Chrome–spinels in gabbro–wehrlite intrusions of the Pechenga area, Kola Peninsula, Russia: emphasis on alteration features
Introduction
Chrome–spinels are common accessory minerals in ultramafic rocks and are often used as an indicator of the petrogenesis of these rocks (Irvine, 1965; Roeder et al., 1979; Cameron, 1975; Barnes and Hill, 1995). However, the petrogenetic interpretation of chrome–spinel compositions can be complicated by post-crystallization re-equilibration (Hamlyn and Keays, 1979; Henderson and Wood, 1981; Roeder and Campbell, 1985; Scowen et al., 1991) and metamorphic alteration (Evans and Frost, 1975; Eales et al., 1988; Burkhard, 1993). Therefore, studying the mechanisms of metamorphic alteration, and in particular their chemical patterns, is critical for the assessment of the applicability of chrome–spinels to petrogenetic reconstructions of strongly metamorphosed ultramafic rocks.
The Pechenga greenstone belt hosts numerous intrusions of mafic and ultramafic composition (Gorbunov et al., 1985). These intrusions contain accessory chrome–spinels of various compositions including unusual Ti,Cr-spinels (e.g., Neradovsky, 1985; Abzalov et al., 1991; Hanski, 1992). Metamorphic grade within this area varies from sub-greenschist to lower amphibolite facies (Petrov et al., 1986) and, therefore, the area provides a unique opportunity to follow prograde metamorphic alteration of spinels from relatively fresh rocks with well-preserved primary magmatic features to completely altered rock types. The present study concerns alteration textures and chemistry of chrome–spinels, aiming at systematic documentation of the alteration mechanisms and their chemical patterns.
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Regional geology
The Early Proterozoic Pechenga belt is situated in the north-western part of the Kola Peninsula, Russia, close to the border with Norway (Fig. 1). It is composed of sedimentary and volcanic rocks and rests unconformably on the Archaean basement. The supracrustal sequence is subdivided into the Northern Pechenga and South Pechenga Groups (Fig. 1) (e.g., Melezhik et al., 1995). The Pechenga Group consists of four megacycles, known as the Ahmalahti, Kuetsjarvi, Kolasjoki and Pilgujarvi Formations (
Sampling and analytical techniques
Samples of the ultramafic rocks for this study were collected from various metamorphic zones in the Pechenga belt, including prehnite–actinolite, greenschist and lower-amphibolite facies. The samples were predominantly taken from drill cores. Additional samples were collected from the nickel mines.
Rocks were studied by standard optical techniques using both transmitted and reflected light. The composition of minerals was analysed using a MS-46 Cameca microprobe. Operating conditions were 22 kV
Prehnite–actinolite and greenschist facies
Two types of chrome–spinels have been identified in the weakly serpentinized ultramafic rocks of sub-greenschist and greenschist facies. These two types are distinguishable by their relationships with magmatic rock-forming silicates, morphology and, particularly, by composition (e.g., Neradovsky, 1985).
The first type, spinel-1, is dark grey in reflected light and represented by euhedral or rounded grains which are partially or fully enclosed in olivine (Fig. 2A,C), or, less commonly, occur
Alteration textures
The Pechenga chrome–spinels show a variety of alteration textures, which can be grouped as follows.
Composition of associated minerals
Ilmenite associated with chrome–spinels in the Pechenga ultramafic rocks contains up to 8.4 wt.% MnO and up to 1.3 wt.% MgO. The MnO content of ilmenites decreases with metamorphic grade (Fig. 9), as has been described for ilmenites from metamorphosed mafic and ultamafic rocks of the Yilgarn Block, Australia (Cassidy et al., 1988) and the Pechenga greenstone belt basalts (Abzalov, 1992). The MgO content, however, is independent of the metamorphic grade (Fig. 9).
Magnetite occurring as outer rims
Primary composition
A specific feature of the Pechenga chrome–spinels is their high-Ti content. The presence of exsolved ilmenite lamellae is clear evidence of the primary magmatic nature of titanium enrichment of the Pechenga chrome–spinels and may reflect the high-Ti nature of the ferropicritic magma. Ti-rich chrome–spinels have been reported from another mafic–ultramafic igneous complexes, such as Mt. Ayliff and Elephant's Head Dyke intrusions of the Karoo province (Eales, 1979; Eales and Snowden, 1979;
Conclusion
(1) Two types of magmatic chrome–spinels were identified in the Pechenga ultramafic rocks. Spinel-1, which crystallizes first is represented by Al,Ti-chromite. Spinel-2 is represented by Ti-rich chrome–spinel, crystallizing after spinel-1. It could be formed by direct crystallization from Ti-rich residual melt and also, by the reaction of earlier crystallized spinel-1 with Ti-rich trapped interstitial liquid. Zoned crystals, where Ti-rich chrome–spinel overgrows a core of spinel-1, are also
Acknowledgements
This study commenced in 1990 when the author worked as Research Fellow of the Geological Institute, Kola branch of the Russian Academy of Sciences. The author expresses sincere gratitude to his former colleagues—Dr. L.I. Polezhaeva for microprobe analyses and L.I. Voloshina for providing unpublished microprobe data of the serpentine minerals. The author is also grateful to S. Barnes, S.E. Haggerty and B. Rees for helpful comments and discussions and to R. Reid who improved English. Critical
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