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Journal of Petrology Volume 41 Number 2 Pages 201-227 2000
© Oxford University Press 2000
Late Devonian Diamondiferous Kimberlite and Alkaline Picrite (Proto-kimberlite?) Magmatism in the Arkhangelsk Region, NW Russia
1DE BEERS CENTENARY (RUSSIA), UL. TVERSKAYA 22A, MOSCOW, 103050, RUSSIA
2DEPARTMENT OF EARTH SCIENCES, UNIVERSITY OF CAMBRIDGE, DOWNING STREET, CAMBRIDGE CB2 3EQ, UK
3DEPARTMENT OF GEOLOGICAL SCIENCES, UNIVERSITY OF DURHAM, SOUTH ROAD, DURHAM DH1 3LE, UK
4INSTITUTE OF ORE DEPOSITS (IGEM), RUSSIAN ACADEMY OF SCIENCES, STAROMONETNY 35, MOSCOW 109017, RUSSIA
5GEOLOGICAL ENTERPRISE ‘ARKHANGELSK GEOLOGY’, TROITSKY PROSPECT 137, ARKHANGELSK, 163001, RUSSIA
Received March 1, 1999; Revised typescript accepted July 14, 1999
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONGEOLOGY OF THE ARKHANGELSK...THE ARKHANGELSK ALKALINE IGNEOUS...PETROGRAPHY AND MINERALOGYGEOCHEMISTRYDISCUSSIONSUMMARYAPPENDIX: ANALYTICAL METHODSREFERENCES |
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Widespread penecontemporaneous igneous activity affected NW Russia (the Kola Peninsula and adjoining areas to the SE around Arkhangelsk) during the Late Devonian (360–380 Ma). Magmatism varies from tholeiitic basalts, erupted in the axial regions of former Middle Proterozoic (Riphean) rifts, to strongly alkaline rock-types on and marginal to Archaean cratons. NNE of Arkhangelsk kimberlites, olivine lamproites and alkaline picrites were emplaced; all these rock-types are diamondiferous to varying extents. Higher TiO2 (and also total Fe) distinguish predominantly mica-poor Eastern Group kimberlites (TiO2 = 2·4–3·1 wt %) and spatially associated alkaline picrites (TiO2 = 3·2–3·7 wt %) from nearby micaceous Western Group kimberlites (TiO2 = 0·8–1·1 wt %). Each rock-type also has distinctive rare earth element (REE) patterns, and
Nd ranges: micaceous kimberlites, (La/Yb)n = 19·1–44·4, Nd = -2·4 to -3·6; olivine lamproites, (La/Yb)n = 76·9, Nd = -4·6 to -4·7; mica-poor kimberlites, (La/Yb)n = 86·3–128·2, Nd = 0·0–2·5; alkaline picrites, (La/Yb)n = 13·1–17·9, Nd = 0·1–1·1. Variations in the petrography and bulk-rock chemistry of the Arkhangelsk kimberlites are superficially similar to South African Group I and II kimberlites. Despite their field proximity, the alkaline picrite REE patterns contrast with those of the kimberlites. Instead, they closely resemble those of ‘protokimberlites’, the hypothetical magmas calculated to have precipitated South African kimberlite subcalcic clinopyroxene, garnet and ilmenite megacrysts at base-of-lithosphere depths (
200 km). Our new data, combined with published studies of Arkhangelsk kimberlites and the silicate inclusions in their diamonds, support a genetic model where protokimberlite magmas separated from sub-lithospheric convecting mantle at several hundreds of kilometres depth. During their uprise through 200 km thick lithosphere, some magma batches dissolved predominantly ilmenite on a minor scale and erupted as mica-poor alkaline picrites and kimberlites. Others reacted wholesale with fusible lithospheric components to produce micaceous alkaline picrites and diamondiferous kimberlites. KEY WORDS: kimberlite; protokimberlite; alkaline picrite; Kola craton; diamondiferous
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONGEOLOGY OF THE ARKHANGELSK...THE ARKHANGELSK ALKALINE IGNEOUS...PETROGRAPHY AND MINERALOGYGEOCHEMISTRYDISCUSSIONSUMMARYAPPENDIX: ANALYTICAL METHODSREFERENCES |
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In both the oceanic and continental environments, it is clear that variations in the ‘local’ (scale comparable with that of the 2000 km diameter plume head) lithospheric thickness will strongly influence the presence, abundance and composition of plume-related magmatism (e.g. Thompson & Gibson, 1991
; Sleep, 1996, 1997). This is because of the sensitivity of both the amount and composition of decompression melts to lithospheric thickness (e.g. White & McKenzie, 1995). Where the lithosphere is thick, the magmatism may (1) be relatively small in volume, and (2) include strongly alkaline rock-types, such as kimberlites, lamproites, nephelinites and melilitites. The highly enriched trace element concentrations of many mafic alkaline igneous rocks require that their contributing parental melts were derived (at least in part) from a metasomatized lithospheric mantle source (e.g. McKenzie, 1989; Gibson et al., 1993, 1995a, 1995b; Thompson et al., 1998).
Tainton & McKenzie (1994) have proposed that the REE patterns of kimberlites and lamproites require a two-stage mantle melting model. They envisaged a lithospheric mantle source, initially depleted by melt extraction and then subsequently enriched in trace elements, before kimberlite genesis. Such models are closely related, in their outcome, to those that postulate that kimberlites begin as melts within the convecting mantle, beneath the lithosphere, and then react with and dissolve lithospheric debris during their uprise (e.g. Haggerty, 1994; Pearson et al., 1995; Nowell & Pearson, 1998). The REE modelling of Tainton & McKenzie (1994) deduced that the kimberlite component derived from convecting mantle (their precursor small-fraction metasomatizing melts) were extracted from depleted upper mantle; the same reservoir as mid-ocean ridge basalt (MORB). Harte (1983) and Jones (1987) approached this matter differently, by analysing the trace elements and Sr–Nd isotopic ratios of the clinopyroxene and garnet megacrysts that occur in some South African kimberlites. They showed that these were not in chemical equilibrium with the kimberlites that contained them. Instead, the megacrysts appeared to have precipitated from alkalic picritic (‘proto-kimberlite’) melts that resembled oceanic alkali–olivine basalts and basanites geochemically and appear to have originated from a primitive rather than a depleted convecting mantle source.
This study is concerned with the Late Devonian Arkhangelsk Alkaline Igneous Province (AAIP), which is itself a subset of widespread penecontemporaneous basaltic to strongly alkaline magmatism in NE Europe (Fig. 1). This igneous activity is believed to have been caused by the sub-lithospheric impact of a Late Devonian mantle plume (Mahotkin et al., 1995, 1997; Parsadanyan et al., 1996; Beard et al., 1998; Marty et al., 1998). The AAIP is situated in the northeast of the East European Platform (Fig. 1) and comprises a wide variety of ultramafic alkaline rock-types, including kimberlites, with a rare olivine lamproite variant, and alkaline picrites. In the middle 1970s richly diamondiferous kimberlite pipes were discovered in the region (Stankovskiy et al., 1977; Stankovskiy et al., 1979) and such discoveries have continued (Verichev et al., 1998). Significantly, some of the alkaline picrites are also diamondiferous. These rocks lack kimberlite macrocrysts and xenoliths and are characterized by pseudomorphs that appear to be after melilitite. We shall suggest that the alkaline picrites resemble geochemically the hypothetical ‘proto-kimberlite’ that has been proposed in previous studies of South African kimberlites (Harte, 1983; Jones, 1987).
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GEOLOGY OF THE ARKHANGELSK REGION |
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TOPABSTRACTINTRODUCTIONGEOLOGY OF THE ARKHANGELSK...THE ARKHANGELSK ALKALINE IGNEOUS...PETROGRAPHY AND MINERALOGYGEOCHEMISTRYDISCUSSIONSUMMARYAPPENDIX: ANALYTICAL METHODSREFERENCES |
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The tectonic structure, age and composition of the basement beneath the AAIP strongly influenced (1) the location of Late Devonian magmatic activity, (2) magma composition and (3) the extent to which it is diamondiferous (Sinitsin et al., 1992
; Sablukova et al., 1995). We shall therefore review the Precambrian and Lower Palaeozoic tectonomagmatic evolution of the Arkhangelsk region before discussing the Devonian magmatism.
Precambrian tectonomagmatic evolution of the Baltic Shield
The East European Platform, a collage of Archaean cratons and Early Proterozoic mobile belts, underlies most of western Russia and the Baltic states. Riphean to Palaeozoic sediments cover most of the platform to the south and east of Scandinavia (Blundell et al., 1992). The Baltic Shield, the NW segment of the platform, has been investigated in considerable detail (e.g. Khain & Bozhko, 1988; Nikishin et al., 1996). In its eastern part there are two Archaean cratons, Kola and Karelia, separated by the Early Proterozoic Belomorian mobile belt (Fig. 1). The cratons are composed of Early Archaean tonalitic gneisses and granulites, surrounded by narrow greenstone belts, containing both sedimentary and volcanic rocks. These units were reworked in the Belomorian belt, with accompanying amphibolite-facies metamorphism, granite emplacement and crustal thickening. The southeastward extensions of both cratons and the Belomorian belt are hidden beneath a Riphean–Palaeozoic sedimentary cover, in the Arkhangelsk area (Fig. 1).
Subsequent development of the Baltic Shield involved the formation of the Middle Proterozoic (Riphean) Kola–Belomorian graben system, an offshoot of the rift system that affected the entire East European Platform (Bogdanova et al., 1996). This extension was crucially important in controlling both the siting and nature of the subsequent Devonian alkaline magmatism; it fragmented the uniformly thick lithosphere of the Baltic Shield in the Kola–Arkhangelsk area into blocks where the lithosphere remained thick, separated by a network of Middle Proterozoic rifts (Fig. 1) where the lithosphere was significantly thinner. In general, the Middle Proterozoic rifts were sited within the Belomorian and other Early Proterozoic former mobile belts but a few (e.g. Leshukonskiy) affected the margins of the cratons. One well-studied graben, Onega–Kandalaksha, provides important information about these rifts in general (Konstantinovskiy, 1977; Salop, 1982; Khain & Bozhko, 1988; Shcheglov et al., 1993). It extends NW–SE from Finland through the head of the Gulf of Kandalaksha to the Onega Peninsula (Fig. 1). Its red-bed dominated sedimentary infilling also includes tholeiitic basalts and ferrobasalts (K–Ar age 1300 Ma; Staritskiy, 1981; Sinitsin et al., 1982). Later Vendian sediments complete the infilling and overlap the graben shoulders. Further Riphean tholeiitic basalt dykes and sills cut the NE margin of the Kola craton, adjacent to another Riphean graben (Staritskiy, 1981; Berkovsky & Platunova, 1989). Within the Kola and Karelia cratons, Riphean magmatism is represented by lamproitic dykes (Proskuriykov et al., 1992), as shown in Fig. 1. Some of these have been recently reclassified as kimberlites (Mahotkin, 1998).
The 1300 Ma tholeiitic basalts are useful in reconstructing the pre-Devonian tectonic framework of the Arkhangelsk–Kola region. Enough is now known about the interrelationships between lithospheric plate thickness, mantle potential temperature and the compositions of basaltic (sensu lato) melts (e.g. McKenzie & Bickle, 1988; White & McKenzie, 1995) to be able to deduce that total lithospheric thickness was reduced by extension to <100 km beneath the Riphean rifts. In a region that has lacked subsequent major compressional tectonic episodes, the thinned lithosphere zones could only have rethickened conductively to 125 km (McKenzie, 1989), a value about half that to be expected (200 km or more; McKenzie, 1989; Pearson, 2000) beneath undisturbed Archaean cratons.
Late Devonian magmatic activity in NW Russia
A short intense, widespread phase of Late Devonian (380–360 Ma) mafic, alkaline-ultramafic and carbonatitic magmatism immediately followed large-scale lithospheric doming of the East European Platform (Staritskiy, 1981; Sablukov, 1984; Kramm et al., 1993; Zaitsev & Bell, 1995; Beard et al., 1996, 1998; Nikishin et al., 1996; Wilson & Lyashkevich, 1996). In the Kola–Arkhangelsk–Timan area (Fig. 1), it was associated with further localized doming, centred on the Kola Peninsula (Nikishin et al., 1996). The style of magmatism varied from the eruption of tholeiitic basalts to the emplacement of large alkaline igneous complexes and relatively small-volume ultramafic alkaline pipes and sills.
Tholeiitic basalts
Late Devonian (Frasnian) tholeiitic lavas and tuffs form a 300 m thick volcano-sedimentary succession (Staritskiy, 1981) that outcrops over an area of 2500 km2 and extends from the Kanin Peninsula to Middle Timan. Outlying tholeiitic plugs of the province are marked in Fig. 2 (Mahotkin et al., 1995; Parsadanyan et al., 1996). Other occurrences of Late Devonian magmatism are found throughout the Timan–Pechora region and along the Murmansk coast of the Kola Peninsula (Staritskiy, 1981; Ishmail-Zadeh et al., 1997). These extensive tholeiitic lavas, referred to as the Timan–Kola flood-basalts by Sinitsin & Kushev (1968), are associated with a major tholeiitic dyke swarm (Berkovsky & Platunova, 1989) that extends 2500 km N–S and links Late Devonian magmatism throughout the East European Platform into a single igneous megaprovince (Wilson & Lyashkevich, 1996).
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Alkaline-ultramafic igneous complexes
The Late Devonian magmatism of the Kola region forms one of the world’s largest intrusive and sub-volcanic alkaline provinces. The Kola Alkaline Province outcrops over an area of 100 000 km2 and comprises 24 igneous complexes (Dudkin & Mitrofanov, 1993; Kogarko et al., 1995). Most of these were intruded during a very short time interval (380–360 Ma) in the Late Devonian (Kramm et al., 1993; Beard et al., 1996, 1998). There are three types: (1) nepheline syenite complexes, e.g. Khibina and Lovozero (Kogarko et al., 1995); (2) alkaline-ultramafic complexes predominantly composed of peridotites, pyroxenites and ijolites, e.g. Kovdor (Zaitsev & Bell, 1995) and Afrikanda; (3) complexes containing significant carbonatite, e.g. Sokli (Vartiainen & Paarma, 1979) and Telyachi Island (Beard et al., 1996). The largest alkaline intrusive complexes were emplaced in the Kontozero graben, Kola Peninsula. Lithospheric extension and sedimentation associated with this major NW-trending rift zone began at 375 Ma (Nikishin et al., 1996). The relatively small-volume carbonatite bearing complexes have a more scattered distribution across the Kola craton (Kramm et al., 1993). Outside the Arkhangelsk area kimberlite diatremes and dykes of kimberlite-like rock-types are known from the Terskii Coast field (Beard et al., 1998; Fig. 1).
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THE ARKHANGELSK ALKALINE IGNEOUS PROVINCE (AAIP) |
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TOPABSTRACTINTRODUCTIONGEOLOGY OF THE ARKHANGELSK...THE ARKHANGELSK ALKALINE IGNEOUS...PETROGRAPHY AND MINERALOGYGEOCHEMISTRYDISCUSSIONSUMMARYAPPENDIX: ANALYTICAL METHODSREFERENCES |
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The small-volume Arkhangelsk igneous activity is mostly in the form of sub-volcanic pipes (diatremes), together with some sills (Sinitsin & Grib, 1995
). Clusters of diatremes form several igneous fields (Fig. 2), each with distinctive petrological characteristics. The fields occur in two groups on the SE side of the White Sea: (1) along and up to 100 km inland from the Zimniy Bereg (i.e. Winter Coast); (2) on the NE side of the Onega Peninsula, separated from the Zimniy Bereg by the Gulf of Dvina (Fig. 1). Table 1 summarizes various features of the AAIP localities discussed in this paper. Although our study is far from comprehensive, the samples come from all the main AAIP igneous fields north of Arkhangelsk and include most of the better-known diamondiferous diatremes (e.g. Sinitsin & Grib, 1995; Parsadanyan et al., 1996). Others have been discussed by Mahotkin et al. (1995, 1997).
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A zone of normal faults with NE–SW trends extends throughout the AAIP. These are not shown in Fig. 2 because, like the Riphean grabens, they are mostly buried beneath Upper Palaeozoic sediments. Both limited crustal thinning and remnants of Upper Devonian to Lower Carboniferous sediments preserved along this zone are considered to be evidence for weak Devonian extension (Grib et al., 1987; Nikishin et al., 1996). Most of the AAIP diatremes are thought to be sited at the intersection of NE–SW faults and the shoulders of former Riphean grabens. Specifically: (1) the Chidvia–Izhmozero alkaline picrites and other rock-types (Table 1) are emplaced at the SW margin of the Keretskiy graben (Fig. 1); (2) most of the kimberlites, lamproites and alkaline picrites of the other Zimniy Bereg fields (Zolotitsa, Mela, Kepino–Pachuga and Verkhotina–Soyana; Table 1) are emplaced in Vendian sediments (Erinchek et al., 1998) that are believed to cover the NE margin of the Keretskiy graben, the west margin of the Leshukonskiy graben and its Padun extension (Fig. 1), and adjacent Archaean Kola cratonic basement—the Ruch’y and Zolotitsa horsts (Sinitsin & Grib, 1995).
Field characteristics of the igneous rocks
The diamondiferous-kimberlite diatremes of the Zolotitsa cluster and the alkaline picrite pipe at Chidvia are typical examples of the Zimniy Bereg pipes. Figure 3 shows a plan and sections of one of these, Pionerskaya (Table 1). Information about these diatremes comes from drillcore samples. The vent deposits consist of either one or two phases of breccia. The first phase comprises diverse polylithic tuffs and breccias, which are often layered and consist of rounded fragments (lapilli) of kimberlites or alkaline picrites, olivine pseudomorphs, xenoliths of country rocks and quartz sand (Sablukov, 1987). The fragments are present in various proportions, giving a range of clastic rock-types from autolithic tuffs to sandstones and sedimentary breccias. Clast sizes in igneous breccias are notoriously difficult to estimate in drillcores. The smallest clasts are encompassed by thin sections (e.g. Fig. 4b) but it is not easy to be sure whether the largest igneous samples are from clasts or later dykes. Subhorizontal microlamination, fragment size-sorting, plant remains (Lower Frasnian) and xenoliths of Early and Middle Palaeozoic sedimentary rocks can be traced down to a depth of 600 m (Sablukov, 1987). The polylithic tuffs and breccias are intruded by kimberlite (or alkaline picrite) breccia that consists of lapilli and altered olivines, cemented by a serpentine–saponite mixture, with additional minor talc, richteritic amphibole and diverse carbonates. The relative volumes of polylithic and autolithic igneous breccias vary amongst the different pipes and within each one.
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Most of the diatremes have well-developed infilled craters that appear to have undergone little erosion. These are trough-like or saucer-shaped and are infilled by flat-lying tuffs and interbedded tuffaceous sediments, sandstones and sedimentary breccias. There are also zones of cross-laminated, fine-grained accretionary lapilli tuff (Sablukov, 1987
). In addition to the main pipes, several 0·5–3 m thick sills of kimberlite, with local carbonate facies, are exposed within Vendian arenites along the River Mela. These closely resemble the Benfontein Sill, South Africa (Dawson & Hawthorne, 1973). |
PETROGRAPHY AND MINERALOGY |
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TOPABSTRACTINTRODUCTIONGEOLOGY OF THE ARKHANGELSK...THE ARKHANGELSK ALKALINE IGNEOUS...PETROGRAPHY AND MINERALOGYGEOCHEMISTRYDISCUSSIONSUMMARYAPPENDIX: ANALYTICAL METHODSREFERENCES |
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Approximately 1200 samples have been collected from the hypabyssal and diatreme crater facies of the pipes in the AAIP together with the Mela sills. Table 1 summarizes the rock-types encountered. They can be classified into two groups: (1) kimberlites (Woolley et al., 1996
), with minor olivine lamproites and carbonatites; (2) alkaline picrites. Sablukova (1995), Sablukova et al. (1995) and Sobolev et al. (1997) have given recent accounts of their included xenoliths and macrocrysts.
Kimberlites
Kimberlites are the predominant rock-type in the Zolotitsa and Mela fields, and a component of the Kepino–Pachuga and Verkhotina–Soyana fields. They can be divided geographically (Fig. 2) into a predominantly mica-poor Eastern Group and a predominantly micaceous Western Group, superficially similar to Group I and Group II South African kimberlites, respectively (Parsadanyan et al., 1996). Nevertheless, as emphasized in the previous section, the AAIP kimberlites are mostly fragmented and hence drastically affected by post-magmatic alteration. Only the Pionerskaya pipe has yielded a massive facies. This occurs in a drill-hole (1490; Fig. 3); between 850 and 1000 m there are several 10–30 m massive kimberlite units amongst the breccias. They may be either exceptionally large clasts or post-breccia dykes.
Western Group kimberlites
Kimberlites in the Western Group are predominantly micaceous (Table 1). Most of the richly diamondiferous diatremes are formed by this rock-type. The freshest samples come from the massive facies of the Pionerskaya pipe. They consist principally of macrocrysts and phenocrysts of fresh olivine, together with microphenocrysts of phlogopite. Subhedral to anhedral olivine macrocrysts range from 2 to 12 mm and form 25–30 modal % of the rock. The macrocrysts have high mg-numbers (92·3–92·6) and Ni contents, and low Ca contents (Table 2), thus resembling olivine macrocrysts from the Udachnaya kimberlite, Yakutia (Sobolev et al., 1989), and olivine inclusions in diamonds (Hervig et al., 1980). Euhedral olivine phenocrysts (0·01–1·2 mm in length) form up to 25% of the rock. They have less-magnesian core compositions (mg-number = 89·9–90·6) and are depleted in Ni and enriched in Ca, relative to the olivine macrocrysts (Table 2). They are zoned, with mg-number increasing and Ni decreasing outwards (Table 2). The smallest crystals are partly or totally altered to serpentine and saponite.
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The modal abundance of phlogopite varies from 15 to 35% of the rock, but in exceptional cases it may constitute up to 60%. Sparse magnesian phlogopite microphenocrysts (up to 0·3 mm), with mg-number 92–93 and low Ti contents (Tables 3 and 4), surround the olivine grains. Phlogopite also occurs as <1·5 mm poikilitic anhedral plates that enclose olivine phenocrysts, perovskite (Table 5) and small crystals of a phase that has been entirely replaced by pectolite and hydroandradite (Table 5). The groundmass phlogopite crystals vary in mg-number from 91·2 to 77 (Tables 3 and 4). Figure 5 is a plot of TiO2 and Al2O3 in AAIP phlogopites, showing their relationship to micas from other kimberlites, mafic ultrapotassic rock-types and mantle xenoliths. This diagram includes microphenocryst phlogopite from the kimberlite clasts of the Arkhangelskaya pipe (Table 4), which illustrate the extensive compositional range of the AAIP kimberlite micas (Parsadanyan et al., 1996).
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A mineral that resembles an alkaline amphibole is present as extremely small crystals in the groundmass (Table 5). Its analysis has relatively high CaO, which may be due to electron beam overlap with adjacent high-Ca phases. Saponite is predominant amongst the secondary groundmass minerals and exhibits a wide compositional range (Table 5). Hydroandradite occurs as inclusions within poikilitic mica and also as fine-grained aggregates in the groundmass. We suspect that hydroandradite and pectolite (Table 5) may have replaced original melilite (but see below) in this rock-type (Skinner et al., 1998). Crystallization of melilite would explain why groundmass phlogopites have lower Al contents than the microphenocrysts (Table 3).
Some of the clasts drilled between 370 and 395 m in the Karpinskiy I diatreme have been classified as olivine lamproites by Mahotkin et al. (1995), using the chemical criteria (see below) of Mitchell & Bergman (1991). They have the same abundant olivine macrocrysts and phenocrysts as the associated kimberlites, in a fine-grained groundmass of red–brown phlogopite, light brown richteritic amphibole and clinopyroxene, lacking both perovskite and hydrogarnet. We emphasize that these rocks are only a very minor and localized part of the AAIP magmatism. The Mela sills are composed of a carbonate-rich variant of kimberlite, comprising serpentinized olivine grains and phlogopite flakes, set in abundant calcite. Locally this becomes carbonatite (Table 1).
Eastern Group kimberlites
Kimberlites in the Eastern Group (Kepino–Pachuga and Verkhotina–Soyana fields) are mica poor, relative to those in the Western Group (Table 1). Phlogopite generally forms 1–7% of the rock and rarely reaches 20%. Perovskite, rutile and Fe–Ti oxides are present as accessory phases in the serpentinized groundmass. Their mineralogy is summarized in Table 1.
The discovery in 1996 of the richly diamondiferous Grib kimberlite pipe (also known as Anomaly 441) has terminated the long-established notion (e.g. Sablukova et al., 1995) that only the AAIP Western Group kimberlites are potentially valuable economically (Verichev et al., 1998; L. Rombouts, personal communication, 1998). The Eastern Group kimberlites are all closely associated with neighbouring diatremes of alkaline picrites (see below).
Alkaline picrites
As the ultramafic diatremes of the AAIP have been discovered and sampled, it has become progressively more apparent that the term kimberlite is inappropriate for all their rock-types, particularly for many in the Verkhotina–Soyana, Kepino–Pachuga and Chidvia groups of diatremes (Fig. 2). Whereas typical AAIP kimberlites (Woolley et al., 1996) are full of rounded olivine macrocrysts, together with other kimberlite indicator minerals (e.g. Kudrjavtseva et al., 1991; Sinitsin et al., 1992; Mahotkin et al., 1995; Sablukova, 1995; Sablukova et al., 1995; Sinitsin & Grib, 1995; Parsadanyan et al., 1996; Griffin et al., 1999) other diatremes contain MgO-rich rock-types that are rich in euhedral or subhedral phenocrysts but poor in rounded macrocrysts. The feature that makes them so difficult to classify is that they contain substantial amounts (up to 40 %) of a former euhedral microphenocryst and groundmass phase that resembles melilite in morphology but has been entirely replaced by aggregates of hydroandradite (± pectolite). As a result, the nomenclature of these enigmatic rocks presents a problem that has to be overcome before their geochemistry and genesis can be discussed without confusion. Their variants have been referred to in several recent publications by such names as olivine melilitite, melilite picrite and melnoite (e.g. Mahotkin et al., 1995; Sinitsin & Grib, 1995; Parsadnayan et al., 1996; Skinner et al., 1998). The problems with such names are that (1) they all imply that fresh melilite occurs in the AAIP whereas, at the time of writing, it has never been analysed by electron microprobe in these rocks (H. Grutter, personal communication, 1999), and (2) mention of AAIP melilitites causes confusion with the contemporaneous melilitites that do indeed contain fresh melilite in the Kandalaksha Gulf (Bell et al., 1996; Beard et al., 1998; Ivanikov et al., 1998), and elsewhere on the Kola Peninsula. We therefore consider that the least controversial approach is to call these AAIP rocks alkaline picrites, following Sinitsin et al. (1994).
Figure 4 shows photomicrographs of the alkaline picrites, emphasizing their abundant olivine phenocrysts, relative lack of rounded olivine macrocrysts and the elongate microphenocryst–groundmass pseudomorphs after supposed melilite. We urge extreme caution in the use of the term ‘melilitite’ in the AAIP, until samples are found in which this mineral is unaltered. Elsewhere world-wide, elongate pseudomorphs and Ca-rich secondary minerals can be seen clearly to replace phlogopite, not melilite, in kimberlitic (sensu lato) rocks. For instance, phlogopite-bearing picritic and kimberlitic (sensu lato) rock-types in the Alto Paranaíba Igneous Province, SE Brazil, show all stages of phlogopite replacement by calcite (Gibson et al., 1995a). The rock-types shown in Fig. 4b and c strongly resemble the phlogopite picrites described by Gibson et al. (1995a), except that the AAIP elongate microphenocryst and groundmass phase is pseudomorphed by pectolite and hydroandradite, rather than unaltered phlogopite.
The clasts in the Anomaly 651 pipe of the Kepino–Pachuga field (Table 1) typify the alkaline picrites (Fig. 4a, b). They contain 15–20% of euhedral (altered) <3 mm olivine phenocrysts and 5% chromite microphenocrysts in a formerly microcrystalline to glassy groundmass that is totally altered to fine-grained serpentine, saponite, opaques and hydroandradite. The amounts of pseudomorphs after supposed melilite vary from negligible (Fig. 4a) to abundant (Fig. 4b), from clast to clast. Figure 4c illustrates a variant that is slightly less rich in olivine phenocrysts and contains phlogopite microphenocrysts in a groundmass rich in pseudomorphs after supposed melilite. Clasts of this variant in the Chidvia pipe (Table 1) contain up to several percent of crustal quartz xenocrysts, plus scattered xenocrystal feldspars. Geographically the majority of the alkaline picrites are associated with the Eastern Group of kimberlites (Fig. 2). The micaceous Chidvia picrites occur to the south of the Western Group Zolotitsa kimberlite field. Finally, Table 1 emphasizes an important feature of the AAIP alkaline picrites: most of them are sparsely diamondiferous.
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GEOCHEMISTRY |
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TOPABSTRACTINTRODUCTIONGEOLOGY OF THE ARKHANGELSK...THE ARKHANGELSK ALKALINE IGNEOUS...PETROGRAPHY AND MINERALOGYGEOCHEMISTRYDISCUSSIONSUMMARYAPPENDIX: ANALYTICAL METHODSREFERENCES |
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Hydrothermal alteration
All of the investigated samples from the AAIP have been altered to some extent (often substantial) by hydrothermal processes. This common feature of kimberlites and other strongly alkaline provinces is enhanced in the AAIP by the brecciated nature of most rock-types. The extensive array of secondary minerals (Table 1) forms
20–80% of each rock. Chemical analyses of the alkaline picrites (Table 6) show that they have total volatile contents (H2O + CO2) of 6–7%, whereas this value rises to 7–12% in the kimberlites (excluding their carbonatitic facies in the Mela sills).
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We have attempted to evaluate the chemical effects of hydrothermal alteration by comparing analyses of the Pionerskaya diatreme massive kimberlites with those of nearby clasts in the breccias (Table 6). This is not an ideal approach because it cannot disentangle hydrothermal effects from magmatic variations amongst the samples but, nevertheless, it gives an indication of the possible chemical effects of wholesale rather than marginal alteration of the clasts. The only elements strongly affected are Rb, which is dramatically reduced in the altered clasts, and K and Cs, which are reduced substantially in the massive facies. The clasts have higher Si, Na and Ba, and lower Mg, Ca, Cr and Nb, than the massive facies (Table 6). Clearly, variation in such a wide range of elements could have magmatic as well as hydrothermal causes. We consider that the prudent approach, in a province where alteration-free samples are as yet unobtainable, is to treat geochemical variations (except those of K, Rb, Cs and probably Na) as magmatic but to do so with great caution.
Major and trace elements
Two matters need to be emphasized at the beginning of this section: (1) we have only one major-element analysis of a mica-poor Eastern Group kimberlite (Table 6) and therefore also use those published by Parsadanyan et al. (1996); (2) we have simplified the discussion below by omitting the kimberlites that are carbonate rich and grade into carbonatites. These are the Mela sills (Table 6) and the Zvezdochka kimberlites of Parsadanyan et al. (1996).
The high mg-number [Mg/(Mg + Fe)] of the micaceous Western Group kimberlites and lamproites places them together in a distinctive field in variation diagrams (Fig. 6). The Eastern Group kimberlites occupy a lower mg-number range (because of higher Fe2O3) and also have notably higher TiO2 than the Western Group kimberlites. There is a varying amount of overlap in the other plots but, in general, kimberlites of the Eastern Group have lower SiO2 and Lu, and higher Cr, than those in the Western Group. The behaviour of Lu in AAIP rocks is clearly significant, in that this element provides a clear discriminant between kimberlites and all the alkaline picrites, which are relatively Lu rich. Heavy rare earth element (HREE) abundances in rocks containing mantle-derived xenoliths and xenocrysts have to be treated with care, because they can be noticeably affected by small amounts of either fragmented or dissolved xenocrystal mantle garnet. We take the view with the AAIP samples that the consistency of Lu concentrations within each magma group probably means that xenocrystal garnet (Sablukova et al., 1995; Griffin et al., 1999) is not the cause of intergroup Lu variations.
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, Western Group kimberlites; •, olivine lamproites; 
, Eastern Group kimberlites; 
, mica-poor alkaline picrites; 
, micaceous alkaline picrites.
Figure 7 focuses on the AAIP alkaline picrites. A selection of petrographically fresh world-wide melilitites shows that CaO is usually >10% in this rock-type (Fig. 7a), whereas its abundance is much less (as low as 2·85%) in the AAIP samples. At first sight, Fig. 7a suggests that the AAIP alkaline picrites are simply Ca poor as a result of high MgO contents, expressed modally in olivine; all their analyses plot along a trend between typical melilitites and their olivine phenocrysts. Nevertheless, Fig. 7b shows that the situation is more complicated. This plot of (CaO + Na2O + K2O) vs (SiO2 + Al2O3) was proposed by Le Bas (1989) as a discriminant between melilitites, nephelinites and basanites. All of the AAIP alkaline picrites fall far from the melilitite field in this diagram. A vector drawn between a typical Hawaiian melilitite and an appropriate olivine phenocryst composition shows clearly that richness in MgO, expressed as olivine, is not a satisfactory explanation for the anomalous major-element compositions of all the AAIP alkaline picrites. The compositions of the mica-poor picrites might, with difficulty, be fitted by such a model but the Chidvia micaceous picrites clearly have implausibly high (SiO2 + Al2O3) contents. We noted above (Table 1) that the Chidvia micaceous picrites contain several percent of xenocrystal quartz, and a vector for quartz addition in Fig. 7b shows that this may explain their compositions. Nevertheless, the amounts of quartz contamination implied by this vector are far higher than the modal quartz in the rocks (<5%). This may be because: (1) several percent of quartz xenocrysts have dissolved in the melts; (2) pervasive hydrothermal alteration has significantly changed the major-element compositions of the AAIP alkaline picrites; (3) the abundant elongate pseudomorphs in these rocks (Fig. 4b, c) may not have originally been melilitite. At present we accept the current view of Skinner et al. (1998) that this phase was indeed formerly melilitite. If so, the total pseudomorphic replacement of abundant melilitite in a rock might reasonably be expected to affect relative abundances of SiO2, Al2O3, CaO and alkalis.
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The REE are particularly helpful in illuminating the nature and sources of the AAIP magmas. Although even these elements are sometimes mobile during hydrothermal alteration of mafic–ultramafic rocks (Lahaye et al., 1995), we consider that their relative abundances in these samples may still be magmatic because, in most samples, the alteration has not strongly affected such REE host minerals as perovskite, phlogopite and clinopyroxene. Figure 8 shows REE data for representative samples, grouped according to their localities. The micaceous kimberlites of the Zolotitsa field (Western Group) all have similar REE patterns that are steep and slightly concave upwards. The steepness of the Karpinskiy I lamproite pattern is similar to those of the mica-poor kimberlites from the Verkhotina–Soyana and Kepino–Pachuga fields. The lamproite, however, has a relatively straight pattern, for REE lighter than Ho, whereas the mica-poor (Eastern Group) kimberlite patterns are sigmoidal to varying extents.
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Despite their close field association, the mica-poor alkaline picrites have REE patterns that are clearly different from those of the Eastern Group kimberlites. Although sigmoidal, the picrite patterns are less steep and with much higher HREE than the kimberlites (see also Fig. 6). The Chidvia micaceous alkaline picrite has an REE pattern with a similar slope to that for the mica-poor alkaline picrites but concave upwards, rather than sigmoidal. Figure 9a shows a wide range of normalized incompatible elements in representative samples of the main strongly alkaline AAIP rock-types. A basanite (RTH31) from Oahu, Hawaii, is also plotted for comparison. The typical ocean-island basalt (OIB) pattern of this sample is convex upwards, peaking at Nb and Ta. The representative AAIP Eastern Group kimberlite and mica-poor alkaline picrite also have essentially OIB-like patterns, once allowance has been made for hydrothermal stripping of Rb and K. The troughs at Sr in both patterns are a fairly common feature of fresh plagioclase-free mafic–ultramafic strongly alkaline rocks (e.g. Foley et al., 1987). The Western Group kimberlite also has an OIB-like pattern but with a peak at P. The lamproite pattern, overall, does not culminate at Nb and Ta, and shows relative enrichment in Ba, Rb and K, together with a trough at Ti. In Fig. 9b the normalized incompatible trace elements of the two types of AAIP alkaline picrites are compared with those of melilitites from the nearby Terskii Coast, Germany (Hegau) and Brazil (Lages). They are clearly very similar, once allowance has been made for the possible effects of hydrothermal alteration on abundances of Rb and K (also Ba and Sr?) in the AAIP samples. The incompatible element differences between the Chidvia micaceous alkaline picrite and the other mica-poor alkaline picrites (e.g. relatively low Zr, Hf and Ti) give the composition of the Chidvia sample resemblances to those of the nearby Zolotitsa field Western Group kimberlites.
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Sr and Nd isotopes
Figure 10 shows a plot of
Nd380 vs initial 87Sr/86Sr for AAIP samples (Table 7), together with the fields of various other oceanic basalts, kimberlites and mafic ultrapotassic rock-types for comparison. The AAIP data form a rather scattered distribution in Fig. 10. Nevertheless, this scatter is not a random one and we think that it contains considerable information about the origin and evolution of the magmas. We have emphasized above that many of the AAIP samples are intensely hydrothermally altered and some have both petrographic and elemental evidence of crustal (sediment) contamination. Both these processes can raise 87Sr/86Sr but, for reasons of mass balance, do not normally change Nd significantly in strongly alkaline igneous rocks. In both Table 7 and Fig. 10 it is clear that samples with relatively high 87Sr/86Sr occur sporadically; i.e. members of the various AAIP rock-types in Table 7 fall in small and distinctive ranges of Nd, whereas those of 87Sr/86Sri are larger and values of >0·706 occur in both positive and negative Nd sub-groups.
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Table 7 and Fig. 10 show how the AAIP Eastern Group kimberlites and mica-poor alkaline picrites both lie within a small range of positive Nd values. The mica-poor alkaline picrite range (Nd = 0 to +0·9) may possibly be slightly lower than that of the Eastern Group kimberlites (Nd = 0 to +2·5 but with only one sample <1·0). Further data would be needed to confirm this point. All the other AAIP samples that we have studied show negative Nd values but, again, each individual rock-type falls within a small Nd range. Amongst the AAIP kimberlites and olivine lamproites, there is a sequence of rising K2O (despite hydrothermal alteration problems) and falling Nd, as follows: (1) Eastern Group kimberlites, K2O = 0·21–1·59, Nd = +1·1 to +2·5; (2) Western Group kimberlites, K2O = 0·71–4·22, Nd = -2·5 to -3·8; (3) olivine lamproites, K2O = 3·90–5·05, Nd = -4·6 to -4·7. This correlation breaks down when the phlogopite–calcite kimberlites and their closely associated carbonatites are considered. Despite comparatively low K2O contents, these rock groups have substantially lower Nd values (apart from one Mela carbonatite) than all the kimberlites and olivine lamproites.
The micaceous alkaline picrites plot fairly close to the Western Group kimberlites and olivine lamproites in Fig. 10, at slightly lower Nd values. Thus in terms of Sr–Nd isotope systematics, as with their elemental compositions, these picrites seem to be more similar to the Western rather than the Eastern Group kimberlites. If this grouping is correct, it appears that each kimberlite type has an associated alkaline picrite type that resembles it geochemically, except for relatively less-steep REE patterns and more-abundant HREE.
In Fig. 10 we have also plotted Late Devonian carbonatites from the Kola Peninsula (Kramm et al., 1993) because these extend the AAIP range of Nd to high values, appropriate, for instance, to OIB plume-derived melts resembling those of Hawaii or Iceland. We have drawn a tentative vector in Fig. 10, linking the Kola carbonatites with highest Nd to AAIP kimberlites and alkaline picrites with the lowest 87Sr/86Sri values in each rock-type. This trend is clearly towards an isotopic component, rich in carbonate and phlogopite with low time-integrated Sm–Nd and Rb–Sr ratios.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONGEOLOGY OF THE ARKHANGELSK...THE ARKHANGELSK ALKALINE IGNEOUS...PETROGRAPHY AND MINERALOGYGEOCHEMISTRYDISCUSSIONSUMMARYAPPENDIX: ANALYTICAL METHODSREFERENCES |
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Tectonic setting of the AAIP diatremes: lithospheric thickness variations and characteristics
The regional tectonic map (Fig. 1) gives firm boundaries to the Archaean Kola craton and the Proterozoic rifts beneath the AAIP. Nevertheless, it must be remembered that all these basement features are entirely hidden beneath a younger sedimentary cover, up to several kilometres thick. Figure 2 shows that the kimberlites and their associated alkaline picrites are confined to a clearly defined region that is surrounded to the south and east by basaltic (sensu lato) pipes. Furthermore, the kimberlite and picrite pipes, including those of the Chidvia field, are mostly diamondiferous to varying extents (Table 1). The P–T constraints of the diamond stability field mean that the lithospheric thickness must average >130 km (see below) beneath the diamondiferous pipes. Thus, Fig. 2 appears to map the SE corner of the buried Kola craton.
The juxtaposition of diamondiferous and basaltic (sensu lato) pipes within a few kilometres of each other (Fig. 2; see also Sablukova et al., 1995, Fig. 2) is of considerable theoretical importance in understanding the nature of craton margins adjacent to subsequent rifts. This, in turn, is relevant to successful diamond exploration. In contrast to the 150 km lithospheric thickness required for diamondiferous magmas, tholeiitic to alkali–olivine basalts (products of relatively dry mantle melting) cannot form deeper than 125 km, even above the axis of a powerful hot mantle plume (e.g. McKenzie & Bickle, 1988; Farnetani & Richards, 1994). Indeed, if Hawaii is taken as one of the hottest current mantle plumes, it appears that 75 km is a more realistic value for the maximum lithospheric thickness during tholeiite and alkali–olivine basalt genesis (Watson & McKenzie, 1991). Therefore we suspect that lithospheric thickness changes very abruptly between the diamondiferous kimberlite and alkaline picrite to the west and basalts (sensu lato) to the east (Fig. 2).
Amongst the AAIP diatremes of predominantly diamondiferous rock-types there is a spatial distribution of chemical variants that has been recognized for a decade in the published literature (e.g. Sablukov, 1990; Mahotkin et al., 1995, 1997; Sablukova et al., 1995). Figure 6 shows that the Western Group kimberlites, olivine lamproites and micaceous alkaline picrites are relatively Ti poor, whereas the Eastern Group kimberlites and mica-poor alkaline picrites are relatively Ti rich. Apart from the Chidvia micaceous alkaline picrites, the two groups also differ substantially in Fe contents (Table 6) and hence mg-number; the Eastern Group kimberlites are called Fe–Ti kimberlites in many published accounts. It is apparent in Fig. 2 that these two geochemical sub-types occur within a few kilometres of each other, in two roughly N–S elongated zones, separated by a line at 41°20'E.The only small-scale exception to this geographical grouping is Anomaly 401 and some adjacent diatremes. These lie to the east of the 41°20'E line but contain rock-types that classify with the Western Group kimberlites.
Sablukova et al. (1995) have focused on these Ti-rich and Ti-poor variants in the Zolotitsa, Soyana and Pachuga pipe fields, making extensive electron- and proton-microprobe analyses of megacryst minerals and fragments in 12 kimberlites. They applied established geothermometry and geobarometry techniques to data for Cr-pyrope garnet and chromite grains. From these results they deduced the following radically different Late Devonian conditions within the lithospheric mantle beneath the Western and Eastern Group kimberlites, despite their proximity (Fig. 2):
- beneath the Zolotitsa field (Western Group) kimberlites the Devonian lithosphere consisted of relatively depleted harzburgite, with phlogopite-dominated metasomatism between 125 and 150 km and minor melt-related metasomatism (Harte, 1983; Green & Pearson, 1986; Harte et al., 1993) at depths >160 km. The palaeogeotherm is relatively cool (37 mW/m2 using a conductive model), reaching
1100°C at 180 km.
- Beneath the Soyana and Pachuga fields (Eastern Group) kimberlites the Devonian lithosphere was also composed of relatively depleted harzburgite but this was extensively affected throughout by melt-related metasomatism. The palaeogeotherm is hotter than that beneath the Western Group kimberlites, reaching 1300°C at 180 km.
Kimberlite genesis and evolution
It is clear from our descriptions above that the Eastern and Western kimberlite groups of the AAIP show chemical resemblance to the South African Group I and II kimberlites. The questions that we seek to answer in this section are: (1) Were any of the AAIP kimberlites ever all-liquid magmas and, if so, at what P, T, etc. did they originate? (2) If not, how far can we disentangle the various contributions to their erupted bulk compositions? These factors and questions are similar to those in recent discussions of South African kimberlites. It is therefore appropriate first to summarize current and recent ideas about the latter:
- it has been proposed that at least some South African kimberlites were originally all-liquid magmas, so that the P–T conditions of their genesis can potentially be defined by experimental petrology, REE inversion or both (e.g. Edgar & Charbonneau, 1993; Tainton & McKenzie, 1994; Dalton & Presnall, 1998). The difference between the Group I and II variants in such magmas could lie in geochemically different types of plumes (spatially or temporally) in the convecting mantle beneath the Kaapvaal craton (Le Roex, 1986).
- Many workers have commented on the elemental and isotopic similarities between South African Group I kimberlites and strongly alkaline OIBs, such as the Honolulu Series, Oahu, Hawaii (e.g. Thompson et al., 1984). These similarities are often considered to exist because the Group I kimberlites originated from convecting mantle sources, beneath the Kaapvaal craton. In contrast, the Group II kimberlites do not resemble any oceanic magmas geochemically and may therefore have partially or entirely lithospheric sources.
- Studies of osmium and hafnium isotopes in kimberlites favour the view that both Group I and II South African kimberlites have substantial contributions from lithospheric mantle; they all seem to be mixtures from sources both within and beneath the Kaapvaal craton (Pearson et al., 1995; Nowell & Pearson, 1998).
Sablukova et al. (1995) emphasized that both mica-poor and micaeous kimberlite in the AAIP are full of finely comminuted crystalline lithospheric material. Therefore option (3) above seems to be the most appropriate way to approach the genesis of these kimberlites. This implies that neither their bulk compositions nor their REE can be used to define specific P–T conditions for their genesis because they were never entirely liquid melts. Previous studies on AAIP and Terskii Coast kimberlites (Mahotkin et al., 1995, 1997; Parsadanyan et al., 1996; Beard et al., 1998) have treated the rocks as having once been all-liquid melts that could be traced by geochemistry to specific mantle sources. We consider that each kimberlite had no specific ‘source’ but was assembled from both liquid and solid contributions from various depths, during magma upwelling. Sablukova et al. (1995) suggested that the elemental differences between the two AAIP kimberlite types relate to their contrasting lithospheric-source components: phlogopite-dominated metasomatized harzburgites for the micaceous kimberlites; ilmenite-dominated, pervasively melt-invaded harzburgite for the mica-poor kimberlites. This type of model is concordant with the complex morphologies and multistage growth histories of AAIP diamonds (Garanin et al., 1998).
Isotopic data can also help to clarify the situation (especially Nd isotopes, where the effects of hydrothermal alteration and crustal contamination are minimized). The low Nd values of the AAIP Western Group kimberlites place most of them far outside the OIB field in Fig. 10 and the olivine lamproites have slightly lower Nd. These data are concordant with the view that the component that gave the Western Group kimberlites and olivine lamproites their distinctive elemental and isotopic compositions was phlogopite rich and had resided in the lithosphere, isolated from the convecting mantle, for a considerable time. Because the Eastern Group kimberlites have Nd values at and slightly above Bulk Earth, mostly within the OIB field (Fig. 10), it could be argued that these were simply melts from sub-lithospheric convecting mantle. The data of Sablukova et al. (1995) show that this cannot be so, and we concur with these workers that the simplest explanation of the isotopic data is that upwelling melts from sub-lithospheric sources incorporated crystalline debris from previous magma batches, which had reacted with and frozen within the lithosphere only a comparatively short time previously. This genetic scheme is very similar to the one proposed for some OIBs by McKenzie & O’Nions (1995). Sablukova et al. (1995) supported this view because their reconstructed palaeogeotherm for the Eastern Group kimberlites is steepened in the lower lithosphere. This is consistent with advective heat transfer into the lithosphere, from the underlying convecting mantle, shortly before kimberlite genesis. Very similar processes appear to have taken place during the Tertiary in parts of the Tanzanian (Nyanza) craton affected by the mantle plume beneath the East African rift (Chesley et al., 1998; Lee & Rudnick, 1998). Nevertheless, the high Nd of Kola carbonatites (Fig. 10) and most other alkaline igneous rocks on the Kola Peninsula leaves open the less-obvious possibility that the sub-lithospheric contribution to the Arkhangelsk Eastern Group kimberlites also had much higher Nd and that the present values, near Bulk Earth, disguise a substantial lower-Nd lithospheric input.
Is there any direct evidence within the AAIP kimberlites for a sub-lithospheric contribution to the magmas? Sobolev et al. (1997) have reported electron microprobe analyses of olivine, chromite, pyroxene and garnet inclusions within AAIP diamonds. Amongst these are a pyrope inclusion containing significant pyroxene solid solution (the first outside South Africa and Brazil) and relatively high K2O (0·65–0·80%) in jadeite-rich pyroxenes. Both these mineralogical features suggest that their host diamonds originated several hundred kilometres deep, well below the base of the sub-AAIP lithosphere (Haggerty, 1994; Sablukova et al., 1995; Harlow, 1997).
Mahotkin et al. (1995, 1997) and Parsadanyan et al. (1996) have proposed that a mantle plume may have been the heat source for this widespread Late Devonian melting event. This hypothesis is supported by a recent study of noble gases (He, Xe and Ne) in Devonian carbonatites from the Kola Peninsula, which has shown that they are similar to those of OIBs and that the carbonatites contain primordial rare gases from the lower mantle (Marty et al., 1998). We propose that the Arkhangelsk magmatism is linked to the impact of this same ‘Kola’ mantle plume.
The alkaline picrites; are they ‘protokimberlites’?
Despite the published suggestion that they contain pseudomorphed melilite (discussed above), it needs to be stressed that both the AAIP alkaline picrites are entirely unlike melilitites from well-known suites elsewhere (e.g. Fig. 7). The latter occur in both oceanic and continental settings where the lithospheric thickness is <100 km and they are widely supposed to originate within the lithosphere (Rogers et al., 1992; Hoernle & Schmincke, 1993; Beard et al., 1998; Gibson et al., 1999). Their diamond content places the sources of the AAIP alkaline picrites much deeper. Bearing in mind their Nd values (Fig. 10), there is no obvious reason why they could not have originated in the convecting mantle beneath the Kola craton (i.e. >180 km; Sablukova et al., 1995).
The spatial relationship between the AAIP mica-poor Eastern Group kimberlites and alkaline picrites (Fig. 2) seems too close to be no more than a coincidence, despite such important chemical differences as REE pattern slopes and curvature (Fig. 8). Published accounts of South African kimberlites may clarify the significance of the AAIP alkaline picrites. Almost all South African Group I kimberlites contain large monomineralic megacrysts from a suite of minerals that includes subcalcic clinopyroxene, orthopyroxene, pyrope garnet, ilmenite and zircon. Detailed geochemical studies by Jones (1987) followed up earlier reports (e.g. Harte, 1983) that these minerals appeared to have crystallized as phenocrysts at depth from a magma, but that this liquid was both elementally and isotopically different from the kimberlites that hosted them. Jones (1987) concentrated on REE in the sub-calcic clinopyroxenes because they are much more REE rich than the other megacryst minerals, but he also showed that the garnets gave concordant results (see also Nowell & Pearson, 1998). When the REE patterns of the magmas that would have been in equilibrium with the sub-calcic clinopyroxene (and garnet) megacrysts were calculated, they were those of oceanic alkali–olivine basalts and basanites; very different from the host kimberlites. Jones (1987) proposed that precursor melts to the Group I kimberlites (called ‘protokimberlites’ by some researchers) originated within convecting mantle beneath the South African lithosphere. As they rose towards the surface, they evolved towards Group I kimberlite compositions by precipitation of the megacryst phases and incorporation, with partial dissolution, of material from the overlying lithospheric mantle. Figure 11 compares the chondrite-normalized REE patterns of AAIP Eastern Group kimberlites and mica-poor alkaline picrites with the range of ‘protokimberlite’ REE patterns, calculated by Jones (1987). The similarity between the AAIP alkaline picrite and the hypothetical ‘protokimberlite’ REE patterns is remarkable. There is a small difference in pattern curvature but it must be emphasized that Jones’ calculations used crystal–liquid partition coefficients from a decade ago and that very small changes in the preferred middle REE values will change calculated pattern curvatures considerably.
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We suggest that the diamondiferous megacryst-free AAIP mica-poor alkaline picrites are a small missing part of the kimberlite jigsaw puzzle, namely, approximations to the hypothetical ‘protokimberlites’ that are deduced from megacryst studies to be widespread precursor melts, at sub-lithospheric depths, to South African Group I kimberlites. This suggestion implies that the genetic model of Jones (1987), Pearson et al. (1995), Nowell & Pearson (1998), and many others for South African kimberlites may apply equally well to the AAIP. In each case, parts of the petrogenetic jigsaw puzzle are currently missing: in South Africa there are Group I kimberlites and their megacrysts but no recognized ‘protokimberlites’; in the AAIP there are Group I kimberlites and ‘protokimberlites’ (mica-poor alkaline picrites) but no appropriate megacrysts yet described. This problem may soon be resolved because Sablukova et al. (1995) noted that the AAIP Eastern Group kimberlites contain abundant Mg–Cr-rich ilmenites that ‘show good magmatic fractionation trends’, as is often the case in such suites elsewhere (Griffin et al., 1997).
Although our new data are a step towards an improved understanding of AAIP magmatism, we consider that we would be over-interpreting them if we attempted to develop a detailed model of how combined fractional crystallization and lithospheric contamination took place during the evolution of AAIP ‘protokimberlite’ to mica-poor (Eastern Group) kimberlite. The normalized incompatible-element data plotted in Fig. 9a give one clue about these processes. The normalized patterns of both AAIP mica-poor kimberlite and alkaline picrite show positive spikes at both Ti and (especially) Nb and Ta, when compared with the pattern for a Hawaiian basanite (RTH31; Fig. 9) or OIBs in general. This feature may be expressed differently by comparing Nb/La in world-wide OIBs (average 1·15; Fitton et al., 1991) with values of this ratio in AAIP Eastern Group kimberlites and alkaline picrites (1·8–5·9; Table 6). It seems reasonable to attribute this chemical difference between OIB-like rock-types in the AAIP and true OIBs to ilmenite, rich in Nb and Ta (Griffin et al., 1997), in the AAIP samples; both as solid macrocryst debris and dissolved(?) in the Eastern Group kimberlite, and dissolved in the alkaline picrite. Greenwood et al. (2000) identified the same phenomenon in the Paranatinga kimberlites of Brazil. Future detailed geochemical studies of AAIP samples less affected by hydrothermal alteration or crustal contamination, or by both, will be needed to add detail to our tentative genetic model. Such studies will also need to consider whether such a model can be extended to show a genetic relationship between the AAIP Western Group kimberlites and their associated micaceous alkaline picrites.
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SUMMARY |
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TOPABSTRACTINTRODUCTIONGEOLOGY OF THE ARKHANGELSK...THE ARKHANGELSK ALKALINE IGNEOUS...PETROGRAPHY AND MINERALOGYGEOCHEMISTRYDISCUSSIONSUMMARYAPPENDIX: ANALYTICAL METHODSREFERENCES |
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- The diatremes and sills of the Late Devonian (380–360 Ma) Arkhangelsk Alkaline Igneous Province (AAIP) are part of a larger area of contemporaneous varied magmatism.
- AAIP rock-types include kimberlites, olivine lamproites, alkaline picrites (±melilite?) and carbonatites. The carbonatites form components of sills and the other rock-types are fragments in diatremes.
- Pervasive hydrothermal alteration makes the role of ‘melilite’ somewhat uncertain in these rocks because it is entirely pseudomorphed. Nevertheless, there is a clear textural separation between the alkaline picrites and the kimberlites; the latter are full of comminuted megacrysts and lithospheric mantle debris.
- Two kimberlite variants occur: predominantly micaceous (Western Group) and predominantly mica poor (Eastern Group). These are <25 km apart. They resemble South African Group I and II kimberlites petrographically and chemically but are far from identical to them.
- AAIP Western Group kimberlites and olivine lamproites are diamondiferous (some richly). The Eastern Group kimberlites are generally poorly diamondiferous to barren. The alkaline picrites are sparsely diamondiferous and the carbonatites are barren.
- The various rock groups are clearly differentiated by the slopes and shapes of their chondrite-normalized REE patterns. The alkaline picrite patterns are much less steep (with notably higher HREE) than those of either kimberlite type.
- Published data (Sablukova et al., 1995) show that both kimberlite types are full of megacryst fragments and lithospheric mantle debris, and that this xenocrystal material is specific to each suite: it is dominated by phlogopite in the Western Group kimberlites, and rich in ilmenite in the Eastern Group kimberlites. Nevertheless, minerals enclosed within AAIP diamonds include pyrope with significant pyroxene solid solution and relatively K-rich jadeiitic pyroxenes (Sobolev et al., 1997), both indicating magma genesis at depths of several hundred kilometres. These data support an AAIP kimberlite genesis model of precursor melts forming in convecting mantle, well beneath the lithosphere; rising through lithospheric mantle with locally variable metasomatic prehistories; and reacting with, dissolving and physically incorporating lithospheric mantle phases, to produce hybrid kimberlite magmas.
- The mica-poor alkaline picrite REE patterns are very similar to those of the ‘protokimberlite’ magmas, calculated to be in equilibrium with sub-calcic clinopyroxene and other megacryst phases in South African Group I kimberlites (Jones, 1987; Nowell & Pearson, 1998). We suspect that this sparsely diamondiferous picrite is a minimally lithosphere-contaminated sample of AAIP deep-source ‘protokimberlite’ magmas. Nevertheless, even the picrites show evidence of post-genesis complexity. They are relatively enriched in Fe, Ti, Nb and Ta (e.g. Nb/La up to 5·9). If the picrite magma originated from the same convecting mantle source as OIB, it appears subsequently to have dissolved ilmenite.
- The widespread broadly synchronous magmatism and abundance of Mg-rich rock-types on the Kola Peninsula and in the AAIP all suggest the impact of a starting mantle plume beneath NW Russia in the Late Devonian. Around Arkhangelsk the pre-existing lithospheric thickness was crucial in controlling how close to the surface the head of the Kola starting plume could rise, and hence the types of magmas that were generated. The end-member products were diamondiferous kimberlites and alkaline picrites on the SE margin of the Kola craton, and tholeiitic basalts where cratonic (>150 km) lithosphere had been thinned by subsequent rifting.
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APPENDIX: ANALYTICAL METHODS |
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TOPABSTRACTINTRODUCTIONGEOLOGY OF THE ARKHANGELSK...THE ARKHANGELSK ALKALINE IGNEOUS...PETROGRAPHY AND MINERALOGYGEOCHEMISTRYDISCUSSIONSUMMARYAPPENDIX: ANALYTICAL METHODSREFERENCES |
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Electron microprobe analyses were obtained from polished-thin sections with a Cambridge Stereoscan S4-10 system with Kevex-7000 analyser operated at 15 kV and 0·02–0·03 mA sample current and Nick-wave correction program. Microprobe work was carried out in the Mineralogy–Petrography Institute, Freiburg University (Germany).
All of the samples were crushed in jaw-crusher and powdered in a corundum disc mill. Analyses of major elements in samples from the Zimniy Bereg pipes were determined by wet chemistry at the Institute of Ore Deposits (IGEM, Moscow). In the same samples Li, Rb, Cs, Sr, Ba, Ni, Co, Cr and V concentrations were determined by atomic absorption and Zr, Nb, Y concentrations were determined by X-ray fluorescence (XRF) at IGEM (Moscow). Analyses of the major and trace elements of the Onega pipe samples were determined on fused discs and pressed powder pellets by XRF techniques at the Freiburg University (Germany). Hf, Sc, Ta, Th, U, REE were determined by instrumental neutron activation analysis (INAA) at IGEM (Moscow). Those samples in Table 6 with complete REE and other trace element data were analysed by inductively coupled plasma mass spectrometry (ICP-MS) at the University of Durham. Sample preparation techniques have been discussed by Thompson et al. (1998).
Sr and Nd isotopes were analysed on a Finnigan-MAT-262 mass-spectrometer, at IGEM (e.g. Zhuravlev et al., 1983). Sr and Nd isotopic ratios were determined on the freshest massive whole-rock samples and separated autoliths from the Zimniy Bereg pipes and on clinopyroxene mineral separates from the Onega Peninsula pipes to avoid the effects of crustal contamination. The mean of 87Sr/86Sr ratios obtained for the E&A Sr standard is 0·708027 (n = 7) with 2
SD of the mean of 0·000025. The mean of 143Nd/144Nd ratios made on the La Jolla Nd standard is 0·511843 (n = 7) with 2 SD of the mean of 0·000016.
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ACKNOWLEDGEMENTS |
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This work was funded by the International Scientific Foundation (Moscow, Grant N5D000) and financial support from the Universities of Cambridge and Durham. We warmly thank Luc Rombouts and Steve Haggerty for information and discussions on the Arkhangelsk kimberlites. We are grateful to Y. Brown, J. C. Greenwood, R. G. Hardy, J. Keller, C. J. Ottley and D. G. Pearson for their help with analytical equipment and techniques. We would also like to thank H. Aliberti and C. Moseley for their assistance with preparation of this manuscript. Keith Bell, Hilary Downes and Lizzy Ann Dunworth are thanked for their thorough and constructive reviews of an earlier draft of this manuscript.
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FOOTNOTES |
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*Corresponding author. Telephone: +44-1223 333400. Fax: +44-1223 333450. e-mail: sally{at}esc.cam.ac.uk
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