Geochemistry of Ultramafic to Mafic Rocks in the Norwegian Lapland: Inferences on Mantle Sources and Implications for Diamond Exploration

Pavel K Kepezhinskas,Glenn M.D Eriksen,Nikita P Kepezhinskas

doi:10.5539/esr.v5n2p148

Abstract

Geology of the Norwegian Lapland is dominated by diverse Archean crystalline basement complexes superimposed with Proterozoic greenstone belts. Isotopic dating of detrital zircons from basement gneisses in the Kirkenes area establishes presence of Early Archean (3.69 Ga) crustal component as well as three major episodes of crustal growth at 3.2 Ga, 2.7-2.9 Ga and 2.5 Ga. Precambrian terranes are intruded by ultramafic-mafic dikes and sills that range in composition from komatiites and ultramafic-mafic lamprophyres to high-Mg basalts and low-Ti subalkaline basalts. Geochemical characteristics of these rocks fall into three principal groups: 1) enriched compositions with high Nd, Nb, Hf, Zr and Th concentrations and elevated La/Th and Nb/Th coupled with low La/Nb, Ba/Nb and U/Nb ratios; 2) compositions depleted in Th, Hf and Nb together with low LREE/HFSE (such as La/Nb) and LILE/HFSE (such as Ba/Nb and U/Nb) ratios; 3) transitional group clearly identified by marked depletions in Ti, Nb and Ta contents coupled with enrichment in Th and U and other large-ion lithophile elements (LILE). These geochemical characteristics are interpreted within the framework of two principal source models: 1) derivation of parental ultramafic-mafic melts from multiple mantle sources (depleted to enriched) inherited from Archaean lithospheric tectonics and 2) a single primitive mantle source which underwent several depletion and enrichment episodes, at least partially associated with subduction zone processes. Subduction modification of depleted lithospheric mantle was assisted by accretion of subducted sediment to depleted mantle source at Archean, Proterozoic or Early Paleozoic convergent margin. Alkaline ultramafic rocks such as lamprophyres and mica picrites display geochemical characteristics supportive of their origin within stability field of diamond in a deep mantle beneath Norwegian Arctic margin which, together with other lithospheric characteristics, suggests its high potential for hosting economic diamond mineralization.

Highlights

Formation of continental crust and evolution of Archean cratons are among the most important fundamental problems of modern geology
Archean boninite-like rocks do share certain geochemical features (Si enrichment at high Mg# numbers, clear large ion lithophile element enrichments coupled with high field strength element depletions) with modern boninite melts emplaced in frontal arc environments of the Bonin, New Caledonia and Tonga arcs, none of these ancient high-Mg lavas appear to match the extreme depletion manifested in unique chemical compositions of modern boninites
We suggest that enriched mafic magmas that are present among the products of ultramafic-mafic magmatism in the Norwegian Lapland could have been generated by adiabatic melting of the within-plate (OIB-type) mantle source (EM2; Fig. 12) that ascended into the mantle wedge beneath an Archean convergent margin possibly through structural discontinuities in the delaminated basaltic crust roughly following the model of Morris & Hart (1983)

Summary

Introduction

Formation of continental crust and evolution of Archean cratons are among the most important fundamental problems of modern geology. One of the ongoing scientific debates related to this problem focusses on the relative role of plate tectonic processes – such as subduction and production of oceanic crust – on early Earth (Kusky & Polat, 1999; Dhuime et al, 2012; Turner et al, 2014; Kamber, 2015; Nutman et al, 2015). Petrologic and geochemical studies of the last two decades established existence of fragmented Archean oceanic crust incorporated into cratonic structure (Szilas et al, 2013; Furnes et al, 2015), confirmed presence within Archean terranes of unique rock associations such as boninite-like volcanics (Kerrich et al, 1998; Polat et al, 2002; Smithies et al, 2004; Szilas et al, 2012a), adakites Archean silica-rich, magnesian magmas could be interpreted as have been formed via melting of a fluid-fluxed depleted mantle in a tectonic environment comparable with modern convergent margins (Smithies et al, 2004)

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