Elsevier

Gondwana Research

Volume 20, Issues 2–3, September 2011, Pages 255-283
Gondwana Research

GR Focus
An overview of the geochemistry of Eoarchean to Mesoarchean ultramafic to mafic volcanic rocks, SW Greenland: Implications for mantle depletion and petrogenetic processes at subduction zones in the early Earth

https://doi.org/10.1016/j.gr.2011.01.007Get rights and content

Abstract

This study reviews the geochemical characteristics of Eoarchean to Mesoarchean ultramafic to mafic volcanic rocks (now amphibolites) in SW Greenland and compares them with those of Cenozoic oceanic island arc basalts, to evaluate Archean subduction zone petrogenetic processes. Emphasis is placed on the Th–REE–HFSE (Zr, Ti, and Nb) systematics of the ca. 3800 and ca. 3700 Ma arc suites in the Isua greenstone belt, the ca. 3075 Ma Ivisaartoq–Ujarassuit greenstone belt, and amphibolites associated with the ca. 2970 Ma Fiskenæsset layered anorthosite complex.

On N-MORB-normalized diagrams, the Isua, Ivisaartoq–Ujarassuit, and Fiskenæsset volcanic rocks are all characterized by depletion of Nb relative to Th and LREE, consistent with a supra-subduction (forearc–arc–backarc) geodynamic setting. Similarly, on the Th/Yb–Nb/Yb projection, these suites plot within the field of Cenozoic oceanic island arc basalts. On log-transformed immobile trace element ratio (Nb/Th, La/Th, Sm/Th, and Yb/Th) diagrams, they display a trend projecting from MORB (Mid-Ocean Ridge Basalt) to IAB (Island Arc Basalt) on the IAB–CRB (Continental Rift Basalt)–OIB (Ocean Island Basalt)–MORB diagram, as for Cenozoic oceanic island arc basalts. Accordingly, these trace element compositions are interpreted as reflecting the enrichment of Archean depleted upper mantle (MORB-source) by subduction-derived melts and fluids following the initiation of intra-oceanic subduction and arc migration.

Concentrations of MgO and Ni in SW Greenland Archean basalts overlap with, but extend to 2 to 4 times higher than, those in Cenozoic oceanic island arc counterparts. In contrast, the majority of Archean basalts have REE and HFSE concentrations 2 to 4 times lower than Cenozoic oceanic island arc basalts, consistent with more depleted sub-arc mantle wedge peridotites in the Archean than in Cenozoic counterparts. Such depletion reflects the extraction of large volumes of mafic to ultramafic melts from hotter Archean mantle. We infer that less refractory compositions of the Present-day depleted upper mantle, the source of MORB and conservative HFSE in arc basalts, have resulted from re-mixing with the less depleted to enriched deeper mantle material transferred to shallower depths by mantle convection and plumes in post-Archean times.

Archean basalts in SW Greenland share the negative Nb and Ti anomalies of Cenozoic oceanic island arc basalts. On average, Archean basalts, however, have lower Th/Nb, La/Nb, Sm/TiO2 and Gd/TiO2 ratios than Cenozoic oceanic island arc counterparts. Given that rutile controls the Nb and Ti budgets in arc magmas, the lower Th/Nb, La/Nb, Sm/TiO2 and Gd/TiO2 ratios in SW Greenland basalts are attributed to the mobility of Nb and Ti in slab-derived melts involving rutile fusion in Archean subduction zones. These elements are less mobile in fluids originating from Cenozoic subducting oceanic crust where rutile appears to be generally stable. Accordingly, it is suggested that higher geothermal gradients in the Archean may have provided optimized conditions for slab melting and metasomatism of the sub-arc mantle wedge by slab-derived melts.

Graphical Abstract

Research Highlights

► This study deals with the geochemistry of Archean volcanic rocks in SW Greenland. ► Trace element characteristics suggest a supra-subduction geodynamic setting. ► On tectonomagmatic discrimination diagrams, they display a trend from MORB to IAB. ► Their mantle sources were metasomatized by subduction-derived melts and fluids. ► The source of Archean arc basalts was more depleted than their Cenozoic counterparts.

Introduction

The geodynamic origin of Archean greenstone belts, or terranes, has been extensively debated in the literature (Windley, 1993, Condie, 1981, Condie, 2005, de Wit, 1998, Kusky, 2004, Cawood et al., 2006, Kerrich and Polat, 2006, Furnes et al., 2009, Ernst, 2009, Herzberg et al., 2010). A number of geological models, including uniformitarian and non-uniformitarian plate tectonic models have been proposed to explain the origin of these greenstone terranes (Windley, 1993, de Wit, 1998, Hamilton, 1998, Kusky and Polat, 1999, Polat and Hofmann, 2003, Cawood et al., 2006, Stern, 2005, Dilek and Polat, 2008, Nutman et al., 2009). Uniformitarian models have invoked Present-day like geodynamic settings including intra-continental rifts, mid-ocean ridge rifts, backarc basins, island arcs, ocean plateaus, plumes in a continental setting, and subduction–accretion complexes, or some combination, to account for the variety of structural, lithological, and geochemical characteristics of Archean greenstone belts. Specifically, the Superior Province, the largest Archean craton in the world, has been interpreted as a series of allochthonous, amalgamated oceanic and continental fragments or tectonostratigraphic terranes, ranging in age from 3700 to 2680 Ma, and accreted from north to south over 40 million years between 2720 and 2680 Ma (Stott, 1997, Percival et al., 2006, and references therein). Similarly, on the basis of detailed field observations, contrasting ages, and metamorphic histories, several studies have shown that the SW Greenland Archean craton is a collage of Eoarchean to Neoarchean oceanic island arcs and continental fragments, assembled in several accretionary tectonothermal events by horizontal tectonics (McGregor, 1973, Bridgwater et al., 1974, Friend et al., 1988, Friend et al., 1996, Friend and Nutman, 2005a, Friend and Nutman, 2005b, Polat et al., 2008, Garde, 2007, Nutman et al., 2002, Nutman et al., 2009, Ordóñez-Calderón et al., 2009, Windley and Garde, 2009).

Over the last two decades, numerous geochemical studies have used trace element data to interpret the geodynamic origin of Archean greenstone belts, specifically associations of volcanic rocks to circumvent ambiguities of geodynamic interpretations arising from single lithotypes that may erupt in a variety of settings (e.g., Xie et al., 1993, Dostal and Mueller, 1997, Polat et al., 1998, 2002, Polat et al., 2005, Polat et al., 2006, Kerrich et al., 1998, Wyman et al., 2000, Parman et al., 2001, Wilson, 2003, Hollings and Kerrich, 2004, Hollings and Kerrich, 2006, Sandeman et al., 2004, Smithies et al., 2004, Smithies et al., 2005, Smithies et al., 2007, Garde, 2007, Manikyamba et al., 2008, Said et al., 2010, and references therein). Geodynamic interpretation in these studies is mainly based on normalized trace element patterns of the volcanic associations.

A number of lines of isotopic evidence from Archean rocks and Hadean zircon grains are consistent with the early differentiation of the mantle (Wilde et al., 2001, Boyet et al., 2003, Caro et al., 2003, Caro et al., 2006 Harrison et al., 2005, Harrison et al., 2008, Bennett et al., 2007, Blichert-Toft and Albarède, 2008, and references therein). However, the consequences of early differentiation of the mantle on the trace element compositions of Archean ultramafic to mafic volcanic rocks have not adequately been addressed. In addition, Th–REE–HFSE (Nb, Ti, and Zr) fractionations in Eoarchean to Mesoarchean subduction zones have, so far, been poorly understood.

In this review, we summarize the geological and geochemical characteristics of the Eoarchean Isua greenstone belt (ca. 3800 and ca. 3700 Ma arcs), Mesoarchean (ca. 3075 Ma) Ivisaartoq–Ujarassuit greenstone belt, and amphibolites associated with the Mesoarchean (ca. 2970 Ma) Fiskenæsset layered anorthosite complex, SW Greenland (Fig. 1, Fig. 2, Fig. 3). New major and trace element data are also reported for eighteen samples from amphibolites associated with the ca. 2970 Ma Fiskenæsset layered anorthosite complex. The geochemical characteristics of Archean volcanic rocks in SW Greenland are compared with those of Cenozoic oceanic island arc basalts, using extensive major and trace element data (> 700 analyses) from the Aleutian, Kuril, Mariana, Kermadec, Tonga, Vanuatu, and Scotia arcs. The main objectives of this study are: (1) to assess the geodynamic significance of Th–REE–HFSE systematics of Archean ultramafic to mafic volcanic rocks (now amphibolites) in SW Greenland, using N-MORB-normalized patterns, Nb/Yb–Th/Yb projection, and recently developed log-transformed trace element ratio (La/Th, Nb/Th, Sm/Th, and Yb/Th) discrimination diagrams (Agrawal et al., 2008); (2) to evaluate the degree of trace element depletion in Eoarchean to Mesoarchean upper mantle, as sampled by arc magmas, and its implications for mantle differentiation processes in the early Earth; (3) to address Th–Nb–REE–Ti fractionations in Archean subduction zones; and (4) in the light of this extensive database revisit the model of Pearce (2008) in which Archean arc volcanic sequences, that otherwise have intra-oceanic characteristics, are viewed as recording contamination by felsic crust.

Given that all Archean supracrustal rocks in SW Greenland are metamorphosed, the prefix ‘meta’ is implicit. Notwithstanding the fact that Archean supracrustal rocks in the Nuuk region were metamorphosed under amphibolite facies conditions, they are called “greenstone belts” to be consistent with the terminology used in our previous publications. Accordingly, the term “greenstone” used in this contribution does not imply greenschist facies metamorphism.

Section snippets

Regional geology and field relationships

The Archean craton of SW Greenland largely consists of Eoarchean to Neoarchean (ca. 3800–2700 Ma) orthogneisses with TTG (tonalite–trondhjemite–granodiorite) compositions (Steenfelt et al., 2005, and references therein). These orthogneisses contain many layers of amphibolite and anorthosite, both of which are up to ca. 2 km thick, together with important but rare sedimentary rocks. This lithological association occurs in alternating granulite and amphibolite facies belts (Bridgwater et al., 1976

Geochemistry and tectonic setting

The geochemical characteristics of Eoarchean to Mesoarchean mafic to ultramafic lavas in SW Greenland and those of Cenozoic oceanic island arc basalts are summarized in Fig. 9, Fig. 10, Fig. 11, Fig. 12, Fig. 13, Fig. 14, Fig. 15, Fig. 16, Fig. 17, Fig. 18, Fig. 19 and Table 1, Table 2, Table 3, Table 4, Table 5, Table 6. Fig. 10, Fig. 11, Fig. 12 include N-MORB-normalized trace element pattern for the average composition (n > 700) of Cenozoic oceanic island arc basalts from the Aleutian, Kuril,

Assessment of crustal contamination

Negative Nb anomalies in the Isua, Ivisaartoq–Ujarassuit, and Fiskenæsset volcanic rocks (Figs. 10–12) could reflect either mantle source characteristics or crustal contamination during magma emplacement. Contamination of these volcanic rocks by continental crust during magma emplacement was ruled out by Polat et al., 2002, Polat et al., 2009a, Polat et al., 2010) on the basis of the following geological, geochemical, and isotopic observations: (1) there is no field evidence that the Isua,

Summary and conclusions

This study reviews the Th–REE–HFSE systematics of ultramafic to mafic volcanic rocks (now amphibolites) in the Eoarchean Isua (ca. 3800 and ca. 3700 Ma arc terranes) and Mesoarchean (ca. 3075 Ma) Ivisaartoq–Ujarassuit greenstone belts, and mafic volcanic rocks (now amphibolites) associated with the Mesoarchean (ca. 2970 Ma) Fiskenæsset layered anorthosite complex.

The ca. 3800 Ma arc terrane contains transitional to tholeiitic basalts, ‘enriched’ basalts, picrites, cherts, and BIF. The ca. 3700 Ma

Acknowledgments

C.R.L. Friend and an anonymous reviewer are acknowledged for their constructive comments, which have resulted in significant improvements to the paper. We thank R. Kerrich for reviewing the initial draft of the manuscript. We thank T. Plank for providing references and invaluable comments on the geochemistry of arc magmas. This contribution is supported by NSERC grants 250926 to A. Polat and 83117 to B. Fryer. Field work was supported by the Bureau of Minerals and Petroleum in Nuuk and the

Ali Polat is professor of Geology at the University of Windsor, Ontario, Canada, since 2002. Polat received his BSc (1988) from Istanbul Technical University, Turkey, MSc (1992) from the University of Houston, USA, and PhD (1998) from the University of Saskatchewan, Canada. He was a recipient of the Alexander von Humboldt (1999–2001) and Max-Planck Society (2000–2001) Research Fellowships to study the geochemistry of the Eoarchean Isua greenstone belt, Greenland, at Max-Planck-Institute, Mainz,

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    Ali Polat is professor of Geology at the University of Windsor, Ontario, Canada, since 2002. Polat received his BSc (1988) from Istanbul Technical University, Turkey, MSc (1992) from the University of Houston, USA, and PhD (1998) from the University of Saskatchewan, Canada. He was a recipient of the Alexander von Humboldt (1999–2001) and Max-Planck Society (2000–2001) Research Fellowships to study the geochemistry of the Eoarchean Isua greenstone belt, Greenland, at Max-Planck-Institute, Mainz, Germany. Polat's main research interests include Archean greenstone belts, anorthosites, ophiolites, accretionary complexes, trace element and radiogenic isotope geochemistry, and geodynamics. He published over 50 research papers.

    Peter W.U. Appel is Senior Research Scientist at the Geological Survey of Denmark and Greenland (GEUS). Graduated from the University of Copenhagen. He has spent more than 10 years with exploration of Greenland for mining companies. In 1976, Appel joined the Department of Air Chemistry at Max-Planck-Institute für Chemie, Mainz, Germany, to work with atmosphere evolution in the early Archaean. This assignment included extensive field work in Greenland in the 3.8 Ga Isua greenstone belt. As a senior researcher at the Geological Institute at the University of Copenhagen, Appel continued his work on the Isua rocks (1980–1982). Since 1982, he has worked at GEUS as a senior researcher primarily with the geochemistry of various greenstone belts in West Greenland. During the last five years, his work has mainly been devoted to mitigating environmental problems caused by extraction of gold by artisanal miners in Third World countries. He has published a number of refereed papers in international journals.

    Brian Fryer graduated from M.I.T. in 1971 with a PhD in geochemistry. He has had academic appointments at the University of Western Ontario, Memorial University of Newfoundland and is currently Professor of Earth and Environmental Science at the University of Windsor and a Fellow of the Royal Society of Canada. His research interests are diverse and range from the early history of Earth and the formation of mineral deposits to environmental processes and biomineralization. Core to his research is the generation of high quality trace element and isotopic data and the developing of new analytical techniques, particularly LA-(MC)ICP-MS for application to natural systems.

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