Sk Area Research Articles

Results of New Field and Geochemical Studies of the Volcanic and Intrusive Rocks of the Maymecha-Kotuy Area, Siberian Flood-Basalt Province, Russia

The Maymecha-Kotuy area, comprising ∼70,000 km2 of the northern part of the Siberian flood-basalt province, is of unusual interest because it appears to be the only such province in the world where high-Ti, alkaline-ultramafic rocks with associated carbonatites predominate over basaltic extrusive and intrusive rocks. New field and geochemical studies of the igneous rocks of this area were initiated with the goals of (1) correlating them with the magmatic formations of the well-studied Noril'sk area and (2) reconstructing the entire magmatic evolution of Siberian flood-basalt volcanism. This report presents the first complete stratigraphic section for the volcanic sequence of the Maymecha River Basin (the most extensive in the Maymecha-Kotuy area), based on flow-by-flow mapping and sample collection. The geochemical and lithologic characteristics of the volcanic and intrusive rocks are thoroughly documented and show an unusually broad range in composition–e.g., SiO2 and MgO contents range from 40 to 70 and from <1 to 38 wt%, respectively. New geochemical data, considered together with earlier paleomagnetic data, indicate that as much as 3000 m of the Maymecha-Kotuy sequence consists of lavas younger than those preserved near Noril'sk. Thus, the combined thickness of the lavas and tuffs that constitute the Siberian flood-basalt province is estimated as ∼6500 m. As at Noril'sk, the volcanic sequence lies with unconformity on sedimentary rocks of the Tungusskaya Series. At Maymecha, the sequence begins with a major, basaltic-tuff unit and an overlying suite of low-Ti, tholeiitic basalts, but the magmatism then became more diverse, with several alternations of low-Ti and high-Ti lavas, and evolved to eruption of high-Ti trachybasalts, melanephelinites, trachyandesites, trachytes, trachydacites, and trachyrhyodacites. The uppermost ∼1400 m of the sequence consists of high-Ti, limburgitic and picritic lavas, as well as Mg-rich (23 to 38 wt%), high-Ti, meymechitic lavas, apparently unique in the world. Graphic evidence of the volcanic nature of the meymechites is presented here for the first time. The numerous dikes and several sills of the Maymecha River Basin are subdivided into seven types, which can be geochemically related to the volcanic sequence and in some cases can be reliably correlated with magmatic rocks of the Noril'sk area. Preliminary indications are that the complex body commonly referred to as the Guli intrusion is, in fact, an intrusive-volcanic complex occupying ∼2000 km2, in which a large, laccolithic intrusive mass of dunitic to peridotitic rocks was breached by a central volcanic edifice from which flowed ankaratrite and picritic ankaratrite lavas. Cutting the ankaratrites and picritic ankaratrites of the complex, but intimately related to it, is a variety of intrusive, alkaline rock types and two carbonatite bodies that may be related to a central feeder zone. We recognize four magma types as products of Siberian flood-basalt volcanism and refer to them herein as low-Ti-I, low-Ti-2, moderate-Ti, and high-Ti. In contrast to the Noril'sk area, where high-Ti magmas constitute <1 vol% of the igneous rocks, their proportion is ∼50 vol% in the Maymecha River Basin. These high-Ti rocks can be subdivided into trachybasaltic, melanephelinitic, and meymechitic rock series; clearly related to these series, judging from trace-element characteristics, are associated trachyandesitic, trachytic, and felsic lavas. In general, the trachyandesitic, trachytic, and felsic lavas appear to be related to the trachybasaltic series through fractionation, but some of the felsic tuffs display characteristics that suggest their relation to the melanephelinitie series. Previous investigators have concluded that meymechitic magma formed by low degrees of mantle-peridotite melting (<7%) at pressures corresponding to depths of 250 to 300 km. The distinctions between the trachybasaltic, melanephelinitic, and meymechitic rock series must relate to different sources and conditions of melting. It is remarkable that no geochemical signature of crustal contamination can be recognized for any of the high-Ti rocks, including the felsic lavas and tuffs. We conclude that the alternation of low-Ti and high-Ti magma types observed in the lower half of the volcanic sequence in the Maymecha River Basin resulted from repetitive expulsion from discrete mantle sources through independent plumbing systems.

A reconnaissance [formula omitted] geochronologic study of ore-bearing and related rocks, Siberian Russia

40Ar 39Ar age spectra of biotite from a mineralized vein in the ore-bearing, Noril'sk I intrusion and from picritic-like gabbrodolerite from the weakly mineralized, Lower Talnakh intrusion show that these bodies were emplaced at 249 ± 2 Ma, which is not significantly different from the age of the Permian-Triassic boundary. The ore-bearing intrusions postdate the lower third of the flood-basalt sequence in the Noril'sk area and, on the basis of geochemistry, can best be correlated with lavas slightly younger than those which they cut. Thus, flood basalt was erupted at the time of the Permian-Triassic mass extinction event, although its role in this event is, as yet, ill defined. Additional new 40Ar 39Ar age data for a group of intrusive and extrusive rocks on the western margin of the Siberian craton indicate that mafic magmatism extended over a period of several tens of million years, whereas paleomagnetic data suggest that the bulk of the Siberian flood-basalt sequence near Noril'sk has been erupted in only a million years or so. 40Ar 39Ar ages of plagioclase from early flood-basalt flows are about 2% younger than those obtained for biotite from the crosscutting, Noril'sk I intrusion, probably because of slight alteration and Argon loss from the plagioclase.

Paleomagnetism of the Siberian Flood Basalts of the Noril'sk Area: A Constraint on Eruption Duration

The volcanic sequence of the Noril'sk area, northern Siberia, provides the most complete section of early Siberian flood-basalt volcanism. Paleomagnetic measurements for more than 4000 samples of lava and tuff indicate that nearly all of this >3500-m-thick sequence was laid down during one interval of normal magnetic polarity. Lavas of the lower third of this sequence are cut by the ore-bearing Noril'sk-I intrusion, which has an age of 251 Ma, identical to that of the Permian-Triassic boundary. Thus, the normal-polarity interval represented by this sequence is inferred to be the first of the Triassic Period. Eruption of this enormous volume of material in a relatively brief period coincident with the earth's greatest mass extinction requires that all aspects of Siberian flood-basalt volcanism be evaluated carefully as possibly contributing to that catastrophe.

Isotopic and trace-element constraints on mantle and crustal contributions to Siberian continental flood basalts, Noril'sk area, Siberia

We present a tightly controlled and comprehensive set of analytical data for the 250-Ma Siberian flood-basalt province. Consideration of major- and trace-element compositions, along with strontium, lead and neodymium isotopic compositions, strongly supports earlier Russian subdivision of this magmatism into three magmatic cycles, giving rise to three assemblages of eleven basalt suites in the ascending order Ivakinsky-Gudchikhinsky, Khakanchansky-Nadezhdinsky and Morongovsky-Samoedsky. Geochemical and isotopic discontinuities of varying magnitude characterize most of the boundaries between the eleven recognized basalt suites in the Noril'sk area. Although we conclude that the dominant volume of erupted magma originated from an asthenospheric mantle plume, none of the lavas is interpreted to directly represent asthenospheric melts, which would have been far more magnesian. On the basis of thermal considerations, we consider it unlikely that vast volumes of basaltic melt were produced directly from the continental lithospheric mantle beneath the Siberian craton. Moreover, there is little evidence from mantle xenoliths that the geochemical signatures of such melts would correspond to those of the Siberian flood basalts. Studies of melt migration lead us to conclude that transport of asthenospheric melt through the lithospheric mantle would be rapid, by fracture propagation. Lavas from the Gudchikhinsky suite have negligible Ta-Nb anomalies and positive ϵ Nd values and their parental magmas presumably interacted little with the continental lithospheric mantle or crust. All other lavas have negative Ta-Nb anomalies and lower ϵ Nd values that we attribute to interaction with continental crust. The model that we have developed requires discrete contributions from the plume and complex processing of all erupted magmas in the continental crust. The earliest magmas represent small percentages of melt formed in equilibrium with garnet. Over time, the percentage of melting in the source region and the volume of magma produced increased, and garnet was no longer stable in the plume source. All of the plume-derived melts initially contained more than 20 wt% MgO and became less Mg rich by fractionation of olivine as they traversed the lithospheric mantle. We conclude, however, that the most significant control on the geochemical and isotopic compositions of all the erupted lavas was processing of mantle-derived magma in crustal reservoirs during periodic replenishment, periodic tapping, continuous crystal fractionation and wallrock assimilation. Rapid eruption of an extremely large volume of processed magma that varied little in chemical and isotopic composition produced the sequence of relatively monotonous tholeiitic basalts that constitute the 2,300-m-thick third assemblage of the Siberian flood-basalt province near Noril'sk.

Siderophile and chalcophile metals as tracers of the evolution of the Siberian Trap in the Noril'sk region, Russia

In this study Cu, Ni, and platinum-group elements (PGE) were determined in a sequence of basaltic and picritic lavas from the Siberian Trap in the Noril'sk area of Russia to constrain genetic relationships between the basalts and the petrogenesis of Ni-Cu-PGE sulfide deposits associated with the Talnakh and Noril'sk intrusions. In the most primitive basalts (8–19 wt% MgO) of the Tuklonsky (Tk) suite, Pt and Pd concentrations range from 4–13 ppb, increasing with decreasing MgO content; whereas Ir contents decrease with MgO from 0.8-0.05 ppb. The contrasting behavior of these elements, which all have very high sulfide-silicate partition coefficients, as well as the primitive mantle-like ratios of Cu/Y and Pd/Y, suggests that these magmas were not sulfide-saturated. The high PGE abundances imply that their parental magmas were also not sulfide saturated during partial melting in the mantle. Due to sulfide segregation, the overlying basalts of the Nadezhdinsky (Nd) series are low in Cu and Ni (52 and 38 ppm, respectively); highly depleted in all PGE; and have very low Cu Y , Pd Y , and Pd Cu ratios. However, in stratigraphically higher levels, Cu, Ni, and PGE concentrations increase systematically through the Morongovsky (Mr) suite to reach a concentration plateau in the uppermost Mokulaevsky (Mk) suite (Pt 8 ppb; Pd: 9 ppb; Ir: 0.12 ppb; Rh: 0.4 ppb). At the same time, ratios such as Cu Y increase and approach primitive mantle values. However, ratios involving PGE, such as Pd Y , remain low, suggesting the removal of small amounts of sulfide (0.01–0.03%). The compositional variations in the basalts and the sulfide liquids can be quantitatively described by fractional segregation of a sulfide liquid in an open- or closed-system magma chamber. The latter model suggests that the basalts represent the eruption products of a zoned magma chamber in which light magma, with crustal components contaminated, overlies less contaminated, denser magma. Crustal contamination caused sulfide saturation, and the resulting sulfide liquids settled through a magma column and accumulated at the bottom of the chamber. In this model, the sulfide liquid is not in equilibrium with the whole magma mass, and sulfide segregation is compared with the zone-refining process of metallurgy. The sulfides become more enriched as they move through the magma; and although the magma left behind is depleted in PGE, Cu, and Ni, their concentrations also increase with depth. Eventually, the magma chamber is emptied from the top to the bottom, producing the flood basalt sequence and the associated intrusions and ore deposits. In the open-system model, sulfide saturation was initially caused by assimilation of crustal material by the Tuklonsky magma. Continuous and simultaneous replenishment, assimilation, and crystallization processes formed the lower Nd lavas. The concurrent removal of 0.5-1% sulfide strongly depleted these magmas in chalcophile and siderophile metals. Due to the continuous replenishment of the magma chamber with uncontaminated PGE-rich magma, succeeding lavas (Mr, Mk) show diminishing signs of crustal contamination and become less sulfide-saturated, as indicated by the increasing Ni, Cu, and PGE abundances. During the evolution of the chamber, the magma remained sulfur-saturated, and sulfides accumulated at the base. The composition of the sulfide ores could be regarded as a mixture consisting of low Ni-, Cu-, and PGE-sulfides derived with a low silicate/sulfide ratio (100) from the Tk-Nd magma and high Ni-, Cu-, and PGE-sulfides formed with a high ratio (10,000) from the Mr-Mk magma.

Geochemistry of the Siberian Trap of the Noril'sk area, USSR, with implications for the relative contributions of crust and mantle to flood basalt magmatism

The sequence investigated of the Siberian Trap at Noril'sk, USSR, consists of at least 45 flows that have been divided into six lava suites. The lower three suites consist of alkalic to subalkalic basalts (the Ivakinsky suite), overlain by nonporphyritic basalts (the Syverminsky suite), and porphyritic and picritic basalts (the Gudchikhinsky suite). The upper three suites are tholeiitic. The uppermost 750 m of dominantly non-porphyritic basalt belong to the Mokulaevsky suite and are characterized by a nearly constant Mg number (0.54–0.56), SiO2 (48.2–49.1 wt%), Ce (12–18 ppm), and Ce/Yb (5–8). The underlying 1100 m of dominantly porphyritic basalt belong to the Morongovsky and Nadezhdinsky suites. There is a continuous increase in SiO2 (48.1–55.2 wt%), Ce (12–41 ppm), and Ce/Yb (5–18) from the top of the Mokulaevsky to the base of the Nadezhdinsky with little change in the Mg number (0.53–0.59). Mokulaevsky magmas have trace element signatures similar to slightly contaminated transitional type mid-ocean ridge basalts. The change in major and trace element geochemistry in the upper three suites is consistent with a decline in the degree of anatexis and assimilation of tonalitic upper crust by Mokulaevsky magma. The Nadezhdinsky and underlaying lavas thicken within and thus appear to be related to an elongate basin centred on the Noril'sk-Talnakh mining camp. The Mokulaevsky and Morongovsky lavas thicken to the east and appear to be related to a basin centred more than 100 km to the east of the Noril'sk region; these magmas may have risen up out of a different conduit system.

Structure of ore field and morphology of ore bodies in southwestern extension of the copper-nickel deposit at Talnakh (Noril'sk area)

(1973). Structure of ore field and morphology of ore bodies in southwestern extension of the copper-nickel deposit at Talnakh (Noril'sk area) International Geology Review: Vol. 15, No. 9, pp. 1047-1053.

The early phase of effusive traprock magmatism in Noril'sk area

(1973). The early phase of effusive traprock magmatism in Noril'sk area. International Geology Review: Vol. 15, No. 6, pp. 721-725.

Regional Planning Problems in the Soviet Far East

A variety of approaches are being recommended in regional plans for future settlements patterns in the Soviet Far East. In Magadan Oblast, the present scattered pattern of small populated places is to be superseded by greater concentration of urban population in a set of subregional service centers. In Khabarovsk Kray, different approaches are being recommended for the future development of the Khabarovsk and Komsomol'sk areas. Because limitation of the future growth of Khabarovsk is desirable, new industrial establishments are to be located in the future in nearby small towns and urban settlements of the Khabarovsk industrial district. In the Komsomol'sk area, satellite towns are to be significantly developed, giving rise to a grouped form of urban settlement, including the tin-mining center of Solnechnyy and the paper and chemical center of Amursk. In Amur Oblast, consideration is being given to the development of an iron and steel plant in the Svobodnyy area, giving rise to a city of up to 200,000 population.