September 2005 LIP of the Month

Corresponds to event #207 in LIP record database.

The Early Paleoproterozoic (2.5-2.36 Ga) Baltic Large Igneous Province (BLIP): An Example of Siliceous High-Magnesian (Boninite-Like) Magmatism in a Within-Plate Setting

Evgenii V. Sharkov
Institute of Geology of Ore Deposits, Petrography, Mineralogy and Geochemistry, Russian Academy of Sciences, Staromonetny per., 35, Moscow, 119017, Russia
Email: sharkov@igem.ru

Introduction

The early Paleoroterozoic (2.5-2.36 Ga) Baltic large igneous province (BLIP) of the siliceous high-magnesian (boninite-like) series (SHMS) occupies almost the entire eastern Baltic Shield, including the Kola Peninsula, Karelia, and northern and central Finland (Fig. 1) (Sharkov et al., 1997, 2005; Vuollo and Huhma 2005). The original size of the province is unknown, but its preserved remnant is 0.8 x 10⁶ km² in area, i.e., comparable with large Phanerozoic igneous provinces. The southern and eastern extensions of the province are overlain by sediments of the Russian Platform; the northern one is buried beneath the Norwegian Caledonides nappes and the Barents Sea; and the western extension is broken by younger Svecofennian orogen (1.95-1.8 Ga). Similar SHMS rocks of the same age are found in the basement of the Russian Platform, in Scotland, in Greenland, and on the Canadian Shield (Matachewan and Mistassini dike swarms in the Superior craton, and volcanic sheets and layered intrusions in the Southern Province; e.g. Heaman, 1997; Buchan and Ernst, 2004). Paleomagnetic, geological, and stratigraphic considerations suggest that all these cratons were parts of a Laurentia-Baltica supercontinent in the Paleoproterozoic. Thus, the original size of the BLIP was not less than 2500 km in length and 1500 km in width.

The BLIP is localized within three major structural provinces of the region. On the rigid Archean Karelian and Kola cratons, the SHMS rocks form volcanosedimentary complexes in rift structures (Vetreny Belt, East-Karelian, Pechenga-Varzuga, etc.) as well as gabbronorite dike swarms and large layered mafic-ultramafic intrusions (Burakovsky, Monchegorsky, etc.). In the Belomorian (White Sea) and Tersk-Lotta mobile belts, which were zones of a gentle crustal-flow from cratons toward the granulite belt (Sharkov et al., 2000), these rocks occur as a series of small mafic-ultramafic intrusions. In the Lapland-Umba granulite belt, located between the Kola and Karelian cratons, the Early Paleoproterozoic magmatism is significantly different and is manifested only as crustal enderbites and charnockites.

Volcanics of the Vetreny Belt Formation in the Baltic Province

Compositionally, the lava flows in volcanic plateaus of rift structures vary from low-Ti picrites and basalts (boninitic picrites and basalts) through andesites to dacites and rhyolites, with basalts predominating. The boninitic nature of the SHMS magmas is the most distinct for mafic-ultramafic volcanics of the Vetreny Belt structure where lava flows (~2.44 Ga) with preserved volcanic glass (basalt to andesite and dacite) occur (Sharkov et al., 2004).

In the Vetreny belt, boninitic basalts are usually of fine-porphyritic texture, with olivine, Cr-spinel, and, less often, clinopyroxene phenocrysts. The basalt groundmass has a specific microspinifex texture, expressed as long-prismatic, diverse acicular, radiate-lamellar, and paniculate aggregates of clinopyroxene and, less often, plagioclase and/or olivine in volcanic glass (Fig. 2,A). At the Bol. Levgora site, a thick differentiated lava flow occurs, which is composed predominantly of boninitic basalts with microspinifex texture formed by long-prismatic crystals (up to 10 cm long) and paniculate aggregates of pyroxene and/or olivine in volcanic glass.

Olivine relics have been found both in microspinifex basalts and in porphyritic segregations (Fig. 2,B). Compositionally, the olivine is usually close to forsterite Fo_80-87. The clinopyroxene phenocrysts are skeletal- and "case-formed" (hollow) and compositionally correspond to augite Wo_41-44En_44-50Fs_9-11 or, seldom, pigeonite Wo₁₁En₆₀Fs₂₉ with Al₂O₃ = 4.3 wt.% and pigeonite-augite Wo_30-42En_44-53Fs_10-29. The clinopyroxene of the microspinifex-basalt groundmass corresponds to augite Wo_45-47En_33-42Fs_12-21 with Al₂O₃ = 6.5-7.5 wt.% (Fig. 3). Chrome-spinel occurs as occasional fine grains up to 0.1 mm in size and is close in composition to subferrialuminochromite with Cr₂O₃ = 42-46 wt.%. Plagioclase occurs only in holocrystalline fine-grained dolerites and compositionally corresponds to labradorite An_62-64.

Also of interest is well-preserved volcanic glass from the Myandukha rocks (upper section of the Vetreny Belt Fm) (Fig. 2). Fresh volcanic glass, isotropic or weakly anisotropic in polarized light, is spotted brown in transmitted light (Sharkov et al., 2004a). In the hyaloclastites, it composes the groundmass and is of fine fluidal-banded structure; in the basalts it forms a groundmass between the olivine and pyroxenes crystals. Microprobe analysis showed that the spotted volcanic glass varies in chemical composition from basalt (SiO₂ = 50-54 wt.%) to andesite-dacite (SiO₂ = 56-62 wt.%). [Dacitic interstitial glass (SiO₂ = 57-65 wt.%) is also present in boninites of the young island-arc systems (Fig. 4), including the ultramafic varieties (SiO₂ = 46.5 wt.%, MgO = 31.2 wt.%) from the inner zone of the Tonga arc (Vysotsky, 1989).] The glass is formed by amorphous silica, containing nano- and microcrystals predominantly of water-bearing silicates (amphiboles and various layered silicates: chlorite, talc, biotite, and kaolinite) and rarely orthopyroxene, α-quartz, tridymite, epidote, halite, anatase, and cuprite (Sharkov et al., 2004a). In their mineral composition these nanophases differ drastically from the lava groundmass, which contains olivine, high-alumina clinopyroxenes, and chromite. The nanophases are, probably, products of crystallization of micelles - partly ordered silicates in the siliceous matrix, which occurred both just after the chilling of glass in the hot material (orthopyroxene, tridymite, and α-quartz), and then during its cooling and long existence in the Earth's crust at much lower temperatures. Most likely, this structure of the SHMS lavas volcanic glass (rigid SiO₂ framework with nano- and micromicelles) ensured their preservation for ca. 2.4 Ga.

In terms of whole rock composition, the rocks of the Vetreny Belt Fm are dominated by siliceous (SiO₂ < 54 wt.%) high-magnesian (MgO >> 8 wt.%) boninitic basalts with low contents of TiO₂ (< 0.8 wt.%). Boninitic picrites and picrobasalts with SiO₂ = 42-45 wt.% and MgO = 33-21 wt.% are subordinate. All the rocks have similar patterns of trace and dispersed elements; in particular, they are enriched in Sr, Zr, and Ba and are depleted in Nb, Y, etc. (Fig. 4,A). Also, they have similar REE patterns, with LREE prevailing over HREE (Ce/Yb)_n = 2.68-3.92), thus differing significantly from both tholeiitic and komatiitic basalts (Fig. 4,B). Additional geochemical features of Vetreny belt volcanics are shown on Figs. 5 and 6. Geochronology using the Sm-Nd, Re-Os, and U-Pb zircon methods yielded the age of these volcanics of 2.45-2.41 Ga and εNd value is -2.6 (Puchtel et al., 1991, 1997, 2001).

Earlier, the Vetreny Belt volcanics was regarded as a representative of the Proterozoic komatiite series on the Baltic Shield (Kulikov, 1988). However, our research has shown that they, although assigned mainly to komatiites and komatiitic basalts according to their contents of major elements, bear an association of magnesian olivine, pyroxenes and Cr-spinel phenocrysts with andesitic and even dacitic glass (Evseeva et al., 2004). This peculiar plagioclase-free composition, along with the specific geochemistry and the presence of intermediate-felsic glass, is typical of the Phanerozoic boninites. Additional characteristics of island arc boninites are as follows. Besides ordinary and olivine boninites, bronzite andesites, hypersthene dacites, and associated quartz dacites and low-K rhyolites (Dobretsov et al., 1980; Bloomer, Hawkins, 1987; Van der Laan et al., 1992; Ohnenstetter, Brown, 1996), the boninite igneous series of young island-arc systems (Izu-Bonin, Mariana, Tonga, etc.) includes more primitive mafic-ultramafic rocks, including picrites and picrobasalts (Sharaskin et al., 1980; Vysotsky et al., 1983; Vysotsky, 1989;). Judging from the high content of phenocrysts (>>50%), some of them have cumulate origin and are the end-members of the boninite series.

In summary, the Vetreny Belt volcanics, which are made up mainly of basaltic and picrobasaltic lava flows, subordinate andesite-basalts, and scarce tuffs and tuffaceous-sedimentary rocks, also include such mafic-ultramafic rocks corresponding in mineral composition and geochemistry to boninite rock series (which are normally considered to form in a subduction setting). However, the Vetreny Belt structure (Fig. 1) formed in an within-plate setting during continental rifting (Kulikov, 1988; Sharkov et al., 1997).

Layered Intrusions

Related to the SHMS volcanics are large layered mafic-ultramafic intrusions. They are localized mainly in the uplifted 'shoulders' of rift volcanosedimentary structures (Pechenga-Varzuga on the Kola Peninsula, Pana-Kuolajarvi in the northern Karelia, Pohjanmaa in eastern Finland, etc.) formed by the SHMS volcanics and, less often, in the basement uplifts between these structures (Alapieti et al., 1990; Sharkov and Smolkin, 1996). The massifs have intrusive contacts with the host Archean rocks and tectonic contacts (via shear zones) with rift rocks. Obviously, these intrusions were 'pulled out' from beneath volcanic plateaus along the system of faults. However, in some cases they may have been emplaced along the contact between basement and Paleoproterozoic volcanics/sediments. The original sizes of the massifs are unknown and, possibly, were greater.

The eastern Baltic Shield is one of the world's largest provinces of Early Paleoproterozoic mafic-ultramafic layered intrusions (hosting more than 12 large massifs) (Fig. 1). All these intrusions were produced from the SHMS melts. They have of similar structure and are formed by the rock series, dunites-harzburgites-norites-gabbronorites-anorthosites-magnetite-gabbrodiorites, although in some features each intrusion is unique.

The largest is the Burakovsky pluton (BP) which is located in the southeast of the Baltic Shield, east of Lake Onega (Chistyakov et al., 2002). It consists of two large bodies, Aganozero and Shalozersko-Burakovsky, which are in contact in their upper parts and form a single massif at the level of the pre-Quaternary erosional surface (Fig. 7). Each body has a distinct internal structure. The Aganozero body (AB) is made up mainly of ultramafic cumulates, which change to mafic cumulates upsection. The mafic rocks form a synclinal structure in the core of the body. In plan, the AB is submeridionally (north-south) elongated, and in section it is funnel-shaped. The Shalozersko-Burakovsky intrusive body (ShBB) is elongated northeastward and is lopolith-shaped. In contrast to the AB, at its pre-Quaternary erosional level, it is composed mainly of mafics, but the deeper horizons are also composed of ultramafics.

In both bodies, the layered series consists of up to five zones from bottom to top: ultrabasic, pyroxenite, gabbronorite, pigeonitic gabbronorite, and magnetite gabbronorite-diorite. The latter zone is observed in the ShBB only. All studied rocks have similar light REE enriched patterns, though the REE level regularly increases from the ultrabasic cumulates at the bottom to gabbroids at the top. In general, all isotopic and geochemical data indicate that the initial SHMS melts that formed both bodies of the Burakovsky pluton are of the same type.

The geological and petrological-geochemical data indicating that the Aganozero and Shalozero-Burakovo bodies are different intrusive bodies are confirmed by isotopic studies: The AB has an Sm-Nd age of 2372 ± 22 Ma (εNd = -3.22 ± 0.13), and the ShBB, 2433 ± 28 Ma (εNd = -3.14 ± 0.14) (Chistyakov et al., 2002). This Nd indicates that the AB is perhaps as much as ~50 Myr younger than the ShBB; however, this result needs to be confirmed with high precision U-Pb dating.

In addition, the formation of each body was accompanied by the intrusion of extra pulses of melt into solidifying magma chambers, as evidenced from the appearance of peridotite horizons. These extra magma pulses have lower ⁸⁷Sr/⁸⁶Sr(T) ratios (0.7019 for the ShBB and 0.7032 for the AB) and higher εNd(T) (-1.32 for the ShBB and 2.35 for the AB).

A similar pattern is observed for the large Monchegorsky complex on the Kola Peninsula (Sharkov et al., 2002,)). It consists on two large bodies: Monchegorsky nickeliferous mafic-ultramafic pluton and the essentially mafic massif Monche-Chuna-Volch'i Tundras (Major Ridge). These massifs are also made up of similar rocks but differ in their abundance and cumulate stratigraphy. According to isotope-geochronological data, the Major Ridge massif is 40 Ma younger than the Monchegorsky pluton (2.46 and 2.5 Ga, respectively). In contrast to the Burakovsky massif, the Monchegorsky complex was involved in tectonic processes along the major Kola fault and is broken into a series of blocks (Fig. 8).

Small Mafic-Ultramafic Intrusions of Mobile Belts

Within the Belomorian and Tersk-Lotta mobile belts, localized between the Karelian and Kola cratons and the Lapland-Umba granulite belt, the SHMS igneous rocks are of the same age (ca. 2.45-2.36 Ga as the large layered plutons of the neighboring cratons (Sharkov et al., 2004b). They comprise numerous small synkinematic mafic-ultramafic intrusions that are compositionally similar to units within the large layered intrusions. However, in contrast to the latter, each rock types forms an individual body with a corresponding chilled zones.

In the Belomorian mobile belt (BMB), these bodies are ubiquitously (but to a variable degree) metamorphosed and transformed into the so-called drusites (coronite metagabbros). Isochemical coronitization is apparent in rims of metamorphic minerals along boundaries of igneous minerals; the gabbroids have preserved relics of the latter and also initial magmatic structures, including layering.

Two major varieties of the BMB mafic-ultramafic intrusions are recognized according to lithology: mainly mafic (gabbronorite-anorthosites) and mainly ultramafic (lherzolite-gabbronorites). The rocks and minerals in the intrusions of the Drusite complex and in the large layered plutons of the Baltic Shield are almost of the same composition. Both types of intrusions (large layered plutons and the Drusite complex) and have two major types of paragenesis: (1) ultramafic cumulates (Ol ± Chr, Ol + Opx ± Chr, Ol + Opx + Cpx ± Chr, Opx ± Cpx, Opx + Pl ± Ol, and Opx + Cpx + Pl ± Ol) in the lower parts of intrusions, and (2) mafic cumulates (Opx + Pl ± Cpx, Pl, and Pig + Pg-Aug + Pl ± Mgt) in the upper parts. In the large layered plutons, rocks with different paragenesis, all occur in a single body, whereas in the Drusite complex each intrusion has only one paragenesis. The compositional trends of the Drusite (coronite) complex rocks are analogous to those of the differentiates of the Burakovsky layered pluton (Sharkov et al., 2004b).

Geochemistry

All igneous rocks of the BLIP are siliceous and often highly magnesian (up to 27 wt.% MgO). Most of rocks have low titanium (TiO2 << 1 wt.%) and low or medium alumina contents, except for anorthosites and gabbronorite-anorthosites with up to 20-27 wt.% Al2O3. The rocks show a great variation in SiO2 content, which reaches 54 wt.%; some of them bear quartz or are quartz-normative. BLIP rocks have similar REE patterns, which is evidence of a homogeneous composition for parental magmas for the entire BLIP. The patterns show only a minor, if any, Eu anomaly. The rocks are enriched in LREE (Ce/Yb =5.65) and exhibit flat HREEs. Their REE patterns almost match with those of the cumulates of the Burakovsky layered pluton and the Drusite complex. The REE composition fields of these both types of rocks overlap the field of boninites of the modern Izu-Bonin arc, which indicates a geochemical similarity between these formations (Fig. 9).

Genesis of Magmas of the Siliceous High-Magnesian Series

Thus, all rocks of the BMB small intrusions and layered plutons of the neighboring cratons are characterized by the same spectrum of rock types (from ultramafic cumulates to magnetite gabbro-diorites) and similar compositions of mineral phases. These rocks of the mafic-ultramafic Drusite complex and Burakovsky layered pluton are characterized by the same regularities: enrichment in incompatible elements (Ba, Zr, Th, La, etc.) and depletion in Ti and HFSE (Nb, Y, etc.), i.e., all these rocks are products of crystallization of the boninite-like igneous series (SHMS) melts (Fig. 10). In the BMB, ascending melts might have been localized in small chambers, which were deformed during high grade deformation of the host rocks.

The high-Mg composition of olivines and pyroxenes (mainly orthopyroxenes) and the presence of chromites in the ultramafic rocks of the Drusite complex suggest that the parental magmas were derived from a mantle, which was depleted in incompatible HFSE (Y << Ti << Zr). The melts were enriched in LREE and lithophile elements (Rb, Ba), possibly as a result of crustal assimilation. This is confirmed by results of isotope studies indicating that the SHMS magmas were generated as a result of the large-scale assimilation of Archean rocks with εNd(T) = -9.5 by mantle-derived melts (Puchtel et al., 1997; Amelin and Semenov, 1996).

In contrast to typical Phanerozoic large igneous provinces, formed predominantly by tholeiitic basalts, the BLIP is made up of boninite-like rocks. In contents of major, trace, and rare-earth elements, with low Nb and Ti, and high Al₂O₃/TiO₂ values they are close to Phanerozoic boninites (Pearce et al., 1992) and are intermediate between typical boninites and island-arc tholeiites (Fig. 9). But there is a major difference in their isotopic composition: the average εNd(T) value in the studied SHMS rocks is from -1 to -3, whereas in Phanerozoic boninites it is from +6 to +8. This indicates the contribution of subduction components (sediments, fluids, etc.) to the generation of Phanerozoic boninites and of the Archean crust to the formation of SHMS melts.

Phanerozoic boninites occur only in transitional continent-ocean zones in an island-arc setting; they are produced during subduction processes. In contrast the SHMS rocks of the BLIP resulted from the large-scale assimilation of crustal material by mantle-derived high-temperature melts ascending through the crust. The assimilation proceeded through the mechanism of zone refinement, i.e., melting of the roof material accompanied by crystallization of high-temperature phases at the bottom of the chamber (Fig. 10).

The Structure of Magmatic Systems of Siliceous High-Magnesian Series

The existence of a large igneous provinces suggests the presence of a mantle superplume beneath them (Dobretsov et al., 2001). The presence of magmatic complexes at different depths in the BLIP makes it possible to reconstruct the structure of the intrusive component of the large igneous province.

Detailed study of layered intrusions of the Kola-Karelian region showed that they were produced as a result of pulses of melt emplaced into intrusive chambers. It suggests that the intrusions were transitional magma chambers, where processes of magma storage, crystallizing differentiation and mixing of fresh and evolved magmas occurred. The final transformation of the primary mantle-derived magmas occurred in shallow subvolcanic sill-like bodies localized just beneath lava plateaus (Fig. 10).

Thus, three major depth levels are recognized in the SHMS magma systems: the deepest level - crust-mantle boundary, with crustal contamination of plume head melts; the intermediate one, with transitional magma chambers which survived as large layered intrusions; and the highest level, with subvolcanic sills and lava plateaus. The dispersed style of magmatism in mobile zones like the BMB seems to be a result of the deformation during emplacement of the magmas (Fig. 11). This suggests that the magma generation zones localized beneath these sites were the same as those beneath the neighboring cratons, and that the high grade deformation zones were no more than 20-25 km thick, judging from the geobarometric data (Sharkov et al., 2004b).

In contrast to the Phanerozoic within-plate magmatic systems, where basaltic-magma underplating resulted in intrusions that then produced the mafic lower crust (Rudnick, 1990), the early Paleoproterozoic SHMS magmatic systems formed at specific magma sites where magma chambers ascended through the Archean lower crust. Actually, this is a kind of underplating when the temperature of primary mantle-derived magmas was extremely high (1600-1700^oC) (Girnis and Ryabchikov, 1988). Earlier studies showed that the lower-crustal xenoliths beneath the BLIP are mainly garnet granulites and eclogites, which were cumulates, formed coeval with the magmatic province from the same magmas and represent the lower level of the magmatic system.

Conclusions

The Early Paleoproterozoic (ca. 2.5-2.4 (2.36) Ga) Baltic large igneous province of the siliceous high-magnesian series is localized in the eastern Baltic Shield. Within the Kola and Karelian cratons, the BLIP is made up of lava plateaus (from low-Ti picrites and basalts through andesites to dacites and rhyolites) in riftogenic structures, gabbronorite dike swarms, and large layered mafic-ultramafic intrusions. Small mafic-ultramafic intrusions (Drusite (coronite) complex) are disseminated in the Belomorian and Tersk-Lotta mobile belts. All of this suggests the existence of a mantle superplume beneath the region at that time.

It should be noted that large layered intrusions can also be fed from dykes (e.g. feeding of Bushveld intrusion in South Africa from the Thabazimbi-Murchison Lineament (LIP of the Month May 2005), and that major dyke swarms can transport magma sideways in the crust for up to about 2500 km (review in Ernst et al. 2005). Therefore, it is possible that the 2.5-2.4 Ga layered intrusions in the Kola-Karelian region could have been fed laterally via associated dykes, from a mantle source(s) (i.e. mantle plume(s)) located on the edges of the Kola-Karelian craton.

The major difference of the BLIP from Phanerozoic large igneous provinces is the composition of magmatic melts: in the BLIP, they were close in geochemistry to subduction-related magmas (boninites) but were generated in within-plate setting. This suggests that the SHMS magmas were generated as a result of the large-scale assimilation of crustal material by mantle-derived high-temperature magmas during their ascent to the surface.

Magmatic systems of the BLIP include four depths of activity levels: (1) head of mantle plumes, where primary mantle-derived melts were generated; (2) lower crust, where mantle-derived magmas were contaminated by crustal matter and transformed into the SHMS magmas; (3) upper crust, where transitional magma chambers (preserved as large mafic-ultramafic layered intrusions) formed; and (4) volcanic sheets at the surface and subvolcanic sills beneath them. Dyke swarms distributed SHMS magma within the crust and were the plumbling system for both volcanics and layered intrusions. The dispersed intrusive mafic-ultramafic magmatism in mobile belts was probably related to the emplacement of magmas into an actively deforming zone which led to many small chambers rather than a single large magma chamber.

References