Late Archean high-Mg granitoids (sanukitoids) in the southern Karelian domain, eastern Finland: Pb and Nd isotopic constraints on crust−mantle interactions

doi:10.1016/j.lithos.2004.05.007

Lithos

Volume 79, Issues 1–2, January 2005, Pages 161-178

https://doi.org/10.1016/j.lithos.2004.05.007 Get rights and content

Abstract

Late Archean (ca. 2.73 Ga) porphyritic high-K and high-Mg granitoids in the Nilsiä and Lieksa areas, eastern Finland, have geochemical signatures similar to those of high-Mg sanukitoid series from other parts of the Karelian domain of the Fennoscandian (Baltic) Shield and the Archean sanukitoid suites from Canada.

The Nilsiä and Lieksa granitoids have low SiO₂ contents (62.7−67.0) and high Mg numbers (0.45−0.52). They are enriched in K₂O (2.40−4.73 wt.%), P₂O₅ (0.25−0.42 wt.%), Sr (610−850 ppm), Ba (1200−2300 ppm), Cr (40−80 ppm) and LREE [(La/Yb)_N=19–65]. They are characterized by a negative Eu anomaly (Eu/Eu* 0.53−0.81) and show strong depletion in U, high Th/U ratio, low U/Pb ratio, high μ₂ values for the source (>9), positive ɛ_Nd (2.7 Ga) values of 0.3−1.4 and Nd depleted mantle model ages 2.75−2.86 Ga. The high μ₂ values pointing to a significantly older crustal source apparently contradicts the positive ɛ_Nd (2.7 Ga) values supporting a mantle origin.

The geochemical and isotopic data suggest that the Nilsiä and Lieksa granitoids originated at ca. 2.73 Ga from a mantle-wedge source enriched in LILE, U, Th and Pb by recycling of continental material in subduction-related slab dehydration processes shortly (up to 200 Ma) before melting. A considerably large amount of crustal lead was contributed from subducting sediments into the overlying mantle wedge. Consequently, the isotopic composition of mantle-wedge lead was overprinted by crustal lead isotope signatures. It is suggested that crustal recycling through subduction zone processes played an important role for the enrichment of the mantle wedge and generation of mantle-derived high-μ granitoids in the Archean.

Introduction

In addition to better-known and more abundant tonalite−trondhjemite−granodiorite (TTG) suites, K-rich granitoids are prominent in many Late Archean cratons (e.g. Sylvester, 1994). Although they comprise a significant part of the Archean crust, only lately has the origin of these granitoids attracted much attention. Recent studies have shown that a significant (but still unknown) proportion of the K-rich granitoids occurring in Archean granite−greenstone terrains belong to sanukitoid suites, i.e. diorites, monzodiorites, granodiorites and granites with high Mg contents and distinctive geochemical characteristics. These rocks are considered to represent the addition of mantle melts directly into the continental crust and are therefore very important in evaluating crustal growth rates and understanding the mechanisms of crust formation during the Archean.

The term Archean sanukitoid suite was first introduced by Shirey and Hanson (1984) for Archean igneous rocks (intrusive and extrusive) with geochemical characteristics similar to 12−15 Ma high-Mg andesites that are termed sanukites in the Setouchi area of Japan (Tatsumi and Ishizaka, 1982). The term sanukitoid originally referred to all textural modifications of the sanukite magma type. Stern et al. (1989) defined sanukitoids as primitive, intermediate igneous rocks with SiO₂ 55−60 wt.%, Mg numbers >0.6, Ni>100 ppm and Cr>200 ppm, K₂O>1 wt.%, Sr and Ba >500 ppm, Rb/Sr ratios <0.1, and strongly LREE enriched REE patterns with minor Eu anomalies. The term sanukitoid suite (or high-Mg diorite suite) refers to sanukitoids and related rocks (diorites through granodiorites) that are not geochemically equivalent but that have similar geochemical characteristics. In the IUGS classification and glossary of terms of Le Maitre et al. (2002), sanukitoid is defined as a plutonic equivalent of sanukite or Archean high-Mg quartz monzodiorite and granodiorite. In this paper, the term sanukitoid is used loosely as a synonym for high-Mg granitoids referring to a series of rocks having relatively high Mg numbers and high Ni, Cr, LILE (Sr, Ba, P) and LREE abundances at any given silica content (sanukitoid series).

The oldest sanukitoids (2.95 Ga) have been found in the granite−greenstone terrain of the Pilbara Craton, Western Australia (Smithies and Champion, 2000), although most known sanukitoids were formed at ca. 2.7 Ga. Sanukitoids are late- to post-kinematic (e.g. Shirey and Hanson, 1984, Stern et al., 1989, Beakhouse et al., 1999) and occur in Archean granite−greenstone terrains, intruding voluminous TTGs, the latter commonly interpreted as slab derived melts. Sanukitoids have been recognized from the Superior Province of the Canadian Shield (Shirey and Hanson, 1984, Shirey and Hanson, 1986, Stern et al., 1989, Stern and Hanson, 1991, Beakhouse et al., 1999, Stevenson et al., 1999), the Fennoscandian (Baltic) Shield (e.g. Lobach-Zhuchenko et al., 2000a, Lobach-Zhuchenko et al., 2005, Kovalenko et al., 2005, Halla, 2002), the South Indian Dharwar Craton (Sarvothaman, 2001, Moyen et al., 2003), the Ukrainian Shield (Artemenko et al., 2003), the Pilbara Craton in Western Australia (Smithies and Champion, 2000) and Greenland (Steenfelt et al., 2005).

All the studies above have established the somewhat paradoxical geochemical features of the sanukitoid-type rocks. Low SiO₂ content, high Mg number, enrichment in Mg, Ni and Cr point to a mantle-wedge peridotite source, whereas enrichment in LILE (Ba, Sr, P) and LREE indicate an enriched (metasomatized) mantle source. The isotopic composition of sanukitoids is also paradoxical: Nd and Sr isotopes point to a mantle origin (Stern and Hanson, 1991), but Pb isotope compositions of K-feldspars (Stevenson et al., 1999; Halla, this study) point to a crustal source. The geochemical features of wedge-derived sanukitoids are explained either by remelting of a mantle-wedge source previously hybridized by significant amounts of slab-derived (TTG) melts (e.g. Smithies and Champion, 2000), or by fluid-mobile elements during slab dehydration processes (Kamber et al., 2002).

Prior to this study, the only sanukitoid-type rocks which have been studied in Finland were the Arola granodiorite in the Kuhmo−Suomussalmi greenstone belt (Querre, 1985) and the Kuittila pluton in the Ilomantsi area (Nurmi et al., 1993). Halla (2002) reported sanukitoid-type granitoids in the southern Karelian Domain, eastern Finland. These are found in the Nilsiä and Lieksa areas in the westernmost part of the Karelian province of the Fennoscandian (Baltic) Shield.

This paper presents the major geochemical features and Pb and Nd isotope geochemistry of the Nilsiä and Lieksa granitoids and show that the granitoids have similar characteristics to sanukitoids reported from the Karelian Province of the Fennoscandian (Baltic) Shield and from the Superior Province, Canada. Because the role of crustal contamination in the genesis of the sanukitoids remains unclear, special emphasis is put on the Pb and Nd isotopic constraints on the crust−mantle interactions. The main conclusion of this paper is that the sanukitoids in eastern Finland originated at ca. 2.73 Ga from a mantle-wedge source enriched in LILE, U, Th and Pb by recycling of continental material in subduction-related slab dehydration processes shortly (up to 200 Ma) before melting.

Section snippets

Geological setting

The bedrock of southern Finland falls into two major domains: the Paleoproterozoic Svecofennian Domain in the west and the Late Archean Karelian Domain in the east. The southern Karelian Domain, which forms the westernmost part of the Karelian Province of the Fennoscandian (Baltic) Shield, consists mainly of Late Archean greenstone−granite belts of low metamorphic grade, and granulite−gneiss belts of high metamorphic grade, along with Paleoproterozoic quartzites, schists, migmatites and

Petrography and geochemistry

The K-feldspar megacrysts of the Nilsiä granitoids are Carlsbad-twinned perthitic microcline. Smaller phenocrysts of antiperthitic plagioclase are also present. Commonly, the original K-feldspar grains have undergone extensive recrystallization leading to the formation of thick, recrystallized mantles around the original grains. The recrystallized mantle around the core of the K-feldspar extends away from the core forming tails or wings in the matrix. In the more deformed parts, the rocks have

Pb isotopes

The Pb isotopic data for whole-rock samples (Table 2a) and K-feldspar fractions (Table 2b) are plotted on Pb−Pb diagrams in Fig. 4, Fig. 5. The two-headed arrow in the figures represent a mixing line between the upper crustal and mantle sources according to the plumbotectonic model of Zartman and Doe (1981, version II) at 2.7 Ga. This model includes also a U-depleted and less radiogenic lower-crust source, which is omitted from the figures because the contribution of this source to the Nilsiä

Nd isotopes

The Nilsiä and Lieksa granitoids have positive ɛ_Nd (2.7 Ga) values of 0.3−1.4 and Nd DM model ages of 2.75−2.86 Ga (Table 3). According to the studies of Lobach-Zhuchenko et al. (2000b) and Kovalenko et al. (2005), the sanukitoid intrusions from the younger Central Karelian Domain of the Fennoscandian (Baltic) Shield have positive initial ɛ_Nd values of 0.7−2.1 and Nd DM model ages of 2.70−2.85 Ga. The intrusions from the older West Karelian Domain have initial ɛ_Nd values of −1.7−+0.7 and older

Discussion and conclusions

U−Th−Pb data for the Nilsiä and Lieksa granitoids show the following characteristics: (1) low U/Pb (μ₃<4) ratios and high Th/U (κ₃>10) ratios for the rocks; (2) a high model μ₂ value relative to Stacey and Kramers (1975) for the source; and (3) a high ²⁰⁸Pb/²⁰⁴Pb ratio and elevated ²⁰⁷Pb/²⁰⁴Pb ratios for deformed K-feldspars, affected by the Paleoproterozoic Svecofennian tectonothermal event at 1.9 Ga. The extremely low U/Pb and high Th/U ratios of the rocks probably formed through U-depletion

Acknowledgements

This paper is based on the doctoral dissertation of the author. The geochemical and isotopic analyses were carried out at the Geolaboratory and the Laboratory for Isotope Geology, Geological Survey of Finland, Espoo. I wish to thank the staff of the Laboratory for Isotope Geology for the skilful assistance in sample preparation and analysis. Geochemical analyses were financially supported by the Geological Museum of the Finnish Museum of Natural History. I sincerely thank M. Whitehouse and T.

References (44)

G.P. Beakhouse et al.
Geochemical and U–Pb zircon geochronological constraints on the development of a Late Archean greenstone belt at Birch Lake, Superior Province, Canada
Precambrian Research
(1999)
E. Hanski et al.
The Os and Nd systematics of c. 2.44 Ga Akanvaara and Koitelainen mafic layered intrusions in northern Finland
Precambrian Research
(2001)
J.P. Hogan et al.
The effect of accessory minerals on the redistribution of lead isotopes during crustal anatexis: a model
Geochimica et Cosmochimica Acta
(1991)
P. Hölttä
Geochemical characteristics of granulite facies rocks in the Archean Varpaisjärvi area, central Fennoscandian Shield
Lithos
(1997)
P. Hölttä et al.
P–T–t development of Archaean granulites in Varpaisjärvi, Central Finland, II: dating of high-grade metamorphism with the U–Pb and Sm–Nd methods
Lithos
(2000)
S.B. Jacobsen et al.
Sm–Nd isotopic evolution of chondrites
Earth and Planetary Science Letters
(1980)
A.V. Kovalenko et al.
Petrogenetic constraints for the genesis of Archaean sanukitoid suites: geochemistry and isotopic evidence from Karelia, Baltic Shield
Lithos
(2005)
S.B. Lobach-Zhuchenko et al.
The Archaen sanukitoid series of the Baltic Shield: geological setting, geochemical characteristics and implications for their origin
Lithos
(2005)
M. Scambelluri et al.
Incompatible element-rich fluids released by antigorite breakdown in deeply subducted mantle
Earth and Planetary Science Letters
(2001)
S.B. Shirey et al.
Mantle heterogeneity and crustal recycling in Archean granite-greenstone belts: evidence from Nd isotopes and trace elements in the Rainy Lake area, Superior Province, Ontario, Canada
Geochimica et Cosmochimica Acta
(1986)

J.S. Stacey et al.

Approximation of terrestrial lead isotope evolution by a two-stage model

Earth and Planetary Science Letters

(1975)

A. Steenfelt et al.

Mantle wedge involvement in the petrogenesis of Archaean gray gneisses in W. Greenland

Lithos

(2005)

R. Stevenson et al.

Assimilation-fractional crystallization origin of Archean Sanukitoid Suites: western Superior Province, Canada

Precambrian Research

(1999)

P.J. Sylvester

Archaean granite plutons

Y. Tatsumi et al.

Origin of high-magnesian andesites in the Setouchi volcanic belt, southwest Japan, I. Petrographical and chemical characteristics

Earth and Planetary Science Letters

(1982)

P.N. Taylor et al.

Crustal contamination as an indicator of the extent of early Archaean continental crust: Pb isotopic evidence from the late Archaean gneisses of West Greenland

Geochimica et Cosmochimica Acta

(1980)

J.L. Wooden et al.

Pb, Sr, and Nd isotopic compositions of a suite of late Archean, igneous rocks, eastern Beartooth Mountains: implications for crust-mantle evolution

Earth and Planetary Science Letters

(1988)

R.E. Zartman et al.

Plumbotectonics—the model

Tectonophysics

(1981)

G.V. Artemenko et al.

Archean high-Mg granitoids (sanukitoids) in the Ukrainian Shield and its comparison with rocks of TTG suite

J.M. Brenan et al.

Experimental evidence for the origin or lead enrichment in convergent-margin magmas

Nature

(1995)

D.J. DePaolo

Neodymium isotopes in the Colorado Front Range and crust-mantle evolution in the Proterozoic

Nature

(1981)

J. Halla

Origin and Paleoproterozoic reactivation of Neoarchean high-K granitoid rocks in eastern Finland. Annales Academiae Scientiarum Fennicae

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Citation Excerpt :
In all geochemical diagrams, the BG from the Ourilândia complex shows affinity with the Xinguara-type granites from the Carajás province (Almeida et al., 2013; Feio et al., 2013; Leite-Santos and Oliveira, 2016; Rodrigues et al., 2014) and those from other Archean terranes, such as the BGs described in the Dharwar and Kaapvaal cratons (Laurent et al., 2014a; Moyen et al., 2003). Meanwhile, the Ourilândia SNKs show compositional affinities with the Rio Maria SNK suite (Oliveira et al., 2011) and SNKs from the Karelian (Finland; Halla, 2005; Heilimo et al., 2010) and Superior (Canada; Stevenson et al., 1999) provinces and those from the Pilbara craton (Australia; Smithies and Champion, 2000), as previously demonstrated by Santos and Oliveira (2016). However, the Ourilândia SNKs are characterized by granodioritic to tonalitic calc-alkaline series (Fig. 4a), while the Rio Maria SNKs are characterized by granodioritic to monzonitic calc-alkaline series (Oliveira et al., 2011).
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