Letter
Setting and origin for problematic rocks from the
\
3.7 Ga
Isua Greenstone Belt, southern west Greenland: Earth’s
oldest coarse clastic sediments
Christopher M. Fedo *
Department of Geology,Bell Hall,George Washington Uni6ersity,Washington,DC20052,USA Received 14 September 1999; accepted 18 November 1999
Abstract
Whether or not coarse detrital sedimentary rocks occur within the \3.7 Ga Isua Greenstone Belt (IGB), southern west Greenland, has been debated for some time. Repeated, regional metamorphic, deformational, and metasomatic events have obscured most protolith lithologies leading to misunderstandings about the stratigraphy and environ-ments of deposition. Rocks here interpreted as meta-conglomerate crop out in a fault-bounded structural domain that is lower in strain relative to adjacent domains. The meta-conglomerate has a strike length of 1 km and is 10 m thick. Bed thickness ranges from 10 cm to more than 1 m, and beds may be either framework- or matrix-supported. A poorly sorted and variably rounded polymict assemblage of framework clasts consisting of meta-chert, BIF, and a variety of mafic volcanic rock fragments are set into a matrix of biotite+quartz+garnet schist; clast compositions indicate reworking of adjacent stratigraphic units. A lack of structural similarity in framework clasts, range of grain sizes, range of rounding, and polymict composition demand a primary sedimentary origin for the deposit. Inferred depositional processes include traction and debris flow, which would be consistent with subaerial or shallow subaqueous environments, although the limited extent of the meta-conglomerate warrants interpretational caution. © 2000 Elsevier Science B.V. All rights reserved.
Keywords:Isua; Archean; Greenstone; Sedimentary; Conglomerate
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1. Introduction
The presence of coarse detrital sedimentary rocks within the ‘stratigraphy’ of the Isua supracrustal succession [Isua Greenstone Belt (IGB) Appel et al., 1998] has been postulated many times beginning with early geologic
descrip-* Corresponding author. Tel.: +1-202-9946964; fax: + 1-202-9940450.
E-mail address:[email protected] (C.M. Fedo)
tions (Allaart, 1976; Bridgwater et al., 1979; Dim-roth, 1982; Nutman et al., 1984). Despite the apparent recognition of such coarse rocks, there consistently has been concern that some of them could in fact represent severely tectonized rocks of uncertain protolith (Bridgwater et al., 1979; Nut-man et al., 1984), whose current outcrop appear-ance merely resembles metamorphosed ‘conglomerate’. Lack of consensus on this very basic matter still exists, and leads to significant confusion when discussing and interpreting the setting in which these historically problematic rocks were formed.
One problem in assessing possible origins for supposed clastic sedimentary rocks at Isua is the occurrence of several types of coarse-grained rocks, especially those termed ‘conglomeratic structures’ by Nutman et al. (1984), who recog-nized three specific types, namely (1) ‘flat pebble conglomerate structure’; (2) ‘round pebble con-glomerate structure’ and (3) ‘conglomeratic struc-ture in the felsic formation of sequence A’. The three different types of conglomerate are exposed in ‘Sequence A’ (Nutman et al., 1984; Nutman, 1986), which represents a poly-deformed and lithologically diverse set of rocks that comprise most of the supracrustal succession of the IGB. Based on new detailed stratigraphic (by C.M. Fedo) and regional structural (by J.S. Myers) mapping in the main study area (Fig. 1), it has become clear that the stratigraphy and protolith lithologies as outlined by Nutman et al. (1984) needed significant revision (Rosing et al., 1996; Appel et al., 1998) in order to better interpret the paleogeography. Utilizing new data collected dur-ing three field seasons (1997 – 1999), the purpose of this letter is to (1) re-examine the stratigraphic setting; (2) critically evaluate whether or not coarse detrital sediments occur within the belt, and (3) discuss the paleogeographic implications.
2. Geologic framework
Detailed outlines of the controversies and geo-logic events that have shaped the Early Archean terrane that includes the IGB region have been presented elsewhere (Nutman et al., 1996;
White-house et al., 1999). Tonalite – trondjhemite – gran-odiorite-suite rocks representing the Amıˆtsoq gneisses intruded the terrane that includes the IGB between about 3650 Ma and \3800 Ma (Nutman et al., 1996; Whitehouse et al., 1999), providing an indirect constraint on the formation age of the belt. Presently, the supracrustal succes-sion/Amıˆtsoq gneiss contacts are highly tec-tonized. The IGB, which forms an arcuate-shaped package of rocks, is the largest of many supracrustal enclaves that occur within the Amıˆt-soq gneiss, and that are termed the Akilia associa-tion (McGregor and Mason, 1977). Field relaassocia-tions show that the Amıˆtsoq gneisses and the IGB were repeatedly deformed and regionally metamor-phosed at amphibolite facies.
Attempts to date the stratigraphy directly have encountered problems concerning protoliths of the sampled material and lack of proper target rocks to examine. Rocks belonging to the ‘A6 felsic unit’ of Nutman et al. (1984) have yielded consistent single crystal U-Pb zircon ages of
3810 Ma (Compston et al., 1986; Nutman et al., 1997), although new mapping shows that the A6 unit is most likely a metasomatized sheet of Amıˆt-soq gneiss (also see Nutman et al., 1996; Rosing et al., 1996). Nutman et al. (1997) also reported a
3710 Ma single crystal U-Pb zircon age from quartzo-feldspathic schist (unit B1), interpreted to be of volcanic origin, west of the main NNE-strik-ing ductile structure that divides part of the IGB stratigraphy (Fig. 1). Kamber et al. (1998) sug-gested a source and depositional age in excess of 3.7 Ga for at least this part of the belt based on a Sm-Nd whole-rock regression age of 3742949 Ma for a collection of 24 schist samples from B1. More recently, Frei et al. (1999) documented a 3691922 Ma date using the PbSL method on magnetite, which was interpreted as the age of metamorphic recrystallization. Collectively, these studies point to a depositional age in excess of 3.7 Ga.
quartzo-feldspathic rock, garnet amphibolite, BIF, chert, and ultramafic rocks, whose primary depositional features have been mostly obliter-ated. Although still very deformed, the central domain is somewhat less metamorphosed and in a comparatively lower strain state relative to the
NW and SE domains, such that primary features are locally preserved (Appel et al., 1998). All litho-types in the central domain show a promi-nent foliation in association with a strong, steeply SE-plunging, stretching lineation that formed dur-ing the last phases of deformation.
3. Stratigraphic dilemma
One of the most significant problems in inter-preting the supracrustal succession in the Isua Greenstone Belt has been the failure to erect a meaningful stratigraphy based on properly inter-preted protoliths (Nutman et al., 1984; Rosing et al., 1996; Appel et al., 1998). Lithologies that comprise the IGB have been severely deformed, and in many cases also profoundly metasoma-tized, so that current appearance, mineralogy, and geochemistry rarely resemble the protolith. Al-though most rocks are highly schistose, locally abundant minerals in the succession, such as quartz and dolomite, have been dynamically re-crystallized so that internal mineralogic strain is not congruent with the overall strain state of the rocks (Bridgwater et al., 1981).
In one case within the study area, a widespread greenschist unit termed the ‘garbenschiefer amphi-bolite’ has long been considered an igneous intru-sion (Allaart, 1976; Gill et al., 1981; Nutman et al., 1984; Nutman, 1997), despite the fact that pillow textures had been recognized in the 1970s, and now are observed throughout the unit occur-ring in a variety of morphologies (Rosing and Rose, 1993; Komiya and Maruyama, 1995; Ros-ing et al., 1996; Appel et al., 1998). Pillow basalt thus forms a significant and integral part of the stratigraphic development of the IGB, especially in the domain that contains supposed conglomer-ates. By contrast, Dimroth (1982), who envisioned that the entirety of the succession at Isua was sedimentary, proposed that much of the iron for-mation and interlayered quartzite with dolomite represented original primary limestone of shallow subtidal origin (replaced by silica). However, nearly all of the structures seen in these rocks represent tectonic features, such as boudins, and carbonate forms a major replacement mineral in many lithologies (Rose et al., 1996). Given such examples, it is not surprising that much about the stratigraphy and surficial conditions during the time of deposition remains highly speculative.
Nutman et al. (1984) and Nutman (1986) erected a stratigraphic hierarchy for the IGB based on a number of transects perpendicular to foliation, including the area shown in Fig. 1. A
premise of that hierarchy is that the stratigraphy was folded into an upright isoclinal syncline early in the deformation history. New mapping indi-cates that no such structure exists, nor are litholo-gies repeated on opposite sides of the belt as would be expected on opposing fold limbs [also noted by Rosing et al. (1996)]. The cornerstone for building a ‘new’ stratigraphy in this part of the IGB has been the recognition of multiple ductile faults that divide the supracrustal rocks into tectonic domains whose internal stratigraphic sections are independent of each other (and most likely formed at different times, Fig. 1)following on the observations of Rosing et al. (1996), I propose that the lithologic units and stratigraphic terminology outlined by Nutman et al. (1984) be entirely abandoned because it no longer accu-rately describes rocks of the IGB. Rocks previ-ously classified as ‘flat pebble conglomerate structure’ and ‘round pebble conglomerate struc-ture’ crop out within the central domain of the study area (Fig. 1), whereas the ‘conglomeratic structure in the felsic formation of sequence A’ (A6 unit described above) is now mapped as a mylonitized, boudinaged, and metasomatized Amıˆtsoq gneiss sheet. Although younging direc-tion cannot be determined, primary stratigraphic units comprising the central domain are roughly orthogonal to the main foliation, which strikes NE and dips steeply to the SE (Fig. 1; Appel et al., 1998).
4. Meta-conglomerate of the Central Domain
4.1. Essential background
con-Fig. 2. Measured stratigraphic sections of the meta-conglomer-ate unit. Thicknesses are considered as structural. Strati-graphic younging direction is not known. (A) Western locality. Note abundance of replacement carbonate. GPS coordinate: N 65° 10.120%W 049° 49.568%. (B) Eastern locality. GPS coordi-nate: N 65° 10.466%W 049°48.004%.
tic sedimentary origin over a tectonic origin for this unit based on textural and compositional parameters discussed below.
Identifying primary sedimentary features in the sequence is limited by the shape (k) and magni-tude (d, Ramsey and Huber, 1983) of the pre-served strain. Dimroth (1982) proposed that the section was shortened by a factor of four based on pebble shapes, but offered no quantitative analysis. I have measured the axial lengths of 90 clasts (98% quartzite, 2% other) from a 3-dimen-sionally exposed outcrop near the western mea-sured section, which have yielded a strain ellipsoid whose axial proportions are 1:0.68:0.34, k=0.47, and d=1.1. TheX-axis is parallel to the regional stretching lineation. I consider this a minimum estimate of strain because it represents measure-ments taken primarily from quartzite (meta-chert) clasts that are more refractory than the biotite-rich volcanic clasts and enclosing matrix, which are highly deformed. The flattening strain has compromised original framework-matrix relation-ships, especially where the rocks are matrix sup-ported and translation of pebbles and cobbles through the ductilely deforming matrix would be expected; where the meta-conglomerate is sup-ported by quartzite clasts, primary depositional textures are better preserved. A number of deci-meter-thick mylonite zones segment the western section in particular giving rise to lithologic trun-cations that Dimroth (1982) interpreted as scour-and-fill structures. Partial to complete carbonate replacement of matrix material and adjacent lithologies is commonplace, especially where in spatial association with mylonite zones.
4.2. Stratigraphy and lithology
Rocks termed ‘round pebble conglomerate structure’ (hereafter referred to as meta-conglom-erate) crop out discontinuously along strike for about 1 km in the southern part of the central domain (Fig. 1). There are two principal expo-sures of the unit, one east of the major Protero-zoic mafic dyke, and the other to the west, with the latter one being the historically most studied (Allaart, 1976; Dimroth, 1982; Nutman et al., 1984; Nutman, 1986). I have measured detailed glomerate structure’ as a possibility to be of
clas-stratigraphic sections (Fig. 2) of the two expo-sures in the direction of structural dip and col-lected observations from a number of less completely exposed outcrops along strike. Both measured sections of meta-conglomerate are about 10 m thick (Fig. 2), but poor exposure at the base of each section precludes determining if more exists. Stratigraphic younging direction can-not be assessed with any certainty for the con-glomerate, nor in the central domain as a whole. One possible example of scour-and-fill structure (Fig. 3A) occurs near the western measured sec-tion hinting the unit there youngs in the dip direction. At the top of the sections, the meta-conglomerate is associated with some iron forma-tion and greenschist/garnet amphibolite that is interpreted as mafic meta-volcanic rocks (Fig. 2). Primary sedimentary layering that broadly defi-nes bedding is inferred from vertical grain size alternations (Fig. 3A,B). Regardless of the amount of matrix, framework clasts are com-monly rotated into the regional cleavage, which can give a false impression of imbricated bedding (Fig. 3C,D). Individual beds range from about 10 cm to more than 1 m in thickness (all measure-ments represent structural thicknesses). Bed ge-ometry occurs as broadly lens shaped to parallel sided with diffuse contacts, which probably results from both depositional and structural processes. A few beds consist of laterally discontinuous peb-ble clusters several grains thick (Fig. 3A,B). The western section is more framework supported (Fig. 3A), whereas the eastern section displays more matrix-supported layers (Fig. 3D). It is com-mon for matrix-supported units to be directly adjacent, and grade into, framework-supported
units (especially in the eastern section). Both sec-tions have thin intercalated pelites and the eastern section preserves a 30 cm thick bed of biotite quartzite interpreted to represent an immature arenite (Fig. 2B). Quartzite layers in the western section (Fig. 2A) could be interpreted as arenite, however a coarsely recrystallized chert origin is just as likely because they are so quartzose.
Framework clasts range in size from sand through cobbles (up to 20 cm in Y-axis dimen-sion) indicating that sorting thoughout the meta-conglomerate is very poor. Clasts are rounded to angular (Fig. 3B – F), although the effects of duc-tile deformation and coarse recrystallization are important, especially in the XZ plane where pointed tails on clasts are common. It is typical for clasts of different dimension, rounding, and composition to be adjacent to each other, which indicates a lack of structural continuity and makes a tectonic origin (such as boudinage) for the fabric very unlikely. The measured sections are traversed by younger quartz veins, some of which have been boudinaged and are readily dis-tinguished from the host conglomerate (Fig. 3A,E).
At least two depositional mechanisms may be inferred from the textural relations outlined above. The organization of framework-supported beds almost certainly represents deposition of bedload detritus by tractive processes. Although clast size has been structurally modified, present dimensions require that signifcant energy existed to transport and deposit such coarse material. Matrix-supported units are tentatively interpreted to represent cohesive debris flows (Lowe, 1979) where the mud matrix provides cohesive strength
to the deposit as it comes to rest. Caution is warranted in extending this interpretation too far because of the intensity of deformation in these units.
4.3. Composition and source
Framework clasts are set in a matrix of bi-otite+quartz+garnet9chlorite9pyrite schist. These represent replacement and metamorphic minerals, not detrital mineralogy. The biotite and chlorite are considered coarsely recrystallized and metasomatized clay mud that formed the matrix to the meta-conglomerate.
Dimroth (1982) noted a restricted framework clast assemblage of chert and biotite schist, while Nutman et al. (1984) considered the deposit as ‘oligomictic’ with quartz nodules and biotite-rich clasts. My analysis reveals more overall clast di-versity than originally reported, which helps to identify the unit as truely of detrital origin. In all examined exposures, clasts of quartzite dominate the gravel-sized composition (Fig. 3A – H). The quartzite is equigranular, very pure, and coarse grained (crystal units 1 – 2 mm across are com-mon), and interpreted as coarsely recrystallized chert. This is consistent with the texture of ‘chert’ interlayers in BIF deposits elsewhere in the study area (Bridgwater et al., 1981). A number of differ-ent colors ornamdiffer-ent the quartzite clasts, but I suggest the coloration represents stains related to later fluid infiltration events, not primary varia-tion. Rare clasts of oxide-facies BIF have coarsely recrystallized chert bands and display internal lay-ering inclined to foliation.
Clasts dominated by biotite are less abundant than homogenous quartzite, appear to blend in with the surrounding matrix, and show an array of textural variation. Common clast variants in-clude: (1) pure biotite flattened into cm-wide lenses (Fig. 3F); (2) light gray clasts of biotite+
quartz schist with mm-scale streaks of pure biotite (Fig. 3F); (3) even gray clasts of quartz+biotite schist (Fig. 3G) and (4) biotite+quartzite schist with clusters of quartz rodded in the direction of stretching (Fig. 3G). I interpret the collection of biotite-rich clasts to represent original mafic vol-canic source rocks, which is consistent with the
mineralogy of basaltic pillow breccia deposits B1 km away (Appel et al., 1998). Current mineralogy and geochemistry represent the cumulative effects of multiple metamorphic and metasomatic events that are known to have occurred (Rosing et al., 1996). Textural variants from above are thought to mimic primary eruptive diversity, especially the quartz-rodded lithology, which looks identical to deformed and recrystallized amygdaloidal pillow basalt fragments seen elsewhere (Appel et al., 1998). Such a polymict assortment of lithotypes would be difficult to generate from tectonic pro-cesses alone, and so a detrital origin is favored. Surrounding rocks of the central domain contain all the lithologies observed as clasts in the con-glomerate, suggesting the meta-conglomerate rep-resents reworking of the adjacent stratigraphy.
5. Paleogeographic implications
I consider the presence of the meta-conglomer-ate in the central domain of the Isua Greenstone Belt enigmatic. It represents a brief episode of very coarse clastic sedimentation in a section oth-erwise comprised of pillow basalt, pillow breccia, chert, and BIF, that resembles other ‘mafic-ultra-mafic volcanic association’ greenstone belt succes-sions (Eriksson et al., 1994), and that is most simply interpreted as having formed under below wave-base conditions (Rosing et al., 1996). How-ever, the rounded and polymict nature of frame-work clasts is more consistent with formation in a shallow subaqueous or emergent setting (Fisher and Schmincke, 1984, pp. 265 – 276).
meta-conglomerate. The overwhelming dominance of basalt in the central domain would indicate that as an important source lithology, which is consis-tent with the biotite+garnet schist composition of the matrix and the difficulty in recognizing basaltic clasts. Weathering of an exposed volcanic source is consistent with the relative abundance of chert clasts (now quartzite) over mafic clasts, which would be far more susceptible to degrada-tion during weathering processes.
Using these indirect lines of evidence, I hypoth-esize that the meta-conglomerate was derived from a subaerially exposed volcanic edifice that underwent chemical weathering processes, which concentrated more chemically resistant chert clasts. It is likely that the production of mud for matrix material was enhanced by early sea-water alteration of the lavas. The weathered material was then transported and deposited, probably near the source (Fisher and Schmincke, 1984, pp. 275), in subaerial or shallow subaqueous environ-ments by a combination of processes. The pres-ence of chert (interpreted as below wave base in origin) as clasts implies that sea-floor material was deposited, lithified, exposed, and eroded. A tec-tonic origin for uplifting of the source would be consistent with early Earth processes (Eriksson et al., 1994), although other mechanisms to expose the source, such as dramatic boiling off of ocean water resulting from a large asteroid impact (Sleep et al., 1989) or primary volcano growth must be considered as possible. An absence of other coarse sedimentary units in the central do-main, or anywhere else in the IGB, implies that exposure of the volcanic-dominated source was a temporally limited, anomalous event in the strati-graphic development of the succession.
6. Conclusions
Repeated episodes of regional metamorphism, deformation, and metasomatism have made it difficult to properly recognize the lithologies (and stratigraphy), and interpret the environments of deposition of rocks that comprise the Isua Green-stone Belt. Ductile faults segment the supracrustal rocks into variably strained domains, each of
which contain an independently formed strati-graphic section. Within a relatively lower strain domain (although still highly strained), about 10 m of nodular-appearing rock, whose origin has been debated, is best interpreted as a metamor-phosed conglomerate. The unit displays both framework- and matrix-supported layers. Frame-work clasts, which consist of meta-chert, BIF, and mafic volcanic rock, are very poorly sorted and rounding ranges from rounded to angular. Dis-tinct layer-perpendicular grain size alternations occur within the section, and are interpreted to represent boundaries of original sedimentary bed-ding. A lack of structural similarity in adjacent framework clasts, range of grain sizes, range of rounding, and polymict composition demand a primary sedimentary origin for the deposit. A subaerial or shallow subaqueous depositional set-ting proximal to source would be consistent with the rounding, coarseness, and organization of the deposit. Clast compositions represent reworking of the adjacent volcanic-dominated stratigraphy, which must have been uplifted and exposed to initiate the sedimentary cycle required to form the meta-conglomerate.
Acknowledgements
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