source to generate the KDBS rocks. Their low ISr=0.70453–0.70535, ɛNd (37.2Ma)=1.54–1.9, and TDM ages ranging from 0.65 to 0.86Ga are consistent with the melting of a Cadomian enriched lithospheric mantle source, metasomatized by fluids derived from the subducted slab or sediments during magma generation. These interpretations are consistent with high ratios of 206Pb/204Pb=18.43–18.67, 207Pb/204Pb=15.59, and 208Pb/204Pb=38.42–38.71, indicating the involvement of subducted sediments or continental crust. The sill is considered to have been emplaced in an environment of lithospheric extension due to the slab rollback in the lower Eocene. This extension led to localized upwelling of the asthenosphere, providing the heat required for partial melting of the subduction-contaminated subcontinental lithospheric mantle beneath the Alborz magmatic belt. Then, the shoshonitic melt generates the entire spectrum of KDBS rocks through assimilation and fractional crystallization during the ascent of the magma.

Disequilibrium dihedral angles in dolerite sills: A new proxy for cooling rate

The geometry of clinopyroxene-plagioclase-plagioclase junctions in mafic rocks, measured by the median dihedral angle Θcpp, is created during solidification. In the solidifying Kilauea Iki (Hawaii) lava lake, the wider junctions between plagioclase grains are the first to be filled by pyroxene, followed by the narrower junctions. The final Θcpp, attained when all clinopyroxene-plagioclase-plagioclase junctions are formed, is 78° in the upper crust of the lake, and 85° in the lower solidification front. Θcpp in the 3.5-m-thick Traigh Bhan na Sgurra sill (Inner Hebrides) is everywhere 78°. In the Whin Sill (northern England, 38 m thick) and the Portal Peak sill (Antarctica, 129 m thick), Θcpp varies symmetrically, with the lowest values at the margins. The 266-m-thick Basement Sill (Antarctica) has asymmetric variation of Θcpp, attributed to a complex filling history. The chilled margins of the Basement Sill are partially texturally equilibrated, with high Θcpp. The plagioclase grain size in the two widest sills varies asymmetrically, with the coarsest rocks found in the upper third. Both Θcpp and average grain size are functions of model crystallization times. Θcpp increases from 78° to a maximum of ∼100° as the crystallization time increases from 1 to 500 yr. Because the use of grain size as a measure of crystallization time is dependent on an estimate of crystal growth rates, dihedral angles provide a more direct proxy for cooling rates in dolerites.

MULTIPLE SILLS OR A LAYERED INTRUSION? TIME TO DECIDE

Below many old continental flood basalt provinces typically there are contemporaneous sills, which are commonly quite thin. Some sills may show internal chilled margins that suggest repeated injection, but only once the previous sill had largely solidified and cooled. Much rarer are very thick intrusive bodies that have cooled sufficiently slowly to allow a layered complex to develop. It is generally accepted that they are the result of multiple intrusions that occurred prior to a significant degree of crystallization of the previous magma body. The Karoo Large Igneous Province and the Bushveld Complex are archetypal examples in South Africa of these processes. Both have volumes in excess of 10 6 km 3 . A consideration of the rate of cooling and crystallization of intrusions may provide an answer as to why the two different products result. Of 160 accurately measured sill thicknesses in the Karoo (from borecore information) relatively few exceed 50 m, and some may be composite. A 50 m thick basic sill will become 50% solidified in 10 years. The possibility of a second injection into the centre of that body in less than 10 years is generally unlikely. For a 100 m thick sill, the time period for the same degree of cooling is 37 years. Once the first sill has cooled by 100°C and become 50% crystalline, a second injection will chill, and form a distinct body within the first. It will not inflate the magma portion of the first sill. Average calculated emplacement rates for both the Karoo dolerites and the Bushveld Complex are far too slow for a layered complex to have developed. However, if there were three or perhaps four injections of magma about 100 m thick (or perhaps more than five injections of 50 m thickness) within 100 years into the same body, then the rate of cooling of that composite body could become sufficiently slow that it would act as a trap for any subsequent magma injection and a thick layered intrusion would result. Intensely pulsed volcanic episodicity in igneous provinces is beginning to be identified with increasing eruption intensity and decreasing time lapses, based on extremely precise measurements of palaeomagnetic directions. Hence, there need be no fundamentally different mechanism for the origin of the two different resultant igneous suites, merely a statistical peak in magma injection rate for a short period of time.

Comparable study on the effect of errors and uncertainties of heat transfer models on quantitative evaluation of thermal alteration in contact metamorphic aureoles: Thermophysical parameters, intrusion mechanism, pore-water volatilization and mathematical equations

Abstract By using a 15 m thick, well-dated basic sill as an example, this study presents an investigation on the effect of errors in thermophysical parameters of igneous intrusions and over-simplification in heat transfer equations as well as uncertainties in pore-water volatilization and intrusion mechanisms of magma on quantitative evaluation of thermal alteration in contact metamorphic aureoles based on heat transfer models and the EASY%Ro model. Our results indicate: 1) Using an under-estimated and temperature-independent specific heat (e.g. ~ 800 J kg− 1 °C− 1) of igneous intrusions instead of the temperature-dependent specific heat can cause the maximum deviation of 68 °C in peak temperature and of 1.3% in vitrinite reflectance, whereas such deviations (less than 26 °C and 0.1%) are small enough to be ignored for the model using a high, invariable specific heat (e.g. ~ 1200 J kg− 1 °C− 1) of igneous intrusions. The specific heat of igneous intrusions is likely a more notable error source in causing model deviations compared to the intrusion temperature and the latent crystallization heat of melted magma, if the latent crystallization heat is allowed for; 2) the effect of the uncertainty in whether pore water volatilized during cooling of intrusive magma on the model prediction can be as notable as that of the use of an under-estimated specific heat. The over-simplification in heat transfer equations (i.e. the variation of thermal conductivity with spatial location is not used) results in an obvious underestimation of the thermal effect of igneous intrusions on host rocks. The model deviation caused by this type of error may be far beyond that caused by the errors in thermophysical parameters of igneous intrusions or the uncertainty in pore-water volatilization; 3) the model deviations due to the errors in the thermophysical parameters and the uncertainty in pore-water volatilization can be large enough to disturb the estimation of the intrusion mechanism based on heat transfer models. By comparing the vitrinite reflectance predicted by different heat transfer models with the measured one, the results from more than one type of model can be found to match well with the observation. Only the model of the finite-time intrusion mechanism can be reasonably regarded as representing natural conditions. It is not fully reliable to validate the availability of the used heat transfer models only by observing whether the model results match with measured geothermometers due to the effect of these uncertainties, errors and over-simplification. More careful model specification and parameter estimation are required in the future use of heat transfer models of igneous intrusions.

The influence of igneous intrusions on the peak temperatures of host rocks: Finite-time emplacement, evaporation, dehydration, and decarbonation

Using a 13-m-thick basic sill and its limestone host rocks of the Permian Irati Formation from the Parana Basin, South America, as an example, this paper presents a numerical investigation based on heat conduction models on the effect of the emplacement mechanism of igneous intrusions, pore-water evaporation, and dehydration and decarbonation of host rocks on the peak temperature ( T peak) of host rocks. Our results demonstrate that: (1) the finite-time intrusion mechanism of magma can lower the predicted T peak of host rocks by up to 100 °C relative to the instantaneous intrusion mechanism, and although pore-water evaporation together with dehydration and decarbonation reactions can also depress the thermal effect of the sill on its host rocks, the maximum effect of these mechanisms on T peak only reaches approximately 50 °C. (2) The effect of pore-water evaporation on T peak is obviously greater than that of the dehydration and decarbonation reactions: the former can cause a maximum deviation of 40 °C in the predicted T peak, whereas the deviation due to the latter is less than 20 °C. Further, the effect of the dehydration and decarbonation reactions on T peak is less than 10 °C if pore-water evaporation is allowed simultaneously in the models and can hence be ignored in thermal modeling. (3) The finite-time intrusion mechanism of magma probably represents the natural condition of the sill. Pore-water evaporation and dehydration and decarbonation of host rocks are also likely to play important roles in lowering the thermal effect of the sill.

Numerical investigation of the effect of volatilization and the supercritical state of pore water on maturation of organic matter in the vicinity of igneous intrusions

This study presents a numerical investigation of the effect of volatilization and the supercritical state of pore water on maturation of organic matter in host rocks based on the heat flow models assuming the instantaneous and finite-time intrusion mechanisms of magma. A 15 m thick, well-dated basic sill in the DSDP 41-368 hole near Cape Verde Rise, eastern Atlantic is selected as an example due to the sufficient thermophysical parameters of rocks and the definite burial and thermal history of the shale host rocks. Results indicate: (1) The effect of the temperature-dependent thermal properties of pore water at a hydrostatic pressure of 414 bar on the predicted vitrinite reflectance ( R r) is less than 0.1% no matter which intrusion mechanism of magma is assumed and can hence be ignored reasonably; (2) The consideration of volatilization of pore water can reduce the predicted R r of host rocks significantly. In case of the instantaneous intrusion mechanism, the maximum deviation of the predicted R r caused by pore-water volatilization reaches 1.3% at the location of half the sill thickness away from the contact (i.e. X / D = 0.5), and the deviation above 0.5% can occur in the region from 0.3 to 1.0 in the form of X / D . In case of the finite-time intrusion mechanism, the maximum deviation of the predicted R r due to pore-water volatilization attains 1.15% at X / D = 0.25, and the region where the deviation is larger than 0.5% lies between 0.15 and 0.6 in the form of X / D ; (3) If hydrothermal convection in the host rocks is allowed for, the predicted R r of the overlying host rocks is less than that of the underlying host rocks at the same X / D in the inner region of the contact aureole of igneous intrusions, whereas the phenomenon is converse in the outer region. In contrast, the measured R r profile shows that at the same X / D , R r of the overlying host rocks is totally higher than that of the underlying host rocks. Thus, it is not the hydrothermal convection in the overlying host rocks that resulted in the asymmetry of the current R r profiles below and above the sill; (4) The predicted R r based on the heat conduction model assuming the finite-time intrusion mechanism and pore-water volatilization matches well with the measured one out of the region where the R r geothermometer is unreliable due to the effect of volatilization of pore water. This demonstrates that the finite-time intrusion mechanism of magma, together with pore-water volatilization, possibly represents natural conditions. ► The temperature dependence of thermal properties of pore water can be ignored. ► Volatilization of pore water can reduce the predicted R r significantly. ► Hydrothermal convection in host rocks can not result in the present R r profile. ► Finite-time intrusion and pore-water volatilization represent natural conditions.

Quantifying 3D crystal populations, packing and layering in shallow intrusions: A case study from the Basement Sill, Dry Valleys, Antarctica

In many shallow level intrusions, complex crystal layering structures can be preserved due to pulses of intrusion and magma batches that arrive with a significant crystal payload. In this study, we investigate crystal layering within the Dais intrusion of the Basement Sill, Dry Valley, Antarctica. A single three-dimensional (3D) Rhythmic layer is imaged using X-ray computed tomography (CT) to successfully measure the phase distributions of pyroxene and plagioclase through the rock, and a combination of textural quantification in 2D and the 3D high-resolution data allows the texture to be further broken down into the key components: orthopyroxene (OPX), cumulate OPX, clinopyroxene (CPX), and plagioclase (PLAG). The texture was formed by an initial OPX cumulate framework of 37%–47%. The OPX in the cumulate overgrew by an additional 7%–10%; with the concomitant growth of CPX and PLAG in the interstices, a mixture of fine plagioclase microcrystals and melt were expelled into the upper layer. Textural development in the upper layer was arrested by the rapid growth of OPX and CPX dendrites forming oikiocrysts containing the initial microcrystal population of existing PLAG nuclei, along with plagioclase overgrowths in the areas where the pyroxenes are absent. Calculations of liquid and solidification development at the distance the sample is from the sill contact suggests that the textures in this part of the sill developed over ∼140 yr or less.

Sequence of Early Precambrian metamorphic events in the southern Prince Charles Mts., Eastern Antarctica

The results of geological study of the mountain framework of the southern part of the Lambert Glacier, Mawson Escarpment, Eastern Antarctica, are discussed. The studied territory is of key importance for understanding the regional geological history. The Ruker and the Lambert rock complexes have been distinguished at the Mawson Escarpment. The former is subdivided into the Mawson and Menzies groups. The polymetamorphic rocks of the Mawson Group comprise granite gneiss, orthopyroxene gneiss, and crystalline schists dated at >3000 Ma combined with tectonic wedges and blocks of the variegated sequence with ultramafic (komatiitic) rocks. The find of those rocks allows us to suggest that an ancient granite-greenstone domain existed in the territory of the Prince Charles Mts.; this domain is retained only as tectonic wedges amongst granite gneisses of the Mawson and Menzies groups composed of polymetamorphic terrigenous rocks with basic sills. The following sequence of metamorphic mineral assemblages in the Menzies Group has been established: (1) And-Crd ± St, (2) Ky-St-Grt-Bt-Ms, (3) Sil-Grt-Crd. The andalusite-type metamorphism of rocks pertaining to the Menzies Group probably has the same age as greenschist metamorphism of rocks belonging to the Collaboration Group (2917 ± 82–2878 ± 65 Ma at Mt. Ruker). The formation of kyanite-staurolite mineral assemblage (mounts Stinear, Maguire, Rymill; South Mawson Escarpment) might be related to a metamorphic event dated at 2400–2350 Ma. The formation of sillimanite-garnet and sillimanite-cordierite assemblages with staurolite relics correlates in time with emplacement of the MacColly granite 600–500 Ma ago. Polymetamorphic rocks of the Lambert Complex are migmatites and gneisses, often with orthopyroxene relics. Blocks of ultramafic rocks are localized amongst granite gneisses. The superimposed metamorphism of amphibolite and granulite facies took place 1800 Ma ago. The model Nd age of ultramafic rocks (2500 Ma) is treated as the time of emplacement of magma into the rocks of the Lambert Complex. Isotopic and geochemical evidence for Early Paleozoic granulite-facies metamorphism is known.