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A Vestige of Earth's Oldest Ophiolite

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Science  23 Mar 2007:
Vol. 315, Issue 5819, pp. 1704-1707
DOI: 10.1126/science.1139170

Abstract

A sheeted-dike complex within the ∼3.8-billion-year-old Isua supracrustal belt (ISB) in southwest Greenland provides the oldest evidence of oceanic crustal accretion by spreading. The geochemistry of the dikes and associated pillow lavas demonstrates an intraoceanic island arc and mid-ocean ridge–like setting, and their oxygen isotopes suggest a hydrothermal ocean-floor–type metamorphism. The pillows and dikes are associated with gabbroic and ultramafic rocks that together make up an ophiolitic association: the Paleoarchean Isua ophiolite complex. These sheeted dikes offer evidence for remnants of oceanic crust formed by sea-floor spreading of the earliest intact rocks on Earth.

Ophiolites represent sections of oceanic crust that were generated by sea-floor spreading and later emplaced onto continental margins (1). Originally, ophiolites were assumed to represent oceanic crust formed at mid-ocean ridges, but this view has changed radically, and it is becoming clear that the majority of ophiolites are generated in supra–subduction-zone environments (1). Depending on their tectonic environment of formation and their structural architecture and geochemical affinities, Phanerozoic ophiolites can be classified into different types, but the majority are genetically related to subduction environments (1, 2). A complete ophiolite consists of submarine basaltic volcanic rocks (mainly pillow lavas), sheeted dikes, a plutonic complex, and upper-mantle rocks. However, many ophiolites lack one or more of these components (2). In Archean greenstone belts, the absence of complete ophiolite pseudostratigraphies (in particular, sheeted dikes and gabbros) has led many workers to conclude that ophiolites are not represented in the earliest stages of Earth's history (35). The oldest purported example is the 2505-million-year-old Dongwanzi ophiolite complex in the North China craton (6), which is a disputed claim (7). It has been suggested that several Archean greenstone belts host dismembered ophiolites (8, 9). Nonetheless, the question of whether Archaean oceanic crust formed by sea-floor spreading was related to Phanerozoic-like plate tectonics has so far remained conjectural because of the absence of compelling kinematic evidence to discriminate between origins through the horizontal motion of plates at divergent plate boundaries or through vertical motion above mantle plumes. Sheeted dikes provide an answer to these questions because they form by sea-floor spreading and accretion during horizontal movement at divergent plate boundaries, and they are considered to be crucial components of ophiolites. Here we report the discovery of a sheeted-dike complex within the Paleoarchean Isua supracrustal belt (ISB). This and the associated rocks, together with their compositions, make up a ∼3.8-billion-year-old ophiolite, which in turn has strong implications about the early tectonic and geochemical evolution of Earth.

The ISB, situated in southwestern Greenland (Fig. 1A), defines an arcuate belt ∼35 km long and 2.5 km thick (Fig. 1B) that contains a variety of igneous and sedimentary rocks (10, 11). In general, the rocks are strongly deformed and metamorphosed to amphibolite facies, and primary features are scarce. The main lithologies of the ISB are metabasalts (amphibolites), metagabbros and ultramafics associated with metapelites, cherts, banded iron formations (BIFs), and felsic rocks (11), now preserved as enclaves within the surrounding plutons (Ikkattoq and Amitsoq gneisses) (Fig. 1B).

Fig. 1.

(A) Location of the ISB in southwestern Greenland. The black square shows the location of the area detailed in (B). (B) ISB and adjacent gneisses. (Inset) Location of the area detailed in (C). (C) Simplified geological map of the western arm of the ISB, showing locations (1 to 3) of the 100% sheeted-dike complex (3: latitude 65.05.335°N, longitude 50.10.661°W) grading into dikes and volcanic rocks (2: latitude 65.07.033°N, longitude 50.09.769°W) and volcanic rocks (1: latitude 65.07.889°N, longitude 50.09.835°W). The geological maps [(B) and (C)] are modified from (10, 13, 15).

Pillow structures and associated hyaloclastites are common within the amphibolites (12, 13) and provide unequivocal evidence of submarine lavas. Another major component (∼50%) of the ISB, a unit described as “garbenschiefer,” was first considered to be derived from mafic intrusions (10), but has been reinterpreted as a volcano sedimentary sequence containing gabbro sills (11, 13, 14). It has been suggested that together these assemblages represent oceanic-like crust that may have been obducted within an accretionary wedge ∼3.7 billion years ago, as a result of plate tectonic–like processes (12).

Radiometric dating (U/Pb and Pb/Pb) has shown that the ISB formed between ∼3800 and 3700 million years ago (Ma) (14). Whole-rock Sm-Nd isochrons define ages of 3779 ± 81 Ma from metasediments and enclosing garbenschiefer (15) and 3777 ± 44 Ma from pillow lavas and metagabbro (16). The latter date is partly based on samples collected from location 1 of this study (Fig. 1C).

We examined a number of sections in the eastern part of the western arm of the ISB, mapped as variegated schists (10) and amphibolites (13, 17) within the area around locations 1 to 3 (Fig. 1C). Location 1 shows well-preserved metabasaltic pillow lava (Fig. 2A), in places with small triangular pockets of interpillow hyaloclastite (Fig. 2A). Locally, the pillows contain felsic ocelli (Fig. 2B) and rare amygdales. The rocks are homogeneously deformed and contain a cleavage that is subparallel to the lithological layering. Deformed ocelli (originally spherical) indicate deformation with 80 to 90% shortening perpendicular to the cleavage and 200 to 250% extension along a well-defined lineation plunging 72°S. At location 2, ∼1.5 km south of location 1 (Fig. 1C), the sequence consists of tabular subparallel dikes with intervening centimeter- to decimeter-thick zones of lenticular-to-irregular screens of volcanic material (Fig. 2, C and D, and fig. S1), and locally, plagiogranite occurs [Fig. 2D and supporting online material (SOM)]. Approximately 500 m further south, at location 3, the mixed dike/volcanic sequence changes structurally downward into a sheeted complex consisting of 100% tabular dikes (Fig. 2E and fig. S2), which to the west is in tectonic contact with metagabbro and ultramafic sheets. Individual dikes range in width from 2 to 50 cm. Dikes have both one- and (mostly) two-way fine-grained chilled planar margins (Fig. 2E). Cross-cutting dikes are also observed (Fig. 2F). We examined a number of sections across a 30–to–50-m-wide subvertical sequence of dikes, which we interpret as part of a sheeted-dike complex with an estimated predeformation width of >200 m.

Fig. 2.

(A) Well-preserved pillow lava exhibiting chilled margins (dark selvages) and pockets of interpillow hyaloclastite (IPH). (B) Ocelli-bearing pillows. The pale gray ocelli, originally spheres, give a measure of the deformation that the rocks have suffered. (A) and (B) are from location 1. (C) Dikes with intervening layers of volcanic rocks (V). (D) Nearly 100% sheeted dikes with minor amounts of interdike volcanic material. Dikes can be traced along strike for more than 20 m. The white lens (bottom right) is a plagiogranite (PG). (C) and (D) are from location 2. (E) 100% sheeted-dike complex. The weathered-out zones are amphibole schist, originally chilled margins. (F) Two crosscutting layers (just below the hammer head). (E) and (F) are from location 3. [(G) and (H)] Photomicrographs of the central part of the dike (G), showing relict subophitic texture (plane-polarized light image), and chilled margin of the dike (H), consisting predominantly of fine-grained amphibole (plane light image).

Petrographically, the central parts of the dikes consist of fine-grained (∼300 μm) plagioclase, amphibole (predominant), and biotite with relic subophitic texture (Fig. 2G). The dark green (commonly schistose) marginal zones, inferred to represent chilled margins (Fig. 2E), consist of dense (∼100 μm) monomineralic zones of amphibole (Fig. 2H). These chilled margins of the dikes are texturally and mineralogically similar to the margins of the pillows.

Mafic gneisses interpreted as metagabbros occur as scattered outcrops within an area of ∼100 by 100 m in the southwestern part of the ISB. They are uniform amphibolites characterized by centimeter-scale discontinuous layers and lenses of plagioclase in a hornblende-quartz-plagioclase matrix (fig. S3), representing a highly deformed and metamorphosed gabbroic texture. This contrasts with most Isua amphibolites, which typically consist of lithological units <1 m across strike (Fig. 2, C and D). Felsic dikes of the Amitsoq gneisses crosscut the metagabbros and define the early Archean age of the Isua supracrustals.

The ultramafic rocks (fig. S4), which occur mainly along the boundaries of the western belt of the ISB, have been variably transformed to serpentinites and calc-silicate rocks by metasomatic processes (11). These layered meta-ultramafic rocks are associated with the sheeted-dike complex (Fig. 1C) and metagabbro, sometimes with uninterrupted transitions from layered ultramafic sequence into gabbros (18).

The geochemistry of pillow lava and dikes from locations 1 and 3 (Fig. 1C), as well as that of the least altered samples of previous studies (17, 19), has been plotted in discriminant diagrams with various combinations of the relatively immobile elements Ti, V, Cr, Y, and Zr (1). The new geochemical data (table S1 and SOM) demonstrate intraoceanic island arc and mid-ocean ridge basalt (MORB) affinities (Fig. 3), as previously concluded (19). Furthermore, the similarities in the concentrations of incompatible elements (Ti, V, Zr, and Y) and their ratios (Zr/Y) strongly suggest that the pillow lavas and the dikes are cogenetic, supporting our field observations regarding their spatial and temporal relationships. It has been demonstrated that the metabasalts of the central garbenschiefer unit are geochemically similar to boninites (17) (Fig. 3). The presence of boninites is important in the evaluation of the tectonic environment, because they are generally associated with modern intraoceanic island arcs and are thought to be related to proto-arc and back-arc spreading (2022). This magmatic progression suggests that ophiolites are geochemically heterogeneous and that their tectonic evolution may have involved initial sea-floor spreading, followed by subduction initiation and one or more episodes of arc splitting and basin opening (20, 21).

Fig. 3.

(A) Ti-V, (B) Zr-Zr/Y, (C) Ti-Zr, and (D) Y-Cr discrimination diagrams (1). The geochemical data from the central-, outer-, and inner-arc tectonic domains (undifferentiated metabasalts) are from (17, 19). The new geochemical data of this study are shown in red. The boninite data are from (21). The Ti/V ratios in (A) are characteristic of the following: 10 to 20, island arc; 20 to 50, MORB; 20 to 30, mixed MORB and island arc; and 10 to 50, back-arc basins. Bon, boninites; IAT, island-arc tholeiite; and WPB, within-plate basalt.

Oxygen isotope data from locations 1 and 3 (Fig. 1C) show that the pillows are more enriched in 18O than the dikes (Table 1 and SOM). Although the δ18O values of the central part of the pillows range between 6.5 and 9.9 (average, 7.2), the dikes show a narrower range between 5.7 and 6.9 (average, 6.3). These rocks do not record primary magmatic oxygen isotope values but may record alteration by 0 to +2 of the δ18O value of seawater at a spreading ridge. The pillows are more enriched in 18O because they altered at lower temperatures than did the dikes, and oxygen isotope fractionation decreases in magnitude with increasing temperature (23). Collectively, these data and the relic subophitic textures in the sheeted dikes are consistent with the seawater/rock interaction during ocean-floor metamorphism that takes place at modern spreading ridges (SOM) and which has been documented in most ophiolites of Phanerozoic and Proterozoic ages (24, 25). This finding is also consistent with fluid-inclusion studies on amygdales in the ISB pillow breccias that indicate alteration during early sea-floor–like hydrothermal metamorphism (26).

Table 1.

Summary of δ18O results.

SampleTypeδ18O (standard mean ocean water)View inline
03-3.5 Pillow core 9.9
03-3.13 Pillow core 6.8
03-3.22 Pillow core 7.3
03-3.23 Pillow core 6.9
03-3.31 Pillow core 6.7
03-3.39 Pillow core 6.5
03-3.40 Pillow core 6.6
1A2-IG-06 Pillow core 6.8
1B4-IG-06 Pillow core 6.8
2A-IG-06 Pillow core 6.9
2B2A-IG-06 Pillow core 7.1
3A2A-IG-06 Pillow core 6.8
5B1-IG-06 Pillow core 7.9
5C-IG-06 Pillow core 7.6
6B-IG-06 Pillow core 6.8
7-IG-06 Dike 5.9
8-IG-06 Dike 6.0
9-IG-06 Dike 6.1
16A-IG-06 Dike 6.0
16C-IG-06 Dike 6.3
16E-IG-06 Dike 6.9
16G-IG-06 Dike 6.9
17A-IG-06 Dike 5.8
17B-IG-06 Dike 5.7
  • View inline* See SOM.

  • We provide three robust lines of evidence for an Isua ophiolite complex as a vestige of Archean supra–subduction-zone oceanic crust. First, the sheeted-dike complex and cogenetic pillow lavas represent the upper-crustal section of a dismembered ophiolite. The sheeted-dike complex provides compelling structural evidence of horizontal extension by dike injection at a spreading ridge (SOM). Second, we reject the scenario of dike injection above a plume head in a non–plate tectonic environment, given the oceanic island arc and MORB geochemical characteristics of the pillow lavas and dikes reported here, together with the data from (19). Further, the boninitic affinity of the central garbenschiefer (17) issimilar to that of Phanerozoic supra–subduction-zone ophiolites with a protracted tectonomagmatic evolution history (20, 22). Third, the oxygen isotope compositions of the pillow lavas and dikes and their petrographic textures are compatible with sea-floor hydrothermal metamorphism at a spreading ridge. Although the strain history of these rocks is not yet sufficiently well known to permit a detailed reconstruction of the Isua ophiolite complex, we contend that the ISB preserves vestiges of Earth's oldest ophiolite and oceanic crust. This implies that sea-floor spreading and subduction processes of Phanerozoic-like plate tectonics were operating ∼3.8 billion years ago, as proposed by Komiya et al. (12).

    Supporting Online Material

    www.sciencemag.org/cgi/content/full/315/5819/1704/DC1

    SOM Text

    Figs. S1 to S4

    Table S1

    References

    References and Notes

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