March 2005 LIP of the Month

Corresponds to events #52-56 in LIP record database

The Volynian Flood Basalt Province and coeval (Ediacaran) magmatism in Baltoscandia and Laurentia

Per-Gunnar Andreasson
Centre for Geobiosphere Dynamics
Lund University, Sölvegatan 13, S-223 62 LUND, Sweden
per-gunnar.andreasson@geol.lu.se

Kenneth L. Buchan
Geological Survey of Canada, Natural Resources Canada, 601 Booth St., Ottawa, Ontario, Canada KIA 0E8
kbuchan@NRCan.gc.ca

Leonid V. Shumlyanskyy
Institute of Geochemistry, Mineralogy and Ore Formation
P.O.Box 291, Kyiv 01001, Ukraine

Richard E. Ernst
Geological Survey of Canada, Natural Resources Canada, 601 Booth St., Ottawa, Ontario, Canada KIA 0E8

Introduction

In Baltica, late Neoproterozoic fragmentation of Rodinia resulted in successful and failed rift basins extending from off-shore of present-day Scandinavia in the west to the Urals in the east (Fig. 1; Kumpulainen and Nystuen 1985; Vidal and Moczydlowska 1995; Bogdanova et al. 1996). Fragments of giant mafic dyke swarms of the Baltoscandian volcanic rifted margin (Baltoscandian Margin) and the early ocean floor occur today within allochthons of the Scandinavian Caledonides (see Andreasson 1994 for review; Andreasson et al. 1998; Bingen et al. 1998). Erosional remnants of continental flood basalts (Volyninan Flood Basalt, VFBP) are preserved in depressions in Ukraine and adjacent Poland, Belorussia and Moldova (e. g. Karpinski 1874; Kruglov and Tsypko 1988; Znamenskaya et al. 1990; Bialowolska et al. 2002).

In Laurentia, late Neoproterozoic-early Cambrian rifting was associated with formation of the Iapetus Ocean. The Iapetus margin is characterized by failed rifts, extensive mafic dyke swarms, volcanics and plutons (Fig. 2; Higgins and van Breemen 1998, McCausland and Hodych 1998, Cawood et al. 2001, Puffer 2002). Giant dyke swarms extend for several hundred km into the craton, with the Grenville swarm paralleling the prominent Ottawa graben (Kumarapeli et al. 1990). Volcanic rocks that represent the erosional remnants of continental flood basalts extend intermittently for up to 2000 km along the margin.

The voluminous magmatism of the three provinces in Baltica and Laurentia qualifies as a Large Igneous Province (LIP) event or events occurring in the latest Neoproterozoic to early Cambrian. This contribution summarizes the characteristics of the provinces and discusses their possible palaeogeographic relations.

Volynian Flood Basalt Province (VFBP)

The VFBP (Fig. 3) occupies an area of >200 000 km2 and attains a thickness of >600 m (Volovnik 1971). The VFBP derived its name from the Volyn-Orsha aulacogen (Fig. 1) which by tradition has been considered as the host structure of volcanism. However, Kruglov and Tsypko (1988) noted that the flood basalts fill a younger depression formed at the mouth of the Volyn-Orsha depression as a result of Rodinia break-up (cf. Znamenska and Chebanenko 1985; Poprawa and Paczesna 2002). Thus the Tornquist margin rather than the Riphean Volyn-Orsha aulacogen may be the main controlling structure. This also implies that the VFBP represents a part of once more widespread flood basalt province, fragmented by Rodinia breakup. 

The volcanic sequence can be subdivided into three units: a lowermost of mainly basalts with subordinate tuff beds (85 to 350 m); a middle unit consisting of <220 m pyroclastic sediments and an upper unit comprising several basalt flows with rather abundant tuff and interlayers of pyroclastics and breccia (150-200 m). A basalt flow is typically 20-30 m thick and consists of a core of well-preserved basalt between upper and basal crusts. Crusts are aphyric and amygdaloidal. Core basalts contain 1-15% of palagonite glass, 45-70% plagioclase, 20-40% pyroxene and 5-10% opaque minerals. Less than 1 % phenocrysts (plagioclase>>pyroxene) and 1-2 % of small amygdules occur. The VFBP is famous for its native copper mineralization (Shumlyanskyy et al. 2002 and other papers in the same volume).

Most of the basalt flows are tholeiitic in composition; a minor group of potassic composition (trachybasalt) also occurs. There is a subdivision into high-Ti and low-Ti basalts at similar Mg-numbers (only a few low-Ti basalt samples being more primitive: #Mg=0.54-0.63). The high-Ti samples derive from the uppermost levels of the volcanogenic sequence and definitively belong to a separate basalt flow. The high-Ti basalt is more enriched also with respect to high field strength elements including rare earth elements (Fig. 4). The VFBP exhibits other geochemical characteristics of continental flood basalts, and properties which may reflect a magma derived from an enriched mantle source, but contaminated by continental crust or subcontinental lithosphere (Condie 2001; Ernst and Buchan 2003). These include:

• similarity to oceanic island basalts with regard to trace elements, but with characteristic peaks for K, Ba, Th 
• markedly enriched in LREE as compared to N-MORB 
• enriched isotope signatures: eNd550 Ma = -2.7- -2.9 and 87Sr/86Sr550Ma = 0.707318-352 (low-Ti group).
• weak negative Ta-Nb anomalies in chondrite-normalized spidergrams
• crustal contamination: Nb/U (27); Ta/U (1.5); Ce/Pb (9-12).

Numerous K-Ar analyses of basalts from the VFBP yielded ages between 560-540 Ma (Staritskyy 1981; Semenenko 1975). Elming et al. (2004) reported 40Ar-39Ar ages of 590-560 Ma for samples from four quarries of the Volyn province. Shumlyanskyy and Derevska (2002) obtained a Rb-Sr four-point isochron age of 552±59 Ma. Employing U-Pb ion microprobe technique (SHRIMP) on zircon, Compston et. al. (1995) obtained an age of 551±4 Ma from a tuff of the Slawatycze Formation on the Lublin Slope of Poland and correlated it with the uppermost Volynian volcanic sequence (cf. stars in Fig. 3). Shumlyanskyy and Andreasson (2004) carried out ion-microprobe analysis (NORDSIM) of eight zircons from samples taken from upper levels of the sequence. All crystals were euhedral and tabular to prismatic, colourless and displayed igneous oscillatory zoning, lacking cores and rims. Preliminary results were as follows: four crystals yielded near-concordant analyses with a weighted average 238U/232Th age of 554±16 Ma; MSWD = 1.49. Four other larger crystals yielded concordant analyses with a weighted average 238U/232Th age of 519±14 Ma; MSWD = 0.74. Th/U ratios were 1.0-1.7. A 206Pb/238U – 207Pb/235U discordia line constructed for all zircons except one corresponds to an age of 549±29 Ma; MSWD = 0.68.

Baltoscandian rift magmatism

Of large areal extent, the allochthons hosting Baltoscandian rift magmatism are still incompletely mapped and magmatism is poorly constrained with regard to geotectonic setting and also age. However, the various dyke swarms can be interpreted in terms of a rifted continental margin with an oceanward zonal variation with regard to age and mantle source (Andreasson 1994; Bingen and Demaiffe 1999). Today hosted by autochthonous basement, the Egersund dyke swarm must derive from inner parts of the rifted continental margin, possibly a failed arm. This 616±3 Ma old (U-Pb baddeleyite; Bingen et al. 2004) dyke swarm includes an alkaline (eNdi < +1; Sri = 0.7046) and a tholeiitic to mildly alkaline suite (+2.0èeNdiè3.1; Sri = 0.7037). A rare lava of continental tholeiitic affinity (Furnes et al. 1983) underlies the <610 Ma old (Bingen et al. 2004) Varangerian glaciogenic deposit of the Hedmark basin (H in Fig. 1) which has only been transported a short distance. The extensive dyke swarms of the rift basins cut an equivalent to the <610 Ma tillite. Dykes are tholeiitic, locally mildly alkaline, and enriched compared to MORB (Fig. 5; +4.8èeNdi è5.0; 0.7039-0.7028). Dykes of the continent-ocean transition have been dated at 608±1 Ma (U-Pb zircon; Svenningsen 2001). They compare to dykes of the rift basins but are less enriched in incompatible elements (eNdi è 6.6). Undated dyke-intruded gabbro and subordinate ultramafics of inferred ocean-floor derivation have OIB affinity and display a distinct positive Nb-Ta anomaly (red line in Fig. 5). The 577-554 Ma old magmatism of the Seiland Igneous Province (U-Pb zircon from three intrusions; Roberts et al. 2004) included emplacement of gabbroic and subordinate ultramafic and carbonatitic rocks cut by an alkaline dyke swarm. Alkaline syenite dykes intruded as late as 523 Ma (U-Pb zircon; Pedersen et al. 1989). The geochemistry of the intrusions is poorly investigated; however, Mg-rich and picritic compositions occur (Robins and Takla 1979; Bennet et al. 1986). Dyke swarms cutting some intrusions are enriched and approach OIB affinity (Fig. 5; Reginiussen et al. 1995). Alkaline and carbonatitic complexes of the platform include the 583±15 Ma old Fen complex (Meert et al. 1998) and the 584±13 Ma Alnon complex (Andersen 1996).

Iapetus margin of Laurentia

Multiple stages of rifting are recorded along the Iapetus margin of Laurentia during the late Neoproterozoic to early Cambrian (e.g. Cawood et al., Waldron and van Staal 2001). Although magmatic activity in some areas dates from as early as ca. 760 Ma (Aleinikoff et al. 1995), we concentrate on the later episodes that began at ca. 615 Ma. Paleomagnetic evidence (McCausland and Hodych 1998) is consistent with the initiation of sea floor spreading at ca. 570 Ma. Another phase of rifting, perhaps involving a microcontinent or microcontinents, may be associated with later magmatism at ca. 550 Ma (Cawood et al. 2001, Waldron and van Staal 2001).

Post-615 Ma magmatism along the Iapetus margin of Laurentia (Fig. 2) is collectively referred to as the Central Iapetus magmatic event. It occurred semi-continuously from 615 to 535 Ma. Many of the magmatic units are now precisely dated and are described below. The oldest units are the extensive 615 ±2 Ma Long Range diabase dyke swarm (Kamo et al. 1989, Stukas and Reynolds 1974) and the coeval 617 ±8 Ma Hare Hill granite (van Berkel and Currie 1988). The Round Pond granite of western Newfoundland followed at 602 ±10 Ma (Williams et al. 1985). At 590 Ma the giant Grenville dyke swarm (Kamo et al. 1995) was emplaced along and parallel to the Ottawa graben, a failed rift that extends for 500 km into the craton (Kumarapeli et al. 1990). Alkalic intrusions, such as the 577±1 Ma Callander complex (Kamo et al. 1995), were emplaced in the Ottawa graben a short time later. Volcanic rocks of similar or slightly younger age are found along the margin, including the 572-564 Ma Catoctin volcanics (Aleinikoff et al. 1995) and 571 ±5 Ma Pinney Hollow volcanics (Walsh and Aleinikoff 1999). The 20,000 cu. km Sept Iles mafic layered intrusion was emplaced at 564 ±4 Ma (Higgins and van Breemen 1998). Volcanic rocks and plutons followed, including the 555 +3/-5 Ma Lady Slipper pluton (Cawood et al. 1996), the 554 +4/-2 Ma Tibbit Hill volcanics (Kumarapeli et al. 1989) and the 550 +3/-2 Ma Skinner Cove volcanics (McCausland and Hodych 1998). The youngest units are the 534 ±1 Ma Rigaud and 533 ±1 Ma Chatham-Grenville syenite intrusions of the Ottawa graben (McCausland et al. 2004).

Kumarapeli (1993) and Seymour and Kumarapeli (1995) examined the tectonic setting and chemistry of the prominent 590 Ma Grenville dyke swarm (Fig. 2). They concluded that the swarm was derived from a mantle plume head located in the vicinity of the Tibbit Hill volcanics, the site of a triple junction related to Iapetus rifting.

The overall geometry of the Grenville and Long Range dyke swarms (Fig. 2) has been interpreted to reflect subswarms of a giant radiating dyke swarm associated with a mantle plume (Ernst and Buchan 1997). Alternatively, the 25 my age difference between these two swarms and their geographic separation could indicate two distinct source regions along the Iapetus margin.

Puffer (2002) compared the chemistry of the volcanic rocks and dyke swarms of the Central Iapetus event of eastern Laurentia and concluded that it is consistent with mantle plume derivation. He divided the magmatic units into an older (615-564 Ma) “Mid-Vendian flood basalt group” and a younger (ca. 554-550 Ma) group which exhibits an ocean island basalt (OIB) affinity characterized by a greater enrichment in high field strength elements. The older group has a composition that resembles continental flood basalts which have been derived from a subcontinental lithospheric mantle source mixed with magma from a plume source. The younger group exhibits a regional variation in TiO2 and Zr, and the values peak just north of the Tibbit Hill triple junction suggesting a plume centre at this location.

Discussion

Volynian-Baltoscandian relations. The VFBP fulfils several critera of a LIP (Ernst and Buchan 2001; 2003); a notable exception is the presence of pyroclastics in the VFBP sequence, which is unlike most LIPs (but similar to the Siberian Traps). Available geochemical data provide little support for a link between Volynian magmatism and coeval Baltoscandian rift magmatism (i. e. the 550-580 Ma old Seiland Igneous Province). Only the lava of the Hedmark basin compares to the Volynian flood basalts; however, the Hedmark basalt is at least 50 Ma older than the VFBP. A hotspot-track relationship between the c. 550 Ma old VFBP and 584 Ma old alkaline-carbonatite complexes in Scandinavia is inconsistent with a presumably ”eastward” movement of Baltica after breakup. Too little is at present known about the magmatism hosted by Neoproterozic rift basins elsewhere in the East European Craton (triangles in Fig. 1) for a discussion of potential links to the VFBP.

Volynian-Baltoscandian-Laurentian relations. The Volynian and Baltoscandian margins of Baltica and the Iapetus margin of Laurentia have in common a rift magmatic evolution characterized by early (ca. 620-590 Ma) mainly tholeiitic dyking (Egersund, Särv, Seve-Kalak, Long Range and Grenville), 570-550 mainly tholeitic volcanism (Catoctin and Volyn) and an overlapping (ca. 580-520 Ma) phase of more alkaline character including plutonism (e.g. Seiland plutonism, Fen and Alnon carbonatite complexes; Skinner Cove, Lady Slipper, Tibbit Hill). It is tempting to interpret this evolution in terms of triple-junction rifting and generated by a single, major plume (Bingen et al. 1998; Ernst and Buchan 2001).

However, the paleogeographic relationship between Baltica and Laurentia at the time of emplacement of the Volyn flood basalts and coeval magmatism elsewhere in Baltica and Laurentia is highly controversial because of uncertainties in the interpretation of paleomagnetic data currently available. Firstly, the paleolatitude of Laurentia is uncertain. For example, McCausland and Hodych (1998) and Hodych et al. (2004) suggest that the present-day eastern margin of Laurentia was at a latitude of about 10-20° at 550 Ma. On the other hand, Hartz and Torsvik (2002, Fig. 2) locate this portion of Laurentia at about 70-80°. In addition the location and azimuthal orientation of Baltica based on paleomagnetic data are equally controversial. Some authors place the present-day western margin of Baltica adjacent to eastern Greenland (e.g. Torsvik et al. 1996; Meert and Van der Voo 1997) prior to breakup of Baltica and Laurentia. Recently, however, Hartz and Torzvik (2002) and Torsvik (2003) have suggested that paleomagnetic results are consistent with the eastern margin of Baltica adjacent to Greenland at this time. More reliable paleomagnetic poles from the two continental blocks are required to establish the relative locations of these blocks in the late Neoproterozoic, and therefore, the relative locations of the Volyn-Baltoscandian magmatism and that of eastern Laurentia. 

References