David Bridgwater

Stratigraphic and geochemical evidence for the depositional environment of the early archaean isua supracrustal belt, southern west greenland☆

The highly deformed c. 3800 Ma Isua supracrustal belt is a fragment of a more extensive Early Archaean sedimentary and volcanic succession intruded by and tectonically intercalated with tonalitic and granitic Amftsoq gneisses in the period 3800-3600 Ma. The supracrustal rocks recrystallised under amphibolite facies conditions between 3800 and 3600 Ma, in the Late Archaean and locally at c. 1800 Ma. Layered sequences of rock of sedimentary and probable volcanic origin form over 50% of the belt. Bodies of high Mgue5f8Al basic rocks and ultramafic rocks were intruded into the layered sequences prior to isoclinal folding and intrusion of Amitsoq gneisses. The layered rocks which are < 1 km thick are divided into two sequences, that are in faulted contact with each other. The way-up of these sequences has been determined from facing-directions of locally-preserved graded layering in felsic metasediments at several localities. The overall upwards change in sedimentary succession is interpreted as showing change from dominantly basic to dominantly felsic volcanism which provided the major clastic component of the sediments. Clastic sedimentation took place against a background of chemical sedimentation, shown by interlayers of banded iron formation, metachert and calc-silicate rocks throughout the sequences. The felsic rocks locally preserve graded bedding and possible conglomerate structures, indicating deposition from turbidite flows and possibly as debris flows. Nodules in the felsic rocks contain structures interpreted as fiamme. There is an irregular enrichment in K2O/Na2O in many of the felsic rocks at constant SiO2 and Al2O3 content, interpreted as owing to alteration of original andesitic to dacitic volcanic rocks. Banded iron formations locally contain conglomeratic structures suggesting sedimentary reworking, possibly under shallow water conditions. Lithological and geochemical characters of the clastic components of the supracrustal sequences are consistent with derivation from felsic and basic volcanic rocks and do not require a continental source.

Early Archaean granulite-facies metamorphism south of Ameralik, West Greenland

Relict high-pressure granulite-facies rocks have been found in the Amiˆtsoq gneisses and inclusions of the older Akilia supracrustal association, on islands south of Godthab. Only amphibolite-facies assemblages have been found in Ameralik dykes and younger rocks from this area. The Amiˆtsoq gneisses are depleted in Rb and U relative to those of Ameralik and Isua. Well-fitted Pb/Pb and Rb-Sr isochrons on Amiˆtsoq granulites indicate that this depletion, correlated with the granulite-facies metamorphism, occurred ca. 3600 Ma ago. Textural features suggest that the present cpx + opc + gnt + plag + qtz + hbl assemblages evolved from earlierintermediate-P assemblages (cpx + opx + plag), probably during cooling from the metamorphic peak. Re-equilibrium of olderintermediate-P assemblages in local environments of low ƒH2O, during the ca. 2800-Ma metamorphism of the Malene supracrustals, is feasible but is considered unlikely. Either interpretation requires crustal thickness of at least 20 km and geothermal gradients of⩽30°C/km, ca.3600Ma ago. The higher heat production of early Archaean times was apparently dissipated through oceanic, rather than continental, areas.

Ion microprobe UPb zircon geochronology and isotopic evidence for a trans-crustal suture in the Lapland–Kola Orogen, northern Fennoscandian Shield

The Lapland–Kola Orogen (LKO; former Kola craton) in the northern Fennoscandian Shield comprises a collage of partially reworked late Archaean terranes with intervening belts of Palaeoproterozoic juvenile crust including the classic Lapland Granulite Terrane. Rifting of Archaean crust began at c 2.5–2.4 Ga as attested by layered mafic and anorthositic intrusions developed throughout the northernmost Fennoscandian Shield at this time. Oceanic separation was centred on the Lapland Granulite, Umba Granulite (UGT) and Tersk terranes within the core zone of the orogen. Importantly, Smue5f8Nd data show that Palaeoproterozoic metasedimentary and metaigneous rocks within these terranes contain an important, generally dominant, juvenile component over a strike length of at least 600 km. Evidently, adjacent Archaean terranes, with negative eNd signatures, contributed relatively little detritus, suggesting a basin of considerable extent. Subduction of the resulting Lapland–Kola ocean led to arc magmatism dated by the NORDSIM ion probe at c 1.96 Ga in the Tersk Terrane in the southern Kola Peninsula. Accretion of the Tersk arc took place before c 1.91 Ga as shown by ion probe Uue5f8Pb zircon dating of post-D1, pre-D2 pegmatites cutting the Tersk arc rocks, juvenile metasediments as well as Archaean gneisses in the footwall of the orogen. Deep burial during collision under high-pressure granulite-facies conditions was followed by exhumation and cooling between 1.90 and 1.87 Ga based on Smue5f8Nd, Uue5f8Pb and Arue5f8Ar data. Lateral variations in deep crustal velocity and Vp/Vs ratio, together with reflections traversing the entire crust observed in reprocessed seismic data from the Polar Profile, may be interpreted to image a trans-crustal structure — possibly a fossilised subduction zone — supporting an arc origin for the protoliths of the Lapland Granulite, UGT and Tersk terranes and the location of a major lithospheric suture — the Lapland–Kola suture.

Early Archaean Amîtsoq tonalites and granites of the Isukasia area, southern West Greenland: development of the oldest-known sial

Amphibolite facies early Archaean Amîtsoq gneisses envelop and intrude the c. 3,800 Ma Isua supracrustal belt, Isukasia area, southern West Greenland. Most of these gneisses are strongly deformed, but in a c. 75 km2 augen of lower deformation, the Amîtsoq gneisses are seen to comprise predominantly 3,750–3,700 Ma tonalitic grey gneisses that were intruded first by thin bodies of mafic to dioritic composition, known collectively as the Inaluk dykes, and then by c. 3,600 Ma white gneisses and finally by sporadic c. 3,400 Ma pegmatitic gneiss sheets. The grey gneisses could have formed by partial melting of crust consisting predominantly of basic rocks. The Inaluk dykes are interpreted as strongly fractionated basic melts of mantle origin, contaminated by crustal material. The white gneisses consist mostly of medium grained granite and occur as lenses and anastomosing sheets throughout their host of grey gneisses with subordinate inclusions of supracrustal rocks. The white gneisses have chemistry compatible with formation by partial melting at depth of a source dominated by grey gneisses. The igneous chemistry, including REE abundances, of the grey gneisses and white gneisses has been modified to varying degrees by metasomatism and assimilation reactions during the crystallisation of the white gneisses and also during subsequent tectonometamorphic events. The white gneisses are evidence for considerable reworking by anatexis of sialic crust in the early Archaean, 150 to 100 Ma after its formation. The white gneisses and the pegmatitic gneisses show that granitic rocks s.s. were important in the earliest Archaean, and are further evidence of the diversity of the oldest-known sial.

U–Pb zircon ages of Kangâmiut dykes and detrital zircons in metasediments in the Palaeoproterozoic Nagssugtoqidian Orogen (West Greenland): Clues to the pre-collisional history of the orogen1

Abstract In the southern marginal zone of the Palaeoproterozoic Nagssugtoqidian orogen and its foreland (West Greenland), Archaean gneisses were intruded by the syn-kinematic Kangâmiut dyke swarm. Leucodioritic cores of two dykes have yielded U–Pb zircon ages of 2036±5xa0Ma and 2046±8xa0Ma (2σ). Dyke emplacement and associated crustal movements thus occurred well before the main plutonic and tectonic activity within the orogen (∼1900–1800xa0Ma). In the central Nagssugtoqidian orogen, pelitic to semi-pelitic metasedimentary rocks were intruded by 1920–1900xa0Ma quartz diorites and tonalites of the Arfersiorfik association, which have chemical and isotopic compositions comparable with igneous suites formed in arc environments. The supracrustal and intrusive rocks are separated from Archaean gneisses by decollements of high-grade mylonite and annealed marble, and are allochthonous units. Detrital zircons in two samples of metasedimentary rocks from these allochthons are mostly large, poorly rounded, 2100–1950xa0Ma old with only a few Archaean grains. Deposition of the metasediments must have taken place between ∼1950 and ∼1920xa0Ma, before the intrusion of rocks of the Arfersiorfik association. The zircons may have been shed from presently unrecognised pre-Arfersiorfik association igneous complexes (arcs?), whose emplacement coincided with intrusion of the ∼2040xa0Ma Kangâmiut dykes into deforming Archaean crust in what later became the southern margin of the orogen. Quartz-rich psammitic metasedimentary rocks are also present in the central Nagssugtoqidian orogen. Detrital zircons from one sample yielded a few 2150–2250xa0Ma grains, but are predominantly Archaean. They include a high proportion of 3300–3400xa0Ma grains (the oldest material yet found in the orogen). Thrust-bounded panels of metasedimentary rocks in Archaean gneisses occur at the boundary between the southern and central parts of the orogen coinciding with the northern limit of the Kangâmiut dykes. Of the detrital zircons in one metasediment sample from these panels a few yielded ages around 2100–2000xa0Ma, some were Archaean, and >50% were ∼2400xa0Ma. Deposition of these two sediment suites took place after ∼2100xa0Ma. The detrital zircon population shows that the sources for these sediments were different from that of the above-mentioned allochthonous rocks. Neither the 3300–3400xa0Ma nor the ∼2400xa0Ma detrital grains can be matched with any rocks in Greenland, indicating a distal source for these sediments.

The iron-rich suite from the Amîtsoq gneisses of southern West Greenland: early Archaean plutonic rocks of mixed crustal and mantle origin

A distinctive group of augen gneisses and ferrodiorites (termed the iron-rich suite) is a component of the early Archaean Amîtsoq gneisses of southern West Greenland. The iron-rich suite outcrops south of the mouth of Ameralik fjord in an area that underwent granulite facies metamorphism in the early Archaean. The iron-rich suite forms approximately 30% of the Amîtsoq gneiss of this area and occurs as sheets and lenses up to 500 m thick. The rest of the Amîtsoq gneisses are predominantly tonalitic-granodioritic, banded grey gneisses. Despite intense deformation and polymetamorphism, there is local field evidence that the iron-rich suite was intruded into the grey gneisses after they had been affected by tectonism and metamorphism. The banded grey gneisses are interpreted as 3,700 to 3,800 Ma old; U-Pb zircon ages from the iron-rich suite give concordia intercepts at circa 3,600 Ma.Coarse grained augen gneisses with microcline mega-crysts are the dominant lithology of the iron-rich suite. They are mostly granodioritic, grading locally into granite and diorite, and are generally rather massive, but locally have well-preserved layering or are markedly heterogeneous. Mafic components are commonly concentrated into “clots” rich in hornblende and biotite and containing apatite, ilmenite, sphene and zircon. Variation in the proportion of these clots is the main reason for the compositional variation of the augen gneisses. The ferrodiorites of the suite occur as lenses in the augen gneisses. Leucocratic granitoid sheets locally cut the iron-rich suite. The augen gneisses and ferrodiorites have geochemical characteristics in common, such as high Fe/Mg values and high contents of FeOt, TiO2, P2O5, Zr, Y and total REE (rare earth elements).The iron-rich suite probably formed as follows:Heating of the lower crust adjacent to mantle-derived basic intrusions caused melting of the lower crust, giving rise to granodioritic magmas. Disruption of partially crystallised basic intrusions caused mixing of the crustal melts and the fractionated mantle melts to produce the augen gneisses with their high FeOt, TiO2, P2O5, Zr, Y and total REE enrichment. Fragmented, crystallised parts of the basic intrusions gave rise to the ferrodiorite inclusions. These heterogeneous plutons rose to higher crustal levels where they crystallised as sheets and possibly were responsible for the local granulite facies metamorphism. The granitoid sheets that cut the iron-rich suite are interpreted as crustal melts of local origin.The iron-rich suite resembles Proterozoic rapakivi granite-ferrodiorite-norite (anorthosite) associations which form characteristic suites in late- to post-tectonic environments in recently thickened sial. The occurrence of this type of magmatism in the early Archaean is evidence of the complex, polygenetic nature of the oldest known continental crust.

Discrepancies between neodymium, lead and strontium model ages from the Precambrian of southern East Greenland: Evidence for a Proterozoic granulite-facies event affecting Archaean gneisses

Abstract Smue5f8Nd (IDM) model ages for gneisses from the coastal region between 66° and 68°N in southern East Greenland range from 3.02 to 2.79 Ga and indicate that these basement rocks were formed in a major late Archaean episode of sialic crust formation between 3.0 and 2.8 Ga ago. Very low concentrations of U have resulted in unradiogenic Pb-isotopic compositions, so that most samples do not yield chronologically useful Pb Pb isochrons. The data have been used to calculate Pb Pb model ages, and in the northern and southern parts of the study area these are in good agreement with the Smue5f8Nd model ages. In the central part of the area, however, Pb Pb model ages are much lower than Smue5f8Nd ages, locally as low as ∼ 1.80 Ga. This discrepancy is attributed to loss of U long after the crust formation event, and probably related to a phase of Proterozoic granulite-facies metamorphism that has not previously been recognised. Rb-Sr isotope data support this interpretation.

The petrogenesis of the Kangâmiut dyke swarm, W. Greenland

Abstract Previous studies have shown that the 2.04 Ga Kangâmiut dyke swarm of SW Greenland was injected into an active tectonic environment associated with the formation of the Nagssugtoqidian orogenic belt. Major and trace element modelling of the swarm shows that its chemical evolution was controlled by simple clinopyroxene–plagioclase fractionation. However, such trends — although typical of continental flood basalts and mafic dyke swarms — are at variance with their mineralogy and petrography, which show that locally hornblende is the dominant primary ferromagnesian mineral. Modelling of intradyke fractionation alone shows that hornblende could locally have been an important crystallising phase within several dykes. Normal basaltic fractionation must have occurred before dyke injection at the exposed crustal levels, where the influx of water into the dykes is believed to be responsible for the transition from clinopyroxene–plagioclase (tholeiitic) to hornblende–plagioclase±oxides (calc–alkaline) crystallisation. Overall geochemical trends are dominated by tholeiitic fractionation because (1) hornblende fractionation tended to buffer chemical composition; (2) the presence of water in the surrounding country rocks may have resulted in the advection of heat away from the dyke and consequently resulted in rapid crystallisation, particularly in thin dykes. There is no evidence from trace element data, and particularly Pb isotopic ratios, of any significant assimilation of country rocks occurring during clinopyroxene–plagioclase fractionation, although this does not preclude contamination of the mantle source prior to magma generation. It is likely that the incompatible element enrichment within the dykes resulted from subduction-related mantle metasomatism. The Kangâmiut dyke swarm was both a syn-tectonic and thermal event, which triggered it may be linked to passage of a slab window underneath the metasomatised region, or a mantle plume ascending under a subduction zone.

Early Proterozoic mafic dykes in the North Atlantic and Baltic cratons: field setting and chemistry of distinctive dyke swarms

Abstract Widespread swarms of basic dykes intruded the Archaean North Atlantic craton (NAC) and the NW Baltic shield between 2.5 and 1.9 Ga. The oldest Mg-rich dykes and sills are 2.4–2.5 Ga and are associated with rift controlled acid and basic magmatism. In SW Greenland the main high-Mg swarms are c. 2.2 Ga. The high-Mg dykes have high Si, LILE and LREE and are low in Ca, Al, Ti and Nb. Isotopic compositions (Sr, Nd and Pb) demonstrate a high degree of crustal contamination. Individual dykes are remarkably constant in composition but there are large variations between adjacent dykes. Tholeiitic swarms were emplaced at c. 2.2 Ga in the southern part of the craton in SW and SE Greenland and Labrador. The majority are normal moderately evolved tholeiites with no marked LILE or LREE enrichment, a few c. 2.2 Ga comparatively mafic tholeiitic dykes show a LILE and LREE enrichment. Dense swarms of distinctive Fe-enriched hornblende-bearing tholeiitic dykes (the c. 2.0 Ga Kangâmiut dykes) intruded the northern part of the NAC in Greenland. Many are composite with intermediate centres. The Kangâmiut dykes were emplaced at depth during shearing in a regional N-S compressional regime and developed metamorphic mineral assemblages at the time of dyke injection. The 1.83 Ga Avayalik dykes from the eastern foreland of the Torngat orogen, northern Labrador show comparable syn-tectonic emplacement and autometamorphic assemblages. Emplacement was controlled by regional sinistral shearing after the first calc-alkaline igneous activity had occurred within the orogen.

Discrepancies between neodymium, lead and strontium model ages from the Precambrian of southern East Greenland: Evidence for a Proterozoic granulite-facies ev

Abstract Sm-Nd (tDM) model ages for gneisses from the coastal region between 66° and 68°N in southern East Greenland range from 3.02 to 2.79 Ga and indicate that these basement rocks were formed in a major late Archaean episode of sialic crust formation between 3.0 and 2.8 Ga ago. Very low concentrations of U have resulted in unradiogenic Pb-isotopic compositions, so that most samples do not yield chronologically useful Pb/Pb isochrons. The data have been used to calculate Pb/Pb model ages, and in the northern and southern parts of the study area these are in good agreement with the Sm-Nd model ages. In the central part of the area, however, Pb/Pb model ages are much lower than Sm-Nd ages, locally as low a.80 Ga. This discrepancy is attributed to loss of U long after the crust formation event, and probably related to a phase of Proterozoic granulite-facies metamorphism that has not previously been recognised. Rb-Sr isotope data support this interpretation.