Geochemical variations in Archean volcanic rocks, southwestern Greenland: Traces of diverse tectonic settings in the early Earth

Ali Polat Department of Earth and Environmental Sciences, University of Windsor, Windsor, Ontario, Canada

The primary driving force behind present-day structural, magmatic, crust were assembled at convergent plate boundaries and intruded pre- sedimentary and metamorphic processes is plate tectonics, resulting dominantly by TTGs, forming Archean greenstone belt-granitoid terranes from the fl ow of matter and energy between the lithosphere and mantle (Kusky and Polat, 1999; Burke, 2011). along divergent, convergent, and transform plate boundaries. Operation Many geochemical studies on Archean volcanic rocks used primi- of plate tectonics and eruption of hot spot lavas from mantle plumes, tive- or MORB-normalized trace element diagrams to interpret their stemming from the core-mantle boundary, appears to be coupled in geodynamic setting (see Puchtel et al., 1999; O’Neil et al., 2011). On that subducting oceanic plates pile up at the core-mantle boundary and N-MORB-normalized diagrams, the Eoarchean Isua, and the Mesoar- then rise as buoyant plumes to feed hot spot volcanoes (see Hofmann, chean Ivisaartoq-Ujarassuit and Fiskenæsset basaltic volcanic rocks in 1997; Burke, 2011). How far back in Earth history were these geo- southwestern Greenland are characterized by large negative Nb anomalies logical processes driven by plate tectonics? Were Archean geological relative to Th and La (Polat et al., 2011), indicating that hydrous fl uids processes dominated by density-driven, vertical crustal overturns and and/or melts originating from the subducted oceanic slabs metasomatized diapirs, without modern analogs? How did Archean continents grow? their mantle sources. These volcanic rocks plot in the arc fi eld on an Nb/ How did Archean oceanic crust form? Did Archean oceanic crust recycle Yb–Th/Yb proxy diagram, as do Phanerozoic supra-subduction zone into the mantle at subduction zones? These questions remain controver- ophiolites and oceanic island arc basalts (Pearce, 2008; Polat et al., 2011). sial. Excellent exposures and a prolonged geological record spanning Archean oceanic island basalts could have not been generated with- 3.85–2.5 Ga in the Archean craton of southwestern Greenland provide out subduction of oceanic crust, but Archean basalts with unambiguous a unique opportunity to test hypotheses proposed for the early evolution MORB-like signatures are extremely rare (Polat and Kerrich, 2006). of Earth. Basalts with OIB-like trace element characteristics have so far been rec- This craton consists mainly of Eoarchean to Neoarchean (ca. 3.8– ognized only in Neoarchean Wawa greenstone belts (Polat et al., 1999). 2.7 Ga) metamorphosed tonalite-trondhjemite-granodiorite suites (TTGs), The scarcity of MORB, OIB, and mélanges in Archean terranes, together amphibolite-dominated greenstone belts (also known as supracrustal with the lack of intact ophiolites and blueschists, have been used to argue belts), and layered anorthosite complexes (Friend and Nutman, 2005; against the interpretation of Nb-depleted Archean rocks as subduction Windley and Garde, 2009; Polat et al., 2011). These all underwent several zone products, and the formation of Archean continental crust at conver- phases of deformation and greenschist to granulite facies metamorphism. gent plate margins. All geological data are consistent with formation through accretion of oce- In this issue of Geology, Jenner et al. (2013, p. 327–330) report OIB- anic island arcs and continental blocks (Friend and Nutman, 2005; Polat like trace element patterns for Eoarchean (ca. 3.75 Ga) basaltic amphibo- et al., 2011). The remnants of oceans closed during accretion are marked lites from Innersuartuut Island, southwestern Greenland. The authors con- by abundant pillow lavas and ultramafi c rocks in the Isua, Ivisaartoq- vincingly show that crustal contamination and metamorphic alteration can Ujarassuit and Tartoq greenstone belts (Polat et al., 2011; Kisters et al., be ruled out, providing geochemical evidence for the existence of hot spots 2012), which occur as fault-bounded lithotectonic assemblages, intruded as early as 3.75 Ga. These are the oldest known, plume-derived intra-plate by contemporaneous TTGs. The structural and lithological characteristics volcanic rocks and appear to have been accreted to a convergent plate mar- of these belts are comparable to those of Phanerozoic accretionary com- gin and intruded by TTGs. In contrast to other Archean basaltic rocks in plexes (see Şengör et al., 1993). southwestern Greenland, which plot in the IAB fi eld on a log-transformed Despite some lithological differences between Archean and Pha- La/Th–Nb/Th–Sm/Th–Yb/Th discrimination diagram, the Innersuartuut nerozoic terranes (i.e., higher abundance of komatiites, TTG intrusions, Island basaltic amphibolites plot predominantly in the OIB fi eld, support- layered anorthosites, banded iron formations in Archean terranes), signa- ing the interpretation of these rocks as OIB (Figs. 1A and 1B). These tures of plate tectonic processes are well preserved in the former (de Wit, rocks probably represent only a tiny relic of a large ocean island fragment 1998; Kerrich and Polat, 2006; Percival et al., 2006; Burke, 2011). The that appears to have been mostly destroyed by deformation, erosion and lithological differences likely resulted from higher mantle temperatures TTG intrusion following its accretion. and an oxygen-poor atmosphere in the Archean. Plate tectonics appear to The presence of Archean IAB and OIB in southwestern Greenland have shaped the evolution of Earth since the formation of the oldest known implies the generation and destruction of oceanic crust at spreading cen- rocks (Burke, 2011). ters and subduction zones, respectively, in those ancient times. The ques- Modern basalts from different tectonic settings, including mid-oce- tion then arises: Why has Archean MORB not been extensively preserved anic ridge basalt (MORB), oceanic island arc basalt (IAB), ocean island in greenstone belts? Given its negative buoyancy, almost all Archean basalt (OIB), and oceanic plateau basalt (OPB), have different trace ele- oceanic crust generated at spreading centers probably was recycled into ment signatures (see Pearce and Peate, 1995; Hofmann, 1997; Kerr, 2003). the mantle by subduction, similar to its modern counterpart. In contrast, This geochemical behavior likely also prevailed in the Archean, given that thicker, buoyant oceanic plateaus, island arcs and fore-arcs would have certain groups of elements behave consistently in specifi c petrogenetic accreted to convergent plate boundaries (Kerrich and Polat, 2006). Fol- settings (Polat and Kerrich, 2006). The chemical composition of basalts lowing their accretion, oceanic islands arcs and plateaus were multiply in Archean greenstone belts carries the fi ngerprints of their geodynamic deformed, metamorphosed, and intruded by TTGs, becoming part of settings (Puchtel et al., 1999; Polat and Kerrich, 2006), suggesting that Archean continental crust. Oceanic island arcs and plateaus thus have they are remnants of Archean oceanic crust. These fragments of oceanic been preferentially preserved.

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