Clark R.L. Friend

The Itsaq Gneiss Complex of southern West Greenland; the world's most extensive record of early crustal evolution (3900-3600 Ma)

The Itsaq Gneiss Complex of southern West Greenland contains the best-preserved occurrences of ⩾ 3600 Ma crust. Its known area is ∼ 3000 km2 with almost continuous exposure in some places. SHRIMP UPb zircon geochronology shows that the gneiss complex had a complicated early history, having been added to, and modified in, several events starting at ∼ 3900 Ma. Supracrustal, mafic and ultramafic rocks comprise approximately 10% of the complex and range in age from ⩾ 3870 to ∼ 3600 Ma. A large portion of the Isua supracrustal belt and some other bodies contain sequences of LREE-enriched, mafic (locally pillow-structured) to felsic volcanic and volcaniclastic rocks (some deposited from turbidity currents) and abundant diverse chemical sediments. These sequences might have formed in an environment analogous to present-day volcanic arcs. Other (mostly ⩾ 3800 Ma?) units dominated by LREE-depleted, high Ti/Zr mafic rocks (as found in komatiites and komatiitic basalts free of crustal contamination) with layers of banded iron formation might have been derived from volcanic edifices formed as a result of plume activity. Only in the youngest supracrustal sequences (∼ 3600 Ma) are detrital sediments derived from mixed-provenance clastic sources an important component. Layered anorthosite, gabbro and peridotite units (some ⩾ 3800 Ma) derived from large deep crustal intrusions are also widespread. In addition there are massive dunites and harzburgites. In the rare cases where their original mineralogy and texture are preserved, these contain aluminous spinel, indicating equilibration in the lowermost crust or upper mantle. Approximately 90% of the complex consists of grey gneisses, the dominant protoliths having been tonalites and less abundant trondhjemites, quartz-diorites, diorites and granodiorites. The protoliths were intruded in several events starting at ∼ 3870 Ma. Like other suites of Archaean grey gneisses, they were formed by partial melting, probably in an arc environment, of buried (subducted?) amphibolite, leaving residual hornblende ± garnet. Granites form approximately 10% of the complex. The oldest are 3650 Ma leucogranites, which probably formed by deep crustal anatexis of predominantly tonalitic gneisses. There are also ∼ 3625 Ma augen granites and ferrogabbros, whose chemistry resembles that of some A-type, within-plate granite suites. The evolution of the Itsaq gneiss complex is marked by increasing compositional diversity with time. Pre-3650 Ma lithologies are predominantly mafic and ultramafic rocks, tonalites and diorites. The first recorded regional metamorphic event occurred at 3650 Ma, and was marked by localised partial melting and intrusion of leucogranites. This might record crustal thickening, brought about by collision of different blocks of tonalite-dominated crust. A further thermal event occurred just after 3600 Ma, as shown by PbPb titanite and feldspar ages and local intrusion of granites. Deposition of sediments at ∼ 3600 Ma which were derived from mixed-age sources suggests their derivation from an extensive block of “continental” crust.

RECOGNITION OF 3850 MA WATER-LAIN SEDIMENTS IN WEST GREENLAND AND THEIR SIGNIFICANCE FOR THE EARLY ARCHAEAN EARTH

A layered body of amphibolite, banded iron formation (BIF), and ultramafic rocks from the island of Akilia, southern West Greenland, is cut by a quartz-dioritic sheet from which SHRIMP zircon 206Pb/207Pb weighted mean ages of 3865 +/- 11 Ma and 3840 +/- 8 Ma (2 sigma) can be calculated by different approaches. Three other methods of assessing the zircon data yield ages of >3830 Ma. The BIFs are interpreted as water-lain sediments, which with a minimum age of approximately 3850 Ma, are the oldest sediments yet documented. These rocks provide proof that by approximately 3850 Ma (1) there was a hydrosphere, supporting the chemical sedimentation of BIF, and that not all water was stored in hydrous minerals, and (2) that conditions satisfying the stability of liquid water imply surface temperatures were similar to present. Carbon isotope data of graphitic microdomains in apatite from the Akilia island BIF are consistent with a bio-organic origin (Mojzsis et al. 1996), extending the record of life on Earth to >3850 Ma. Life and surface water by approximately 3850 Ma provide constraints on either the energetics or termination of the late meteoritic bombardment event (suggested from the lunar cratering record) on Earth.

∼ 3710 and ⪖ 3790 Ma volcanic sequences in the Isua (Greenland) supracrustal belt; structural and Nd isotope implications

Abstract The Isua supracrustal belt of southern West Greenland is the largest body of early Archaean supracrustal rocks, and as such has been the focus of numerous studies to characterise early Earth processes. Based on new geochronological constraints we re-interpret the Isua supracrustal belt stratigraphy and re-evaluate Nd isotopic constraints for the early Earth from Isua supracrustal belt rocks. Part of the belt (∼ 50%) consists of a package of mafic chloritic schists (the garbenschiefer unit), layered amphibolites and felsic rocks, mica (±kyanite) schists and banded iron formation (BIF), and is interpreted as a sequence of mafic to felsic volcanic/volcanosedimentary rocks, turbidites and pelites with minor gabbro. Graded felsic volcanic, in this package have yielded a UPb zircon age of ∼ 3710 Ma. Along the southern side of the belt, a package of amphibolites and ultramafic schists with BIF layers, interpreted as a mafic to ultramafic volcanic sequence, is cut by tonalite sheets with U-Pb zircon ages of 3790–3800 Ma. Thus the belt contains at least two unrelated supracrustal packages of different ages (⪖ 3790 Ma and ∼ 3710 Ma), that were probably juxtaposed in the early Archaean. Felsic volcanic rocks from a single ∼ 3710 Ma unit have consistent initial ϵ Nd values of +0.8 to + 1.8, suggesting minimal fractionation of Nd from Sm during later metamorphism. They may have formed from a depleted (but not ultra-depleted) mantle source only a short time before 3710 Ma, with or without a contribution from LREE-enriched crust of 3750–3870 Ma with ϵ Nd (3800) of + 2 to +4. There is debate as to whether the 3806 ± 2 Ma ‘A6’ felsic unit with ϵ Nd(3800) of + 1.0 to +2.9 is derived from volcanic rocks or from granitoid sheets. Thus in the published Isua supracrustal belt Nd database there are no samples which are definitely > 3710 Ma supracrustal or clearly mantle-derived rocks. Given the problems of locally intense alteration, the presence of supracrustal rocks of two or more ages and of older inherited crustal components in some samples, the Isua supracrustal belt is not the ideal locality to find definitive answers on early Archaean mantle evolution.

Rapid emergence of life shown by discovery of 3,700-million-year-old microbial structures

Biological activity is a major factor in Earth’s chemical cycles, including facilitating CO2 sequestration and providing climate feedbacks. Thus a key question in Earth’s evolution is when did life arise and impact hydrosphere–atmosphere–lithosphere chemical cycles? Until now, evidence for the oldest life on Earth focused on debated stable isotopic signatures of 3,800–3,700 million year (Myr)-old metamorphosed sedimentary rocks and minerals from the Isua supracrustal belt (ISB), southwest Greenland. Here we report evidence for ancient life from a newly exposed outcrop of 3,700-Myr-old metacarbonate rocks in the ISB that contain 1–4-cm-high stromatolites—macroscopically layered structures produced by microbial communities. The ISB stromatolites grew in a shallow marine environment, as indicated by seawater-like rare-earth element plus yttrium trace element signatures of the metacarbonates, and by interlayered detrital sedimentary rocks with cross-lamination and storm-wave generated breccias. The ISB stromatolites predate by 220 Myr the previous most convincing and generally accepted multidisciplinary evidence for oldest life remains in the 3,480-Myr-old Dresser Formation of the Pilbara Craton, Australia. The presence of the ISB stromatolites demonstrates the establishment of shallow marine carbonate production with biotic CO2 sequestration by 3,700 million years ago (Ma), near the start of Earth’s sedimentary record. A sophistication of life by 3,700 Ma is in accord with genetic molecular clock studies placing life’s origin in the Hadean eon (>4,000 Ma).

Timing of late Archaean terrane assembly, crustal thickening and granite emplacement in the Nuuk region, southern West Greenland

A breakthrough in understanding the structural and metamorphic evolution of Archaean gneiss complexes occurred with the recognition that the Nuuk region of southern West Greenland comprised separate terranes assembled in the late Archaean. From northwest to southeast these are: the Akia (3220-2970 Ma), Akulleq (3870-3600 and 2820 Ma) and Tasiusarsuaq (2920-2860 Ma) terranes. The minimum time of assembly is recorded by the first event common to all component terranes. Using SHRIMP UPb zircon geochronology the oldest events common to all terranes (including emplacement of crustally derived granites, contemporaneous metamorphism and anatexis) have been dated at 2710–2725 Ma. In the Akulleq terrane areas where in situ diatexite formed, abundant granitoid sheets were intruded and common growth of metamorphic zircon (mostly low ThU) in most lithologies occurred. In the Akia and Tasiusarsuaq terranes there was only intrusion of a lesser number of ∼ 2720 Ma granitoid sheets, because metamorphic zircons of that age have not been found. The ∼ 2720 Ma event is interpreted as marking, or shortly following, terrane assembly, when the diverse components of the Akulleq terrane were tectonically juxtaposed with the other two. This new documentation of metamorphic and associated igneous events within an accreting cratonic region is an illustration that the stabilisation of extensive areas of Archaean gneisses can be due to accretionary tectonics long after the individual components were first formed.

The early Archaean Itsaq Gneiss Complex of southern West Greenland: the importance of field observations in interpreting age and isotopic constraints for early terrestrial evolution

Geochemical and isotopic studies of small volumes of variably preserved≥ 3600 Ma rocks in gneiss complexes are crucial for documenting early Earth history. In the Itsaq Gneiss Complex of the Nuuk region, West Greenland, there is dispute whether the granitic (sensu lato) orthogneisses dominating it are mainly products of a single ca. 3650 Ma crust formation “super event,” or whether they formed in several unrelated events between ca. 3850 and 3560 Ma. Which of these interpretations of the dates is correct has major implications regarding what the whole rock radiogenic isotopic record (Pb/Pb, Sm/Nd, Rb/Sr) reveals about continental crust formation and early terrestrial differentiation. There is also debate whether some West Greenland metasedimentary rocks with 12C/13C data interpreted as evidence for life are≥ 3850 Ma or only≥ 3650 Ma old. Establishing the correct age for these rocks is important for debates concerning early surficial environments and origin of life. Controversies have arisen because of different approaches taken by different workers, specifically with respect to how much emphasis is placed on field geology in interpreting dates and isotopic data. In this paper, field observations and sampling from low strain zones, where the origin and geological context of the rocks are best preserved and understood, are closely integrated with U-Pb zircon dates and cathodoluminescence (CL) imagery of the zircons. This approach shows that most single-phase, well-preserved, meta-granitoid samples have simple zircon populations dominated by oscillatory-zoned prismatic grains formed when their host magmas crystallized. On the other hand, migmatites and some strongly deformed-banded gneisses have much more complex zircon populations. The combined field evidence and zircon geochronology on the Itsaq Gneiss Complex demonstrate that 1) some areas contain exposed orthogneisses formed during multiple magmatic/thermal events between ca. 3850 and 3560 Ma and are not (as suggested by Kamber and Moorbath, 1998) dominated by ca. 3650 Ma granitoids containing abundant> 3650 Ma zircons inherited from cryptic, unexposed, older rocks; 2) abundant,≥ 3750 Ma granitoids are present, which are locally well-preserved; 3) some water-lain sediments reported as showing C isotope evidence for life were deposited as early as 3850 Ma; 4) the whole-rock Sm/Nd isochron approach fails to distinguish with any confidence 3650 Ma from 3800 Ma rocks, 5) however, it reinforces previous indications for markedly depleted (≥ + 2.5 ϵNd) domains in the pre-3750 Ma mantle.

Provenance of the Moine Supergroup of NW Scotland: evidence from geochronology of detrital and inherited zircons from (meta)sedimentary rocks, granites and migmatites

Detrital and inherited zircons from rocks of the Moine Supergroup from structural and stratigraphic positions above and below the Sgurr Beag and Naver thrusts have been dated by ion microprobe. 207Pb/206Pb ages for 65 detrital zircons from the Moine Nappe (Morar Group) range between 2707 and 947 Ma, with a bimodal distribution with clusters at c. 1650 and 1400 Ma. Five grains (8% of the analyses) were Archaean. Analyses of 97 inherited zircons from two Caledonian migmatites and two granites from the Naver Nappe yield 207Pb/206Pb ages between c. 2940 and 926 Ma. They have a dominant cluster at c. 1650 Ma with significant clusters at c. 1400 and c. 1050 Ma. Eight Archaean grains were discovered (8.5% of analyses). Three rocks above the Sgurr Beag Thrust have been examined and a total of 42 analyses obtained. 207Pb/206Pb ages for 35 inherited zircons from two outcrops of the essentially in situ West Highland granite gneiss within the Glenfinnan and Loch Eil Groups range from c. 1889 to 947 Ma. A sample of the Lochailort pelite provided a further seven detrital analyses. These data have a distribution with clusters at c. 1500 and 1100 Ma. The combined datasets indicate that the Moine Supergroup was deposited in a post-Grenvillian basin(s) with detritus derived predominantly from a late Palaeoproterozoic source, e.g. Gothian or Labradorian with only a very small proportion from older sources. The data indicate that the Naver and Sgurr Beag nappes had different sediment sources and so may not correlate. The Morar Group is dominated by late Palaeoproterozoic detritus with lesser amounts of Grenville-aged detritus. The Glenfinnan and Loch Eil Groups appear to have roughly equal proportions of detritus from the two sources. The few Archaean grains indicate that the source comprised comparatively little material of this age, precluding the Lewisian Complex or sub-Moine basement as a significant source and arguing against correlation of the Moine with the Torridonian.

New evidence for protolith ages of Lewisian granulites, northwest Scotland

The granulite facies Scourian rocks in the central region of the Lewisian complex of northwest Scotland play an important role in the understanding of the development of mid-Archean high-grade gneiss complexes. Whereas the later, largely Proterozoic history of the Scourian has been clarified in a detailed U-Pb study, unraveling the Archean ages of the protoliths using conventional isotope techniques has proved impossible. In order to advance our understanding of the deep crustal processes responsible for these granulites, it is essential to establish the age(s) of accretion of the protoliths and their subsequent tectonothermal history. A combined cathodoluminescence (CL) and sensitive high-mass resolution ion microprobe (SHRIMP) single-zircon study has revealed a hitherto unrecognized morphological complexity in zircons from the type localities of the granulites. Use of CL allowed identification of relict oscillatory igneous zoning, metamorphic overgrowths, and irregular areas of recrystallization. From the SHRIMP data, an age of ca. 2960 Ma is inferred for the gneiss protoliths, which were altered considerably during an important metamorphic event ca. 2490 Ma.

Late-Archaean tectonics in the Færingehavn–Tre Brødre area, south of Buksefjorden, southern West Greenland

The Færingehavn–Tre Brødre area consists of three distinct terranes tectonically jux-taposed by a previously unrecognized event. Contacts between the terranes are mylonitic shear zones truncating lithological units in adjacent terranes. The terrenes are: (1) Færingehavn terrane, largely composed of early-Archaean Amîtsoq gneiss cut by younger Archaean granitic gneiss defined here as the Sätut gneiss; (2) Tre Brødre terrane comprising mid-Archaean Malene supracrustal rocks, anorthosite complex and polyphase Nûk gneisses; (3) Tasiusarsuaq terrane largely comprising mid-Archaean Nûk gneisses affected by c. 2800 Ma granulite facies metamorphism. The Tasiusarsuaq terrane is structurally above both the Færingehavn and Tre Brødre terranes which at c. 2800 Ma experienced a lower grade of metamorphism. Juxtaposition of the terranes took place between 2800 and 2500 Ma and involved thrusting and crustal thickening. Subsequent re-equilibration involved folding, steeply inclined shear belts, intrusion of synkinematic granitoids under amphibolite facies conditions and retrogression of granulite facies assemblages. This thrusting post-dates the 2800 Ma granulite facies metamorphism. It is younger and distinct from the thrusting postulated to explain the intercalation of the early-Archaean Amîtsoq gneisses and the mid-Archaean Malene supracrustal rocks, associated with intrusion of the Nûk gneiss precursors described from Godthåbsfjord. The tectonic breaks provide, for the first time, marker horizons which can be used to assess the amount and type of late Archaean deformation in the southern Godthåbsfjord region.

Anatomy of an Early Archean gneiss complex: 3900 to 3600 Ma crustal evolution in southern West Greenland

The Early Archean complex of southern West Greenland consists of polyphase, tonalitic-trondhjemitic-granodioritic (TTG) and granitic Amitsoq gneisses with inclusions of volcanic and sedimentary rocks, gabbros, and ultramafic rocks. In this complex, rocks of similar appearance and composition were found to be of different ages by U-Pb zircon dating; the Amitsoq gneisses comprise 3870,3820-3810, 3760, 3730, 3700, and 3625 Ma TTG and 3660-3650 and 3625 Ma granites, and their inclusions belong to several supracrustal sequences with a similar spread of ages. These results show that the complex grew by episodic addition of new TTG and welding together of rocks of different ages. A possible plate-tectonics scenario is as follows: Melting of subducted mafic (oceanic) crust formed ≥3700 Ma microcontinents consisting of TTG suites with predominantly mafic inclusions. At 3650 Ma, collision between microcontinents caused crustal thickening, high-grade metamorphism, and emplacement of leucogranites. At 3625 Ma,subduction at the edge of the >3625 Ma continental mass created a new crustal addition comprising both TTG and granite, while granites were emplaced into the >3625 Ma continental mass.