Elsevier

Chemical Geology

Volume 353, 30 August 2013, Pages 210-229
Chemical Geology

Emplacement age and Sr–Nd isotopic compositions of the Afrikanda alkaline ultramafic complex, Kola Peninsula, Russia

https://doi.org/10.1016/j.chemgeo.2012.09.027Get rights and content

Abstract

The Kola Peninsula is characterized by diverse alkaline magmatism, including alkaline ultramafic, carbonatite, “kimberlite” and agpaitic rocks, although many aspects of the ages of emplacement and petrogenesis remain unresolved. In this study, rocks from the Afrikanda complex (pyroxenite, ijolite, melteigite, carbonatite, and perovskite ore) were selected for U–Pb age determination and Sr–Nd isotopic analyses using in situ ion probe and laser ablation techniques. The perovskite U–Pb ages are 377 ± 6 (pyroxenite), 379 ± 5 to 385 ± 5 (calcite-bearing perovskite ore) and 376 ± 5 (ijolite–melteigite) Ma, indicating that the different phases of the complex were contemporaneously emplaced at ~ 380 Ma. These ages are comparable to those obtained previously for Afrikanda rocks, and other alkaline ultramafic, carbonatitic, “kimberlitic” and agpaitic complexes in the area; suggesting that the majority of alkaline magmatism in the Kola Peninsula occurred at ~ 375–380 Ma. Sr–Nd isotopic analyses of perovskite, titanite, apatite, and calcite indicate that the Afrikanda complex was derived from a depleted mantle. However, these data suggest that silicate and carbonatitic magmas are not related by simple crystal fractionation within a closed magmatic system. The silicate magma has an initial 87Sr/86Sr isotopic ratio of ~ 0.7034 to 0.7038 and εNd(t)380 value of ~+2.0 to + 4.9, whereas the carbonatitic magma was more primitive with the above values of ~ 0.7033 to 0.7035 and εNd(t)380 ~ + 5.1 to + 5.8, suggesting that either both magmas were derived from two distinct mantle sources or by contamination of a melts derived from a single source. In combination with other geochronological and geochemical data for other complexes in the area, it is proposed that the Kola alkaline magmatism was controlled by mantle plume activity at ~ 380 Ma.

Highlights

► Perovskite U–Pb ages are determined for the alkaline Afrikanda complex in Russia. ► The various phases were synchronously emplaced at ~ 380 Ma. ► Sr–Nd isotopic data suggest that the magma was derived from depleted mantle. ► The Kola Paleozoic magmatism was triggered by mantle plume activity.

Introduction

Continental alkaline magmatism is represented by alkaline rocks that have higher concentration of alkalis than can be accommodated in feldspars alone, and characterized by the presence of feldspathoids, sodic pyroxene and amphibole, and other alkali-rich minerals (Sørensen, 1974). Although alkaline rocks account volumetrically for less than one percent of all igneous rocks, their remarkable diversities in mineralogy, petrology and geochemistry have made them the subject of many scientific studies during the past decades (Sørensen, 1974, Menzies, 1987, Mitchell, 2006, Tappe et al., 2007), mainly because these rocks can provide information regarding the composition and evolution of the continental lithospheric mantle. In addition, alkaline rocks are closely associated with many kinds of mineralization, and account for most of the world's resources of niobium, rare earth elements, phosphorus, zirconium and titanium.

Alkaline magmatism in the Kola Peninsula of Russia is represented by numerous Paleozoic (Devonian) intrusions, collectively termed the Kola Alkaline Province (KAP, Fig. 1). These intrusions have been classified as alkaline mafic–ultramafic rocks (turjaites, ijolites), agpaitic rocks (lujavrite), carbonatitic and rocks with “kimberlitic–lamproitic” affinities (some of the kimberlite-like rocks are actually lamprophyres, see Tappe et al. (2005)) (Downes et al., 2005). Current hypotheses for the petrogenesis of these rocks include derivation from a common primary magma by differentiation and/or liquid immiscibility, or from different batches of mantle melts (Harmer, 1999, Mitchell, 2005, Woolley and Kjarsgaard, 2008). For the Kola Alkaline Province, it has been proposed that the wide range of Sr–Nd isotopic compositions between the carbonatitic and silicate rocks suggested that they are not co-genetic, and the isotopic variation is a consequence of mixing of different batches of mantle-derived magma, but not from crustal contamination because the carbonatites have extremely high Sr and Nd contents (Kramm, 1993, Kramm and Kogarko, 1994, Zaitsev and Bell, 1995, Verhulst et al., 2000, Dunworth and Bell, 2001, Zaitsev et al., 2002, Sindern et al., 2004, Lee et al., 2006). However, most carbonatites display uniform and identical Sr–Nd isotopic compositions to those of associated silicate rocks, thus favoring petrogenesis from a single batch of homogeneous mantle-derived melt that crystallized and fractioned in a closed system (Kogarko, 1987, Bell, 1996, Brassinnes et al., 2005, Balaganskaya et al., 2007, Bell and Simonetti, 2010). An alternative process is liquid immiscibility between a carbonatitic melt and an alkaline silicate melt (Ivanikov et al., 1998). Experimental work in the system of CaO–Na2O–(MgO + FeO)–(SiO2 + Al2O3)–CO2) shows that, depending on the system parameters (P, T, x), a miscibility gap can be reached during crystallization of a parental carbonated-silicate melt (Lee and Wyllie, 1998). However, typically there is no actual evidence for immiscibility in the generation of carbonatite from a carbonated silicate magma.

Another question concerning the petrogenesis of alkaline ultramafic igneous rocks is the nature of the mantle from which alkaline magmas are derived. Carbonatitic, agpaitic and lamproitic rocks are typically considered to be derived from the subcontinental lithosphere (Bell, 1996, Mitchell, 2006, Bell and Simonetti, 2010), although derivation from convecting mantle has been proposed (Bell and Blenkinsop, 1987, Bell and Blenkinsop, 1989, Simonetti and Bell, 1994a, Simonetti and Bell, 1994b, Simonetti et al., 1997). It remains unknown as to why different mantle sources are partially melted coevally during a specific thermal event, how the magmas interact, and their subsequent evolution during magmatic crystallization.

To evaluate the above proposed genetic processes, it is necessary to assess the geochemical relationships between different contemporaneous rocks. For this purpose, the Afrikanda complex was selected for isotopic study as the occurrence consists of diverse rock types within a single intrusion.

Section snippets

Geological setting

The KAP, one of the largest alkaline  ultramafic–carbonatite provinces, is located in the Fennoscandian Shield and occupies an area of > 100,000 km2 (Fig. 1). In the northeastern part of this shield the Precambrian basement can be divided into three main units which are characterized by different times of continental crust formation. The Karelian Province and the Murmansk Terrane represent parts of the Early Archean cratons which are separated by the Lapland–Kola–Belomorian collision zone (

Analytical methods

Separated mineral grains of perovskite, apatite, titanite and calcite were handpicked, mounted in epoxy resin, and polished until the grain centers of the grains were exposed. Before isotopic analysis, back-scattered electron (BSE) images were obtained using a JEOL JXA8100 electron microprobe, in order to assess internal compositional variation and textures, and identify potential target sites for U–Pb and Sr–Nd analyses. All analyses were conducted at the Institute of Geology and Geophysics,

Mineral compositions

In this study, sixteen samples were selected for analysis (Table 1). Their compositions are listed in Table 3 and the rare earth element (REE) distribution patterns are shown in Fig. 5. Of these samples, most are euhedral perovskite with a grain-size of ~ 200 μm (Fig. 3). Some perovskites are mantled by later-crystallizing loparite (Fig. 3c). From these data, the perovskites can be seen to have similar compositions to those occurring in kimberlites (Chakhmouradian and Mitchell, 2001a,

Emplacement age of the Afrikanda complex

Whole-rock Rb–Sr analyses for a clinopyroxenite yielded an age of 364 ± 3 Ma (Kramm et al., 1993), this being previously considered as the emplacement time of the complex. Recently, Reguir et al. (2010) obtained U–Pb ages of 374 ± 10 and 371 ± 8 Ma for perovskite extracted from pyroxenite and carbonatite, respectively. Initial Sr isotopic ratios reported by them are 0.70333 ± 3 for pyroxenite and 0.70337 ± 2 for carbonatite. Recently, Wu et al. (2010c) obtained 207Pb/206Pb ages of 382 ± 12 and 379 ± 6 Ma for

Conclusions

Comprehensive age determinations and Sr–Nd isotopic analyses of perovskite, apatite, titanite and calcite from the Afrikanda complex lead to the following conclusions:

  • 1)

    Perovskites from pyroxenite, calcite-bearing perovskite ore and ijolite–melteigite yield identical U–Pb ages of 377 ± 6, 379 ± 5–385 ± 5 and 376 ± 5 Ma, respectively with an average value of 381 ± 2 Ma, indicating that the different phases of the complex were contemporaneously emplaced at ~ 380 Ma;

  • 2)

    Sr–Nd isotopic analyses of perovskite, apatite,

Acknowledgments

Hilary Downes is thanked for kindly providing perovskite from the Arkhangelsk kimberlite. Wei-Qiang Ji, Qin Zhou, Zhi-Chao Liu, Chuan-Zhou Liu and Liang-Liang Zhang are thanked for their assistance during sample preparation and analyses. Kevin R. Chamberlain helped to run the TIMS analyses for the AFK perovskite. Constructive reviews by Sebastian Tappe, Anton Chakhmouradian, Keith Bell and Christopher McFarlane improved the manuscript, hence are highly appreciated, as well. This work was

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