Trap Intrusions Research Articles

Paramagnetism of Rock-Forming Dolerite Minerals of Intraformational Trap Intrusions in West Yakutia as the Basis for Dolerite Correlation

This paper shows that the mineral composition of dolerites affects their specific paramagnetic susceptibility. Two methods for calculating the specific paramagnetic susceptibility were proposed: one used the thermomagnetic analysis (TMA) curve and the other the Mi(B) curve. There was no significant difference between the results obtained with these two approaches. Application of the proposed methods to dolerite core samples showed that specific paramagnetic susceptibility can be used as the basis for correlation of sills. The composition of titanomagnetites in dolerites can be used as an accessory parameter.

Signs of the record of geomagnetic reversal in permian–triassic trap intrusions of the Ergalakhsky complex, Norilsk region

Alternating intervals of normal and reversed polarity are revealed in the sections of two Permian–Triassic trap intrusions of the Ergalakhsky complex, Norilsk region. The near-contact zones of the intrusions are magnetized reversely, whereas magnetization in the central zones has normal polarity. The arguments are presented that this change in the polarity along the intrusions section is not due to the postmagmatic remagnetization or self-reversal of remanence but marks the reversal of the geomagnetic field that occurred during the emplacement of the intrusive bodies. Highly accurate age determination for the Ergalakhsky intrusions – the oldest intrusive trap complex in the Norilsk region – is vital for time correlation of the initial stage of magmatic activity. According to the paleomagnetic data, the studied sills intruded directly at the Permian–Triassic boundary at the very end of the Ivakinsky stage. The existing estimates for the durations of the reversals indicate that the cooling of the intrusions could last a few thousand years. In the future, the examined sills of the Ergalakhsky complex can be used as a unique object for exploring the structure of the geomagnetic field during the reversals, for reconstructing the thermal history of intrusions’ cooling, and as a reference for estimating the total duration of trap magmatism.

Signs of the Record of Geomagnetic Reversal in Permian–Triassic Trap Intrusions of the Ergalakhsky Complex, Norilsk Region

Alternating intervals of normal and reversed polarity are revealed in the sections of two Permian–Triassic trap intrusions of the Ergalakhsky complex, Norilsk region. The near-contact zones of the intrusions are magnetized reversely, whereas magnetization in the central zones has normal polarity. The arguments are presented that this change in the polarity along the intrusions section is not due to the postmagmatic remagnetization or self-reversal of remanence but marks the reversal of the geomagnetic field that occurred during the emplacement of the intrusive bodies. Highly accurate age determination for the Ergalakhsky intrusions – the oldest intrusive trap complex in the Norilsk region – is vital for time correlation of the initial stage of magmatic activity. According to the paleomagnetic data, the studied sills intruded directly at the Permian–Triassic boundary at the very end of the Ivakinsky stage. The existing estimates for the durations of the reversals indicate that the cooling of the intrusions could last a few thousand years. In the future, the examined sills of the Ergalakhsky complex can be used as a unique object for exploring the structure of the geomagnetic field during the reversals, for reconstructing the thermal history of intrusions’ cooling, and as a reference for estimating the total duration of trap magmatism.

Lithology, organic geochemistry, and petroleum potential of the northern areas of the Kureika syneclise

We present results of lithological and geochemical studies of Paleozoic deposits stripped by drilling within the Ledyanskaya area in the north of the Tunguska syneclise. The studied section has a terrigenous sulfate-carbonate composition and is complicated by trap intrusions. We have established that the Ordovician-Devonian sedimentation proceeded within an epicontinental basin, with its depth varying from supralittoral to lower sublittoral zones. In the Carboniferous and Permian, coastal-continental sedimentation was predominant; in the Late Permian it was accompanied by volcanic activity. Analysis of the poroperm properties of rocks has distinguished three potential oil reservoirs: Silurian reef carbonate deposits and Ordovician and Carboniferous sand horizons. The Lower Silurian argillaceous-carbonate rocks and Devonian carbonate-sulfate-clay members, halite beds, dolerite bodies, and tuffaceous rocks are probably confining beds of these reservoirs. Geochemical study of organic matter has shown several possible sources of hydrocarbons: Upper Proterozoic deposits, Lower and Middle Cambrian Kuonamka complex, Middle Devonian Yukta Formation, and Upper Paleozoic coal-bearing deposits.

Paleomagnetism of trap intrusions, East Siberia: Implications to flood basalt emplacement and the Permo–Triassic crisis of biosphere

Despite decades of concentrated studies, large igneous provinces (LIPs) are still among the most enigmatic phenomena of the Earth evolution. Among the myriad questions facing researchers, the duration of eruptive magmatic activity and its distribution through time are probably the most mysterious ones. Although the accuracy of modern dating techniques offers relatively high resolution, it is impossible to discriminate whether magmatic activity was more or less uniform or concentrated in few powerful outbursts. Consequently, many “trap-related” questions, from their origin to possible impact on the biosphere, still cannot be answered. Some of the above questions can be addressed with paleomagnetic data as have been effectively done in the case of the end-Cretaceous Deccan traps. The Siberian traps are one of the largest LIPs in the Phanerozoic and are thought, by some, to be the triggering event for the end-Permian mass extinction. We conducted a paleomagnetic study of trap intrusions along two long profiles in the eastern part of the province and found that most site-mean directions are grouped several times tighter than is common for the data on thick lava series and dyke swarms; similar over-grouping is found in other parts of the Siberian trap province too. We argue that this phenomenon is the most likely related to nearly simultaneous emplacement of trap intrusions over areas of several ten to few hundred kilometers in dimension for few millenniums or even faster. Moreover, such brief events account for all, or nearly all, magmatism in each area; how such events correlate over the entire Siberian LIP, however, remains an open question. Still, our data together with paleomagnetic results on thick lava series in Siberia strongly indicate that magmatic activity consisted of a number of brief pulses. In comparison to the other LIPs, the Siberian one is underlain by the thickest succession of carbonates, evaporites and coal-bearing clastics that are capable of producing huge amount of green-house and poisonous gases. We hypothesize that these very fast injections of magma into thick underlying sediments and the unique geological setting of the Siberian LIP are the major reasons for the Permo–Triassic mass extinction being the strongest in Earth history.

Native iron (–platinum) ores from the Siberian Platform trap intrusions

Native iron occurs in two forms: terrestrial (telluric) and meteoritic. The former was considered to be poor in trace elements, whereas the latter contains varying amounts of Ni–Fe alloys, including kamacite, tanite and awaruite. In the late 1950s, native iron was discovered in the Siberian Platform trap intrusions, although it is only recently that geologists became interested in native iron from the traps. We provide a brief description of intrusions containing native iron and a detailed description of its occurrence, species, mineralogy and geochemistry of ore-forming and accompanying typomorphic chemical elements. A study of Siberian native iron shows wide variations in composition from Ni-free iron to awaruite and its intergrowth with a number of minerals. Globules of glass with a ‘liquid-in-liquid’ structure were recognised in native iron and analysed. Immiscible liquids have the compositions of ferruginous and high-silicon melts which are similar to that of ferrogabbro and granophyre. Isotopic compositions, such as 4He/3He, δ13CPDB, 87Sr/86Sr, 87Rb/86Sr of native iron and host-rocks suggests participation of continental-crust material in ore formation. Native iron was found to contain high concentrations of Cu, Ni, Co, Pt, Pd, Rh and Ge. The existence of a new Fe (–Pt) type of ore is established for the first time. Native iron and related Fe (–Pt) ores formed in feeder columns as a result of the interaction of tholeiite-basalt melt with fluid reservoirs of sedimentary rocks in the platform cover. Hydrocarbon fluids and organometallic compounds play a leading role in ore formation. The finding of Fe (–Pt) ores open up new possibilities for discovering new types of ores in large igneous provinces, in which hydrocarbons are of major importance.

The first 40Ar/39Ar age data for trap intrusions of the western part of the Siberian Platform

Based on geological and age data it is known [1, 2,etc.] that basite (trap) magmatism at the Siberian Platform was manifested during the wide time intervalfrom the Proterozoic to the Mezozoic. It has beenestablished that the maximum volumes of lava extrusions, tuffogenic formations, and subvolcanic intrusions formed during the short time interval at theboundary between the Permian and Triassic periods[3–5, etc.]. The maximum number of age datings,based on K–Ar and other modern methods was madefor the northern part of the Siberian Platform, wherelarge manifestations of trap magmatism of differentfacies and associated industrial copper–nickel sulfideore deposits are located [6, etc.]. It should be notedthat idea of the shorttime interval of intrusion of trapsinto the deposits of the platform cover and extrusion oflavas on the Earth’s surface has not been confirmedrecently by new age dating. In particular, the majorityof U–Pb age data from different zircon groups of theintrusions from the Norilsk region fall into the intervalof 230–270 Ma. In the same place, the less distributedgroups of zircons form two more ancient age clustersof ~300 and ~340 Ma [7].In the western part of the Siberian Platform, occupying an extensive area between the rivers of Nizhnyaya Tunguska and Podkamennaya Tunguska, thetrap magmatism and the age sequence of its formationhave been studied insufficiently. Structurally, this areacorresponds to the Tunguska syneclise, made up ofterrigenious–carbonate deposits of the platform coverand manifestations of the trap magmatism of differentfacies (Fig. 1).V.S. Sobolev [8] was the first to take into accountthe wide development of intrusive traps in the basin ofthe Nizhnyaya Tunguska River. Further, intrusivemagmatism of this extended territory has been studiedduring geological prospecting and thematic works.Findings of ore “boulders” and fragments of ore bodies of the Norilsk type, as well as recent geologicalprospecting works in this area, have raised interest inthe age sequence of the development of oremagmaticsystems associated with magnesian traps of the MiddleYenisei Province [9].The most complete information on wide development of trap intrusions in the thick sedimentary cover(4.2 km) of the Siberian Platform was obtained as aresult of geologicalprospecting works for oil and gas.

Permian ultramafic-mafic magmatism and accompanying Cu-Ni mineralization in the Gobi-Tien Shan belt as a result of the Tarim plume activity

The paper reports data on the age, composition, and geodynamic environments of Permian ultramafic-mafic complexes in the Zaisan-Gobi zone in the eastern Gobi-Tien Shan rift belt. The synchronous formation of these complexes with Early Permian lava alkali and tholeiitic basalts discovered beneath the cover of the Tarim and Junggar platform blocks suggests that they are derivates of the Tarim plume related to the Permo-Triassic Siberian superplume. This is evidenced from the composition and metallogeny of the complexes similar to those of magnesian trap intrusions on the western periphery of the Siberian Platform. In the area of influence of the Tarim plume, including the Zaisan-Gobi zone and other structures of the Gobi-Tien Shan belt, diverse Cu-Ni-PGE, Ni-Co-As, Au-Hg, Cu-Mo, and ore deposits related to plume magmatism formed. Their composition, formation conditions, and relationship with magmatism are discussed.

A note on evanescent behavior of Arctic thermohaline intrusions

Temperature/salinity interleaving is a signature of thermohaline transition in the Arctic Ocean. These interleaving features, or intrusions, are observed to decrease in amplitude as they spread laterally from warmer toward cooler water. Here this phenomenon is investigated by considering the effect of a nonlinear equation of state on intrusion structure and behavior. The analysis shows that large-scale gradients of the thermal expansion coefficient (a) can induce a spatial decay of intrusion temperature, salinity, and velocity amplitudes toward cooler water. Spatial decay implies a recirculating flow between adjacent layers, which induces a slow vertical propagation of the intrusions. The temperature-dependence of a provides a mechanism which may act to trap intrusions in the vicinity of warm Arctic boundary currents, inhibiting ventilation of cooler waters.

Palaeomagnetism of East Siberian traps and kimberlites: two new poles and palaeogeographic reconstructions at about 360 and 250 Ma

SUMMARY We studied basalt sills and dykes and kimberlite pipes from the eastern part of the Siberian platform. These are spread out over a few hundred kilometres and are divided into two age groups: Late Permian–Early Triassic (Mir, Aikhal and Olenyek regions) and Late Devonian–Early Carboniferous (Viluy and Markha basins, Aikhal region). The palaeomagnetic poles determined for each sill or dyke are statistically different from each other, but averaging all corresponding directions from these sills and dykes with those from kimberlites of different generations yields results with acceptable statistics. This is probably due to the averaging out of secular variation and /or changes in magnetic polarity. The resulting palaeomagnetic poles allow one to reconstruct the palaeopositions of Siberia during the two large flood basalt events at about 360 and 250 Ma. A significant rotation of the Siberian platform during the Devonian and Carboniferous is suggested. The new data are in general agreement with some earlier palaeomagnetic poles for the Late Permian–Early Triassic traps of the Tungusska basin, and significantly reduce uncertainties for previous Late Devonian–Early Carboniferous Siberian poles, which were very scattered. Together with geological and absolute age data, the palaeomagnetic data support the hypothesis that kimberlite magmatism and trap intrusions were both rapid and relatively close in time. The 360 Ma event is emerging as a magmatic event with considerable extent over the Russian and Siberian platforms, related to incipient continental break-up and triple junction formation, possibly at or near the time of the Frasnian–Famennian mass extinction.

Estimating formation conditions of ore-bearing trap intrusions (example of Noril'sk I intrusion)

Estimating formation conditions of ore-bearing trap intrusions (example of Noril'sk I intrusion)

MAFIC MINERALS IN THE DIFFERENTIAL TRAPROCK INTRUSIVES OF THE NORIL'SK REGION

This paper gives a description of crystal optics of olivines, clinopyroxenes and orthopyroxenes from differentiated trap intrusions of the Noril'sk region. The Fe-content of these minerals increases during differentiation from-the picrite horizon up to the uppermost differentiation products. In the contaminated lower layer the Fe-content increases likewise. At the initial stages of crystallization of every rock the formation of pyroxenes high and low in Ca occurs simultaneously, but in the final stage only pyroxenes low in Ca remain stable, indicating an exhaustion of Ca in the melt. --auth. English summ.